Tailor-Made Pt Catalysts with Improved Oxygen Reduction Reaction

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Tailor-Made Pt Catalysts with Improved Oxygen Reduction Reaction Stability/Durability Kiran Pal Singh, Emmanuel Batsa Tetteh, Ha-Young Lee, Tong-Hyun Kang, and Jong-Sung Yu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01420 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019

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ACS Catalysis

Tailor-Made Pt Catalysts with Improved Oxygen Reduction Reaction Stability/Durability

Kiranpal Singh#, Emmanuel Batsa Tetteh#, Ha-Young Lee, Tong-Hyun Kang and Jong-Sung Yu* Department of Energy Science and Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Republic of Korea # These authors contributed Equally

ABSTRACT With recent technological advances, the demand for renewable energy is ever growing. Polymer electrolyte membrane fuel cells (PEMFCs) have shown great promises in subduing various environmental and energy issues. However, the use of platinum (Pt) electrocatalyst at both cathode and anode sides of the PEMFCs has infused economical and sustainability hurdles in the commercialization of such devices. To alleviate these problems linked with Pt catalyst, various avenues have been researched and used. In the present review article, we have tried to look into the problems associated with the stability of the Pt-based electrocatalysts. Here, the scope of the current review article is categorized into three issues regarding the stability of Pt electrocatalysts: shape-controlled structure, alloy and core-shell structure, and supporting materials for Pt-based electrocatalysts. Major factors influencing the stability of the Pt-based electrocatalysts have been discussed, and various parameters needed for increasing the stability are also pondered upon. Keywords: Platinum, Pt alloy, catalyst support, PEMFC, stability

1. INTRODUCTION With the advancement in technology, the prospect for environmentally friendly energy storage and conversion technology is increasing day by day. Fuel cells have found a special place in energy conversion devices due to the use of hydrogen (H2) and oxygen (O2) as fuel and oxidant, respectively, and the formation of the environmentally benign renewable reaction product, H2O. Polymer electrolyte membrane fuel cells (PEMFCs) have been recognized as one attractive solution to the growing concerns on environmental and energy-related issues, such as climate change and depletion of fossil fuels. The major components of the PEMFCs are anode, cathode, and the membrane. During fuel cell operation, H2 (g) fuel oxidizes at the anode to generate electron (e-) and proton (H+). The 1 ACS Paragon Plus Environment

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generated H+ then travels through the proton conducting membrane towards the cathode where it combines with O2 (g) to form water. Even though theoretically PEMFC proposes great promises in fighting global climate change problems, as with all rising technologies, researchers have faced many obstacles in establishing economical and profitable fuel cells. O2, being highly stable in atmospheric conditions, requires very high activation energy for its reduction. To bring down this energy, precious catalysts (Platinum (Pt)-based) are being used at PEMFC cathode. This material comes with the price tag of $52,000 per Kg, which is prone to large market fluctuations owing to their monopolized distribution.1 Therefore, maximum investment is required for the catalyst component, and finding cheaper alternatives for ORR catalysis can significantly bring down the overall PEMFC cost. Reducing the Pt loading at the cathode without performance loss is the subject of most electrocatalytic studies. To realize this, many efforts have been made such as replacing Pt with other non-precious transition metals, metal nitrides, and nanoscale carbon-based metal-free electrocatalysts.2,3 However, poor stability and comparatively limited performance of these catalysts remain unsolved for PEMFC application. Most of the recent techniques mainly focus on manipulating the surface structure (by varying shape) and surface states (by varying composition) of Pt nanoparticles (NPs) to achieve the higher activity. Great heights have already been achieved in preparing an excellent Pt-based electrocatalyst for fuel cells. However, the stability of such catalysts still requires great attention. Department of Energy (DOE) 2020 technical target mandates the cycling stability of membrane electrode assembly (MEA) with only 10% voltage decay at 1.0-1.5 A/cm2 current density for 5000 h test.4 In particular, the durability of cathode catalyst is a prime concern, as the majority of the activity decay is directly correlated to the stability of the Pt and the carbon support at cathode electrode. Activity loss in Pt-based catalysts over the course of PEMFC operation can be expressed by four mechanisms: 1) electrochemical Ostwald ripening, 2) electrochemical dissolution and deposition in the ionomer phase, 3) migration of Pt on the carbon support followed by coalescence and 4) detachment from the support. Even though Pt is quite stable, thermodynamically it dissolves in acid media via the following process.5 Pt Pt2+ + 2e-

EO = 1.18 V vs. SHE

OR Pt + H2O  PtO + 2H+ + 2e-

EO = 0.98 V vs. SHE

Chemical dissolution of PtO PtO + 2H+ Pt2++ H2O

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From the above equations, it can be seen that the Pt losses its activity during PEMFC operation, either by dissolution or by the irreversible Pt oxide formation. Moreover, during the Pt oxide (PtO) reduction, the formed Pt(II) ions get dissolved into the electrolyte. Apart from Pt dissolution, carbon corrosion (Pt support) is another major factor which reduces the cathode catalyst performance over the course of time. C+ 2H2O  CO2 + 4H+ + 4e- EO = 0.2 V vs. SHE Therefore, to achieve an efficient catalyst for PEMFC, it is of great importance to focus on the durability of the cathode catalyst. Pt-based electrocatalysts can be broadly divided into three classes: 1) Pure Pt, 2) Pt core-shell and 3) Pt alloys.6 In recent years, for the improvement of the stability of Pt-based catalysts without compromising its activity, various methodologies and concepts have been developed. It has been observed that the surface structural characteristics like d-band, strain, and coordination number, which have played an important role in the enhancement of catalytic activity, have similarly been used to generate electrochemically stable ORR catalysts. Certain Pt shapes engender enhanced durability while others are more prone to Pt oxidation, dissolution, and general catalyst deactivation. Particularly, architectures that can provide a large contact area with a carbon support surface, such as nanoplates, nanowires, and nanorods, significantly enhance anchoring of Pt on carbon support to prevent detachment and migration. Well-defined Pt morphologies like nano frames, octahedral, icosahedral and cubic nanostructures have also capitalized on the inherent resistance to oxidation of the displayed special facets. On the other hand, in alloyed Pt-metal catalyst, the loss in activity is accredited to the dissolution or oxidation of the non-noble metals (generally transitional metals) in the detrimental and corrosive PEMFC operating conditions. To improve the stability of such Pt-based alloys, strategies mainly pertaining to the stabilization of the non-noble metal have been adopted. Furthermore, modifying Pt-alloy surface with organic/metal-metalloid species has been shown to change the electronic charge distribution over the Pt atom, hence significantly influencing the Pt’s long-term durability along with its poison resistance capability. The degradation of catalyst support which is another bottleneck in the commercialization of PEMFC has also been tackled in many ways. Almost all the Pt-based PEMFC catalysts require some form of support, usually carbon black because of its large surface area, high electrical conductivity, and well-developed pore structure. Recently, advanced carbon nanomaterials including carbon blacks, heteroatom-doped carbon, carbon nanotubes, graphene nanosheets, and carbon xerogel with different specific surface areas, degrees of graphitization, and extent of surface functionalization have been evaluated as support materials for Pt-based catalyst. These supports provide enhanced metal-support interaction which is key to preventing electrical isolation of active materials. Furthermore, well-anchoring of Pt metal particles on the support 3 ACS Paragon Plus Environment

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inhibits Pt particle migration and aggregation. Non-carbon catalyst supports such as transition metal oxides, carbides, nitrides, borides, etc. have also been studied for high durability support material. In the present review article, we have tried to establish a relationship between the structure, composition, synthesis conditions, electrocatalytic activity, and most importantly the electrochemical stability of carbon or non-carbon supported Pt-based electrocatalysts. Furthermore, various methodologies which have been adopted to alleviate the degradation of the Pt catalyst or the carbon support will be reviewed. In particular, this review article will be highly important and informative for those working in catalysts overall and particularly in the fuel cell catalysts. The present article is proper and timely as the fuel cell technology regains its importance in relation to upcoming hydrogen society and the durability issue has been one of major obstacles for commercialization of the fuel cell technology.

2. STABILITY OF SHAPE-CONTROLLED Pt The loss of Pt surface area during fuel cell operation mainly account for the performance loss of the PEMFC over time. The surface area loss could occur via two mechanisms; dissolutionreprecipitation (Ostwald ripening) and particle migration.7 Although strong metal-support interaction can limit Pt particle migration, the Pt dissolution and reprecipitation are unavoidable in PEMFC. The dissolution rate largely depends on the specific operating environments of the MEA such as temperature, electrode potential and the concentration of reactants and product.7– 10 Pt dissolution has been confirmed by the detection of Pt in the polymer membrane after extended life testing and the presence of soluble Pt species in the water exiting the cell.11 It has been reported that shape-controlled catalysts exist only in a “metastable” state and will inevitably change to the thermodynamically preferred spherical shape during voltage cycling, hence indicating low stability when subjected to voltage cycling. Despite such happening, to date, the shape-controlled Pt and Pt alloy nanostructures have demonstrated the highest mass activities in rotating disc electrode (RDE) as they have the closest ties to the fundamental singlecrystal studies.12 Furthermore, recent studies suggest that the emerging shape-control Pt is capable of enhancing the durability of the catalyst. Shape-controlled catalysts mainly include nanopolyhedra, nanowires, nanotubes, Nanorods, nanoframes, and other special shapes in the form of aerogels or porous architectonics. Here we summarize three major areas of shapecontrolled Pt and their corresponding durability enhancement.

2.1. Nanopolyhedra. Reports about polyhedron-engineered Pt nanocrystals or NPs and their synthesis began in the mid-1990s by El Sayed group for application in high-temperature liquid and gas phase catalytic reactions like olefin hydrogenations.12–15 Electrocatalytic 4 ACS Paragon Plus Environment

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applications of these Pt nanocrystals began from the 2000s in Feliu and Xia groups as pioneers to focus on such applications.12,16–20 The ORR on shape-controlled Pt NPs was firstly reported by Inaba et al.21 Today these nanocrystals have demonstrated extraordinary performance as ORR catalyst, because of their precise structure-reactivity maneuvering.22,23 It is known that specific shapes are bound by specific atomic packing facets, and therefore each shape presents a unique catalytic surface with different chemical states (Figure 1a).12,23,24 Huang group designed a strained PtPb/Pt core/shell intermetallic nanoplates (Figure 1b) with high ORR activity and excellent durability (Figure 1c) after 50K voltage cycles of the accelerated deteriorating test (ADT) between 0.6 and 1.1 V versus RHE in room temperature liquid half-cell with no apparent structure and composition changes.25 The special 2D nanoplate structure with uniform four top layers of stable Pt(110) was found to underscore high endurance of these catalysts. Via Density functional theory (DFT), oxygen adsorption energy (EO) on the PtPb nanoplates with Pt (110) surface, as a function of strain in the [001] and [110] directions were calculated (Figure 1d). It is generally believed that compressive strain can weaken the Pt-O binding on Pt (111) surface in M/Pt core/shell catalyst, whereas lowcoordinated (110) surface atoms have stronger Pt-O binding. However, interestingly, the DFT calculations reveal that the tensile strains on Pt (110) facet can increase the ORR activity, hence optimizing the Pt-O binding energy and reducing Pt oxidation. Previously-reported PtPb/Pt core/shell nanocatalyst without the unique hexagonal plate structure suffered from electrocatalytic activity loss.26,27 The majority of shape-controlled Pt nanocrystals have a size greater than 3 nm (bigger than conventional Pt NPs), and thus currently, the shape-controlled pure Pt nanocrystals rather than alloy are not practical for fuel cell application due to low Pt utilization. Computational studies have therefore been adopted to screen various shapes of pure Pt nanocrystal for stability.


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Figure 1. (a) Illustration of various 3D polyhedral configurations shown as a function of low index (100), (111), and/or (110) facets and high index (hkl) facets. Reproduced with permission from ref 23 Copyright 2018 The Royal Society of Chemistry. (b) Atomic model of the hexagonal nanoplate, (c) ORR activity and durability performance of the PtPb nanoplates/C catalyst before and after ADT. (d) DFT calculations of oxygen adsorption energy. Reproduced with permission from ref 25 Copyright 2016 Science.

Huang et al. presented a systematic investigation on the structure and stability of polyhedral Pt nanocrystals with both low- index and high-index facets using theoretical atomistic simulations related to cohesive energies and surface atomic bonding states.28 It was found that the stability of Pt nanocrystals depended strongly on the surface structures, and Pt nanocrystals with highindex facets [i.e., (310), (311), and (331)] exhibited better structural and thermal stability. Furthermore, the surface with high dangling bond density can provide a high chemical reactivity for nanocrystal catalysis. Most importantly, the octahedral nanocrystals with (111) facets showed the best structural and thermal stability although they presented the lowest dangling bond density of the surface.28 Such stability was later observed experimentally by Huang et al. who prepared various surface-doped Pt3Ni octahedral structures bounded exclusively by Pt (111).29 In particular, the Mo-doped Pt3Ni octahedral structures displayed exceptional activity and durability 6 ACS Paragon Plus Environment

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for the ORR in liquid half-cell. Li et al. attempted transferring such RDE stability of octahedral NPs to the MEA by testing a 15 nm octahedral PtNi alloy under the galvanostatic condition of 1,000 mA/cm2 for 100 h in an MEA at 80 °C. Although the octahedral alloy displayed better retention of maximum power density compared to commercial Pt/C, they could not maintain their octahedral shapes after stability testing even though the cathode potential at 1,000 mA/cm2 is relatively mild.30 It has been already established that the temperature in the MEA and high cathodic potential close to OCV can accelerate platinum dissolution.31 Despite all the efforts in shape-controlled nanopolyhedra for ORR catalysis, it is difficult to compare the stability of various shapes of Pt nanocrystals without considering the particle size effect and degree of strain. Hence it is currently impossible to experimentally confirm which type of polyhedra is most stable. More importantly, the exceptional durability obtained in this polyhedral catalyst has not yet been confirmed in MEA. Ultimately, MEA studies are required to confirm the RDE-observed durability. Moreover, most polyhedral Pt nanocrystals are supported on carbon, and as such, particle migration and agglomeration are unavoidable due to carbon corrosion. Furthermore, these polyhedral Pt structures provide limited contact with the carbon support, due to which the problems associated with particle detachment and migration can occur inevitably.

2.2. Nanowires, Nanotubes, and Nanorods. In order to overcome the particle detachment drawback of nanopolyhedra, a large contact area is required for firm anchoring of Pt on the carbon support, and this is the most prominent feature of the nanowires (NWs) and a similar class such as nanotubes (NTs) and nanorods (NRs). This class of structures is endowed with physical and chemical properties such as inherent anisotropic morphology, high flexibility, high surface area, and high conductivity needed for enhanced stability. Li et al. demonstrated the stability of Pt NWs (PtNW/C) as cathode catalyst in a 15 cell PEMFC stack with an active area of 250 cm2.32 Initially, in-situ electrochemical active surface area (ECSA) values were estimated under fully humidified H2/N2 conditions. The potential was then held at 1.5 V under fully humidified air/air for 14 h, and after completion, ECSA values were measured again under fully humidified H2/N2 (Figure 2a). The PtNW/C catalyst displayed only a minor loss of ECSA compared to commercial Pt/C (Figure 2b). The catalyst was further evaluated in a 1.5 kW fuel cell stack under practical operating conditions, particularly a simulated driving cycle situation derived from an actual FC vehicle drum test data.32,33 For 420 h of continuous operation of the dynamic driving cycle at 70 oC, the average voltage of every single cell based on PtNW/C cathode electrodes reduced from 0.619 V to 0.53 V at 800 mA/cm2, representing an average cell decline rate of 14.4% (Figure 2c). On the other hand, when using commercial Pt/C as cathode electrocatalysts, the average decline rate of every single cell after 420 h testing was 17.9%, which was higher than that of PtNW/C. Although the performance degradation of each stack can be caused by several factors including membrane degradation, ionomer decomposition, and 7 ACS Paragon Plus Environment

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electrode degradation at both the anode and cathode, Identical fabrication processes and testing protocols suggest the improved durability of the stack using PtNW/C (14.4% voltage loss) can be attributed to the PtNW/C cathode catalyst. The durability of PtNW/C is due to the onedimensional nanostructure with lengths in the 20–40 nm range that makes Pt less vulnerable to migration, aggregation, detachment, dissolution, and Ostwald ripening during fuel cell operation. Recently, Li et al. synthesized Pt/NiO core/shell NWs and transformed it into PtNi alloy NWs, via thermal annealing. Electrochemical dealloying of the PtNi alloy NWs led to jagged Pt nanowires (J-Pt NWs).34 These J-Pt NWs demonstrated an ECSA of 118 m2/gPt, a specific activity of 11.5mA/cm2 and a mass activity of 13.6 A/mgPt at 0.9 V. Under ADT with a sweep rate of 100 mV/s between 0.6 V and 1.0 V in O2-saturated 0.1 M HClO4 for 6000 CV cycles at ambient temperature, the ECSA dropped by only ~ 7%, the specific activity dropped by only ~ 5.5%, and together the mass activity dropped by only ~ 12 % (Figure 2d). ECSA loss for the commercial Pt/C catalyst under similar ADT was found to be ~ 30 % despite its lower initial ECSA. The surface atoms in the J-Pt NWs were found to be under-coordinated with coordination number (CN) between 6 and 8, relative to the typical Pt crystal surface with CN of 8 for Pt (100) and 9 for Pt (111) facets. These surface atoms also exhibited high values of Cauchy atomic stress, about 10 times that for regular (100) or (111) facets. Such mechanical strain could decrease the binding energy of adsorbents on close-packed surfaces, which should make the surface more resistant to oxidation, hence yielding excellent catalytic stability.


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Figure 2. CV curves of before and after 14 h ADT (hold 1.5 V) for MEA prepared with (a) PtNW/C and (b) commercial Pt/C. (c) I–V and I–P curves of PtNW/C. Reproduced with permission from ref 32 Copyright 2014 Elsevier B.V (d) ORR polarization curves and mass activity Tafel plot (inset) for the J-Pt NWs before and after 6000 cycles of ADT. Reproduced with permission from ref 34 Copyright 2016 Science.

Stability was also observed for other sub-nanometer Pt alloy NWs such as PtNiCo and PtNi NWs by Huang group.35 ADT conditions were the same as the former, except a higher number of potential cycling (30000) as well as larger potential cycling window of 0.6 V - 1.1 V vs RHE. Commercial Pt/C catalyst was found to be quite unstable and severe Pt particle aggregation was found under these conditions, with only 59.6% of the initial ECSA and 38.5% of the initial mass activity maintained after 30,000 ADT. Interestingly, the PtNiCo NWs showed higher ECSA loss (27.6%) than the PtNi NWs (11.6%) after 30,000 ADT. The poor stability of PtNiCo NWs has been attributed to the higher transition metal content in PtNiCo NWs. Here, catalyst degradation in the alloyed Pt-based NWs is attributed mainly to the dissolution of non-noble transition metal rather than detachment, migration, and agglomeration. Doping of certain transition metals like Mo, Rh, V and Cr into Pt-based alloys can improve resistance to oxidation of the non-noble 9 ACS Paragon Plus Environment

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transition metal alloying elements.29,36–38 The one-nanometer-thick PtNiRh NWs, after 10,000 cycles of ADT showed a loss of 12.8% of the initial mass activity.39 In contrast, the loss in the mass activities of similarly-prepared PtNi bimetallic NWs/C and commercial Pt/C catalysts was 54.3%, and 73.7% respectively. This observation confirms that metal dissolution is the cause of ECSA loss in NWs rather than detachment and agglomeration. Ultrathin or sub-nanometer NWs of Pt enable high mass activity,40 whereas increasing NW thickness resulted in better stability, therefore establishing a trade-off between Pt utilization and durability.41 Various research groups have corroborated the superior durability of thick NWs of Pt.41–43 Apart from NWs, hollow NWs or NTs also revealed similar properties and showed enhanced durability. Furthermore, these structures also demonstrated relatively higher Pt utilization due to the exposure of both exterior and interior surfaces for catalytic processes.44 Low aspect ratio NRs also retain the benefits available to NWs. Liang et al. investigated the ORR activity and stability of Pt3M (M = Au, Ag, and Pd) NRs, both experimentally and computationally.45 They found that the rod-like structure can lessen the dissolution effect, Ostwald ripening and aggregation of the NRs in acidic conditions when compared to carbonsupported NP-type structure. Yoon et al. also synthesized bare Pt3Ni NRs from PtNi@Ni core-shell NRs by acid etching and confirmed the durability of the highly active NR catalyst under 20,000 cycles of ADT.46 Similar findings have been also made for pure Pt NRs47, porous Pt NRs48,49 and metallic-nonmetallic composite NRs.50,51 The advantages of the unique one-dimensional geometry of NWs, NTs, and NRs were reflected in the ability of these catalysts to suppress the Ostwald ripening and improve the contact between the metal and carbon support by providing multipoint contacts, resulting in reduced movement and aggregation usually observed in spherical NPs. These properties also enhance the thermal stability during annealing to transform disordered alloys into ordered intermetallic. These recently discovered high-performance NWs, however, need to be analyzed under harsh cathode conditions in an MEA to justify their real durability and practicality for the PEMFC as well.

2.3. Nanoframes/Nanocages. Nano-frames (NFs)/cages represent a newly emerged class of promising electrocatalysts. The unique morphology of these catalyst exposes a high proportion of active surfaces, consequently saving the amounts of expensive Pt used. In general, the term ‘‘frames’’ refers to nanostructures composed of only edges without facets, while ‘‘cages’’ refer to nanostructures whose side facets have large cavities.52 These structures have surfaces with three-dimensional (3D) molecular accessibility and therefore expose both the interior and exterior surface of the nanostructures to reactants. In 2014, Stamenkovic and coworkers reported the preparation of Pt3Ni NFs catalyst.53 Crystalline PtNi3 polyhedra, with rhombic dodecahedron morphology, was transformed in solution, by interior erosion, into Pt3Ni NFs (Figure 3a-c). Subsequent thermal treatment engendered the desirable smooth Pt-skin type of structure (Figure 3d) with a top-most Pt-skin thickness of at least 2 monolayers. It is known that 10 ACS Paragon Plus Environment

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defect sites in nanoparticles are more susceptible to dissolution.54 The NFs exhibited excellent durability in electrochemical potential cycling between 0.6 and 1.0 V up to 10K potential cycles (Figure 3e) at 25°C in RDE. The similar test resulted in substantial loss of specific surface area (~ 40 %) for the state-of-the-art Pt/C electrocatalysts. STEM (dark-field and bright-field) images confirmed that the frame structure was preserved after the 10K potential cycles (Figure 3f and g). DFT calculations depicted the influence of strain on catalytic behavior in which the dependence of activity versus Pt-skin thickness was estimated to be optimal for 2 to 3 monolayers. These findings shed light on the origin of the high catalytic activity as well as the durability of the catalyst. The enhanced durability was ascribed to the smooth Pt skin and the electronic structure of the Pt-skin surface which can effectively limit the oxygenated intermediate coverage, because of the weaker oxygen binding strength, consequently diminishing the probability of Pt dissolution. In subsequent work, they demonstrated the control of the architecture to produce either completely hollow nanoframes (H-NF) or excavated nanoframes (E-NF) of PtNi (Figure 3h).55 The E-NF displayed twice the performance of the H-NF with regards to mass activity in RDE measurements. In such study, both catalysts possessed a Pt skeleton structure rather than a Pt Skin structure due to lack of annealing. They, however, maintained their morphology after the electrochemical test, suggesting that this newly excavated NFs would demonstrate superior durability if endowed with a smooth Pt skin structure. The superior activity and most importantly the durability of these hollow nanostructures have been observed by different research groups.56–62 However, a recent study suggests that the NFs lose their structural integrity during catalysis. Such structures get collapsed during electrochemical application due to undesirable phase segregation.52 As such, several strategies have been adopted to improve the durability of the NFs.56 Structural fortification is one of these strategies which involves growing of additional metals on the active but highly vulnerable vertices of the NFs.63 Kwon et al. reported the fortification of the vertices of a PtCu NFs by Co doping to enhance the structural robustness of the catalyst toward the ORR.64 Wu et al. also developed a trimetallic NFs catalyst via galvanic deposition of Au Islands on PtNi for enhanced durability.65 Ternary and quaternary multi-metallic alloy phases have also been sought to enhance stability.66– 68 The structural fortification strategy and the inconsistency in stability, observed in RDE, are issues that need to be addressed. These inconsistencies may depict the fledgling nature of this class of materials, as they are the most recent addition to the Pt-based catalyst family. It also suggests that there is more to be achieved in this field by further intensive research.


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Figure 3. (a-d) Schematic illustration for the evolution process from polyhedra to NFs. (e) ORR polarization curves (inset corresponding Tafel plots) of Pt3Ni frames before and after ADT. (f) Bright-field STEM image and (g) dark-field STEM image of Pt3Ni NFs/C after ADT. Reproduced with permission from ref 53 Copyright 2014 Science (h) Schematic illustration of the synthesis of hollow nanoframe (H-NF) and excavated nanoframe (E-NF). Reproduced with permission from ref 55 Copyright 2017 American Chemical Society.

3. STABILITY OF ALLOY AND CORE-SHELL STRUCTURES Theoretically, for a high electrochemical activity of Pt NPs, its binding strength towards intermediates, O*, HO* and HOO* should be reduced since the intermediates bind to Pt, causing dissolution of Pt. In particular, these intermediates usually accelerate the segregation of alloyed non-precious transition metals (NPTMs) from subsurface layer to the surface by directly interacting with the subsurface metal, and upon reaching the surface, the solute metal atoms tend to dissolve into the electrolyte. Alloying Pt with different metals can bring about the aforementioned changes.69 This process is not new, and alloys such as PtxTi or PtxCr were used in the cathodes of phosphoric acid fuel cells over 30 years ago.70–72 Taking a note from this, in the 1990s, PtxNi, PtxCo, and PtxCr based catalysts were reported as a cathode catalyst for PEMFC, with significantly improved ORR activity over pure Pt catalysts.73–75 This improvement in the ORR electrocatalysis of Pt alloys has been ascribed to structural changes caused by alloying, such as modifications in the geometrical (decrease of the Pt-Pt bond distance)76 or electronic (an increase of Pt d-electron vacancy)77 structure of Pt metal. Even though Pt alloyed catalysts showed great 12 ACS Paragon Plus Environment

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promises in improving the ORR kinetics compared with pure Pt catalysts, severe acidic (pH ~ 0) and oxidizing atmosphere of PEMFC have rendered most of the alloyed NPTMs unstable and unsuitable. Pourbaix diagrams indicate that most TMs such as Co, Cr, Fe, Ni, Mn, Cu, and V are soluble at a potential between 0.3 and 1 V versus SHE and at pH of around 0.0 which is a typical pH of the medium to which Pt alloys are subject with Nafion as an electrolyte in PEMFCs.78 Hence, the stability improvement of Pt and Pt alloy catalysts is found to be more challenging than improving their activity in the PEMFCs. Consequently, various research efforts have been involved in improving the stability of these Pt-alloys. In the following sections, we will review the findings of some basic research strategies that have been opted to improve the stability of Pt alloys.

3.1. Temperature Effect. Stability of Pt-alloys depends greatly on the conditions used for their preparation. It has been observed that the alloying extent between transition metal (TM) and Pt can severely influence the alloy stability. If not alloyed properly, a large part of the TMs will be present in the unalloyed form79 and therefore a remarkable dissolution of these metals in the oxide form will occur in the acid environment, eventually reducing the PEMFC durability. 78 In one study, Beard and Ross investigated the effect of Pt-Co alloying on the rate of Co metal dissolution and on the activity.80 The alloys were prepared by drying and carbonization of dissolved Co and Pt precursor mixtures at 700, 900 and 1200 0C under an inert atmosphere. As expected, as-prepared catalysts with least Pt and Co alloying showed the highest Co dissolution during the fuel cell test. Interestingly, in the carbonized catalyst, the rate of dissolution decreases with increasing carbonization temperature. Through XRD and XPS studies, it was confirmed that the carbonized sample at 1200 0C showed the highest Pt-Co alloying and least un-alloyed Co particle, due to which the dissolution of Co was minimum. Similarly, in a study conducted by Strasser et al. on the structure-activity-stability relationships of Pt-Co alloys, it was observed that the catalysts carbonized at 950 0C showed much better stability than the catalyst carbonized at 600 0C.81 In their study, face-centered cubic (fcc) Pt-Co alloy phases of various stoichiometry were obtained for the catalyst carbonized at 600 0C. The Co-rich fcc phase suffers from severe chemical and electrochemical corrosion. However, such an fcc phase is associated with the most favorable ORR active sites. On the other hand, in the catalyst carbonized at 950 0C, ordered face-centered tetragonal (fct) Pt-Co alloy phase was obtained. These less catalytically active fct Pt-Co phases showed better chemical stability in Nafion-containing electrode layers.81 Through these studies, authors have indicated that there exists a trade-off between catalyst activity and degradation in multiphase materials. In another study, the effects of the concentration of TMs on the stability of alloys were studied. Four TMs (Fe, Co, Ni, and Cr) were selected and the concentration of TMs was varied from 1 to 1/3, relative to the Pt concentration. As expected, alloys with lower TMs concentration Pt: M (3:1) showed the best stability, whereas highest activity loss was estimated for Pt: M 13 ACS Paragon Plus Environment

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(1:1).82,83 On the basis of this observation, it was concluded that 3:1 (Pt: M) is the optimum amount for the proper Pt and TM alloying, and the material prepared with higher TM concentration may lead to the improper alloying which might increase the concentration of unalloyed TM. To reduce the concentration of un-alloyed TMs, Gasteiger et al. proposed a preleaching procedure of the prepared alloy.84 This procedure led to the minimum contamination of the MEA owing to TM dissolution during operation. As a result, the MEAs based on pre-leached Pt alloy catalyst, stayed stable throughout the durability test, signifying no degradation of the membrane due to TM contamination.84 Through various characterization techniques, it was understood that in Pt alloys, the surface layer is almost always composed of pure Pt, and this process of transformation is independent of the concentration of TM.84–87 As non-precious TMs are thermodynamically unstable in the PEMFC working conditions, their dissolution is pronounced. Stamenkovic et al. denoted the non-precious TMs-leached Pt-TM alloy as a ‘‘Pt-skeleton’’ surface.86 In such structures, a disordered Pt overlayer of typically 1-2 nm thickness is usually observed.88,89 Pt overlayer can also be formed by vacuum annealing in an inert or reducing atmosphere. This methodology produces Pt alloy with “Pt-skin” structure, consisting of 1 nm thick Pt overlayer. The driving force for the formation of the Pt skin is ascribed to the lower surface energy of Pt, relative to the TM. Markovic et al. analyzed the stability of these near-surface structures, formed from the Pt alloy, in the perchloric acid and demonstrated that the surface with Pt-skin is much stable than one containing Ptskeleton.90 As the formation of Pt-skeleton occurs due to the dissolution of near-surface nonnoble metals, their lower stability is understandable. In addition, better Pt-skin stability implies that the ordered Pt surface acts as a protective layer for the atomic layers underneath.

Figure 4. Normalized surface area for different Pt/C and PtCo/C catalysts upon cycling in a PEMFC between 0.6 and 1 V at 20 mV s-1 and 80 0C. Reproduced with permission from ref 91. Copyright 2007 ElectroChemical Society. 14 ACS Paragon Plus Environment

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The most likely cause of the increased stability of Pt-skin based alloy is ascribed to the hightemperature annealing used in their preparation.91,92 Application of high temperature can cause Pt particles to sinter, which can increase its particle size and its ordering, which can stabilize the catalysts. Markovic et al. have also noted the presence of crystalline Pt (111) phase in the formed Pt-skin structure.90 Figure 4 shows the effect of cycling on the evolution of the normalized surface area for several different types of Pt/C catalysts, in comparison to a PtCo/C catalyst. It can be noted that the unannealed, well dispersed Pt/C, 2–3 nm showed the least surface area retention, whereas the stability of the catalysts improved once either the particle size increased or heat treatment was done. Clearly, subjecting the PtCo/C catalyst to a similar heat treatment to Pt/C provides it with the same degree of stability. Furthermore, it can be seen that the annealed Pt/C, with similar Pt particle size to that of unannealed Pt/C, shows better stability. It simply suggests that heat treatment improves the stability of the catalyst. Chorkendorff et al., in one of their review article, suggested that the annealing procedure could remove the defects or the undercoordinated sites most prone to corrosion, consequently reducing the propensity of the catalyst to corrode.93 In summary, the stability of the Pt alloys depends greatly on the integrity of the Pt overlayer and can be improved by pre-heat treatment.

3.2. Alloying with Transition Metals. Alloying with Late NPTM: Late non-precious transition metals belong to the group of metals which are present on the right side of the d-block, i.e. from group 8 to group 11. As we have discussed earlier, alloying non-precious metals can change the Pt surface energy, hence often making O2 reduction facile. However, even the wellalloyed catalysts don’t guarantee the improved stability of the catalysts in an acidic environment, compared with the state-of-the-art Pt/C. It has been observed that after cycling test, the mass activity of both Pt/C and PtCo alloys/C decreased slightly, whereas the specific activity of Pt/C remained the same or increased as opposed to that of the specific activity of the PtCo alloys. The decreased specific activity of Pt alloys clearly indicates the harmful impact of the leaching of TMs from the Pt alloy during ORR on the oxygen reduction capability of the catalyst. These observations further point towards the necessity of improving the stability of the alloying metal in the Pt alloys.94–98 Based on various studies done with different Pt-TM alloys, it was observed that the stability of Pt-TM alloy in an acid environment depends mainly on the type of TM used. Pt-Cr and Pt-Co were found to be more stable than Pt-V, Pt-Ni, and Pt-Fe.72,75,99,100 Gonzalez et al. proposed that stability of these catalysts depends on the degree of alloying of the TM, and the materials which can form better alloys with Pt (in this case Co and Cr) shows better stability. Nørskov et al. also suggested that the metals, which show negative alloy formation energy with Pt, have a tendency to better stabilize the Pt. Hence, such metals can improve Pt stability upon alloying.101 From these


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experimental annotations, it can be concluded that the extent of TM alloying with Pt is significantly important in dictating the stability of the alloy. The loss in the specific activity of Pt alloys in PEMFC working conditions has been ascribed to the dealloying of the NPTMs. The primary reason for extreme susceptibility of late NPTMs to the PEMFC oxidizing conditions is their lower oxidation potential.92,102,103 The PEMFC cathode operating potential is far above the dissolution potentials of the non-precious TM.104 Therefore, severe PEMFC operation condition oxidizes the TMs present at the catalyst surface, creating a concentration gradient between the surface and the catalyst core, which enables the diffusion of sublayer NPTMs to the surface, causing the segregation of the TM at the surface. This NPTM segregation from subsurface layer to the surface may be accelerated by direct interactions between adsorbed O* or HO* and the subsurface metal, and upon reaching the surface, the solute metal atoms will dissolve into the electrolyte.105,106 This vicious cycle of NPTM dissolution keeps on repeating until catalyst loses almost all of the alloying metal. As we have discussed earlier, the formation of crystalline Pt skin can stabilize the Pt alloy structure and might enhance the stability of the alloy in the acidic environment. Greeley and Nørskov showed that in Pt3Co (111), Pt3Ni (111) and Pt3Fe (111) alloys, the Pt skin could increase the stability.104 This improvement is found to be associated with the improved surface energy of bulk-terminated alloy surface. Therefore, it can be expected that the Pt skin structure should show much better stability than the Pt skeleton structure. However, as has been discussed earlier in a highly acidic environment and above 0.8 V, Pt also starts dissolving. This can create defects in Pt skin structure as well, hence exposing the underlying NPTMs. Therefore, to stabilize the alloying NPTMs it is highly desirable to use the metal, which has better stability in acidic condition and can sustain the PEMFC oxidizing condition even at high potential application. Given to the superior stability of Cr in the acidic environment and its better alloying extent than other late NPTMS, Hunsom et al. examined the effect of incorporating late NPTMS (Cr, Co, Ni) and Pd into the Pt lattice on PEMFC stability. Among all the studied catalysts, Pt-Cr based catalyst was found to outperform Pt/C and other NPTM based catalyst as far as activity and stability in PEMFC is concerned. It was observed that Pd once mixed with Pt showed much-improved activity, however, given to its lower stability then PtCr and higher cost, Cr-based alloys were chosen to be most optimum transition metals.107 Yu et al. have shown through dynamic fuel cell operation the dissolution propensity of Pt in Pt/C and Pt and Co in PtCo/C.94 It was observed that in Pt/C, due to smaller Pt particle size the catalyst showed good ECSA before cycling, but after 1200 cycles at 65 oC between 0.87-1.2 V a drastic decrement in ECSA was observed. Through cross-sectional SEM Images, it was observed that during cycling, the Pt particles tend to detach from the carbon support followed by the redeposition on the membrane. This phenomenon then leads to the ECSA decrement. However, interestingly in PtCo, no Pt deposition was observed on the Nafion membrane, even though a 16 ACS Paragon Plus Environment

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ACS Catalysis

great deal of Co loss was observed. This has then lead to lower ECSA loss even after 2400 cycles. It was later concluded that the large particle size of PtCo in PtCo/C lead to higher Pt stability in PEMFC conditions. Alloying with Early NPTM: Early transition metals are present on the left side of the periodic table i.e. from group 3 to group 7. Nørskov et al. filtered some of the most stable and active Pt alloys through theoretical screening study.83 The parameters for screening included 1) catalyst surface should have lower energy for the adsorption of oxygen and its intermediates, 2) catalyst should form Pt overlayer and 3) the alloy formation should be extremely stable. Figure 5a shows the main outcome of the study conducted by them. Here, the positive alloying energy (i.e. ∆E) indicates the unstable alloy, while the negative ∆E value points to the stable alloying materials. Furthermore, ∆GO* (is equal to ∆Eo - ∆EoPt in Figure. 5a) dictates the free energy for the adsorption of reactant and the intermediate. The alloys showing ∆GO* lower than Pt are considered to be favorable for ORR. As expected, it can be seen that Pt alloys with late NPTMs show optimal ∆GO* for efficient ORR, which is true as Ni, Co, and Cu-based Pt alloys are among the best-known ORR catalysts. However, the ∆E value of these catalysts are not optimum, and it can be seen that the solid solution of Pt and late NPTMs yields unstable alloy. This instability of alloy can explain the leaching out of late NPTMs from the Pt lattice during PEMFC operation. On the other hand, Pt3Y and Pt3Sc show great promises as far as lowering the energy barrier for O2 reduction and alloy formation is concerned. Both of these alloys show the most negative alloy formation energy among all the TMs studied.93


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Figure 5. (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 83 Copyright 2009 Nature Publishing Group. (b) Correlation between stability and heat of alloy formation, and between stability and absorption edge peaks of XANES spectra (∆APtM alloy/∆APt) of Pt3M (M = Y, Zr, Ti, Ni, and Co) catalysts. Reproduced with permission from ref 110 Copyright 2012 American Chemical Society.

The high stability of these early NPTM-based alloys was supposed to be the result of completely filled bonding state and empty antibonding state. Both Y and Sc are early TMs and have one d electron, whereas Pt is late TM and therefore has 9 d electrons. In the solid solution of both the metals, the metal-metal d-band will be half filled, corresponding to the filled bonding state and empty antibonding state. It is observed that the favorable heat of formation of Pt3Y and Pt3Sc is still not sufficient to make these alloys thermodynamically stable against oxidation


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or dissolution under the PEMFC conditions. However, it is expected to affect the kinetic stability of these compounds.88 Chorkendorff et al. demonstrated that even Pt skin structure is not stable in the Pt-Sc alloy and Sc is found to get segregated at the surface as scandium oxide. This was proposed to be due to the strong interaction between subsurface Sc and the oxygen molecule, which can induce only a small barrier for the Sc segregation.108 However, in Y-based Pt alloys, it was observed that due to the large covalent radius of Y, an overlayer of Pt is formed over the Pt-Y core. Very high activity and much better stability have been demonstrated on the Pt-Y alloy compared with other alloys.109 Followed by these findings, Yoo et al. showed that the sputter-deposited Pt-Y alloy showed very high stability in the three-electrode system, and the catalyst activity remained unchanged up to 3000 cycles, illustrating the direct correlation between the heat of formation and the stability of the alloy in the ORR working atmosphere.110 Eventually it was observed that the Pt3Y showed the lowest heat of formation and the highest stability among all the prepared catalysts. The superior stability of the Pt3Y-based catalyst was attributed to its smallest Pt d-band vacancy (Figure 5b). As the bulk electronegativity of Y (1.22) is considerably smaller than that of Pt (2.28), an electron transfer from Y to Pt can be expected. This electron density flow can decrease the d-band vacancy in Pt and can influence the dissolution potential of Pt and Y. Due to the low standard reduction potential of yttrium compared to Pt, the synthesis of carbon-supported PtxY catalysts is quite challenging. Extended electrochemical testing in actual PEMFCs remains elusive, especially with respect to catalyst degradation upon voltage-cycling. Gasteiger et al. in their study presented the facile synthesis of a bimetallic PtxY/C catalyst via yttrium halide precursor impregnation in the commercial Pt/C and their subsequent heattreatment in H2 at 1200 oC. The prepared catalyst showed a high specific ORR activity due to its comparably low ECSA, along with a similar mass activity to Pt/C. The as-prepared catalyst also showed much-enhanced stability in an accelerated stress test (AST) based on voltage-cycling between 0.6 and 1.0 vs RHE at 50 mV s−1, compared to a standard Pt/C catalyst. These improved attributes of the synthesized catalyst were assigned to the large particle size of the synthesized PtxY/C catalyst (≈10 nm), due to which the crystalline property of the catalyst was improved.111 Alloying with lanthanides: As has been discussed earlier in case of both late and early TMs, a Pt overlayer is bound to form and the stability of this layer depends greatly on the alloying energy of the underlying TMs. Chorkendorff et al. have systematically studied activity and stability trends of Pt5La, Pt5Ce, Pt5Sm, Pt5Gd, Pt5Tb, Pt5Dy, Pt5Tm, and Pt5Ca in RRDE configuration (23°C, 1600 revolutions per minute in O2-saturated 0.1 M HClO4). It was observed that alloys based on lanthanides exhibited activities almost 3 to 6 factors higher than Pt.112 The effect of cycling test on the stability of these lanthanides-based alloys is presented in Figure 6a. They found that the mean thickness of the Pt layer over all the catalyst was almost similar before the stability test. However, after stability test, the thickness increased from Pt5La to Pt5Tb. This provides the 19 ACS Paragon Plus Environment

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direct observation of the stability of La towards the dissolution during ORR. Furthermore, the cycling stability of these alloys was found to be governed mainly by the compressive strain on the Pt overlayer. The strain was used as a stability descriptor, and it was observed that stability decreases as the compressive strain increases. Lanthanum (La) with the smallest atomic radii among all the lanthanides infuses least compressive strain and does not destabilize the Pt overlayer, which might reduce the surface diffusion of the lanthanides from the bulk. Due to this factor, the La-based alloy showed the best stability among all the studied lanthanides.

Figure 6. (a) Kinetic current density, jk, of Pt5M and Pt, 1600 revolutions per minute in O2-saturated 0.1 M HClO4, before and after a stability test consisting of 10,000 cycles at 23°C between 0.6 and 1.0 V versus RHE at 100 mV s−1. Reproduced with permission from ref 112. Copyright 2016 Science. (b) Experimental enthalpies of formation specified per formula unit. Values for the different Pt alloys. Reproduced with permission from ref 113. Copyright 2014 Royal Society of Chemistry.

It can be seen from Figure 6b that the heat of formation for La and Ce are very negative, and there is an even stronger driving force towards the surface oxidation of La and Ce once exposed to air.113 However, it was observed through XPS that once the Pt-La or Pt-Ce alloy forms, a kinetic barrier against deep oxidation is introduced. In both the alloy catalysts, a thick Pt layer is bound to form. The formed, few monolayers of Pt overlayer, provides kinetic stability to the catalyst and the diffusion of La or Ce from the bulk to the overlayer would be very slow, due to the strong Pt– La and Pt–Ce interactions. The effect of higher La concentration in Pt-La alloys (Pt3La) is also studied and it was found that due to lower Pt concentration, the prepared catalyst tends to corrode during ORR.113 In summary, the aforementioned alloys of Pt with early, and late TMs and lanthanides in their polycrystalline form, in contrast to single crystalline form, owe their stability to the dissolution of 20 ACS Paragon Plus Environment

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the non-noble metals from the alloy surface under the ORR conditions.114 During this process, the electroactive non-noble metal can be dissolved out through the process of selective dissolution.115,116 Dealloying is much more prominent at polycrystalline surfaces, as the process takes place primarily at defect sites.116,117 Therefore, polycrystalline surfaces generally show much lower stability toward dealloying compared to ordered alloy surface.118 This results in a structure, which consists of an ordered Pt-rich shell (Pt-Skin) covering a Pt alloy core.116,119–121 However, it was observed that the stability of such alloys depend on the alloying energy and further dissolution of the non-noble metal from the alloy core. Due to low oxidation potential and high heat of formation, late TMs show the least stability in PEMFC, particularly in acidic media. However, early TMs and Lanthanides showed the least heat of formation and hence creates a stable alloy with Pt. Still, the driving force for the dissolution of the alloying metal was found to be very high under acidic media, but due to the formation of much stable Pt overlayer, these catalysts showed improved stability. Furthermore, the alloys based on La and Ce showed improved stability due to the strong Pt-La and Pt-Ce interactions present at the core of the alloy.

3.3. Core-shell Structure. Due to superior stability of ordered Pt overlayer on the Pt-alloy, researchers started to create core-shell structure consisting of Pt shell and a significant proportion of another noble metal, such as Ir, Au or Pd or non-noble metal core. It was observed that the alloying metals with lattice parameter larger than Pt, such as Au, induce tensile strain on the Pt overlayer, and increase its d-band center.122 Therefore, even though Au has high reduction potential and good chemical stability, the catalysts based on the alloy of Pt and Au are not suitable as they show very low activity. On the contrary, alloying metals with lower lattice parameter than Pt, such as Cu and Pd, induce compressive strain on Pt overlayer and consequently decrease its d-band center, which is supposed to increase the stability of the alloy. Interestingly, however, the impact on the activity is only minimal.122 Balbuena et al. investigated the surface segregation properties and trends for dissolution in acid medium and ORR activity of different core-shells consisting of a fixed monolayer of Pt shell and core based on 3d, 4d and 5d late TMs. In their study, they observed that the core consisting of pure Au, Ir and Ag were quite stable as far as the surface segregation is concerned. However, for the cores based on Cu, Ni and CO it was found that these metals tend to segregate on the Pt surface to cover up to 50% of the surface. After 50% coverage, the propensity of these metals toward segregation was found to reduce.123 In the presence of O2, the tendency for surface segregation was found to be enhanced for the 3d late TM-based cores. The mechanism of these metals segregation/dissolution in the presence of surface-adsorbed O2, O and OH has been discussed in the earlier section. However, the cores containing 4d and 5d late TMs showed much better resistance towards surface segregation. Even though cores containing 4d and 5d late TMs show better stability in ORR, their high price does not make them suitable as an alloying material candidate. To reduce the concentration of 21 ACS Paragon Plus Environment

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precious alloying metal, alloy of late NPTMs and Pd as a core and pure Pt as a shell was prepared, such as Pt on PdFe/C, Pt on PdCo/C, and Pt on PdNi/C.114,120,124,125 It was observed that usage of Pd and NPTMs alloy not only reduces the quantity of Pd used but also improves the stability of NPTMs in the ORR working atmosphere. Figure 7a and b show the ORR activity and stability of Pt on PdFe catalysts. It can be seen that the prepared catalyst shows extremely high stability for ORR in an acidic environment.115 The exact mechanism of this improvement is still unknown, but it was proposed that initial Pd dissolution from the core causes a contraction of the Pt overlayer due to which propensity of NPTMs and Pd towards further dissolution decreases.

Figure 7. (a) ORR polarization curves for three kinds of PdFe@Pt NP and the commercial Pt NP catalysts. The current was normalized against the total mass of NPs used, and the rotation rate was 1600 rpm. (b) Comparative ORR activities of 5 nm/1 nm PdFe@Pt NPs before and after 10,000 potential cycles. Reproduced with permission from ref 115. Copyright 2010 American Chemical Society. (c) massnormalized Tafel region of ORR measurement for Pt/C, Au@Pt/C, and AuCu@Pt/C catalysts in O2saturated 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. (d) Linear sweep voltammograms of AuCu@Pt/C before (black) and after (red) 30,000 cycles of stability testing. Reproduced with permission from ref 129. Copyright 2012 Royal Society of Chemistry.


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Apart from Pd, the alloys of Au with other late NPTMs are also reported.126–128 However, as we have discussed earlier due to the larger lattice parameter of Au than Pt, Au as a core material imparts tensile strain and hence is expected to show low ORR activity. It was found that the lattice parameters of Au could be manipulated by alloying it with different NPTMs, such as Cu, Ni, Co and Fe.129 Figure 7c and d shows the effect of alloying Au with Cu on the stability of the prepared core-shell particle catalyst. It can be seen that the prepared catalyst showed exceptionally high stability after the accelerated durability test in acidic media. The improved stability of such alloys was attributed to the inhibition of the surface oxide formation on the catalyst surface in the presence of Au and the improved stability of Pt shell in the presence of Au core, due to the significant d-orbitals coupling between Pt atoms and Au substrate.126,130

3.4. Surface Modification. Strmcnik et al. first coined the concept of Pt surface patterning in their system.131 They have patterned the Pt surface using the CN- ion. This model system was particularly adapted to inhibit the adsorption of secondary/spectator species (SO4 2− and PO4 3− anions in H2SO4 or H3PO4 solutions) on the Pt surface. It was proposed that the negatively charged CN- ions could repel the incoming SO42− and PO43− anions, and therefore, the poisoning effect caused due to the adsorption of such species could be greatly suppressed.131 Based on this concept, other researchers have also developed patterned Pt surfaces to achieve high selectivity and ORR activity.132–135 In addition to surface modification of Pt surface, the passivation of the alloying TM surface has been investigated for improved stability of the Pt-TM alloys. As we have discussed earlier, Ptbased alloys have practical limitations due to the high predisposition of NPTMs, which can be subject to surface segregation and dissolution. The electrochemical dissolution and dealloying effects are well known factors for fuel cell catalyst degradation. Researchers have realized that selective passivation of the TM surface in Pt-TM alloy can drastically reduce the inclination of the TMs towards oxidation and consequently its dissolution in acid electrolytes. To suppress the selective leaching of NPTMs during ORR, Lee et al. have synthesized the halide-treated octahedral PtNi NPs. It was observed that the halides (Cl, Br, I, and F) could adsorb on the Ni surface more strongly than on the Pt. Therefore, they can protect the Ni site from the dissolution in acidic media during ORR. Among all the halides, the effect of Br treatment was found to be most beneficial. It was observed that the Br-treated octahedral PtNi NPs maintained their activity and their structure even after 10K cycles at 0.6-1.1V as shown in Figure 8.136 Similarly, the effect of N coordination with Co metal present in PtCo alloy was studied by Jung et al.137 For this purpose, Poly(N-isopropylacrylamide) (PNIPAM) was chemically functionalized on the carbon surface, and the PNIPAM-functionalized carbon (C-PNIPAM) was then used as a support during PtCo alloy preparation. It was observed that the N atom in the amide group of PNIPAM selectively interacted with Co precursor than with Pt precursor. This interaction then led to the formation of Co−Nsur, which was found to be more resilient than the Co atom towards oxygen species 23 ACS Paragon Plus Environment

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adsorption. As a result, the obtained structure was found quite stable and the prepared catalyst maintained not only its activity but also its structure after 10K ADT cycles.

Figure 8. Improvement in PtNi alloy stability due to halide treatment. Reproduced with permission from ref 136. Copyright 2016 Elsevier.

Stability of the alloy material is also greatly influenced by the Pt overlayer crystallinity. It was found that the face-centered tetragonal (fct) structure of Pt alloy NPs shows higher durability for ORR compared with the face-centered cubic (fcc) structure. However, such crystalline Pt overlayers or alloys are obtained only after high temperature treatment, and due to such high temperature treatment, Pt alloy particles tend to get aggregated. To avoid such scenario and for obtaining well dispersed Pt alloy particles on a carbon support, Chung et al.138 have coated the alloy surface using N-doped carbon. The coating of Pt-Fe alloy was accomplished by dipping the alloy in polydopamine solution followed by carbonization at high temperature. High temperature carbonization led to the formation for fct-Pt alloy and the conversion of polydopamine to Ndoped carbon. In addition, this carbon layer on Pt alloy prevented the agglomeration of Pt alloy particles, and led to finely dispersed Pt alloy particles. It was found that such carbon shell impeded the adsorption of O2 on the catalyst surface, and hence the activity of the prepared catalyst significantly depended on the thickness of the carbon shell. Carbon shell of 1 nm showed the best activity for ORR and extremely high stability in acidic condition. The dissolution propensity of the NPTM alloys was also found to be reduced due to carbon shell coating. The MEA performance at 80 0C of the catalyst was also evaluated, and the catalyst has demonstrated 24 ACS Paragon Plus Environment

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a significantly improved activity and stability over its disordered PtFe/C and pristine Pt/C counterpart. The prepared catalyst has shown to perform for 100h without significant loss of its activity. The effect of sodium oleate was also studied on stabilizing the Pt alloy structure at high temperature carbonization. For this, Chung et al. have synthesized PtNi NPs using oleic acid and sodium borohydride (NaBH4).139 It was observed that due to the reaction between oleic acid and NaBH4, sodium oleate species is formed on the PtNi alloy surface. Thermal annealing of sodium oleate-coated PtNi NPs at 700 0C led to nicely dispersed PtNi NPs. It has been observed that the Pt degradation in both Pt/C and Pt alloy/C primarily depends on the crystallinity of the Pt outer layer. Smaller catalyst particles (Pt and Pt alloy) tend to degrade faster than larger particles, due to the presence of higher defect sites in the smaller particles. In nanoscale Pt alloys, due to thinner Pt overlayer and resulting poor crystallinity of the Pt shell, the dissolution of non-precious metal becomes prominent due to higher oxidation potential of nonprecious metals (early and late transition metals and Lanthanides) compared to the Pt. Due to this, the possibility of activity decay is higher in nano-sized Pt alloys as compared to pure Pt nanoparticles with similar size. However, through nano architecturing, scientists have tried to develop the methods to keep the crystalline Pt overlayer of the Pt alloy catalyst intact, and in such case, the stability of the Pt alloys was found to be much better than the pure Pt particles of almost similar size.138 Benefit of alloying Pt with other non-precious metals becomes evident at the large scale (such as films) since in large Pt alloys, the formation of Pt overlayer is quite prominent, due to which the dissolution/oxidation of underlying non-precious metals is reduced. Apart from this, such alloying also reduces the binding strength of Pt towards intermediates, O*, HO* and HOO*, and hence the oxidation of Pt surface is further delayed and restricted. Due to these factors, the stability of catalyst containing large Pt alloy particles can be improved better than the catalyst containing large Pt particles with similar size. It should be also noted here that even though these alloy catalysts and the core-shell counterparts of the Pt/C catalyst have shown great promises in maintaining the activity even after hours of working in PEMFC, their applicability is restricted during the shut-down and startoff conditions of the PEMFC. Especially in such conditions, the potential at the cathode can reach up to 1.8 V, and hence almost all the TMs can be at risk of being oxidized. Apart from catalyst activity, the durability of the catalyst also affects the functioning of the membrane. It has been reported that the dissolved NPTM cations can interact with the sulphonic groups of the Nafion and can decrease its conductivity, which eventually will lead to poor cell performance.75 Hence, the NPTMs incorporation into Pt lattice have its own advantages and disadvantages, and the choice of the alloying metal should be done very cautiously.

4. STABILITY OF Pt SUPPORT


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Although unsupported Pt NPs can be used in the fuel cell in the form of Pt black, the development of highly dispersed Pt on various porous carbon supports brought considerable progress in fuel cell electrocatalysts.140-144 The basic requirements for a viable support material for Pt NPs, include but not limited to excellent electronic conductivity, high surface area, chemical and electrochemical stability, and strong metal support interaction.141-144 Carbon black has frequently been used, but recently, numerous alternatives including modified carbon materials (heteroatom-doped, graphitized, core-shell, NTs), non-carbon materials (metal oxides, nitrides, carbides) and hybrid nanomaterials have been developed as effective supporting materials to attain the desired stability of Pt-based catalyst.144-148 Here, we discuss recent advances in using carbon and non-carbon support materials with intrinsic resistance to the harsh and detrimental ORR conditions and their ability to stabilize Pt during ORR electrocatalysis.

4.1. Carbon-Based Support. The low price and availability of carbon black have made them popular among support materials for Pt NPs. They are available with different surface area, conductivity as well as surface functionality. Table 1 presents a summarized list of commonly used carbon blacks and their respective properties.149 Among them, Vulcan carbon possesses the most defective surface with abundant organic surface groups (CO, COOH, and CN).143 This property of Vulcan carbon enhances particle dispersion and therefore makes it the most frequently-used Pt support at lab scale research, and also for some commercialized Pt/C catalyst.150,151 Whiles the surface defect is desirable for particle dispersion and activity enhancement, it is rather detrimental to the catalyst durability as it enhances the thermochemical instability. Kim et al. based on spectroscopic characterization, revealed that oxygen functionalization in the Pt/C catalyst support can improve initial ORR activity, but exerts an adverse effect on its long-term durability. The electrons from the Pt NPs could be withdrawn by highly electronegative neighboring oxygen group, which can partially oxidize the Pt NPs, and trigger its dissolution and Ostwald ripening (Figure 9a).152 Furthermore, Speder et al. witnessed significant carbon corrosion and ECSA loss in Vulcan samples compared with Ketjenblack samples after 15 h ADT treatment.153 Such high corrosion on Vulcan carbon was attributed to the high number of defect sites. A comparative study on Black Pearls 2000 and Vulcan XC72 by Wang et al. also indicated more significant Pt dissolution and growth in Black Pearls 2000 support due to its lower corrosion resistance.154 Through these studies it was concluded that the defect densities in carbon support can decrease its electrochemical stability. Hence, the graphitization of carbon support can improve the corrosion resistance of Pt/C catalyst in fuel cell application.155–157


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Table 1. Common carbon blacks employed as Pt catalyst support Carbon

Type of carbon

BET surface area(m2/g)

DBP* adsorption

Supplier

Vulcan XC72

Furnace black

250

190

Cabot Corporation

Ketjen EC300J

Furnace black

800

360

Ketjen Black International

Ketjen EC600JD

Furnace black

1270

495

Ketjen Black International

Black Pearls 2000

Furnace black

1500

330

Cabot Corporation

*DBP: dibutyl phthalate number (a measure of carbon void volume)

Figure 9. (a) Schematic illustrations of changes in Pt particle size distribution before and after the ADTs. Reproduced with permission from ref 152 Copyright 2016 Elsevier Ltd) (b) Pt 4f XPS spectra of Pt/SH-CNTs, Pt/COOH-CNTs and Pt/pristine-CNTs catalysts, (c) Normalized Pt ECSA of Pt/SH-CNTs, Pt/pristine-CNTs and Pt/COOH-CNTs catalysts after CV cycling in the potential range of 0 – 1.2 V vs. RHE in N2-purged 0.5 M H2SO4 at room temperature. Reproduced with permission from ref 170 Copyright 2011 The Royal Society of Chemistry.

Graphene is a two-dimensional sheet of carbon atoms chemically bonded in the hexagonal pattern typical of graphite which is an allotrope of carbon. It is endowed with all the requirements of an ideal Pt support, including high surface area, electronic conductivity, and electrochemical stability. Higgins et al. reported a comprehensive review of graphene for ORR.158 Although 27 ACS Paragon Plus Environment

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graphene has outstanding features for support material, its pristine surface is not favorable for depositing well-dispersed, uniformly sized Pt NPs. This means graphene surface functionalization is required to facilitate good NPs dispersion.159,160 Graphene can be functionalized by doping with heteroatoms (N, P, B, S, and Se).3,145 Based on computational studies, Tian et al. showed that due to strong hybridization between Pt orbitals and dangling bonds at the N-doped graphene (NG) (N with higher electronegativity than carbon) defect sites, the stability of the Pt/NG catalyst can be enhanced.161. Doping of graphene by P/B (lower electronegativity than carbon) or S/Se (similar electronegativity to carbon) has also recorded similar improvement in durability of Pt catalyst.3,145,146,158,162–166 Carbon NTs, quite similar in properties to graphene, are cylindrical rolled layers of graphene. They are generally categorized into single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). Their purity, conductivities, corrosion resistance, mechanical and electrochemical properties are better compared to commonly used carbon blacks. Their high curvature, however, renders their surface extremely inert with few binding sites for anchoring the NPs even relative to graphene. Heteroatoms doping such as N, P, S, B and F atoms has been used to enhance the adsorption of NPs to CNTs surface with the effect being similar to graphene.146,162–164,167–169 Introduction of oxygen-containing functional groups (CO, COOH, and OH) and defects on the CNTs surface can also activate the surface. The presence of these defect sites and functional groups destroys the graphitized CNT surface, hence reducing the conductivity whiles significantly accelerating the corrosion during ORR in a manner analogous to carbon black mentioned earlier. Careful selection of surface functional group has, however, eliminated this shortcoming with the thiol (SH), amine (NH2), and aniline(Phenylamine) groups being the best candidates for enhanced stabilization of Pt NPs.170–175 Chen et al. confirmed the strong interaction between Pt NPs and SH functionalized CNTs (SH-CNTs) through X-ray photoelectron spectroscopy (XPS). Pt 4f (XPS) spectrum of the Pt/SH-CNTs catalyst (Figure 9b) shows a shift in the Pt 4f7/2 peak to higher binding energy as compared with those for the Pt/pristine-CNTs and Pt/COOH-CNTs. This shift is attributed to the strong ligand effect between Pt NPs and SH-CNTs. Consequently, the S 2p peak of Pt/SH-CNTs shifted toward lower binding energy. Such Pt/SHCNTs displayed the best durability with the Pt/COOH-CNTs displaying significant degradation (Figure 9c), confirming the adverse effect of oxygen-containing functional groups (CO, COOH, and OH) on stability.170 Subsequent computational and experimental study on Pt/SH-CNTs and Pt/OHCNTs by Li et al. also corroborated the previous findings.171 The DFT study indicates that the –SH group enhances the oxidation resistance of the Pt cluster and the CNTs and restricts the Pt migration on the CNTs by depressing the d-band center of the Pt whiles increasing interaction between Pt and SH-CNTs. Experimentally, they found that the Pt on OH-CNTs has a stronger tendency to aggregate than the Pt on SH-CNTs. However, carbon-based supports themselves do not provide total immunity to carbon corrosion, and hence other non-carbon based support materials have also been investigated. 28 ACS Paragon Plus Environment

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4.2. Non-Carbon Based Support. Non-carbon materials can offer robust and corrosion resistance support needed for extended operation of PEMFC. To date, none of the studied carbon-based supports have been able to alleviate completely the electrochemical carbon corrosion during extended operation and repeated PEMFC cycling. Over the past few decades, various transition metal oxides, transition metal nitrides, and transition metal carbides/borides have been exhaustively explored as non-carbon Pt catalyst supports, due to their high corrosion resistance and strong metal–support interaction. Among the various metal oxides, only limited materials such as TiO2, CeO2, Ta2O3, WO3, MoO3, and SnO2 display the requisite chemical stability suitable for PEMFCs conditions.176–181 This choice is, however, narrowed when high electrical conductivity is required. TiO2, nontoxic and cheap oxide, known for its photocatalytic properties has been considered as PEMFC catalyst support due to its relatively high electrical conductivity compared to other oxide materials.182 Huang et al. examined the electrochemical stability and performance of mesoporous TiO2 supported Pt NPs (Pt/TiO2) and state-of-the-art commercial Pt/C (Tanaka) in an MEA at 80 oC by holding the cell potential at 1.2 V for 200 h and 80 h, respectively (Figure 10a and b).183 While the Pt/C electrocatalyst showed a severe decrease in performance after 80 h due to carbon corrosion and subsequent detachment and agglomeration of Pt particles, the Pt/TiO2 displayed unaltered performance over the entire 200 h test period due to the superior stability of the TiO2 support. It has long been reported that TiO2 can anchor Pt NPs due to the strong metal-support interaction between the Pt NPs and the TiO2 support, thereby inhibiting Pt migration and agglomeration.184 As mentioned earlier, electron conductivity is very important and to widen the range of applicable metal oxides, doping procedures have been adopted to meet the required electrical conductivity. Esfahani et al. developed a multifunctional titanium suboxide doped with Mo and Si (Ti3O5Mo0.2Si0.4) simply referred to as TOMS with remarkably high electronic conductivity for a metal oxide.180 The prepared Pt/TOMS catalyst displayed excellent activity towards ORR and extraordinary durability in ADT, losing only 10% of its active surface area over 5000 cycles. The enhanced activity and durability is attributed to strong electronic interaction between the Pt NPs and the TOMS support.180 Several reports have indicated enhanced conductivity and stability on Ta185, Mo186, Nb187, and Ru188 doped TiO2. Ioroi et al. prepared a sub-stoichiometric titanium oxide Ti4O7 with high electrical conductivity and demonstrated the inherent oxidation resistance of the Ti4O7 support material in an MEA operated at 80°C.189 Only a small anodic current was observed for Ti4O7 even up to 1.8 V. On the other hand, conventional Vulcan XC-72, recorded a rapid increase in anodic current at potentials beyond 0.9V clearly indicating that the Ti4O7 support is more oxidation-resistant under PEMFC operation conditions (Figure 10c).189 Even though Pt corrosion is inversely proportional to Pt NPs size, recently Mohamed et al. observed good stability, under PEMFC relevant conditions, for small Pt NPs (2.5–3.5 nm), supported on a commercial antimony-doped tin oxide (ATO) nano-powder after ADT in RDE at 29 ACS Paragon Plus Environment

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room temperature in a potential range of 0.5 – 1.5 V vs RHE.179 It was later revealed that an electrochemical transistor effect in ATO, where a potential-dependent switching of metal oxide surface conductivity occurs can protect supported Pt NPs at high potentials. Strasser group also observed that Pt agglomeration, particle growth, dissolution, or detachment is not the cause for observed losses in catalytic ORR activity of Pt supported on ruthenium− titanium mixed oxide (RTO). Instead, Pt surface is poisoned by the gradual growth of a thin oxide layer, which was found to be impermeable for O2, on the Pt NPs due to strong metal-support interaction.190

Figure 10. PEMFCs Polarization curves measured at 80 oC for (a) Pt/TiO2 and (b) Pt/C electrocatalysts after the ADT protocol for 0-200 and 0-80 h, respectively. Reproduced with permission from ref 183 Copyright 2009 American Chemical Society. (c) Cyclic voltammograms of the Ti4O7 and XC-72 electrodes without Pt loading bonded to a Nafion membrane in a PEMFC single cell configuration under flowing N2 Reproduced with permission from ref 189 Copyright 2004 Elsevier B.V

Transition metal nitrides are another class of interesting support materials for Pt. They have high thermal stability with a high melting point, electrochemical stability, and exhibit exceptional corrosion resistance under PEMFC operation conditions.191 They are metallic in nature which gives them excellent conductivity and simultaneously exhibits great catalyst–support interactions, affording electrocatalysts with high ORR activity and good electrochemical stability.191 Pan et al. developed a flexible 1D porous TiN nanotube (NTs) as a support for Pt NPs.192 ADT cycles between the potential of 0.6 to 1.05 V in 0.5 M H2SO4 solution revealed that the TiN support can enhance the durability of the catalyst and maintain the ECSA of Pt. While the final ECSA of Pt/C (E-TEK) dropped by 71% of the initial value after 12,000 cycles, the Pt/TiN showed only 23% loss of the initial ECSA.192 Earlier work by Soon and co-workers utilized firstprinciple DFT calculations to investigate TiN supports for single-atom Pt catalysts. It was found that under typical PEM fuel cell operational conditions, i.e. strongly oxidizing conditions, TiN surface vacancies play a crucial role in keeping the Pt atoms anchored.193


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Transition metal carbides (TMCs) have similar structures and physicochemical properties to their nitride counterparts. However, a couple of differences exist such as the stoichiometry and coordination of the lattice, mainly dictated by the different stable oxidation state for C (–4) and N (–3). N has a higher electronegativity than C and as a result, the magnitude of electron transfer between the C/N and metal is different.145 Interestingly, the similarity in electronic structures of TMCs and noble metals near the Fermi level, can promote electron transfer between the catalyst and its support to enhance the stability of supported Pt NPs and simultaneously enhance its intrinsic activity for the ORR.194 Kimmel et al. studied the electrochemical stability of various TMCs to identify their potential use in electrochemical and photo-electrochemical applications.195 They found that generally, all of the TMCs are stable for HER/HOR, and most are stable for alcohol oxidations but for the ORR or OER, only TaC, TiC, and ZrC show stability.195,196 Later, Wei and coworkers developed an advanced surface Al-leached Ti3AlC2 particles (e-TAC) with excellent electrical conductivity as support material for Pt NPs (Figure 11a).197 Electrochemical measurements confirm that the supported Pt/e-TAC electrocatalyst shows much-improved activity and enhanced durability toward the ORR when compared with the commercial Pt/C catalyst (Figure 11 b). DFT calculations conducted with a Pt13 cluster loaded on the Ti3C2 and C indicated a stronger interaction between Pt13 and Ti3C2 compared to that between Pt13 and C.197 The partial density of state (PDOS) of Pt/Ti3C2 and Pt/C before and after adsorption also confirmed the strong interaction between Pt and Ti3C2 with a more considerable overlap near the Fermi level between the Pt-d states and the Ti-d states. Various reports have, however, indicated severe electrochemical instability of the supposedly stable TMCs under the ORR condition.194,195,198,199 Even though highly resilient non carbon-based supports have been developed, their surface area and porosity are still not comparable to the carbon based supports. These make it challenging to deposit well dispersed Pt particles and hence affect the overall catalyst performance.


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Figure 11. (a) Schematic of the Pt/e-TAC catalyst formation. (b) Normalized Pt ECSA of Pt/e-TAC and Pt/C catalysts as a function of the number of CV cycles. Reproduced with permission from ref 197 Copyright 2014 The Royal Society of Chemistry.

4.3. Composite/Hybrid/Special Support. A critical look at the different classes of Pt support materials makes it obvious that each specific system has its own drawbacks. Merging different classes of materials in the form of composite and hybrid affords some synergistic effect to overcome the drawbacks of individual systems.147,182,199–202 Kou et al. reported the stabilization of Pt at the unique triple-junction hybrid structure (Pt-ITO-graphene).203 They initially synthesized indium tin oxide (ITO) nanocrystals directly on functionalized graphene sheets, forming an ITOgraphene hybrid. After that, Pt NPs were deposited, forming the unique triple-junction structure (Pt-ITO-graphene). The graphene sheets acted as a scaffold that provides the high surface area and greatly increases the electrical conductivity, whereas the ITO NPs were evenly dispersed and protected graphene from corrosion, improving the durability of the substrate. Periodic-DFT calculations reveal that the deposition of Pt NPs is thermodynamically favored and hence, stable at the metal oxide-graphene junctions (Figure 12a-d).203 Experimentally, the Pt-ITO-graphene after ADT, showed only a 14 mV degradation in half-wave potential whereas Pt-graphene displayed a 40 mV degradation. Pt-ITO-graphene retained 77.8% of its initial mass activities after ADT, whereas Pt-graphene could retain only 33.3%, indicating a significant improvement in stability as well as activity for the hybrid Pt-ITO-graphene structure. Post ADT TEM analysis shows significant agglomeration of Pt NPs in Pt-graphene, but much less Pt agglomeration in Pt-ITOgraphene.


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Figure 12. (a-d) The top views (left) and side views (right) of (a) Pt6 cluster on graphene (b) ITO (In9Sn1O15) cluster on graphene (c) Pt6−ITO−graphene triple junction structure (d) Pt6 cluster on ITO (222) surface. Reproduced with permission from ref 203 Copyright 2011 American Chemical Society.

Li et al. demonstrated that the carbon-carbon composite catalytic structure of reduced graphene oxide(RGO) and carbon black (CB) can dramatically enhance ORR activity and stability compared with the simple RGO or CB support.204 ADT cycling between 0.6 and 1.1 V in 0.1 M HClO4, which was exposed to the atmosphere, showed that the Pt/RGO/CB loses only 5% of its ECSA after 20 000 cycles, while the commercial JM Pt/C catalyst loses almost 50% of its ECSA after the same number of cycles. They further suggested that the flexible 2D profile of RGO might function as a “mesh” that prevents leaching of dissolved Pt species into the electrolyte, while the CB can serve as an active site for recapture or re-nucleation of small Pt clusters.204 Silicon carbide (SiC) is a popular material with many desirable properties, including high thermal conductivity and stability in acidic and oxidative environments.205–207 However, they lack the electronic conductivity required for use in the PEMFC. By the addition of carbon, Lv et al. increased the electrical conductivity of SiC support.208 Firstly, homogeneously dispersed Pt NPs were deposited on SiC (Pt/SiC) using an ethylene glycol reduction method then the carbon (Vulcan XC-72) was introduced into this Pt/SiC catalyst to form the hybrid Pt/SiC/C catalyst. After 1000 cycles of ADT test, the ECSA loss of Pt/C is 68.5%, whereas the Pt/SiC/C has only degraded 42.7%. Interestingly after 4000 cyclings, only 8.9% of the initial ECSA of Pt/C remained, whereas 22.5% of the initial ECSA for Pt/SiC/C was retained. It is evident that the degradation rate of the Pt/SiC/C is lower than that of the commercial Pt/C. Similar synergy has been observed in other hybrid and composite systems.209–214 The present review article covers and compiles various methodological aspects, which directly/indirectly influence the stability/durability of Pt-based catalysts for PEMFC. The low ORR kinetics problem associated with the pristine Pt catalyst can be partially overcome by alloying Pt with other transition metals (TMs) or lanthanides. However, in the race of achieving better activity, the stability of the catalyst was compromised. Due to high oxidation potential of these metals, the Pt alloys suffer from weak durability and thus, their applicability in PEMFC working conditions is limited even though they show excellent ORR activity. Through various studies, it was observed that the stability of these Pt-alloys was mainly governed by the extent of alloying and the crystallinity of the Pt overlayer, and therefore the proposed core-shell structure with crystalline Pt shell and metal core has shown great promises in improving the catalyst stability in the PEMFC. Similarly, carbon-based supports can offer high conductivity, porosity and surface area necessary to achieve high catalytic performance but they are susceptible to corrosion at higher potentials rendering them less stable relative to non-carbon based supports. Pt nanowires, which offer high contact area on the support, can display better catalytic stability although they have less catalytic surface area and thus reduced Pt utilization because majority of 33 ACS Paragon Plus Environment

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the metal atoms are buried deep within the wires. Reduction in nanowire thickness to enhance Pt utilization can reduce the stability. Ideally, nanorod and nanotube types are better choices for stability as well as activity. It can be understood that there exists a trade-off between the stability and the activity. With the current review, a generalized pattern can be carefully drawn and more clear directives can be provided for the scientists. Figure 13 summarizes the methods for each of three categorized issues in catalysts and supports regarding the stability of Pt-based electrocatalysts. As a rough measure, for all three sections, the activity is generally increasing from first point and becomes maximum at the last one. Similarly, the stability is decreasing from the first one to the last, illustrating an inverse relationship between activity and stability.

Figure 13. Three categorized issues regarding the stability of Pt-based electrocatalysts.

5. SUMMARY In the present review article, various strategies adopted for attaining a stable Pt-based ORR catalyst for PEMFC, have been discussed. In the scope of this review article we have tried to cover shape-controlled Pt catalyst, Pt alloy and core-shell catalyst, and the supports for these catalysts. In summary, it has been shown that shape control of Pt nanomaterials can optimize the surface reactivity of Pt, thereby regulating the rate of Pt dissolution and eventually Ostwald ripening. Moreover, the shapes that provide more contact area and degree of interaction between the metal and support can prevent particle detachment from the support, hence resulting in less migration and agglomeration. Since the potential of a detached and isolated platinum particle is equivalent to open circuit voltage (OCV)31, detached and isolated NPs are more easily dissolved to form ionic species. As such, large contact area between support and metal can eventually 34 ACS Paragon Plus Environment

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improve metal resistance to oxidation. A newly burgeoning class of Pt shapes, i.e. nanoframes and nanocages, can offer excellent stability and high Pt utilization due to uniform crystal surfaces and three-dimensional (3D) molecular accessibility. Their final physicochemical properties depend on the initial polyhedra from which they were derived and the degree of post-synthesis treatment. However, there is currently little concrete evidence of the RDE-based stability of shape-controlled nanostructures in the MEA which is a major challenge that needs to be addressed. Further studies are also required to delineate their structural integrity. The lower ORR kinetics problem associated with the pristine Pt catalyst was subdued by alloying Pt with other NPTMS or lanthanide. However, in the race of achieving the better activity, the stability of the catalyst was compromised. Through various studies, it was observed that the stability of these Pt alloys was mainly governed by the extent of alloying and the crystallinity of the Pt overlayer. It was perceived that most of the alloying materials present on the catalyst surface tend to oxidize and hence dissolve during PEMFC. This dissolution process leaves behind an overlayer of pure Pt atom and the subsurface with alloy material. Therefore, the NPTMs, having a poor alloying tendency (late NPTMs), can etch from the Pt matrix during PEMFC operation. The movement of such species from the Pt subsurface was primarily depended on the reactivity of these species towards O2 and its intermediates. The movement of NPTMs from the subsurface was also affected by the crystallinity of the Pt overlayer. The crystalline overlayer showed superior stability than the poor crystalline counterpart. Among all the studied catalysts, early TMs and lanthanides showed the most negative heat of formation and consequently the formed alloy demonstrates the best stability. It was observed that the Y and La-based Pt alloys showed the least deactivation during ORR due to their superior alloying. Due to superior stability of ordered Pt overlayer on the Pt alloy, researchers have adopted a technique to grow crystalline Pt overlayers over a core containing a significant proportion of another noble metals, such as Ir, Au or Pd or non-noble metals. It was observed that the cores containing 4d and 5d late TMs showed much better resistance towards surface segregation and much better stability in PEMFC. However, the precious nature of the 4d and 5d late TMs make them an inappropriate candidate for the PEMFC. To circumvent this situation, researchers have devised the methodology to incorporate both NPTMS and 4d and 5d late TMs alloy at the core and ordered Pt shell. It was observed that usage of 4d and 5d late TMs and NPTMs alloy not only reduces the quantity 4d and 5d late TMs but also improves the stability of NPTMs in the ORR working atmosphere. Even though such core-shell catalysts display very high stability, the usage of precious metal renders them inappropriate for PEMFC. The high propensity of NPTMs for surface segregation and dissolution have led the researchers to adopt the surface patterning technique. Through these techniques, researchers have realized the selective passivation of TMs surface in the Pt-TM alloy, which can significantly reduce the predisposition of TMs towards oxidation and consequently its dissolution in water or 35 ACS Paragon Plus Environment

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acid electrolytes. This surface patterning technique has found to be quite useful in improving not only the stability of the catalyst but also its activity. Even though the intrinsic stability of Pt and its alloys have been significantly improved, the substrate on which they are supported (carbon-based, non-carbon based, and composite or hybrid mixtures) significantly affects the overall stability of Pt catalyst. Carbon black is popular due to its low price, high surface area and easy availability. Its surface is however endowed with various defects and oxygen-containing functionalities (CO, COOH, and OH), which although desirable for high Pt dispersion and improved catalytic activity, can accelerate carbon degradation during electrochemical cycling. In view of these, graphene and carbon NTs with a more pristine surface and fewer defects have shown improved durability over the conventional carbon black. It was also revealed that heteroatom (N, P, B, S) doping and surface functionalization (thiol [SH], amine [NH2], and aniline[Phenylamine]) of the carbon-based supports significantly enhance metal support interaction by changing the electronic structure of the carbon irrespective of the electronegativity of the doping element. This resulted in reduced Pt migration on the support and hence higher catalyst stability. Nevertheless, the inherent oxidative nature of the carbon-based supports in the presence of Pt has led researchers to noncarbon materials, which are more corrosion resistant. Among them, significant advancement has been made with metal oxides, nitrides and carbides. These materials, however, despite being highly resistant to corrosion, lack the requisite conductivity and surface area required for excellent fuel cell performance. In order to develop the ideal support, researchers have explored the synergistic combination of carbon and non-carbon based supports. It would, however, be advantageous to develop a highly conductive non-carbon support with surface area, porosity and conductivity comparable to that of the commercial carbon black. This would go a long way to advance the field of ORR electrocatalysis.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. ORCID Jong-Sung Yu: 0000-0002-8805-012X Emmanuel Batsa Tetteh: 0000-0002-5241-9976

Author Contributions 36 ACS Paragon Plus Environment

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The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. K.P. Singh and E. B. Tetteh contributed equally. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was generously supported by Global Frontier R&D Program on Centre for Multiscale Energy System (NRF-2011-0031571) and NRF grant (NRF 2016M1A2A2937137) funded by the Ministry of Science, ICT & Future Planning.

REFERENCES (1)

Takahashi, I.; Kocha, S. S. Examination of the Activity and Durability of PEMFC Catalysts in Liquid Electrolytes. J. Power Sources 2010, 195, 6312–6322.

(2)

Tran, T.-N.; Song, M. Y.; Singh, K. P.; Yang, D.-S.; Yu, J.-S. Iron–Polypyrrole Electrocatalyst with Remarkable Activity and Stability for ORR in Both Alkaline and Acidic Conditions: A Comprehensive Assessment of Catalyst Preparation Sequence. J. Mater. Chem. A 2016, 4, 8645–8657.

(3)

Zheng, Y.; Yang, D.-S.; Kweun, J. M.; Li, C.; Tan, K.; Kong, F.; Liang, C.; Chabal, Y. J.; Kim, Y. Y.; Cho, M.; Yu, J.-S.; Cho, K. Rational Design of Common Transition Metal-Nitrogen-Carbon Catalysts for Oxygen Reduction Reaction in Fuel Cells. Nano Energy 2016, 30, 443–449.

(4)

Department of Energy. DOE Technical Targets for Polymer Electrolyte Membrane Fuel Cell Components https://www.energy.gov/eere/fuelcells/doe-technical-targets-polymerelectrolyte-membrane-fuel-cell-components. (Accessed: January 24, 2019).

(5)

Darling, R. M.; Meyers, J. P. Kinetic Model of Platinum Dissolution in PEMFCs. J. Electrochem. Soc. 2003, 150, A1523.

(6)

Sui, S.; Wang, X.; Zhou, X.; Su, Y.; Riffat, S.; Liu, C. A Comprehensive Review of Pt Electrocatalysts for the Oxygen Reduction Reaction: Nanostructure, Activity, Mechanism and Carbon Support in PEM Fuel Cells. J. Mater. Chem. A 2017, 5, 1808–1825.

(7)

Ferreira, P. J.; la O’, G. J.; Shao-Horn, Y.; Morgan, D.; Makharia, R.; Kocha, S.; Gasteiger, H. A. Instability of Pt∕C Electrocatalysts in Proton Exchange Membrane Fuel Cells. J. Electrochem. Soc. 2006, 152, A2256.

(8)

Dam, V. A. T.; de Bruijn, F. A. The Stability of PEMFC Electrodes. J. Electrochem. Soc. 2007, 154, B494. 37 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 55

(9)

Ferreira, P. J.; Shao-Horn, Y. Formation Mechanism of Pt Single-Crystal Nanoparticles in Proton Exchange Membrane Fuel Cells. Electrochem. Solid-State Lett. 2007, 10, B60.

(10)

Yasuda, K.; Taniguchi, A.; Akita, T.; Ioroi, T.; Siroma, Z. Platinum Dissolution and Deposition in the Polymer Electrolyte Membrane of a PEM Fuel Cell as Studied by Potential Cycling. Phys. Chem. Chem. Phys. 2006, 8, 746–752.

(11)

Xie, J.; Wood, D. L.; Wayne, D. M.; Zawodzinski, T. A.; Atanassov, P.; Borup, R. L. Durability of PEFCs at High Humidity Conditions. J. Electrochem. Soc. 2005, 152, A104.

(12)

Strasser, P.; Gliech, M.; Kuehl, S.; Moeller, T. Electrochemical Processes on Solid Shaped Nanoparticles with Defined Facets. Chem. Soc. Rev. 2018, 47, 715–735.

(13)

El-Sayed, M. A. Some Interesting Properties of Metals Confined in Time and Nanometer Space of Different Shapes. Acc. Chem. Res. 2001, 34, 257–264.

(14)

Narayanan, R.; El-Sayed, M. A. Shape-Dependent Catalytic Activity of Platinum Nanoparticles in Colloidal Solution. Nano Lett. 2004, 4, 1343–1348.

(15)

Narayanan, R.; El-Sayed, M. A. Changing Catalytic Activity during Colloidal Platinum Nanocatalysis Due to Shape Changes: Electron-Transfer Reaction. J. Am. Chem. Soc. 2004, 126, 7194–7195.

(16)

Vidal-Iglesias, F. J.; Solla-Gullón, J.; Rodrı́guez, P.; Herrero, E.; Montiel, V.; Feliu, J. M.; Aldaz, A. Shape-Dependent Electrocatalysis: Ammonia Oxidation on Platinum Nanoparticles with Preferential (1 0 0) Surfaces. Electrochem. Commun. 2004, 6, 1080– 1084.

(17)

Solla-Gullón, J.; Vidal-Iglesias, F. J.; Feliu, J. M. Shape Dependent Electrocatalysis. Annu. Reports Sect. “C” Physical Chem. 2011, 107, 263.

(18)

Xiong, Y.; Wiley, B. J.; Xia, Y. Nanocrystals with Unconventional Shapes—A Class of Promising Catalysts. Angew. Chemie Int. Ed. 2007, 46, 7157–7159.

(19)

Chen, J.; Lim, B.; Lee, E. P.; Xia, Y. Shape-Controlled Synthesis of Platinum Nanocrystals for Catalytic and Electrocatalytic Applications. Nano Today 2009, 4, 81–95.

(20)

Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angew. Chemie Int. Ed. 2009, 48, 60–103.

(21)

Inaba, M.; Ando, M.; Hatanaka, A.; Nomoto, A.; Matsuzawa, K.; Tasaka, A.; Kinumoto, T.; Iriyama, Y.; Ogumi, Z. Controlled Growth and Shape Formation of Platinum Nanoparticles and Their Electrochemical Properties. Electrochim. Acta 2006, 52, 1632–1638.

(22)

Zhang, C.; Shen, X.; Pan, Y.; Peng, Z. A Review of Pt-Based Electrocatalysts for Oxygen Reduction Reaction. Front. Energy 2017, 11, 268–285.

(23)

Wang, Y.-J.; Long, W.; Wang, L.; Yuan, R.; Ignaszak, A.; Fang, B.; Wilkinson, D. P. Unlocking the Door to Highly Active ORR Catalysts for PEMFC Applications: Polyhedron-Engineered Pt-Based Nanocrystals. Energy Environ. Sci. 2018, 11, 258–275. 38 ACS Paragon Plus Environment

Page 39 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(24)

Vidal-Iglesias, F. J.; Arán-Ais, R. M.; Solla-Gullón, J.; Herrero, E.; Feliu, J. M. Electrochemical Characterization of Shape-Controlled Pt Nanoparticles in Different Supporting Electrolytes. ACS Catal. 2012, 2, 901–910.

(25)

Bu, L.; Zhang, N.; Guo, S.; Zhang, X.; Li, J.; Yao, J.; Wu, T.; Lu, G.; Ma, J.-Y.; Su, D.; Huang, X. Biaxially Strained PtPb/Pt Core/Shell Nanoplate Boosts Oxygen Reduction Catalysis. Science. 2016, 354, 1410–1414.

(26)

Ghosh, T.; Vukmirovic, M. B.; DiSalvo, F. J.; Adzic, R. R. Intermetallics as Novel Supports for Pt Monolayer O 2 Reduction Electrocatalysts: Potential for Significantly Improving Properties. J. Am. Chem. Soc. 2010, 132, 906–907.

(27)

Roychowdhury, C.; Matsumoto, F.; Zeldovich, V. B.; Warren, S. C.; Mutolo, P. F.; Ballesteros, M.; Wiesner, U.; Abruña, H. D.; DiSalvo, F. J. Synthesis, Characterization, and Electrocatalytic Activity of PtBi and PtPb Nanoparticles Prepared by Borohydride Reduction in Methanol. Chem. Mater. 2006, 18, 3365–3372.

(28)

Huang, R.; Wen, Y.-H.; Zhu, Z.-Z.; Sun, S.-G. Structure and Stability of Platinum Nanocrystals: From Low-Index to High-Index Facets. J. Mater. Chem. 2011, 21, 11578.

(29)

Huang, X.; Zhao, Z.; Cao, L.; Chen, Y.; Zhu, E.; Lin, Z.; Li, M.; Yan, A.; Zettl, A.; Wang, Y. M.; Duan, X.; Mueller, T.; Huang, Y. High-Performance Transition Metal-Doped Pt3Ni Octahedra for Oxygen Reduction Reaction. Science. 2015, 348, 1230–1234.

(30)

Li, B.; Wang, J.; Gao, X.; Qin, C.; Yang, D.; Lv, H.; Xiao, Q.; Zhang, C. High Performance Octahedral PtNi/C Catalysts Investigated from Rotating Disk Electrode to Membrane Electrode Assembly. Nano Res. 2019, 12, 281–287.

(31)

Ioroi, T.; Siroma, Z.; Yamazaki, S.; Yasuda, K. Electrocatalysts for PEM Fuel Cells. Adv. Energy Mater. 2018, 9, 1801284.

(32)

Li, B.; Higgins, D. C.; Xiao, Q.; Yang, D.; Zhng, C.; Cai, M.; Chen, Z.; Ma, J. The Durability of Carbon Supported Pt Nanowire as Novel Cathode Catalyst for a 1.5 KW PEMFC Stack. Appl. Catal. B Environ. 2015, 162, 133–140.

(33)

Li, B.; Lin, R.; Yang, D.; Ma, J. Effect of Driving Cycle on the Performance of PEM Fuel Cell and Microstructure of Membrane Electrode Assembly. Int. J. Hydrogen Energy 2010, 35, 2814–2819.

(34)

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.; Huang, Y.; Duan, X. Ultrafine Jagged Platinum Nanowires Enable Ultrahigh Mass Activity for the Oxygen Reduction Reaction. Science. 2016, 354, 1414–1419.

(35)

Jiang, K.; Zhao, D.; Guo, S.; Zhang, X.; Zhu, X.; Guo, J.; Lu, G.; Huang, X. Efficient Oxygen Reduction Catalysis by Subnanometer Pt Alloy Nanowires. Sci. Adv. 2017, 3, e1601705.

(36)

Beermann, V.; Gocyla, M.; Willinger, E.; Rudi, S.; Heggen, M.; Dunin-Borkowski, R. E.; Willinger, M.-G.; Strasser, P. Rh-Doped Pt–Ni Octahedral Nanoparticles: Understanding 39 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 55

the Correlation between Elemental Distribution, Oxygen Reduction Reaction, and Shape Stability. Nano Lett. 2016, 16, 1719–1725. (37)

Jia, Q.; Zhao, Z.; Cao, L.; Li, J.; Ghoshal, S.; Davies, V.; Stavitski, E.; Attenkofer, K.; Liu, Z.; Li, M.; Duan, X.; Mukerjee, S.; Mueller, T.; Huang, Y. Roles of Mo Surface Dopants in Enhancing the ORR Performance of Octahedral PtNi Nanoparticles. Nano Lett. 2018, 18, 798–804.

(38)

Cao, L.; Mueller, T. Theoretical Insights into the Effects of Oxidation and Mo-Doping on the Structure and Stability of Pt–Ni Nanoparticles. Nano Lett. 2016, 16, 7748–7754.

(39)

Li, K.; Li, X.; Huang, H.; Luo, L.; Li, X.; Yan, X.; Ma, C.; Si, R.; Yang, J.; Zeng, J. One-NanometerThick PtNiRh Trimetallic Nanowires with Enhanced Oxygen Reduction Electrocatalysis in Acid Media: Integrating Multiple Advantages into One Catalyst. J. Am. Chem. Soc. 2018, 140, 16159–16167.

(40)

Huang, H.; Li, K.; Chen, Z.; Luo, L.; Gu, Y.; Zhang, D.; Ma, C.; Si, R.; Yang, J.; Peng, Z.; Zeng, J. Achieving Remarkable Activity and Durability toward Oxygen Reduction Reaction Based on Ultrathin Rh-Doped Pt Nanowires. J. Am. Chem. Soc. 2017, 139, 8152–8159.

(41)

Bu, L.; Guo, S.; Zhang, X.; Shen, X.; Su, D.; Lu, G.; Zhu, X.; Yao, J.; Guo, J.; Huang, X. Surface Engineering of Hierarchical Platinum-Cobalt Nanowires for Efficient Electrocatalysis. Nat. Commun. 2016, 7, 11850.

(42)

Luo, M.; Sun, Y.; Zhang, X.; Qin, Y.; Li, M.; Li, Y.; Li, C.; Yang, Y.; Wang, L.; Gao, P.; Lu, G.; Guo, S. Stable High-Index Faceted Pt Skin on Zigzag-Like PtFe Nanowires Enhances Oxygen Reduction Catalysis. Adv. Mater. 2018, 30, 1705515.

(43)

Zhang, N.; Bu, L.; Guo, S.; Guo, J.; Huang, X. Screw Thread-Like Platinum-Copper Nanowires Bounded with High-Index Facets for Efficient Electrocatalysis. Nano Lett. 2016, 16, 5037– 5043.

(44)

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, 1–6.

(45)

Liang, Y.-T.; Lin, S.-P.; Liu, C.-W.; Chung, S.-R.; Chen, T.-Y.; Wang, J.-H.; Wang, K.-W. The Performance and Stability of the Oxygen Reduction Reaction on Pt–M (M = Pd, Ag and Au) Nanorods: An Experimental and Computational Study. Chem. Commun. 2015, 51, 6605– 6608.

(46)

Yoon, J.; Park, J.; Sa, Y. J.; Yang, Y.; Baik, H.; Joo, S. H.; Lee, K. Synthesis of Bare Pt 3 Ni Nanorods from PtNi@Ni Core–Shell Nanorods by Acid Etching: One-Step Surfactant Removal and Phase Conversion for Optimal Electrochemical Performance toward Oxygen Reduction Reaction. CrystEngComm 2016, 18, 6002–6007.

(47)

Xiong, X.; Liao, Y.; Du, C.; Luo, W.; Chen, S. Reaction-Kinetics-Tuned Synthesis of Platinum Nanorods and Nanodendrites with Enhanced Electrocatalytic Performance for Oxygen Reduction. ChemElectroChem 2016, 3, 2281–2287. 40 ACS Paragon Plus Environment

Page 41 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(48)

Lu, Y.; Jiang, Y.; Chen, W. PtPd Porous Nanorods with Enhanced Electrocatalytic Activity and Durability for Oxygen Reduction Reaction. Nano Energy 2013, 2, 836–844.

(49)

Li, C.; Sato, T.; Yamauchi, Y. Electrochemical Synthesis of One-Dimensional Mesoporous Pt Nanorods Using the Assembly of Surfactant Micelles in Confined Space. Angew. Chemie Int. Ed. 2013, 52, 8050–8053.

(50)

Liu, C.; Ma, Z.; Cui, M.; Zhang, Z.; Zhang, X.; Su, D.; Murray, C. B.; Wang, J. X.; Zhang, S. Favorable Core/Shell Interface within Co 2 P/Pt Nanorods for Oxygen Reduction Electrocatalysis. Nano Lett. 2018, 18, 7870–7875.

(51)

Fu, K.; Wang, Y.; Mao, L.; Yang, X.; Jin, J.; Yang, S.; Li, G. Facile Morphology Controllable Synthesis of PtPd Nanorods on Graphene-Multiwalled Carbon Nanotube Hybrid Support as Efficient Electrocatalysts for Oxygen Reduction Reaction. Mater. Res. Bull. 2018, 108, 187–194.

(52)

Park, J.; Kwon, T.; Kim, J.; Jin, H.; Kim, H. Y.; Kim, B.; Joo, S. H.; Lee, K. Hollow Nanoparticles as Emerging Electrocatalysts for Renewable Energy Conversion Reactions. Chem. Soc. Rev. 2018, 47, 8173–8202.

(53)

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.

(54)

Adzic, R. R. Platinum Monolayer Electrocatalysts: Tunable Activity, Stability, and SelfHealing Properties. Electrocatalysis 2012, 3, 163–169.

(55)

Becknell, N.; Son, Y.; Kim, D.; Li, D.; Yu, Y.; Niu, Z.; Lei, T.; Sneed, B. T.; More, K. L.; Markovic, N. M.; Stamenkovic, V. R.; Yang, P. Control of Architecture in Rhombic Dodecahedral Pt– Ni Nanoframe Electrocatalysts. J. Am. Chem. Soc. 2017, 139, 11678–11681.

(56)

Oh, A.; Baik, H.; Choi, D. S.; Cheon, J. Y.; Kim, B.; Kim, H.; Kwon, S. J.; Joo, S. H.; Jung, Y.; Lee, K. Skeletal Octahedral Nanoframe with Cartesian Coordinates via Geometrically Precise Nanoscale Phase Segregation in a Pt@Ni Core–Shell Nanocrystal. ACS Nano 2015, 9, 2856–2867.

(57)

Zhang, Z.; Luo, Z.; Chen, B.; Wei, C.; Zhao, J.; Chen, J.; Zhang, X.; Lai, Z.; Fan, Z.; Tan, C.; Zhao, M.; Lu, Q.; Li, B.; Zong, Y.; Yan, C.; Wang, G.; Xu, Z. J.; Zhang, H. One-Pot Synthesis of Highly Anisotropic Five-Fold-Twinned PtCu Nanoframes Used as a Bifunctional Electrocatalyst for Oxygen Reduction and Methanol Oxidation. Adv. Mater. 2016, 28, 8712–8717.

(58)

Fu, T.; Fang, J.; Wang, C.; Zhao, J. Hollow Porous Nanoparticles with Pt Skin on a Ag–Pt Alloy Structure as a Highly Active Electrocatalyst for the Oxygen Reduction Reaction. J. Mater. Chem. A 2016, 4, 8803–8811.

(59)

Hong, J. W.; Kang, S. W.; Choi, B.-S.; Kim, D.; Lee, S. B.; Han, S. W. Controlled Synthesis of Pd–Pt Alloy Hollow Nanostructures with Enhanced Catalytic Activities for Oxygen 41 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 55

Reduction. ACS Nano 2012, 6, 2410–2419. (60)

Han, L.; Wang, P.; Liu, H.; Tan, Q.; Yang, J. Balancing the Galvanic Replacement and Reduction Kinetics for the General Formation of Bimetallic CuM (M = Ru, Rh, Pd, Os, Ir, and Pt) Hollow Nanostructures. J. Mater. Chem. A 2016, 4, 18354–18365.

(61)

Park, J.; Wang, H.; Vara, M.; Xia, Y. Platinum Cubic Nanoframes with Enhanced Catalytic Activity and Durability Toward Oxygen Reduction. ChemSusChem 2016, 9, 2855–2861.

(62)

Niu, Z.; Becknell, N.; Yu, Y.; Kim, D.; Chen, C.; Kornienko, N.; Somorjai, G. A.; Yang, P. Anisotropic Phase Segregation and Migration of Pt in Nanocrystals En Route to Nanoframe Catalysts. Nat. Mater. 2016, 15, 1188–1194.

(63)

Wang, K.; Du, H.; Sriphathoorat, R.; Shen, P. K. Vertex-Type Engineering of Pt-Cu-Rh Heterogeneous Nanocages for Highly Efficient Ethanol Electrooxidation. Adv. Mater. 2018, 30, 1804074.

(64)

Kwon, T.; Jun, M.; Kim, H. Y.; Oh, A.; Park, J.; Baik, H.; Joo, S. H.; Lee, K. Vertex-Reinforced PtCuCo Ternary Nanoframes as Efficient and Stable Electrocatalysts for the Oxygen Reduction Reaction and the Methanol Oxidation Reaction. Adv. Funct. Mater. 2018, 28, 1706440.

(65)

Wu, Y.; Wang, D.; Zhou, G.; Yu, R.; Chen, C.; Li, Y. Sophisticated Construction of Au Islands on Pt–Ni: An Ideal Trimetallic Nanoframe Catalyst. J. Am. Chem. Soc. 2014, 136, 11594– 11597.

(66)

Ud Din, M. A.; Saleem, F.; Ni, B.; Yong, Y.; Wang, X. Porous Tetrametallic PtCuBiMn Nanosheets with a High Catalytic Activity and Methanol Tolerance Limit for Oxygen Reduction Reactions. Adv. Mater. 2017, 29, 1604994.

(67)

Zhang, C.; Sandorf, W.; Peng, Z. Octahedral Pt 2 CuNi Uniform Alloy Nanoparticle Catalyst with High Activity and Promising Stability for Oxygen Reduction Reaction. ACS Catal. 2015, 5, 2296–2300.

(68)

Saleem, F.; Ni, B.; Yong, Y.; Gu, L.; Wang, X. Ultra-Small Tetrametallic Pt-Pd-Rh-Ag Nanoframes with Tunable Behavior for Direct Formic Acid/Methanol Oxidation. Small 2016, 12, 5261–5268.

(69)

Greeley, J.; Nørskov, J. K. Combinatorial Density Functional Theory-Based Screening of Surface Alloys for the Oxygen Reduction Reaction. J. Phys. Chem. C 2009, 113, 4932–4939.

(70)

Ross, P. N. J. Oxygen Reduction on Supported Pt Alloys and Intermetallic Compounds in Phosphoric Acid; 1980.

(71)

Jalan, V. M.; Landsman, D. A.; Hartford, W. Noble Metal-Refractroy Metal Alloys as Catalysts and Method for Making. US4186110A, 1980.

(72)

Landsman, D. A.; Hartford, W.; Luczak, F. J. Nobel Metal-Chromeum Alloy Catalysts and Electrochemical Cell. US4316944A, 1982. 42 ACS Paragon Plus Environment

Page 43 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(73)

Mukerjee, S.; Srinivasan, S. Enhanced Electrocatalysis of Oxygen Reduction on Platinum Alloys in Proton Exchange Membrane Fuel Cells. J. Electroanal. Chem. 1993, 357, 201–224.

(74)

Mukerjee, S.; McBreen, J. Effect of Particle Size on the Electrocatalysis by CarbonSupported Pt Electrocatalysts: An in Situ XAS Investigation. J. Electroanal. Chem. 1998, 448, 163–171.

(75)

Vielstich, W.; Lamm, A.; Gasteiger, H. A. (Hubert A.; Yokokawa, H. Handbook of Fuel Cells : Fundamentals, Technology, and Applications; Wiley, 2003.

(76)

Jalan, V. Importance of Interatomic Spacing in Catalytic Reduction of Oxygen in Phosphoric Acid. J. Electrochem. Soc. 1983, 130, 2299.

(77)

Toda, T. Enhancement of the Electroreduction of Oxygen on Pt Alloys with Fe, Ni, and Co. J. Electrochem. Soc. 1999, 146, 3750.

(78)

Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; Pergamon Press: New York, 1966.

(79)

Antolini, E. Formation of Carbon-Supported PtM Alloys for Low Temperature Fuel Cells: A Review. Mater. Chem. Phys. 2003, 78, 563–573.

(80)

B. C. Ross, P. N. B. The Structure and Activity of Pt-Co Alloys as Oxygen Reduction Electrocatalysts. J. Electrochem. Soc. 1990, 137, 3368.

(81)

Koh, S.; Leisch, J.; Toney, M. F.; Strasser, P. Structure-Activity-Stability Relationships of Pt−Co Alloy Electrocatalysts in Gas-Diffusion Electrode Layers. J. Phys. Chem. C 2007, 111, 3744–3752.

(82)

Antolini, E.; Salgado, J. R. C.; Gonzalez, E. R. The Stability of Pt–M (M=first Row Transition Metal) Alloy Catalysts and Its Effect on the Activity in Low Temperature Fuel Cells. J. Power Sources 2006, 160, 957–968.

(83)

Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1, 552–556.

(84)

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.

(85)

Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Nørskov, J. K. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chemie Int. Ed. 2006, 45, 2897–2901.

(86)

Stephens, I. E. L.; Bondarenko, A. S.; Perez-Alonso, F. J.; Calle-Vallejo, F.; Bech, L.; Johansson, T. P.; Jepsen, A. K.; Frydendal, R.; Knudsen, B. P.; Rossmeisl, J.; Chorkendorff, I. Tuning the Activity of Pt(111) for Oxygen Electroreduction by Subsurface Alloying. J. Am. Chem. Soc. 2011, 133, 5485–5491. 43 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 55

(87)

Strasser, P.; Koh, S.; Anniyev, T.; Greeley, J.; More, K.; Yu, C.; Liu, Z.; Kaya, S.; Nordlund, D.; Ogasawara, H.; Toney, M. F.; Nilsson, A. Lattice-Strain Control of the Activity in Dealloyed Core–Shell Fuel Cell Catalysts. Nat. Chem. 2010, 2, 454–460.

(88)

Chen, S.; Ferreira, P. J.; Sheng, W.; Yabuuchi, N.; Allard, L. F.; Shao-Horn, Y. Enhanced Activity for Oxygen Reduction Reaction on “Pt 3 Co” Nanoparticles: Direct Evidence of Percolated and Sandwich-Segregation Structures. J. Am. Chem. Soc. 2008, 130, 13818– 13819.

(89)

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.

(90)

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.

(91)

Makharia, R.; Kocha, S.; Yu, P.; Sweikart, M. A.; Gu, W.; Wagner, F.; Gasteiger, H. A. Durable PEM Fuel Cell Electrode Materials: Requirements and Benchmarking Methodologies. In ECS Transactions; ECS, 2006; Vol. 1, pp 3–18.

(92)

Neyerlin, K. C.; Srivastava, R.; Yu, C.; Strasser, P. Electrochemical Activity and Stability of Dealloyed Pt–Cu and Pt–Cu–Co Electrocatalysts for the Oxygen Reduction Reaction (ORR). J. Power Sources 2009, 186, 261–267.

(93)

Stephens, I. E. L.; Bondarenko, A. S.; Grønbjerg, U.; Rossmeisl, J.; Chorkendorff, I. Understanding the Electrocatalysis of Oxygen Reduction on Platinum and Its Alloys. Energy Environ. Sci. 2012, 5, 6744.

(94)

Yu, P.; Pemberton, M.; Plasse, P. PtCo/C Cathode Catalyst for Improved Durability in PEMFCs. J. Power Sources 2005, 144, 11–20.

(95)

Ball, S. C.; Hudson, S. L.; Theobald, B. R.; Thompsett, D. PtCo, a Durable Catalyst for Automotive PEMFC? In ECS Transactions; ECS, 2007; Vol. 11, pp 1267–1278.

(96)

Ball, S. C.; Theobald, B.; Thompsett, D.; Hudson, S. Enhanced Stability of PtCo Catalysts for PEMFC. In ECS Transactions; ECS, 2006; Vol. 1, pp 141–152.

(97)

Wolf.; Lamm, H. A. (Hubert A.; Yokokawa, H. Handbook of Fuel Cells :Advances in Electrocatalysis, Materials, Diagnostics and Durability, Volumes 5 & 6; Wiley, 2009.

(98)

Escudero-Cid, R.; Hernández-Fernández, P.; Pérez-Flores, J. C.; Rojas, S.; Garcia-Rodríguez, S.; Fatás, E.; Ocón, P. Analysis of Performance Losses of Direct Methanol Fuel Cell with Methanol Tolerant PtCoRu/C Cathode Electrode. Int. J. Hydrogen Energy 2012, 37, 7119– 7130.

(99)

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 44 ACS Paragon Plus Environment

Page 45 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Catalysts. J. Phys. Chem. B 2002, 106, 4181–4191. (100) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity Benchmarks and Requirements for Pt, Pt-Alloy, and Non-Pt Oxygen Reduction Catalysts for PEMFCs. Appl. Catal. B Environ. 2005, 56, 9–35. (101) Matthey, J.; Ralph, T. R.; Keating, J. E.; Collis, N. J. Oxygen Reduction Electrocatalysts in Solid Polymer Fuel Cell Membrance Electrode Assemblies. ETSU F/02/00038/REP Contractor Prepared By; 1997. (102) Chen, S.; Gasteiger, H. A.; Hayakawa, K.; Tada, T.; Shao-Horn, Y. Platinum-Alloy Cathode Catalyst Degradation in Proton Exchange Membrane Fuel Cells: Nanometer-Scale Compositional and Morphological Changes. J. Electrochem. Soc. 2010, 157, A82. (103) Dubau, L.; Durst, J.; Maillard, F.; Guétaz, L.; Chatenet, M.; André, J.; Rossinot, E. Further Insights into the Durability of Pt3Co/C Electrocatalysts: Formation of “Hollow” Pt Nanoparticles Induced by the Kirkendall Effect. Electrochim. Acta 2011, 56, 10658–10667. (104) Greeley, J.; Nørskov, J. K. Electrochemical Dissolution of Surface Alloys in Acids: Thermodynamic Trends from First-Principles Calculations. Electrochim. Acta 2007, 52, 5829–5836. (105) Menning, C. A.; Chen, J. G. General Trend for Adsorbate-Induced Segregation of Subsurface Metal Atoms in Bimetallic Surfaces. J. Chem. Phys. 2009, 130, 174709. (106) Menning, C. A.; Chen, J. G. Regenerating Pt–3d–Pt Model Electrocatalysts through Oxidation–Reduction Cycles Monitored at Atmospheric Pressure. J. Power Sources 2010, 195, 3140–3144. (107) Kaewsai, D.; Hunsom, M. Comparative Study of the ORR Activity and Stability of Pt and PtM (M = Ni, Co, Cr, Pd) Supported on Polyaniline/Carbon Nanotubes in a PEM Fuel Cell. Nanomaterials 2018, 8, 299. (108) Johansson, T. P.; Ulrikkeholm, E. T.; Hernandez-Fernandez, P.; Malacrida, P.; Hansen, H. A.; Bandarenka, A. S.; Nørskov, J. K.; Rossmeisl, J.; Stephens, I. E. L.; Chorkendorff, I. Pt Skin Versus Pt Skeleton Structures of Pt3Sc as Electrocatalysts for Oxygen Reduction. Top. Catal. 2014, 57, 245–254. (109) Hernandez-Fernandez, P.; Masini, F.; McCarthy, D. N.; Strebel, C. E.; Friebel, D.; Deiana, D.; Malacrida, P.; Nierhoff, A.; Bodin, A.; Wise, A. M.; Nielsen, J. H.; Hansen, T. W.; Nilsson, A.; Stephens, I. E. L.; Chorkendorff, I. Mass-Selected Nanoparticles of Pt x Y as Model Catalysts for Oxygen Electroreduction. Nat. Chem. 2014, 6, 732–738. (110) 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. (111) Schwämmlein, J. N.; Harzer, G. S.; Pfändner, P.; Blankenship, A.; El-Sayed, H. A.; Gasteiger, H. A. Activity and Stability of Carbon Supported Pt x Y Alloys for the ORR Determined by 45 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 55

RDE and Single-Cell PEMFC Measurements. J. Electrochem. Soc. 2018, 165, J3173–J3185. (112) Escudero-Escribano, M.; Malacrida, P.; Hansen, M. H.; Vej-Hansen, U. G.; VelazquezPalenzuela, A.; Tripkovic, V.; Schiotz, J.; Rossmeisl, J.; Stephens, I. E. L.; Chorkendorff, I. Tuning the Activity of Pt Alloy Electrocatalysts by Means of the Lanthanide Contraction. Science. 2016, 352, 73–76. (113) Malacrida, P.; Escudero-Escribano, M.; Verdaguer-Casadevall, A.; Stephens, I. E. L.; Chorkendorff, I. Enhanced Activity and Stability of Pt–La and Pt–Ce Alloys for Oxygen Electroreduction: The Elucidation of the Active Surface Phase. J. Mater. Chem. A 2014, 2, 4234. (114) Zhang, J.; Mo, Y.; Vukmirovic, M. B.; Klie, R.; Sasaki, K.; Adzic, R. R. Platinum Monolayer Electrocatalysts for O 2 Reduction: Pt Monolayer on Pd(111) and on Carbon-Supported Pd Nanoparticles. J. Phys. Chem. B 2004, 108, 10955–10964. (115) 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. (116) Alonso-Vante, N., S. D. Bhat, Mary Clare Sison Escan˜o, Sadia Kabir, Y. Luo, Javier Parrondo, Vijay Ramani, A. K. Sahu, Alexey Serov, P. Sridhar, Lihui Wang, Z. W. Electrochemistry; Banks, C., McIntosh, S., Eds.; Electrochemistry; Royal Society of Chemistry: Cambridge, 2017; Vol. 14. (117) 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. (118) 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. (119) Wang, X.; Choi, S. Il; 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, 1–8. (120) Wang, D.; Xin, H. L.; Yu, Y.; Wang, H.; Rus, E.; Muller, D. A.; Abruña, 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. (121) Wang, D.; Xin, H. L.; Wang, H.; Yu, Y.; Rus, E.; Muller, D. A.; DiSalvo, F. J.; Abruña, 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. (122) Sharma, M.; Jung, N.; Yoo, S. J. Toward High-Performance Pt-Based Nanocatalysts for 46 ACS Paragon Plus Environment

Page 47 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Oxygen Reduction Reaction through Organic–Inorganic Hybrid Concepts. Chem. Mater. 2018, 30, 2–24. (123) Ramírez-Caballero, G. E.; Ma, Y.; Callejas-Tovar, R.; Balbuena, P. B. Surface Segregation and Stability of Core–Shell Alloy Catalysts for Oxygen Reduction in Acid Medium. Phys. Chem. Chem. Phys. 2010, 12, 2209. (124) Wang, J. X. X. J. X.; Inada, H.; Wu, L.; Zhu, Y.; Choi, Y. Y. M. M.; Liu, P.; Zhou, W. P. P. W. P.; Adzic, R. R. R. R. Oxygen Reduction on Well-Defined Core- Shell Nanocatalysts: Particle Size, Facet, and Pt Shell Thickness Effects. J. Am. Chem. Soc. 2009, 131, 17298–17302. (125) Lim, B.; Jiang, M.; Camargo, P. H. C.; Cho, E. C.; Tao, J.; Lu, X.; Zhu, Y.; Xia, Y. Pd-Pt Bimetallic Nanodendrites with High Activity for Oxygen Reduction. Science. 2009, 324, 1302–1305. (126) Wang, C.; van der Vliet, D.; More, K. L.; Zaluzec, N. J.; Peng, S.; Sun, S.; Daimon, H.; Wang, G.; Greeley, J.; Pearson, J.; Paulikas, A. P.; Karapetrov, G.; Strmcnik, D.; Markovic, N. M.; Stamenkovic, V. R. Multimetallic Au/FePt 3 Nanoparticles as Highly Durable Electrocatalyst. Nano Lett. 2011, 11, 919–926. (127) Gong, K.; Su, D.; Adzic, R. R. Platinum-Monolayer Shell on AuNi 0.5 Fe Nanoparticle Core Electrocatalyst with High Activity and Stability for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2010, 132, 14364–14366. (128) Sasaki, K.; Naohara, H.; Cai, Y.; Choi, Y. M.; Liu, P.; Vukmirovic, M. B.; Wang, J. X.; Adzic, R. R. Core-Protected Platinum Monolayer Shell High-Stability Electrocatalysts for Fuel-Cell Cathodes. Angew. Chemie Int. Ed. 2010, 49, 8602–8607. (129) 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. (130) 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. Chemie 2010, 122, 10395–10399. (131) 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. (132) 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. (133) 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. (134) 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. 47 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 55

(135) 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. (136) 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. (137) 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–e237. (138) Chung, D. Y.; Jun, S. W.; Yoon, G.; Kwon, S. G.; Shin, D. Y.; Seo, P.; 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. (139) 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. (140) Auer, E.; Freund, A.; Pietsch, J.; Tacke, T. Carbons as Supports for Industrial Precious Metal Catalysts. Appl. Catal. A Gen. 1998, 173, 259–271. (141) Yu, J.-S.; Kang, S.; Yoon, S. B.; Chai, G. “Fabrication of Ordered Uniform Porous Carbon Networks and Their Application to a Catalyst Supporter” J. Am. Chem. Soc. 2002, 124, 9382-9383 (142) Chai, G. S.; Shin, I. S.; Yu, J.-S. Synthesis of Ordered, Uniform, Macroporous Carbons with Mesoporous Walls Templated by Aggregates of Polystyrene Spheres and Silica Particles for Use as Catalyst Supports in Direct Methanol Fuel Cells. Adv. Mater. 2004, 16, 2057–2061. (143) Fang, B.; Kim, J. H.; Kim, M.-S.; Yu, J.-S. Hierarchical Nanostructured Carbons with Meso– Macroporosity: Design, Characterization, and Applications. Acc. Chem. Res. 2013, 46, 1397–1406. (144) Chai, G. S.; Yoon, S. B.; Kim, J. H.; Yu, J.-S. Spherical Carbon Capsules with Hollow Macroporous Core and Mesoporous Shell Structures as a Highly Efficient Catalyst Support in Direct Methanol Fuel Cell. Chem. Commun. 2004, 23, 2766-2767. (145) Yang, D.-S.; Bhattacharjya, D.; Inamdar, S.; Park, J.; Yu, J.-S. Phosphorus-Doped Ordered Mesoporous Carbons with Different Channel Lengths as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reaction in Alkaline Media. J. Am. Chem. Soc. 2012, 134, 16127-16130. (146) Yang, D.-S.; Chaudhari, S.; Rajesh, K. P.; Yu, J.-S. Preparation of Nitrogen-Doped Porous Carbon Nanofibers and the Effect of Porosity, Conductivity, and Nitrogen Content on Their Oxygen Reduction Performance. ChemCatChem 2014, 6, 1236-1244. 48 ACS Paragon Plus Environment

Page 49 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(147) Sung, I. K.; Yoon, S. B.; Yu, J.-S.; Kim D. P. Fabrication of Macroporous SiC from Templated Preceramic Polymers. Chem. Commu. 2002, 14, 1480-1481. (148) Yoon, S. B.; Chai, G. S.; Kang, S.; Yu, J. -S.; Gierszal, K. P.; Jaroniec, M. Graphitized pitchbased carbons with ordered nanopores synthesized by using colloidal crystals as templates. J. Am. Chem. Soc. 2005, 127, 4188-4189. (149) Li, L.; Hu, L.; Li, J.; Wei, Z. Enhanced Stability of Pt Nanoparticle Electrocatalysts for Fuel Cells. Nano Res. 2015, 8, 418–440. (150) Tran, T.-N.; Song, M. Y.; Kang, T.-H.; Samdani, J.; Park, H.-Y.; Kim, H.; Jhung, S. H.; Yu, J.-S. Iron Phosphide Incorporated into Iron-Treated Heteroatoms-Doped Porous Bio-Carbon as Efficient Electrocatalyst for the Oxygen Reduction Reaction. ChemElectroChem 2018, 5, 1944–1953. (151) Singh, K. P.; Bae, E. J.; Yu, J.-S. Fe–P: A New Class of Electroactive Catalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137, 3165–3168. (152) Kim, J. H.; Cheon, J. Y.; Shin, T. J.; Park, J. Y.; Joo, S. H. Effect of Surface Oxygen Functionalization of Carbon Support on the Activity and Durability of Pt/C Catalysts for the Oxygen Reduction Reaction. Carbon 2016, 101, 449–457. (153) Speder, J.; Zana, A.; Spanos, I.; Kirkensgaard, J. J. K.; Mortensen, K.; Hanzlik, M.; Arenz, M. Comparative Degradation Study of Carbon Supported Proton Exchange Membrane Fuel Cell Electrocatalysts – The Influence of the Platinum to Carbon Ratio on the Degradation Rate. J. Power Sources 2014, 261, 14–22. (154) Wang, J.; Yin, G.; Shao, Y.; Zhang, S.; Wang, Z.; Gao, Y. Effect of Carbon Black Support Corrosion on the Durability of Pt/C Catalyst. J. Power Sources 2007, 171, 331–339. (155) Selvaganesh, S. V.; Sridhar, P.; Pitchumani, S.; Shukla, A. K. A Durable Graphitic-Carbon Support for Pt and Pt 3 Co Cathode Catalysts in Polymer Electrolyte Fuel Cells. J. Electrochem. Soc. 2013, 160, F49–F59. (156) Kim, S.-M.; Heo, Y.-K.; Bae, K.-T.; Oh, Y.-T.; Lee, M.-H.; Lee, S.-Y. In Situ Formation of Nitrogen-Doped Onion-like Carbon as Catalyst Support for Enhanced Oxygen Reduction Activity and Durability. Carbon N. Y. 2016, 101, 420–430. (157) Meier, J. C.; Galeano, C.; Katsounaros, I.; Witte, J.; Bongard, H. J.; Topalov, A. A.; Baldizzone, C.; Mezzavilla, S.; Schüth, F.; Mayrhofer, K. J. J. Design Criteria for Stable Pt/C Fuel Cell Catalysts. Beilstein J. Nanotechnol. 2014, 5, 44–67. (158) Higgins, D.; Zamani, P.; Yu, A.; Chen, Z. The Application of Graphene and Its Composites in Oxygen Reduction Electrocatalysis: A Perspective and Review of Recent Progress. Energy Environ. Sci. 2016, 9, 357–390. (159) Ando, F.; Tanabe, T.; Gunji, T.; Kaneko, S.; Takeda, T.; Ohsaka, T.; Matsumoto, F. Effect of the D-Band Center on the Oxygen Reduction Reaction Activity of Electrochemically Dealloyed Ordered Intermetallic Platinum–Lead (PtPb) Nanoparticles Supported on TiO 2 49 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 55

-Deposited Cup-Stacked Carbon Nanotubes. ACS Appl. Nano Mater. 2018, 1, 2844–2850. (160) Fujigaya, T.; Nakashima, N. Fuel Cell Electrocatalyst Using Polybenzimidazole-Modified Carbon Nanotubes As Support Materials. Adv. Mater. 2013, 25, 1666–1681. (161) Tian, Y.; Liu, Y.; Zhao, J.; Ding, Y. High Stability and Superior Catalytic Reactivity of NitrogenDoped Graphene Supporting Pt Nanoparticles as a Catalyst for the Oxygen Reduction Reaction: A Density Functional Theory Study. RSC Adv. 2015, 5, 34070–34077. (162) Hoque, M. A.; Hassan, F. M.; Higgins, D.; Choi, J.-Y.; Pritzker, M.; Knights, S.; Ye, S.; Chen, Z. Multigrain Platinum Nanowires Consisting of Oriented Nanoparticles Anchored on Sulfur-Doped Graphene as a Highly Active and Durable Oxygen Reduction Electrocatalyst. Adv. Mater. 2015, 27, 1229–1234. (163) Li, Y.-H.; Hung, T.-H.; Chen, C.-W. A First-Principles Study of Nitrogen- and Boron-Assisted Platinum Adsorption on Carbon Nanotubes. Carbon 2009, 47, 850–855. (164) Higgins, D.; Hoque, M. A.; Seo, M. H.; Wang, R.; Hassan, F.; Choi, J.-Y.; Pritzker, M.; Yu, A.; Zhang, J.; Chen, Z. Development and Simulation of Sulfur-Doped Graphene Supported Platinum with Exemplary Stability and Activity Towards Oxygen Reduction. Adv. Funct. Mater. 2014, 24, 4325–4336. (165) Wang, R.; Higgins, D. C.; Hoque, M. A.; Lee, D.; Hassan, F.; Chen, Z. Controlled Growth of Platinum Nanowire Arrays on Sulfur Doped Graphene as High Performance Electrocatalyst. Sci. Rep. 2013, 3, 2431. (166) Park, J.; Jang, Y. J.; Kim, Y. J.; Song, M.; Yoon, S.; Kim, D. H.; Kim, S.-J. Sulfur-Doped Graphene as a Potential Alternative Metal-Free Electrocatalyst and Pt-Catalyst Supporting Material for Oxygen Reduction Reaction. Phys. Chem. Chem. Phys. 2014, 16, 103–109. (167) Muhich, C. L.; Westcott, J. Y.; Morris, T. C.; Weimer, A. W.; Musgrave, C. B. The Effect of N and B Doping on Graphene and the Adsorption and Migration Behavior of Pt Atoms. J. Phys. Chem. C 2013, 117, 10523–10535. (168) Zhu, J.; Wei, P.; Li, K.; He, S.; Pan, Z.; Nie, S.; Key, J.; Shen, P. K. Self-Assembled Nanofiber Networks of Well-Separated B and N Codoped Carbon as Pt Supports for Highly Efficient and Stable Oxygen Reduction Electrocatalysis. ACS Sustain. Chem. Eng. 2019, 7, 660–668. (169) Higgins, D. C.; Meza, D.; Chen, Z. Nitrogen-Doped Carbon Nanotubes as Platinum Catalyst Supports for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells. J. Phys. Chem. C 2010, 114, 21982–21988. (170) Chen, S.; Wei, Z.; Guo, L.; Ding, W.; Dong, L.; Shen, P.; Qi, X.; Li, L. Enhanced Dispersion and Durability of Pt Nanoparticles on a Thiolated CNT Support. Chem. Commun. 2011, 47, 10984. (171) Li, L.; Chen, S. G.; Wei, Z. D.; Qi, X. Q.; Xia, M. R.; Wang, Y. Q. Experimental and DFT Study of Thiol-Stabilized Pt/CNTs Catalysts. Phys. Chem. Chem. Phys. 2012, 14, 16581. (172) Kim, Y.; Mitani, T. Surface Thiolation of Carbon Nanotubes as Supports: A Promising Route 50 ACS Paragon Plus Environment

Page 51 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

for the High Dispersion of Pt Nanoparticles for Electrocatalysts. J. Catal. 2006, 238, 394– 401. (173) Park, S.-A.; Kim, D.-S.; Kim, T.-J.; Kim, Y.-T. Strong Interaction between Pt and Thiolated Carbon for Electrocatalytic Durability Enhancement. ACS Catal. 2013, 3, 3067–3074. (174) He, D.; Zeng, C.; Xu, C.; Cheng, N.; Li, H.; Mu, S.; Pan, M. Polyaniline-Functionalized Carbon Nanotube Supported Platinum Catalysts. Langmuir 2011, 27, 5582–5588. (175) Hsu, C.-H.; Liao, H.-Y.; Kuo, P.-L. Aniline as a Dispersant and Stabilizer for the Preparation of Pt Nanoparticles Deposited on Carbon Nanotubes. J. Phys. Chem. C 2010, 114, 7933– 7939. (176) Murphin Kumar, P. S.; Ponnusamy, V. K.; Deepthi, K. R.; Kumar, G.; Pugazhendhi, A.; Abe, H.; Thiripuranthagan, S.; Pal, U.; Krishnan, S. K. Controlled Synthesis of Pt Nanoparticle Supported TiO 2 Nanorods as Efficient and Stable Electrocatalysts for the Oxygen Reduction Reaction. J. Mater. Chem. A 2018, 6, 23435–23444. (177) Mirshekari, G. R.; Rice, C. A. Effects of Support Particle Size and Pt Content on Catalytic Activity and Durability of Pt/TiO 2 Catalyst for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells Environment. J. Power Sources 2018, 396, 606–614. (178) Martins, M.; Milikić, J.; Šljukić, B.; Soylu, G. S. P.; Yurtcan, A. B.; Bozkurt, G.; Santos, D. M. F. Mn2O3-MO (MO = ZrO2, V2O5, WO3) Supported PtNi Nanoparticles: Designing Stable and Efficient Electrocatalysts for Oxygen Reduction and Borohydride Oxidation. Microporous Mesoporous Mater. 2019, 273, 286–293. (179) Mohamed, R.; Binninger, T.; Kooyman, P. J.; Hoell, A.; Fabbri, E.; Patru, A.; Heinritz, A.; Schmidt, T. J.; Levecque, P. Facile Deposition of Pt Nanoparticles on Sb-Doped SnO 2 Support with Outstanding Active Surface Area for the Oxygen Reduction Reaction. Catal. Sci. Technol. 2018, 8, 2672–2685. (180) Alipour Moghadam Esfahani, R.; Ebralidze, I. I.; Specchia, S.; Easton, E. B. A Fuel Cell Catalyst Support Based on Doped Titanium Suboxides with Enhanced Conductivity, Durability and Fuel Cell Performance. J. Mater. Chem. A 2018, 6, 14805–14815. (181) Huang, S.-Y.; Ganesan, P.; Popov, B. N. Electrocatalytic Activity and Stability of TitaniaSupported Platinum–Palladium Electrocatalysts for Polymer Electrolyte Membrane Fuel Cell. ACS Catal. 2012, 2, 825–831. (182) Chung, D. Y.; Yoo, J. M.; Sung, Y.-E. Highly Durable and Active Pt-Based Nanoscale Design for Fuel-Cell Oxygen-Reduction Electrocatalysts. Adv. Mater. 2018, 30, 1704123. (183) Huang, S.-Y.; Ganesan, P.; Park, S.; Popov, B. N. Development of a Titanium DioxideSupported Platinum Catalyst with Ultrahigh Stability for Polymer Electrolyte Membrane Fuel Cell Applications. J. Am. Chem. Soc. 2009, 131, 13898–13899. (184) Shih, C.-C.; Chang, J.-R. Pt/C Stabilization for Catalytic Wet-Air Oxidation: Use of Grafted TiO2. J. Catal. 2006, 240, 137–150. 51 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 55

(185) Kumar, A.; Ramani, V. Strong Metal–Support Interactions Enhance the Activity and Durability of Platinum Supported on Tantalum-Modified Titanium Dioxide Electrocatalysts. ACS Catal. 2014, 4, 1516–1525. (186) Ho, V. T. T.; Pan, C.-J.; Rick, J.; Su, W.-N.; Hwang, B.-J. Nanostructured Ti 0.7 Mo 0.3 O 2 Support Enhances Electron Transfer to Pt: High-Performance Catalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2011, 133, 11716–11724. (187) Kim, M.; Kwon, C.; Eom, K.; Kim, J.; Cho, E. Electrospun Nb-Doped TiO2 Nanofiber Support for Pt Nanoparticles with High Electrocatalytic Activity and Durability. Sci. Rep. 2017, 7, 44411. (188) Parrondo, J.; Han, T.; Niangar, E.; Wang, C.; Dale, N.; Adjemian, K.; Ramani, V. Platinum Supported on Titanium-Ruthenium Oxide Is a Remarkably Stable Electrocatayst for Hydrogen Fuel Cell Vehicles. Proc. Natl. Acad. Sci. 2014, 111, 45–50. (189) Ioroi, T.; Siroma, Z.; Fujiwara, N.; Yamazaki, S.; Yasuda, K. Sub-Stoichiometric Titanium Oxide-Supported Platinum Electrocatalyst for Polymer Electrolyte Fuel Cells. Electrochem. Commun. 2005, 7, 183–188. (190) Hornberger, E.; Bergmann, A.; Schmies, H.; Kühl, S.; Wang, G.; Drnec, J.; Sandbeck, D. J. S.; Ramani, V.; Cherevko, S.; Mayrhofer, K. J. J.; Strasser, P. In Situ Stability Studies of Platinum Nanoparticles Supported on Ruthenium−Titanium Mixed Oxide (RTO) for Fuel Cell Cathodes. ACS Catal. 2018, 8, 9675–9683. (191) Yang, M.; Cui, Z.; DiSalvo, F. J. Mesoporous Titanium Nitride Supported Pt Nanoparticles as High Performance Catalysts for Methanol Electrooxidation. Phys. Chem. Chem. Phys. 2013, 15, 1088–1092. (192) Pan, Z.; Xiao, Y.; Fu, Z.; Zhan, G.; Wu, S.; Xiao, C.; Hu, G.; Wei, Z. Hollow and Porous Titanium Nitride Nanotubes as High-Performance Catalyst Supports for Oxygen Reduction Reaction. J. Mater. Chem. A 2014, 2, 13966. (193) Zhang, R.-Q.; Lee, T.-H.; Yu, B.-D.; Stampfl, C.; Soon, A. The Role of Titanium Nitride Supports for Single-Atom Platinum-Based Catalysts in Fuel Cell Technology. Phys. Chem. Chem. Phys. 2012, 14, 16552. (194) Liu, Y.; Kelly, T. G.; Chen, J. G.; Mustain, W. E. Metal Carbides as Alternative Electrocatalyst Supports. ACS Catal. 2013, 3, 1184–1194. (195) Kimmel, Y. C.; Xu, X.; Yu, W.; Yang, X.; Chen, J. G. Trends in Electrochemical Stability of Transition Metal Carbides and Their Potential Use As Supports for Low-Cost Electrocatalysts. ACS Catal. 2014, 4, 1558–1562. (196) Qiu, Z.; Huang, H.; Du, J.; Tao, X.; Xia, Y.; Feng, T.; Gan, Y.; Zhang, W. Biotemplated Synthesis of Bark-Structured TiC Nanowires as Pt Catalyst Supports with Enhanced Electrocatalytic Activity and Durability for Methanol Oxidation. J. Mater. Chem. A 2014, 2, 8003–8008. 52 ACS Paragon Plus Environment

Page 53 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(197) Xie, X.; Xue, Y.; Li, L.; Chen, S.; Nie, Y.; Ding, W.; Wei, Z. Surface Al Leached Ti3AlC2 as a Substitute for Carbon for Use as a Catalyst Support in a Harsh Corrosive Electrochemical System. Nanoscale 2014, 6, 11035–11040. (198) Avasarala, B.; Haldar, P. Durability and Degradation Mechanism of Titanium Nitride Based Electrocatalysts for PEM (Proton Exchange Membrane) Fuel Cell Applications. Energy 2013, 57, 545–553. (199) Chen, S.; Huang, Q.; Yang, W.; Zou, H.; Mai, H.; Wang, J. PtCoN Supported on TiN-Modified Carbon Nanotubes (PtCoN/TiN–CNT) as Efficient Oxygen Reduction Reaction Catalysts in Acidic Medium. Int. J. Hydrogen Energy 2018, 43, 14337–14346. (200) Kwon, T.; Jeon, K.-W.; Dutta, S.; Lee, I. S. Conversion Chemistry of Nanoscopically Confined Manganese Silicate: Solid-State Route toward Porous Metal Oxide Catalyst–Support. Chem. Mater. 2018, 30, 8070–8078. (201) Ramesh, P.; Itkis, M. E.; Tang, J. M.; Haddon, R. C. SWNT−MWNT Hybrid Architecture for Proton Exchange Membrane Fuel Cell Cathodes. J. Phys. Chem. C 2008, 112, 9089–9094. (202) Grewal, S.; Macedo Andrade, A.; Nelson, A. J.; Thai, K.; Karimaghaloo, A.; Lee, E.; Lee, M. H. Critical Impact of Graphene Functionalization for Transition Metal Oxide/Graphene Hybrids on Oxygen Reduction Reaction. J. Phys. Chem. C 2018, 122, 10017–10026. (203) Kou, R.; Shao, Y.; Mei, D.; Nie, Z.; Wang, D.; Wang, C.; Viswanathan, V. V.; Park, S.; Aksay, I. A.; Lin, Y.; Wang, Y.; Liu, J. Stabilization of Electrocatalytic Metal Nanoparticles at Metal−Metal Oxide−Graphene Triple Junction Points. J. Am. Chem. Soc. 2011, 133, 2541– 2547. (204) Li, Y.; Li, Y.; Zhu, E.; McLouth, T.; Chiu, C.-Y.; Huang, X.; Huang, Y. Stabilization of HighPerformance Oxygen Reduction Reaction Pt Electrocatalyst Supported on Reduced Graphene Oxide/Carbon Black Composite. J. Am. Chem. Soc. 2012, 134, 12326–12329. (205) Summers, J. C.; Van Houtte, S.; Psaras, D. Simultaneous Control of Particulate and NOx Emissions from Diesel Engines. Appl. Catal. B Environ. 1996, 10, 139–156. (206) Ledoux, M. J.; Meunier, F.; Heinrich, B.; Pham-Huu, C.; Harlin, M. E.; Krause, A. O. I. Part I. n-Butane Dehydrogenation on Unsupported Carbon Modified MoO3 (MoOxCy): Effect of Steam on the Catalyst Stability. Appl. Catal. A Gen. 1999, 181, 157–170. (207) Narushima, T.; Goto, T.; Hirai, T. High-Temperature Passive Oxidation of Chemically Vapor Deposited Silicon Carbide. J. Am. Ceram. Soc. 1989, 72, 1386–1390. (208) Lv, H.; Mu, S.; Cheng, N.; Pan, M. Nano-Silicon Carbide Supported Catalysts for PEM Fuel Cells with High Electrochemical Stability and Improved Performance by Addition of Carbon. Appl. Catal. B Environ. 2010, 100, 190–196. (209) Gómez-Marín, A. M.; Bott-Neto, J. L.; Souza, J. B.; Silva, T. L.; Beck, W.; Varanda, L. C.; Ticianelli, E. A. Electrocatalytic Activity of Different Phases of Molybdenum Carbide/Carbon and Platinum-Molybdenum Carbide/Carbon Composites toward the 53 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 54 of 55

Oxygen Reduction Reaction. ChemElectroChem 2016, 3, 1570–1579. (210) Elbaz, L.; Kreller, C. R.; Henson, N. J.; Brosha, E. L. Electrocatalysis of Oxygen Reduction with Platinum Supported on Molybdenum Carbide–Carbon Composite. J. Electroanal. Chem. 2014, 720–721, 34–40. (211) Li, Y.; Zhang, X.; Wang, S.; Sun, G. Durable Platinum-Based Electrocatalyst Supported by Multiwall Carbon Nanotubes Modified with CeO 2. ChemElectroChem 2018, 5, 2442–2448. (212) Wang, Y.-J.; Wilkinson, D. P.; Neburchilov, V.; Song, C.; Guest, A.; Zhang, J. Ta and Nb CoDoped TiO 2 and Its Carbon-Hybrid Materials for Supporting Pt–Pd Alloy Electrocatalysts for PEM Fuel Cell Oxygen Reduction Reaction. J. Mater. Chem. A 2014, 2, 12681–12685. (213) Millán, M.; Zamora, H.; Rodrigo, M. A.; Lobato, J. Enhancement of Electrode Stability Using Platinum–Cobalt Nanocrystals on a Novel Composite SiCTiC Support. ACS Appl. Mater. Interfaces 2017, 9, 5927–5936. (214) Ma, X.; Meng, H.; Cai, M.; Shen, P. K. Bimetallic Carbide Nanocomposite Enhanced Pt Catalyst with High Activity and Stability for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134, 1954–1957.


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Table of Contents graphic

Tailor-Made Pt Catalysts with Improved Oxygen Reduction Reaction Stability/Durability

Kiranpal Singh, Emmanuel Batsa Tetteh, Ha-Young Lee, Tong-Hyun Kang and Jong-Sung Yu* Department of Energy Science and Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Republic of Korea


Tailor-Made Pt Catalysts with Improved Oxygen Reduction Reaction

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