Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

Review pubs.acs.org/CR

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction Minhua Shao,*,† Qiaowan Chang,† Jean-Pol Dodelet,‡ and Regis Chenitz‡ †

Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ‡ INRS-Énergie, Matériaux et Télécommunications, 1650, boulevard Lionel Boulet, Varennes, Quebec J3X 1S2, Canada ABSTRACT: The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core−shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbonbased materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

CONTENTS 1. Introduction 2. ORR Activity Screening Technique Based on Rotating Disk Electrode 3. ORR on Pure Pt Surfaces 3.1. Structure Effect on Bulk Single Crystals 3.2. Particle Size/Shape Effect 4. Pt Alloys 4.1. Pt−Late Transition Metal Alloys 4.1.1. Activity Enhancement Mechanisms and Surface Segregation 4.1.2. Ternary and Quaternary Pt Alloys 4.1.3. Particle Size Effect 4.2. Pt−Early Transition Metal Alloys 4.3. Ordered Pt Alloys 4.4. Pt Alloys with NSTF Structure 4.5. Porous Pt Alloys 4.6. Shape-Controlled Pt Alloys 4.6.1. Synthesis 4.6.2. ORR Activity 4.7. Nanowires, Nanorods, and Nanotubes 5. Core−Shell Structures 5.1. Cu-Mediated Deposition 5.1.1. Core Material Effect 5.1.2. Core Structure Effect 5.1.3. Scale-Up and MEA Testing 5.2. Chemical Reduction 5.3. Dealloying 5.4. Other Methods 6. Pd-Based Electrocatalysts 6.1. Structure Dependence 6.2. Pd Alloys 7. Metal Oxides, Nitrides, Oxynitrides, and Carbonitrides 7.1. Metal Oxides © 2016 American Chemical Society

7.2. Metal Nitrides and Oxynitrides 7.3. Metal Carbonitrides 8. Metal Chalcogenides 8.1. Noble Metal-Based Chalcogenides 8.2. Non-noble Metal-Based Chalcogenides 9. Carbon-Based Non-noble Metal and Metal-Free Catalysts 9.1. Initial Performance in H2/O2 PEM Fuel Cells 9.2. Synthesis of Non-noble Metal Catalysts 9.3. Initial Performance in H2/Air PEM Fuel Cells 9.4. Durability of Non-noble Metal Catalysts 9.5. Origin of Activity Loss in Fuel Cells 9.6. Comparison with Pt 10. Conclusions Author Information Corresponding Author Notes Biographies Acknowledgments References

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1. INTRODUCTION Similar to batteries, fuel cells convert chemical energy of fuel and oxidant into electric energy. Yet unlike batteries, they do not need recharging as long as fuel and oxidant are continuously supplied. When hydrogen is fed as fuel, the fuel cell only generates electricity, water, and some heat. As compared to thermal engines, the advantages of fuel cells are high efficiency, no environmental pollution, and unlimited sources of reactants. Therefore, fuel cells are expected to come into widespread commercial use in the areas of transportation,

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Received: August 7, 2015 Published: February 17, 2016 3594

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Figure 1. Price of the elements (in $/kg) versus their annual production (in kg/yr).1Reprinted with permission from ref 1. Copyright 2012 Royal Society of Chemistry.

focus on low- and nonplatinum electrocatalysts including advanced Pt alloys, core−shell structures, carbon-based non-noble metal catalysts, Pd-based catalysts, metal oxides, and chalcogenides. The progress of catalyst supports has been reviewed thoroughly recently12,13 and will not be included here.

stationary, and portable power generation, and thus will help solve the global problems of energy supply and clean environment. Among all of the existing fuel cells, the proton exchange membrane fuel cell (PEMFC) has been actively developed for use in vehicles, portable electronics, and combined heat and power (CHP) systems due to its simplicity, low working temperature, high power density, and quick start-up. PEMFCs are especially well suited as the main power sources for automobiles and buses. Fuel cell vehicles (FCVs) have been considered as one of the final solutions for automotive business and have profound advantages over battery powered electric vehicles (EVs). Indeed, the first mass produced FCVs, the Toyota Mirai (“future” in Japanese), have been commercially sold in Japan since 2014 and are going to be available in North America in 2015 at a price of ∼57 000 US dollars. One of the main reasons for the high sale price of the Mirai is the high Pt loading in the fuel cell stacks. At the anode of a PEMFC, hydrogen is oxidized to produce electrons and protons that are transferred to the cathode through an external circuit and the proton exchange membrane, respectively (H2 → 2H+ + 2e−). At the cathode, oxygen is reduced by reaction with protons and electrons to produce water (1/2O2 + 2H+ + 2e− → H2O). Both the anode and the cathode electrodes consist of highly dispersed Pt-based nanoparticles on carbon black to promote the reaction rates of the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR). The reaction rate of HOR on Pt is extremely fast so that the Pt loading at the anode can be reduced to less than 0.05 mg cm−2. On the other hand, at the cathode, the sluggish reaction kinetics of ORR even on the best Pt-based catalyst requires much higher Pt loading (∼0.4 mg cm−2) to achieve a desirable fuel cell performance. Pt is a scarce and expensive metal as shown in Figure 1.1 Therefore, reducing its loading or even completely replacing it with an abundant and cheap metal would be advantageous. Recent intensive research efforts have led to the development of less expensive and more abundant electrocatalysts for PEMFCs. These include advanced Pt alloys, core−shell structures, transition metal oxides and chalcogenides, and carbonbased non-noble metal composite catalysts. Some of the progress has been summarized in several reviews.2−11 This Review aims to summarize recent advances in the past eight years of electrocatalysis in oxygen reduction in acidic media, with a particular

2. ORR ACTIVITY SCREENING TECHNIQUE BASED ON ROTATING DISK ELECTRODE Ideally, newly developed ORR catalysts should be evaluated in a fuel cell environment and compared to the benchmark, for instance, the state-of-the-art Pt/C. In most cases, this approach is impractical because the membrane electrode assembly (MEA) fabrication and test require special skills, equipment, and abundant materials. Fast screening techniques are more suitable to characterize the electrochemical behaviors of newly developed materials at the lab scale. Rotating disk electrode (RDE) with a porous catalyst layer has been the most widely used technique to characterize the supported catalysts in liquid electrolytes since it was proposed by Stonehart and Ross at the United Technologies Corp. (UTC) in 1976.14 The general recipe of fabricating the thin film-RDE commonly used today was developed by Gloaguen et al. in 1994.15 The history of the development of this technique was summarized by Schmidt and Gasteiger.16 Catalyst powders are typically dispersed in a water/ alcohol mixture forming a uniform ink, which is then deposited on glassy carbon electrodes to form catalyst films. To mitigate the mass transfer effect during ORR activity measurements, glassy carbon electrodes are rotated to increase the mass transfer rates of O2 at the electrode surface. The intrinsic activity (kinetic current without mass transfer effect) of the catalysts can be derived according to the Koutecky−Levich equation: 1 1 1 1 1 = + = + * j jk jl,c jk 0.62nFAC0 D02/3v−1/6 ϖ1/2

(1)

where j, jk, and jl,c are the measured current density, kinetic current density, and diffusion-limited current density, respectively. The diffusion-limited current is determined by the number of electrons transferred (n), the Faraday constant (F), the electrode’s geometric area (A), the concentration of dissolved O2 in solution (C*0 ), the diffusion coefficient of O2 (D0), the kinetic viscosity of the solution (v), and the rotation speed of the electrode (ϖ). jk is generally extrapolated from the Koutecky−Levich 3595

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plot (j−1 vs ω−1/2) at various ration speeds. Alternatively, it can also be derived by using the measured diffusion-limited current at a single rotation speed (typically 1600 rpm). In practice, catalyst films usually consist of Nafion ionomer as a binder to keep the catalysts on the RDE when the electrode is rotated. Diluted Nafion solution can be added into the powder/solvent mixture during the ink preparation step or dropped on the catalyst film to form a thin Nafion layer. In either case, the Nafion content should be as low as possible to minimize extra diffusion resistance of O2 and IR drop caused by the Nafion film.17,18 As a general rule, the Nafion film should not exceed 0.2 μm when it is casted on the top of the catalyst layer, or the content of solid Nafion is less than 20 wt % in the catalyst film when it is mixed in the ink. It is important to bear in mind that the Koutecky−Levich equation is based on smooth electrode surfaces under laminar flow hydrodynamics.19,20 Therefore, the quality of a given catalyst film has a great impact on the accuracy of kinetic current calculation in the RDE measurements. A good catalyst film is thin, uniform, and smooth. Thick films lead to increased mass-transport resistance through the film and incomplete utilization of the catalyst. For Pt-based catalysts, the thickness of the catalyst layer is mainly determined by the amount of carbon black. For catalysts with different Pt loadings, it was recommended that the weight of carbon black stayed constant on the RDE so that the thicknesses of the catalyst films were roughly the same.21 It is obvious that irregularly built-up films (nonuniform coverage, very rough surface, etc.) must be avoided in RDE measurements because the Koutecky−Levich equation is not valid anymore under these conditions. To obtain reproducible results in RDE measurements, the first task is to prepare a well-dispersed ink. Because of different surface properties of carbon black (and other noncarbon supports) and catalyst nanoparticles, it is difficult to design a universal recipe for all catalysts. In addition to water, alcohol (typically isopropanol and ethanol) is usually added into the ink to help wet the carbon surfaces. The alcohol/water ratios are highly dependent on the types of the carbons and catalysts, and pretreatment conditions (annealing, acid treatment, etc.), and should be tuned for different catalysts. Even with a fine ink, a uniform catalyst film is not always guaranteed. The dry conditions play a great role in the film quality. Recently, Garsany et al.22,23 were able to reproducibly fabricate good quality catalyst films by rotating the glassy carbon electrode at 700 rpm with the ink droplet on the top. The standard deviations in the electrochemcial active area (ECA) and ORR measurements were much smaller with catalyst films made by the rotational drying method, and the activities were also 70% higher than those measured with ununiformly covered films. Ke et al.24 also developed an “intermittently microcontact-coating finedroplets” method to uniformly cover the electrode with more than 3000 tiny droplets (3 nL per droplet) to overcome the reproducibility issue. Similarly to previous work, higher activities were obtained with catalyst films prepared by the finedroplets methods. Shinozaki et al.25 found that drying under the isopropanol gas environment could also produce a highquality film and in turn good activity in ORR measurement. In the same study, the Nafion ionomer that has been commonly used as the binder in the catalyst film was confirmed to negatively affect the measured activity. The contribution of the block effect from Nation ionomer to the specific activity at 0.9 V for Pt/C was about 0.15 mA cm−2Pt.

In addition to the ink and film qualities, other important factors include the purity of the electrolyte, the potential scanning rate, the flow rate of O2, and the position of the reference electrode.26 It is well-known that ORR of Pt-based catalysts is very sensitive to the anions in the electrolytes.27 Even the purest HClO4 available on the market has a tiny amount of Cl− in it, which will definitely impact the ORR polarization curves. Shinozaki et al.26 found the specific activity of poly-Pt electrode measured in a regular ACS grade HClO4 solution was 3 times lower than that measured in a high-purity Veritas Doubly Distilled (GFS) solution. The specific adsorption of anions in turn is responsible for the scanning rate-dependent activity in RDE measurements.26,28 Fast scanning rates result in higher ORR activities due to fewer amounts of accumulations of poison species and oxide formation.29 A scanning rate of 10 or 20 mV has been recommended.28 The background currents including the capacitive currents and Faradaic currents associated with H adsorption/desorption and oxide formation/reduction processes are recommended to be subtracted from the ORR polarization curves, especially for the high carbon loadings on the electrode and scanning rates. The IR drop in the RDE measurement not only depends on the concentration of the electrolytes and temperature, but the position of the reference electrode. The Ohmic resistance and distance of Luggin capillary from the electrode surface follow Newman’s disk model.20 Fortunately, the resistance does not increase significantly from 2 mm (28.1 Ω) to 20 mm (30.4 Ω) as measured in a 0.1 M HClO4 solution at room temperature.30 Therefore, a slight change of the reference position does not affect the polarization curve much. The IR drop is proportional to the current, that is, larger at the higher currents close to the diffusion-limited current. The IR drop at half-wave potential for a 5 mm diameter RDE at 1600 rpm (jl,c ≈ 1.2 mA) is 18 mV assuming a 30 Ω resistance, causing a considerable error in the calculation of kinetic current.30 Thus, it is strongly recommended to correct the IR drops especially when the measured ORR curves are far away from the benchmark curve. Taken together, it is important to list all of the measurement conditions and data analysis method when reporting the activity data. Table 1 lists the ECAs and activities of two commonly used benchmark Pt/C catalysts (50% Pt on Ketjen black from Tanaka Kikinzoku Kogyo (TKK), and 20% Pt on Vulcan from E-TEK) measured by different groups with different data processing approaches. Without background current and IR drop corrections, the Pt mass and specific activities of 50% Pt/C at 0.9 V are around 0.22 A mg−1 and 0.25 mA cm−2, respectively. For 20% Pt/Vul, they are around 0.2 A mg−1 and 0.35 mA cm−2. After corrections, these values increase in general but need more data to confirm the absolute values. Finally, the ECA calculation is not as straightforward as one thinks. Most of the ECA values in the literatures were calculated from the hydrogen adsorption/desorption (HUPD) charges for Pt-based catalysts.31,32 In a standard practice of HUPD charge calculation, a constant double layer current was extrapolated into the hydrogen adsorption region. However, it was pointed out by Mayrhofer et al.33and Binninger et al.34 that the double layer current was not a constant for the high surface area catalysts due to the influence from the high surface area supports. The effect from the supports was particularly significant when they were metal oxides. Thus, the CV in the hydrogen region has to be corrected by the capacity from the support, which can be obtained by recording the CV in a CO saturated electrolyte. Some recent studies found that the areas 3596

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Table 1. ECAs and Activities of Benchmark Pt/C Catalysts at 0.9 V Taken from the Literaturea catalysts ∼50% Pt/C (TKK)

∼20% Pt/Val (E-TEK)

ECA (m2 g−1) 80 96 72 85 72 79 78 105c 80d 91 101 61d 72

MA (A mg−1) 0.21 0.22 0.23 0.20 0.06 0.40 0.28 0.27 0.43 0.51 0.87 0.31 0.19

b

SA (mA cm−2)

background correction

IR drop compensation

scanning rate (mV s−1)

refs

0.26 0.23 0.31 0.24 0.08 0.51 0.36 0.29 0.54 0.60 0.86 0.51 0.26

N N N N N Y Y Y Y Y Y Y N

N N N N N N N N Y Y Y N N

20 10 10 10 5 10 10 10 20 25 20 20 20

28 39 40 41 42 24 43 44 23 45 25 46 28

Assuming a charge density of 210 μC cm−2 for H adsorption, in 0.1 M HClO4 solution, room temperature, and 1600 rpm unless otherwise mentioned. bAt 60 °C. cCharge density was not disclosed. dAssuming 200 μC cm−2. a

Figure 2. Cathodic current density of the ORR at 0.90 V against the step atom density in 0.1 M HClO4: surfaces with (111) terrace (A), surfaces with (100) terrace (B). The value of n shows the number of terrace atomic rows. Possible active sites for the ORR on the surfaces with (111) terrace (C). Solid and broken rectangulars show the (111) terrace edge and the (111) terrace atomic row neighboring to the edge, respectively. Circles, triangles, and squares present the position of on-top, 3-fold, and bridged sites, respectively.52 Reprinted with permission from ref 52. Copyright 2013 Elsevier.

(RRDE) experiments.38 The hydrogen adsorption was significantly suppressed in Pt alloys due to the alternated electronic properties of Pt surfaces resulting in an underestimation of ECA by nearly 50% using HUPD. In addition to the alloying effect, the shape/structure of the nanoparticles also made the calculation more complicated. The surface area could be

derived from the HUPD were underestimated especially for Pt alloys,35−37 and the estimations from CO stripping and Cu underpotential deposion (UPD) charges were more accurate.37 In the case of Cu UPD, the small interference on the UPD charges from the coadsorption of anions (SO42−, HSO4−, etc.) can be compensated by conducting rotating ring disk electrode 3597

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underestimated by more than 2 times using HUPD on octahedral shape Pt alloy nanocrystals.37 Thus, caution has to be taken in calculating the ECAs of noble metal-based catalysts with various compositions and structures.

(111) and (100) terrace sites decrease dramatically, while the low coordination number edges and kinks become predominant sites in the surface. Because of the much stronger oxygen binding energies on the later, the ORR activity is expected to be lower than that of large particles. As a result, the specific activity of Pt nanoparticles decreases sharply when they are smaller than 3 nm, as observed by many groups, and a typical result is shown in Figure 3A. In an attempt to minimize the effects from

3. ORR ON PURE Pt SURFACES 3.1. Structure Effect on Bulk Single Crystals

The ORR behaviors of low index Pt surfaces, that is, Pt(111), Pt(100), and Pt(110), have been extensively studied. The results were summarized in several comprehensive reviews27,47 and are not discussed here. The principal conclusion is that the activity of ORR in a weakly adsorbed electrolyte, such as HClO4 solution, follows the order of Pt(100) ≪ Pt(111) ≈ Pt(110). Feliu et al.48−51 and Hoshi et al.52 systematically studied the structural effects of high index Pt surfaces and concluded that the ORR activities were highly dependent on the orientation of the steps and terraces on the surfaces. It was found that the activity increased with the increase of terrace density (or the decrease of terrace width) on high index planes n(hkl)-(mno) except for the n(110)-(111) surfaces (n represents the number of terrace atomic rows, (hkl) and (mno) present the structures of the terrace and step, respectively).48−51 In a later study, Hoshi et al.52 confirmed this general trend on (111) terrace except for the surface with the number of the terrace atomic rows n = 2, as shown in Figure 2A. In contrast, the ORR activity does not depend on the step density on (100) terrace as shown in Figure 2B. The active sites on high index surfaces with (111) terrace were proposed to locate at the (111) terrace edge and its neighboring terrace row, as illustrated in Figure 2C.52 The mechanism of higher ORR activity on high index planes, however, has not been well understood.53 These results may be important guidelines in the development of more active pure Pt catalysts, which should possess a stepped surface instead of a smooth (111) surface because Pt(111) is less active than its vicinal stepped surfaces. According to Figure 2A, Pt nanocrystals with high index facets such as (221) = 4(111)-(111) and (331) = 3(111)-(111) are expected to exhibit activities over 3 times higher than (111). Attempts to synthesize such high index facets Pt nanocrystals have been taken.54−58 The stability of these specially designed structures during potential cycling is a big concern.

Figure 3. (A) Specific (blue ◆) and mass (red ■) activities as a function of Pt particle size in a 0.1 M HClO4 solution at 0.93 V without background or IR correction. Scanning rate = 10 mV s−1. The particle size was controlled by a Cu-UPD-Pt-replacement method.41 The specific (blue ◇) and mass (red □) activities of state-of-the-art Pt/C from TKK (TEC10E50E, 46.7 wt %) with an average particle size of 2.5 nm were also included for comparison. (B) Specific and mass activities as a function of electrochemical active area (ECA) in different electrolytes measured at room temperature at 0.9 V. Activities were analyzed from the IR compensated positive-going sweeps at 50 mV s−1, after subtraction of the capacitive background.95 Reprinted with permission from ref 41. Copyright 2011 American Chemical Society. Reprinted with permission from ref 95. Copyright 2011 American Chemical Society.

3.2. Particle Size/Shape Effect

Inspired from the ORR activity trend obtained on Pt single crystals, that is, high index planes > (111) > (100), efforts have been taken to synthesize shape-controlled nanostructures to optimize the structure effect. Octahedral Pt nanoparticles bound by (111) facets were found to be more active in ORR than cubic ones bound by (100) facets, consistent with the bulk single crystal work.35 Nanoparticles with high-index planes including tetrahexahedron (hk0), trapexezohedron (hkk), and trisoctahedron (hhk) with at least one Miller index being larger than unity have demonstrated higher activity than (111) or (100).48,49,59−61 This activity enhancement was assigned to the high density of low-coordinated atoms situated on steps, ledges, and kinks.51,62 The main issue with the shape-controlled Pt nanoparticles is their stability under ORR condition as they tend to evolve to thermodynamically equilibrated shape.63 The Pt particle size effect on ORR has been a long-standing problem that has yet to be solved.63−89 The structural dependence activity observed on bulk Pt single crystals has been used to predicate the particle size and shape effects on the ORR.90 As the particle size changes from 5 to 1 nm, the distributions of

size distribution of different samples and errors in RDE measurements, the activity of Pt particles ranging from 1 to 5 nm was measured using one Pt/C thin film electrode with different sizes being synthesized by layer-by-layer growth using a Cu-UPD-Pt-replacement method.41 The maximum mass activity was observed around 2.2 nm (Figure 3A). Similar conclusions have been drawn on the basis of density functional theoretical calculations.91−94 On the other hand, other researchers argued that the specific activity does not depend on the particle size, even in the range 3598

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of 1−5 nm. Nesselberger et al.95 found that the difference in specific activity between carbon supported Pt particles with various size between 1 and 5 nm was very small, and the mass activity increased linearly with increasing catalyst dispersion regardless of the electrolytes used, as shown in Figure 3B. This trend certainly cannot be explained by the ideal single crystal facet distribution model. A possible reason can be the change of the effective reaction pathway of the ORR with the particle size. Because of the increase of oxygen binding energy on the surface of the smaller Pt particles, the rate-determining step may change from the first proton and electron transfer on large particles to O−O bond breaking on particles smaller than 3 nm.93,95 Other studies implied that the specific activity does not depend on the particle size but on the interparticle distance.78−81,96,97 Nesselberger et al.96 prepared and loaded small Pt clusters (0.6, 0.8, and 2.3 nm) on glassy carbon electrodes using a mass-selection technique. The interparticle distance was well controlled by adjusting the coverage of Pt clusters on the electrode. It was found that the ORR activities of these well-defined Pt clusters increased with decreasing the interparticle distance. Decreasing the interparticle distance overlapped the electric double layers between neighboring particles, leading to a potential drop within the compact layer and consequently weaker adsorption energy on Pt surfaces. This particle proximity effect has been also observed on Pt particles (∼2 nm) supported on high surface area carbon with various Pt loadings.97 Fabbri et al.98 confirmed that the dispersion of Pt nanoparticles on carbon support significantly affected the further reduction of H2O2. The H2O2 yield increased dramatically as the interparticle distance increased from extended layer to welldispersed particles. Pt nanoclusters (less than 1 nm in diameter) that are not normally used in fuel cells have shown some interesting activity trend.88,96,99−101 For instance, Pt12 (∼0.9 nm) clusters were over 10 times more active than that of 2.5 nm Pt/C (TKK).88 By just adding one Pt atom to form Pt13, the ORR activity decreased by more than 2 times.99 This significant activity loss was attributed to the structural evolution from C2v Pt12 to icosahedral Pt13, which had a much stronger oxygen binding energy than that of the former. By adding more Pt atoms to the icosahedral Pt13 core, the resulting Pt17 and Pt19 with edge structure showed higher activity than Pt13.101 This result further confirmed that the stable icosahedral Pt13 structure was not active toward ORR. Because of a lack of image and probe techniques to directly identify the arrangement of surface atoms of a Pt particle, it is almost impossible to establish a real relationship between the ORR activity and the shape/size of nanoparticles. Even on the surface of a well-defined nanostructure, an octahedron, for example, TEM images reveal that there are numerous defects/ steps instead of smooth terraces.63,102 Angelopoulos et al.91 tried to correlate the surface active sites with the ORR activity on Pt nanoparticles in the size range of 1.8−6.9 nm using the Bi and Ge specific adsorption technique. It was concluded that the predominant active sites were (110) and (311) rather than (111) terraces atoms that were historically believed to be the main active sites for ORR. The role of the stepped surface atoms in a Pt nanoparticle needs to be further investigated to fully understand the size and shape effects on the ORR.

4. Pt ALLOYS 4.1. Pt−Late Transition Metal Alloys

Since the discovery of Pt alloys as superior ORR catalysts for fuel cells at UTC in the 1980s,103−105 they have attracted great attention and been considered as the second generation fuel cell catalysts after pure Pt.28,106−111 Indeed, they have been used in the UTC’s stationary PAFCs and the Toyota Mirai FCVs due to their higher activity and durability than pure Pt. The membrane and ionomer contamination caused by the transition metals dissolved during fuel cell operation delayed their applications in PEMFCs in early years. With proper posttreatment (acid washing, for example) of Pt alloys, their better activity and durability than Pt/C in PEM fuel cells have been confirmed.28 4.1.1. Activity Enhancement Mechanisms and Surface Segregation. Various reasons have been proposed to explain the higher ORR activity of Pt alloys. They include compressive strains due to shorter Pt−Pt bond distances,112−114 higher surface roughness caused by the transition metal dissolution,115 downshifting the d-band center of Pt116 or changing the d-band vacancy117 due to strain and ligand effects, delayed formation of oxide species,33,118 etc. It has been generally agreed that a Pt-skin like surface is formed during the initial acid treatment and potential cycling. The structural and electronic effects of transition metals in the core and subsurfaces play a significant role in weakening the adsorption of oxygen containing species. The lower coverage of these oxygen containing species has been thought to be beneficial to enhance the ORR activity because they poison the active sites. This argument has been challenged recently by a few studies.119−121 Using the electrochemical quartz crystal microbalance (EQCM), Omura et al.119 found that the coverage of oxygen containing species on Pt-skin/Pt3Co thin film was higher than that on pure Pt in the potential range of 0.86−0.96 V (RHE). This observation opposed the conventional model of delayed formation of oxides on Pt alloys as the primary reason for the ORR activity enhancement. The higher oxide coverage and faster oxide growth rate on PtCo than Pt were also observed by Huang et al. in their potential-hold measurements (both RDE and MEA).120 These results implied that other mechanisms play a role in determining the activity of Pt surface besides the oxide coverage. The activity enhancement of Pt alloys originates from the transition metals. Thus, the type and amount of the transition metals certainly have pronounced effects on the ORR activity, which was summarized by Wang et al. recently.5 PtM alloys consisting of different metals M (M = Co, 122−137 Ni, 1 2 3 , 1 3 3 , 1 3 8 − 1 4 2 , 1 1 5 , 1 2 8 , 1 3 3 , 1 3 6 , 1 3 7 , 1 4 3 − 1 4 6 Fe, 1 4 7 − 1 5 0 Cu,123,133,145,151−159 Ag,160−163 Au,161,164−169 Pd,161,170−174 Cr,175,176 Mo,177 Mn,178 Al,179 etc.) have been synthesized, and their activities have been compared. Among them, Co, Ni, and Fe have been studied more intensively due to their superior activities. Stamenkovic et al.116found that the activities of the as-sputtered polycrystalline films followed the order of Pt < Pt3Ti < Pt3V < Pt3Ni < Pt3Fe ≈ Pt3Co. A recent study of Han et al. indicated that the ORR activity and stability of Pt alloys correlated with the dissolution potentials of the alloying elements.180 A low dissolution potential of a transition metal resulted in a high ORR activity but low chemical stability. During electrochemical measurements, the surfaces of Pt alloys became pure Pt due to dissolution of non-noble metals (noted as Pt-skeleton). After a mild thermal annealing at 1000 K, 3599

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the same time besides particle size. Wang and co-workers227 synthesized Pt3Co nanoparticles with different sizes from 3 to 9 nm via an organic solvothermal method with synthesis temperatures between 225 and 145 °C. No further thermal annealing was conducted except for removing surfactants at 185 °C in an oxygen environment. The specific activity of Pt3Co increased with increasing particle size as smaller nanoparticles were oxidized at a lower potential, which led to a stronger adsorption of oxygenated species and thus a lower ORR activity. The size-dependent mass activities presented volcano-shape behavior, and the maximum mass activity was found around 4.5 nm due to the two opposite trends in specific surface area and specific activity with particle size (Figure 4). On the contrast,

Pt atoms segregated to the surface, while non-noble metal atoms moved to the sublayers resulting in a Pt-skin type surface. The Pt specific activity trends of the Pt-skin and -skeleton were slightly different with the former following the order of Pt < Pt3Ti < Pt3V < Pt3Fe < Pt3Ni < Pt3Co. The specific activity enhancement factor of annealed Pt3Co was ∼5, while it was 3 for the nonannealed one. This result emphasized the importance of the post treatment of Pt alloys in tuning the ORR activity by engineering the surface structures of Pt alloys.28,169,181,182 The Pt atom segregation was also observed on Pt3Co nanoparticles (5 nm) by annealing the acid washed samples at 727 °C.183,184 The Pt-rich surface and Co-rich layer beneath it were confirmed by scanning transmission electron microscopy (STEM). The reduction of the Pt−Pt bond distance among the Pt surface atoms might be the main reason for the enhanced ORR activity. During thermal annealing, the particle size of Pt alloys and the properties of carbon support might be changed, which could have a negative impact on the fuel cell performance. Mayrhofer et al.185 developed a CO annealing method either in a gas or in a liquid phase. By annealing the Pt3Co/C in CO stream at 200 °C, or electrochemical cycling in a CO saturated alkaline solution, Pt atoms could segregate to the top layer due to a higher adsorption enthalpy of CO on Pt than on Co. During the CO annealing in the liquid cell, there was no Co dissolution into the electrolyte. After CO annealing, the specific activity was increased by ∼50% due to the surface reconstruction. Ciapina et al. found that CO-induced surface segregation could also occur in acid solution.181 A similar Pt segregation phenomenon was observed in Pt3Au186 and PtAu169 nanoparticles. Recently, the post-treatment of Pt−Ni NPs has been studied intensively.187−190 In addition to the surface segregation, thermal annealing can also induce the reorientation of the surface. Chung et al. found that the ratio of (111) facet to all surface atoms was 35% after annealing in a mixture of H2 and Ar at 700 °C, while this value was only 25% at 300 °C.189 The Pt- and (111)-enriched surface of the former resulted in a 2-fold activity enhancement over the latter. The composition of the subsurface of the alloy also has a significant impact on the ORR activity. With the assistance of theoretical calculations, Gao and Muller discovered that Pt3Ni(111) with the first three layers and the fourth layer being pure Pt and Ni, respectively, had the highest activity. The ORR activity of the structure with the first and second layers being pure Pt and Ni, respectively, was 3 orders of magnitude lower.191 4.1.2. Ternary and Quaternary Pt Alloys. Besides bimetallic alloys, ternary,151,178,192−222 quaternary,178 and quinary223 alloys also have been studied in recent years. Various approaches, such as combinatorial high throughput screening,224 DFT calculations,196 and facile synthesis strategy,178,225 have been applied to optimize compositions of alloys. The combinations of Pt−MN (M, N = Fe, Co, Ni, Ti, V, Sn, Cr, Mn, Mo, Ag, Au, Pd, Ir) have been synthesized and evaluated by different groups. Because of the possible synergetic effects, the activity and stability of the ternary alloys might be higher than the corresponding binary ones.133,225,226 4.1.3. Particle Size Effect. The particle size effect of Pt alloys on ORR activity is even more complicated than that of pure Pt. In addition to effects purely from size, other parameters like composition, degree of alloying, annealing temperature, and shape also play roles in determining the activity.227−229 It is difficult to make a meaningful conclusion if other parameters (annealing temperature, composition, etc.) are also changed in

Figure 4. Specific and mass activities of Pt3Co/C at 0.9 V with different particle sizes measured in 0.1 M HClO4 solutions at a scanning rate of 20 mV s−1 and rotation speed of 1600 rpm.227 Reprinted with permission from ref 227. Copyright 2009 American Chemical Society.

Loukrakpam et al.230 found that the mass activity of Pt3Co/C decreased with particle size increasing from 3 to 8 nm. In their study, the nanoparticles were synthesized via a similar solvothermal method but annealed in 7% H2 at 400 °C. The trend, however, was not observed on Pt3Ni/C, the mass activity of which increased gradually up to 8 nm. Several studies focused on the size effect of Pt alloys after a heat treatment.200,228,231,232 With an increase in annealing temperature, the particle size and the degree of alloying increase. Wang et al.228 annealed the Pt3Co/C with an initial particle size of 4.5 nm from 300 to 800 °C. The sintering did not occur until the temperature was 500 °C or higher, while the specific activity continuously increased with annealing temperature. It was believed that the activity enhancement below 500 °C was caused by surface smoothing, removing surface defects, and Pt segregation. Wanjala and co-workers200 studied the thermal treatment results of PtCoNi with the temperature range from 400 to 926 °C. The particle size did not change dramatically between 400 and 800 °C (within 1 nm), while the lattice constant decreased by 2%. Both the specific and the mass activities of the catalysts increased with temperature. The enhanced activity toward ORR could be ascribed to the lattice shrinkage. 4.2. Pt−Early Transition Metal Alloys

By alloying with some early transition metals or rare earths, such as Y,233−238 Sc,235,239 Hf,235 La,240−244 Ce,241,243,245 Ga,246 and Gd,247 the activity and stability of Pt can be significantly improved. In some studies, their activities were even higher than that of Pt late-transition metal alloys (Pt−Co, Pt−Ni, 3600

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Pt−Fe, etc.). Sung and co-workers117 found that the activities of Pt3M (M = Y, Zr, Ti, Ni, and Co) films followed the order of Pt3Ti < Pt < Pt3Zr < Pt3Co < Pt3Ni < Pt3Y. In another study carried out by the same group, Pt3La was also found to show a high activity.113,242 Malacrida et al.,241 however, demonstrated that the activity of Pt3La was only comparable to that of Pt. Instead, Pt5La, Pt5Ce, Pt5Gd, and Pt3Y had much higher activity than that of Pt3La, as shown in Figure 5. The activity

temperature with excess NaCl,258 SiO2,261 or MgO coating260 annealed in an inert262 or reducing259 atmosphere. It was found that Pt3Fe is more stable than PtFe. Wang et al.255 obtained the ordered Pt3Co/C by annealing the mixture of Pt/C with Co salts in a H2/N2 mixed atmosphere at 700 °C. The activity and stability of ordered Pt3Co/C were found to be higher than that of disordered Pt3Co/C prepared at 400 °C. They believed that that the unique structure consisting of a Pt-rich shell and a stable intermetallic Pt3Co core were responsible for its superior performance. Even starting with a transition metal enriched composition, the ordered structure still can be maintained using a proper post-treatment method. Wang et al.265 found that the ordered Cu3Pt structure was maintained as the core, and a Pt thin layer formed as Cu atoms leached from the near surface layers during potential cycling. This unique core−shell structure had higher activity that that prepared by acid washing during which the ordered structure was completed destroyed. Theoretical studies have been carried out to explain the different electrochemical properties between ordered and disordered Pt alloys.266,270 The results suggested that activity enhancement might be due to the stronger Pt−metal covalent bond and more negative formation heat of ordered Pt alloys as compared to that of the disordered one. The claim of higher activity with an ordered structure is not conclusive because the absolute activities of ordered Pt alloys reported so far were not that high and the portion of ordered structure based on the XRD data was unknown. A systematic comparison between fully ordered and disordered Pt alloys with the same composition, particle size, morphology, and annealing temperature has to be conducted.

Figure 5. Specific activities of Pt−early transition metal alloy thin films as compared to polycrystalline Pt at 0.9 V.113 Reprinted with permission from ref 113. Copyright 2012 Royal Society of Chemistry.

enhancement of Pt5La and Pt5Ce over polycrystalline Pt is about 3.5,241 while for Pt5Gd and Pt3Y, the enhancement is even higher, about 5.247,248 A thick Pt overlayer (∼3 monolayers) was formed on the Pt5M by removing La, Ce, and Gd in top layers. This thick Pt overlayer played a significant role in the enhanced activity and stability by protecting active elements, while La continued dissolving from Pt3La without forming a stable core−shell structure.241 DFT work suggested that the enhanced activity of Pt−Y or La was due to the ligand effect from the early transition metals in the sublayers.248,249 The pure Pt overlayer is ∼1 nm thick,235 which excludes the possibility that the enhanced activity is from ligand effect.250 Recently, Stephens et al. proposed that the compressive strain effect might be the reason for the superior activity in Pt−Y, −Ce, and −Gd.235,241 It was found that despite a larger atomic radius of Y than Pt, Pt−Y alloys have smaller Pt−Pt bond distances than pure Pt, resulting in compressive strains. The favorable negative heats of formation of Pt−early transition metal alloys lead to energy barriers in the diffusion of transition metals from the core to the surfaces of catalysts and improve their stability under fuel cell operation conditions.

4.4. Pt Alloys with NSTF Structure

Another unique category of Pt alloys is based on 3M’s nanostructured thin film (NSTF). Pt alloys were sputtered on nonconductive polymer (N,N-di(3,5-xylyl)perylene-3,4:9,10 bis(dicarboximide)) whiskers forming a continuous catalyst film. Such core−shell structure catalysts eliminate carbon corrosion issues and contact resistance between carbon and catalyst that would lead to poor utilization and degradation of the catalyst layer. More importantly, the unique thin film structure significantly reduces the population of the low coordination number atoms and hence increases the specific activity. It was found that the fully formed Pt-based whiskerette films consist of pyramid-like pillars with a cross-section of ∼6 nm. The surface of the catalyst film was dominated by the (111) facets with four extended {111} side facets truncated by a single {100} facet.274 This structure has been demonstrated to have higher chemical and electrochemical stability, and 5−10-fold higher specific activity of ORR as compared to that of carbon supported highly dispersed nanoparticles.275,276 Various Pt−M (M = transition metals) including binary and ternary compositions have been prepared and tested for ORR. For instance, Pt−Co,4,277 Pt−Ni,277−281 Pt−Ir,282 Pt−Co−Ni, Pt−Co−Mn,197 Pt−Co−Fe,277 etc., all showed higher specific activity than conventional catalysts. The compositions and ratios of Pt to the transition metals played a significant role in the ORR activity. Pt3Ni7 with a lattice constant of 0.371 nm showed the highest activity. The severe Ni dissolution in acid, however, led to an increased flooding of the NSTF cathode due to an excess of Ni cations in the membrane enhancing the net water transport across the membrane from anode to cathode.277 Post-treatment such as acid washing may minimize the negative effect from the Ni cations. Another approach is to

4.3. Ordered Pt Alloys

The study of Pt intermetallic (ordered) alloys started in the late 1980s and developed slowly in the 1990s for the lack of research interest due to the poor performance of ordered PtCr251 and PtCo252,253 in both PAFC and PEMFC. Recently, Pt intermetallic alloys have been revisited and argued to be better electrocatalysts than their corresponding disordered ones for ORR, in the aspect of both activity and stability.254 Many ordered compositions, including PtCo,255−257 PtFe,258−262 PtNi,263,264 PtCu,265 PtAl,266 PtZn,267,268 PtFeCo,269,270 PtFeCo,271 PtIrCo,272 and PtAuCu,273 have been synthesized successfully and evaluated for ORR. The ordered structures were usually achieved during high temperature annealing. For instance, PtFe intermetallic alloys could be formed at high 3601

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was reported by Oezaslan et al., although the critical particle size was slightly different due to the differences in composition and synthesis method.297 Synder et al. found that the ORR activity of nanoporous Pt−Ni particles (15 nm) formed by dealloying was slightly higher than that of solid ones.145 When these porous particles were encapsulated with a hydrophobic protic ionic liquid, their ORR activities were further enhanced by 30 mV as measured in both aqueous HClO4 solutions (Figure 7) and fuel cells.298,299

form a Pt-skin type surface with low Ni content. Van der Vliet et al.279 annealed the Pt3Ni NSTF film at relatively low temperature. At 300 °C, the organic whiskers started to evaporate and were completely removed at 400 °C. Meanwhile, the lowcoordinated sites were diminished with (111) facets prevailed on the surface, as shown in Figure 6A−C. This mesostructured

Figure 6. Schematic illustration and corresponding HRTEM images of the mesoscale ordering during annealing and formation of the mesostructured thin film started from the as-deposited Pt−Ni on whiskers (A), annealed at 300 °C (B), and 400 °C (C). Specific activities of Pt−Ni NSTF as compared to polycrystalline Pt and PtNSTF at 0.9 V (D).279 Reprinted with permission from ref 279. Copyright 2012 Nature Publishing Group.

Figure 7. Comparison of oxygen reduction curves of Pt/C, dealloyed Pt−Ni (np-NiPt/C), and ionic liquid encapsulated dealloyed Pt−Ni (nm-NiPt/C + IL) in O2-saturated 0.1 M HClO4 solution at 60 °C, scanning rate = 20 mV s−1, rotation speed = 1600 rpm. The inset is the cartoon illustrating the ionic liquid encapsulated porous nanoparticles.298 Reprinted with permission from ref 298. Copyright 2013 Wiley-VCH.

thin film was 20 times more active than Pt/C (Figure 6D) and showed better durability. The improved activity and durability were assigned to the high percentage of (111) domains and stable Pt-skin surface. Despite their unique advantages, the NSTF thin film structures have severe issues such as flooding and poor proton conductivity in the electrode due to low Pt surface area and pore volume, they are extremely thin, and they have an ionomer free electrode.283−285 Several approaches including coating an ionomer and silica nanoparticles on the NSTF surface, and introducing an additional carbon or Pt/C layer adjacent to the NSTF layer, have been proposed to improve the proton conductivity, water removal, and storage capabilities.286−288

The high solubility of O2 in the ionic liquid and the confined environment of the porous structures were proposed to be the main reasons for the activity enhancement. Chen et al.300 created Pt3Ni nanoframes by dispersing solid PtNi3 polyhedral particles with an average size of 20 nm in hexane or chloroform under ambient conditions for 2 weeks. The transformation process was illustrated in Figure 8. The free-standing nanoframes were then supported on carbon black and further heat-treated at 400 °C to obtain a Pt-skin surface. The hollow structure consisting of 24 edges with width of ∼2 nm was maintained after the heat treatment. The nanoframes showed 22- and 16-fold enhancement over 5 nm Pt/C on mass and specific activities, respectively. The positive effect of ionic liquid in ORR on porous catalysts was also confirmed in this work. After ionic liquid treatment, the enhancement factors increased to 36 and 22 for mass and specific activities, respectively. Dealloying Pt alloy thin films could result in hierarchical porous structures that also showed high ORR activity.301−304 Galvanic displacement of non-noble metal particles (Co, Ni, etc.) with Pt has also been explored to fabricate porous Pt alloys.305−307 The best porous Pt−Ni/C showed a 6-fold enhancement in Pt mass activity over Pt/C and excellent durability in potential cycling.307

4.5. Porous Pt Alloys

As compared to the solid particles, catalysts with nanoporous structures offer more reaction sites and enhance activity via the so-called nanoconfinement effect.289 One of the most efficient ways to prepare porous Pt alloys is dealloying. In this process, the less noble metals are selectively dissolved from an alloy by either a chemical or an electrochemical method.290−294 The dissolution of the less noble atoms on the surface of the alloy generates defects and vacancies, which result in reduction of coordinated numbers of noble atoms. As a result, the mobility of the noble atoms increases. The morphology of the dealloyed material depends on the competition between the dissolution rate of less noble metals and the surface diffusion of noble metals. In bulk alloys, porous structures are generally formed due to the slow diffusion rate of noble atoms on the surface. For Pt-based alloy nanoparticles, the morphology of the final dealloying products strongly correlates with the particle size. For instance, Strasser et al. found a critical particle size of 15 nm for PtNi3, below which the particles tend to form a nonporous (core−shell) structure, while particles larger than 15 nm showed a porous structure.132,295,296 A similar observation

4.6. Shape-Controlled Pt Alloys

4.6.1. Synthesis. It has been known that the activity enhancement on Pt alloys depends on their crystalline orientations. For example, the Pt3Ni(111) is much more active than Pt3Ni(100) and (110).35 The surfaces of conventional Pt alloy particles consist of mixed facets, edges, corners, and other defects that result in low activities. To maximize the structural effect of the Pt alloys, one may want to synthesize shape-controlled 3602

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Figure 8. Schematic illustrations and corresponding TEM images of the samples obtained at four representative stages during the evolution process from polyhedra tonanoframes. (A) Initial solid PtNi3 polyhedra. (B) PtNi intermediates. (C) Final hollow Pt3Ni nanoframes. (D) Annealed Pt3Ni nanoframes with Pt(111)-skin-like surfaces dispersed on high-surface area carbon.300 Reprinted with permission from ref 300. Copyright 2014 American Association for the Advancement of Science.

resulting in a reduced nucleation rate of Pt at the early stage of the synthesis. Fang and co-workers proposed that the yielded W0 from the thermal-decomposition of W(CO)6 could help reduce Pt ions, resulting in a rapid nucleation of Pt clusters in the early stage of the synthesis.309 On the other hand, the work from the Yang group suggested that the CO gas released from the W(CO)6 may play a significant role in shape control of NCs by preferentially binding to certain Pt facets.146 New synthesis protocols based on the use of CO gas were subsequently developed to make shape controlled Pt alloy NCs including cubes, (truncated-)octahedral, and icosahedra.146 The effect of CO gas was further confirmed by a recent study by Choi et al., who demonstrated that only an irregular shape of Pt−Ni nanoparticles was formed when CO gas generated from W(CO)6 was diluted with Ar.317 It was thought that CO preferentially adsorbed on the {100} facets of Pt, resulting in the formation of cubic Pt NCs. The introduction of Ni(acac)2 may alter the adsorption preference of CO from {100} facets of Pt to {111} facets of Pt−Ni. Zhang et al.318 developed a simple impregnation method to obtain octahedral Pt−Ni alloys by heating the dried metal precursors and carbon black mixture in a CO/H2 environment at 200 °C. The critical role of CO in determining the shape of Pt alloys was again demonstrated. Further studies are still necessary to clarify the mechanism of CO adsorption in the development and growth of a particular shape. One of the common properties shared by octahedral Pt−M NCs synthesized by different methods is their large crystallite size with edge length ranging from 9 to 15 nm depending on the methods, precursors, and additives, which is 2−3 times larger than the commercially available conventional Pt alloy catalysts (4−5 nm). As a result, more than 80% of Pt atoms are wasted inside of the particles. One strategy to further improve the Pt mass activity is to synthesize octahedral Pt−M NCs with smaller crystallite sizes by modifying the synthesis protocols. Recent studies have reported Pt−Ni octahedra with 4−6 nm edge length.318,320,321 4.6.2. ORR Activity. The cubic Pt−M NCs only showed limited ORR activity. As compared to conventional Pt−Mn nanoparticles, cubic Pt−Mn NCs were much less active in the HClO4 solutions.310 The cubic Pt3Ni and Pt3Co/C NCs showed slightly higher activities than conventional Pt/C.309 These results are not surprising and consistent with work on

Pt alloy nanocrystals (NCs) with only {111} facets exposed. The Sun group at Brown University first reported the synthesis of cubic PtFe NCs in 2006.308 Following this pioneering work, other Pt−M (M = Mn, Co, Ni, Pd) bimetallic alloy NCs were synthesized.141,146,190,309−319 The synthesis of octahedral Pt−M NCs with particle size in the range of 4−15 nm has been reported by several groups with or without the assistance of CO containing chemicals. In synthesis protocols without CO containing chemicals, Pt(acac)2 and Ni(acac)2 mixed with dimethylfomamide (DMF) in a sealed autoclave were heated to a certain temperature (typically between 120 and 200 °C) to produce sub-10 nm octahedral Pt−Ni.141,314,315 It was found that the precursor ligands played a critical role in controlling the size and shape of Pt−Ni NCs.141 The formation of Pt−Ni octahedra was not observed by mixing K2PtCl6 with Ni acetate, suggesting that acetyl acetonate may modify the nucleation and particle growth kinetics by specific interactions with the nucleation seeds and facets. The detailed mechanisms of the formation of octahedral Pt−Ni NCs, however, have not been studied. Wu et al.314 improved the shape selectivity of octahedral Pt−Ni NCs by adding capping agents poly(vinylpyrrolidone) (PVP) and benzoic acid in the reaction solution, and replacing DMF with benzyl alcohol as the solvent. The complete removal of remaining PVP adsorbed on the surface might be difficult. Huang et al. recently reported a synthesis of ∼4 nm Pt−Ni octahedra using only Pt(acac)2, Ni(acac)2, DMF, and benzoic acid.320 In the synthesis involving CO containing chemicals, the mixture of Pt(acac)2, Ni(acac)2, oleylamine (OAm), and oleic acid (OA) in the presence of W(CO)6 was heated to 230 °C, resulting in 12 nm octahedral Pt−Ni NCs.309 The residual OAm and OA that served as capping agents had to be removed by Ar plasma or acid washing treatments. By introducing a proper solvent (benzyl ether) in the reaction mixture, Choi et al.317 achieved a high yield of octahedral Pt−Ni NCs with negligible capping agents on the surface. The importance of the synergistic combination effects of OAm, OA, and W(CO)6 has been recognized by different groups, but their individual role in the shape selective synthesis of Pt bimetallic NCs is still under debate. The OAm may act not only as a surface stabilizer slowing the NCs growth and preventing them from agglomeration, but also as a coordination ligand with Pt ions 3603

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Table 2. Synthesis Conditions of Pt−Ni Octahedra and Their Corresponding Physical and Electrochemical Properties methods without assistance of CO containing chemicals

with assistance of CO containing chemicals

solvent DMF DMF benzyl alcohol DMF none none benzyl ether diphenyl ether DMF

capping/shape directing agents

temp (°C)

shape selectivity

edge length (nm)

none none PVP, benzoic acid

200 120 150

fair good excellent

8−9 9 12

benzoic acid none OAm, OA, W(CO)6 Oam, OA, W(CO)6 OAm, CO

160 200 230

good fair excellent

230

excellent

210

fair

benzoic acid, Mo(CO)6

170

good

Pt/Ni molar ratio

specific activity (mA cm−2)

Pt mass activity (A mg−1)

1 1 0.5

3a 3.14b

0.68 1.45

315 141 314

4 5.8 11

3 1.5 3

2.2b 3.99a 1.2a

1.80 1.96 0.3

320 318 309

9

2.5

10.1b

3.3

317

ref

10

3

1.26

0.44

146

4

3

8.2b

6.98c

320

a c

Normalized to the surface areas derived from the H adsorption charges. bNormalized to the surface areas derived from the CO stripping charges. Mo-doped Pt3Ni octahedra.

Figure 9. Comparison of ORR polarization curves of the octahedral Pt−Ni/C catalysts with different compositions in O2-saturated 0.1 M HClO4 solutions. Scanning rate = 10 mV s−1. Rotation speed = 1600 rpm (A). The specific activity (SA) and mass activity (MA) at 0.9 V as a function of Ni atom % for the octahedral Pt−Ni/C catalysts (B).321 Reprinted with permission from ref 321. Copyright 2014 Wiley-VCH on behalf of ChemPubSoc Europe.

the bulk Pt alloy single crystals.35 Because of much weaker adsorptions of SO42− and HSO4− on the (100) than on (111) and (110) planes, the cubic Pt−M NCs showed significantly higher activity than conventional Pt−M and Pt nanoparticles in H2SO4 solutions.309,310 A wide range of Pt mass activities of octahedral shape Pt−M (mainly Pt−Ni) NCs in HClO4 solutions were reported.322 The absolute value ranges from as low as 0.3 A mg−1 to as high as 6.98 A mg−1 at 0.9 V, as shown in Table 2. The low mass activities can origin from a few sources, imperfection of the structure, measurement condition, unoptimized composition, and particle size. Choi et al. found that the ORR activity of octahedral Pt−Ni NCs was highly dependent on the Pt:Ni ratio.321 For Pt−Ni NCs with an edge length of 9 nm, Pt2.5Ni had higher activity than Pt1.4Ni, Pt2Ni, Pt3.2Ni, and Pt3.7Ni, as shown in Figure 9. If the Ni content was too low, the oxygen binding energy would be rather high, resulting in a slow step of further reduction of adsorbed oxygen adsorbates. On the other hand, if the Ni content was too high, the binding energy would be too weak to facilitate the breaking of O−O bond and charge transfer. The oxygen binding energy on Pt2.5Ni might be not too weak nor too strong, resulting in a maximum mass activity. A similar composition dependence was reported by Chou et al.323 and Zhang et al.,318 although the optimized composition was PtNi and Pt1.5Ni in their respective studies. The PtNi was also reported to have the highest activity by Cui et al.190 The discrepancy may come from the differences in synthesis and post-treatment methods, final structure, and composition of the catalysts. Cui et al.190 found that the Pt and

Ni distribution was nonuniform in Pt−Ni octahedra. Pt localized at the corners and edges, while Ni enriched in the facets. During initial electrochemical treatment, Ni was easily dissolved resulting in a structure evolution. The sample with a high Ni content (PtNi1.5) formed a concave structure and lost most of its active (111) facets. On the other hand, the sample with a low Ni content (Pt1.5Ni) maintained its octahedral structure but with a thick Pt shell (5−12 atomic layers). The sample with a balanced Pt and Ni composition (PtNi) retained most of the (111) active sites and formed a relatively thin Pt shell (1−4 atomic layers). As expected, the activity of octahedral Pt−Ni NCs is also dependent on the crystallite size. Choi et al. found that the 9 nm Pt2.5Ni NCs showed higher Pt mass activity than the 6 and 12 nm ones.321 One interesting finding reported by Huang et al.320 is that the ORR activity and stability can be enhanced by doping the Pt3Ni octahedra surface with a tiny amount of other transition metals, which were introduced by adding corresponding metal carbonyls in the synthesis solutions. For instance, the Pt mass activity at 0.9 V increased from 1.80 to 6.98 A mg−1 by doping 1.6 molar % of Mo in the octahedral Pt3Ni. Theoretical study revealed that Mo atoms preferred to sit near the edges and vertices of the particles. They stabilized both Ni and Pt atoms against dissolution by forming relatively stronger Mo−Pt and Mo−Ni bonds. The activity enhancement was also observed by doping other transition metals (Cr, Co, Fe, V, Mn, W, et al.). This study implied that previous synthesis of Pt−Ni octahedra involving W(CO)6 might dope a small amount of W in the 3604

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Figure 10. TEM images of 2.5 nm wide Fe56Pt44 NWs (A),331 2 nm wide Fe29Pt41Cu30 NRs (B),334 and Pt5Cu76Co11Ni8 NTs (C).338 Reprinted with permission from ref 331. Copyright 2013 Wiley-VCH. Reprinted with permission from ref 334. Copyright 2013 American Chemical Society. Reprinted with permission from ref 338. Copyright 2011 Wiley-VCH.

of atoms are on the surface with the rest of the 70% wasted inside of the particles. The idea of core−shell structure is to improve the utilization of Pt atoms by depositing a thin Pt-based shell around a less expensive core, such as Pd-, Ru-, and Re-based nanoparticles. There are various approaches to synthesize core−shell catalysts including but not limited to Cu-mediated Pt deposition,346−348 chemical reduction, spontaneous deposition,349 dealloying,316,350 electrochemical deposition, and atomic layer deposition.351,352

surface of Pt−Ni, resulting in an enhanced activity. This hypothesis needs further confirmation. 4.7. Nanowires, Nanorods, and Nanotubes

One-dimensional (1-D) nanostructures such as nanowires (NWs), nanorods (NRs), and nanotubes (NTs) have attracted great attention due to their unique anisotropic structure beneficial to many catalytic reactions.324 The exterior surfaces of 1-D nanostructures consist of smooth and low energy facets, on which the oxygen binding energy is weaker than that on conventional nanoparticles resulting in higher ORR activities.325−327 The 1-D nanowires can form a 3-D interconnected porous aerogel structure. Liu et al.328,329 found that Pt80Pd20 bimetallic aerogel had 5 times higher Pt mass activity than commercial Pt/C. One concern of NWs in fuel cell applications is their low surface area/volume ratio. Great efforts have been taken to synthesize thin and ultrathin Pt alloy NWs. Pt−M (M = Fe, Co, Ni, Pd, etc.) NWs with small diameters have been synthesized by electrospinning, microwave irradiation, and polyol methods, and their ORR performances were evaluated.330−333 For instance, Pt−Fe and Pt−Co NWs with diameter in the range of 2.5−6.3 nm were prepared by thermal decomposition of Fe(CO)5 or Co2(CO)8, and reduction of Pt(acac)2 in an organic solvent in the presence of capping agents, as shown in Figure 10A. The Pt mass and specific activities of 2.5 nm Pt80Fe20 NWs were 0.84 A mg−1 and 1.53 mA cm−2, respectively, at 0.9 V.331 The specific activity could be further increased to 3.9 mA cm−2 by increasing the diameter to 6.3 nm.331 Zhu et al. found that the dissolution of transition metals was dramatically reduced by adding a third metal, for example, Cu, in Pt−Fe to form NRs. The morphology of the Pt−Fe−Cu NRs is shown in Figure 10B.334 Other NRs with various compositions including Pt−Ni,335 Pt−Cu, Pt−Ni−Fe,214 and Pt−Ni−Cu336 have been studied as well. The surface areas of NTs are typically high because both inner and outer walls in NTs are available for reactions.155,337−341 For instance, quaternary Pt5Cu76Co11Ni8 NTs (Figure 10C) prepared by a one-step direct electrodeposition approach using a porous anodic aluminum oxide (AAO) membrane as the template showed an ECA of 104 m2 g−1.338 Its Pt mass activity was about 5-fold that of Pt/C. The nanoporous NWs and NTs fabricated by dealloying (see section 4.5) or sacrificial displacement of a Cu NW template were also evaluated for ORR.342−345

5.1. Cu-Mediated Deposition

In the Cu-mediated deposition method, a Cu monolayer is electrochemically deposited on a noble metal core at potentials lower than its bulk deposition (underpotential deposition, UPD). The Cu monolayer then is displaced by Pt atoms via a spontaneous reaction Cu + Pt2+ → Pt + Cu2+. Ideally, a Pt monolayer (ML) is coated on the core. The Cu UPD on a Pd core and Pt displacement reactions are illustrated in Figure 11.

Figure 11. Illustration of Pt monolayer deposition on a foreign metal core (Pd as an example) involving the Cu UPD and subsequent Pt displacement.

The Adzic group at Brookhaven National Laboratory pioneered this approach and has conducted an extensive study.346−348,353,354 The advantages of this Pt ML structure include not only high Pt utilization (100% in theory), but also possible activity enhancement due to property changes caused by the structural and electronic effects from the core materials. 5.1.1. Core Material Effect. Because of the mismatch of the lattice constant, the Pt ML undergoes either a compressive or a tensile strain when it is deposited on a foreign substrate. The presence of the strain affects the d-band center of the Pt ML.354 For instance, a large tensile strain exists in the Pt ML supported on Au(111), while a compressive strain is expected when it is deposited on Ru(0001). The tensile and compressive strains result in upshifting and downshifting of the d-band center of Pt ML, respectively.355 The binding energies of adsorbates have a strong correlation with the position of d-band centers. In addition, the electronic coupling between the Pt ML and the substrate causes additional electronic (ligand) effect. Both strain and ligand effects have been confirmed by the density functional theory (DFT), and their roles in controlling the electrocatalytic activities of Pt MLs have been demonstrated.354,356,357 Recently, Stolbov et al. proposed that the core−shell interlayer hybridization was the main factor

5. CORE−SHELL STRUCTURES The Pt dispersion (utilization) in conventional nanoelectrocatalysts is low due to the fact that only the surface atoms expose to the electrolytes and participate in the electrocatalytic reactions. For example, for a 3 nm Pt nanoparticle, only about 30% 3605

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was only 0.15 A mg−1. In comparison, with a Pd interlayer, the activity of 1.18 A mg−1 was considerably higher.375 The Pd interlayer plays an essential role in achieving high catalytic activity by adjusting the electronic interaction of the Pt ML with the IrCo core, resulting in a weaker oxygen binding energy. Zhang et al. demonstrated that PtML on Ru(0001) bulk single crystal had a significantly lower ORR activity than that on Pd(111) due to a too weak oxygen binding energy.354 If two to three Pt MLs instead of only one were deposited on Ru nanoparticles, the Pt mass activity of core−shell catalyst was comparable to that of [email protected] The activity enhancement was attributed to the proper oxygen binding energy on a thicker Pt shell based on DFT calculation results. 5.1.2. Core Structure Effect. The structure of the core materials, including the shape, particle size, porosity, and surface morphology, has a significant effect on the ORR activity of the core−shell catalysts.346,377−380 The activity of Pt shell on Pd octahedra was 3.5 times higher than that on Pd cubes, suggesting the importance of maintaining high coverage of (111)-oriented facets.378 Cai et al. were able to smooth the conventional Pd nanoparticle surfaces and increase the fraction of {111} facets by using Br− adsorption/desorption approach.381 The low coordinated Pd atoms formed a Pd−Br2 pair in solution that was redeposited onto the Pd surface in the following anodic scan, accompanied by the oxidative adsorption of bromide. As a result, the activity increased by 25−50% after the Pd core was treated by Br−.381 Pd NW has less defects than nanoparticle and is expected to be a better choice as a core. Koenigsmann et al. confirmed the high activity of Pt ML supported on Pd NWs (1.83 A mg−1Pt).382 The degree of enhancement resulting from the Au interlayer also depends on the structure of the Pd core. The Au interlayer improved the activity of Pt MLs on Pd cubes and octahedra by 3 and 1.2 times, respectively; that is, its effect on ORR activity was much smaller on (111) surface than that on (100).374 The larger enhancement factor at the (100) sites may be due to the larger decrease of oxygen binding energy caused by Au interlayer (0.275 eV) than that at (111) sites (0.075 eV). In the systems of Pt MLs on dealloyed nanoporous Pd−Cu and Pd−Ni nanoparticles, in addition to the strain and ligand effects from the transition metals, the unique porous structure was believed to change the electronic properties of the Pt shell, and consequently enhance the activity.360,362 This argument was supported by the work of Zhang et al.,383 who found that the Pt MLs on hollow Pd and Pd−Au nanoparticles formed by galvanically displacing Ni nanoparticles were 2 times more active than that on corresponding solid particles. Instead of a tensile strain (0.46%) in the Pt ML on a solid Pd9Au core, a 0.46% compressive strain was found on a hollow core. The hollow-induced lattice contraction, together with the smooth surface morphology and the mass-saving geometry of the hollow particles, improved the total PGM activity by 2−3 times. 5.1.3. Scale-Up and MEA Testing. Most of the core−shell catalysts made with the Cu-mediated method were synthesized on a microgram scale, that is, with the Cu UPD and Pt displacement reactions occurring on a RDE tip. When this process was transferred to a gram scale, the high activity of core−shell catalyst was not fully realized.384 One of the main reasons is that the quality of the Pt shell is more sensitive to the reaction conditions in a large reactor such as concentration and injection speed of Pt solutions and rotation speed of solution (transport speed of Pt cations). It is possible that Pt cluster rather than a uniform Pt shell was deposited on the core in large batches, as demonstrated by in situ XRD results in an H2 environment.385

controlling the surface reactivity, whereas the contribution from the strain effect was not significant.358 Zhang et al. compared the ORR activities of Pt MLs supported on different substrates including Ir, Ru, Rh, Pd, and Au, and found a strong correlation between the activities and the d-band centers of Pt MLs, which were dependent on the type of the substrates.354 Following Sabatier’s principle, a good ORR catalyst should exhibit a moderate metal−O interaction.359 If the oxygen binding energy is too weak, the O−O bond cleavage and electron transfer will be difficult. On the other hand, the subsequent reduction of adsorbed oxygen containing species will be slow if the oxygen binding energy is too strong. Furthermore, the tardiness in removing oxygen containing species results in blocking the adsorption and dissociation sites for oxygen molecules. It has been known that the Pt(111) surface binds oxygen too strongly and the ORR kinetics is limited by the rate of removal of strongly adsorbed oxygen containing species. A slightly lower oxygen binding energy, as in the case of Pt ML on Pd(111), achieved by lowering the d-band center of Pt, is expected to enhance the activity. The electronic properties of Pt ML can be further tuned resulting in even higher ORR activities by alloying the Pd core with other metals such as Co,348 Ni,360 Fe,361 Cu,362,363 Ir,363,364 Au,365 etc. Shao et al. achieved the highest ORR activity (2.8 A mg−1Pt) of core−shell catalysts by depositing the Pt shell on nanoporous Pd−Cu nanoparticles obtained by electrochemical dealloying.362 Later, it was found that Cu leached from the dealloyed core during the fuel cell operation poisoning the ionomer and membrane by occupying the SO3− group, even affecting the anode performance. The poisoning issue was largely mitigated by using a chemically dealloyed Pd−Ni core due to the fact that Ni2+ does not redeposit on the anode.360 According to Zhang et al., Au(111) is not a good support for Pt ML due to the larger lattice constant of Au than that of Pt.354 However, contrary to this observation, a recent study of Pt overlayers supported on Au single crystals demonstrated that Au(111) is indeed a good support.366 An atomic layer thick Pt shell on Au(111) was 2 times more active than bulk Pt(111) surface. The ORR activities of Pt overlayers on Au(100) and Au(110) were much lower due to easier alloying and reconstruction of these surfaces. The significant compressive strain in the surface of small Au nanoparticles results in a 1.6-fold increase of ORR activity on Pt ML supported on 3 nm Au nanoparticles over that supported on Pd particles with the same size.367 The activity enhancement from small Au core has also been confirmed by other groups.40,349,368−371 The structural and electronic properties of the Au-based core can be further tuned by alloying with transition metals. For example, Gong et al. synthesized core−shell AuNi0.5Fe nanoparticles consisting of 3−5 atomic layers of Au on the multimetallic alloy core.372 The combination of lattice contraction of Au and electron transfer from Ni and Fe atoms to Au might weaken the oxygen binding strength on the Pt ML, resulting in a high ORR activity. Another improvement on core−shell catalysts is to introduce an interlayer between the Pt shell and the core. The interlayers consisting of Pd, Au, or their alloys were deposited via a Cumediated process before the Pt ML was deposited. Xing et al. found that the ORR activity of Pt ML on Pd core enhanced by 3 times by introducing a Pd9Au interlayer (Table 3).373 A similar enhancement (2-fold) was observed with an incomplete Au layer.367,374 Pt monolayers on Ir−Co-based alloy cores did not show a high ORR activity due to strong oxygen binding energies. For example, the Pt mass activity of Pt ML on IrCo 3606

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Table 3. Summary of ORR Activities of Core−Shell Catalysts Synthesized by the Cu-Mediated Method (Measured at 0.9 V) core materials Pt benchmark 48% Pt/C TKK Pd NPs Pd NWs Pd NRs Pd tetrahedra Pd octahedra Pd cubes Pd NPs Pd NPs Pd9Ru NPs IrCo NPs IrCo NPs with Pd interlayer PdIr alloy NPs Ir2Re NPs with Pd interlayer Au NPs Au NPs Ru NPs (2 ML) Os NPs (2 ML) dealloyed PdCu NPs dealloyed PdNi NPs dealloyed IrCu AuNi0.5Fe NPs Ir2Re NPs PdNi NPs PdIrNi2 NPs AuPdNi NPs Pd9Au NPs Pd2Co NPs hollow Pd NPs hollow Pd20Au NPs a

Pt mass activity RDE/MEA (A mg−1)

PGM mass activity (A mg−1)

PGM mass activity (cost adjusted) (A mg−1)a

specific activity (mA cm−2)

0.2

0.2

0.2

0.24

0.75 1.9 1.7

0.18 0.2 0.4

0.38 0.6 0.9

0.31 0.8 0.75

2.2 0.64 0.95 0.60/0.30 0.38 0.15 1.18

0.036 0.16

2.17 0.60

0.13 0.18

1.06 1.2 0.95 0.70 2.8 1.40 1.35 1.38 0.38 1.1 0.9 1.35 0.9/0.36 0.72 1.50 1.62

0.45 0.46 0.53 0.57 0.18 0.12 0.43

1.2 0.99

0.53 0.32 0.45 0.57

ref 317

0.26 0.069 0.45

RDE RDE RDE RDE RDE RDE gram gram RDE RDE RDE

385 382 389 380 378 378 360 387 390 375 375

0.94 0.25

RDE RDE

391 364

0.42 0.51 0.65 1.33 1.18 0.93 0.71 1.12 0.16 0.6 0.79 0.7 0.47/0.38 0.50 0.90 0.85

RDE RDE RDE RDE RDE gram RDE RDE RDE RDE RDE RDE gram gram RDE RDE

368 367 376 392 362 360 393 372 364 394 395 394 388 348 383 383

0.93 0.27 0.6

0.43

batch size

Assuming the cost of Pd is 1/3 of that of Pt.

The problem associated with the Pt cluster formation is that the Pt−Cu displacement involves the electron transfer from the core to Pt cations, rather than direct electron exchange from Cu, as recently proposed by Thambidurai et al.386 That means electrons generated anywhere on the surface can move freely through Pd substrate, reducing Pt cations wherever their activity and surface energy are the greatest. In other words, the Pt atom may not deposit on the same site left by the Cu dissolution, but rather on Pt deposited previously leading to the formation of Pt clusters. The mass transport of Pt cations in large batches exacerbates this problem. As a result, there are only a limited number of core−shell materials that have been synthesized at a gram scale and tested in MEAs. Naohara et al. first reported the synthesis of Pd@Pt on a gram scale with reproducible Pt mass activity around 0.6 A mg−1.387 Zhou et al. synthesized Pd2Co@Pt with the same protocol resulting in a slightly higher activity (0.72 A mg−1).348 By adding special additives in the Pt−Cu displacement solutions, the coverage of the Pt shell could be improved significantly due to selective adsorption of additives on Cu and Pt surfaces but not on Pd surfaces; that is, the Pt atoms were forced to deposit on the Pd core rather than Pt atoms already deposited.385 With this improvement, the Pt mass activities of Pd@Pt and dealloyed Pd−Ni@Pt could reach 0.95 and 1.40 A mg−1, respectively.360

The optimization of MEA performance of core−shell catalysts has not been seen in public domain. The existing data showed that there were significant activity gaps between MEA and liquid cell measurements for core−shell catalysts synthesized in gram batches.384 For example, the activities dropped from 0.6 (liquid cell) to 0.3 A mg−1 (MEA) and from 0.7 (liquid cell) to 0.15 A mg−1 (MEA) for Pd@Pt and Pd−Co@Pt, respectively.348 The dealloyed Pd−Ni@Pt outperformed Pt/C by 40 mV (i.e., 4-fold enhancement over Pt/C) through the whole potential range in the MEA testing.360 The enhancement was also smaller than the expected 7-fold. The low performance of core−shell catalysts may simply be due to unoptimized MEA fabrication parameters. To optimize the performance of core−shell catalysts in fuel cells, a significant amount of work is required to determine the optimal ink and catalyst layer composition, and preparation techniques. The durability of core−shell catalysts during fuel cell operation is a big concern in the fuel cell community. Sasaki et al. demonstrated that the activity decay of Pd@Pt indeed was much slower than that of the benchmark Pt/C.59,388 After 100 000 potential cycles (0.7−0.9 V, with 30 s dwell time at 80 °C), the Pt mass activity of Pd@Pt decreased by 37%. For comparison, Pt/C lost its initial activity by 70% after only 60 000 cycles. During potential cycling, a large amount of Pd dissolved from the core and migrated to the membrane and the 3607

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Figure 12. Distribution of Pd, Pt, and Au in the core−shell nanoparticles as synthesized (A), after 100 000 potential cycles between 0.6 and 1.0 V (B), and after 20 000 potential cycles between 0.6 and 1.4 V (C) obtained by the line-scan analysis using EDS/STEM. (D) The Pt mass activity of the Pt/Pd9Au/C catalysts at 0.9 V as a function of number of potential cycles during fuel cell testing. The result of Pt/C is included for comparison. Absolute pressure for H2/O2 = 150 kPa. Relative humidity = 100%. Pt loadings in the cathodes: 0.105 mg cm−2 for Pt/Pd9Au/C, 0.102 mg cm−2 for Pt/C.388 Reprinted with permission from ref 388. Copyright 2012 Nature Publishing Group.

anode. The formed Pd2+ was reduced by H2 diffusing from the anode forming a Pd band in the membrane. The rest of the Pd2+ diffused further to the anode and was reduced in the anode catalyst layer. With continuous removal of Pd in the core, the particle gradually shrinks and the Pt atoms on the surface were able to form bi-/multilayers. With a thicker Pt shell, the stability of the core−shell structure was further improved. The Pd dissolution prevented Pt shell via a cathodic protection mechanism. The stability of the core−shell structure was further improved by alloying Pd core with a small amount of Au (Pd9Au).388 As shown in Figure 12D, after 200 000 cycles (0.6−1.0 V), the Pt mass activity only decreased by ∼30%. Even under a much harsher cycling condition (0.6−1.4 V), about 30% of activity was maintained after 20 000 cycles. In both cases, the core−shell structure was maintained (Figure 12B and C). The only difference is that the Pt shell of the latter became thicker. DFT calculations suggested that Au atoms segregated preferentially at defect sites in Pt shell suppressing Pd dissolution from the core. In addition, the Au clusters might also delay the Pt oxidation/dissolution and thereby stabilize the Pt surfaces.388

Thin and uniform Pt shells were formed by this approach as shown in Figure 13A and B, and their thickness (from as low as 0.38 nm to over 1 nm) could be tuned by varying the amount of Pt precursors. The activities of Pd@Pt catalysts, however, were not high. The low ORR activity may be due to the negative effect from the PVP, which is not easy to remove from the Pt surfaces. Other groups explored deposition method without PVP. Zhang et al.401 synthesized Pd@Pt core−shell catalysts with different Pd:Pt ratios using ascorbic acid as the reducing agent in the presence of PEO106PPO70PEO106 as capping agent. In general, the ORR activity of Pd@Pt was low (up to 2 times improvement over Pt/C) in the RDE testing. Interestingly, the fuel cell performance of Pd@Pt improved significantly (70 mV at 600 mA cm−2) after 40 000 potential cycles (0.65−1.05 V, 100 mV s−1) instead of decay, and the activity was 4.5 times over Pt/C after cycling. The same group also used amphiphilic triblock poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) copolymer as the reducing and capping agent to synthesize Pd@Pt core−shell nanoparticles with various shell thickness.402 When more Pt atoms were deposited, the layerby-layer growth mode was changed to island-on-wetting-layer growth mode. The RDE and fuel cell testing results showed 2 and 3 times activity improvement over Pt, respectively. The core−shell transferred to a nanocage structure resulting from Pd dissolution during electrochemical cycling. The activity of the nanocage was found to be higher than that of the solid [email protected] The core−shell structure was also synthesized in ethanol solvent, which reduced the Pt cations at 70 °C. DFT calculations showed that the 2-D rather than 3-D growth was energetically favorable for Pt on Pd nanoparticles. The uniform

5.2. Chemical Reduction

In the chemical reduction method, the Pt salts are reduced in either aqueous or organic solvents by various reduction agents and deposited on the cores, which serve as the seeds for the shell growth. Different types of cores, solvent, capping, and reduction agents have been explored. 264,370,396−399 For instance, Pd@Pt core−shell nanoparticles were synthesized by coating the Pd nanoparticles in an ethylene glycol and/or diethylene glycol solution with PVP as the capping agent.172,400 3608

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developed a simple approach to coat Pt shell on Pd NPs. After treatment with H2, the Pd NPs suspension was mixed with a solution containing Pt2+ cations, which were spontaneously reduced by Pd hydride forming a uniform thin Pt shell. In an alternative process (as shown in Figure 14),418,419a sacrificial

Figure 14. Illustration of the synthesis of Pd@Pt NPs based on a Cu thin layer formed by chemical reduction of Cu2+ by absorbed H2 in Pd (Pd hydride).419 Reprinted with permission from ref 419. Copyright 2011 American Chemical Society.

Cu thin layer was deposited first deposited on Pd NPs with the assistance of Pd hydride. The Cu layer was then displaced by Pt to form a 0.3 nm thick Pt shell. The specific activity and Pt mass activity were double those of Pt/C. Liu et al. synthesized a sandwich structure with a Y layer between Pd core and Pt layer utilizing the same mechanism.420 The Sun group developed a new strategy by epitaxial overgrowth of Pt-based bimetallic shell on Pd and Au NPs core.421−424 Pd, Au, or even Ni NPs seeds were first synthesized with the polyol methods and mixed with Pt(acac)2 solvent at 110−120 °C. Fe(CO)5 was injected into the mixture, which was further heated to 200 °C. The composition of the Pt−Fe shell could be controlled by adjusting the molar ratio of Fe:Pt precursors. The thickness of the bimetallic shell varied from 1 to 3 nm depending on the amount of meal precursors added. Figure 15 showed the synthesis process, HAADF-STEM, and element mapping of Au@Pt3Fe NPs consisting of 7 nm Au cores and 1.5 nm Pt3Ni shells.423 The initial Pt mass and specific activities were 5- and 3-fold higher than those of Pt/C. In addition to the higher activities, the core−shell catalysts was also more durable. After 60 000 potential cycles between 0.6 and 1.1 V, the activity improvement factors increased to 17 and 7 for mass and specific activities, respectively.423 The stable icosahedral Au core was hypothesized to be responsible for the excellent durability of Au@Pt3Fe. The Au−Cu alloy NPs were also explored as the core to introduce compressive strain.425 Using a similar approach, Pt−Fe binary shell with tunable thickness ranging from 0.3 to 1.3 nm was successfully coated on FePtPd(Au) NWs.426 The core−shell structure with the shell thickness of 0.8 nm showed the highest activity and 2−3 times more activity than the core (Pt−Fe NWs). As compared to Pt/C catalyst, the Pt mass and specific activities were 14.5 and 12.5 times higher, respectively.426 The Pt-based bimetallic shells discussed above are mixed facets. As discussed in section 3.2, the {111} facet has the highest activity. If one can control the final structure with only {111} facets exposed, the ORR activity of the core−shell catalysts will be maximized by the combination of the following three aspects: a high Pt utilization, electronic effect from the alloying elements, and desired structure (Table 4). Choi et al.427 deposited a ∼1 nm Pt2.5Ni shell on a 5 nm Pd seed using a similar method in the synthesis of PtNi octahedron,317 forming an octahedral core−shell nanoparticle. The Pt mass and specific activities were enhanced by 12.5- and 14-fold, respectively. A similar approach has been conducted by Zhao et al. to synthesize [email protected] octahedra with a somewhat lower activity.428

Figure 13. HAADF-STEM image (A) and HAADF-STEM-EDS mapping images of Pd@Pt nanoparticles (B). Scale bars in (A) and (B) are 2 and 5 nm, respectively.400 HAADF-STEM image of Pd@Pt (Pd and Pt are the core and shell, respectively) cubes with a Pt shell thickness of 2−3 monolayers (C),415 and after chemically removing the Pd core (D).417 Reprinted with permission from ref 400. Copyright 2013 Wiley-VCH on behalf of ChemPubSoc Europe. Reprinted with permission from ref 415. Copyright 2014 American Chemical Society. Reprinted with permission from ref 417. Copyright 2015 American Association for the Advancement of Science.

coating of 1 or 2 Pt MLs on Pd was promoted by the slower kinetics of ethanol oxidation.404 The Pt mass activity of Pd@Pt synthesized in ethanol was 0.64 A mg−1, which was higher than those discussed above and comparable to that synthesized via the Cu-mediated method. The high activity could be attributed to the thin, uniform, and clean Pt shell. Using the proprietary method, Ball et al. deposited Pt shell on Pd−Co alloy nanoparticles and achieved a Pt mass activity of 0.7 A mg−1.405,406 The same material, however, only showed a Pt mass activity of 0.14 A mg−1 in the fuel cell testing. Alia et al. coated Pt overlayers with various thicknesses (1.1, 1.7, and 2.2 Pt MLs) on Pd nanotube by reducing Pt salts with PVP at 108.7 °C. The Pt mass activity was as high as 1.8 A mg−1 for the thinnest core−shell sample.407A few works also synthesized core−shell structures consisting of Pt nanobranches (dendrites, nanoparticles, etc.), which showed moderate ORR activities.408−410 Another promising synthesis protocol is epitaxial overgrowth of Pt on Pd nanoparticles.411−415 A smooth and thin Pt shell on the scale of one monolayer to 2 nm could be deposited on Pd cores. Recent reports of 1−3 monolayers of Pt shells on 20 nm Pd cubes415 (as shown in Figure 13C) and 2−5 monolayers on Pd octahedra416 showed 3- and 5-fold enhancement over Pt/C in mass activities, respectively. The same group further reduced noble metal loading by etching away most of the Pd atoms in the core in a FeCl3/HCl mixed solution, resulting in a cubic nanocage structure (Figure 13D).417 The mass activity enhancement factor was slightly increased from 2 to 3. For the octahedral nanocage, the mass activity was higher with an enhancement factor of 5 over Pt/C. By utilizing the ability of Pd to absorb hydrogen to form a Pd hydride, which is a strong reduction agent, Wang et al. 3609

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Figure 15. Schematic illustration of Au@Pt3Fe nanoparticle synthesis (A), HAADF-STEM characterization (B), and elemental distribution analysis of Au@Pt3Fe (red, Pt; green, Au) (C), and the elemental distribution analysis of a single core−shell particle (D).423 Reprinted with permission from ref 423. Copyright 2010 American Chemical Society.

5.3. Dealloying

surface at a depth of 2−3 nm, as shown in Figure 16. The competition between the Ni segregation to the surface induced by the adsorption of oxygen species and the formation of a protective Pt shell might be the reason for the accumulation of Ni atoms below the Pt shell. The presence of the Ni-enriched layer resulted in a larger compressive strain in the Pt shell and hence a higher ORR activity.438 The dealloying condition also plays a critical role in determining the structure, activity, and durability of the catalysts.142,439 Take PtNi3 as an example,439 dealloying in a hot diluted H2SO4 solution resulted in a thicker Pt shell and a higher Ni content in the core as compared to that in a more oxidative HNO3 solution. The denser core−shell structure obtained in the H2SO4 solution led to a higher ORR activity and durability revealed in MEA testing. When the particle size is large enough, a porous structure instead of a core−shell structure is easy to form. Wang et al. also studied the core−shell structure as a function of initial compositions of the Pt−Ni alloys.440 After 100 potential cycles (0.06−1.1 V), the Pt shell of dealloyed PtNi3 (1 nm) was thicker than that of dealloyed PtNi (0.5 nm) due to easier removal of Ni in the former. The thicker Pt shell, however, was not beneficial for oxygen reduction due to significant relaxation of the compressive strain in it. This argument is consistent with the fact that dealloyed PtNi was more active than PtNi3.440 They also found that the ORR activity of as-dealloyed Pt−Ni nanoparticles could be enhanced 2-fold by heat treatment at 400 °C.114 The heat treatment was believed to smooth the surface and reduce the low coordination

As discussed in section 4.5, dealloying can not only form porous structures, but also solid core−shell structures. The unique core−shell structure consisting of a Pt alloy core and a pure Pt shell showed higher ORR activity than conventional Pt alloy catalysts.128,134,222,431−433 In addition, a solid core−shell structure is preferred from the durability point of view because the nanoporosity may result in a faster activity decay during ORR.132 The compressive strain formed in the Pt shell was found to play a critical role in the activity improvement. Strasser et al. extensively studied the dealloying of Pt−Cu nanoparticles.434−437 A few atomic layers (0.6 nm) of Pt-rich shell was formed on a PtCu3 core after 200 potential cycles in acid media. The measured lattice distance in the Pt shell was smaller than that of pure Pt due to the lattice mismatch between the Pt shell and the Cu-rich alloy core.350 The magnitude of the compressive strain was determined by the Pt:Cu ratio and annealing temperature of the alloys. The highest ORR activity was achieved on a dealloyed PtCu3 with a Pt mass activity of 0.56 A mg−1. The activity enhancement should mainly come from the surface strain effect because the ligand effect from the core was negligible due to the thick Pt shell. The same group studied the dealloying of Pt−Ni alloy nanoparticles with different Ni contents.438 The dealloyed PtNi3 showed the highest Pt mass activity (0.81 A mg−1) and specific activity (2.27 mA cm−2). Unlike a conventional core− shell structure observed in the dealloyed PtNi, a Ni-enriched layer between the Pt shell and the Pt alloy was formed near the 3610

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3611

a

2.7c

1.6 0.48

2.5

0.79

0.85 0.9

3−4 times over Pt/C 0.17−0.31

0.5 2.20

0.58 0.4−0.5 0.25−0.55

0.24

specific activity (mA cm−2)

1.5

0.47 0.49

0.2

PGM mass activity (cost adjusted)a (A mg−1)

0.55

0.15

0.18 0.25

0.26

0.64 0.08−0.21 0.16−0.37

0.2 1.80 0.70/0.14 2−3 times over Pt/C 0.15−0.22 0.57 0.94 0.35 0.49

0.2

0.2

PGM mass activity (A mg−1)

427 428

oleylamine, oleic acid, and W(CO)6 at 200 °C hydrazine hydrate, citric acid, and PVP at 65 °C

172 407 406 397 429 425 430 415 416

404 400 402

317

refs

423

method

ethanol at 70 °C ethylene glycol and diethylene glycol with PVP at 200 °C amphiphilic triblock poly(ethylene oxide)−poly(propylene oxide)− poly(ethylene oxide) copolymer at 80 °C ethylene glycol with PVP at 140 °C aqueous solution in the presence of PVP at 108.7 °C proprietary methods 1,2-propanediol, oleic acid, KBH4 reduction at 138 °C ultrasound-assisted polyol ethylene glycol reduction oleylamine at 240 °C aqueous solution in the presence of L-ascorbic acid under ambient conditions ethylene glycol in the presence of ascorbic acid, KBr and PVP at 200 °C aqueous solution in the presence of citric acid and PVP at 95 °C/or ethylene glycol in the presence of ascorbic acid, KBr and PVP at 200 °C oleic acid and oleylamine at 200 °C

Assuming the cost of Pd is 1/3 of that of Pt. bMeasured at 0.85 V. cNormalized to the surface area derived from the charge associated with Cu UPD.

Au NPs (Pt3Ni shell) Pd NPs (octahedral Pt2.5Ni shell) Pd NPs (octahedral Pt1.8Ni shell)

Pd NPs Pd NTs Pd3Co NPs Nib Pd−Co Au−Cu Pd−Au Pd cube Pd octahedron

Pt benchmark 48% Pt/C TKK Pd NPs Pd NPs Pd NPs

core materials

Pt mass activity RDE/MEA (A mg−1)

Table 4. Summary of ORR Activities of Core−Shell Catalysts Synthesized by Chemical Methods (Measured at 0.9 V)

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6. Pd-BASED ELECTROCATALYSTS Pd and Pt are in the same group and share similar electronic properties. Indeed, Pd shows a reasonable good activity toward ORR, which is about 5 times lower than that of Pt.457 This, combined with the fact that the historical cost of Pd is only 1/2−1/4 of Pt, makes it an attractive alternative to Pt.457 6.1. Structure Dependence

Pd is a more reactive metal than Pt, and binds oxygen more strongly. It oxidizes at more negative potentials than Pt and is expected to be less active for ORR.458 The study of ORR on Pd bulk electrodes in acidic media has received less attention than that on Pt due to the lower activity of the former, difficulty of preparing Pd single crystals, and their poor stability. Kondo et al.459 reported the activity trend of ORR on low index planes of Pd single crystals recently. They found that the ORR activity strongly depended on the structure of the Pd surface. In contrast to Pt(hkl) surfaces, where Pt(110) and Pt(111) show much higher activity than Pt(100) in a HClO4 solution, the activity of the Pd(100) surface is the highest in the same solution. The comparison of the kinetic current densities on different Pd and Pt crystal orientations at 0.9 V is shown in Figure 18A. Surprisingly, Pd(100) is 14 and over 2 times more Figure 16. HAADF images (insets) and EELS compositional line profiles of individual dealloyed PtNi (A) and PtNi3 (B) nanoparticles. Red, Pt; green, Ni. A Ni-enriched shell is observed between the Pt shell and Pt−Ni alloy core in (B).438 Reprinted with permission from ref 438. Copyright 2012 American Chemical Society.

Figure 17. Illustration of the formation of a smooth Pt shell surrounding a Pt−Ni shell by dealloying and subsequent thermal annealing.114 Reprinted with permission from ref 114. Copyright 2011 American Chemical Society.

surface atoms, as shown in Figure 17. The activity and durability enhancement by postannealing was also observed by Han et al.439 5.4. Other Methods

Other methods to deposit Pt-based shell on foreign metal cores have been explored, including the spontaneous galvanic displacement,441−446 electrochemical deposition,447−450 thermal annealing,451,452 evaporation,453 atomic layer deposition (ALD),454 microwave-,443 and ultrasound-assisted reduction.429,455 Because the equilibrium electrode potential of PtCl42−/Pt couple (0.775 V vs SHE) is higher than that of most other couples, such as PdCl42−/Pd (0.591 V), Cu2+/Cu (0.24 V), and Co2+/Co (−0.28 V), the galvanic displacement reaction of the core consisting of these metals by Pt can occur spontaneously.441−443,456 For instance, the Pd−Cu@Pt core− shell catalysts fabricated by this method showed a 3-fold enhancement in Pt mass activity over Pt/C.441 Liu et al. developed a self-terminating process, in which the Pt deposition on Au(111) substrate was terminated with the assistance of Hupd resulting a 2D Pt monolayer without formation of 3D Pt clusters.448 It will be of interest to check whether the same mechanism also works on nanostructure substrates.

Figure 18. Comparison of specific activity of oxygen reduction on low index facets of Pd and Pt single crystals (A), and Pd and Pt nanostructures (B) at 0.9 V.457,461 Reprinted with permission from ref 457. Copyright 2011 Elsevier. Reprinted with permission from ref 461. Copyright 2011 Royal Society of Chemistry.

active than Pd(111) and Pt(111), respectively. Their results suggest that (100) plane is the most active site for ORR on Pd crystals, which has been confirmed by several studies reporting Pd nanocrystals enriched with {100} facets with exceptional ORR activities in acidic solutions.460,461 Erikson et al. demonstrated that cubic Pd particles with an average size of ∼27 nm had a higher ORR activity than spherical Pd particles 3612

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(2.8 nm).460 A systematic study on the structural dependence of electrochemical behavior of Pd nanocrystals with a much smaller particle size (5−6 nm) was conducted by Shao et al.461 The specific activities of cubic, octahedral, and conventional Pd/C at 0.9 V in a 0.1 M HClO4 solution were compared in Figure 18B. The activity enhancement of Pd/C cubes was about 10 times over octahedra, consistent with the bulk surface study. The high ORR activity on Pd cubes can be contributed to its lower oxide coverage and consequently more available reaction sites than octahedra.461 Pd-based nanorods are also more active than conventional Pd nanoparticles. For instance, Pd nanorods prepared by electrodepostion methods showed a 10-fold activity enhancement over Pd nanoparticles.462,463 The problem with Pd nanocubes is their extremely low stability. Upon potential cycling, the well-defined structure converts to an irregular shape quickly. By alloying with a small amount of Rh, Yan et al.464 discovered that the activity of Pd octahedron was improved significantly. The addition of Rh also enhanced the stability of the octahedral structure. A recent DFT study revealed a slightly weaker oxygen binding energy (0.04 eV) on Pd(100) than that on Pd(111).378 A weaker oxygen binding energy helps improve the ORR kinetics on Pd surfaces by decreasing the coverage of oxygen containing species and increasing their removal rates. However, the difference in binding energy between Pd(100) and Pd(111) surfaces is rather too small and may not be the only reason for the activity enhancement. More experimental and theoretical studies are needed to further understand the mechanisms of structural dependence of ORR activity on Pd and Pd alloy facets.378,465,466

Dealloying has been also applied to the Pd-based alloys to create solid core−shell structure. Yang et al. tried to prepare core−shell type Pd−Cu structure by electrochemical dealloying PdCu3 alloy films. A thick pure Pd overlayer (∼2 nm) was formed on the PdCu3 substrate after Cu atoms were removed from the near-top surface. The activity enhancement, however, was only 2-fold due to the thick Pd overlayer, which limited the strain and ligand effects from the PdCu3 substrate.489 Nanoporous Pd-based structures can also be fabricated by (electrochemical or chemical) dealloying methods starting with transition metal enriched Pd alloys nanoparticles, such as PdNi6 and PdCu6. Even though the electronic properties of the surfaces of these dealloyed nanoparticles were significantly different from those of pure Pd nanoparticles, there ORR activities have not been measured. Xiong et al.490 prepared a 3D-porous Pd−Cu bimetallic film by partial displacing Cu foam deposited on Au substrate with Pd salts. However, its ORR activity could not be accurately measured by RDE due to the thick (20 μm) and rough porous structure. Other nanostructures and compositions including spongelike PdAu,491 Rh−Pd alloy nanodendrites,522 nanocubes and octahedra,464 Pd−Co−Ni, and carbon nitride composite472 have been explored. The ORR activity of Pd alloys can be enhanced by proper supports like metal carbides and oxides. The Shen group explored the WC prepared by the intermittent microwave heating method on carbon black as the support for Pd alloys nanoparticles.493 Pd−Au and Pd−Fe nanoparticles supported on WC/C showed significant improvement over Pd/C evidenced by the synergy effect from the support. By adding a small amount of Ce in the synthesis of Pd3Co alloy, the activity of catalyst was enhanced by 100 mV.494 The promotion effect of ceria (CeO2) contributed to the electronic interaction between PdCo and CeO2 particles, and Pd enrichment on the alloy surface caused by Ce modification. Kwon et al. also observed the synergy effect from ceria on PdCo alloy with a positive shift of the ORR onset potential by 50 mV.495 The stability of most of the binary alloys is poor. By incorporating a more corrosion resistant metal, like Au, Mo, and Mn, the stability of the alloy could be enhanced.496−499 Another approach to improve the durability of Pd-based nanomaterials is to fabricate nanocomposites with exfoliated montmorillonite (ex-MMT). Wei et al.500 found that the Pd/ex-MMT catalysts were considerably more stable in an acidic solution than Pd/C. The Pd-d states and O(AlO6)-p states of MMT had similar energy levels. As a result, electrons were easy to transfer between these levels to form a strong Pd−O(AlO6) bond. During the attack by the Oads, the Pd atoms remain linked together through a Pd−Pd bond and anchored tightly on the ex-MMT. The origin of the activity enhancement of Pd alloys has been studied by several groups. Bard et al.501,502 suggested that for Pd−M alloys the reactive metal M constitutes the site for breaking the O−O bonds, forming Oads that would migrate to the hollow sites dominated by Pd atoms, where it would be readily reduced to water. On the basis of this mechanism, the alloy surface should consist of a relatively reactive metal such as Co, and the atomic ratio of this metal should be 10−20% so that there are sufficient sites for reactions of O−O bond breaking on M and Oads reduction at hollow sites formed by Pd atoms. Similar thermodynamic guidelines were proposed by Balbuena et al.503 and Savadogo et al.504 However, the reactive metals are unstable and easily leach from the alloy surface

6.2. Pd Alloys

Similar to Pt, the activity of Pd can be enhanced by alloying with many other metals. Pd−M (M = Co, Fe, Ni, Cr, Mn, Ti, V, Sn, Cu, Ir, Ag, Rh, Au, Pt) alloys showed much higher ORR activities than pure Pd, and some were comparable to Pt.467−476 As expected, the compositions of Pd alloys play an important role in ORR activity. The optimum Pd:M ratios for ORR were strongly dependent on the alloying elements, synthesis methods, and annealing temperatures. Shao et al.477 found that Pd−Co/C nanoparticles synthesized by the impregnation method at 900 °C exhibited the highest ORR activity when Pd:Co = 2:1. The same optimum ratio was reported by Wang et al.478 for Pd−Co/C synthesized by the coreduction method in ethylene glycol followed by thermal annealing at 500 °C. On the other hand, different optimum rations (30−40 at. %479 or 10−20 at. %480 Co) were reported for Pd−Co/C synthesized by coreduction method in aqueous solutions. In the studies of Pd−Fe/C systems, the highest activity was commonly observed at Pd:Fe = 3:1.481−485 The high activity of Pd3Fe was confirmed by Zhou et al., who demonstrated that a well-prepared Pd3Fe(111) surface had an ORR activity comparable to that of Pt(111).361 Similar to Pt-based alloys, it is expected that pure Pd skin- and skeleton-like surfaces are formed during annealing and electrochemical measurements. The systematic study of Pd-based alloy single crystals on ORR with various morphology and compositions has yet to be reported. A small amount of Ir can further improve the activity of Pd3Fe.482 For Pd−W alloys annealed at 800 °C, the maximum activity was observed when the alloy consisted of only 5% W.486 Some Pd−nonmetallic element alloys487,488 were also studied with limited activity improvement. 3613

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during electrochemical measurements. It will be of interest to confirm the existence of transition metals on the surface in acidic media. Theoretical calculations and experimental data suggested that Pd−M alloys underwent Pd segregation, in which Pd atoms migrated to the surface to form a pure Pd skin on the bulk alloys upon annealing at elevated temperatures. The Pd lattice contracted upon alloying with 3d metals, generating compressive strain in the Pd skin. For example, the compressive strain alone weakened the Pd−O binding by 0.1 eV for Pd3Fe(111).505 The electronic structure of the Pd skin is further modified by transition metals via electron transfer (ligand effect). In Pd3Fe(111), the ligand effect further weakened the Pd−O binding by 0.25 eV. The combined strain and ligand effects were the main reasons for the ORR activity enhancement of Pd alloys. Similar results were obtained for the Pd−Co and Pd−Ir−Co alloys.506,507 The durability of Pd-based ORR catalysts is their biggest concern. One way to introduce them into the electrodes of practical fuel cells is to modify their surfaces with Pt-based overlayers. This approach was discussed in detail in section 5.

Figure 19. ORR−current curves of various RuO2- and IrO2-based electrodes supported on Ti substrate: (a) RuO2, (b) IrO2, (c) RuLaO2, (d) IrVO2, and (e) a Pt electrode. Electrolyte is 0.5 H2SO4, sweep rate is 5 mV s−1, and temperature is 60 °C.517 Reprinted with permission from ref 517. Copyright 2009 Elsevier.

Another approach to improve the catalytic activities of metal oxides is to reduce their crystalline sizes,520 which can increase the available reaction sites, surface defects, and electrical conductivity. TaOx powders (particle size not characterized) prepared by heat treating the Ta precursors at 450 °C in air showed high ORR onset potential (0.9 V vs RHE) and current density.516 As compared to bulk TaOx oxides, the Ta 4f7/2 peak of the powder was 0.5 eV lower (26.0 eV vs 26.5 eV), suggesting a lower valence of Ta ions in the powder form. Recently, Seo et al.521,522 were able to deposit highly dispersed fine metal oxides nanoparticles including NbOx, ZrOx, and TaOx on carbon black using a potentiostatic electrodepostion method. Because group IV and V metal precursors are insoluble in aqueous solutions, the electrodeposition was conducted in ethoxide ethanol solutions. These oxide nanoparticles showed much higher ORR activities than their bulky particles/films with high onset potentials of 0.96 VRHE (NbOx), 1.02 VRHE (ZrOx), and 0.93 VRHE (TaOx), respectively. By adjusting the deposition conditions, such as deposition potential and time, the particle sizes of the metal oxides could be well controlled ranging from 1 to 14 nm. As shown in Figure 20, the mass specific activity increased with decreasing of particle sizes in

7. METAL OXIDES, NITRIDES, OXYNITRIDES, AND CARBONITRIDES 7.1. Metal Oxides

Many metal oxides, particularly group IV and V metal oxides, are chemically stable in acidic electrolytes and proposed for catalyst supports to replace carbon. The problems associated with them are low electrical conductivity and lack of adsorption sites for oxygen species on metal oxides’ surfaces resulting in extremely low ORR activity in their bulk form. Extensive efforts have been made to solve these issues by surface modification, doping, alloying, and forming highly dispersed nanoparticles.508−511 The Ota group found that many metal oxides,512−514 including ZrO2−x, Co3O4−x, TiO2−x, SnO2−x, and Nb2O5−x, prepared by sputtering with respective metal oxide targets in the Ar atmosphere showed clear ORR activities in H2SO4 solutions. The introduction of surface defects such as O vacancies in sputtering was believed to be the main reason for the enhanced catalytic activities. The ORR activity on metal oxides may also depend on the surface structure. For instance, it was found that the ORR activity on Ti oxide catalysts increased with the increase of the percentage of TiO2 (rutile) (110) plane.515 Takasu et al. prepared TiOx, ZrOx, and TaOx, with corresponding binary oxide films on a Ti substrate by a dip-coating method and annealing at temperatures between 400 and 500 °C in air.516 The ORR activity of the pure TiOx was improved by adding certain amounts of Zr and Ta into TiOx to form binary oxides such as Ti0.7Zr0.3Ox and Ti0.5Ta0.5Ox. The same group also tested RuOx, IrOx, RuM (M = La, Mo, V), and IrM (M = La, Ru, Mo, W, V)Ox films prepared by the same method.517−519 The dramatic improvement of ORR activity in Ru−LaOx and Ir−VOx binary oxides as compared to RuOx and IrOx films in 0.5 M H2SO4 was observed, as shown in Figure 19.517 Note that the similar current density of Ir−VOx film and a Pt plate does not mean that these two materials have comparable activities because the former has a much higher surface area due to high porosity of the coated film. The specific adsorption of (bi)sulfates on Pt surface but not on metal oxides might be another reason. The mechanisms underlying the enhanced activity in binary oxides are not clear.

Figure 20. Linear sweep voltammograms normalized to the mass of TaOx nanoparticles deposited on carbon black electrodes with an average particle size of 1.0 (a), 2.6 (b), 4.6 (c), and 13.5 (d) nm.522 Reprinted with permission from ref 522. Copyright 2014 Royal Society of Chemistry. 3614

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7.3. Metal Carbonitrides

TaOx. The enhanced ORR activities with smaller particles are mainly due to increased surface areas, densities of surface defects, and electrical conductivities. The changes of the surface structures and electronic properties such as oxygen vacancies and work functions as particles become smaller need more studies to confirm. The Ota group managed to deposit small Ta2O5 particles ( RuxCrySez > RuxWySez.613 When the Ru−Se/C catalyst was diluted with Fe, the activity was unchanged or slightly higher than the origin with Fe content as high as 50%.585 So far, the maximum power density for Ru−Fe−Se/C was 140 mW cm−2 at 80 °C and 250 KPa (0.4 mgRu cm−2). The stability of Ru−Fe−Se/C during potential cycling is a big concern due to significant Fe dissolution leading to an activity drop together with an increased H2O2 yield.585 Even though RuxSey catalysts showed good ORR activity, the toxicity of Se remains one of the concerns for this type of materials. The feasibility of the RuxSy as an alternative to RuxSey has been studied. It was concluded that the ORR activity improvement of the former over bare Ru is very limited (∼20 mV).589 The synthesis and evaluation of other noble metal-based chalcogenides have also been carried out, even though they were not as intensive as Ru-based ones. Figure 22 predicts that Rh-based selenides and sulfides were more active than pure Rh and comparable to Pt. However, Cao et al.608 demonstrated that Rh particles modified with Se and S showed much worse activity as compared to the unmodified particles. The Se and S atoms most likely poisoned the catalysts rather than enhanced the activity by covering the active Rh sites. Ir-based selenides and sulfides have also been synthesized by different methods. The modification of Ir particles with Se and S did not change the activity dramatically (within 10−20 mV) in RDE measurements.615 In another study, Ir85Se15/C with an average particle size less than 2 nm synthesized via a microwave-assisted polyol process showed the best fuel cell performance among the chalcogenide catalysts, with a maximum power density of 500 mW cm−2 at 80 °C and 200 kPa (0.4 mgIr cm−2).616 The maximum power density of the Ir85Se15/C catalysts was further improved by 80% by adding Nafion in the synthesis, possibly due to a higher active surface area through better particle size control and catalyst utilization.617

In addition, the activity also depends on the catalytic centers with Co, and its alloys are superior to others. Both theoretical and experimental results confirmed that Co selenides were less active than its sulfides by ∼0.2 V.629 Two kinds of crystal structures having different activities were discovered on CoSe2 by Alonso-Vante and co-workers. An orthorhombic structure was observed at a heat treatment temperature of 250−300 °C, while a cubic structure was present at high temperatures (400− 430 °C). The latter had a higher ORR activity by 30 mV in H2SO4.620 Despite intensive development and recent advances, the activities of NPM-based chalcogenides are still significantly lower than that of RuxSey. As shown in Figure 23, the half-wave potential of CoSe2 is about 200 mV lower than that of RuxSey.

9. CARBON-BASED NON-NOBLE METAL AND METAL-FREE CATALYSTS Another possible replacement for Pt at the cathode of PEM fuel cells with Fe-based catalysts has indeed shown interesting properties for the electroreduction of O2 in acid medium. As seen in Figure 1, Fe is a cheap metal, and its production is currently the most abundant of all metals.1 The Fe-based catalysts are said to be of the type Fe/N/C because they are produced by the pyrolysis of an iron precursor, a nitrogen precursor, and a carbon support at high temperature. The latter may eventually be replaced by a carbon precursor as well. Co-based catalysts of the type Co/N/C and catalysts based on Co and Fe (Co−Fe/N/C) have also shown interesting properties, even if, according to Figure 1, Co is a much less common metal than Fe. Several book chapters and review articles have already been written about Fe- and/or Co-based catalysts made to perform the oxygen reduction reaction (ORR) in fuel cells.11,630−638 In this literature, it is also reported that, besides the non-noble metal-based catalysts, some metal-free catalysts made of carbon doped with several heteroatoms (the most common one being nitrogen) may also reduce O2 electrochemically in acid medium.639−644 To replace Pt with a non-noble catalyst at the cathode of a PEM fuel cell, it is necessary that the catalyst fulfills the following three requirements: (i) be able to provide a power equivalent to that provided by Pt in the low power regime of the fuel cell; this requirement essentially depends on the ORR kinetics at the surface of the non-noble catalyst; (ii) be able to provide a power equivalent to that provided by Pt when the fuel cell delivers the work for which it has been designed (for instance, replace the internal combustion engine in an automobile by providing the necessary power to an electrical motor); and (iii) demonstrate a stability of at least 1000 h when the fuel cell is running at useful power. In this Review, we will not discuss the kinetics of oxygen reduction at the surface of non-noble catalysts, nor the structure of the active center(s) as these topics have already been covered in many of the previous reviews. Instead, we will focus on PEM fuel cell results only and will (i) review what has been published recently on the use of non-noble (metallic or metal-free) catalysts at the cathode of PEM fuel cells; (ii) document their performance and stability in fuel cells; and (iii) compare them with those of Pt. A series of papers published after 2006, all presenting the synthesis of non-noble catalysts, their characterization, and their use in PEM fuel cells, at the beginning of life of the catalysts, were selected.639,645−691 The stability in fuel cells has also been studied for some of these catalysts.682,692−737 The variables of such studies are multiple. Most of the catalysts are synthesized according to novel procedures. The inks used to prepare the

8.2. Non-noble Metal-Based Chalcogenides

The family of NPM (Co, Ni, Fe)-based chalcogenides has been studied as ORR catalysts for more than four decades.618−627 As compared to Ru and Rh, these transition metals have a lower cost and higher abundance on earth. A general conclusion can be drawn on the basis of the previous studies that the ORR activity follows the trend MxSy > MxSey > MxTey.628 3617

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rectangles marked (MOF) in Figure 24 represent non-noble cathode catalysts that were made with a nitrogen precursor and a Co and/or Fe precursor. Their ORR properties are mainly attributed to metal-containing active sites (usually labeled FeNx or even FeN4 or their Co equivalent). These active site structures are derived from experimental evidence and supported by theoretical considerations.640,738,739 A typical example of such catalyst is obtained as follows:676 Fe-8CBDZ-DHT-NH3 was prepared by a sacrificial support method. Carbendazin (a nitrogen precursor) and iron nitrate (the iron precursor) in a 8:1 ratio and for a nominal iron loading of 15 wt % were added to a dispersion of fumed silica in water. After evaporation of the solvent, the obtained solid was then subjected to a first heat-treatment for 1.5 h at 800 °C under N2 atmosphere. The resulting material was leached by 25 wt % HF overnight to remove the silica sacrificial support and the leachable metal. After being rinsed with DI water, the material was then subjected to a second 30 min heat-treatment, at 950 °C under NH3 atmosphere, to obtain Fe-8CBDZ-DHT-NH3. The catalyst has a Fe content of 0.1−0.3 at. % (determined by XPS) and a total N content up to 7.7 at. % (also determined by XPS). The ORR catalytic activity of Fe-8CBDZ-DHT-NH3 is attributed to FeNx centers formed during the heat-treatments. This catalyst has an initial peak power of 0.56 W cm−2 in H2/O2 fuel cell and appears therefore in Figure 24 as a blank rectangle between 0.55 and 0.60 W cm−2. All light-blue rectangles in Figure 24 represent catalysts that were made with a nitrogen precursor and with either Co and/or Fe precursors, but for which, according to their authors, the ORR properties do not derive from metal-containing sites, but are the result of the nitrogen-doped carbon obtained during their synthesis. In this case, all of the ORR active sites in these catalysts, according to the authors, are of the CNx type, and the metals are only intermediates to initiate, during the pyrolysis step, larger CNx concentrations in the catalyst. The metals used during the synthesis of the catalysts and identified as Co or/and Fe in the rectangle that represent each of these catalysts in Figure 24 are, according to their authors, completely leached at the end of the synthesis or are inaccessible to O2 behind a protecting carbon layer. A typical example of such catalyst is obtained as follows:702 NMCC-SiO2-800-3 was prepared by a sacrificial support method. Ethylenediamine (EDA; the nitrogen precursor) was added to a solution of cobalt nitrate and iron nitrate (the metal precursors) in isopropyl alcohol to create Co−Fe−EDA complexes. Next, silica was added to the mixture. After evaporation of the solvent, the resulting powder was pyrolyzed for 1 h at 800 °C in Ar atmosphere. The pyrolyzed sample was washed with NaOH to remove silica and with H2SO4 to remove surface metal. The resulting material was pyrolyzed a second time for 3 h at 800 °C in Ar to obtain NMCC-SiO2-800-3. A similar catalyst, NMCC-C-800-3, was synthesized by using carbon black (Ketjenblack EC-300) instead of silica. In this case, the carbon support cannot be removed. Both NMCC-SiO2-800-3 and NMCC-C-800-3 have identical nitrogen-containing groups identified by XPS. The only difference between these catalysts is the total nitrogen content (5.93 and 1.64 at. %, respectively). Metal particles formed during the synthesis are encapsulated within the carbon substrates. The authors conclude that the metal (Fe or Co) in the final NMCC catalysts is likely not part of the active sites. Hence, the nitrogen doped in the carbon matrix is presumably at the origin of the main active site for ORR. NMCC-SiO2-800-3 is characterized by an initial peak power of 0.45 W cm−2, while the

membrane-electrode-assemblies (MEAs) are made according to various recipes, especially with regards to the weight ratio of the ionomer to the catalyst loaded at the cathode. Furthermore, the thickness of the ionomer membrane, the fuel cell temperature, the nature of the gases (H2/O2 or H2/Air), their humidity, and pressure are also variables. When the non-noble catalysts are tested for their stability in fuel cell, other variables are added to those already enumerated above for the measurement of their initial fuel cell performance. For instance, durability experiments are not necessarily performed with the same gases (and the same experimental conditions) as those used to determine the catalyst’s performance. Furthermore, stability tests may be run for different time lengths, either at constant potential or at constant current density, and the values of the potential or the current density chosen may vary from one catalyst to another. It is therefore difficult to compare published performance and durability results among non-noble metal catalysts. 9.1. Initial Performance in H2/O2 PEM Fuel Cells

The emergence of several general trends in all reported results is seen by first ordering all of these catalysts according to their initial maximum performance (expressed in W cm−2) in H2/O2 PEM fuel cells (a value very often available from the published results). This is illustrated in Figure 24, where the abscissa,

Figure 24. Initial peak power for non-noble catalysts tested in H2/O2 PEM fuel cells. The nature of the main active site at work in these catalysts has been proposed by their authors.

divided into 0.05 W cm−2 peak power increments, reports initial peak powers of 0.05−0.10 W cm−2 to 0.95−1.00 W cm−2 for all catalysts used at the cathode of H2/O2 fuel cells. Each individual rectangle in Figure 24 represents the initial maximum performance for one catalyst. A column made with several rectangles is obtained when several catalysts are characterized with an initial maximum performance falling into the same specific power increment. For instance, 12 catalysts were reported to have an initial peak power between 0.10 and 0.15 W cm−2, but only one was reported to have an initial peak power between 0.70 and 0.75 W cm−2. A list of non-noble cathode catalysts for H2/O2 fuel cells, with their associated review code, their initial peak power, and their reference, is provided in Tables 5 and 6. The distribution of initial peak powers in Figure 24 seems to follow a Poisson distribution with a maximum at rather low initial peak power and a decreasing probability for obtaining higher initial peak powers. All blank rectangles and blank 3618

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Table 5. List of Non-noble Cathode Catalysts for H2/O2 Fuel Cells, Giving Their Code Used in This Review, Their Initial Peak Power, and Their Reference polarization curve only

Table 6. List of Non-noble Cathode Catalysts for H2/Air Fuel Cells, Giving Their Code Used in This Review, Their Initial Peak Power, and Their Reference

polarization curve and durability

code

initial max power (W cm−2)

ref

code

08#22 08#37a 08#37b 08#54c 08#54d 08#61 09#03 09#22 09#36 10#10 10#28 10#29 10#76 10#98 11#64 11#69 11#127 12#70 12#100 12#147 12#156 12#160 12#180 13#96 13#113 13#152 13#206 14#01 14#12 14#31 14#56 14#157 14#184 14#217 15#07 15#28 15#30 15#51 15#58 15#59 15#61 15#62

0.12 0.15 0.11 0.33 0.51 0.26 0.16 0.05 0.36 0.21 0.13 0.38 0.20 0.18 0.36 0.16 0.15 0.35 0.26 0.48 0.30 0.42 0.30 0.08 0.32 0.33 0.77 0.64 0.15 0.56 0.10 0.35 0.46 0.62 0.30 0.12 0.46 0.60 0.17 0.07 0.49 0.46

645 646 646 647 647 649 651 652 653 654 655 656 657 658 660 661 662 663 664 665 666 667 668 670 671 672 673 674 675 676 677 679 681 682 683 684 685 686 687 688 689 690

07#08 07#32 08#03e 08#03f 08#15 08#34 09#06 09#34 09#74 10#01 10#11 10#29 11#39 11#73 11#79 11#87 11#97 11#109 11#137 11#138 12#15 12#44 12#79 12#104 13#02 13#02 13#23 13#30g 13#124 14#04 14#38 14#45 14#84 14#97 14#114 14#133 14#151 14#nic 14#214 15#01 15#01 15#11 15#54 15#60 15#63 15#66

initial max power (W cm−2) 0.06 0.12 0.12 0.23 0.53 0.09 0.18 0.36 0.44 0.14 0.45 0.44 0.30 0.35 0.42 0.33 0.34 0.12 0.55 0.91 0.37 0.28 0.27 0.29 0.05 0.10 0.32 0.73 0.62 0.20 0.20 0.36 0.17 0.60 0.24 0.14 0.47 0.98 0.17h 0.23i 0.18 0.90 0.94 0.11 0.23

polarization curve only

polarization curve and durability

ref

code

initial max power (W cm−2)

ref

code

initial max power (W cm−2)

ref

692 693 694 694 696 697 698 699 700 701 702 703 705 706 707 708 709 710 711 712 713 714 715 716 717 717 720 721 722 682 723 724 725 726 727 728 729 730 731 732 732 733 734 735 736 737

08#37a 08#37b 08#56 08#64 11#42 11#69 13#206 14#112 14#162 15#52

0.11 0.08 0.14 0.07 0.25 0.07 0.30 0.30 0.26 0.38

646 646 648 650 659 661 673 678 680 691

08#03 08#13 10#84c 10#84d 11#109 11#137 13#11 13#18 14#84 14#114 14#151 14#nic 14#214 15#11

0.05 0.10 0.14 0.18 0.06 0.14 0.33 0.16 0.18 0.32 0.07 0.20 0.41 0.07

694 695 704 704 710 711 718 719 725 727 729 730 731 733

a

08#37: Backpressure = 200 kPag. b08#37: Backpressure = 0 kPag. 10#84: In Figure 2b of the original article. d10#84: In Figure 4a of the original article. c

All dark-blue rectangles in Figure 24 represent catalysts that are made without any Co or/and Fe precursors, but only with nitrogen precursor and carbon (or carbon precursor). Here, according to the authors, the only sites able to reduce oxygen in these catalysts are CNx sites. A typical example of such catalyst is obtained as follows:698Ketjenblack (EC-300) was prewashed with HCl to remove metallic impurities and then chemically oxidized with HNO3. Urea formaldehyde (UF) and melamine (two nitrogen precursors) were condensed onto the functionalized Ketjenblack. The resulting material was then heat-treated for 1.5 h at 800 °C under N2 atmosphere to obtain UF-C. The amount of N detected by XPS in UF-C was 2.2 wt %. No Fe nor Co was detected in the catalyst by ICP-MS. The initial maximum peak power of UF-C in H2/O2 fuel cell is 0.18 W cm−2, which appears in Figure 24 as a dark-blue rectangle between 0.15 and 0.20 W cm−2. It is obvious from Figure 24 that the average initial peak power developed by the catalysts, represented by dark-blue rectangles, is lower than the average peak power of the catalysts, represented by light-blue rectangles. The fact that, according to the authors, only CNx sites are ORR active in the latter catalysts has been challenged in a recent review640 where it is said that, even if catalysts represented by light-blue rectangles have more CNx sites to reduce O2 than catalysts represented by dark-blue rectangles, the ORR activity of CNx sites is rather low (more than 1 order of magnitude in solution for CNx sites as compared to FeNx or CoNx sites). Therefore, CNx sites alone cannot account for the power difference seen in Figure 24 between light- and dark-blue rectangles and their associated catalysts. Instead, it is proposed that the extra performance of catalysts represented by light-blue rectangles has its origin in a low concentration of FeNx or CoNx sites that appear in these catalysts as the result of the use of these metal precursors during their synthesis. Indeed, acid leaching of the reduced metallic species obtained during the high temperature heattreatment is never fully complete as some metallic ions adsorb on the carbon support during the leaching step instead of being rinsed away from these catalysts, hence becoming equivalent to

a

08#37: Backpressure = 200 kPag. b08#37: Backpressure = 0 kPag. 08#54: Nafion 117 membrane. d08#54: Nafion 112 membrane. e 08#03: Backpressure = 0 kPag. f08#03: Backpressure = 200 kPag. g 13#30: No polarization curve was provided in this paper, only a durability curve. h15#01: For N-G-CNT, 0.5 mg cm−2 + KB 2 mg cm−2. i15#01: For N-G-CNT, 2 mg cm−2 + KB 2 mg cm−2. c

initial peak power of NMCC-C-800-3 is 0.35 W cm−2. Only the best catalyst was used to build Figure 24, where it appears as a light-blue rectangle (marked Co Fe) between 0.45 and 0.50 W cm−2. 3619

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being important variables identified for the fuel cell tests. Figure 25A presents the influence of the cathode non-noble catalyst loading on the initial peak power for H2/O2 fuel cell tests. The color intensity of each individual rectangle increases incrementally with the catalyst loading. Note that, for each power increment in Figure 25A, there is no relation between the order of the catalysts in each column in Figure 25A and the catalyst located at the same place in the equivalent column in Figure 24. For instance, the catalyst identified by a light-blue rectangle at the bottom of the fifth column (from 0.25 to 0.30 W cm−2) in Figure 24 is not necessary the same as that loaded between 8 and 10 mg cm−2 at the bottom of the fifth column in Figure 25A. The only common point between both catalysts is that their initial peak power is in the range of 0.25−0.30 W cm−2. This remark will also apply to the other panels of Figure 25. From Figure 25A, one sees that the most common catalyst loading at the cathode of the MEAs using non-noble catalysts ranges between 3.8 and 4.1 mg cm−2 (a blue rectangle with a white spot). As this loading is usually obtained by building several layers of catalyst on top of each other, the thickness of the cathode may become important. When each layer of catalyst containing 1 mg cm−2 of catalyst has a thickness of about 20−25 μm, the cathode becomes 80−100 μm thick. This is much larger than cathode layers using high Pt loadings on carbon, which enable one to get about 10 μm (and even lower) thick cathodes,2,28,741 but this higher loading is necessary for non-noble catalysts to compensate for their ORR activity lower than that of Pt. Doing so, it has been shown that, at low power, a non-noble catalyst is able to deliver as much current as Pt/C in H2/O2 PEM fuel cell.700 Of course, such a thick electrode necessarily involves mass transport problems at the cathode when the cathode will deliver currents larger than 0.1 A cm−2. It is therefore important to develop non-noble catalysts that would roughly be 10 times more active to be able to reduce the cathode thickness. Figure 25B presents the temperature at which fuel cell measurements were recorded for all of the catalysts of Figure 24. Evidently, 80 °C is the most popular temperature. It is also the temperature suggested by the DOE for fuel cell experiments.742 As expected, lower temperatures yield lower initial peak powers. Figure 25C presents the back-pressures used at the cathode of H2/O2 PEM fuel cells. The most popular back-pressures are comprised between 150 and 200 kPa (1.5−2.0 atm). Here, lower back-pressures (100−138 kPa) do not necessarily mean lower initial peak performance as evidenced by the six catalysts providing the highest peak powers. However, no or very low back pressures (0−30 kPa) seem to lead to low peak powers. The last panel of Figure 25 presents the effect of membrane thickness used with each catalyst on the peak power initially developed in H2/O2 fuel cell by this catalyst. It is obvious from Figure 25D that higher peak powers are obtained with thinner membranes. The most popular membrane seems, however, to be about 50 μm thick. Another parameter that would be very interesting to plot versus maximum peak power would be the catalyst to ionomer ratio of the inks used to obtain the MEA cathode layers. There was, however, not enough data reported in the literature about this parameter to obtain valuable statistics.

load a metallic precursor on the already N-doped carbon structure. Furthermore, in many cases for the catalysts represented by light-blue rectangles, there is even a last pyrolysis step performed after acid leaching during their synthesis. This is ideal to activate FeNx or CoNx sites from the adsorbed iron or cobalt ions and the nitrogen functionalities of the N-doped carbon structure, adding therefore an additional activity to that of the CNx sites already present at the surface of these catalysts represented by light-blue rectangles. Even for catalysts that are represented by dark-blue rectangles and are supposedly made with nitrogen and carbon precursors devoid of Fe, it is important to stress that iron is a very common impurity present at the ppm level in many chemicals, and it may be unintentionally added during the catalyst synthesis. This is not the case for the example of synthesis reported above, because zero metal content was found by ICP-MS, but it could be the case for the most performing dark-blue catalysts. Only very powerful analytical techniques like neutron activation analysis (NAA) or induced coupled plasma (ICP) analyses are able to detect these traces of iron in the otherwise reputed metal-free CNx-based catalyst.740 Finally, there are four yellow rectangles in Figure 24 that have not yet been identified. They are characterized by encapsulated metal particles that are, according to their authors, at the origin of the ORR activity of these catalysts. The first example is a catalyst containing pea-pod-like carbon nanotubes with enclosed metal particles and N-doped carbon walls. They were synthesized according to the following procedure:717 Ferrocene (the iron precursor) and sodium azide (the nitrogen precursor) were reacted at 350 °C in N2 atmosphere in a stainless-steel autoclave. The resulting sample (FeOx/Pod-Fe) was leached with HCl to remove FeOx particles on the outside wall of the carbon nanotubes and yield the Pod-Fe catalyst. It contains 12.8 wt % Fe detected by ICP and 0.8 wt % N detected by XPS. Its initial peak power is 0.05 W cm−2. This catalyst is represented by a first yellow rectangle surrounded by a thick black line in Figure 24. A second version of this type of catalyst, but this time containing more N than Pod-Fe, was obtained by heating Co2Fe(CN)6 for 2 h at 600 °C in Ar. The resulting sample (Pod(N)-FeCo) was also leached with HCl to produce Pod(N)-FeCo, which is also a pea-pod-like carbon nanotube containing 7.9 wt % Fe, 8.2 wt % Co (both measured by ICP), and 3.3 wt % N (measured by XPS). The initial peak power of Pod(N)-FeCo is 0.10 W cm−2. It is represented by the second yellow rectangle surrounded by a thick black line in Figure 24. According to the authors and their DFT calculations, the ORR activity of these catalysts arises from electron transfer from Fe (or Co) encapsulated particles to the carbon nanotube leading to a decrease of the local work function on the carbon surface and resulting in some interaction with O2. The latter is activated and reduced on the carbon nanotube outer wall. N-doping of the carbon nanotube walls would enhance this effect. So far, however, powers obtained using this procedure are quite low as seen in Figure 24. Several catalysts at the high end of the initial peak power have been identified as MOF (for metal−organic framework). MOF is a material made by repetition in three dimensions of (−metal−organic−metal−) chains. The use of MOFs in the synthesis of cathode catalysts for ORR will be discussed later. The initial peak power representation of Figure 24 is now used to determine the influence of (i) the catalyst loading at the cathode, (ii) the fuel cell temperature, (iii) the backpressure of O2 at the cathode, and (iv) the membrane thickness, all of them

9.2. Synthesis of Non-noble Metal Catalysts

We previously said that, to obtain a Me/N/C catalyst that will have some ORR properties, we need to start from a metal precursor (either Fe or/and Co), a nitrogen precursor, and a 3620

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Figure 25. Influences of various parameters on the initial peak power for non-noble catalysts tested in H2O2 PEM fuel cells: the cathode loading (A), the fuel cell temperature (B), the back-pressure (C), and the membrane thickness (D).

first activated at 800 °C, then at 1000 °C in NH3 to become a catalyst. The latter was characterized by an initial peak power of 0.98 W cm−2. (2) The second best performing catalyst is Fe/N/C-SCN.735 It was prepared on carbon black (KJ600) that was first functionalized with sulfophenyl groups. Polymethylphenylenediamine (PmPDA) was coated on the functionalized carbon black through oxidative polymerization of mPDA monomer by (NH4)2S2O8. After Fe(SCN)3 was added, the resulting material was subjected to the sequence of a first pyrolysis step at 950 °C under Ar, acid leaching, and a secondary pyrolysis step again at 950 °C under Ar to obtain the catalyst. Its initial peak power was 0.94 W cm−2. (3) The third best performing catalyst is NC Ar + NH3.712 It was obtained by mixing ZIF-8 (a commercial ZnII zeolitic imidazolate framework of formula ZnN4 C8 H12)743 with 1,10phenathroline and ironII acetate in methanol and water. The dried slurry was then ball-milled (planetary) in N2 in a steel vial with chromium-steel balls. The resulting material was first heattreated at 1050 °C in Ar, then a second time at 950 °C in NH3 to give NC Ar + NH3 characterized by an initial peak power of 0.91 W cm−2. (4) The fourth best performing catalyst is Fe/N/CF.734 It was prepared by electrospinning a mixture in dimethylformamide (DMF) of PAN, polymethylmethacrylate, and ZIF-8 ballmilled with tris-1,10-phenanthroline iron(II) perchlorate. This precursor mixture was electrospun to form polymeric nanofibers. The latter were cured in air at 200 °C before being first heated in flowing Ar at 1000 °C, then submitted to a flow of

carbon support (which can also be replaced by a carbon precursor). A high temperature pyrolysis of the mix of all of these precursors is, however, necessary to obtain the catalytic sites that will be able to perform the oxygen reduction reaction in the acid medium of PEM fuel cells. Noticeably, after looking to Figure 24, one may conclude that some precursors and synthesis procedures are able to yield catalysts in a large range of initial peak powers. So the question is: How can we prepare a catalyst that is highly performing at the beginning of its life? In Figure 24, several high peak power catalysts were made with MOFs, which are essentially used as highly porous carbon (and nitrogen) precursors. It means that no carbon black was initially involved in the synthesis of these catalysts. To find out if there are other hints leading to highly performing catalysts, we will herein describe the synthesis procedure used for 12 selected catalysts appearing in Figure 24, starting from the most performing one and decreasing in performance until a last catalyst yielding an initial peak power of 0.35 W cm−2 is reported. The elemental analysis of each catalyst, its porosity, and its content in ink used to prepare the cathode are summarized in Table 7 (when available) to find out whether there are common points and practices that are preferred to obtain performing catalysts. (1) The highest performing catalyst is PAN-Fe031000NH3.731 It was made by dispersing polyacrylonitrile (PAN) in a solution of FeCl2·4H2O in tetrahydrofuran. The initial Fe loading was 0.3 wt % to obtain PAN-Fe03. The resulting material was pyrolyzed at 600 °C for carbonization. It then was ball-milled for dispersion. The dispersed sample was 3621

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a

Fe(PI-1000-III-NH3)

Fe-8 CBDZ-DHTNH3

PANI FeCo-C

15#54 0.90

13#124 0.73

14#04 0.62

14#114 ≥0.60

14#31 ≥0.56

11#137 0.55

08#15 0.53

08#54 0.51

10#11 0.45

4

5

6

7

8

9

10

11

12

no

no

Ketjenblack

no

yesa + sacrificial SiO2

NMCC C-800−3

yes Ketjenblack

no

NMCC-SiO2-800-3

yes Ketjenblack yes

no

no

no

no

no

no

yesa + sacrificial SiO2

yes (PI-composite)

yes (MOF)

yes (POP)

yes (PAN + PMMA)

no

yes (MOF)

catalyst-900

C-compositeCo3Fe1Nx

PFeTPP-1000

Fe/N/CF

NC Ar + NH3

yes KJ600

no

C black (or similar) in synthesis

no

yes (PAN)

only C precursor(s) in synthesis

See details about the nature of the carbon precursor in the synthesis described in the text.

0.35

Zn(mIm)2 TPIP

11#138 0.91

3

Fe/N/C-SCN

15#60 0.94

PAN-Fe03-1000NH3

14#214 0.98

2

catalyst’s name

1

code/initial peak power (W cm2)

(1.64) 1.91

(5.93) 6.85

(0.70) 0.80

3.9

?

(7.7) 8.9

3.1

2.22

1.92

9.03

(5.3) 6.0

4.4

(3.22) 3.7

N

?

?

(0.66) 3.00

1.4

?

(0.3) 1.4

1.1

1.07

4.95

0.56

(0.78) 3.51

1.4

(0.24) 1.11

Fe

?

?

4.6

?

Co

Zn 1.86

S 1.14

Zn ?

Zn (0.01) 0.05

S 2.1

other

elemental analysis wt % (or at. %)

Table 7. Characterization of Performing Non-noble Cathode Catalysts Used in H2/O2 PEM Fuel Cells

?

?

?

?

?

400−600

1050

1277

758

809

964

751

1096

BET

?

?

?

?

?

?

844

?

mostly

780

814

414

?

micropores

?

?

?

?

?

?

206

?

?

8

184

337

?

mesopores

porosity (m2 g−1)

75−25

75−25

77−23

75−25

65−35

55−45

63−37

?

50−50

40−60

40−60

50−50

?

ink composition (wt % cat.−wt % Nafion)

Chemical Reviews Review

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NH3 at 900 °C to produce a material that was acid washed. The dried sample was heat-treated again at 700 °C in NH3 to produce a final catalyst with an initial peak power of 0.90 W cm−2. (5) The next best performing catalyst is PFeTPP-1000 with an initial peak power of 0.73 W cm−2.722 It was obtained from FeTPP POP, where POP is a porous organic polymer made from the 3D polymerization of repetitive units containing a functionalized iron tetraphenylporphyrin. The catalyst precursor was heat-treated at 1000 °C in N2. (6) The following catalyst is Zn (mIm)2TPIP with an initial peak power of 0.62 W cm−2.682 Here, ZIF-8 was synthesized from ZnO and 2-methyl imidazole in an autoclave at 180 °C for 18 h. The iron precursor was (1,10-phenanthroline)3 iron perchlorate. The resulting material was pyrolyzed at 1050 °C in Ar. It then was acid leached before being pyrolyzed a second time at 950 °C in NH3. (7) The next catalyst is Fe/PI-1000-III-NH3 with an initial peak power of ≥0.60 W cm−2.727 Here, the symbol ≥ is used as the reported polarization curve of that the catalyst stops at 0.6 V and 1.0 A cm−2. A polyimide (PI) was first obtained by reacting pyromellitic acid dianhydride with 4,4′-oxidianiline. A composite material then was made using −PI−/Fe(acac)3, where acac is acethylacetonate. The resulting material was first pyrolyzed at 600 °C in N2, then at 800 °C in 50% NH3, then again at 1000 °C in 50% NH3. (8) The following catalyst is Fe-8 CBDZ-DHT-NH3 with an initial peak power of ≥0.56 W cm−2.676 Again, the polarization curve stopped at 0.4 V and 1.4 A cm−2. The synthesis of this catalyst was already described as a typical example of catalyst whose ORR activity resulted from the presence of FeNx sites. (9) The next catalyst is PANI FeCo-C with an initial peak power of 0.55 W cm−2.711 This catalyst used Ketjenblack [EC-300] as carbon support. The carbon black was first treated with HCl to remove its metal impurities. It then was mixed with aniline oligomers, cobalt nitrate, and iron chloride, as well as with ammonium persulfate. The latter chemical was used to polymerize aniline and obtain PANI onto the carbon support. The resulting material was first pyrolyzed at 900 °C in N2, leached with H2SO4, then pyrolyzed a second time in N2 to obtain PANI FeCo-C. (10) This catalyst labeled C-composite-Co 3Fe1Nx is characterized by an initial peak power of 0.53 W cm−2.696 Carbon black was first treated with HCl to remove metal impurities before being refluxed with HNO3 to introduce O- and N-bearing functional groups on the carbon. The oxidized carbon surface was first modified by reaction with nitrogen-containing organic precursors followed by the deposition of Co and Fe complexes with ethylene diamine. The resulting material was first heat-treated in Ar, then leached in H2SO4. (11) The following catalyst, labeled catalyst-900, is characterized by an initial peak power of 0.51 W cm−2.647 To prepare this catalyst, carbon black (Ketjenblack EC-300) was first functionalized with N- and O-functionalities by refluxing it in HNO3. It then was mixed with cyanamide (CN−NH2) and iron sulfate. The resulting material was first heat-treated at 900 °C in N2, leached in H2SO4, then re-heat-treated at 900 °C in N2. (12) The last catalysts are NMCC SiO2-800-3 and NMCC C-800-3, which are displaying initial peak powers of 0.45 and 0.35 W cm−2, respectively.702 Their synthesis has already been

described as typical examples of catalysts for which, according to the authors, the main active sites are CNx. It is apparent from the description of the synthesis and from Table 7 that the most common factor to obtain high performing catalysts is the use of a carbon precursor like a MOF (here ZIF-8; a Zn imidazolate framework), a POP (a porous organic polymer made of functionalized tetraphenyl porphyrin units), or N-bearing polymers like polyacrylonitrile, poly m-phenylenediamine, or a polyimide composite. All of these carbon (and nitrogen) precursors yield catalysts with very high BET (total) surface area (between 758 and 1277 m2 g−1). These catalysts also contain a large fraction of micropores, whose surface area ranges between 780 and 844 m2 g−1 (according to the values reported in Table 7). In these catalysts, the active sites are made at the same time as the carbon support and are therefore well integrated in the catalyst structure. These active sites are, at least initially, in contact with O2 gas diffusing through the cathode. Furthermore, the high microporous surface area of these catalysts is also an important factor in their performance because it is known that highly active catalytic FeN4 sites are located in micropores.739,744 The use of a sacrificial support like silica (for the catalysts Fe-8 CBDZ-DHT-NH3 and NMCC-SiO2-800-3 in Table 7), which is also a way to produce catalysts from nitrogen and carbon precursors, does not seem, however, to be as efficient to synthesize high performing catalysts as is the use of nitrogen and carbon precursors like ZIF-8, POP, or N-bearing polymers. So far, when a silica sacrificial support has been used to prepare Fe-8 CBDZ-DHT-NH3, the BET did not increase to the level of that measured for catalysts 1−7 in Table 7. However, when both types of catalysts 12 are compared, one may conclude that replacing SiO2 with Ketjenblack does involve a decrease of the initial peak power of NMCC C-800-3 as compared to that of the same catalyst (NMCC SiO2-800-3) made with a sacrificial support, all other parameters of the synthesis being the same. From Table 7, it seems also that, as long as there is enough nitrogen and metal in the catalyst, neither nitrogen nor metal content seems to be a factor limiting the peak power performance of these catalysts. Finally, a last conclusion from Table 7 is that higher ORR performance in fuel cells is obtained with a higher percentage of Nafion in the ink used to prepare the cathode, as this percentage decreases similarly to the peak power from the top to the bottom of Table 7. It is expected that a large Nafion content in the ink will result in a better coverage of the non-noble catalyst in the cathode, and this should facilitate the access of protons to active sites located at the surface and in the micropores of these catalysts. 9.3. Initial Performance in H2/Air PEM Fuel Cells

Some of the catalysts that were tested for their beginning of life performance in H2/O2 PEM fuel cells (Figure 24) were also tested in H2/air PEM fuel cells. Their initial peak power distribution is shown in Figure 26. The same figure also includes the initial peak power of the catalysts that were only tested in H2/air fuel cells. It seems evident from Figure 26 that reporting performance of non-noble catalysts in H2/air is not as common as reporting their performance in H2/O2 fuel cells. Note that each power increment in Figure 26 is only one-half that of each power increment in Figure 24. This indicates that switching from O2 to air reduces the initial peak power by more than a factor of 2. This reduction factor is larger than for Pt/C for which switching from O2 to air only decreases the peak power of the Pt-based fuel cell from 1.36 to 0.96 W cm−2 (as it will be 3623

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(2) The second best performing catalyst is Fe Phen@MOFArNH3. It was made by mixing a solution A of 2-methylimidazole in methanol with a solution B of Zn(II) nitrate and 1,10-phenanthroline in water. Iron(II) acetate was then added to the reaction. Zn(II) nitrate and 2-methylimidazole are the necessary ingredients to obtain ZIF-8. Doing so, it is claimed that the complex Fe phenanthroline is trapped in the micropores of ZIF-8. The latter material is then heat-treated first in Ar at 1050 °C, then in NH3 at 1050 °C. The resulting catalyst is Fe Phen@MOF-ArNH3. It is characterized by an initial peak power of 0.38 W cm−2 in H2/air.691 (3) The synthesis and characterization of catalyst 3 in Table 8 have already been described. It is catalyst 7 (14#114)727 in Table 7. When measured in H2/air, the initial peak power of catalyst 3 in Table 8 is ≥0.32 W cm−2 (the polarization curve in H2/air stops at 0.8 A cm−2 at 0.4 V). (4) Catalyst 4 in Table 8 is Fe-PANI/C-Mela with an initial peak power of 0.33 W cm−2.718 This catalyst was made by dispersing melamine, aniline, and iron chloride in an HCl solution. Ammonium peroxidisulfate was added to induce aniline polymerization. After drying, the resulting mixture was first pyrolyzed at 900 °C in Ar. The sample then was leached in H2SO4 before being pyrolyzed a second time at 900 °C in Ar to obtain Fe-PANI/C-Mela. (5) Catalyst 5 in Table 8 is C700/950 with an initial peak power of 0.30 W cm−2.678 This catalyst was made from a Fe-based MOF obtained from the reaction of anhydrous ferrous chloride and 1,3,5 tris (2H-tetrazol-5-yl) benzene hydrochloride in a mixture of dimethylformamide and dimethyl sulfoxide. The dried Fe-based MOF was heat-treated at 700 °C in Ar, then leached in H2SO4, then heat-treated a second time at 950 °C in NH3. The conclusions that were reached for catalysts 1−12 tested in H2/O2 fuel cells, and for which the synthesis was previously described and the characterizations were summarized in Table 7, are the same as the conclusions that may be reached now for the three catalysts of Table 8 that were tested in H2/air. For the latter fuel cell tests, one sees that (i) the highest performances are again obtained for catalysts that are made by using one or several carbon precursors in their synthesis; (ii) the porosity of these catalysts is high with a major contribution of the micropore surface area (when available) to the total BET surface area; and (iii) nitrogen and Fe contents are in the same range as all other catalysts already reported in Table 7. Finally, and contrary to what was observed in Figure 24 for other catalysts whose ORR activity is claimed to be

Figure 26. Initial peak power for non-noble catalysts tested in H2/Air PEM fuel cells. The nature of the main active site at work in these catalysts has been proposed by their authors.

shown later on). This is noticeably an effect of mass transport problems that are more acute in the thick cathode of non-noble catalysts than in the thin cathode made with Pt/C. A list of non-noble cathode catalysts for H2/air fuel cells, their review code, their initial peak power, and their reference is provided in Table 6. On one hand, there are not enough results reported for H2/air fuel cell tests to obtain statistics similar to those discussed in Figure 25 for H2/O2 fuel cell tests. However, we may assume that the conclusions reached about catalyst loading, fuel cell temperature, back pressure, and membrane thickness will be the same for H2/air as for H2/O2 PEM fuel cells. On the other hand, we may now enlarge the previous discussion about how to prepare an initially performing catalyst because the important characteristics of the catalysts providing an initial peak power ≥0.30 W cm−2 in H2/air fuel cells may now be added to those already found from H2/O2 fuel cell experiments. (1) The highest performing catalyst is PAN-Fe031000NH3.731 Its synthesis has already been described as it is also the highest performing catalyst in H2/O2 in Table 7. In H2/air, the initial peak power of PAN-Fe03-1000NH3 is 0.41 W cm−2.

Table 8. Characterization of Performing Non-noble Cathode Catalysts Used in H2/Air PEM Fuel Cells porosity (m2 g−1)

elemental analysis wt % (or at. %) code/initial peak power (W cm2) 1

a

14#214 0.41

2

15#52 0.38

3

14#114 ≥0.32

4

13#11 0.33

5

14#112 0.30

catalyst’s name PAN-Fe031000NH3 Fe Phen @ MOFArNH3 Fe(PI1000-IIINH3) Fe-PANI/ C-Mela C700-950

only C precursor(s) in synthesis yes (PAN) yes

a

C black (or similar) in synthesis

N

Fe

no

(3.22) 3.7

(0.24) 1.11

Co

other

micropores

mesopores

1096

?

?

?

1200

?

?

40−60

no

?

3.1

yes (PIcomposite)

no

3.1

1.1

1050

844

206

63−37

yesa

no

?

?

702

?

?

75−25

yesa (MOF)

no

5.3

687

516

171

40−60

(2.2) 2.5

Zn 0.13

BET

ink composition (wt % cat.− wt % Nafion)

See details about the nature of the carbon precursor in the synthesis described in the text. 3624

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Review

based on encapsulated metal, catalyst 15#52 in Table 8 displays an exceptionally high ORR performance with an initial peak power of 0.38 W cm−2 in H2/air, as seen in Figure 26, where it is represented by a yellow rectangle. 9.4. Durability of Non-noble Metal Catalysts

The first part of this Review about non-noble catalysts was focusing on the initial performance of these catalysts. If performance is important for non-noble catalysts to become serious contenders to Pt, their durability is important as well. The latter will now be analyzed. The 2017 DOE durability target for Pt catalysts to be used in automotive application is 5000 h.742 However, to accelerate the durability tests, Pt-based catalysts are frequently probed in cycling conditions, either in MEA or in half cell (RDE). Two potential-cycle durability tests have been designed: (i) the start/stop cycle, and (ii) the load cycle.745 On the one hand, for the start/stop cycle, the protocol consists of applying a 2 s per cycle triangular wave from 1.0 to 1.5 V vs RHE with a sweep rate of 0.5 V s−1. The reason for these conditions is that during the shut-down of a fuel cell, the gas in the anode flow field is replaced with air. Thus, at the next start-up, the potential at the cathode is increased to about 1.5 V vs RHE.746 Under these conditions, the electrochemical oxidation of the carbon support is believed to play an important role in the degradation of Pt-based catalysts. On the other hand, for the load cycle, it is known that Pt dissolution occurs during potential change.745 In an actual PEM fuel cell vehicle, the potential range of each cell in the stack will approximately change between 0.6 and 1.0 V vs RHE. The former potential corresponds to the maximum load, and the latter to the open circuit potential during idle stop operation. The load cycle protocol consists of applying rectangular waves from 0.6 to 1.0 V vs RHE (3 s at 0.6 V and 3s at 1.0 V) after an initial potential hold at 0.6 V for 30 s. It has already been shown that applying potentials of ≥1.2 V to MEAs containing non-noble catalysts at the cathode results in serious degradation of the non-noble catalyst.728,747 Therefore, start/stop cycles up to 1.5 V applied to non-noble catalysts will inevitably degrade them. The question now is knowing that load cycles, occurring at potentials between 0.6 and 1.0 V vs RHE, are able to replace much longer durability tests for Pt-based catalysts, are similar load cycles performed in acid solution able to replace fuel cell durability tests for non-noble catalysts? As will be seen in Figure 27 and discussed below, the answer to this question is no! Figure 27A and B show a series of RDE curves recorded at 25 °C, in Ar (a) and in (b) O2-saturated 0.1 M HClO4, for multiple cycles followed by polarization curve measurements of FeIM/ZIF-8. This catalyst was made from a mixture of an iron imidazolate framework and ZIF-8, which was first pyrolyzed at 1050 °C in Ar, followed by a second pyrolysis at 950 °C in NH3. The initial peak performance of FeIM/ZIF-8 was 0.29 W cm−2 measured in H2/air fuel cell.716 It is obvious from Figure 27A and B that 10 500 cycles from 0.0 to 1.1 V at 50 mV s−1 in Ar or 2000 cycles in O2 at 10 mV s−1 have nearly no influence on the RDE properties of the catalyst in acid solution. However, Figure 27C shows that the catalyst is unstable at 0.5 V and 80 °C in a 100 h durability test in fuel cell. Similar conclusions were also reached in another paper from the same group722 and in a paper from a different group718 that used Fe-PANI/C-Mela, whose synthesis and properties were already reported as catalyst 4 in Table 8. Here, cycling was performed up to 10 000 cycles and at different potentials

Figure 27. ORR polarization curves of FeIM/ZIF-8 measured during RDE stability test. The test conditions include cycling from 0.0 to 1.1 V at 50 mV s−1 in (A) 0.1 M Ar-purged HClO4 or (B) 0.1 M O2-purged HClO4. At 25 °C for multiple cycles, followed by polarization curve measurements in O2-purged HClO4 at the scan rate of 10 mV s−1. (C) 100-h stability test by measuring the current density at 0.5 V of a single cell with FeIM/ZIF-8 as the cathode catalyst (Nafion 117 membrane) operated with H2/air. For all measurements, the temperature of the cell was kept at 80 °C.

(from 0.55 to 0.95 V) in air-saturated 0.1 M HClO4. Although Fe-PANI/C-Mela is a rather stable catalyst at all potentials in solution, a drastic decrease of current density, measured at 0.6 V in H2/air fuel cell, is however observed. Another group748 also reports similar results with a catalyst prepared by the sacrificial SiO2 support method, where a mesoporous ordered silica was mixed with two prophyrins (ClFeTMPP and CoTMPP). The resulting material was heat-treated at 800 °C in N2, and then the silica was leached out with HF to increase the total surface area of the catalyst to 930 m2 g−1. Although this is also an interesting catalyst, as far as its initial peak power is concerned, it was not included in Figure 24 because its polarization curve was reported as iR-free potential and the value of R was not provided in the paper. The uncorrected cell potential could therefore not be determined and compared to the other catalysts reported in Figure 24. It is, however, shown in the article that 10 000 cycles in O2-saturated 0.1 M HClO4, recorded between 0.6 and 1.0 V vs RHE at 50 mV s−1, leave this catalyst practically unchanged in solution, while it is unstable over 100 h at 0.5 V in the H2/O2 fuel cell. 3625

DOI: 10.1021/acs.chemrev.5b00462 Chem. Rev. 2016, 116, 3594−3657

Chemical Reviews

Review

Figure 28. Time evolution in H2/O2 fuel cell for catalysts 1−6 (A), and catalysts 7−11 (B), which were measured at constant potential of 0.5 V. Time evolution in H2/O2 fuel cell for all catalysts (C,D) that were measured at constant potential of 0.4 V. In (D), one of the catalysts was tested for 1100 h. The properties of the catalysts are detailed in Table 9. The numbers appearing at the end of each curve in (A) are the microporous surface area/total BET surface area of each catalyst and their ratio.

Now that it was demonstrated that the durability of nonnoble catalysts in MEAs in fuel cell cannot be extrapolated from RDE measurements in acid solution, it is therefore imperative to carry out durability experiments in fuel cell to obtain the true durability behavior of non-noble catalysts. Durability tests can be performed either in H2/O2 or in H2/air fuel cells. Other variables like constant potential or constant current have also to be considered in the durability tests. Only a few catalysts were measured for their durability at constant current, while most of them were reported at constant potential, especially at 0.4 V. In the following, we will first report durability tests in H2/O2 followed by tests in H2/air. Figure 28A and B presents the time evolution, for a maximum of 200 h and at 0.5 V in H2/O2 fuel cells, of catalysts 1−6 and catalysts 7−11 of Table 9, respectively. Figure 28C presents the time evolution for a maximum of 200 h of all of the catalysts that were measured at 0.4 V in H2/O2 fuel cell, while the time evolution for a maximum of 1100 h at 0.4 V in H2/O2 fuel cell is shown in Figure 28D for the same catalysts. A maximum of 200 h was chosen for all of the catalysts of Figure 28A−C, to compare them on the same time scale. For the sake of comparison, all catalysts of Figure 28 are also shown on the same power scale, from 0 to 0.5 W cm−2. In Table 9, the catalysts measured either at 0.5 V or at 0.4 V appear in decreasing order according to the initial power they display in Figure 28. Besides reporting the catalyst’s name given by their authors, Table 9 also reports (when available) important data

about the synthesis procedure of the catalysts, their elemental composition, their main catalytic site according to their authors, their porosity, the composition of the ink used to produce the cathodes, and also if any carbon etching, either with NH3 or with CO2, was used during their synthesis. Figure 29A−C presents the time evolution for a maximum of 200 h of all catalysts of Table 10, but this time at 0.6, 0.5, and 0.4 V in H2/air fuel cell, respectively, while the time evolution for a maximum of 850 h is only shown in Figure 29D for the catalysts probed at a constant potential of 0.4 V in H2/air fuel cell. Figure 30A presents the time evolution of all catalysts that were measured in H2/O2 fuel cell at a constant current of 0.1 or 0.2 A cm−2 and whose properties are gathered in Table 11. Figure 30B is the equivalent figure for all catalysts that were also measured at a constant current, but this time in H2/air fuel cell. Their properties are detailed in Table 12. It is important to note that, to compare the power obtained at constant current with those obtained at constant potential, all power axes in Figures 28, 29, and 30 are the same with powers ranging from 0 to 0.5 W cm−2 for H2/O2 and H2/air fuel cells. Note also that durability results at constant potential or constant current cannot be deduced from one another because the correlation between potential and current in fuel cell (the polarization curve) is not linear. Furthermore, it will also be shown later that the durability behavior of a catalyst in fuel cell is not the same at all potentials but is better at lower potentials. When durability is measured 3626

DOI: 10.1021/acs.chemrev.5b00462 Chem. Rev. 2016, 116, 3594−3657

a

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

2 3 4 5 6 7 8 9

3627

0.4 0.4

0.4

0.4

0.4

0.4 0.4

1 2

3

4

5

6 7

FeCo-EDA-600 10#29 UF-C 09#-06

bNGr 14#97

Co-ED/Ppy-CNF-10 12#44 Fe-NSG 15#66

Co-ED/PPy-CNF 11#97 P-CNF 11#87

Fe/N/CF 15#54 PFeTPP-1000 13#124 NC Ar + NH3 11#138 Zn(mIm)2 TPIP 14#04 SCI Ar + NH3 09#74 NC Ar 11#138 Fe/melamine/KB 11#39 N-G-CNT: 2/0.5/0.15 + KB 15#01 Fe/Phen/N-C flakes 13#30 CHb 350 900 08#03

Fe/N/C-SCN 15#60

catalyst’s name ref

no no

no

no

no

no no

yes

no

yes yes yes yes no yes no no

no

(MOF)

(PAN + PMMA) (POP) (MOF) (MOF)

only C precursor(s) in synthesis

yes functional graphite oxide yes ball-milled graphite oxide yes Ketjenblack yes Ketjenblack

no + vulcan after synth yes carbon nanofiber yes platelet C nanofiber yes carbon nanofiber

yes carbon nanoflakes

yes Ketjenblack (KJ600) no no no no yes black pearls no yes Ketjenblack yes NCNT

C black (or similar) in synthesis

(2.18) 2.51 2.2

7.3

(3.7) 3.9

4.5

4.5 3.8

?

(1.88) 2.14

9.0 1.92 (5.3) 6.0 2.22 (2.4) 2.7 (3.7) 4.2 4.1 (4.1) 4.7

4.4

N

4.14 0.0

1.7

(1.1) 4.8

0.47

a

(0.28) 1.28

0.56 4.95 (0.78) 3.51 1.07 (0.44) 2.01 (0.65) 2.92 ? 0a

1.4

Fe

4.31 0.0

0.3

2.7

2.7 ?

Co

(FeNx)? (EFeNc)? CNx (Co/ Fe) Fe/Co- Nx CNx (0 Me)

CNx (Co)

CNx (Co) CNx (Co)

FeNx

FeNx

FeNx FeNx FeNx FeNx FeNx FeNx FeNx CNx

FeNx

main catalytic site

483 321

?

?

?

? ?

453

?

809 758 964 1277 767 550 ? 422

751

BET

Deduced from TGA (no metal residue in N-G-CNT). b(Catalyst + Ketjenblack)/Nafion = 1/1. cEncapsulated Fe-containing nanoparticles.

0.5

11

10

0.5

1

potential (V)

elemental analysis wt % (or at. %)

136 ?

?

?

?

? ?

?

?

780 mostly 814 ? 605 504 ? ?

414

micropores

347 ?

?

?

?

? ?

?

?

8 ? 184 ? 162 46 ? ?

337

mesopores

porosity (m2 g−1)

Table 9. Characterization of Non-noble Cathode Catalysts Used for Durability Tests in H2/O2 PEM Fuel Cells: Constant Potentials

50−50 ?

33−67

?

?

? ?

29−71

?

40−60 50−50 40−60 ? 40−60 40−60 67−33 50−50b

50−50

ink catalyst− Nafion (wt %)

no no

no

no

no

no no

CO2

NH3

NH3 no NH3 NH3 NH3 no no NH3

no

use of NH3 or CO2

Chemical Reviews Review

DOI: 10.1021/acs.chemrev.5b00462 Chem. Rev. 2016, 116, 3594−3657

Chemical Reviews

Review

Figure 29. Time evolution in H2/air fuel cell for all catalysts that were measured at constant potential of either 0.6 V (A) or 0.5 V (B). Time evolution in H2/air fuel cell for all catalysts that were measured at constant potential of 0.4 V for 200 h (C), and up to 900 h (D). The properties of the catalysts are detailed in Table 10.

are highly porous. Furthermore, all of these catalysts are characterized by a very high porosity with a total surface area (BET) comprised between 751 and 1277 m2 g−1. Most of the pores in these catalysts are micropores, which are known to host highly active FeNx sites.744 The numbers appearing at the end of each curve in Figure 28A are the microporous surface area/total BET surface area of each catalyst and their ratio. It seems that the higher is this ratio, the lower will be the power at which the catalyst will reach some stability. All durability tests stop at 100 h. It seems, however, that the curve of catalyst 15#54 with a microporous to total surface area ratio of 0.96 will cross for t > 100 h the durability curve of the catalyst 11#38_Ar + NH3. From the behavior of all durability curves in Figure 28A, one may conclude that, if a high microporous surface area is beneficial to promote the initial peak performance of a catalyst, the same high microporous surface area will be detrimental for its stability! Catalyst 11#138 Ar in Figure 28B is typical of behavior 2. Here, the performance of the catalyst goes through a maximum, then decreases. The synthesis of catalyst 11#138 Ar + NH3, which is a highly performing catalyst in this Review with an initial peak power of 0.91 W cm−2, has been described under catalyst 3 in section 9.2. The synthesis of 11#138 Ar is exactly the same as that of 11#138 Ar + NH3, except for the last pyrolysis step in NH3 that is missing in 11#138 Ar. A 57Fe Mössbauer study of these two catalysts revealed that both catalysts contained about the same number of Fe-based catalytic sites, but that many of these sites were initially secluded in 11#138 Ar. This was not the case for 11#138 Ar + NH3 for

galvanostatically in fuel cell for an unstable catalyst, a decrease in current is compensated by a decrease in potential, which means a change in potential toward a potential of better stability. Therefore, a durability test at constant current (starting at an initial potential Vstart) will give the impression that the catalyst is more stable than if the durability test of the same catalyst was measured using Vstart as constant potential. Let us discuss now the durability results illustrated in Figures 28 and 29. These are results obtained at constant potential either in H2/O2 or in H2/air fuel cells. Focusing first on catalysts tested in H2/O2 fuel cells that deliver initial powers ≥0.10 W cm−2, we notice essentially three behaviors: (i) behavior 1 illustrated in Figure 28A, where all catalysts are initially very performing (with an initial power around 0.5 W cm−2 at 0.5 V) but are also very quickly losing their activity; (ii) behavior 2 illustrated by 11#138 Ar and 11#39 in Figure 28B, where the catalysts’ performance first increases, goes through a maximum (between 0.20 and 0.30 W cm−2 at 0.5 V), then decreases; to a certain extent, it is also the durability behavior of the three first catalysts listed in Figure 28C (it is easier to see the negative slopes of the same three curves on Figure 28D, drawn on an extended scale); and (iii) behavior 3 illustrated in Figure 28 D by catalyst 14#97 that is able to deliver a rather stable but low power of 0.13 W cm−2 for 1100 h at 0.4 V. Looking to Table 9 for properties of catalysts displaying behavior 1, one can see that four out of six of these catalysts were made without any addition of carbon black during their synthesis, and, when carbon black was used in the synthesis, either Ketjenblack or black pearls were chosen as both carbon supports 3628

DOI: 10.1021/acs.chemrev.5b00462 Chem. Rev. 2016, 116, 3594−3657

0.8

0.6

0.6

0.6 0.5

0.5 0.5 0.5 0.5

0.5

0.5 0.4 0.4

0.4

0.4

0.4

0.4

0.4

0.4 0.4

0.4

0.4

1

1

2

3 1

2 3 4 5

6

7 1 2

3

4

5

6

7

8 9

10

11

potential (V)

PAN-Fe031000NH3 14#214 Fe-PANI-Mela 13#11 NC Ar F(90) 14#NIC PANI-Fe-C 13#18 NC Ar + NH3 11#138 NC Ar 11#138 15#54 FeIM/ZIF-8 12#104 CHbMg-1000-5 min 14#151 CHb Mg 3s 400 900 11#109 CHb200 900 08#03 Py-B12/C 12#15 SCI Ar + NH3 09#74 PyP-CoTMPP-600 10#84 NMCC-SiO2-800-3 10#11-800C PANI FeCo-C 11#137 NMCC-SiO2-1000-3 10#11-1000C CoFe(1:3)N-C 11#79 PANI-Fe-C 13#18 PyP-CoTMPP-700 08#13 Py-Co-corrole/ C-700 12#79 Fe/C 15#11

catalyst’s name ref

3629

yes

no yes + sacrificial SiO2 no

yes + sacrificial SiO2 no

yes + sacrificial SiO2 yes + sacrificial SiO2 no

yes hemoglobin no no

no

yes vulcan

yes Ketjenblack no

yes Ketjenblack

no

yes Ketjenblack

no

no

no no + vulcan after synth no + vulcan after synth no yes vulcan yes black pearls

yes (MOF) yes hemoglobin

yes hemoglobin

no

yes 25 wt % C fibers yes Ketjenblack no

no

no

C black (or similar) in synthesis

yes (MOF)

yes 75 wt % (MOF) no yes (MOF)

yes

yes (PAN)

only C precursor(s) in synthesis

∼4.0

?

? N/Co= (2.6)

(6.0) 6.7

(2.38) 2.76

?

(5.93) 6.85

?

? ? (2.4) 2.7

?

4.50 2.35

(3.7) 4.2

? (5.3) 6.0

(3.90) 4.4

?

(3.22) 3.7

N

∼8.0

7.4 (t = 0 h) 1.6 (t = 200 h)

(0.6) 2.7

?

?

?

(0.44) 2.01

0.41

0.25

5.29 0.32

(0.65) 2.92

7.4 (t = 0 h) 1.6 (t = 200 h) (0.78) 3.51

2.75

?

(0.24) 1.11

Fe

elemental analysis wt % (or at. %)

1.50

?

(0.0) 0.0

?

?

?

?

0.95

Co

EFeN

CoNx

FeNx CoNx

FeNx/CNx (Co)

CNx (Co/Fe)

Fe-CoNx

CNx (Co/Fe)

CoNx

FeNx CoNx FeNx

FeNx

FeNx FeNx

FeNx

FeNx FeNx

FeNx

FeNx

FeNx

main catalytic site

375

?

? ?

370

?

?

?

?

831 ? 767

1110

572 1562

550

? 964

338

702

1096

BET

?

?

? ?

?

?

?

?

?

? ? 605

?

mostly ?

504

? 814

310

?

?

?

?

? ?

?

?

?

?

?

? ? 162

mainly

? mainly

46

? 184

28

?

?

micropores mesopores

porosity (m2 g−1)

Table 10. Characterization of Non-noble Cathode Catalysts Used for Durability Tests in H2/Air PEM Fuel Cells: Constant Potentials

40−60

33−67

70−30 ?

65−35

75−25

65−35

75−25

?

29−71 67−33 40−60

?

50−50 39−61

40−60

70−30 40−60

40−60

?

?

ink catalyst− Nafion (wt %)

no

no

no no

no

no

no

no

no

CO2 no NH3

no

NH3 NH3

no

no NH3

no

no

NH3

use of NH3 or CO2

Chemical Reviews Review

DOI: 10.1021/acs.chemrev.5b00462 Chem. Rev. 2016, 116, 3594−3657

Chemical Reviews

Review

Ar + NH3 and also to increase somewhat its total N content (from 3.7 at. % for 11#138 Ar to 5.3 at. % for 11#138 Ar + NH3). It was proposed that the activation of 11#138 Ar during its durability test was the result of the dissolution of some inactive material, giving access to O2 and to the electrolyte and to increasingly more catalytic sites that were once secluded in the material. However, once activated, these sites begin to perform ORR and show a rapid decay as they do for 11#138 Ar + NH3.749 To obtain behavior 2, it is, however, necessary that the rate at which sites are activated is faster than that of their decay. We believe that this explanation may also hold for the three best catalysts of Figure 28C (or D), a fit to the experimental curve being obtained by balancing activation and decay rates for these catalysts. According to their authors, all of the catalysts of Figure 28C and D (except for 10#29 and 15#66) are supposed to reduce oxygen on CNx sites, but they are all made (except for 09#06) with Co or/and Fe precursors. The only catalyst also believed to function with CNx catalytic sites but made without Co or Fe precursors is 09#06. It is, however, also a poor performing, yet rather stable catalyst. The catalyst that displays a stable 0.13 W cm−2 for 1100 h (14#97) was made by first ball-milling graphite oxide, then mixing this material with dicyanamide (a nitrogen precursor) with CoCl2 and FeCl2. The mixture was pyrolyzed at 900 °C in Ar, then leached in H2SO4 at 80 °C, then pyrolyzed again at 900 °C in Ar. According to their authors, the main catalytic sites of catalyst 14#97 are CNx (with some minor contribution of MeNx sites).726 Catalyst 10#29 is supposed to reduce oxygen on Fe/CoNx sites. However, it does not show the typical decay observed for the catalysts reputed to reduce oxygen on FeNx or CoNx active sites. This is because their authors waited until the catalyst displayed some stability before recording its durability behavior.703 As far as catalyst 15#66 is concerned, it is not obvious to know what type of catalytic site is responsible for the ORR activity and performance attributed to this catalyst.737 Looking now at the durability tests recorded for H2/air, we first note in Figure 29A that catalyst 13#11 displays a strong typical behavior 1 at a constant potential of 0.6 V. Again, according to Table 10, this catalyst was made from carbon precursors only, like most of the catalysts presented in Figure 28A. Catalyst 13#11 is characterized by a high total BET surface area of 702 m2 g−1, a value similar to the BET surface area of many catalysts of Table 9, also made without any carbon black in their synthesis. Catalyst 13#18 displays a mild behavior 1, while catalyst 14#Nic730 displays

Figure 30. Time evolution for all catalysts that were measured at constant current in H2/O2 (A) and H2/air (B) fuel cells. The properties of the catalysts are detailed in Tables 11 and 12.

which most of the sites were initially accessible to perform ORR.749 Looking to Table 9, one sees that the BET surface area of 11#138 Ar is only 550 m2 g−1 (with a microporous surface area of 504 m2 g−1), as compared to the BET surface area of 11#138 Ar + NH3 (964 m2 g−1) with a microporous surface area of 814 m2 g−1. The effect of NH3 is therefore to etch 11#138 Ar to practically double its surface area in 11#138

Table 11. Characterization of Non-noble Cathode Catalysts Used for Durability Tests in H2/O2 PEM Fuel Cells: Constant Currents elemental analysis wt % (or at. %) only C current catalyst’s name precursor(s) in −2 (A cm ) ref synthesis 1

0.2

2

0.2

3

0.2

4

0.2

5

0.1

NMCC-800 09#34 CoTETA/C 10#01 NMCC-1100 09#34 C-compositeCo3Fe1Nx 08#15 MNC-3 15#63

no no no no no

C black (or similar) in synthesis yes Ketjenblack yes Ketjenblack yes Ketjenblack yes Ketjenblack yes Ketjenblack

porosity (m2 g−1) ink catalyst− Nafion micropores mesopores (wt %)

use of NH3 or CO2

N

Fe

Co

main catalytic site

?

?

?

CNx (Co/Fe)

?

?

?

75−25

no

?

α-Co?

?

?

?

?

no

?

BET

?

?

?

CNx (Co/Fe)

?

?

?

75−25

no

3.9

1.4

4.6

CNx (Co/Fe)

?

?

?

75−25

no

873

?

?

70−30

no

3.7

FeNx 3630

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Table 12. Characterization of Non-noble Cathode Catalysts Used for Durability Tests in H2/Air PEM Fuel Cells: Constant Currents elemental analysis wt % (or at. %) porosity (m2 g−1) current (A cm−2)

catalyst’s name ref

1

0.2

2

0.25

1000-III-NH3-g 14#84 Fe(PI-1000-IIINH3) 14#114

only C precursor(s) in synthesis

C black (or similar) in synthesis

N

Fe

yes

no

1.9

1.6

yes

no

3.1

1.1

Co

main catalytic site CNx (Fe) CNx (Fe)

BET

ink catalyst− Nafion micropores mesopores (wt %)

use of NH3 or CO2

918

698

220

?

NH3

1050

844

206

63−37

NH3

highest initial peak power (0.98 W cm−2) recorded in this Review (see catalyst 1 in Table 7). Another point that is important to note in Figure 29C is the behavior of catalyst 10#11 pyrolyzed at two temperatures, either 800 or 1000 °C. For this catalyst, 10#11_800C practically displays a behavior 1-like type curve, while the decay curve for 10#11_1000C looks more like a behavior 2 type of curve. The catalyst was made using sacrificial SiO2 support on which complexes of ethylene diamine with cobalt nitrate and iron sulfate were impregnated. After a first pyrolysis at 800 °C (or 1000 °C) in Ar, the SiO2 sacrificial support was removed using NaOH. The resulting material was acid leached, then re-heat-treated at 800 (or 1000 °C) in Ar. According to the authors, CNx are the main active sites in these two catalysts, despite the use of Co and Fe precursors in their synthesis. Therefore, the pyrolysis temperature greatly influences the decay rate of a catalyst, slowing it for higher pyrolysis temperatures. This positive effect is, however, counterbalanced by some loss of power for the resulting catalyst pyrolyzed at the highest of both temperatures (800 or 1000 °C). The last durability (Figure 30A and B) has been measured under constant current either in H2/O2 or in H2/air fuel cell, respectively. As it was already mentioned, it is not possible to compare durability results obtained at constant current with those obtained at constant potential. Furthermore, except for the two catalysts tested under H2/air, the initial power of all catalysts tested under H2/O2 is quite low (≤0.10 W cm−2) and therefore of little interest. Both 14#84 and 14#114 catalysts were made with a carbon precursor (instead of a carbon black), and they have again a high BET surface area with a large fraction of micropores. It is expected that, if the same catalysts were tested at constant potential, they would display a fast initial decay starting at 0.15 W cm−2 for 14#114 or 0.10 W cm−2 for 14#84. According to their authors, CNx is the main ORR site in these catalysts, even if an iron precursor was used in their synthesis.

a behavior 2 durability curve. In Figure 29B, which presents the durability behavior at a constant potential of 0.5 V, we also recognize a behavior 1 in the durability curves of 11#138_Ar + NH3, while catalyst 11#138 Ar displays a behavior 2 durability curve. Both catalysts are the same as those already described in Figure 28A. However, after 100 h, both 11#138 + NH3 Ar and 11#138 Ar catalysts deliver the same power (∼0.14 W cm−2) at 0.5 V in Figure 29B, catalyst 11#138 Ar + NH3 starting at 0.3 W cm−2 at t = 0 and 11#138 Ar starting at 0.15 W cm−2. This is quite different from the behavior of the same catalysts measured in H2/O2 durability tests, where catalyst 11#138 Ar + NH3 started at 0.45 W cm−2 and dropped to 0.08 W cm−2 after 100 h of test, while catalyst 11#138 Ar started at 0.23 W cm−2 and dropped to about 0.16 W cm−2 after 100 h (see Figure 28A). Therefore, the use of pure O2 gas at the cathode exacerbates the power decay during the durability tests of the catalysts. The last figures to discuss in the series of experiments performed at constant potential are Figure 29D and the detail of its first 200 h test presented in Figure 29C. Here, the potential of the catalysts has been maintained constant at 0.4 V. In these figures, catalysts 11#137 and 13#18 display rather stable durability behaviors (during 200 h for catalyst 13#18 and about 700 h for catalyst 11#137). The synthesis of catalyst 11#137 was already described as catalyst 9 in the previous section reporting on the initial performance of the catalysts. Alike 11#137, catalyst 11#18 is also made with PANI as nitrogen precursor. Both catalysts use Ketjenblack as carbon black in their synthesis. Here, it is important to note that if catalyst 13#18 displays a stable behavior for 200 h at about 0.11 W cm−2 when it is measured in H2/air fuel cell at a constant potential of 0.4 V, the same catalyst is not stable in H2/air fuel cell when it is measured at a constant potential of 0.6 V. In that case, the power delivered at t = 0 is about 0.12 W cm−2. It drops to about 0.07 W cm−2 (a 40% power drop), after 200 h after a durability test at 0.6 V. No wonder durability experiments are most popular in H2/air fuel cell tests at a constant potential of 0.4 V! In these conditions, the authors are able to report larger and more stable powers than at higher potential or in H2/O2 fuel cells. It is particularly important to report here on the durability of catalyst 14#214, which was the only catalyst measured at 0.8 V and should therefore decay very quickly at that high potential. However, this catalyst only loses 10% of its initial performance after 7 h of test in H2/air fuel cell. This is a power loss comparable to that of catalyst 13#18 presented in Figure 29A, but at 0.6 V instead of 0.8 V. It is a pity that the durability test performed on catalyst 14#214 was not longer as this catalyst seems quite promising. Indeed, in H2/O2 fuel cell, the same catalyst was characterized by the

9.5. Origin of Activity Loss in Fuel Cells

Durability tests for ORR catalysts measured in fuel cell are very often reported without any comments. When some comments are provided, a list of possible hypotheses to explain the lack of durability is also provided, most frequently without any elaborate explanation. These hypotheses are, for instance: (i) the electrooxidation of the catalyst carbonaceous support causing the loss of ORR catalytic sites, which are known to be integrated for Me/N/C and for CNx in the carbonaceous support of the catalysts; (ii) the loss of the metallic ion in the Me/N/C sites, leaving behind free metal ions, like Fe or Co ions, and also pyridinic type CNx sites; (iii) the generation of H2O2 by less performing ORR sites that may further oxidize the 3631

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cell cathode when it is fed with O2 instead of air. Moreover, the hypothesis of flooding the micropores would also explain the difference in stability behavior (presented in Figure 27) for the same catalyst between RDE and fuel cell tests. For RDE tests, where the electrolyte is an aqueous acid solution, the micropores of the catalyst are instantly filled with water from the acid solution; it is therefore difficult to measure a decay in performance similar to behavior 1 in that case. On the contrary, in fuel cell tests, where the electrolyte is a protonic ionomer, water flooding of micropores is the result of the ORR; in this case, the performance decay becomes measurable. This hypothesis of flooding of micropores has been put to test in a recent paper where the hydrophobicity of the carbon support was modified by changing the synthesis procedure to obtain such catalysts.755 In our recent attempt, we produced catalysts with various hydrophilicity by ball-milling ZIF-8 with an iron porphyrin: ClFeTMPP. The mixture was then heat-treated at high temperature in Ar. The temperature of this first pyrolysis varied from 850 to 1150 °C in steps of 100 °C to provide four different catalysts labeled: NC ClFeTMPP-T Ar, where T was either 850, 950, 1050, or 1150 °C. All four catalysts then were submitted to a second pyrolysis, this time identical for all catalysts and involving a heat-treatment at 950 °C in NH3. The final catalysts were labeled: NC ClFeTMPP-T Ar + NH3 with T representing the temperature of the first pyrolysis in Ar. The nominal Fe loading of all of these catalysts was always 0.8 wt % as Fe in ClFeTMPP. Figure 31 illustrates the power durability behavior of the four catalysts. It is clear from this figure that the catalyst delivering

ionomer and/or the catalyst, especially if metallic ions like Fe or Co ions are around, because the latter are able to generate Fenton’s type reagents that are highly oxidizing species;750,751 (iv) Fe or Co ions released from catalytic Me/N/C sites may also exchange for protons in the ionomer, increasing therefore its resistance; (v) water flooding of the catalyst pores impeding the transport of oxygen in the cathode; and (vi) reaction of protons with pyridinic CNx sites poisoning the activity of these sites, etc. An excellent review about the lack of durability of non-noble metal and metal-free catalysts and its potential causes has recently been published.752 Note that it is possible that the lack of durability of most Me/N/C catalysts may have more than one cause, complicating therefore the resolution of this problem. Let us now focus on some of the hypotheses for which the authors have provided elaborate information. For Popov and collaborators who defend the idea that CNx are ORR catalytic sites, even if Co or Fe precursors are used as metal precursors during the preparation of the catalysts, the metal is only present during the synthesis to favor N-doping of the carbon support.639 There are, however, several kinds of CNx sites depending on where N substitutes for C. A pyridinictype N substitutes for a carbon atom at the edge of graphene platelets (or carbon sheets), while a graphitic N substitutes for carbon on the base planes of the carbon support. Both pyridinic and graphitic type CNx sites are providing some ORR activity. However, pyridinic-type N is a basic functionality that will react with protons in the acid medium of the fuel cell to form NH+ species, which then become ORR inactive. Durability experiments showing a fast decrease of ORR with time are therefore interpreted as the result of the protonation of pyridinic-type CNx sites. Furthermore, Popov and collaborators also advocate that preparing catalysts at 800 or 1000 °C, as it was done for the two types of 10#11 catalysts in Figure 29C, results in the loss of more of pyridinic N-type sites, which are mainly found at 800 °C than of graphitic N-type sites, explaining therefore why the catalyst prepared at 1000 °C is initially less performing but more durable than that prepared at 800 °C. For Ozkan and collaborators, who recognize the existence of CNx sites and that of FeNx sites in Fe/N/C catalysts, the loss of durability of Fe/N/C catalysts results from the loss of Fe ions from the FeNx catalytic sites. Because FeNx sites are resulting from the coordination of a Fe ion between pyridinic type nitrogen atoms,640,738,739a demetalation of the FeNx sites will leave behind pyridinic type N, increasing therefore the total density of CNx sites in the catalyst.643,753,754 Nothing is said, however, in Ozkan’s group publications about the reason why FeNx sites in Fe/N/C catalysts are demetalated when these catalysts are in contact with the acidic medium of a PEM fuel cell. Our explanation for behavior 1 durability curves mainly illustrated by all of the curves in Figures 28A, for Me/N/C catalysts, is that these materials are highly microporous and therefore also highly prone to water flooding. As the main catalytic sites in our catalysts are hosted in micropores,744 flooding them with water will oblige O2 to change its mode of transport to the sites from a gaseous to a dissolved mode in the now water-filled micropores. This would explain the drop with elapsed time (behavior 1) in the power delivered by these catalysts, being more drastic when the microporous surface area becomes a larger fraction of the total BET surface area. It would also explain why using O2 at the cathode is much more efficient to flood the micropores of the catalysts than using air, because more water is provided for the same volume of gas at the fuel

Figure 31. Chronoamperometry curves at 0.6 V in H2/O2 fuel cell for NC ClFeTMPP-T Ar + NH3 catalysts, where T is the temperature (between 850 and 1150 °C) of the first pyrolysis in Ar. A second pyrolysis is then performed in NH3 at 950 °C. The open symbols are the current density values read at 0.6 V on the polarization curves measured either initially or between two segments of chronoamperometry curves.

the largest initial power is NC ClFeTMPP-1050 Ar + NH3, but the catalyst showing the best durability is that produced at the highest first pyrolysis temperature (1150 °C) in Ar. These four NC ClFeTMPP-T Ar + NH3 catalysts have been characterized for the change in their graphitization when the pyrolysis temperature in Ar was modified from 850 to 1150 °C. Changes in their total nitrogen and oxygen contents, in the type of nitrogen 3632

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content at the surface of its carbon support, as well as by their lowest surface concentration of pyridinic and pyrrolic nitrogen atoms. The same catalysts displays a high total surface area, mainly composed of micropores. It has been shown that the behavior of the four catalysts in Figure 31 cannot be explained neither by using the hypothesis proposed by Popov and his collaborators nor by that provided by Ozkan and her collaborators.755 On the contrary, the variable lack of durability of the four catalysts illustrated in Figure 31 can be rationalized in terms of water flooding of the catalyst’s micropores: NC ClFeTMPP-1150 Ar + NH3 being the least hydrophilic and the most graphitic of all four catalysts. It remains now to be checked if the same behavior cannot be explained as well by some general electrooxidation of the catalyst’s support or by an electrooxidation limited only to their active sites. As far as an increase of the MEA resistance with time is concerned, we checked this hypothesis to explain behavior 1 decay, while recording the data necessary for Figure 31, as a complete polarization curve and an impedance measurement were recorded at specific times marked by symbols on each curve in Figure 31. The impedance measurements for all catalysts indicated that the resistance of the MEA (mostly related to Nafion ionomer and Nafion membrane) in fuel cell remained practically constant and even slightly decreased with time during the durability test. For instance, for NC ClFeTMPP-1150 Ar + NH3, the initial resistance was 0.070 Ω cm2, while it was 0.064 Ω cm2 after 88 h at the end of the durability test. This observation suggests that if iron was eventually released from the catalyst into the MEA, it should be negligible as otherwise it would have increased the resistance of the MEA by exchange with protons. The most difficult cause for a lack of durability to be investigated in fuel cell is certainly the possible release of peroxide in an MEA. However, if some amount of peroxide is indeed released, it should be low because the presence of any free iron ions in the MEA would result in a strong chemical oxidation of the catalyst’s carbon support by the Fenton’s reagent produced by the chemical reaction of free iron ions with hydrogen peroxide. It remains to be checked in fuel cell whether the electronic conductivity around catalytic sites is not drastically affected by chemical corrosion of the carbon structure of the catalysts.

functionalities doping their carbon support, and in their porosity (total BET and microporosity) were also determined. All of this information is summarized in the three panels of Figure 32, where it can be seen that NC ClFeTMPP-1150 Ar + NH3 is the most graphitic catalyst among the four ones. It is also characterized by the lowest heteroatom (N and O)

9.6. Comparison with Pt

It is important to compare performance and durability measured for non-noble catalysts with those of Pt-based catalysts. As far as performance is concerned, we have seen in Figure 24 that the catalyst displaying the largest initial peak power in H2/O2 fuel cell was 14#214 with a maximum initial peak power of 0.98 W cm−2.731 The durability in H2/O2 and H2/air fuel cells was not reported in the same experimental conditions for at least 100 h at any potential for the same catalyst, but this was done for catalyst 11#38_Ar + NH3, the third catalyst on the list in Table 7 at a constant potential of 0.5 V (Figure 28A, H2/O2; and Figure 30, H2/air). The original publication about catalyst 11#38_Ar + NH3 was also reporting on the activity of that catalyst measured in A cm−3 at 800 mViR‑free in H2/O2 fuel cell and in experimental conditions recommended by the DOE for non-Pt catalysts activity per volume of supported catalyst (for transportation applications).742 The volumetric activity of 11#138 Ar + NH3 was 39 A cm−3 (measured at 800 mViR‑free) and 230 A cm−3 (extrapolated at 800 mViR‑free from a linear Tafel slope between 1 and 0.9 V (both iR-free)). The 2020 volumetric activity target set by the DOE is 300 A cm−3.742

Figure 32. (A) Changes with the pyrolysis temperature in Ar, in fwhm of the D band and D/G surface area ratio, for all of the NC ClFeTMPP-T_Ar + NH3 catalysts first pyrolyzed in Ar then in NH3. (B) Changes with the first pyrolysis temperature in Ar, in the total nitrogen and oxygen contents of NC ClFeTMPP-T_Ar + NH3 catalysts. (C) Left axis: Changes with the first pyrolysis temperature in Ar, in the individual types of nitrogen contents for NC Por_0.8-T Ar + NH3 catalysts. Right axis: Changes with the first pyrolysis temperature in Ar, in the microporous surface area (lines and points) and total surface area (points only) of NC ClFeTMPP-T_Ar + NH3 catalysts. 3633

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Note that the highest volumetric activity (450 A cm−3), measured at 800 mViR‑free, has been reported for catalyst 15#54.734 The latter is the fourth catalyst on the list in Table 7. Coming back to the performance and the durability of nonnoble catalysts, it is certain that the first applications of these catalysts will not be in transportation but rather in lower power applications like in portable PEM fuel cells for which the Pt-group metal loading in the fuel cell MEAs will not be as restrictive as those recommended by the DOE for transportation applications. In the latter case, the 2020 DOE target for Pt group metal-based catalysts is 0.125 mg Pt group metal per cm2 of electrode area (total loading). In this Review, we used a cathode loading of 0.3 mg Pt cm−2 to make our comparison. The polarization curves and power curves for a H2/O2 and a H2/air fuel cell using that Pt loading are given in Figure 33,

the power loss of the Pt/C catalyst in Figure 33 should be 0.0297 W cm−2 per 100 h and 0.0259 W cm−2 in H2/air fuel cell. Therefore, after 1100 h in H2/O2 fuel cell, the power delivered by Pt/C of Figure 33 should be 1.318 W cm−2. In these conditions, the stable behavior of catalyst 14#97 at 0.13 W cm−2 represents 9.8% of the power delivered by Pt/C after 1100 h. Accordingly, after 700 h in H2/air fuel cell, the power delivered by Pt/C of Figure 33 should be 0.898 W cm−2, and the stable behavior of catalyst 11#137 at 0.14 W cm−2 represents 15.6% of the power delivered by Pt/C after 700 h. At the end of this Review of the carbon-based non-noble metal and metal-free catalysts, one may wonder how much further these catalysts need to go to become serious contenders as Pt-based catalysts in automotive application. According to the manufacturing cost analysis of Fuel Cell Systems published in 2011 by the DOE for an estimated annual production rate of 500 000 units for automotive application, a 0.125 mg cm−2 Pt-based cathode catalyst would represent ca. 30% of the total cost of the stack (25$/kWnet) or 15% of the system cost (51$/kWnet).756 The calculation of this study was based on MEAs of 0.125 mg Pt cm−2 of a nanostructured thin film platinum catalyst producing 833 mW cm−2 at 55% cell efficiency (0.676 V) under H2/air. In this Review, the most performing non-noble metal catalyst produces 230 mW cm−2 at 55% cell efficiency (0.676 V) under the same conditions.731 However, in a PEM fuel cell, the catalyst is not the only element composing the stack. A decrease in performance from a non-noble metal catalyst as compared to a Pt cathode will inevitably imply an increase in the number of bipolar plates, membranes, gaskets, and current collectors to keep the same power output. Therefore, by using the cathode of 500 000 units of Fuel Cell Systems, the most performing non-noble metal catalyst, and assuming that this catalyst will be as durable as Pt and that its production cost will be 10 times less expensive than that of a Pt cathode, the stack cost will nevertheless jump to 51$/kWnet or twice the cost of a Pt stack! From this analysis, it is then obvious that the future research axes for non-noble (or metal free) catalysts for ORR will be (i) continue to improve their initial peak power performance; (ii) improve their durability to at least that of Pt; and (iii) decrease their production cost to the smallest possible fraction of the cost of Pt-based catalysts.

Figure 33. Polarization and power curves obtained at 80 °C in H2/O2 and H2/air PEM fuel cells using a Pt-based commercial catalyst with a cathode loading of 0.3 mgPt cm−2. The membrane was Nafion NR211. Backpressures of 15 psig and 0.3 lpm for either H2, O2, or air were used in these experiments.

where it is seen that, with a loading of 0.3 mg Pt cm−2 at the cathode, a peak power of 1.36 W cm−2 is obtained at 0.41 V and 3.4 A cm2 in a H2/O2 fuel cell, and 0.96 W cm−2 is obtained at 0.45 V and 2.13 A cm−2 in a H2/air fuel cell. The maximum initial peak power of 0.98 W cm−2, measured in H2/O2 fuel cell for a cathode using the non-noble catalyst 14#214 PAN-Fe031000NH3,731 represents therefore 72% of the maximum peak power generated by the Pt/C electrode of Figure 33. As far as H2/air is concerned, the maximum initial peak power of the best catalyst in Figure 26 is 0.41 W cm−2. It is also reported for catalyst 14#214.731 This is only 43% of the maximum peak power generated in H2/air by the Pt/C cathode catalyst in Figure 33. We have seen that the best durable non-noble catalyst in H2/O2 and H2/air fuel cells is 14#97 that delivers in H2/O2 and at 0.4 V a constant power of 0.13 W cm−2 over 1100 h,726 while 11#137 delivers about 0.14 W cm−2 over 700 h at 0.4 V in H2/air.711 At 0.4 V in H2/O2 fuel cell, the Pt/C cathode of Figure 33 yields 3.37 A cm−2 or 1.35 W cm−2 of power. At 0.4 V in H2/air fuel cell, the Pt/C cathode of Figure 33 yields 2.31 A cm−2 or 0.92 W cm−2 of power. The 2015 DOE target for durability of portable fuel cell systems is defined as a maximum of 20% of power loss over 5000 h. If we assume (i) that the entire loss of power may be attributed to loss of the Pt/C catalyst performance, and (ii) that the decline of power loss is linear with elapsed time, then at 0.4 V in H2/O2 fuel cell,

10. CONCLUSIONS This comprehensive Review covers almost all of the state-ofthe-art developments of low cost and high performance electrocatalysts for oxygen reduction reactions in acidic media. For the past eight years, great progress has been made in developing more active electrocatalysts based on both Pt-based nanomaterials and non-noble metal compounds/composites. For the former, the size, composition, morphology, porosity, surface structure, synthesis method, and post-treatment play significant roles in determining their activity and stability. In general, the activity of Pt-based electrocatalysts can be improved by: (1) Incorporating proper transition metals to increase the dispersion of Pt atoms and specific activity. The Pt mass activity improvement in conventional Pt alloys is typically limited up to 4-fold due to large particle size and imperfect structure. (2) Forming a core−shell structure to improve the utilization of Pt atoms and modify the electronic properties by strain and ligand effects from the core. On the microgram scale synthesis, the highest Pt mass activity improvement factor can reach as high as 14-fold (2.8 A mg−1).362 On the gram scale synthesis, it is limited to 7-fold.360 (3) Forming and maintaining structures 3634

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also the catalysts that are characterized by the largest microporous surface area being practically equivalent to the total BET surface area of the material. This may have important consequences because the most active catalytic sites (the MeNx sites) are known to be hosted in the micropores of the carbonaceous support. A large microporous surface area will therefore lead to a large number of highly active sites, but those may easily be flooded if the carbonaceous catalyst support becomes too hydrophilic in a running fuel cell. Rendering this carbonaceous support less hydrophilic by improving its graphitization inevitably leads to a decrease of the number of heteroatoms (like N and O) on the carbon support and therefore to a lower site density and catalyst performance. An electrooxidation of the carbonaceous support of the catalyst and the accumulation of some H2O2 from an incomplete oxygen reduction in the cathode may also be potential causes of the lack of durability of non-noble catalysts. The latter potential causes need to be studied in detail (in fuel cell absolutely, not in cycling tests in solution) to determine if they have an important influence on the durability of non-noble catalysts in actual fuel cells. So far, the most stable catalysts are those made with a preexisting carbon support to host the catalytic sites. For 14#97, which has been tested during about 1100 h at 0.4 V and in H2/O2 fuel cell, the stable power delivered represents 9.8% of the power that should be delivered (according to DOE) by Pt/C after the same test duration, while the power of the most stable catalyst (11#137) that has been tested during about 700 h at 0.4 V in H2/air fuel cell represents 15.6% of the power that should be delivered by Pt/C after the same test duration. In 14#97, the authors believe that the main catalytic sites are CNx, while in 11#137, the ORR activity has been mainly attributed to MeNx sites. As 9.8% or 15.6% represents only a small fraction of the 2015 targeted performance for Pt/C in portable applications, it is important, as a first step, to be able to stabilize the best non-noble catalysts at their initial peak power. However, this will not be enough for these catalysts to become serious contenders to Pt/C at the cathode of PEM fuel cells. Their performance still needs to be improved to reach that of Pt catalysts, and their production cost needs to be drastically decreased to a very small fraction of the Pt catalyst production cost.

with only the highest active facets exposed to electrolytes. This strategy so far showed the highest activity of 6.98 A mg−1, that is, 35-fold enhancement with a Mo-doped Pt3Ni octahedral structure.320 (4) Creating a porous structure by dealloying to increase the surface area and strain. A 20-fold activity improvement has been observed with a Pt−Ni nanoframe structure.300 The activity can be further improved by post surface modification via annealing or acid treatment. In summary, the highest activities according to RDE evaluations have been observed on the shape-controlled Pt alloys and core−shell structures so far. Whether these highly active ORR electrocatalysts can exhibit the same enhancement factor in a real fuel cell is still an open question. Great efforts on demonstrating their feasibility in fuel cell applications with desired performance and durability are required. Even though significant progress has been made on metal oxides, nitrides, oxynitrides, carbonitrides, and chalcogenides, their activities are still not comparable to those of Pt-based materials. In the frame of this Review, we also reported recent results for the behavior of non-noble catalysts of the type Fe/N/C and Co/N/C at the cathode of PEM fuel cells. More particularly, we focused on the performance of these catalysts in H2/O2 and H2/air fuel cells as well on their durability at the cathode of the same cells. Our observations and comparisons are made in terms of electrical power delivered by the PEM fuel cells. For the performance, the initial peak power of the nonnoble catalysts was listed, discussed, and compared to Pt/C. It was found that in H2/O2 fuel cells, the initial peak power for non-noble catalysts is comprised between 0.05 and 0.98 W cm−2, depending on their mode of synthesis. The most performing catalysts are those made using precursors for the metal and the nitrogen atoms and either very porous carbon supports or carbon precursors that will also yield very porous carbon supports upon pyrolysis. Doing so, the density of sites able to reduce oxygen is maximized as those sites are disseminated on the entire surface area of the catalyst material. An intermediate possibility with intermediate initial peak power performance is obtained when a carbon precursor is used with a sacrificial support. The latter has to be removed chemically, once the catalyst is obtained, after precursors and sacrificial support have been heat-treated at high temperature. For all catalysts, this Review also lists the main catalytic site(s) able to perform ORR as reported by the authors of these catalysts. These sites are of three types: MeNx, CNx, and encapsulated metal. Results reporting the initial peak power performance of non-noble catalysts in H2/air fuel cells are much less numerous than those obtained in H2/O2 fuel cells. However, the conclusions drawn from H2/air experiments also confirm what was reported about the initial peak power performance of non-noble catalysts in H2/O2 PEM fuel cells. As compared to a Pt/C cathode loaded at 0.3 mg Pt per cm2, the initial peak power of the most performing catalyst in H2/O2 (14#214) is 72% of that of Pt/C, while in H2/air, the initial peak power of the most performing catalyst (14#214) is only 43% of that of Pt/C. This is the sign of an important mass transport problem of non-noble catalysts used at the cathode of H2/air fuel cells. As far as durability is concerned, it was seen that it is absolutely necessary to test non-noble catalysts in fuel cells as results of RDE cycling may indicate a durability, which is not confirmed in fuel cell. The most important result about the lack of durability of most of the non-noble catalysts is that the most performing ones are also the least durable ones. The latter are

AUTHOR INFORMATION Corresponding Author

*Tel.: +852-34692269. E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Minhua Shao earned his B.S. (1999) and M.S. (2002) degrees in chemistry from Xiamen University, and a Ph.D. degree in materials science and engineering from the State University of New York at Stony Brook (2006). He joined UTC Power in 2007 to lead the development of advanced catalysts and supports for PEMFC and PAFC. He was promoted to UTC Technical Fellow and Project Manager in 2012. In 2013, he joined Ford Motor Company to conduct research on lithium-ion batteries for electrified vehicles. He then joined the Hong Kong University of Science and Technology in the Department of Chemical and Biomolecular Engineering as an Associate Professor in 2014. He received the Supramaniam Srinivasan Young Investigator Award from the ECS Energy Technology Division 3635

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(2014) and the Student Achievement Award from the ECS Industrial Electrochemistry and Electrochemical Engineering Division (2007). His research mainly focuses on electrocatalysis and advanced batteries. Qiaowan Chang received a B.S. in chemical engineering from Tianjin University and B.S. (Double Degree) in finance from Nankai University in 2014. She is now undertaking a MPhil degree at the Hong Kong University of Science and Technology under the supervision of Professor Minhua Shao. Her main research interests are advanced materials for fuel cells. Jean-Pol Dodelet got his Ph.D. in 1969 in Physical-Chemistry from “L’Université Catholique de Louvain”, Belgium. The same year, he left for Canada where he became Postdoctoral Fellow (from 1969 to 1971), then Research Associate (from 1971 to 1976) in Radiation Chemistry at “The University of Alberta”, Edmonton, Alberta, Canada. In 1976, he became Professor of Physical-Chemistry at “L’Université du Québec à Trois Rivières” in Québec, Canada, where he worked on the properties of molecular photoconductors. In 1981 he became Professor at INRS, which stands for “Institut National de la Recherche Scientifique”, in Québec, Canada, where he was still working until his retirement in 2015. At INRS, he became interested in electrocatalysis, especially in non-noble electrocatalysts for the reduction of oxygen in PEM fuel cells, a research topic that he has now pursued since 1990. In the last several years, he collaborated with General Motors in the frame of an NSERC Industrial Research Chair in electrocatalysis to develop non-noble electrocatalysts. The chair ended in December 2009. Since that time, he continued to be involved in research and development on the same topic with a focus on the durability of these non-noble electrocatalysts at the cathode of H2/air PEM fuel cells. Regis Chenitz was born in 1980 in Paris, France. He received his “diplôme d’ingénieur” in Physics and Chemistry from ENSCBP and his M.Sc. in Chemistry and Materials from “L’Université de Bordeaux I”, France, in 2006. He then moved to Canada and obtained his Ph.D. in Energy and Materials Science from “L’INRS-EMT”, Canada, in 2012 under the supervision of Professor Jean-Pol Dodelet. Presently, he is a Research Associate at “L’INRS-EMT”, Canada. His research interests involve catalysis and polymer membranes for PEMFC, direct methanol and formic acid fuel cells, non-noble metal catalyst for ORR in acid and alkaline media, and nanomaterial synthesis and characterization.

ACKNOWLEDGMENTS Work on this Review at the Hong Kong University of Science and Technology (HKUST) was supported by the Research Grant Council of the Hong Kong Special Administrative Region (IGN13EG05 and 26206115) and a startup fund from the HKUST. Work on this Review at INRS Énergie, Matériaux et Télécommunications, was supported by funds provided from MESRST (Gouvernement du Québec) and from NSERC, the Canadian funding agency. REFERENCES (1) Vesborg, P. C. K.; Jaramillo, T. F. Addressing the Terawatt Challenge: Scalability in the Supply of Chemical Elements for Renewable Energy. RSC Adv. 2012, 2 (21), 7933−7947. (2) Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486 (7401), 43−51. (3) Jung, N.; Chung, D. Y.; Ryu, J.; Yoo, S. J.; Sung, Y.-E. Pt-Based Nanoarchitecture and Catalyst Design for Fuel Cell Applications. Nano Today 2014, 9 (4), 433−456. (4) Nie, Y.; Li, L.; Wei, Z. Recent Advancements in Pt and Pt-Free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44 (8), 2168−2201. 3636

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