Toward High-Performance Pt-Based Nanocatalysts for Oxygen

Review Cite This: Chem. Mater. 2018, 30, 2−24

pubs.acs.org/cm

Toward High-Performance Pt-Based Nanocatalysts for Oxygen Reduction Reaction through Organic−Inorganic Hybrid Concepts Monika Sharma,† Namgee Jung,*,† and Sung Jong Yoo*,‡ †

Graduate School of Energy Science and Technology (GEST), Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea ‡ Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea ABSTRACT: Pt-based multistructured nanocatalysts such as alloy, core−shell, and surface Pt-rich nanoparticles have been extensively studied for hydrogen fuel cell applications, and their catalytic performances for oxygen reduction reactions have been significantly upgraded for decades. Due to these technical enhancements, Pt-based nanoarchitectures have turned out to be compatible with commercially accessible fuel cell systems. In addition, based on physical and electrochemical backgrounds for the basic catalyst nanoarchitectures, novel catalyst designs with organic−inorganic hybrid concepts have been recently developed to more effectively improve the electrochemical reaction activities and durabilities. In this review, the typical class of Pt-based nanocatalysts are systematically explained according to their compositions and structures, and the emerging class of organic− inorganic hybrid catalyst designs are then thoroughly introduced. It is expected that the most recent improvements of Pt-based nanoarchitectures will have great effects on the future works for the commercialization of fuel cell catalysts.

1. INTRODUCTION Fuel cell can be used as an environmentally friendly alternative to fossil fuels due to direct conversion of chemical energy stored in small organic molecules and hydrogen into electrical energy with high efficiency. There are various fuel cell types1−3 such as proton exchange membrane fuel cells (PEMFCs),4 alkaline fuel cells (AFCs),5 phosphoric acid fuel cells (PAFCs),6 solid oxide fuel cells (SOFCs),7,8 and molten carbonate fuel cells (MCFCs).9 Among them, PEMFC has been extensively developed since it has wide working power ranges from ∼50 W to ∼100 kW for various applications, for instance, laptop computers, stationary power generation, and fuel cell vehicles (FCVs). PAFC is also an effective power and heat generation system because it can use hydrogen gas directly reformed from natural gases and be operated at relatively high temperature as much as 120−200 °C. In addition, it is expected that AFC will have great advantages using cheaper catalysts (Co- and Febased catalysts or carbon-based materials) than Pt. However, Pt is mainly utilized as cathode catalysts for oxygen reduction reaction (ORR) in PEMFC, AFC, and PAFC so far, although it is one of the most expensive materials. Thus, to accelerate the commercial viability of fuel cells, many problems still need to be solved such as high cost and low ORR kinetics of Pt electrocatalysts in the cathodes. The sluggish kinetics of this reaction cause high overpotential and require too much high loading of Pt. Nevertheless, the catalytic activities of Pt-based catalysts have been significantly upgraded for decades, and the electrodes using the catalysts have shown © 2017 American Chemical Society

high power densities, although Pt loading in the electrodes have been considerably decreased. These technical enhancements have resulted in Pt-based catalysts suitable for use in commercially available fuel cell systems. At present, it is requisite to conglomerate recent progress in the development of fuel cell catalysts and advanced nanoarchitecture designs. In addition, based on the physical and electrochemical backgrounds for the basic nanoarchitectures, novel Pt-based fuel cell catalysts designed by organic−inorganic hybrid technologies have been recently developed to more effectively improve the electrochemical catalytic activities. In this review, a typical class of Pt-based nanocatalysts are systematically explained according to their compositions and structures, and an emerging class of organic−inorganic hybrid catalyst designs are intensively introduced. The typical fuel cell catalysts basically include pure Pt, Pt-based alloy, core−shell, and surface Pt-rich nanoparticles. It is very important to understand the fundamental structures and catalytic properties of the typical Pt-based nanocatalysts. Emerging catalyst design strategies are categorized according to the main catalyst properties changed by elaborate organic−inorganic hybridization techniques. It is expected that the most recent improvements of nanoarchitectures for fuel cell catalysts will Received: August 13, 2017 Revised: November 26, 2017 Published: November 28, 2017 2

DOI: 10.1021/acs.chemmater.7b03422 Chem. Mater. 2018, 30, 2−24

Review

Chemistry of Materials

can be attached to the carbon surface. Such development for high dispersion of metal nanoparticles on carbon blacks worked well in fuel cell technology and caused a significant decrease in Pt loading and increase in power density.29 Further progress for support materials revealed that, in comparison to carbon blacks, CNTs and CNFs have better electrical conductivities and corrosion resistance; therefore, they are another tempting material for support.30,31 However, it was hard to acquire high Pt loadings on the carbon, because they have few functional groups on the surface due to the high crystallinity of carbon structure.27 Initially, scientists tried to make the surface defects have lots of functional groups through harsh acid treatments by using HNO3 and H2SO4, as shown in Figure 2.32−37 However, these acid treatments spoiled the

have great effects on the future works for the commercialization of fuel cell catalysts.

2. TYPICAL CLASS OF NANOCATALYSTS FOR FUEL CELL APPLICATIONS 2.1. Pt Nanoparticles. During the ORR process, oxygen species such as O2, OH, and O act as reactants and intermediates produced on the catalyst surface. Therefore, an appropriate electronic structure of catalyst to adsorb and to desorb the oxygen species is a prerequisite for the enhancement of the ORR activity, and Pt meets this requirement which makes Pt the best ORR catalyst over other elements (e.g., Pd, Ag, Ir, Au, Co, Ni, Ru, and Cu).10−12 As shown in Figure 1, the ORR activities of catalyst candidates are significantly dependent on the O and OH

Figure 1. Trends in ORR activity as a function of O binding energy. Reproduced with permission from ref 12. Copyright 2004 American Chemical Society.

Figure 2. Fusion of hydrophilic functional groups through typical surface defects on CNT. (a) Bend in the tube due to five- or sevenmembered rings in the C framework, instead of the normal sixmembered ring. (b) sp3-hybridized defects (R = H and OH). (c) Hole lined with −COOH groups produced by damaged framework due to oxidative conditions, (d) open end of the CNT, terminated with −COOH groups or other possible terminal groups such as −NO2, OH, H, and O. Reproduced with permission from ref 33. Copyright 2002 John Wiley and Sons.

binding energies, and Pt shows the highest ORR activity among them.12 Therefore, in the initial stage of development of Pt catalyst, Pt black without a support material was used as a fuel cell catalyst.13 However, in practice, Pt black showed very poor fuel cell performance because of particle aggregation in membrane electrode assembly (MEAs). Pt nanoparticles formed large size of agglomerates in catalyst layers due to their high surface energy, which resulted in significant loss of available active surface area.14−16 In addition, the catalyst layers had highly dense structures since those were composed of Pt nanoparticle agglomeration and Nafion ionomers only. The dense electrode structures have suffered from high concentration overpotential in high current density regions, and power density of the fuel cell became much too low. Consequently, to compensate the low performance of Pt catalyst, high loading of Pt nanoparticles was inevitable at that time. Therefore, to increase the active surface area and to fabricate porous electrode structures, Pt nanoparticles have been supported onto carbon materials such as carbon blacks17,18 carbon nanotubes (CNTs),19,20 carbon nanofibers (CNFs),21,22 and graphene23−25 with high surface areas and electrical conductivities.26 First of all, carbon black is one of the most used carbon support materials since it is much cheaper than other carbon materials. Due to the existence of a large number of surface functional groups (e.g., −CO, −COOH, and −CN) and their interactions with metal precursors in catalyst synthesis,27,28 Pt nanoparticles with particle sizes of 2−10 nm

desirable conductivities of carbon support materials and caused severe carbon corrosion during fuel cell operation.31 Therefore, to solve the critical problem of the surface treatments, noncovalent functionalization methods using electrostatic properties and π−π/σ−π interactions have been proposed.38−40 Oh et al. obtained enhanced Pt loading on the CNFs by using small organic molecules containing −COOH groups, namely, 1-pyrenecarboxylic acid (PCA) for the surface-functionalization of CNFs as shown in Figure 3.31 The π−π interactions between the phenyl or pyrene group of PCA and the side walls of CNFs are responsible for these results. Similarly, Li et al.41 prepared a series of carboxylic acid-, amine-, and thiol-terminated CNTs by “covalent reflux mediated” and “noncovalent sonicationinduced” methods. Both methods provided completely different electronic structures and behaviors. In covalent-based systems, electron transfer likely occurs while in sonication-induced systems, and favorable π−π stacking interactions take place from the attached moieties to the multiwalled CNTs. Moreover, they used these functionalized CNTs as a support 3


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Accordingly, alloy catalysts of Pt with 3d transition metals such as Fe, Ni, and Co etc. (PtM, where M stands for 3d transition metals) have been studied to reduce the Pt usage and to enhance the ORR activity.52−55 In terms of crystalline structures, the Pt lattice structure can be compressed by alloying it with the second metal having a smaller lattice parameter than Pt, which is called strain effect of alloying. The overlap between the d-orbitals of neighboring Pt atoms results in the d-band broadening (Figure 4)56 and the decrease in the

Figure 3. Surface functionalization of CNF via noncovalent π−π interaction by benzyl mercaptan, 1-aminopyrene, and 1-pyrenecarboxylic acid. Reproduced with permission from ref 31. Copyright 2011 John Wiley and Sons.

for synthesized ultrathin Pt nanowires. The noncovalent method offers more beneficial electronic modification of Pt for largely enhanced ORR activity. Thus, CNTs and CNFs in which carbon surface has been modified by noncovalent π−π interactions can be utilized as support materials for fuel cells. Graphene also has great potential as a support material for fuel cell catalysts due to its special chemical and mechanical stability and electrochemical properties.42−46 However, graphene also has similar problems regarding low catalyst loading on the surface. Accordingly, graphene oxide (GO) with functional groups is generally used to attach Pt nanoparticles on the surface and then is changed to reduced graphene oxide (rGO). Through the synthetic efforts, Li et al. showed improvements of durability of fuel cell catalyst with a graphene, which was attributed to the composite structure of graphene and nanoparticles.47 In spite of promising support material, processing graphene often leads to layer stacking; therefore, modified ultrafine carbon black was additionally pillared between graphene sheets to reduce stacking and result in eminent durability. Research emphasis has also been shifted to alternative supports, i.e., noncarbon-supported materials, such as carbides, oxides, borides, nitrides, etc. It has been found that some of these noncarbon-supported Pt-based catalysts have stronger metal−support interaction which results in both higher ORR activity and stability. In recent years, Jackson et al.48 compared the Pt nanoparticle supported on graphite-rich boron carbide (BC) with Pt supported on Vulcan XC 72R. Observed improved kinetic current and stability of the BC supported catalysts had been explained by stronger electronic interaction between the catalyst and support, which in turn reduces the Pt dissolution and/or agglomeration. Grubb et al.49 and Lv et al.50 also reported the electronic interactions between the catalyst and support responsible for promising ORR activity. 2.2. Pt-Based Alloy Nanoparticles. Although recent attempts for loading a small amount of Pt nanoparticles on carbon support materials with high surface areas have reduced Pt usage and maximized the electrochemically active surface area (ECSA), Pt catalysts still account for ∼55% of the cost of a fuel cell stack.51 Besides, to enhance the ORR activity, it is necessary to alter the Pt electronic structure to control the adsorption of different oxygen species on the catalyst surfaces in the ORR.

Figure 4. Origin of the valence d-band shift (a) in the presence of tensile or compressive strain but absence of charge transfers and (b) when there is no strain but instead there is charge transfer to the adlayer sp valence levels. Reproduced with permission from ref 56. Copyright 2015 American Chemical Society.

oxygen binding energy on PtM alloy nanoparticle surfaces. In addition, as the electronic effect of alloying, the electronegativity difference between the second metal and Pt induces the charge transfer from the second metal to Pt. As a result, the Pt d-band center is downshifted (Figure 4), and the oxygen binding energy of Pt in PtM alloy catalyst generally becomes weaker compared to that in a bare Pt catalyst. The adsorption energies of the reaction intermediates on Pt active sites in PtM alloy catalyst are properly tuned, and the ORR activity can be considerably enhanced.57−59 On the other hand, PtM alloy catalysts composed of Pt and the second metal having larger lattice parameter (e.g., Au) are expected to show much lower activities than a bare Pt catalyst. In this case, upshifted d-band center causes enhanced surface coverage of ORR intermediates due to strong oxygen binding energy, which results in deactivation of the active surfaces. The volcano plots to determine the best secondary alloying metal with Pt for the ORR have been theoretically and experimentally proved, as shown in Figure 5.60−63 In addition, various composition ratios of PtM alloy catalysts such as Pt3M1, Pt1M1, and Pt1M3 have been investigated to find out the best alloy catalyst having the highest ORR activity. The reported results prove that the proper oxygen adsorption energy is required as a prior condition for the highest ORR activity. However, further fuel cell applications of PtM alloy catalysts have been inhibited due to their low durabilities, although metal alloying significantly enhanced the initial catalyst performance. In acidic medium having pH lower than 1 and at high potentials 4


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Figure 5. Relationships between experimentally measured specific activity for the ORR on Pt3M surfaces in 0.1 M HClO4 at 333 K versus the d-band center position for the (a) Pt-skin and (b) Pt-skeleton surfaces. Reproduced with permission from ref 63. Copyright 2007 Nature Publishing Group.

Figure 6. (a) Output of computational screening, showing the oxygen binding energy relative to that of Pt, on a Pt or Pd skin surface as a function of alloying energy. Reproduced with permission from ref 60. Copyright 2009 Nature Publishing Group. (b) Correlation between stability and heat of alloy formation and absorption edge peaks of XANES spectra of Pt3M catalyst (M = Y, Zr, Ti, Ni, and Co) catalysts. Reproduced with permission from ref 55. Copyright 2012 American Chemical Society.

greater than 0.7 V (vs RHE)), the 3d transition metals severely dissolved and the catalyst durability was also considerably affected. Therefore, early transition metal alloys (PtSc, PtTi, PtLa, PtY, etc.) have been proposed as promising alloying compositions to simultaneously enhance the ORR activity and durability of fuel cell catalysts (Figure 6a).60,61,64,65 The electronic perturbations in early transition metal alloy catalysts, the changes in the heat of alloy formation energy and the dband filling, ensure their high catalytic activities and durabilities. It has been extensively confirmed by experimental data and theoretical calculations, as shown in Figure 6b.55 In-depth studies about interrelations among the d-band filling, heat of formation energy, and durability of early transition metal alloy catalysts again revealed the prominence of elemental combinations in the ORR. Nevertheless, the chemical synthesis of early transition metal alloy nanoparticles still remains as challenging work because of very low standard reduction potentials of the second transition metals. Meanwhile, to maximize the catalytic activities of alloy catalysts, many researchers have taken interest in restraining their shapes. Pt3Ni octahedra having (111) facets displayed five times greater ORR activity than cubic Pt3Ni nanoparticles having (100) facets (Figure 7).52 Stamenkovic et al. also proved the importance of geometric orientation of electrocatalysts through a detailed critical inspection of Pt3Ni single-crystal with distinct (hkl) values.66 The Pt3Ni(111) had superior and 10 times higher activity than Pt3Ni(100) and Pt(111), respectively. The enhanced activity was elucidated by the electronic structure and rearrangement of the surface structure. The adsorption energies of the reaction intermediates for ORR on Pt(hkl) were properly controlled by the geometric orientation of catalysts, which significantly affected the catalytic activity.

2.3. Core−Shell Nanoparticles. As mentioned earlier, although extensive research about PtM alloy systems has been conducted, they have rather low stabilities at high potentials and in acidic medium.67−69 In addition, extremely reduced Pt usage in PtM alloy structures is needed for commercialization of fuel cell systems. Therefore, in terms of nanoparticle structures, core−shell catalysts with cheap core material and Pt shell layer are very attractive. Zhang et al.70 correlated the ORR activities of core−shell catalysts with the d-band structure and position within the Pt shell layer. The shift of the d-band center position of Pt was attributed mainly to the strain effect depending on the core materials. As tensile strain is applied to the Pt surface by using a core material with larger lattice parameters such as Au rather than Pt, the overlap between d orbitals of neighboring Pt atoms decreases which results in a decrease in the bandwidth and upshifted d-band center of the Pt shell layer. On the other hand, Ru, Ir, Cu, and Pd have smaller lattice parameters than Pt; thus, on using them as core materials, compressive strain works on Pt, which results in an increase in the bandwidth and downshifted d-band center of the Pt shell layer. Pd has been proposed as a decent core material due to its greater stability relative to Co, Ni, and Fe.71 Further, its noble nature is beneficial to control the size of its nanoparticle. Literature reveals the chemical deposition of 1 to 3 nm thick Pt and PtFe shells on Pd nanoparticle seeds as shown in Figure 8.72,73 On comparing with Pt nanoparticles, the core−shell catalyst with 1 nm thick shell demonstrated higher ORR activity and durability, which in turn indicates the importance 5


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Figure 7. Images for Pt3Ni nanoctahedra (a−e) and for Pt3Ni nanocubes (f−j). (a, f) Field-emission SEM images. (b, g) High-resolution SEM images. (c) 3-D image of an octahedron. (d, i) TEM images. (e, j) High-resolution TEM images of single nanocubes. (h) 3-D image of a cube, (k) specific and mass activity measured at 0.9 V (vs RHE) at 295 K on Pt3Ni nanoctahedra, Pt3Ni nanocubes, and Pt nanocubes. Reproduced with permission from ref 52. Copyright 2010 American Chemical Society.

been used to alloy with Pd. On the basis of core material composition, a volcano plot has been developed depending on the oxygen binding energy of the Pt shell with varying Pd/Co ratio.76,77 In addition, carbon-supported Pd3Cu@Pt core−shell nanoparticles with Pt shell layers were chemically synthesized by using Hantzsch esters, as shown in Figure 9a.78 As compared to other core−shell nanoparticles like Pd3Ni@Pt, Pd3Co@Pt, and Pd3Fe@Pt, better catalytic activity and durability of Pd3Cu@Pt due of its proper oxygen adsorption and vacancy formation energy has also been depicted by theoretical calculations (Figure 9b). In a recent publication, Yoon et al.79 constructed Mo-doped PdPt@Pt core−shell octahedrals immobilized on ionic block copolymer-functionalized rGO, which showed high durability and activity for the ORR due to the Mo doping and the combination with the functional graphene supports. In addition, due to high reduction potential and chemical durability of Au, it has been extensively studied as the prominent component of metal alloy core materials. However, the use of Au as a core material imparts tensile strain because of the larger lattice parameter than Pt and hence is expected to show a negative impact on the ORR activity. On the contrary, Au has great advantages to exhibit superior durability in acidic electrolytes. Actually, the lattice parameter of Au can be compressed by alloying it with other metals such as Ni, Co, Fe, and Cu with small lattice parameters. Yang et al.80 tuned the surface strain of Pt shell as beneficial compressive strain by using the AuCu alloy core as shown in Figure 10. In an accelerated durability test (ADT), AuCu@Pt/C exhibited excellent stability indicated by its unchanged half wave potential

Figure 8. (a) ORR polarization curves for three kinds of Pd/FePt nanoparticles and the commercial Pt nanoparticle catalyst. The current was normalized against the total mass of nanoparticles used, and the rotation rate was 1600 rpm. (b) Comparative ORR activities of 5 nm/ 1 nm Pd/FePt nanoparticles before and after 10 000 potential cycles. Reproduced with permission from ref 72. Copyright 2010 American Chemical Society.

of shell thickness. In addition, it was confirmed that core−shell nanoparticles containing a single atomic layer of Pt shell showed higher performance than analogues having multiple atomic Pt layers through fine-tuning of Pt shell layers in Pd@ PtnL (n = 1−6).74 Namely, with increasing the Pt shell thickness, the catalytic activity of the shell approaches those of pure Pt. With a similar concept, Choi and co-workers synthesized Pd−Pt core−shell icosahedra catalyst and reported 7.8 times greater ORR activity of this alloy catalyst than that of the commercial Pt/C catalyst.75 Second, simultaneously to limit the use of expensive Pd and to bring alteration in electronic and structural properties of the Pt shell layer, 3d transition metals like Co, Ni, Fe, and Cu have

Figure 9. (a) Two-step synthetic schemes for Pt-based core−shell catalysts by using benzyl ether as a solvent and only surfactant. The Hantzsch ester is capable of reducing the shell precursors selectively on the Pd core surface. (b) The computational screening of Pt-coated suitable core materials for the high activity and durability. Reproduced with permission from ref 78. Copyright 2013 Nature Publishing Group. 6


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transition metal alloy catalysts as starting materials through the annealing process, acid and heat treatments, chemical tuning method, and electrochemical dealloying process. Pt-rich surfaces were first developed by a simple annealing process to induce the segregation energy differences between host and solute materials (Figure 11).86 The Pt3Ni(111) alloy

Figure 11. Surface segregation energies of transition-metal impurities (solute) for the closed-packed surfaces of transition metal hosts. Reproduced with permission from ref 86. Copyright 1999 American Physical Society.

Figure 10. (a) TEM, (b) HRTEM, and (c) HAADF-STEM images of AuCu@Pt nanoparticles, (d, e) Au, Cu, and Pt elemental profiles along the red and blue line across the AuCu@Pt nanoparticle in (c), (f) mass-normalized Tafel region of ORR measurement for Pt/C, Au@ Pt/C, and AuCu@Pt/C catalysts in O2-saturated 0.1 M HClO4 at sweep rate 20 mV s−1; 1600 rpm; room temperature. Pt loading was 15 mg cm−2 for Pt/C and Au@Pt/C and 7.5 mg cm−2 for AuCu@Pt/C. (g) Linear sweep voltammograms of AuCu@Pt/C before (black ) and after (red ) 30 000 cycles of stability testing. Reproduced with permission from ref 80. Copyright 2012 Royal Society of Chemistry.

surface was converted to a Pt-skin structure by high temperature annealing in an inert environment, and the electrochemical properties of the catalyst surface were significantly enhanced (Figure 12).66 Therefore, Cai et al.87 designed chemically ordered Pt3Co and PtCo nanoparticles with stable Pt rich shell for high catalytic activity and superior stability for the ORR. The improved catalytic performance of 700 °C annealed samples (Pt3Co-700 and PtCo-700) was attributed to the synergistic effect of compact arrangement of surface Pt atoms and ordered core. The existence of the compact Pt shell to protect the relatively active ordered core and tuning of the electronic properties by the ordered core improved the catalytic activity toward the ORR. Marković et al. also used acid treatment and subsequent heat treatment to develop nanoparticles with Pt-skin surfaces from PtNi alloy nanoparticles, as shown in Figure 13.88 The acid treatment leaches out surface Ni atoms from the PtNi alloy nanoparticles, and then alloy nanoparticles with a corrugated Pt-skeleton surface were produced. An atomic rearrangement of the Ptskeleton structures through the heat treatment generates a Ptskin surface structure which had more downshifted d-band center. Ni XANES spectra also confirmed that the Pt-skin layer completely prevented Ni dissolution from the nanoparticle interior compared to the Pt-skeleton layer. An alternative chemical tuning method was also proposed to synthesize more active and durable PtNi alloy nanoparticles with Pt-skin surfaces (Pt@PtNi catalyst)89 by additional deposition of small amounts of Pt on PtNi nanoparticles with

after 30 000 cycles (Figure 10g). The Au component in the AuCu alloy core is crucial toward stabilizing the Pt shell during ORR because a significant coupling of d-orbitals of Pt atoms to the Au substrate stabilizes the Pt shell layer.81−83 Further, Gong et al.84 reported that, during core material synthesis of AuNiFe nanoparticles, Au moves to surfaces as a result of its segregation energy and compressive strain. It was found that a number of Au atoms distributed near the core surface and the surface Au atoms prevented the dissolution of the second metal in the core material. However, for commercialization of core−shell catalysts, the development of more stable and cheaper core materials is needed since Pd and Au are still too high in price, despite their great physical advantages. 2.4. Surface Pt-Rich Alloy Nanoparticles. After intensive studies on the core−shell catalysts, the importance of the surface compositions of PtM alloy catalysts has been brought into focus again. The Marković group explicated that the surface composition and structure on PtM alloy nanoparticles have a great effect on their catalytic activities rather than the bulk compositions by an extended crystallographic study.85 Therefore, surface Pt-rich materials were fabricated from Pt-3d 7


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Figure 13. (a) Representative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images taken along the zone axis (110), as confirmed by the fast Fourier transfer (FFT) patterns of the STEM images (shown as insets). (b) Background subtracted, normalized intensity line profiles extracted for the regions marked in (a). (c) Composition line profiles (normalized for Pt L peaks) obtained by energy-dispersive X-ray spectroscopy (EDX) with an electron beam (∼2 Å in spot size) scanning across individual catalyst particles. (d) Overview and (e) cross-section views of the nanostructures depicted by atomistic particle simulation. Reproduced with permission from ref 88. Copyright 2011 American Chemical Society.

Figure 12. In situ characterization of the Pt3Ni(111) surface in 0.1 M HClO4 at 333 K. (a) SXS data and (a′) concentration profile revealed from SXS measurements. at.%, atomic %. (b) Cyclic voltammetry in the designated potential region (red curve) as compared to the voltammetry obtained from the Pt(111) surface (blue curve). (c) Surface coverage calculated from cyclic voltammograms of Pt3Ni(111) (red curve) and Pt(111) (blue curve); polarization curves obtained from rotating ring disk electrode (RRDE) measurements. θOxide, surface coverage by adsorbed spectator oxygenated species. (d) Green scale refers to hydrogen peroxide production in the designated potential region and (e) ORR currents measured on Pt3Ni(111) (red curve), Pt(111) (blue curve), and polycrystalline Pt(gray curve) surfaces. The arrows indicate positive potential shift of 100 mV in electrode half-potential (ΔE1/2) between ORR polarization curves measured on Pt-poly and Pt3Ni(111) surfaces. I, II, and III represent potential region of Hapd adsorption/desorption processes, double-layer region, and region of OHad layer formation, respectively. Reproduced with permission from ref 66. Copyright 2007 The American Association for the Advancement of Science.

Pt-skeleton surfaces to form a smooth Pt-skin layer. These chemically tuned Pt@PtNi catalysts showed significant enhancement of the ORR activity (Figure 14) due to more ordered surface Pt facets and higher Pt coordination numbers which is caused by existence of fewer oxygen species near the Pt and Ni atoms in sublayers under the Pt-skin layer. Recently, Chen et al. presented a novel class of catalysts by creating Pt3Ni nanoframes with three-dimensional (3D) molecular accessible surface. To synthesize Pt3Ni nanoframes, they dispersed oleylamine capped PtNi3 polyhedra (20 nm) in hexane or chloroform under ambient conditions for 2 weeks. Further, heat treatment at 400 °C of these free-standing nanoframes results in smooth Pt-skin surface structures. The transformation process was illustrated in Figure 15. The nanoframes exhibit 16- and 22-times enhancement over 5 nm

Figure 14. ORR plots of PtNi nanoparticle with Pt-skin surface, fabricated by the acid-heat treatment, and chemically tuned PtNi nanoparticle with Pt-skin surface, prepared by Pt chemical deposition after the acid treatment. Reproduced with permission from ref 89. Copyright 2013 Royal Society of Chemistry.

Pt/C on specific and mass activities, respectively. They also reported the positive effect of ionic liquid in ORR on porous 8


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Figure 15. Schematic illustrations and corresponding TEM images of the samples obtained at four representative stages during the evolution process from polyhedra to nanoframes. (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. Reprinted with permission from ref 90. Copyright 2014 American Association for the Advancement of Science. Figure 16. Structural analysis of the J-PtNWs by ReaxFF reactive molecular dynamics and X-ray absorption spectroscopy. (a) Illustrations of a J-PtNW, (b) J-PtNW with colored atoms to show the 5-fold index, and (c) J-PtNW showing distribution of atomic stress (in atm·nm3). (d) Pt−Pt radial distribution function (RDF) of the SMA predicted J-PtNW (red) compared with the peaks of the RDF for the regular PtNW (black). (e) Pt L3 edge FT-EXAFS spectrum (black) collected ex situ and the corresponding first-shell least-squares fit (red) for the J-PtNWs. (f) Distribution of the absolute values of the average atomic stress on surface rhombi for the R-PtNWs (black) and the JPtNWs (red). A rhombus is an ensemble of four atoms arranged as two equilateral triangles sharing one edge, as shown in the inset. Reprinted with permission from ref 91. Copyright 2016 American Association for the Advancement of Science.

catalysts. Ionic liquid treatment increases the enhancement factors to 22 and 36 for specific and mass activities, respectively.90 On the other hand, Li et al. synthesized first Pt/NiO core/shell nanowires which converts into PtNi alloy nanowires upon thermal annealing process in Ar/H2 mixture at 450 °C. Finally, theses PtNi alloy nanowires were transformed into jagged Pt nanowires (J-Pt NWs) via electrochemical dealloying. These synthesized J-Pt NWs have a mass activity of 13.6 A/mgPt with an ECSA of 118 m2/g and a specific activity of 11.5 mA/cm2 for ORR (at 0.9 VRHE). Highly stressed, undercoordinated rhombus-Pt-rich surface configurations of the J-Pt NWs is the prime factor for enhanced ORR activity as shown by reactive molecular dynamics simulations (Figure 16).91 Moreover, by using the selective chemical etching approach, porous Pt3Ni nanowires (NWs) with high index facets, highly open porous structure, 1D structure, and ultrathin Pt-rich surface has been synthesized by Jiang et al.92 from phase and composition segregated Pt−Ni NWs as the starting material. Extraordinary ORR activities of porous Pt3Ni NWs result due to their unique structures.

inorganic hybrid structures are not only physical mixtures, but instead they involve intimate mixing of organic and inorganic components where at least one of the component domains has a dimension ranging from a few angstroms to several nanometers. On the basis of the nature of their inner interface, there are two types of organic−inorganic hybrid structures. Class I involves embedded organic and inorganic components, and only weak bonds such as van der Waals, hydrogen, or ionic bonds provide cohesion to the entire structure. While in Class II, strong chemical bonds such as covalent or noncovalent bonds help the two phases to partially connect together.93,94 Considering the multiplicity of combinations of components and their properties, organic−inorganic hybrids represent an intriguing class of materials with a large spectrum of applications in the field of optics, electronics, ionics, energy, mechanics, biology, medicine, etc. These hybrid materials also open promising applications in new catalysts and new generation of fuel cells.95 Recently, interesting organic−inorganic hybrid concepts to improve ORR performances have been proposed. The hybrid concepts represent a creative alternative to design new catalysts with improved or unusual features compared to typical Pt-based catalysts. The advantageous characteristic of metals can be combined with those of organic molecules within a molecularscale composite. For example, molecular patterning by the presence of organic molecule on the metal surface can result in improved selectivity and performance during electrochemical reactions by affecting the adsorption of spectator species as well as reactant species owing to their steric, electrostatic, and

3. CONCEPT OF ORGANIC−INORGANIC HYBRID STRUCTURE IN FUEL CELL CATALYSTS As the previous section explains that the synthesis of novel catalysts having better properties and performance for fuel cell applications is constantly developing at the interface of material science and chemistry, therefore, in this pursuit, the ability to control the surface structure on the atomic and macroscopic level is a crucial parameter to design catalysts with preprogrammed activity. However, the developed Pt-based catalysts still have many problems in terms of catalytic activity, durability, and cost for commercialization. Therefore, creative strategies to totally change the electrochemical properties of catalyst nanoparticles should be continuously proposed. Development of unconventional methods and catalyst structures can provide critical clues to overcome the limitations of classical approaches to fuel cell catalysts. Especially, significant advances in the synthesis of organic− inorganic hybrid materials have inspired many scientists. The concept of the organic−inorganic hybrid can be defined as the control of inorganic structure/properties by an organic component or combination of the properties of organic and inorganic components within a single material. Organic− 9


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Figure 17. Proposed models for selective adsorption of reactants and spectator species and scheme representing the accessibility of Pt surface atoms for O2 adsorption on CN-free and CN-covered Pt(111). (a) For SO42−/PO43− anions covered Pt(111), O2 approaches surfaces atoms only through a small number of holes in the adsorbate anion adlayer. (b) Availability of large number of holes for O2 adsorption on the Pt(111)−CNad surface due to suppressed adsorption of SO42−/PO43− anions by the CN adlayer. (c) Alkaline solutions involves noncovalent interactions between K+ and covalently bonded CNad to form CNad−M+(H2O)x clusters which cause suppressed adsorption of O2 (and H2O) on the Pt(111)−CN surface. For clarity, the hydrated K+ ions are shown as not being part of the double-layer structure. In reality, however, they are quasi-specifically adsorbed, with the locus centered between the covalently bonded CNad and hydrated K+. Reproduced with permission from ref 104. Copyright 2010 Nature Publishing Group.

hydrophilic/hydrophobic properties. In addition, by appropriate selection of functional organic molecules and optimization of their hybrid structures, the surface electronic structure of metal nanoparticles can be systematically tuned. These organic−inorganic hybrid catalysts have unique combinations of new properties that are absent in typical Pt-based catalysts. Therefore, in the next section, the emerging class of Pt-based catalysts will be introduced according to their physical and electrochemical properties modified through the organic− inorganic hybrid techniques.

reactions but affect the reaction rates. From kinetic studies, the rate expression for the ORR is i = nFKCO2(1‐θad)x exp( −βFE /RT ) exp(−γ ΔGad /RT )

where n, F, K, CO2, R, x, β, and γ are constants, T is the temperature, E is the electrode potential, θad is the total surface coverage of adsorbates, and ΔGad is the Gibbs free energy of adsorption.102,103 This equation shows that both the total coverage of adsorbate as well as Gibbs free energy strongly influence the activity. Since some of the Pt catalyst surfaces are already covered with spectator ions and reaction intermediates during the ORR, O2 molecules can adsorb only on vacant Pt sites. When Pt is used as a fuel cell catalyst in KOH or H3PO4 solution, the spectator ion (OH− or phosphoric acid anion) adsorption with high surface coverage (θad) deactivates the active sites on the Pt surface, which in turn decreases in the ORR current. Therefore, it is very important to conserve the active Pt sites (1 − θad) available for O2 adsorption without preoccupation of other spectator ions. Simultaneously, to decrease the effects of spectator ions and to make a more active Pt surface, the surface coverage (θad) of adsorbates on Pt catalyst should be controlled. In this regard, new ideas using molecular patterning techniques have been recently developed. In molecular patterning, organic molecules are intentionally adsorbed on Pt catalyst surfaces. Although the electrochemically active surface area of Pt is decreased as much as surface coverage of adsorbate molecules, the superior catalytic activity of the remaining Pt sites can fully compensate the initial loss of active surface area. First, Strmcnik et al.104 proposed a model electrocatalyst for the ORR based on patterning of Pt surfaces with cyanide (CN) adsorbate, i.e., Pt(111)−CNad. Cyanide anions (CN−) can

4. EMERGING CLASS OF NANOCATALYSTS FOR FUEL CELL APPLICATION 4.1. Enhanced Pt Surface Properties through Organic−Inorganic Hybrid. In AFC and PAFC, it is well-known that spectator ions in electrolytes considerably affect the electrochemical properties of Pt catalyst. First, in AFC, KOH solution is used as an electrolyte and OH− ions are produced by the ORR at the cathode. Therefore, there are a number of OH− ions near Pt nanoparticles. During the ORR, these OH− ions can easily adsorb on the Pt surface and disturb the adsorption of O2 molecules due to excess of OH− ions in KOH solution.96−98 Second, phosphoric acid anions in H3PO4 solution are strongly preoccupied on the Pt surface, imposing retardation of ORR due to lack of available Pt sites for O2 adsorption in PAFC.99−101 Especially, the effects of phosphoric acid anion adsorption on the ORR activity of Pt catalyst in PAFC is more considerable than those of OH− anion adsorption in AFC since phosphoric acid anion has a bulky molecular size. These ions in electrolytes are called spectator ions which do not directly participate in the electrochemical 10


Review


Figure 18. Electrochemical measurements on CN-free and CN-covered Pt(111) surfaces in solutions containing SO42−/PO43− anions (a−d), cyclic voltammograms for the CNad-free (black) and CNad-covered (red) Pt(111) surfaces in Ar-saturated H2SO4 or H3PO4 (a and c), and the corresponding ORR polarization curves in O2 saturated solutions (b and d). Gray-dashed curves represent ORR activity in 0.1 M HClO4. A small pseudocapacitance observed for Pt(111)−CNad between 0.6 and 0.85 V corresponds to SO42− adsorption (5 μC cm−2) and PO43− (10 μC cm−2). The charge in the OH adsorption region (from 0.85 to 1.0 V) increases from 55 μC cm−2 (in the presence of PO43−) to 70 μC cm−2 (in the presence of SO42−). Conditions: scan rate 50 mV s−1, 1600 rpm, and 20 °C. The potential regions of Hupd adsorption, anion adsorption, and OH adsorption are specified by I, II, and III, respectively. The dashed vertical line in (b) shows easy comparison of the current density at 0.9 V for the CN-free and CN-covered Pt surfaces. Reproduced with permission from ref 104. Copyright 2010 Nature Publishing Group.

and anionic CNad adlayer. These clusters on the Pt surface block the active sites for the O2 adsorption. It was a critical limitation of molecular patterning using CN− with a short chain length. On the basis of the above report, it was also concluded that electrostatic interaction should be considered to utilize the molecular patterning technique for electrochemical catalyst applications. The same strategy has been practically applied by Liu et al.105 to achieve selectivity of hollow PtNi nanospheres for ORR. They synthesized cyanide (CN−)-functionalized hollow PtNi nanospheres (PtNi@CN HNSs) with a high alloying degree by a facile cyanogel reduction method. Substantial enhancement of onset and half-wave potentials of the PtNi@CN HNSs (Eonset = 0.948 V; E1/2 = 0.86 V) compared with commercial Pt black (Eonset = 0.918 V; E1/2 = 0.84 V) as well as 1.86 times increased specific activity in comparison to Pt black depicts a dramatically improved ORR activity of PtNi@CN HNSs in an acid medium. Alloying Pt with Ni as well as CN− functionalization to PtNi alloy surface both contributed for their improved ORR activity. In the case of PtNi@CN HNSs, CN− groups uniformly bounded to PtNi alloy surface reduced the high activity of PtNi alloy electrocatalysts for methanol oxidation reduction (MOR) because, for the MOR, three adjacent Pt sites are necessary but the presence of CN− groups interrupt the accessibility of contiguous Pt atoms as shown in Figure 19. Analogous to above concept, Lu et al.106 also modified the surface of commercial Pt/C electrocatalysts using ·CN radical chemisorption via a green photo-Fenton cyanation process. Unlike the traditional cyanation process, the reported ·CN radical chemisorption process avoids the formation of undesirable detrimentals such as OHad−K+(H2O)x, CN− K+(H2O)x, or CN−Na+(H2O)x clusters on the Pt catalyst surface in alkaline solution owing to covalent interactions

strongly adsorb on the Pt surface, and they form a well-defined pattern as shown in Figure 17. In the case of HClO4, the adsorption of ClO4− is negligible, but in H2SO4 or H3PO4 solutions, SO42− and PO43− anions have a strong tendency to adsorb on Pt surfaces and consequently obstruct the O2 adsorption by blocking desirable surface sites for ORR. Accordingly, the Pt catalysts have reduced ORR activity in H2SO4 or H3PO4 compared with in HClO4. Molecular patterning by CN− prevents the adsorption of SO42− and PO43− on the Pt surface during the ORR in H2SO4/H3PO4 solution (Figure 18a,c, region II). Bulky phosphoric acid anions could not access the clean Pt surface due to the third-body effect of the molecular pattern. Phosphoric acid anions are hardly adsorbed on the Pt surface since they need a 3-fold Pt site for the adsorption through the three oxygen bridges. The third-body effect is very similar to the steric hindrance by molecular structures in organic chemistry. On the other hand, O2 molecules could adsorb and react on remaining Pt surfaces since they have small enough molecular size to penetrate the CN− molecular pattern. Consequently, the Pt surface with CN− molecular pattern had significantly increased ORR activity compared to a bare Pt surface without any pattern. This is a very interesting result because, despite the very small active surface area, the specific activities of the remaining Pt sites were considerably enhanced only by blocking the adsorption of bulky spectator ions. As a result, 25- and 10-times higher ORR activities on Pt(111)−CNad compared to bare Pt have been measured in H2SO4 and H3PO4 solutions, respectively (Figure 18b,d). Increase of ORR activities in H3PO4 solutions is lower due to less suppression of PO43− adsorption. However, in KOH solution CN-modified Pt surface showed poor catalytic activity due to formation of large K+(H2O)x−CNad clusters as the result of noncovalent interaction between the hydrated K+ cations 11


Review


Meanwhile, Jung et al.107 focused on how to positively utilize the direct electrostatic interactions between a charged Pt nanoparticle surface and oppositely charged spectator ions for the enhancement of ORR activity in both KOH and H3PO4 solutions. The changes in Pt surface property through strong electrostatic interaction between organic adsorbate (L-cysteine) and spectator ions in KOH and H3PO4 solutions had significant effects on the ORR activity. L-Cysteine has interesting functional groups such as thiol (-SH), amine (−NH2), and carboxylic acid (−COOH). The L-Cysteine molecule could strongly and chemically adsorb on the Pt surface through the formation of thiolate (Pt-SR). Due to both −COOH and −NH2 groups, L-cysteine-decorated Pt catalyst showed Janus surface property in KOH and H3PO4 solutions. The −COOH and −NH2 groups of L-cysteine adsorbed onto Pt nanoparticle surfaces indicated negative (−COO−) and positive charges (−NH3+) through deprotonation and protonation of the functional groups in KOH and H3PO4 solutions, respectively. As shown in Figure 21a, in KOH solution, negatively charged Lcysteine on the Pt surface could repulse excess OH−, which interrupts O2 adsorption during the ORR. In addition, Lcysteine attracted hydrated K+ ions and thus prevents the bulky cation from blocking the Pt active sites through the noncovalent interaction by keeping it away from the Pt surface at a distance corresponding to the molecular chain length. In H3PO4 solution, phosphoric acid anions were attracted by both electrostatic interaction and strong hydrogen bonding between the phosphoric acid oxyanion and −NH3+ of L-cysteine, which resulted in protection of pristine Pt surfaces. The attracted phosphoric acid anions could also be positioned away from the Pt surface due to the chain length of L-cysteine, which resulted in the significant enhancement of ORR activity as shown in Figure 21b,c. As improvement of electrochemical activity by obstructing the adsorption of poisonous spectator species on electrocatalysts is the most promising approach, therefore, this concept was also implemented by Chung et al.108 in their report. They directly synthesized oleylamine (OA)-adsorbed Pt/C catalyst (sPt−OA) via colloidal reduction methods using oleylamine as a surfactant excluding the surfactant removal by conventional thermal/chemical treatment. Adsorbed oleylamine molecules lead to modification of their electronic structures indicated by smaller d-band vacancy of sPt−OA than that of commercial Pt/C catalyst (cPt). Further, higher ORR activity of sPt−OA and OA-adsorbed commercial Pt/C catalysts (cPt−OA) than untreated cPt can mainly be attributed to third-body effect or steric hindrance of oleylamine which

Figure 19. ORR polarization curves of (a) unfunctionalized PtNi and (b) PtNi@CN HNSs in O2-saturated H2SO4 (0.5 M) with and without methanol (0.25 M) at 5 mV·s−1, 1600 rpm. K2PtCl4 and NiCl2 precursors were reduced by conventional NaBH4 reduction to form PtNi nanoparticles. Reproduced with permission from ref 105. Copyright 2016 Springer.

between ·CN radical and the Pt surface as represented in Figure 20. In contrast to CN− anion modified Pt catalyst which

Figure 20. Scheme for (a) the CN radical chemisorption and (b) CN− anion surface modification processes. Reproduced with permission from ref 106. Copyright 2016 American Chemical Society.

resulted 50-fold decrease of the ORR activity in KOH solution, these ·CN−Pt/C electrocatalysts showed a slightly decreased ORR activity (half-wave potentials: Pt/C (0.865 V), ·CN−Pt/ C-1h (0.860), ·CN−Pt/C-10h (0.858), and ·CN−Pt/C-20h (0.850 V)) with an increase of ·CN coverage. Further, substantial methanol tolerance has also been observed in both alkaline and acidic medium after ·CN chemisorption on the Pt surface in a counter-claim to CN− decorated Pt catalysts.

Figure 21. (a) Schematic illustration of the electrostatic interactions between charged L-cysteine molecules on Pt surface and spectator ions in KOH and H3PO4, respectively. Electrochemical characterizations of untreated and L-cysteine-decorated Pt polycrystalline surfaces in 0.1 M KOH and 0.05 M H3PO4. (b) Anion adsorption charge densities obtained from the CVs and (c) specific activities (at 0.9 VRHE) for the ORR. Reproduced with permission from ref 107. Copyright 2015 Elsevier. 12


Review

Chemistry of Materials arises due to selective blockage of adsorption of large anions and passage of smaller O2 molecules onto the Pt surface through bulky aliphatic hydrocarbon chains of OA as illustrated in Figure 22. Effective adsorption of OA blocked about 70% of

Figure 23. Schematic image of OA/PA (7/3)-Pt nanoparticle. One organic protective molecule occupies 1.32 surface Pt atoms. Reproduced with permission from ref 109. Copyright 2014 American Chemical Society.

significantly improved durability than Pt/CB catalyst (Figure 24). The Pt surface modification by the PA group could suppress agglomeration and Ostwald ripening by preventing migration of Pt nanoparticles on carbon black.

Figure 22. Scheme illustrating third-body effects of specifically adsorbed anions on a modified Pt surface for oxygen reduction reaction. Reproduced with permission from ref 108. Copyright 2015 Royal Society of Chemistry.

the surface sites on the Pt nanoparticles resulting in decreased ECSA, but the third-body effect of cPt−OA or sPt−OA in comparison with cPt is perceptible by electrochemical analysis using HClO4 electrolyte solutions that contained H3PO4 at various concentrations. Thus, tolerance of OA-modified electrode to the specifically adsorbed anions such as conjugate bases of H3PO4 extended their application in high-temperature PEMFCs with PBI/PA membranes where electrocatalyst is severely poisoned by specific anion adsorption with H3PO4. Similarly, Miyabayashi et al.109 depicted the surface modification of Pt nanoparticles by using octylamine (OcA) and a pyrene group containing alkylamine (PA) such as 8(pyrene-1-ylmetoxy)octane-1-amine with controlled ratios of OcA/PA (9:1, 8:2, and 7:3) by the two-phase liquid reduction method (Figure 23). Consequently, OcA-Pt/CB and OcA/PAPt/CB showed more positive onset potential and higher ORR activities than Pt/CB. Besides, higher ECSA values (76.7−77.8 m2 g−1) for OcA-Pt/CB and OcA/PA-Pt/CB than that for Pt/ CB (63.7 m2 g−1) originates due to enhanced dispersion of OcA and PA protected Pt nanoparticles on carbon. It also speculates about the weaker adsorption of OcA and PA on Pt and their nonintervention with H+ adsorption at electrode surface. The presence of PA altered the polarity of the interfacial region (less polar) which in turn caused a change of adsorption kinetics of the reaction intermediates or polar components on the Pt surface, i.e., preferable O2 adsorption on the Pt compared with other polar components of electrolyte. Therefore, it has been found that an increase in PA ratio produced enhanced mass activity, specific activity, and

Figure 24. Area (jksp) and mass (jkm) specific activities after accelerated potential cycle test for OA-Pt/CB, OA/PA(9/1)-Pt/CB, OA/PA(8/ 2)-Pt/CB, OA/PA(7/3)-Pt/CB, and 30% Pt/CB catalysts at 0.9 V. Dotted lines show their initial activities for comparison. Reproduced with permission from ref 109. Copyright 2014 American Chemical Society.

As described earlier, alloying Pt with transition metals such as Pt−Ni alloy significantly enhances the ORR activity, but during ORR reaction in acidic medium, easy leaching of Ni from Pt− Ni alloy based catalyst causes low durability; therefore, the present paper reported an approach to enhance durability of the Pt−Ni alloy based catalyst. They adsorbed the halides (Cl, Br, I, and F) on the surface of the octahedral PtNi nanoparticles to synthesize halide-treated octahedral PtNi nanoparticles. Halides could be adsorbed on Ni sites more strongly than on Pt sites; therefore, they protected Ni from being leached out and preserved the high activity of the PtNi surface (Figure 25). Upon the ADTs, both as-prepared PtNi and halide-treated PtNi nanoparticles showed a slight change in ECSA, but it was confirmed that the mass activity of as-prepared PtNi nanoparticles for the ORR (52.6%) was significantly reduced as compared to that of Br-treated PtNi nanoparticles (15.0%). In addition, the reduction in specific activity is 49.8% for the asprepared PtNi nanoparticles and 16.4% for Br-treated PtNi nanoparticles. Thus, the Br-treated PtNi catalyst preserved a large amount of surface Ni and thereby maintained the effect of electronic structure modification by the underlying Ni.110 13


Review


can preserve more electrons due to the Co−Nsurf bonding formation compared to the case of Co−Osurf bonding (Co oxides) formation because the electronegativity of N (3.04) is lower than that of O (3.44), as shown in Figure 26. These

Figure 25. Enhanced stability of octahedral PtNi nanoparticles for oxygen reduction reaction after halide treatment. Reproduced with permission from ref 110. Copyright 2016 Elsevier.

Figure 26. Schematic diagram of the electronic ensemble effect selectively tailoring the surface Co atoms in top sites with the N moiety on the PtCo nanoparticle surface. Reproduced with permission from ref 112. Copyright 2016 Nature Publishing Group.

4.2. Enhanced Pt Electronic Structures through Organic−Inorganic Hybrid. While Pt-based alloy nanoparticles are very attractive electronic structures for the ORR compared to Pt nanoparticles, they have practical limitations due to the inevitable surface oxide formation and dissolution of TMs on the surface. When PtM alloys are chemically synthesized, the surface TMs on the alloy nanoparticles are easily oxidized by water, although Pt is intact due to low oxophilicity. The surface TM oxides can also be formed because they are exposed to humid air after the catalyst synthesis. This inevitable TM oxide formation on PtM alloy nanoparticles has been identified through XPS analysis.111 Owing to the surface TM oxide formation, PtM alloy catalysts may lose the intrinsic catalytic activities theoretically expected since highly electronegative oxygen species can withdraw electrons from the surface TM atoms and the charge transfer from TM to Pt is retarded.112 Consequently, the electronic effect by alloying Pt and TMs is weakened, and the intrinsic ORR activity becomes much lower than expected. In addition, the surface TM oxides on PtM alloy nanoparticles are rapidly dissolved during the ORR in the cathode of fuel cells since the fuel cell cathode is severely operated at high potentials. This electrochemical dissolution and dealloying effects are well-known to be critical factors for fuel cell catalyst degradation. However, in practice, it is hard to prevent the surface TMs from being naturally oxidized in water, air, or acid electrolytes without using additional passivation materials. Therefore, to minimize the effects of the surface oxidation of TM atoms on PtM alloy nanoparticles, interesting organic− inorganic hybrid structures have been recently proposed. Jung et al.112 reported the electronic ensemble effects between Pt and Co−Nsurf through the selective functionalization of Co atoms on PtCo nanoparticles by using N-containing polymer. Poly(N-isopropylacrylamide) (PNIPAM) was chemically functionalized on the carbon surface, and the PNIPAMfunctionalized carbon (C-PNIPAM) was utilized as a supporting material when carbon-supported PtCo catalyst was fabricated. Nitrogen in the amide group of PNIPAM more strongly interacted with Co precursor than with Pt precursor, which resulted in the formation of Co−Nsurf bonding. Through the Co−Nsurf bonding formation, the surface Co atom can be protected against the oxide formation by oxygen species in solvent, air, and electrolyte. Accordingly, the surface Co atoms

electrons can transfer from Co to Pt, which resulted in more downshift of the Pt d-band center in PtCo nanoparticles selectively functionalized with PNIPAM compared to a conventional PtCo catalyst. As a result, the ORR activity of PtCo catalyst selectively functionalized with PNIPAM was considerably enhanced through the electronic ensemble effects between Pt and the surrounding Co−Nsurf. In addition, the Co passivation with PNIPAM enhanced the catalytic durability of PtCo nanoparticles during the ORR. By the electronic ensemble effect, the oxygen binding energy of Pt was decreased which also prevented the dissolution of Pt. Consequently, the particle size, morphology, and ORR activity of PtCo nanoparticles with PNIPAM could be almost maintained even after the ADT, while the conventional PtCo nanoparticles were significantly degraded by particle dissolution and agglomeration and showed poor ORR activity as illustrated in Figure 27. Accessibility of minimal active sites because of strong adsorption of capping organic molecules on the surface of nanoparticles reflects capping organic molecules as undesirable species. Nevertheless, Chung et al.113 reported that the electrocatalytic activity can be promoted by surface coordinated species such as oleylamine (OA) despite an initial decrease in surface free sites by ascertaining the amount of adsorption. Conventionally, enhanced ORR activity can be achieved by tailoring the design of the electrocatalysts or/and total surface coverage. These alterations can be accomplished by changes in the electronic structure of the frontier d-band and by controlling the adsorption of spectator species. Hence, they investigated the d-band characteristics of OA-modified Pt/C having different coverage. Modification with OAs exhibited a lower d-band center relative to unmodified Pt, and decrease in the d-band center was proportional to the surface coverage. This modification of the frontier d-band structure can be explained by electronic effect involving in the donation of electrons from OAs to Pt nanoparticles. In the presence of H3PO4, adsorption of phosphate anions severely deactivated ORR, but alteration in the frontier d-band structure of OAmodified Pt/C lowered the adsorption strength of phosphate 14


Review


and 1.9 nm) and different TPPTP coverage (0.33 and 0.41), respectively, through ligand exchange reactions to examine the impacts of TPPTP on the ORR electrocatalytic performance of Pt nanoparticles. They compared as-synthesized Vulcan carbon supported Pt2.1 nmTPPTP/VC, Pt1.9 nmTPPTP/C to lab synthesized 2.4 nm bare Pt/VC (Pt2.4 nm/VC) and commercial 4.4 nm bare Pt/VC (Pt4.4 nm/VC). Electrochemical cyclic voltammograms showed weaker hydrogen adsorption on Pt2.1 nmTPPTP/ VC, and Tafel plots of area-specific ORR activities showed enhanced area-specific activity for Pt2.1 nmTPPTP/VC while mass-specific activity was inferior as shown in Figure 29. It is evident from Table 1 that smaller TPPTP-capped Pt particles performed better than slightly larger bare Pt catalyst (2.4 nm) and almost similar to commercial Pt catalyst (4.4 nm). They proved by TEM, NMR, and X-ray absorption spectroscopy including XANES and EXAFS that only the inner core of the atoms resembled bulk metal, while the majority of the Pt atoms were nonmetallic in nature due to strong electronic coupling with surface-bound ligands.117 Moreover, a critical difference of 40 mV observed in the case of Pt2.1nmTPPTP/VC for onset potentials for adsorption as well as desorption of oxygen species as compared to the corresponding values for Pt2.4nm/VC indicated the weaker oxygen adsorption on Pt by TPPTP ligands. On the basis of density functional theory (DFT) calculations, Oudenhuijzen et al.118 reported that hydrogen and oxygen coverage on Pt particles are strongly determined by the ionicity and acid/base properties of the support. They predicted a significant weakening of the Pt−Oad bonds (between Pt and adsorbed oxygen species) when Pt nanoparticles were supported on “ionic” metal oxides with electronrich coordinating support oxygen on Pt nanoparticles whereas when the metal oxide support was “covalent” and the coordinating support oxygens are electron-poor, a strengthening of the Pt−Oad bond is predictable. Since triarylphosphine based ligands coordinate Pt particles through an electron-rich phosphorus,119,120 it caused weakening of Pt−Oad bonds at PtTPPTP. Thus, the reduced strength of Pt−O bonds is possibly the cause of their improved ORR as the diminished desorption of oxygen species is the rate-defining factor. Although the mechanism for the Pt−Oad bond weakening in Pt-TPPTP needs further study, but it may be attributed to change in electronic energy of the Pt d-band orbitals by the surface ligands. Alternatively, the hydrophobic nature of the aryl groups existing in TPPTP restrains the water accessibility and inhibits Pt-oxide formation. Chen and co-workers reported the chemical functionalization strategy to achieve selectively functionalized zero-dimentional (0D) and three-dimensional (3D) Pt-based nanostructures with enhanced durability. First, they synthesized polyallylamine (PAA) functionalized Pt nanolance assemblies (PAA-Pt NLAs),121 Pd−Pt bimetallic core−shell nanodendrites (PAAPdPt@PtPd CSNDs),122 Pt nanocubes (Pt-NCs),123 and PtPd alloy nanoflowers (PAA−Pt-Pd ANFs)124 in the presence of poly(allylamine hydrochloride) (PAH) by varying the medium pH and temperature. Although most of PAA-functionalized catalysts had much lower ECSA, they showed greater specific activity and more positive shift of half-wave potentials compared to commercial Pt black. Moreover, PAA-Pt NLAs and PAA-PdPt@PtPd CSNDs showed much higher stabilities during the ADTs. As in their preceding works, they further used PAH to synthesize Pt(Pt-LSSUs) nanostructures having shapes like long-spined sea-urchin (Figure 30).125 The as-prepared Pt(Pt-LSSUs) @PAA provided superior Pt usage instead of the

Figure 27. (a) ORR polarization curves and (b) the specific activities at 0.9 VRHE of catalysts before and after ADTs. Reproduced with permission from ref 112. Copyright 2016 Nature Publishing Group.

anions on Pt resulting in low surface coverage by phosphate anions. In addition, as phosphate adsorption requires preferentially a 3-fold site of the Pt, phosphate adsorption could be effectively hindered by the presence of OA on Pt during the ORR which leads to enhancement of ORR kinetics despite a loss of the ECSA. In short, both electronic and structural modifications of the Pt surface induced by organic molecule capping improved the ORR activity, as shown in Figure 28. In view of firm proneness of organophosphine ligands to bind over metal surfaces,114,115 peculiarly for Pt, Pietron and co-workers116 synthesized triphenylphosphine triphosphonate (TPPTP)-capped Pt nanoparticles of two different sizes (2.1

Figure 28. (a) Schematic presentation of tailoring the d-band structure by surface-capping organic molecules, (b) third-body effect of OA for hindering adsorption of phosphate ions, and ORR polarization curves of Pt/C and OA-modified Pt/C in O2-saturated (c) 0.1 M HClO4 and (d) 0.1 M HClO4 + 0.1 M H3PO4 at 20 °C and 1600 rpm. Reproduced with permission from ref 113. Copyright 2013 American Chemical Society. 15


Review


Figure 29. (a) CV of Pt2.4 nm/VC (0.0245 mgPt/cmgeometric2) and Pt2.1 nm−TPPTP/VC (0.0245 mgPt/cmgeometric2) on 0.196 cm2 GC electrodes in Arsaturated 0.1 M HClO4 at 25 °C. Scan rate 20 mV/s. (b) Tafel plots of the specific ORR activities (is) at Pt2.4 nm/VC, Pt2.2 nm TPPTP/VC, and commercial Pt4.4 nm/VC. Reproduced with permission from ref 116. Copyright 2008 The Electrochemical Society.

Table 1. Effect of TPPTP Functionalization on ORR (adapted from ref 116) sample

im (mA mg−2Pt at +0.90 V)

is (mA cm−2 at +0.90 V)

Pt1.9 nmTPPTP/C Pt2.1 nmTPPTP/VC Pt2.4 nm/VC Pt4.4 nm/VC

0.20 0.20 0.30 0.21

0.36 0.39 0.32 0.40

Figure 30. (a) SEM and (b) TEM images of Pt-LSSUs@PAA. (c) TEM image of single Pt-LSSUs@PAA. (d) Picture of lime urchin. (e) SAED pattern of single Pt-LSSUs@PAA. (f) The magnified SEM image of Pt-LSSUs@PAA. Reproduced with permission from ref 125. Copyright 2016 American Chemical Society.

Figure 31. (a) Cyclic voltammograms and (b) ORR polarization plots for Pt-LSSUs@PAA and Pt black in N2-saturated 0.5 M H2SO4 at 50 mV s−1 and in O2-saturated 0.5 M H2SO4 at 5 mV s−1 and 1600 rpm, respectively. Inset shows ik values of Pt black and Pt-LSSUs@PAA at varying potentials. Cyclic voltammograms and ORR polarization plots of (c) Pt-LSSUs@PAA and (d) Pt black in 0.5 M H2SO4 before and after ADT. (e) Cyclic voltammograms and (f) ORR polarization plots for Pt black and Pt-LSSUs@PAA in the presence of 1 M CH3OH. Reproduced with permission from ref 125. Copyright 2016 American Chemical Society.

above-mentioned Pt nanostructures due to high branching degree and sheet morphology of the branch. Therefore, it showed comparable ECSA values to that of commercial Pt black despite much bigger particle size of Pt-LSSUs@PAA nanostructures (180 nm) than that of Pt black (8.6 nm). Meanwhile, both the onset potential of −OHad formation and reduction potential of surface oxide at the Pt-LSSUs@PAA surface appeared at higher potentials compared to those of Pt black as shown in Figure 31a. In addition, Pt-LSSUs@PAA nanostructures showed much higher catalytic activity and excellent durability for the ORR, as shown in Figure 31b−d. The enhanced electrochemical properties of Pt-LSSUs@PAA were attributed to the 3D interconnected structure with effective antioxidation features and strong interaction between the metal and the N group of PAA. Lone pair electrons of −NH2 groups in PAH facilitate the transfer of electrons from the neighboring electron-rich N to Pt, which caused the

decrease in 5d vacancies in Pt atoms resulting in excellent antioxidation ability of Pt due to the weaker adsorption of oxygen species. In this situation, uniformly adsorbed PAAfunctionalized nanostructures also act as barrier networks to restrain accessibility of a particular alcohol on Pt surface due to molecular size sorting as shown in Figure 31e,f. Chen and co-workers126 tuned the electrocatalytic performance of noble metal surfaces by chemical functionalization with electron-withdrawing ligands. They synthesized carbon supported chlorophenyl (ArCl)-stabilized Pt nanoparticles (Pt− ArCl) and also chemically modified commercial Pt/C with the same chlorophenyl groups (Pt/CArCl). Chlorophenyl groups 16


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In general, the face-centered tetragonal (fct) structure of PtM alloy nanoparticles shows higher activity and durability for ORR compared to the face-centered cubic (fcc) structure. Zhang et al.129 reported Pt-rich fcc-FeCuPt and fct-FeCuPt alloy nanoparticles developed by heat treatment and the electrochemical dealloying process. They observed that the catalytic activities of the Pt surface were varied in accordance with crystalline structures of the core materials. The fct-FeCuPt/Pt nanoparticles showed higher ORR activity due to the release of the overcompressed Pt strain by the core material with a fct structure as shown in Figure 32. In addition, the Abruña

form a multilayer structure bonded onto the Pt nanoparticles surface. The Pt−ArCl/C and Pt/CArCl exhibited a more positive shift in ORR curves than untreated Pt/C catalysts. Pt−ArCl/C manifested ∼2.3 times higher specific activity and ∼2.8 times higher mass activity of ORR as measured by the kinetic current density at 0.9 V (vs RHE) with respect to that of the commercial untreated Pt/C whereas Pt/CArCl doubled the mass activity and tripled the specific activity for ORR. Strong electronic interactions between the aromatic rings and the Pt nanoparticles are accountable toward improved ORR activity. As a continuation of their previous work, Chen and coworkers127 showed that the ORR activity of Pt nanoparticles could be effectively manipulated by varying the parasubstituent’s electronegativity that can be measured by the Hammet substituent constant (σ). The ORR activity of Pt nanoparticles with similar particle sizes (1.85 nm) but capped with different substituent aryl groups (Pt−Ar−R where R is CH3, F, Cl, OCF3, CF3) increases with higher electronwithdrawing ability/electronegativity of the para-group. From Table 2, as can be seen, the mass activities of three samples Table 2. Physical Parameters of Pt−Ar−R Nanoparticles and Their Electrocatalytic Activity for ORR (adapted from ref 127) substituent (R) σ particle size (nm) area per ligand (Å2) specific ECSA (m2 g−1Pt) jk (mA cm−2 at +0.90 V) jm (mA mg−2Pt at +0.90 V)

CH3

F

Cl

OCF3

CF3

Pt/C

−0.017 2.1 8.0

+0.05 2.1 4.5

+0.23 1.85 8.0

+0.35 2.5 15.7

+0.54 2.2 13

3.3

54

54

93

47

59

80

0.15

0.30

0.47

0.52

0.65

0.20

0.082

0.162

0.437

0.244

0.384

0.16

Figure 32. (a) ORR polarization curves of Pt, fct-FePt/Pt, fccFeCuPt/Pt, and fct-FeCuPt/Pt nanoparticles. (b) The specific activities of the catalysts at 0.531 V (E1/2 of the commercial Pt catalyst). (c) ORR polarization curves for fct-FeCuPt/Pt before and after 10 000 cycles between 0.4 and 0.7 V. (d) TEM image of the fctFeCuPt/Pt nanoparticles after 10 000 cycles. Reproduced with permission from ref 129. Copyright 2014 American Chemical Society.

group130 reported the great long-term stability of fct-PtCo nanoparticles due to the strong Pt−Co bonding in the core material. Therefore, it was confirmed that the crystalline structures of alloy nanoparticles had critical effects on the catalytic activity and durability of Pt-based catalysts. However, high annealing temperature at ∼700 °C is necessary to produce ordered fct-PtM alloy nanoparticles since as-synthesized PtM nanoparticles have disordered fcc structures. Consequently, fctstructured PtM alloy nanoparticles inevitably have large particle sizes and poor particle size distributions. Although alternative approaches using inorganic protective shells were developed to prevent the agglomeration during the high temperature annealing process,131−133 they needed an additional step of removing the shell layers from the nanoparticle surface to expose the electrochemically active sites. Therefore, recently Chung et al.134 evolved an organic− inorganic hybrid intermetallic fct-PtM nanoparticle coated with a N-doped carbon shell layer as shown in Figure 33. First, the carbon shell layer fabricated by carbonization of polydopamine coating layers effectively prevented the coalescence of PtFe alloy nanoparticles during a high temperature annealing process at 700 °C. Accordingly, fct-PtFe nanoparticles could be very small, as little as 6.5 nm, and show good particle distribution on carbon support materials. However, the catalytic activities of fct-PtFe nanoparticles were much different depending on the thickness of the carbon layers. When the carbon shell layers

(ArCl, ArOCF3, and ArCF3) were much larger than that of the commercial Pt/C. The enhanced ORR electrocatalytic activity can be explained by the fact that the presence of electronwithdrawing organo ligands on the Pt surface may change the electronic structure of the Pt nanoparticles. For oxygen adsorption on the Pt surface, electron is transferred from Pt atoms to oxygen adatom.128 But, the interaction between Pt and electron-withdrawing ligand caused the decrease in the electron density of the Pt surface atoms, which in turn weakened the adsorption of oxygen-containing species (Oads or OHads) on the Pt surface. 4.3. Enhanced Pt Crystalline Structures through Organic−Inorganic Hybrid. In terms of crystalline structure in PtM alloy nanoparticles, novel organic−inorganic hybrid catalyst structures have also been investigated. Practical use of nanoparticles with high surface areas for fuel cell catalysts was impeded due to their low durability by oxidation and dissolution of the nanoparticles. As mentioned before, under the fuel cell operating conditions Pt-based alloy nanoparticles can be easily oxidized, dissolved, and agglomerated into large particles resulting in considerable activity loss during cycling. In this regard, ordered intermetallic nanoparticles have been considered as one of the most promising catalysts to enhance both catalytic activity and durability in practical fuel cell applications. 17


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Figure 33. Synthesis of ordered intermetallic fct PtFe/C. (a) Scheme for the synthesis of carbon-supported and N-doped carbon-coated ordered fctPtFe nanoparticles. (b, c) TEM images of thermally annealed NPs without (b) and with dopamine coating (c) at 700 °C. (d) HRPD data of carbonsupported pure Pt(Pt/C) and PtFe nanoparticles before annealing (PtFe/C, before) and after annealing (PtFe/C, after). (e−g) Cs-corrected HAADF-STEM image of a fct-PtFe nanoparticle after annealing (e), a model structure of fct-PtFe structure (f), and FFT patterns of the image (g). The Pt columns look brighter than the Fe columns due to the Z-contrast. Reproduced with permission from ref 134. Copyright 2015 American Chemical Society.

durability (Figure 34c,d) for 10 000 potential cycles between 0.6 and 1.0 V (vs RHE). Indeed, the enhanced ORR activity and durability of intermetallic fct-PtM alloys compared to fccPtM are attributed to several factors such as strong alloy effects in well-ordered intermetallic structure, strong interaction between 3d transition metal and Pt via their spin−orbit coupling and d-orbital hybridization,135 and existence of an essential structural feature which involves some ideal catalytic facets around each nanoparticle with Pt sitting on the top and the 3d transition metal lying under the Pt layer.130 Moreover, the Pt shell is strained to the ordered intermetallic core, which has a smaller lattice constant than the disordered alloy.131 It was also confirmed through the first-principles calculation that about half of the values of the vacancy formation energy, Evac, in the fct-PtFe structure were higher than those in the fcc-PtFec structure. Therefore, the intermetallic fct structure could effectively prevent the formation of Fe vacancies and dissolution of other remaining Fe atoms resulting in high durability. It was also identified that the N-doped carbon shell encapsulating PtFe nanoparticles could stabilize the Pt-rich surface of the nanoparticles due to strong adsorption of nitrogen to Pt atoms, based on the calculation result of higher binding energy of the Pt55-cluster on N-doped graphene compared to that on undoped graphene. Chung et al.136 also depicted the effects of structural reconstruction of Pt based nanoparticles including bimetallic ordering and surface reorientation on the electrochemical properties of ORR. They synthesized PtNi nanoparticles by colloidal reduction method using oleic acid and sodium borohydride (NaBH4). After the preparation, they treated the as-synthesized PtNi nanoparticles by thermal annealing at 300

were very thin, less than 1 nm, fct-PtFe nanoparticles showed superior ORR activities (Figure 34a,b) due to the high

Figure 34. (a) Oxygen reduction reaction activity of ordered fct-PtFe/ C, disordered fcc-PtFe/C, and Pt/C. (b) Mass and specific activities of the electrodes measured at 0.9 V. (c) ORR polarization curves and (d) changes in half wave potentials of Pt/C, fcc-PtFe/C, and fct-PtFe/C before and after ADT of 10 000 cycles. Reproduced with permission from ref 134. Copyright 2015 American Chemical Society.

permeability of O2 molecules through the carbon layers. On the other hand, the catalytic activities of fct-PtFe nanoparticles with thick carbon layers (thicker than 1 nm) were significantly low. In addition, Fe atoms in ordered fct-PtFe nanoparticles were more stable and less dissolved than those in disordered fcc-PtFe. As a result, fct-PtFe nanoparticles showed superior 18


Review

Chemistry of Materials and 700 °C (PtNi/C_300 and PtNi/C_700) to enhance the degree of alloying. In general, nanoparticles get severely agglomerated during heat treatment. However, in their report, PtNi nanoparticles were not significantly agglomerated even after thermal annealing at 700 °C due to the formation of sodium oleate, derived from the oleic acid and NaBH4, on the surface of particles137 (Figure 35). More precisely, the presence

of oleic acid causes the formation of well-ordered PtNi nanoparticles without altering the particles size. In addition, formation of a Pt-rich surface is observed in case of PtNi/ C_700 rather than PtNi/C_300. As COad oxidation analysis is a superior technique for investigating the features of Pt surface orientation,138,139 COad oxidation analysis was performed to confirm the reorientation of the surface facet (Figure 36a). The first peak around 0.75 VRHE is related to the Pt(111) facet, and an increment in the ratio of the (111) facet after heat treatment has been observed. Hence, thermal annealing also accounts for reorientation of the surface direction, i.e., thermal energy induces bimetallic ordering with Pt shell and the (111) direction reorientation in curves in proportion to the annealing temperature. Therefore, the polarization demonstrates much higher ORR activity for PtNi/C_700 nanoparticles than for the reference Pt/C (Figure 36b) due to the electronic tuning of the Pt dband structure by alloying Pt with a 3d metal and the surface reconstruction of the Pt shell.

5. CONCLUSIONS Although fuel cells are promising clean energy conversion devices, an inability of large scale synthesis as well as the poor performance of the electrocatalysts for ORR severely hindered the fuel cell commercialization. The sluggish kinetics of the reaction leads to substantial losses in the fuel cell efficiency, even though a high amount of Pt-based catalysts was used. Thus, enormous attempts have been devoted to improving the catalytic efficiency of Pt and Pt-based catalysts in the preceding decades. Traditionally, the studies of fuel cell electrocatalysts have focused on the design and manipulation of the composition (alloys, core−shell structures, and Pt-skin surfaces), particle sizes, and shapes (crystal planes exposed). Accordingly, the present review shortly summarized the scientific achievements in the development of typical Pt-based nanocatalysts for fuel cells. Apart from the above, recently there emerges another promising strategy involving organic−inorganic hybrid concepts to tune the performances of Pt-based nanocatalysts. Therefore, we highlighted new progress regarding the impacts of surface functionalization of Pt or Pt-based catalysts with small organic molecules, molecular fragments, radicals, and ions on their electrocatalytic activity in fuel cell applications. Modification of Pt surfaces with CN−, ·CN, L-cysteine, OA, PAA, etc. positively controlled the selective interactions with spectator ions or methanol during the ORRs. Moreover, the surface modification by halides such as Br− resolved the

Figure 35. (a) Schematic illustration of the surface reconstruction of PtNi nanoparticles by thermal annealing. TEM images of (b) PtNi/ C_300 and (c) PtNi/C_700: Particle size distributions (inset), highresolution images (top right), and their reduced FFT pattern (bottom right). Reproduced with permission from ref 136. Copyright 2014 American Chemical Society.

Figure 36. Electrochemical measurements of PtNi_300 and PtNi_700: (a) CO stripping curves after full adsorption of CO molecules on the PtNi surface, (b) ORR polarization curves in O2-saturated 0.1 M HClO4, and (c) cyclic voltammograms in Ar2-saturated 0.1 M HClO4 at 20 °C. Reproduced with permission from ref 136. Copyright 2014 American Chemical Society. 19


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(7) Stambouli, A. B.; Traversa, E. Solid Oxide Fuel Cells (SOFCs): A Review of an Environmentally Clean and Efficient Source of Energy. Renewable Sustainable Energy Rev. 2002, 6, 433−455. (8) Singhal, S. C. Advances in Solid Oxide Fuel Cell Technology. Solid State Ionics 2000, 135, 305−313. (9) Steele, B. C. H.; Heinzel, A. Materials for Fuel-Cell Technologies. Nature 2001, 414, 345−352. (10) Panchenko, A.; Koper, M. T. M.; Shubina, T. E.; Mitchell, S. J.; Roduner, E. Ab initio Calculations of Intermediates of Oxygen Reduction on Low-index Platinum Surfaces. J. Electrochem. Soc. 2004, 151, A2016−A2027. (11) Vasić, D.; Pašti, I.; Gavrilov, N.; Mentus, S. DFT Study of Interaction of O, O2, and OH with Unreconstructed Pt(hkl) (h, k, l= 0, 1) Surfaces-Similarities, Differences, and Universalities. Russ. J. Phys. Chem. A 2013, 87, 2214−2218. (12) Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-cell Cathode. J. Phys. Chem. B 2004, 108, 17886−17892. (13) Litster, S.; McLean, G. PEM Fuel Cell Electrodes. J. Power Sources 2004, 130, 61−76. (14) Tseung, A. C. C.; Dhara, S. C. Loss of Surface Area by Platinum and Supported Platinum Black Electrocatalyst. Electrochim. Acta 1975, 20, 681−683. (15) Stonehart, P.; Zucks, P. A. Sintering and Recrystallization of Small Metal Particles. Loss of Surface Area by Platinum-Black FuelCell Electrocatalysts. Electrochim. Acta 1972, 17, 2333−2351. (16) Kinoshita, K.; Routsis, K.; Bett, J. A. S.; Brooks, C. S. Changes in the Morphology of Platinum Agglomerates during Sintering. Electrochim. Acta 1973, 18, 953−961. (17) Antolini, E. Formation, Microstructural Characteristics and Stability of Carbon Supported Platinum Catalysts for Low Temperature Fuel Cells. J. Mater. Sci. 2003, 38, 2995−3005. (18) Park, Y.-C.; Tokiwa, H.; Kakinuma, K.; Watanabe, M.; Uchida, M. Effects of Carbon Supports on Pt distribution, Ionomer Coverage and Cathode Performance for Polymer Electrolyte Fuel Cells. J. Power Sources 2016, 315, 179−191. (19) Hasché, F.; Oezaslan, M.; Strasser, P. Activity, Stability and Degradation of Multi Walled Carbon Nanotube (MWCNT) Supported Pt Fuel Cell Electrocatalysts. Phys. Chem. Chem. Phys. 2010, 12, 15251−15258. (20) Wang, C.; Waje, M.; Wang, X.; Tang, J. M.; Haddon, R. C.; Yan, Y. Proton Exchange Membrane Fuel Cells with Carbon Nanotube Based Electrodes. Nano Lett. 2004, 4, 345−348. (21) Rodriguez, N. M.; Kim, M. S.; Baker, R. T. K. Carbon NanoFibers: A Unique Catalyst Support Medium. J. Phys. Chem. 1994, 98, 13108−13111. (22) Lin, Z.; Ji, L.; Krause, W. E.; Zhang, X. Synthesis and Electrocatalysis of 1-Aminopyrene-functionalized Carbon Nanofibersupported Platinum−Ruthenium Nanoparticles. J. Power Sources 2010, 195, 5520−5526. (23) Kou, R.; Shao, Y.; Wang, D.; Engelhard, M. H.; Kwak, J. H.; Wang, J.; Viswanathan, V. V.; Wang, C.; Lin, Y.; Wang, Y.; Aksay, I. A.; Liu, J. Enhanced Activity and Stability of Pt Catalysts on Functionalized Graphene Sheets for Electrocatalytic Oxygen Reduction. Electrochem. Commun. 2009, 11, 954−957. (24) Liu, S.; Wang, J.; Zeng, J.; Ou, J.; Li, Z.; Liu, X.; Yang, S. Green” Electrochemical Synthesis of Pt/Graphene Sheet Nanocomposite Film and its Electrocatalytic Property. J. Power Sources 2010, 195, 4628− 4633. (25) Qu, L.; Liu, Y.; Baek, J. B.; Dai, L. Nitrogen-doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano 2010, 4, 1321−1326. (26) 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, 433−456. (27) Antolini, E. Carbon Supports for Low-Temperature Fuel Cell Catalysts. Appl. Catal., B 2009, 88, 1−24.

durability issues, observed in the case of PtM alloys, by preserving transition metals from being leached out. Functionalization of Pt with electron donating or withdrawing molecules significantly enhanced the ORR activities and durabilities of Ptbased nanocatalysts because of strong electronic interactions (electron transfer) between Pt and the organic molecules. PtM alloy nanoparticles could also have highly active and stable crystalline structures without change of the particle size by utilization of carbon layers made from polymers coated on the nanoparticle surface or by formation of sodium oleate complex during the nanoparticle synthesis. In short, organic−inorganic hybrid structures have unique combinations of new properties that are absent in typical Pt-based catalysts. Consequently, the present new structural concept ensures the evolution of extremely efficient and economical Pt-based catalysts for fuel cells. In addition, it is expected that more effective catalysts with organic−inorganic hybrid structures will be continuously proposed in the near future. In this regard, the present review imparts the positive prospect for future works on the elaboration of new types of fuel cell catalysts. Although there exist numerous advantages of organic−inorganic hybrid designed catalysts, it is also noteworthy that in-depth understanding of what happens on the atomic and electronic scale during electrochemical processes is required as a prior condition for real progress in the field of organic−inorganic hybrid catalysts for fuel cells. In this respect, organic−inorganic hybrid catalysts pose a challenge.

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AUTHOR INFORMATION

Corresponding Authors

*(N. J.) E-mail: [email protected]. *(S.J.Y.) E-mail: [email protected]. ORCID

Sung Jong Yoo: 0000-0003-1556-0206 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation (NRF) (2016M3A6A7945505). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (Nos. 2015R1C1A1A01053367, 2017R1A2B2003363). This work was also supported by the KIST Institutional Program (Project No. 2E27301-17-035) and by the Research Fund of Chungnam National University.

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REFERENCES

(1) Perry, M. L.; Fuller, T. F. A Historical Perspective of Fuel Cell Technology in the 20th Century. J. Electrochem. Soc. 2002, 149, S59− S67. (2) Larminie, J.; Dicks, A. Fuel Cell Systems Explained, 2nd ed.; John Wiley & Sons: 2003. (3) O’Hayre, R.; Cha, S.-W.; Colella, W.; Prinz, F. B. Fuel Cell Fundamentals, 3rd ed.; John Wiley & Sons: 2016. (4) Peighambardoust, S. J.; Rowshanzamir, S.; Amjadi, M. Review of the Proton Exchange Membranes for Fuel Cell Applications. Int. J. Hydrogen Energy 2010, 35, 9349−9384. (5) Gülzow, E. Alkaline Fuel Cells: A Critical View. J. Power Sources 1996, 61, 99−104. (6) King, J. M.; Kunz, H. R. Phosphoric Acid Electrolyte Fuel Cells, Handbook of Fuel Cells; John Wiley & Sons: 2010. 20


Review


(48) Jackson, C.; Smith, G. T.; Inwood, D. W.; Leach, A. S.; Whalley, P. S.; Callisti, M.; Polcar, T.; Russell, A. E.; Levecque, P.; Kramer, D. Electronic Metal-support Interaction Enhanced Oxygen Reduction Activity and Stability of Boron Carbide Supported Platinum. Nat. Commun. 2017, 8, 15802. (49) Grubb, W.; McKee, D. Boron Carbide, A New Substrate for Fuel Cell Electrocatalysts. Nature 1966, 210, 192−194. (50) Lv, H.; Peng, T.; Wu, P.; Pan, M.; Mu, S. Nano-boron Carbide Supported Platinum Catalysts with Much Enhanced Methanol Oxidation Activity and CO tolerance. J. Mater. Chem. 2012, 22, 9155−9160. (51) Guo, S.; Zhang, S.; Sun, S. Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52, 8526−8544. (52) Zhang, J.; Yang, H.; Fang, J.; Zou, S. Synthesis and Oxygen Reduction Activity of Shape-Controlled Pt3Ni Nanopolyhedra. Nano Lett. 2010, 10, 638−644. (53) Cui, C.; Gan, L.; Li, H.-H.; Yu, S.-H.; Heggen, M.; Strasser, P. Octahedral PtNi Nanoparticle Catalysts: Exceptional Oxygen Reduction Activity by Tuning the Alloy Particle Surface Composition. Nano Lett. 2012, 12, 5885−5889. (54) Guo, S.; Li, D.; Zhu, H.; Zhang, S.; Markovic, N. M.; Stamenkovic, V. R.; Sun, S. FePt and CoPt Nanowires as Efficient Catalysts for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52, 3465−3468. (55) Hwang, S. J.; Kim, S.-K.; Lee, J.-G.; Lee, S.-C.; Jang, J. H.; Kim, P.; Lim, T.-H.; Sung, Y.-E.; Yoo, S. J. Role of Electronic Perturbation in Stability and Activity of Pt-based Alloy Nanocatalysts for Oxygen Reduction. J. Am. Chem. Soc. 2012, 134, 19508−19511. (56) Ontaneda, J.; Bennett, R. A.; Grau-Crespo, R. Electronic Structure of Pd Multilayers on Re (0001): The Role of Charge Transfer. J. Phys. Chem. C 2015, 119, 23436−23444. (57) Stephens, I. E. L.; Bondarenko, A. S.; Gronbjerg, U.; Rossmeisl, J.; Chorkendorff, I. Understanding the Electrocatalysis of Oxygen Reduction on Platinum and its Alloys. Energy Environ. Sci. 2012, 5, 6744−6762. (58) Wang, C.; Chi, M.; Wang, G.; van der Vliet, D.; Li, D.; More, K.; Wang, H.-H.; Schlueter, J. A.; Markovic, N. M.; Stamenkovic, V. R. Correlation between Surface Chemistry and Electrocatalytic Properties of Monodisperse PtxNi1‑x Nanoparticles. Adv. Funct. Mater. 2011, 21, 147−152. (59) Bing, Y.; Liu, H.; Zhang, L.; Ghosh, D.; Zhang, J. Nanostructured Pt-alloy Electrocatalysts for PEM Fuel Cell Oxygen Reduction Reaction. Chem. Soc. Rev. 2010, 39, 2184−2202. (60) Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Norskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1, 552−556. (61) Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Norskov, J. K. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chem., Int. Ed. 2006, 45, 2897− 2901. (62) Paulus, U. A.; Wokaun, A.; Scherer, G. G.; Schmidt, T. J.; Stamenkovic, V.; Radmilovic, V.; Markovic, N. M.; Ross, P. N. Oxygen Reduction on Carbon-Supported Pt−Ni and Pt−Co Alloy Catalysts. J. Phys. Chem. B 2002, 106, 4181−4191. (63) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Trends in Electrocatalysis on Extended and Nanoscale Pt-Bimetallic Alloy Surfaces. Nat. Mater. 2007, 6, 241−247. (64) Yoo, S. J.; Hwang, S. J.; Lee, J. G.; Lee, S. C.; Lim, T. H.; Sung, Y.-E.; Wieckowski, A.; Kim, S. K. Promoting Effects of La for Improved Oxygen Reduction Activity and High Stability of Pt on Pt− La Alloy Electrodes. Energy Environ. Sci. 2012, 5, 7521−7525. (65) Yoo, S. J.; Kim, S. K.; Jeon, T.-Y.; Hwang, S. J.; Lee, J. G.; Lee, S. C.; Lee, K.-S.; Cho, Y.-H.; Sung, Y.-E.; Lim, T. H. Enhanced Stability and Activity of Pt−Y Alloy Catalysts for Electrocatalytic Oxygen Reduction. Chem. Commun. 2011, 47, 11414−11416.

(28) Roy, S. C.; Harding, A. W.; Russell, A. E.; Thomas, K. M. Spectroelectrochemical Study of the Role Played by Carbon Functionality in Fuel Cell Electrodes. J. Electrochem. Soc. 1997, 144, 2323−2328. (29) Auer, E.; Freund, A.; Pietsch, J.; Tacke, T. Carbons as Supports for Industrial Precious Metal Catalysts. Appl. Catal., A 1998, 173, 259−271. (30) Yang, W.; Wang, Y.; Li, J.; Yang, X. Polymer Wrapping Technique: An Effective Route to Prepare Pt Nanoflower/Carbon Nanotube hybrids and Application in Oxygen Reduction. Energy Environ. Sci. 2010, 3, 144−149. (31) Oh, H.-S.; Kim, H. Efficient Synthesis of Pt Nanoparticles Supported on Hydrophobic Graphitized Carbon Nanofibers for Electrocatalysts using Noncovalent Functionalization. Adv. Funct. Mater. 2011, 21, 3954−3960. (32) Xing, Y. Synthesis and Electrochemical Characterization of Uniformly-Dispersed High Loading Pt nanoparticles on Sonochemically-treated Carbon Nanotubes. J. Phys. Chem. B 2004, 108, 19255− 19259. (33) Hirsch, A. Functionalization of Single-Walled Carbon Nanotubes. Angew. Chem., Int. Ed. 2002, 41, 1853−1859. (34) Li, Y.; Hu, F. P.; Wang, X.; Shen, P. K. Anchoring Metal Nanoparticles on Hydrofluoric Acid Treated Multiwalled Carbon Nanotubes as Stable Electrocatalysts. Electrochem. Commun. 2008, 10, 1101−1104. (35) Zhang, J.; Zou, H.; Qing, Q.; Yang, Y.; Li, Q.; Liu, Z.; Guo, X.; Du, Z. Effect of Chemical Oxidation on the Structure of Single-Walled Carbon Nanotubes. J. Phys. Chem. B 2003, 107, 3712−3718. (36) Mawhinney, D. B.; Naumenko, V.; Kuznetsova, A.; Yates, J. T.; Liu, J.; Smalley, R. E. Infrared Spectral Evidence for the Etching of Carbon Nanotubes: Ozone Oxidation at 298 K. J. Am. Chem. Soc. 2000, 122, 2383−2384. (37) Yin, S.; Shen, P. K.; Song, S.; Jiang, S. P. Functionalization of Carbon Nanotubes by an Effective Intermittent Microwave Heatingassisted HF/H2O2 Treatment for Electrocatalyst Support of Fuel Cells. Electrochim. Acta 2009, 54, 6954−6958. (38) Kim, M.; Park, J.-N.; Kim, H.; Song, S.; Lee, W.-H. The Preparation of Pt/C Catalysts using Various Carbon Materials for the Cathode of PEMFC. J. Power Sources 2006, 163, 93−97. (39) Yang, W.; Wang, X.; Yang, F.; Yang, C.; Yang, X. Carbon Nanotubes Decorated with Pt Nanocubes by a Noncovalent Functionalization Method and Their Role in Oxygen Reduction. Adv. Mater. 2008, 20, 2579−2587. (40) Wang, D.; Lu, S.; Jiang, S. P. Tetrahydrofuran-Functionalized Multi-Walled Carbon Nanotubes as Effective Support for Pt and PtSn Electrocatalysts of Fuel Cells. Electrochim. Acta 2010, 55, 2964−2971. (41) Li, L.; Liu, H.; Wang, L.; Yue, S.; Tong, X.; Zaliznyak, T.; Taylor, G. T.; Wong, S. S. Chemical Strategies for Enhancing Activity and Charge Transfer in Ultrathin Pt Nanowires Immobilized onto Nanotube Supports for the Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2016, 8, 34280−342942. (42) Zhou, X.; Qiao, J.; Liu, Y. In Graphene: Energy Storage and Conversion Applications; Liu, Z., Zhou, X., Eds.; CRC Press: Taylor and Francis Group, 2015; Chapter 7. (43) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (44) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweld- ebrhan, D.; Miao, F.; Lau, C. N. Superior Thermal Conductivity of Single Layer Graphene. Nano Lett. 2008, 8, 902−907. (45) Chen, D.; Tang, L.; Li, J. Graphene-based Materials in Electrochemistry. Chem. Soc. Rev. 2010, 39, 3157−3180. (46) Li, S. S.; Zheng, J. N.; Ma, X.; Hu, Y. Y.; Wang, A. J.; Chen, J. R.; Feng, J. J. Facile Synthesis of Hierarchical Dendritic PtPd Nanogarlands Supported on Reduced Graphene Oxide with Enhanced Electrocatalytic Properties. Nanoscale 2014, 6, 5708−5713. (47) Li, Y.; Zhu, E.; McLouth, T.; Chiu, C. Y.; Huang, X.; Huang, Y.; Li, Y. Stabilization of High-Performance Oxygen Reduction Reaction Pt Electrocatalyst Supported on Reduced Graphene Oxide/Carbon Black Composite. J. Am. Chem. Soc. 2012, 134, 12326−12329. 21


Review


(84) Gong, K.; Su, D.; Adzic, R. R. Platinum-Monolayer Shell on AuNi0.5Fe Nanoparticle Core Electrocatalyst with High Activity and Stability for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2010, 132, 14364−14366. (85) Stamenković, V.; Schmidt, T. J.; Ross, P. N.; Marković, N. M. Surface Composition Effects in Electrocatalysis: Kinetics of Oxygen Reduction on Well-Defined Pt3Ni and Pt3Co Alloy Surfaces. J. Phys. Chem. B 2002, 106, 11970−11979. (86) Ruban, A. V.; Skriver, H. L.; Nørskov, J. K. Surface Segregation Energies in Transition-Metal Alloys. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 15990−16000. (87) Cai, Y.; Gao, P.; Wang, F.; Zhu, H. Carbon Supported Chemically Ordered Nanoparicles with Stable Pt Shell and Their Superior Catalysis Toward the Oxygen Reduction Reaction. Electrochim. Acta 2017, 245, 924−933. (88) Wang, C.; Chi, M.; Li, D.; Strmcnik, D.; Van Der Vliet, D.; Wang, G.; Komanicky, V.; Chang, K. C.; Paulikas, A. P.; Tripkovic, D.; Pearson, J.; More, K. L.; Marković, N. M.; Stamenkovic, V. R. Design and Synthesis of Bimetallic Electrocatalyst with Multilayered Pt-Skin Surfaces. J. Am. Chem. Soc. 2011, 133, 14396−14403. (89) Jung, N.; Chung, Y.-H.; Chung, D. Y.; Choi, K.-H.; Park, H.-Y.; Ryu, J.; Lee, S.-Y.; Kim, M.; Sung, Y.-E.; Yoo, S. J. Chemical Tuning of Electrochemical Properties of Pt-Skin Surfaces for Highly Active Oxygen Reduction Reactions. Phys. Chem. Chem. Phys. 2013, 15, 17079−17083. (90) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J. A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.; Stamenkovic, V. R. Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces. Science 2014, 343, 1339−1343. (91) Li, M.; Zhao, Z.; Cheng, T.; Fortunelli, A.; Chen, C.-Y.; Yu, R.; Zhang, Q.; Gu, L.; Merinov, B. V.; Lin, Z.; Zhu, E.; Yu, T.; Jia, Q.; Guo, J.; Zhang, L.; Goddard, W. A., III; Huang, Y.; Duan, X. Ultrafine Jagged Platinum Nanowires Enable Ultrahigh Mass Activity for the Oxygen Reduction Reaction. Science 2016, 354, 1414−1419. (92) Jiang, K.; Shao, Q.; Zhao, D.; Bu, L.; Guo, J.; Huang, X. Phase and Composition Tuning of 1D Platinum-Nickel Nanostructures for Highly Efficient Electrocatalysis. Adv. Funct. Mater. 2017, 27, 1700830−1700836. (93) Sanchez, C.; Ribot, F. Design of Hybrid Organic-Inorganic Materials Synthesized via Sol-gel Chemistry. New J. Chem. 1994, 18, 1007−1047. (94) Gomez-Romero, P.; Sanchez, C. Functional Hybrid Materials; Wiley-VCH: Weinheim, 2004. (95) Sanchez, C.; Julian, B.; Belleville, P.; Popall, M. Applications of Hybrid Organic−Inorganic Nanocomposites. J. Mater. Chem. 2005, 15, 3559−3592. (96) Grgur, B. N.; Marković, N. M.; Ross, P. N., Jr. Underpotential Deposition of Lead on Pt (111) in Perchloric Acid Solution: RRDPt(111)E Measurements. Langmuir 1997, 13, 6370−6374. (97) Marković, N. M.; Gasteiger, H. A.; Ross, P. N., Jr. Oxygen Reduction on Platinum Low-Index Single-Crystal Surfaces in Alkaline Solution: Rotating Ring DiskPt(hkl) Studies. J. Phys. Chem. 1996, 100, 6715−6721. (98) Marković, N. M.; Schmidt, T. J.; Stamenković, V.; Ross, P. N. Oxygen Reduction Reaction on Pt and Pt Bimetallic Surfaces: A Selective Review. Fuel Cells 2001, 1, 105−116. (99) He, Q.; Yang, X.; Chen, W.; Mukerjee, S.; Koel, B.; Chen, S. Influence of Phosphate Anion Adsorption on the Kinetics of Oxygen Electroreduction on Low Index Pt(hkl) Single Crystals. Phys. Chem. Chem. Phys. 2010, 12, 12544−12555. (100) He, Q.; Shyam, B.; Nishijima, M.; Ramaker, D.; Mukerjee, S. Mitigating Phosphate Anion Poisoning of Cathodic Pt/C Catalysts in Phosphoric Acid Fuel Cells. J. Phys. Chem. C 2013, 117, 4877−4887. (101) Kaserer, S.; Caldwell, K. M.; Ramaker, D. E.; Roth, C. Analyzing the Influence of H3PO4 as Catalyst Poison in High Temperature PEM Fuel Cells using In-Operando X-ray Absorption Spectroscopy. J. Phys. Chem. C 2013, 117, 6210−6217.

(66) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Improved Oxygen Reduction Activity on Pt3Ni (111) via Increased Surface Site Availability. Science 2007, 315, 493−497. (67) Mayrhofer, K. J. J.; Hartl, K.; Juhart, V.; Arenz, M. Degradation of Carbon-Supported Pt Bimetallic Nanoparticles by Surface Segregation. J. Am. Chem. Soc. 2009, 131, 16348−16349. (68) Oezaslan, M.; Hasche, F.; Strasser, P. Oxygen Electroreduction on PtCo3, PtCo and Pt3Co Alloy Nanoparticles for Alkaline and Acidic PEM Fuel Cells. J. Electrochem. Soc. 2012, 159, B394−B405. (69) Gan, L.; Heggen, M.; O’Malley, R.; Theobald, B.; Strasser, P. Understanding and Controlling Nanoporosity Formation for Improving the Stability of Bimetallic Fuel Cell Catalysts. Nano Lett. 2013, 13, 1131−1138. (70) Zhang, J.; Mo, Y.; Vukmirovic, M. B.; Klie, R.; Sasaki, K.; Adzic, R. R. Platinum Monolayer Electrocatalysts for O2 Reduction: Pt Monolayer on Pd(111) and on Carbon-Supported Pd Nanoparticles. J. Phys. Chem. B 2004, 108, 10955−10964. (71) Banks, C.; Mclntosh, S. Electrochemistry; Royal Society of Chemistry: Cambridge, U.K., 2017; Vol. 14. (72) Mazumder, V.; Chi, M.; More, K. L.; Sun, S. Core/shell Pd/ FePt Nanoparticles as an Active and Durable Catalyst for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2010, 132, 7848−7849. (73) Taufany, F.; Pan, C.-J.; Rick, J.; Chou, H.-L.; Tsai, M.-C.; Hwang, B.-J.; Liu, D.-G.; Lee, J.-F.; Tang, M.-T.; Lee, Y.-C.; Chen, C.I. Kinetically Controlled Autocatalytic Chemical Process for Bulk Production of Bimetallic Core−Shell Structured Nanoparticles. ACS Nano 2011, 5, 9370−9381. (74) Xie, S.; Choi, S.-I.; Lu, N.; Roling, L. T.; Herron, J. A.; Zhang, L.; Park, J.; Wang, J.; Kim, M. J.; Xie, Z.; Mavrikakis, M.; Xia, Y. Atomic Layer-by-Layer Deposition of Pt on Pd Nanocubes for Catalysts with Enhanced Activity and Durability Toward Oxygen Reduction. Nano Lett. 2014, 14, 3570−3576. (75) Wang, X.; Choi, S.; Roling, L. T.; Luo, M.; Ma, C.; Zhang, L.; Chi, M.; Liu, J.; Xie, Z.; Herron, J. A.; Mavrikakis, M.; Xia, Y. Palladium-Platinum Core-Shell Icosahedra with Substantially Enhanced Activity and Durability Towards Oxygen Reduction. Nat. Commun. 2015, 6, 7594−7602. (76) Wang, D.; Xin, H. L.; Yu, Y.; Wang, H.; Rus, E.; Muller, D. A.; Abruna, H. D. Pt-Decorated PdCo@Pd/C Core−Shell Nanoparticles with Enhanced Stability and Electrocatalytic Activity for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2010, 132, 17664−17666. (77) Wang, D.; Xin, H. L.; Wang, H.; Yu, Y.; Rus, E.; Muller, D. A.; DiSalvo, F. J.; Abruna, H. D. Facile Synthesis of Carbon-Supported Pd−Co Core−Shell Nanoparticles as Oxygen Reduction Electrocatalysts and Their Enhanced Activity and Stability with Monolayer Pt Decoration. Chem. Mater. 2012, 24, 2274−2281. (78) Hwang, S. J.; Yoo, S. J.; Shin, J.; Cho, Y.-H.; Jang, J. H.; Cho, E.; Sung, Y.-E.; Nam, S. W.; Lim, T.-H.; Lee, S.-C.; Kim, S.-K. Supported Core@Shell Electrocatalysts for Fuel Cells: Close Encounter with Reality. Sci. Rep. 2013, 3, 1309. (79) Cho, K. Y.; Yeom, Y. S.; Seo, H. Y.; Kumar, P.; Lee, A. S.; Baek, K.-Y.; Yoon, H. G. Molybdenum-doped PdPt@Pt Core-shell Octahedral Supported by Ionic Block Copolymer-functionalized Graphene as a Highly Active and Durable Oxygen Reduction Electrocatalyst. ACS Appl. Mater. Interfaces 2017, 9, 1524−1535. (80) Yang, J.; Chen, X.; Yang, X.; Ying, J. Y. Stabilization and Compressive Strain Effect of AuCu Core on Pt shell for Oxygen Reduction Reaction. Energy Environ. Sci. 2012, 5, 8976−8981. (81) Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Stabilization of Platinum Oxygen-Reduction Electrocatalysts using Gold Clusters. Science 2007, 315, 220−222. (82) Kim, Y.; Hong, J. W.; Lee, Y. W.; Kim, M.; Kim, D.; Yun, W. S.; Han, S. W. Synthesis of AuPt Heteronanostructures with Enhanced Electrocatalytic Activity Toward Oxygen Reduction. Angew. Chem., Int. Ed. 2010, 49, 10197−10201. (83) Roudgar, A.; Groß, A. Local Reactivity of Supported Metal Clusters: Pdn on Au (111). Surf. Sci. 2004, 559, L180−L186. 22


Review

Chemistry of Materials (102) Marković, N. M.; Gasteiger, H. A.; Grgur, B. N.; Ross, P. N. Oxygen Reduction Reaction on Pt(111): Effects of Bromide. J. Electroanal. Chem. 1999, 467, 157−163. (103) Tarasevich, M. R.; Sadkowski, A.; Yeager, E. In Comprehensive Treatise in Electrochemistry; Bockris, J. O. M., Conway, B. E., Yeager, E., Khan, S. U. M., White, R. E., Eds.; Plenum Press: New York, 1983; p 301. (104) Strmcnik, D.; Escudero-Escribano, M.; Kodama, K.; Stamenkovic, V. R.; Cuesta, A.; Marković, N. M. Enhanced Electrocatalysis of the Oxygen Reduction Reaction Based on Patterning of Platinum Surfaces with Cyanide. Nat. Chem. 2010, 2, 880−885. (105) Liu, H.; Liu, X.; Li, Y.; Jia, Y.; Tang, Y.; Chen, Y. Hollow PtNi Alloy Nanospheres with Enhanced Activity and Methanol Tolerance for the Oxygen Reduction Reaction. Nano Res. 2016, 9, 3494−3503. (106) Lu, L.; Li, R.; Fujiwara, K.; Yan, X.; Kobayashi, H.; Yi, W.; Fan, J. Cyanide Radical Chemisorbed Pt Electrocatalyst for Enhanced Methanol-Tolerant Oxygen Reduction Reactions. J. Phys. Chem. C 2016, 120, 11572−11580. (107) Jung, N.; Shin, H.; Kim, M.; Jang, I.; Kim, H.-J.; Jang, J. H.; Kim, H.; Yoo, S. J. Janus Pt Surfaces Derivatized with Zwitterionic Molecules for Oxygen Reduction Reactions in Alkaline and Acid Electrolytes. Nano Energy 2015, 17, 152−159. (108) Chung, Y.-H.; Kim, S. J.; Chung, D. Y.; Park, H. Y.; Sung, Y.-E.; Yoo, S. J.; Jang, J. H. Third-Body Effects of Native Surfactants on Pt Nanoparticle Electrocatalysts in Proton Exchange Fuel Cells. Chem. Commun. 2015, 51, 2968−2971. (109) Miyabayashi, K.; Nishihara, H.; Miyake, M. Platinum Nanoparticles Modified with Alkylamine Derivatives as an Active and Stable Catalyst for Oxygen Reduction Reaction. Langmuir 2014, 30, 2936−2942. (110) Choi, J.; Lee, Y.; Kim, J.; Lee, H. Enhancing Stability of Octahedral PtNi Nanoparticles for Oxygen Reduction Reaction by Halide Treatment. J. Power Sources 2016, 307, 883−890. (111) Duong, H. T.; Rigsby, M. A.; Zhou, W.-P.; Wieckowski, A. Oxygen Reduction Catalysis of the Pt3Co Alloy in Alkaline and Acidic Media Studied by X-ray Photoelectron Spectroscopy and Electrochemical Methods. J. Phys. Chem. C 2007, 111, 13460−13465. (112) Jung, N.; Bhattacharjee, S.; Gautam, S.; Park, H.-Y.; Ryu, J.; Chung, Y.-H.; Lee, S.-Y.; Jang, I.; Jang, J. H.; Park, S. H.; Chung, D. Y.; Sung, Y.-E.; Chae, K.-H.; Waghmare, U. V.; Lee, S.-C.; Yoo, S. J. Organic-Inorganic Hybrid PtCo Nanoparticle with High Electrocatalytic Activity and Durability for Oxygen Reduction. NPG Asia Mater. 2016, 8, e237. (113) Chung, Y.-H.; Chung, D. Y.; Jung, N.; Sung, Y.-E. Tailoring the Electronic Structure of Nanoelectrocatalysts Induced by a SurfaceCapping Organic Molecule for the Oxygen Reduction Reaction. J. Phys. Chem. Lett. 2013, 4, 1304−1309. (114) Ye, E. Y.; Tan, H.; Li, S. P.; Fan, W. Y. Self-Organization of Spherical, Core−Shell Palladium Aggregates by Laser-Induced and Thermal Decomposition of [Pd(PPh3)4]+. Angew. Chem., Int. Ed. 2006, 45, 1120−1123. (115) Son, S. U.; Jang, Y.; Yoon, K. Y.; Kang, E.; Hyeon, T. Facile Synthesis of Various Phosphine-Stabilized Monodisperse Palladium Nanoparticles Through the Understanding of Coordination Chemistry of the Nanoparticles. Nano Lett. 2004, 4, 1147−1151. (116) Pietron, J. J.; Garsany, Y.; Baturina, O.; Swider-Lyons, K. E.; Stroud, R. M.; Ramaker, D. E.; Schull, T. L. Electrochemical Observation of Ligand Effects on Oxygen Reduction at LigandStabilized Pt Nanoparticle Electrocatalysts. Electrochem. Solid-State Lett. 2008, 11, B161−B165. (117) Kostelansky, C. N.; Pietron, J. J.; Chen, M.-S.; Dressick, W. J.; Swider-Lyons, K. E.; Ramaker, D. E.; Stroud, R. M.; Klug, C. A.; Zelakiewicz, B. S.; Schull, T. L. Triarylphosphine-Stabilized Platinum nanoparticles in Three-Dimensional Nanostructured Films as Active Electrocatalysts. J. Phys. Chem. B 2006, 110, 21487−21496. (118) Oudenhuijzen, M. K.; van Bokhoven, J. A.; Ramaker, D. E.; Koningsberger, D. C. Theoretical Study on Pt Particle Adsorbate

Bonding: Influence of Support Ionicity and Implications for Catalysis. J. Phys. Chem. B 2004, 108, 20247−20254. (119) Stelzer, D. In Aqueous-Phase Organometallic Catalysis: Concepts and Applications; Cornils, B., Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, 1998; Chapter 3. (120) Schull, T. L.; Butcher, R.; Dressick, W. J.; Brandow, S. L.; Byington, L. K.; Knight, D. A. Cis-[PtCl2(p-TPPTP(H)7)2]Cl2·4H2O· (CH3CH(OH)CH3): A Platinum Phosphine Complex Exhibiting Both Inter- and Intra-Molecular Hydrogen Bonding. Polyhedron 2004, 23, 1375−1378. (121) Fu, G.; Jiang, X.; Gong, M.; Chen, Y.; Tang, Y.; Lin, J.; Lu, T. Highly Branched Platinum Nanolance Assemblies by Polyallylamine Functionalization as Superior Active, Stable, and Alcohol-Tolerant Oxygen Reduction Electrocatalysts. Nanoscale 2014, 6, 8226−8234. (122) Li, F.-M.; Gao, X.-Q.; Li, S.-N.; Chen, Y.; Lee, J.-M. Thermal Decomposition Synthesis of Functionalized PdPt Alloy Nanodendrites with High Selectivity for Oxygen Reduction Reaction. NPG Asia Mater. 2015, 7, e219. (123) Fu, G.; Wu, K.; Jiang, X.; Tao, L.; Chen, Y.; Lin, J.; Zhou, Y.; Wei, S.; Tang, Y.; Lu, T.; Xia, X. Polyallylamine-Directed Green Synthesis of Platinum Nanocubes. Shape and Electronic Effect Codependent Enhanced Electrocatalytic Activity. Phys. Chem. Chem. Phys. 2013, 15, 3793−3802. (124) Fu, G.; Wu, K.; Lin, J.; Tang, Y.; Chen, Y.; Zhou, Y.; Lu, T. J. One-Pot Water-Based Synthesis of Pt−Pd Alloy Nanoflowers and Their Superior Electrocatalytic Activity for the Oxygen Reduction Reaction and Remarkable Methanol-Tolerant Ability in Acid Media. J. Phys. Chem. C 2013, 117, 9826−9834. (125) Xu, G.-R.; Wang, B.; Zhu, J.-Y.; Liu, F.-Y.; Chen, Y.; Zeng, J.H.; Jiang, J.-X.; Liu, Z.-H.; Tang, Y.-W.; Lee, J.-M. Morphological and Interfacial Control of Platinum Nanostructures for Electrocatalytic Oxygen Reduction. ACS Catal. 2016, 6, 5260−5267. (126) Zhou, Z.-Y.; Kang, X.; Song, Y.; Chen, S. Enhancement of the Electrocatalytic Activity of Pt nanoparticles in Oxygen Reduction by Chlorophenyl Functionalization. Chem. Commun. 2012, 48, 3391− 3393. (127) Zhou, Z.-Y.; Kang, X.; Song, Y.; Chen, S. Ligand-Mediated Electrocatalytic Activity of Pt Nanoparticles for Oxygen Reduction Reactions. J. Phys. Chem. C 2012, 116, 10592−10598. (128) Pang, Q.; Zhang, Y.; Zhang, J. M.; Xu, K. W. Structural and Electronic Properties of Atomic Oxygen Adsorption on Pt(111): A Density-Functional Theory Study. Appl. Surf. Sci. 2011, 257, 3047− 3054. (129) Zhang, S.; Zhang, X.; Jiang, G.; Zhu, H.; Guo, S.; Su, D.; Lu, G.; Sun, S. Tuning Nanoparticle Structure and Surface Strain for Catalysis Optimization. J. Am. Chem. Soc. 2014, 136, 7734−7739. (130) Wang, D.; Xin, H. L.; Hovden, R.; Wang, H.; Yu, Y.; Muller, D. A.; DiSalvo, F. J.; Abruña, H. D. Structurally Ordered Intermetallic Platinum−Cobalt Core−Shell Nanoparticles with Enhanced Activity and Stability as Oxygen Reduction Electrocatalysts. Nat. Mater. 2013, 12, 81−87. (131) Kim, J.; Lee, Y.; Sun, S. Structurally Ordered FePt Nanoparticles and Their Enhanced Catalysis for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2010, 132, 4996−4997. (132) Lee, D. C.; Mikulec, F. V.; Pelaez, J. M.; Koo, B.; Korgel, B. J. Synthesis and Magnetic Properties of Silica-Coated FePt Nanocrystals. J. Phys. Chem. B 2006, 110, 11160−11166. (133) Cheng, N.; Banis, M. N.; Liu, J.; Riese, A.; Li, X.; Li, R.; Ye, S.; Knights, S.; Sun, X. Extremely Stable Platinum Nanoparticles Encapsulated in a Zirconia Nanocage by Area-Selective Atomic Layer Deposition for the Oxygen Reduction Reaction. Adv. Mater. 2015, 27, 277−281. (134) Chung, D. Y.; Jun, S. W.; Yoon, G.; Kwon, S. G.; Shin, P.; Seo, D. Y.; Yoo, J. M.; Shin, H.; Chung, Y.-H.; Kim, H.; Mun, B. S.; Lee, K.S.; Lee, N.-S.; Yoo, S. J.; Lim, D.-H.; Kang, K.; Sung, Y.-E.; Hyeon, T. Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137, 15478− 15485. 23


Review

Chemistry of Materials (135) Li, Q.; Wu, L.; Wu, G.; Su, D.; Lv, H.; Zhang, S.; Zhu, W.; Casimir, A.; Zhu, H.; Mendoza-Garcia, A.; Sun, S. New Approach to Fully Ordered fct-FePt Nanoparticles for Much Enhanced Electrocatalysis in Acid. Nano Lett. 2015, 15, 2468−2473. (136) Chung, Y.-H.; Chung, D. Y.; Jung, N.; Park, H. Y.; Yoo, S. J.; Jang, J. H.; Sung, Y.-E. Origin of the Enhanced Electrocatalysis for Thermally Controlled Nanostructure of Bimetallic Nanoparticles. J. Phys. Chem. C 2014, 118, 9939−9945. (137) Mandal, M.; Das, B.; Mandal, K. Synthesis of Co(x)Pt(1‑X) Alloy Nanoparticles of Different Phase by Micellar Technique and Their Properties Study. J. Colloid Interface Sci. 2009, 335, 40−43. (138) Chung, D. Y.; Chung, Y.-H.; Jung, N.; Choi, K.-H.; Sung, Y.-E. Correlation Between Platinum Nanoparticle Surface Rearrangement Induced by Heat Treatment and Activity for an Oxygen Reduction reaction. Phys. Chem. Chem. Phys. 2013, 15, 13658−13663. (139) Urchaga, P.; Baranton, S.; Coutanceau, C.; Jerkiewicz, G. Electro-Oxidation of COchem on Pt Nanosurfaces: Solution of the Peak Multiplicity Puzzle. Langmuir 2012, 28, 3658−3663.

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Toward High-Performance Pt-Based Nanocatalysts for Oxygen

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