Modifying Pt-based catalysts with organic molecules offers a promising strategy to enhance oxygen reduction reaction (ORR) performance. In this study, dopamine (DA) was used to modify Pt/C, and its impact on electrochemical behavior and ORR activity was systematically investigated. Through DA modification, the ORR activity can be improved to 1.5 times that of pristine Pt/C. This enhancement is linked to DA's ability to maintain a more metallic Pt surface by preventing excessive oxidation. Density functional theory (DFT) calculations indicate that DA binds moderately strongly to the Pt surface, based on π-bonds between Pt and the aromatic carbons in a flat configuration that partially covers the surface while leaving active sites accessible. DA effectively inhibits the formation of surface-blocking OHtop–OHbridge pairs on (111) terraces, thus preserving O2 adsorption sites at (110) edges and unpaired OHtop sites, which are necessary in the ORR. Thus, this work provides mechanistic insight into DA's function and suggests a practical route to improving fuel cell performance through organic–inorganic interface engineering.

Iridium oxide (IrOx) catalysts supported on Nb-doped SnO2 (Nb dopant content of 4 at. %) were prepared with different Ir loadings. The microstructure of IrOx on this support material transforms from dispersed nanoparticles into interconnected nanoparticle networks, accompanied by improved conductivity and reduced work function. These electronic and structural evolutions enhance the oxygen evolution reaction (OER) activity, with both mass and specific activities nearly doubling as the Ir loading increases from less than 10 wt % to over 30 wt %. The results highlight how tuning IrOx morphology and interfacial energetics enable efficient OER catalysis at optimized Ir loadings, offering a pathway toward cost-effective and scalable proton exchange membrane (PEM) water electrolysis.

The scavenging effect to mitigate the membrane degradation by H2O2 is confirmed by the newly developed Pt catalyst supported on M-CeO2 [Pt/M-CeO2 (M = Gd, Zr)]. The production of H2O2 via a two-electron pathway of the oxygen reduction reaction is reduced to less than half that of Pt/CB. The Ce3+ on the M-CeO2 (M = Gd, Zr) surrounded on the Pt catalyst may play a role in the generation of the scavenging effect. The single cell using a thinner Gore-Select membrane and a Pt/Zr-CeO2 anode operates for longer periods at high temperatures (120 degrees C), while maintaining low resistive overpotential and low crossover current compared to those using commercial Pt catalyst supported on the carbon black (Pt/CB) anode. We consider that the application of the newly functionalized Pt/Zr-CeO2 anode would help realize heavy-duty vehicles equipped with polymer electrolyte fuel cells.

The global hydrogen demand from the water electrolysis systems will increase rapidly within the next decade. One of the water electrolysis systems of proton exchange membrane water electrolysis (PEM WE) has been receiving important attention owing to the advantages of high energy conversion efficiency. PEM WE has several unique characteristics, such as highly purified hydrogen produced directly by suppling pure water to the anode-side, but it is vulnerable to the issue of global Ir availability. We proposed that the low Ir loading anodic catalyst layers of IrO x nanoparticles supported on the specially designed oxide nanoparticles. The specially designed oxide nanoparticles of Nb doped SnO 2 (Nb-SnO 2 ) and Sb doped SnO 2 (Sb-SnO 2 ) were synthesized in the flame oxidation technique. The obtained particles have several attractive characteristics, including relatively high surface area, carbon-like network structure and high crystallinity, which result in high catalyst loading amounts, electrical conductivity and chemical stability while maintaining effective gas transport pathways. An IrO x catalyst supported on Sb or Nb doped SnO 2 (IrO x /Nb-SnO 2 or IrO x /Sb-SnO 2 ) showed a ten-fold higher activity for the oxygen evolution reaction (OER) than that of a commercial IrO x . The single cell performance using these anodic catalyst layer (Ir loading amount: 0.20 mg cm -2 ) achieved a current density of over 1.0 A cm -2 at 1.6 V with chemical stability (@80 o C). The cell performance and material characterization (XRD, XPS, XAFS etc.) will be discussed in detail.

We have developed a novel Pt/C electrocatalyst for polymer electrolyte fuel cells (PEFCs) by utilizing an ordered mesoporous carbon with a 3D network structure (nw-OMC). In this study, in situ transmission electron microscopy and scanning transmission electron microscopy measurements were conducted to investigate the structural changes of the nw-OMC support and dispersion behavior of Pt nanoparticles at elevated temperatures under various atmospheres. The results revealed that structural changes of the catalyst significantly depend on both temperature and atmosphere. Notably, under a nitrogen or hydrogen atmosphere at 180 degrees C, Pt particles within the nanopores migrated to deeper regions, enabling precise control of their positions in the pores. This adjustment optimizes the catalyst structure, contributing to enhanced activity and durability under PEFC operating conditions.

The oxygen evolution reaction (OER) is a key reaction in the process of water electrolysis, where water is split into hydrogen and oxygen. Hydrogen is a clean fuel that can be used in fuel cells or for energy storage. Efficient catalysts are required to lower the energy barrier for the OER, thereby making hydrogen production more energy-efficient and economically viable. Nickel-based catalysts provide a cost-effective, abundant, and highly efficient alternative to iridium for the OER, especially in alkaline environments. By improving the catalyst design, alloying it with other metals, and optimizing the catalyst structure, the efficiency and durability of the catalyst can be further improved. Here, we developed a new catalyst material, a NiFe alloy hetero-structured with Ni oxide, and investigated the effect of their structuring on the alkaline OER activity. A two-step procedure was used to prepare the Ni oxide/NiFeO x (NiO x /NiFeO x ) electrocatalysts. Firstly, the NiFe oxide particles was synthesized by flame pyrolysis, 1-4 followed by H 2 reduction heat treatment to generate NiFe alloy surface. Ni cations were then deposited on the reduced NiFe oxide particle surface by an impregnation method, heat treated in H 2 atmosphere and then exposed to O 2 atmosphere at room temperature. Figure 1a shows a TEM image of the NiO x /NiFeO x catalyst (treated at 200 o C). The catalyst particles interconnect with each other to form a network structure, which is expected to be favorable for electronic conduction and mass transport during OER electrocatalysis. The high-resolution TEM image (Figure 1b) shows that a uniform amorphous NiO x layer is constructed on a crystalline phase with a lattice space of 0.205 nm, which corresponds to the (111) plane of the face-centered cubic (FCC) NiFe alloy as interlayer. Figure 1c shows the OER polarization curves measured in 1 M KOH. The OER activity is shown to be greatly enhanced on the NiO x /NiFeO x catalyst, with the mass activity (@1.53 V) being 6.2 and 2.4 times higher than that of NiFeO x and commercial IrO x catalysts, respectively. The high OER activity of NiO x /NiFeO x will help to reduce the overpotential in water electrolysis, thereby improving the efficiency of the process. The structure-activity relationship was further elucidated based on a combination of experiments and density functional theory (DFT) calculations. These results are expected to aid in the design of highly efficient and durable OER catalysts for hydrogen production by alkaline water electrolysis. Acknowledgements This work was partly supported by the JSPS KAKENHI (23H02059), Concert-Japan framework by the JST and the Federal Ministry of Education and Research (BMBF) (FKZ: 01DR21028 and FKZ: 01DR22004), and the projects from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe. Electrochim. Acta, 56, 2881-2887 (2011). G. Shi, T. Tano, D. A. Tryk, M. Yamaguchi, A. Iiyama, M. Uchida, K. Iida, C. Arata, S. Watanabe, and K. Kakinuma. ACS Catal., 12, 14209-14219 (2022). G. Shi, T. Tano, D. A. Tryk, T. Uchiyama, A. Iiyama, M. Uchida, K. Terao, M. Yamaguchi, K. Tamoto, Y. Uchimoto, and K. Kakinuma. ACS Catal., 13, 12299-12309 (2023). G. Shi, T. Tano, D. A. Tryk, A. Iiyama, M. Uchida, K. Terao, H. Osada, M. Yamaguchi, K. Tamoto, and K. Kakinuma. ACS Catal., 14, 9460-9468 (2024). Figure 1

INTRODUCTION A Pt/carbon-based catalyst using an ordered mesoporous carbon with a network structure (nw-OMC) as the support was recently developed by our group, demonstrating higher ORR activity than commercial Pt/CB catalysts. In this study, the effects of the hierarchical porous structure—comprising nanopores, primary pores, and secondary pores—on the I–V characteristics of the catalyst layer were investigated. Furthermore, the effect on catalytic performance of the Pt dispersion and deposition location within the nanopores was examined in detail. EXPERIMENTAL nw-OMC powder was synthesized using resol–nonionic surfactant micelles as both the carbon source and the structure-directing agent. Resol–F127 micelles were prepared by mixing phenol, formaldehyde, water, and F127 with sodium hydroxide. After hydrothermal treatment, the resulting solid was filtered, thoroughly washed, and dried under vacuum. The obtained powder was carbonized at 700 ℃ and subsequently annealed at 1000 ℃. Pt was deposited onto the nw-OMC powder via selective deposition techniques. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were conducted in N₂- and O₂-saturated 0.1 M HClO₄ solution at 25 ℃ using a conventional rotating disk electrode (RDE) setup. Potential step cycling measurements simulating load cycling between 0.6 V and 1.0 V vs. RHE were performed following the FCCJ protocol. Catalyst ink was applied to a glassy carbon disk substrate using an electrospray (ES) coating technique developed by our group. The morphology of Pt/nw-OMC was characterized by scanning transmission electron microscopy (STEM). Catalyst-coated membranes (CCMs) were fabricated using a pulse-spray technique. Nafion NRE212 was employed as the membrane, and TEC10E50E was used as the anode catalyst. MEA measurements were conducted using a JARI single cell (1 cm²). Morphological analyses before and after electrochemical evaluation were performed by STEM, TEM, and cross-sectional SEM. RESULTS AND DISCUSSION Previous studies have shown that the synthesized nw-OMC consists of ~80 nm OMC nanoparticles interconnected to form a network with well-developed primary and secondary pores (100–several hundred nm). On the surface of the OMC particles, nanopores (~5 nm in diameter) were found to be regularly spaced (~9 nm), and selective deposition of Pt nanoparticles inside the nanopores was achieved at a ratio of 60–70%. RDE testing at room temperature indicated that the Pt/nw-OMC catalyst exhibits superior ORR mass activity and durability compared to a commercial Pt/CB (TEC10E50E). In the present work, further improvements were achieved by decreasing the primary particle size of the OMC and enhancing the Pt dispersion. STEM measurements revealed that the average OMC particle size was below 50 nm, forming a well-developed network structure (Fig. 1). The average Pt particle size was decreased to 2.5 nm, in contrast to 4 nm in previous samples. RDE measurements showed a mass activity of 475 A g⁻¹ at 0.9 V, more than double that of the commercial Pt/CB. However, although the catalyst exhibited excellent mass activity in RDE testing, the performance in the MEA evaluation was hindered by excessive void formation within the catalyst layer. This issue was attributed to the tendency of the network-structured OMC to form secondary aggregates during ink preparation, especially under non-optimized mixing conditions. These voids negatively affected the uniformity of the pore structure and the interfacial contact between the catalyst layer and the membrane. In addition to this issue, insufficient mixing between the ionomer and the Pt/OMC catalyst resulted in non-uniform ionomer coverage on the catalyst surface during CL formation, further limiting MEA performance. To overcome this issue, the ink formulation process was carefully optimized by tuning the dispersion and mixing conditions to prevent secondary aggregation of the nw-OMC particles. This optimization effectively suppressed the formation of coarse voids, resulting in a more uniform and well-connected pore structure and enhanced interfacial adhesion between the catalyst layer and the membrane. As a result of the improved pore structure and interfacial contact, the I–V performance of the MEA surpassed that of commercial Pt/CB across the entire current density range (Fig. 2), with a particularly notable lowering of the concentration overpotential during high current density operation. Acknowledgement: This work was partially supported by the ECCEED’30-FC project from NEDO. Figure 1

Current-voltage performance and durability in polymer electrolyte fuel cells should be improved further toward the application of heavy-duty vehicles. Pt and Pt-alloy catalyst supported on the porous carbon is one of the essential catalysts, and is in progress to obtain these performances. One of the other interesting catalysts is the Pt catalysts supported on doped SnO 2 (Pt/Nb-SnO 2 ) without carbon additives. The durability (startup/shutdown, load cycling) and oxygen reduction reaction (ORR) activity of the Pt/Nb-SnO 2 are superior to those of commercial Pt catalysts supported on carbon black (Pt/CB) etc. [1-8] These catalysts have a unique carbon-like microstructure of a fused-aggregate network structure, which supply the essential function of the electronically constructing pathways via necking of each support particle and the gas diffusion pathways via the open pores surrounded by particles. The meta-support interaction at the interface between Pt and these oxide supports affects the electronic state of the Pt catalysts, which enhance the catalytic activity and durability [5] . The IV performance of the single cell using the Pt /Nb-SnO 2 cathode catalyst layers at operating temperatures from 80 o C to 120 o C approaches the I-V performance at the end of life of NEDO target. The Pt/Nb-SnO 2 is one of the candidate catalysts for application of the heavy-duty vehicles operating at wide-temperature range (< 120 o C) [9] . Acknowledgments This work was partially supported by funds for the project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, and JSPS KAKENHI Grant Number (23H02059) from the Ministry of Education, Culture, Sports, Science and Technology. Referencies K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, M. Watanabe, Electrochim. Acta , 56 , 2881 (2011). Y. Senoo, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, M. Watanabe, RSC Adv. , 6 , 321800 (2014). Y. Chino, K. Taniguchi, Y. Senoo, K. Kakinuma, M. Watanabe, M. Uchida, J. Electrochem. Soc. , 162 , F736 (2015). K. Kakinuma, R. Kobayashi, A. Iiyama, M. Uchida, J. Electrochem. Soc ., 165 , J3083 (2018). K. Kakinuma, K. Suda, R. Kobayashi, T. Tano, C. Arata, I. Amemiya, S. Watanabe, M. Matsumoto, H. Imai, A. Iiyama, M. Uchida, ACS Appl. Mater. Interfaces 11 , 34957 (2019). K. Kakinuma, M. Hayashi, T. Hashimoto, A. Iiyama, M. Uchida, ACS Appl. Energy Mater. , 3 , 6922 (2020). G. Shi, T. Tano, D.A. Tryk, A. Iiyama, M. Uchida, K.Kakinuma, ACS Catal ., 11 , 5222 (2021). G. Shi, T. Tano, D.A. Tryk, A. Iiyama, M. Uchida, Y. Kuwauchi, A. Masuda, K. Kakinuma, J. Catal. , 407 , 300 (2022). K. Kakinuma, H. Taniguchi, T. Asakawa, T. Miyao, M. Uchida, Y. Aoki, T. Akiyama, A. Masuda, N. Sato, A. Iiyama J. Electrochem. Soc. 169 , 044522 (2022).

For polymer electrolyte fuel cells (PEFCs), platinum nanoparticles supported on carbon black (Pt/C) serve as the commonly used hydrogen anode catalyst, exhibiting high activity for the hydrogen oxidation reaction (HOR), while the carbon support is susceptible to corrosion under PEFC operation. Here, a highly active HOR anode catalyst of Pt nanorod supported on niobium (Nb)-doped ceria without using corrosive carbon support was developed, which exhibits high durability at high potentials associated with hydrogen starvation. The production of hydrogen peroxide (H2O2), which can degrade the polymer electrolyte membrane, was also found to be significantly suppressed on the Pt nanorod/doped ceria catalyst. Density functional theory (DFT) calculations suggests that the Pt nanorod geometry and interaction with Nb and Ce favor HOR activity and stability while suppressing H2O2 production by modulating the adsorption of key reaction intermediates. This new catalyst has the potential to be used as an anode for high-performance and high-durability PEFCs.

Development of highly durable and active electrocatalysts for polymer electrolyte fuel cells (PEFCs) is an important research topic for future hydrogen energy applications. In PEFCs used for fuel cell vehicles (FCVs), it is known that the high cathode potential causes corrosion of the catalyst support carbon, especially during startup and shutdown operations. It is also known that PFSA-based ionomers directly contacting the Pt surface cause poisoning and deactivation of catalytic activity for the oxygen reduction reaction. There has been interest in immobilization of Pt particles inside nanopores at adequate depth regions (accessible region) at which direct contact with ionomers is suppressed, without any significant limitation of mass transport of reactants and products1). We have recently developed a new Pt/C electrocatalyst using an ordered mesoporous carbon having a network-structure (Net-OMC) with a three-dimensional macrostructure and ordered nanopore (several-nm size) arrangement with ca. 5 nm diameter2)3). In this study, morphologic changes of a Pt/Net-OMC electrocatalyst were studied using in situ TEM/STEM at elevated temperatures and in various gas atmospheres. The Net-OMC was prepared as previously reported2). In this study, the annealing temperature of the Net-OMC sample was 1000 °C. Loading of Pt on the Net-OMC support was carried out using a colloidal technique2)3). The loading amounts of Pt for Pt/Net-OMC were 20 to 30 wt.%. The high-resolution analytical TEM (H-9500, Hitachi High-Tech) was used to observe the characteristics of the catalyst samples at elevated temperatures in various gas atmospheres. The sample was mounted on a heating element of a direct-heating type sample heating holder equipped with a gas supply injector4). During TEM measurements, the total pressure in the vicinity of the catalyst sample was controlled to around 0.2 Pa, and, in order to minimize damage caused by electron beam irradiation, the latter was limited to a short period of time. After sample measurement and cooling down to room temperature, the sample was transferred to the high-resolution STEM with a cs-corrector lens system (HD-2700, Hitachi High-Tech), without detaching the sample from the sample holder in order to observe secondary electron (SE) images at identical locations of the samples. In situ TEM observations were performed from room temperature to 200 °C under various gas atmospheres. At the initial stage, highly dispersed Pt particles were predominantly located at the near outermost surface of nanopores of the Net-OMC support particles. It was found that most Pt particles in the nanopores of Net-OMC migrated 2-5 nm from their original depth positions with increasing temperature. Figure 1 shows the results of SE images for the Pt/Net-OMC catalyst after in situ TEM observations. SE images observed after heating in oxygen (Fig. 1(b) and hydrogen (Fig. 1(c)) flow revealed that Pt particles in the nanopores migrated from the surface to greater depths. Although corrosion of the Net-OMC support progressed to some extent, it was found that agglomeration and growth of Pt nanoparticles hardly progressed. When this carbon support is used as an electrocatalyst, even if catalyst deterioration progresses, it is expected to suppress aggregation of Pt particles due to the feature of accessible nanopores of the Net-OMC support. In the previous study by our group, the Pt/Net-OMC catalyst exhibited remarkably higher durability than a commercial Pt/C during potential step cycling measurement2)3). These results suggest that the more stable characteristics of the catalyst can be easily created by pre-heat treatment in the appropriate atmosphere, and indicate that the Net-OMC is a particularly effective support to combat Pt migration and aggregation. This work was partially based on results obtained from project JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO) Japan. References 1) V. Yarlagadda, M. Carpenter, T. Moylan, R Kukreja, R. Koestner, W. Gu, L. Thompson, A. Kongkanand, ACS Energy Lett., 3, 618 (2018). 2) T. Miyao, H. Nishino, H. Yamazaki, S. Sato, K. Tamoto, M. Uchida, A. Iiyama, K. Shibanuma, N. Koizumi, 242nd ECS meeting Abstract, #I01D-1578(2022). 3) S. Sato, H. Yamazaki, H. Nishino, K. Tamoto, M. Uchida, A. Iiyama, K. Shibanuma, M. Sodeno, N. Koizumi, Y. Hoshikawa, T. Miyao, 244th ECS meeting Abstract, #I01D-1999(2023). 4) T. Kamino, H. Saka, Micros. Microanal. Microstructure, Y4, 219 (1993). Figure 1

A global hydrogen economy (GHE) was envisioned by Haldane in 1923 but has only recently captured wide attention. As the movement toward a GHE accelerates, the supply vs. demand of precious metals such as platinum and iridium will inevitably force their prices skyward. In order to ensure the future of the GHE, it is crucial to intensify the R&D of earth-abundant element-based electrocatalysts. We have recently focused on electrocatalysts for the oxygen evolution and hydrogen evolution reactions (OER, HER) based on Fe, Co, Ni and Mo-containing oxides.1-3 The use of density functional theory (DFT) calculations has enabled us to understand more deeply the origins of high catalytic activity. From OER experimental results, we have found that the activity is higher for Ni-Co oxides with lower structural order.1 This was explained by calculations indicating that the edges of NiOOH sheets are the active sites, at which the rate-determining step M-O + MOH  M-OOH-M is facilitated by the favorable geometry, in which the initial ) O—O distance is significantly shorter than that in the ideal structure (Fig. 1A). The presence of Co ("b" structure) further lowers the activation energy, to 0.15 eV. With the addition of Mo to NixCo1-xOOH, the rds M-O + M-O  M-O-O-M at the layer edges was found to become spontaneous, due to the electron-withdrawing effect of Mo.2 However, it was found that Mo slowly leached out of the structure during actual long-term operation. Ni-Fe oxide was found to be much more stable, and the corresponding DFT calculations showed that the activation energy for the M-O + M-OH  M-OOH-M step was lowered to 0.042 eV.3 In our work on the HER with Ni-Fe alloy/Ni-Fe oxide, we found experimentally that the activity was highest when the alloy was present in contact with a more crystalline form of the oxide.3 The DFT calculations showed that it is necessary to consider both the water dissociation step on the surface of the NixFe1-x(OH)2 sheets and the transfer of a proton to a metallic Ni-Fe alloy particle, on which two adsorbed hydrogens combine to produce H2 (Fig. 1B). This mechanism, which is similar to the bifunctional mechanism proposed by Markovic and coworkers for Pt/NiO,4 we have termed "reverse spillover." Thus, through the combination of experiment and theory, we are developing an understanding of how further increases in electrocatalytic activity for the OER and HER can be achieved with earth-abundant element-based materials. Acknowledgments This work was partially based on results obtained from project JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan and a JSPS grant, KAKENHI (23H02059). References G. Shi, et al., ACS Catal., 12, 14209 (2022). G. Shi, et al., ACS Appl. Energy Mater., 6, 10742 (2023). G. Shi, et al., ACS Omega, 8, 13068 (2023). Z. Zeng, et al., Nature Energy, 2, 17070 (2017). Figure 1. (A) Reaction profile for the rate-determining step M-O + M-OH  M-O-OH-M on defective NixCo1-xOOH sheets with (a) only Ni and (b) Ni and Co in the active site (taken from Ref 1). (B) Reverse spillover reaction in which a proton is transferred from a water molecule at the surface of the Ni(OH)2 sheet to the Ni7Fe cluster that has a high coverage of H, from which H2 can subsequently desorb (taken from Ref. 3). Figure 1

The hydrogen evolution reaction (HER) as part of water splitting is a fundamental electrocatalytic process and plays a crucial role in hydrogen-based energy conversion. Although platinum and its group metals rank among the most active electrocatalysts for the HER, the slow reaction kinetics under alkaline media conditions has hindered their application in alkaline water electrolyzers. Therefore, much effort has been devoted to exploring highly active electrocatalysts for the alkaline HER. Transition-metal hydroxides/oxides are efficient for the prior water-dissociation step of alkaline HER, and therefore, combining these materials with Pt may be promising candidates for alkaline HER catalysts. Here, we demonstrate that Pt nanoparticles supported on interconnected NiFe oxide particles can serve as enhanced catalysts for alkaline HER. The NiFe oxide was synthesized by the flame pyrolysis method.1,2 The Pt nanoparticles were loaded on the NiFe oxide by a modified colloidal method.3,4 The as-prepared Pt/NiFe oxide sample was reduced in 5% H2 (N2 balance) at 100 oC for 1h. The Pt loading was determined to be 28.3 wt% by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The scanning transmission electron microscopy (STEM) image was shown in Figure 1 ((a) secondary electron (SE) image and (b) transmission electron (TE) image). It can be seen that Pt nanoparticles (some interconnected to form rod/wire-like structures) are dispersed on the NiFe oxide support. Figure 1c shows the HER polarization curves of Pt/NiFe oxide compared with a commercial catalyst Pt/C (TEC10E50E, TKK). The HER specific activity (j s) and mass activity (MA) were summarized in Figure 1d. The HER activity is shown to be greatly enhanced on the Pt/NiFe oxide catalyst, with the specific activity and mass activity (@-0.1 V) being approximately 3.9 and 2.4 times higher than that of Pt/C, respectively. The high mass activity of Pt/NiFe oxide would make it possible to lower the Pt usage, and thus cost, when implemented as a cathode in a practical alkaline water electrolyzer. In addition, the Pt/NiFe oxide catalyst was further heat-treated at high temperatures to obtain different nanostructures and elucidate the structure-activity relationship, achieving optimal HER activity at a moderate hydrogen binding energy. These results are expected to aid in the design of highly efficient HER catalysts for hydrogen production by alkaline water electrolysis. Acknowledgement This work was partially supported by the JSPS KAKENHI (23H02059) and the projects from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta, 56, 2881 (2011). G. Shi, T. Tano, D. A. Tryk, M. Yamaguchi, A. Iiyama, M. Uchida, K. Iida, C. Arata, S. Watanabe, and K. Kakinuma, ACS Catal., 12, 14209 (2022). G. Shi, T. Tano, D. A. Tryk, A. Iiyama, M. Uchida, and K. Kakinuma, ACS Catal., 11, 5222 (2021). G. Shi, T. Tano, D. A. Tryk, T. Uchiyama, A. Iiyama, M. Uchida, K. Terao, M. Yamaguchi, K. Tamoto, Y. Uchimoto, and K. Kakinuma, ACS Catal., 13, 12299 (2023). Figure 1

Highly durable and active cathode catalyst is required toward the wide spread use of polymer electrolyte fuel cells, such as heavy-duty vehicles. Pt supported on porous carbon is one of the essential catalysts, and is considered to improve the durability and activity. One of the other interesting catalysts is the Pt catalysts supported on doped SnO2 and CeO2 (Pt/M-SnO2, M = Nb, Zr) without carbon additives. The durability (startup/shutdown, load cycling) and oxygen reduction reaction (ORR) activity of the Pt/M-SnO2 is superior to those of commercial Pt catalysts supported on carbon black (Pt/CB) etc. [1-8]. The Pt/M-CeO2 also supply the new function of scavenging effect to mitigate the membrane degradation even though the higher temperature operation over 100oC. These catalysts have a unique carbon-like microstructure of a fused-aggregate network structure, which supply the essential function of the electronically constructing pathways via necking of each support particles and the gas diffusion pathways via the open pores surrounded by particles. The interface between Pt and these oxide supports also has an essential electronic interaction as shown in the catalyst design concept (Fig. 1)[5]. The IV performance of the single cell using the Pt /Nb-SnO2 cathode catalyst layers and Pt/Zr-CeO2 anode ones at operating temperatures from 80oC to 120oC is quite high to approach the NEDO target performance with keeping high durability. The catalyst is considered to have possibilities for application of the heavy-duty vehicles operating at wide-temperature range (< 120oC)[9]. Acknowledgments This work was partially supported by funds for the project “Electrolytes, Catalysts and Catalyst layers with Extraordinary Efficiency, power and Durability for PEFCs-2030 (ECCEED’30) from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, and JSPS KAKENHI Grant Number (23H02059) from the Ministry of Education, Culture, Sports, Science and Technology. References K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, M. Watanabe, Electrochim. Acta, 56, 2881 (2011). Y. Senoo, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, M. Watanabe, RSC Adv., 6, 321800 (2014). Y. Chino, K. Taniguchi, Y. Senoo, K. Kakinuma, M. Watanabe, M. Uchida, J. Electrochem. Soc., 162, F736 (2015). K. Kakinuma, R. Kobayashi, A. Iiyama, M. Uchida, J. Electrochem. Soc., 165, J3083 (2018). K. Kakinuma, K. Suda, R. Kobayashi, T. Tano, C. Arata, I. Amemiya, S. Watanabe, M. Matsumoto, H. Imai, Iiyama, M. Uchida, ACS Appl. Mater. Interfaces 11, 34957 (2019). K. Kakinuma, M. Hayashi, T. Hashimoto, A. Iiyama, M. Uchida, ACS Appl. Energy Mater., 3, 6922 (2020). G. Shi, T. Tano, D.A. Tryk, A. Iiyama, M. Uchida, K.Kakinuma, ACS Catal., 11, 5222 (2021). G.Shi, T.Tano, D.A.Tryk, A.Iiyama, M.Uchida, Y.Kuwauchi, A.Masuda, K.Kakinuma, J. Catal., 407, 300 (2022). K. Kakinuma, H. Taniguchi, T. Asakawa, T. Miyao, M. Uchida, Y. Aoki, T. Akiyama, A. Masuda, N. Sato, A. Iiyama J. Electrochem. Soc. 169 (2022) 044522. Figure 1

Gas diffusion layers (GDLs) in proton exchange membrane fuel cells (PEMFCs) are responsible for diffusion of reactant gases into the catalyst layers, current collection and the removal of produced water. An accumulation of generated liquid water within the GDL, known as flooding, impedes the supply of reactant gas and results in the increase of concentration overpotential. Therefore, understanding of oxygen transport and produced water removal characteristics is required to enhance cell performance. The objective of this study is to investigate the effect of GDL structure and operation conditions on PEMFC performance and liquid water removal, with a particular focus on comparing a novel GDL to a conventional GDL. The results of a simultaneous evaluation of cell performance and liquid water behavior in the GDLs by means of X-ray imaging under operational conditions are presented.

Hydrogen peroxide is inevitably produced at the hydrogen electrode in both the proton exchange membrane fuel cell (PEMFC) and the proton exchange membrane water electrolyzer (PEMWE) when platinum-based catalysts are used. This peroxide attacks and degrades the membrane, seriously limiting its lifetime. Here we review some of our previous efforts to suppress peroxide production using PtFe as a hydrogen evolution reaction (HER) catalyst and PtCo as a hydrogen oxidation reaction (HOR) catalyst. The mechanisms, which involve the chemical reaction of adsorbed hydrogen with oxygen, are examined using density functional theory. The onset of excess peroxide production at 0.1 V above the reversible potential has not been adequately explained thus far, and therefore a new mechanism is proposed here. This involves a unique reaction site including hydrogen adsorbed at (110) step edges adjacent to (111) terraces on the Pt surface, as well as on Pt alloys and other metals such as Rh and Ir. This mechanism helps explain the recent finding of the Wadayama group that Ir single crystal surfaces such as Ir(111) and Ir(110) produce little peroxide during the HOR. It also points the way toward the design of new catalysts for the hydrogen electrode that suppress peroxide production while retaining high HOR and HER activity.

The hydrogen evolution reaction (HER) is a key chemical reaction as part of water splitting to produce clean hydrogen. Pt is the best catalyst for HER in acidic media, but its activity is greatly reduced under alkaline conditions. Therefore, improving the sluggish alkaline HER kinetics is urgently needed to achieve highly efficient hydrogen production by alkaline electrolyte membrane (AEM) water electrolysis. Herein, Pt nanocatalysts supported on Ni-Fe oxides were synthesized by a modified colloidal method, which exhibit approximately 4 times higher HER specific activity and more than 2 times higher mass activity compared to a commercial Pt/C catalyst in alkaline solution, showing the potential to reduce Pt loading by half on the cathode side of AEM water electrolyzers. The high HER activity corresponds to the large amount in the Ni-Fe oxide phase accompanied by a moderate hydrogen binding energy with Pt. Keeping Pt and Ni-Fe oxides in separate phases is more effective than forming a Pt-Ni-Fe alloy in improving HER activity. Experimental results and density functional theory calculations indicate that the NiFe oxide not only promotes water dissociation but also facilitates the hydrogen adsorption/desorption process on Pt, thereby accelerating the overall kinetics of alkaline HER. The identification of these electrocatalytic trends for metal/oxide modified Pt catalysts provides fundamental insights that can guide the design and synthesis of efficient alkaline HER catalysts for hydrogen production.

Anion exchange membrane fuel cells (AEMFCs) hold the key to future mass commercialisation of fuel cell technology, even though currently, AEMFCs perform less optimally than proton exchange membrane fuel cells (PEMFCs). Unlike PEMFCs, AEMFCs have demonstrated the capability to operate independently of Pt group metal-based catalysts. Water characterization inside the membrane is one factor that significantly influences the performance of AEMFCs. In this paper, different water species inside an anion exchange membrane (AEM), QPAF-4, developed at the University of Yamanashi, were studied for the first time using micro-Raman spectroscopy. Spectra of pure water, alkaline solutions, and calculations based on density functional theory were used to identify the water species in the AEM. The OH stretching band was deconvoluted into nine unique Gaussian bands. All the hydrogen-bonded OH species increased steadily with increasing humidity, while the CH and non-H-bonded OH remained relatively constant. These results confirm the viability of micro-Raman spectroscopy in studying the various water-related species in AEMs. The availability of this technique is an essential prerequisite in improving the ionic conductivity and effectively solving the persisting durability challenge facing AEMFCs, thus hastening the possibility of mass commercialisation of fuel cells.

Recently, a new concept was developed for the design of polymer electrolyte fuel cells, combined a flat separator and a porous gas diffusion layer (GDL) with interdigitated gas-flow channels.1 This new design cell has demonstrated higher performances than that of a conventional cell combined a solid separator with serpentine flow-channels and a flat GDL.2 Conventional interdigitated flow-channel designs, which consist of a solid separator with interdigitated gas-flow channels and a flat GDL, have been known for their higher efficiency of the oxygen supply to the catalyst layer, in comparison with serpentine or parallel flow-channels in combination with a flat GDL, because of the forced convection of the supplied gas in the GDL. However, conventional interdigitated flow-channels have faced two issues associated with low performance: one occurs under high-humidity conditions because of nonuniform gas flow due to accumulated water in the GDL; the other is caused by forced water discharge from the GDL under low-humidity conditions. In this study, to investigate the possibility of the new cell design to overcome such performance issues of conventional interdigitated cells, both conventional and new cell designs were tested with single cells of 1 cm2 active area, and the performances were compared at high and low humidity with various conditions of gas supply. From these results, we have found that the new design of the GDL with interdigitated channels has a clear advantage over that of the conventional separator with interdigitated channels, being able to maintain higher performance under conditions of both water excess and water shortage.3 To reveal the mechanism of the improvement in the cell performance, the temperature and gas flow distributions in the GDL of the new and the conventional interdigitated cells were calculated by numerical simulation, and the water distribution was visualized by X-ray imaging.4 From comparisons of these experimental and numerical results in the two cells, the porous ribs in the newly designed cell were found to play several important roles, as follows: under conditions of excess water, the porous ribs with relatively low thermal conductivity help to alleviate water accumulation in the GDL by acting as a reservoir for excess water and also by increasing the temperature in the GDL adjacent to the catalyst layer; and, under conditions of water shortage, the porous ribs help to alleviate the dry-out of the GDL by withdrawing water from the reservoir shortly and also by decreasing the rate of gas flow forced through the GDL because the porous ribs act as the short-cut pathway for the gas flow. Based on these mechanistic and performance analyses, it is becoming clearer that the new cell design, with interdigitated flow-channels and porous ribs, has the potential to overcome the performance issues of conventional interdigitated cells. Acknowledgement This work was partially based on results obtained from project JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References Watanabe et al., J. Electrochem. Soc., 166, F3210 (2019). Nasu et al., J. Power Sources, 530, 231251 (2022). Inoue et al., J. Electrochem. Soc., 169, 114504 (2022). Inoue et al., J. Power Sources, 585, 233623 (2023). Figure 1

Proton exchange membrane water electrolysis (PEMWE) is being actively developed as a promising technology to produce high-purity hydrogen. While PEMWE has been commercialized, a serious roadblock for wider deployment is the high cost associated with the use of high loadings of the noble metal Ir-based anode catalysts for the oxygen evolution reaction (OER). To lower the Ir loading amount, it is critically important to develop efficient catalysts with increased Ir utilization and higher OER activity. Herein, we report the preparation of one-dimensional Ir oxide nanorod catalysts supported on Sb-doped SnO2 and demonstrate their extremely high activity in the OER catalysis, with the Ir mass-specific activity being 10 times higher than that of commercial IrOx catalyst at 1.5 V vs reversible hydrogen electrode (RHE), showing great promise in dramatically reducing the Ir loading in PEMWE cells. The experiment also found that the OER activation energy was greatly reduced for the Ir oxide nanorod/Sb-SnO2 catalyst. It was proposed, based on experimental results and density functional theory (DFT) calculations, that the nanorod geometry and the interaction with SnO2 support rendered a surface with a lower degree of Ir oxidation, which allows the surface terminal oxygens to be closer together while allowing facile desorption of oxygenated species, which could play an important role in facilitating the crucial step of the OER, thereby enhancing the OER activity.