Pt-based Catalysts For Oxygen Reduction Reaction Research Articles

Tailoring Zirconia Supported Intermetallic Platinum Alloy via Reactive Metal-Support Interactions for High-Performing Fuel Cells.

Developing efficient and anti-corrosive oxygen reduction reaction (ORR) catalysts is of great importance for the applications of proton exchange membrane fuel cells (PEMFCs). Herein, we report a novel approach to prepare metal oxides supported intermetallic Pt alloy nanoparticles (NPs) via the reactive metal-support interaction (RMSI) as ORR catalysts, using Ni-doped cubic ZrO2 (Ni/ZrO2) supported L10-PtNi NPs as a proof of concept. Benefiting from the Ni migration during RMSI, the oxygen vacancy concentrations in the support are increased, leading to an electron enrichment of Pt. The optimal L10-PtNi-Ni/ZrO2-RMSI catalyst achieves remarkably low mass activity (MA) loss (17.8 %) after 400,000 accelerated durability test cycles in a half-cell and exceptional PEMFC performance (MA=0.76 A mgPt -1 at 0.9 V, peak power density=1.52/0.92 W cm-2 in H2-O2/-air, and 18.4 % MA decay after 30,000 cycles), representing the best reported Pt-based ORR catalysts without carbon supports. Density functional theory (DFT) calculations reveal that L10-PtNi-Ni/ZrO2-RMSI requires a lower energetic barrier for ORR than L10-PtNi-Ni/ZrO2 (direct loading), which is ascribed to a decreased Bader charge transfer between Pt and *OH, and the improved stability of L10-PtNi-Ni/ZrO2-RMSI compared to L10-PtNi-C can be contributed to the increased adhesion energy and Ni vacancy formation energy within the PtNi alloy.

Recent progress of Pt-based oxygen reduction reaction catalysts for proton exchange membrane fuel cells

Recent progress of Pt-based oxygen reduction reaction catalysts for proton exchange membrane fuel cells

Ultradurable Pt-Based Catalysts for Oxygen Reduction Electrocatalysis

An oxygen reduction reaction (ORR) is the key half reaction of proton exchange membrane fuel cells (PEMFCs), and is highly dependent on Pt-based nanocrystals as core electrocatalysts. Despite the exceptional ORR activity from adjusting the electronic structures of surface or near-surface atoms, several serious issues, including the corrosion of carbon supports, the preferential leaching of active metal elements, the instability of surface low-coordinated atoms and the sintering/agglomeration of nanocrystals, still exist, challenging the ORR durability of developed Pt-based ORR catalysts. From the point of view of the catalyst structure design, in this review, we summarized the state-of-the-art structural regulation strategies for improving the ORR durability of Pt-based catalysts. The current limitation of Pt-based binary catalysts for ORR electrocatalysis is firstly discussed, and the detailed strategies are further classified into the optimization of supports, metal-doped alloys, core/shell structures, intermetallics and high-entropy alloys, etc. The structure–performance relationship is detailedly explained, especially emphasizing the elimination of the above restrictions. Finally, the existing challenges and future research direction are further presented, aiming at practicing the PEMFC devices of the ultradurable Pt-based catalysts.

Origin of Stability and Activity Enhancements in Pt-based Oxygen Reduction Reaction Catalysts via Defect-Mediated Dopant Adsorption.

Platinum alloys are highly efficient electrocatalysts for the oxygen reduction reaction (ORR) in acidic conditions. However, these alloys are susceptible to metal loss through leaching and degradation, leading to reduced catalyst stability and activity. Recently, it has been shown that doping with oxophilic elements can significantly alleviate these problems, with a prominent example being Mo-doped Pt alloys. Here, to achieve atomic scale understanding of the exceptional activity and stability of these alloys, we present a detailed density functional theory description of the dopants' structures and impact on electrocatalyst properties. Beginning with the Mo/Pt system, we demonstrate that Mo can be stabilized in the form of low-dimensional oxyhydroxide moieties on Pt defects. The resulting structures enhance stability and activity via distinct physical processes, with the Mo moieties both directly inhibiting Pt dissolution at defects and indirectly enhancing ORR activity by generation of strain fields on surrounding Pt terraces. We then generalize these analyses to other metal dopant elements, and we demonstrate that similar low-dimensional oxyhydroxide structures control the electrocatalytic properties through an intricate interplay of the structures' acid stability, intrinsic activity for the ORR, and ability to induce ORR-promoting strain fields on Pt.

Micro-Regulate Ionomer Distribution to Increase Pt Utilization for PEMFC

In the past decade, numerous Pt-based oxygen reduction reaction catalysts with high specific and mass activities have been developed, promising to reduce the cost of proton exchange membrane fuel cells (PEMFCs). However, the high activities are mostly achieved in rotating disk electrode (RDE) measurement and have rarely been accomplished at the membrane electrode assembly (MEA) level [1].One of the key reasons is the resistance caused by complex mass transport pathways, especially at higher current densities. The size and distribution of the ionomer aggregates in catalyst layers are found to be not uniform and easily affected by the concentration of the ionomer [2]. The proton transport is limited when there is no ionomer on the catalyst surface, while the oxygen transport is hindered when the ionomer film is too thick. Therefore, it is very important to control the ionomer distribution on the surface of catalysts.We designed a research protocol as shown in the figure: mask the Pt surface with carbon monoxide (CO) to change its surface energy and subsequently control the ionomer arrangement in the catalyst layer. The Knudsen diffusion resistance and ionomer diffusion resistance results suggest that the mass transport in the catalyst layer was suppressed by blocking and ionomer lean, indicating micro-construction of the catalyst layer to increase the active Pt area such as the thin and uniform ionomer distribution is an effective solution.

Shape and composition evolution of Pt and Pt3M nanocrystals under HCl chemical etching

Controlling the shape and composition of Pt-based nanocrystals is essential to improve electrocatalytic performance. In this work, we have carefully investigated the evolution process of morphology and composition for Pt and Pt3M (M = Ni, Co) nanocrystals by hydrochloric acid (HCl) etching. As a result, only Pt3Ni nanocrystals successfully formed unsaturated step-like atoms on the surface and then constructed high-index facets (HIFs), while Pt and Pt3Co preserved a good octahedron shape. Density functional theory (DFT) calculation suggests that Cl− ions can be tightly adsorbed on the surface of Pt3Ni rather than other nanocrystals, which hinders the deposition of newly-reduced atoms and thus regulating the surface morphology. Besides, the etching of surface transitional metals further accelerates the formation of HIFs. Boosted by the active sites on the surface, HCl-Pt-Ni exhibited a ∼10.8 and ∼11.3 times higher oxygen reduction reaction (ORR) mass and specific activities than commercial Pt/C catalyst, and possessed a good durability after 10,000 cycles test. This work gives a deep insight into the design of high-performance Pt-based ORR catalysts.

Atomically Dispersed Ni-N4 Sites Assist Pt3 Ni Nanocages with Pt Skin to Synergistically Enhance Oxygen Reduction Activity and Stability.

Currently, the rarity and high cost of platinum (Pt)-based electrocatalysts seriously limit their commercial application in fuel cells cathode. Decorating Pt with atomically dispersed metal-nitrogen sites possibly offers an effective pathway to synergy tailor their catalytic activity and stability. Here active and stable oxygen reduction reaction (ORR) electrocatalysts (Pt3 Ni@Ni-N4 -C) by in situ loading Pt3 Ni nanocages with Pt skin on single-atom nickel-nitrogen (Ni-N4 ) embedded carbon supports are designed and constructed. The Pt3 Ni@Ni-N4 -C exhibits excellent mass activity (MA) of 1.92 A mgPt -1 and specific activity of 2.65mA cmPt -2 , together with superior durability of 10mV decay in half-wave potential and only 2.1% loss in MA after 30000 cycles. Theoretical calculations demonstrate that Ni-N4 sites significant redistribute of electrons and make them transfer from both the adjacent carbon and Pt atoms to the Ni-N4 . The resultant electron accumulation region successfully anchored Pt3 Ni, that not only improves structural stability of the Pt3 Ni, but importantly makes the surface Pt more positive to weaken the adsorption of *OH to enhance ORR activity. This strategy lays the groundwork for the development of super effective and durable Pt-based ORR catalysts.

Recent developments in Pt-based alloy nanoparticles for oxygen reduction reaction in fuel cells

The significance of proton exchange membrane fuel cells (PEMFs) in economic recovery and climate action initiatives following a pandemic is growing. PEM fuel cells depend on precious-metal catalysts (often Pt or Pt-based alloys) to catalyze the reactions occurring at the electrodes. This is especially true for the cathodic oxygen reduction reaction (ORR), which is slow, represents a large source of energy loss, and brings the overall efficiency of fuel cells down. The performance, cost, and durability of these Pt-based catalysts for ORR hinder their widespread commercial application. As a realistic technique for decreasing Pt usage and related costs, researchers have concentrated intensively in recent years on producing more active and stable Pt-based alloy electrocatalysts. This review article mainly focuses on recent literature of Pt-based alloy electrocatalysts for ORR, especially the effect of nanoparticles (NPs) size, and shape and, therefore, the activity and stability of the generated electrocatalysts. The ORR activity and stability of Pt-based alloys such as Pt-Ni, Pt-Cu, and Pt-Fe, as well as other nano electrocatalysts, will be reviewed along with various synthetic approaches.

Micromodification of the Catalyst Layer by CO to Increase Pt Utilization for Proton-Exchange Membrane Fuel Cells.

Improving the utilization of platinum in proton-exchange membrane (PEM) fuel cells is critical to reducing their cost. In the past decade, numerous Pt-based oxygen reduction reaction catalysts with high specific and mass activities have been developed. However, the high activities are mostly achieved in rotating disk electrode (RDE) measurement and have rarely been accomplished at the membrane electrode assembly (MEA) level. The failure of these direct translations from RDE to MEA has been well documented with several key reasons having been previously identified. One of them is the resistance caused by complex mass transport pathways in the MEA. Herein, we improve the proton and oxygen transportations in the MEA by building a thin and uniform distribution of ionomer on the catalyst surface. As a result, a PEM fuel cell design is capable of showing a current density improvement of 38% at the same voltage (0.6 V) under the H2/air operation.

The graphene-supported Lanthanum oxide cluster as efficient bifunctional electrocatalyst for oxygen reaction

The development of bifunctional oxygen electrode electrocatalysis for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) plays a vital role in the solution of the energy crisis as well as the environmental pollution. The purpose of this paper is to investigate the potential of graphene-supported lanthanum oxide clusters (La2O3-Gra) as a bifunctional electrocatalyst; the results show that the optimal catalytic site (up-down1) of the most stable configuration (O-Gra-1) modified by OH group shows outstanding bifunctional catalytic activity with ηORR of 0.51 V and ηOER of 0.52 V, which can be compared to the commercial Pt-based ORR catalysts and the Ru/Ir-oxides OER catalysts. Furthermore, there is a strong scale-dependent relationship between the adsorption free energy (ΔGads) of reaction intermediates, resulting in volcano plots between the catalytic activity and ΔGads, which provides a valuable description for future performance prediction of structurally similar catalysts.

Pt-Based Oxygen Reduction Reaction Catalysts in Proton Exchange Membrane Fuel Cells: Controllable Preparation and Structural Design of Catalytic Layer.

Proton exchange membrane fuel cells (PEMFCs) have attracted extensive attention because of their high efficiency, environmental friendliness, and lack of noise pollution. However, PEMFCs still face many difficulties in practical application, such as insufficient power density, high cost, and poor durability. The main reason for these difficulties is the slow oxygen reduction reaction (ORR) on the cathode due to the insufficient stability and catalytic activity of the catalyst. Therefore, it is very important to develop advanced platinum (Pt)-based catalysts to realize low Pt loads and long-term operation of membrane electrode assembly (MEA) modules to improve the performance of PEMFC. At present, the research on PEMFC has mainly been focused on two areas: Pt-based catalysts and the structural design of catalytic layers. This review focused on the latest research progress of the controllable preparation of Pt-based ORR catalysts and structural design of catalytic layers in PEMFC. Firstly, the design principle of advanced Pt-based catalysts was introduced. Secondly, the controllable preparation of catalyst structure, morphology, composition and support, and their influence on catalytic activity of ORR and overall performance of PEMFC, were discussed. Thirdly, the effects of optimizing the structure of the catalytic layer (CL) on the performance of MEA were analyzed. Finally, the challenges and prospects of Pt-based catalysts and catalytic layer design were discussed.

Understanding the growth N–Fe3C from assembly of carbon-coated iron nanoparticles on rGO as efficient oxygen reduction electrocatalysts

The oxygen reduction reaction (ORR) is an essential reaction in metal-air battery systems. However, the traditional noble metal Pt-based catalysts for ORR limit the commercial applications of air battery systems due to their high price, low abundance and poor tolerance. In recent studies, nitrogen–doped transition metal carbides have shown promise as an alternative to traditional noble metal catalysts. In this paper, the thermal cracking and calcination method was used to successfully obtain the growth N-doped carbon coated iron carbide nanoparticles (Fe3C@NC/rGOx NP) from assembly of carbon-coated iron oxide Nano-crystals on rGO (Fe3O4@NC/rGOx NCs). Transmission electron and scanning electron microscopy of the fabricated specimens indicated the presence of many carbon shells and pores in the etched samples. The electrochemical test results of the fabricated specimens suggested that the Fe3C@NC/rGO 100 NP annealed at 800 °C exhibited a better ORR behavior than that of Pt/C in a basic electrolyte.

N-Doped Colloid Imprinted Carbons As Promising ORR Catalysts for Alkaline Applications

The oxygen reduction reaction (ORR) has long been of interest in relation to its many energy applications and interesting multi-pathway mechanisms. The ORR is a key reaction in a range of electrochemical energy conversion and storage devices, such as hydrogen fuel cells and metal-air batteries, respectively. These devices are expected to play an ever-increasing role in the global transition to net zero emissions. Metal-air batteries, such as Zn/air batteries, can operate at room temperature, use recyclable materials, are environmentally friendly, and are preferred in relation to consumer safety than batteries relying on organic solvents and reactive electrode species.1 High rates of the ORR are crucial for the development of high performance Zn/air batteries and catalysts play a significant role, with lowering of the catalyst cost also of increasing importance.The costs of conventional Pt-based ORR catalysts are high. Therefore, metal-free carbon-based ORR electrocatalysts are viewed as increasingly promising alternatives, especially as they are lower in cost due to the availability of the precursor materials. Carbon is also a good electrical conductor and support material, is chemically stable, and can have large surface areas. However, the ORR kinetics are sluggish on carbon and chemical and physical modifications are required to enhance its activity. In typical Zn/air batteries, the ORR occurs at the three-phase boundary (TPB) formed between the solid electrode, liquid electrolyte, and gaseous oxygen. The porosity of the catalyst layer, the wettability of the catalyst/electrolyte interface, and the gas permeability and hydrophobicity of the gas diffusion layer (GDL) are thus also important, significantly influencing the cathode performance and durability. The catalyst layer (CL) must therefore be constructed with both a high-performance catalyst and an optimized TPB length to provide high performance without compromising durability.In the current work, we have doped nitrogen into the lattice of a family of nanoporous colloid imprinted carbon (CIC) powders to increase its ORR activity. The CICs are unique for their versatility in terms of pore size control and ease of surface functionalization.2 Pore sizes in the range of 12 to 100 nm were examined and their effect on the ORR activity and mass transport limitations were investigated. To carry out N-doping, the CICs were exposed to ammonia at 800 ˚C for 7 hr. Catalyst inks were then prepared by mixing the CICs with a binder in an isopropyl alcohol/water solution. Aliquots of the ink were drop-casted on the disc of an RRDE system, or were spray coated or drop-casted on a GDL to determine the ORR performance in an in-house Zn/air battery testing cell, with the N-doped CIC catalyst layer sandwiched between an electrolyte chamber and a graphite current collector. In this setup, O2 gas was flowed through the pores in the GDL to the catalyst/electrolyte interface, a Zn wire installed in the electrolyte chamber was used as the reference electrode, and a Ni sponge was used as the counter electrode.The RRDE experiments showed that, after N doping of the CIC powders, the production of peroxide decreased significantly and the ORR onset potential increased to a very respectable value of ca. 0.9 V vs RHE, indicating the successful activation of the ORR. Electron transfer numbers were found to be greater than 3.5, indicating that either a direct or pseudo- 4 electron transfer ORR pathway is dominant. In agreement with the literature, the ORR currents in the kinetic regions increased linearly with mass loading, expected to be proportional to the total active N-doped CIC surface area.3 The N-doped CIC samples retained excellent performance up to a loading of 0.350 μg/cm2 without losing mechanical stability.Similar N-doped CICs and binders of different hydrophobicity were tested in the Zn/air battery testing system. Electrodes made with a hydrophobic NCS microporous layer (MPL) showed much better ORR performance and durability than hydrophilic NCS MPLs. Although electrodes made using hydrophobic polytetrafluoroethylene (PTFE) as the binder in the catalyst layer showed a similar initial performance to those made using hydrophilic Nafion binders, the PTFE based electrodes exhibited better durability. Furthermore, the temperature and pressure used during electrode fabrication were also found to have a significant impact on the binder distribution and ORR performance. Once optimized, a very good correlation was obtained between the N-doped CIC catalyst performance in the RRDE setup and in the Zn/air battery testing system. References J. Pan et al., Adv. Sci., 5, 1700691 (2018).X. Li et al., ACS Appl. Mater. Interfaces, 10, 2130–2142 (2018).N. Gavrilov et al., J. Power Sources, 220, 306–316 (2012).

Investigation of Oxygen Reduction Reaction of Graphene Supported Metal-N4 Catalysts via Density Functional Theory

The high-dense metal-air batteries are difficult to commercialize on a large scale mainly because of sluggish kinetics of air electrode. The catalysts are of crucial importance for the rate of oxygen reduction reaction (ORR), among which Pt-based catalysts for ORR have shortcomings in stability and cost, and the kind of catalysts with adding C and N to transition metals receive more attention. Here we analyze catalytic performance of graphene supported transition metals-N4(M-N4@G) for ORR based on density functional theory (DFT), verifying rationality of such catalysts with five different transition metals (Pt, Fe, Co, Pd and Ni) embedded in the graphene, and demonstrating that Fe-N4@G has better ORR performance than Pt-N4@G. Moreover, a proposed mechanism of ORR (generating free *O and *OH) is explored to optimize ORR by means of transition-state search in the DFT calculation. Additionally, a novel phenomenon is observed that graphene has a strong attraction to hydrogen atoms, which is facilitated to promote hydrogen evolution reaction of graphene supported catalysts.

Metal-free catalyst for efficient pH-universal oxygen reduction electrocatalysis in microbial fuel cell

• A metal-free and efficient pH-universal ORR catalyst is prepared by simple pyrolysis. • ZIF-8#1000 exhibited ORR electrocatalytic activity, compared with that of 20% Pt/C. • ZIF-8#1000 showed high-performance in air–cathode MFCs. • Three single-chamber MFCs in series can light a light-emitting diode. Developing and utilization of clean and renewable energy is the key to alleviate energy crisis and environmental problems. As a biomass energy utilization, microbial fuel cell (MFC) suffers from low power generation performance caused by the sluggish electronic transfer kinetics of cathodic oxygen reduction reaction (ORR). It is of significance to develop efficient ORR catalysts to get a commercial-scale version in fuel cells. ZIF-8 was usually joined other metal elements (Pt, Fe, Co and etc) to improve ORR catalytic ability. Unfortunately, these catalysts are unstable under acidic conditions. Therefore, it is of great significance to develop an efficient pH-universal ORR catalyst. Herein, a kind of metal-free and efficient pH-universal ORR catalyst are fabricated via a simple method. The catalyst (ZIF-8#1000) exhibits excellent pH-universal ORR electrocatalytic activity and high-performance in single-chamber MFC (distinct output voltage 780 mV and maximum power density 600 mW m −2 ), comparable to those of 20% Pt/C. Interestingly, three single-chamber MFCs in series can light a light-emitting diode. Thus, the as-prepared ZIF-8#1000 has the potential to be alternative of Pt-based ORR catalysts.

Standardized Protocols for Platinum Group Metal-Free Fuel Cell Catalysts for Oxygen Reduction Reaction

In spite of very significant progress in performance of platinum group metal-free (PGM-free) electrocatalysts for oxygen reduction reaction (ORR) in the past two decades, the activity and especially the stability and durability of these catalysts remain insufficient. This applies even to the catalysts derived from transition metals, nitrogen, and carbon via a high-temperature synthesis path, which have consistently shown the best performance among all PGM-free ORR catalysts developed to date. A very important requirement for accelerating the much needed progress in PGM-free electrocatalysis is the development of a common catalyst test platform, especially, since it became apparent some years ago that protocols developed for Pt-based ORR catalysts are inadequate for their non-precious metal counterparts due to the very different performance degradation routes in these two cases.In response to the growing need for uniform PGM-free catalyst test protocols, U.S. Department of Energy delegated the task of completing their development and validation to the Electrocatalysis Consortium (ElectroCat), created in 2016 as part of DOE-EERE’s Energy Materials Network (EMN). In this presentation, we will introduce protocols and performance test metrics, which have been approved by DOE’s Hydrogen and Fuel Cell Technologies Office. We will also briefly summarize the results from the round-robin testing of two PGM-free catalysts, a commercial material from Pajarito Powder and a Fe-N-C catalyst synthesized at Los Alamos, recently completed at U.S. three national laboratories.We are using this opportunity to disseminate PGM-free test protocols and also justify their adoption in the electrocatalysis community with experimental data, which will be reported in greater detail in a separate presentation at this ECS meeting. Acknowledgements This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office under the auspices of the Electrocatalysis Consortium (ElectroCat).

Stabilizing Pt-Based Electrocatalysts for Oxygen Reduction Reaction: Fundamental Understanding and Design Strategies.

Proton exchange membrane fuel cells (PEMFCs) with high efficiency and nonpollution characteristics have attracted massive attention from both academic and industrial communities due to their irreplaceable roles in building the future sustainable energy system. However, the stability issue of Pt-based catalysts for oxygen reduction reaction (ORR) has become a central constraint to the widespread deployment of the devices relative to the catalytic activity. This review aims to provide comprehensive insights into how to improve the stability of Pt-based catalysts for ORR. First, the basic physical chemistry behind the catalyst degradation, including the fundamental understandings of carbon corrosion, catalyst dissolution, and particle sintering, is highlighted. After a discussion of advanced characterization techniques for the catalyst degradation, the design strategies for improving the stability of Pt-based catalysts are summarized. Finally, further insights into the remaining challenges and future research directions are also provided.

Recent developments in Pt–Co catalysts for proton-exchange membrane fuel cells

Development of Pt-based oxygen reduction reaction catalysts with high efficiency and high durability is central to the application of proton-exchange membrane fuel cell systems. Pt–Co bimetallic catalysts have drawn extensive attention owing to their capability of delivering high performance and long lifetime for fuel cell applications including light-duty and heavy-duty vehicles. However, further improvements in durability and performance are needed to meet market requirements. To fully exploit the potential of Pt–Co catalysts, new insights into the relationship between catalyst properties and fuel cell performance and durability are needed, and more effective methods to tailor the features of Pt–Co catalysts need to be developed. This review provides a summary and perspective on recent efforts, including work on customizing the Pt shell and Pt:Co ratio, tailoring the crystal structure, and improving carbon support properties, with a particular emphasis on mechanisms leading to enhancement of mass activity, power density, and durability in membrane electrode assembly testing.

Carbon-Free Connected Platinum–Iron Catalysts with Enhanced Chemically Ordered Structures As Durable Oxygen Reduction Electrocatalysts for PEFCs

To realize the widespread uses of polymer electrolyte fuel cells (PEFCs), the improvement of the activity and durability of Pt-based electrocatalysts for oxygen reduction reaction (ORR) is essential. In our group, a carbon-free connected Pt–Fe catalyst with a porous hollow capsule structure has been developed. [1–3] This catalyst consists of a nanosized beaded network by the connection of Pt–Fe nanoparticles. The beaded metal network is electrically conductive, enabling the removal of carbon supports form a catalyst layer. The carbon-free cathode catalyst layer using a connected Pt–Fe catalyst exhibits high durability against start/stop cycles, because of the elimination of carbon corrosion problems. In addition, the ORR specific activity of a connected Pt–Fe catalyst is about 9 times higher than that of a commercial Pt nanoparticle catalyst supported on carbon black (Pt/C). Thus, a connected Pt–Fe catalyst provides a high ORR activity and excellent start/stop durability. On the other hand, the dissolution of catalyst metals during a load cycle operation is one of critical issues for Pt-based ORR catalysts. Our previous studies using Pt-alloy nanoparticle catalysts on carbon black demonstrate that chemical ordered (superlattice) structures improve load cycle durability due to the suppression of metal dissolution. [4–8] Therefore, as shown in Figure 1A, this study developed a carbon-free connected Pt–Fe catalyst with a high chemical ordering structure to achieve high durability against both start/stop and load cycle operations.In this study, we proposed a new synthesis method using the combination of a SiO2 coating and an annealing treatment (Figure 1B) to control chemically-ordered (face centered tetragonal: fct) structures in a connected Pt–Fe catalyst. The SiO2 layer coated on catalyst surfaces prevents the detachment of Pt–Fe nanoparticles and the large catalyst agglomeration and coalescence during the high-temperature annealing. Using this method, the connected Pt–Fe catalyst having a nano-sized network and a high fct degree (70–80%) was successfully prepared. This catalyst exhibited ca. ten times higher ORR specific activity, compared with that of the Pt/C. Furthermore, as shown in Fig. 1C, the connected Pt–Fe catalyst with the high fct degree (76%) showed the higher retention of the ORR specific activity after 10000 load cycles (0.6 V for 3 s and 1.0 V for 3 s in N2-saturated 0.1 M HClO4 solution at 60 °C), compared to the catalyst with the low fct degree (47%). The results of the STEM-EDX line mapping indicated that a highly ordered fct structure greatly contributes to the suppression of metal dissolution from the catalyst.This study demonstrated, for the first time, the synthesis method as shown in Figure 1B to prepare a connected Pt–Fe catalyst with a nanosized network and a high chemical ordering structure. The carbon-free connected Pt–Fe catalyst with an enhanced chemically ordered structure exhibited a high ORR activity as well as improved durability against both start/stop and load cycle operations. The results obtained in this study provide useful guidelines for the design of high-performance ORR catalysts for PEFCs. Acknowledgements This work was financially supported by Kanagawa Institute of Industrial Science and Technology (KISTEC) and Core Research for Evolutionary Science and Technology at the Japan Science and Technology Agency (JST-CREST, JPMJCR1543).

Facile Preparation of a Surface-Enriched Pt Layer Over Pd/C as an Efficient Oxygen Reduction Catalyst With Enhanced Activity and Stability

Abstract Pt-enriched surface layer formation on Vulcan carbon-supported Pd (Pt@Pd/C) was successfully prepared through a simple and one-pot formic acid reduction approach without any stabilizing agent. The electrocatalytic performance of Pt@Pd/C catalyst toward an oxygen reduction reaction (ORR) in alkaline medium was studied and also compared with standard carbon-supported Pt (Pt/C) and Pd (Pd/C) catalysts. The Pt@Pd/C exhibits higher electrochemical active surface area (74.7 m2/g) and mass activity (1.38 mA/µg) than Pt/C, Pd/C, and contending with standard reported catalysts. In durability tests, Pt@Pd/C showed negligible loss of intrinsic activity (∼10%) after 10,000 cycles which confirmed improved stability than Pt-based catalysts for ORR in KOH medium. This improved electrocatalytic performance could be attributed to their structural characteristics of the Pt-enriched surface layer on Pd/C-core and the compressive lattice strain on Pt. The present investigation demonstrates the simple preparation procedure for surface-enriched Pt on Pd/C and its improved performance for ORR, suggesting that it is a promising contender to benchmark ORR catalysts for alkaline fuel cells.