It has long been recognized that the oxygen reduction reaction occurs more readily on Pt(111) surfaces that include steps, both (111) and (100), than on near-perfect Pt(111). Theoretical models were developed involving the water structure in the electric double layer and its interactions with adsorbed OH, with the actual O2 reduction occurring on the (111) terraces adjacent to the steps. However, the present density functional theory (DFT) calculations confirms that O2 adsorbs strongly at the steps and can undergo dissociation aided by adjacent water molecules to produce adsorbed OH. OH produced at the steps can move to the (111) terraces, where it can be more readily reduced to H2O and desorbed. This model avoids the scaling relation, which predicts that all oxygen-containing reactants and intermediates are proportional to each other on any given surface, i.e., strong O2 adsorption at steps compared with water ensures that the reaction can proceed. Efforts to develop new O2 reduction catalysts have been hampered by the assumption that the reaction rate can be increased by decreasing OH adsorption strength, even though decreased OH adsorption strength is accompanied by decreased O2 adsorption strength on any given crystallographic facet. This proposed model can explain the experimental results on stepped surfaces as well as nanoparticle catalysts, particularly the higher ORR activity on alloys such as PtFe, but with the obligatory presence of steps. The results may also be important for the development of Pt nanoparticle catalysts.

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 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

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.

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 first report of the photo-induced oxygen evolution reaction on UV-irradiated TiO2 was in 1969.1 That year, the results were also presented at the Annual Meeting of the Electrochemical Society of Japan. At the time, many people did not believe the results. This is a testament to their revolutionary character. It was not until the results were published in the Bulletin of the Chemical Society of Japan two years later and in Nature three years later that the ideas began to be accepted.2,3 The results are still recognized as being revolutionary and have been cited over 30,000 times, according to the Web of Science, and over 38,000 times, according to Google Scholar, earning it a place in the list of the 100 most highly scientific papers of all time. The results then became the subject of intense investigation in the early 1970s, with important confirmatory studies being published, for example, by Wrighton, et al.4 It became apparent that the band-gap of TiO2 was too large for the effective utilization of sunlight in water splitting, but, nevertheless, many researchers have been continuing to develop new types of semiconductors and compound structures. Thus, water can now be split effectively with sunlight. In the meantime, Fujishima and coworkers began to make use of the oxidative power of TiO2 in photocatalytic self-cleaning and self-sterilization. This work has led to numerous basic studies, as well as applications, from surgical catheters to window glass, to whole buildings.5 Notably, photocatalytic coatings have recently been intensively developed for the decomposition of viruses by many researchers. Twenty-five years after the original paper in Nature, a second ground-breaking paper appeared, this time on the phenomenon of photo-induced hydrophilicity.6 This phenomenon has some relationship to traditional photocatalysis but is distinct from it, although some researchers have claimed that they are in fact identical. This debate has continued up to the present, with fundamental surface science studies and theoretical calculations being carried out, but there is increasing recognition that it is a separate phenomenon, involving the photo-induced dissociation of water molecules at the TiO2 surface, greatly enhancing its hydrophilicity. Thus, Fujishima’s work has given major impetus to the fields of artificial photosynthesis, the production of solar fuels, and photo-induced self-cleaning and anti-fogging, as well as having a tremendous impact on the development of antibacterial and antiviral coatings. References Fujishima, K. Honda, and S. Kikuchi, Kogyo Kagaku Zasshi, 72, 108 (1969). Fujishima and K. Honda, Bull. Chem. Soc. Japan, 44, 1148-1150 (1971). Fujishima and K. Honda, Nature, 238, 37-38 (1972). S. Wrighton, D. S. Ginley, P. T. Wolczanski, A. B. Ellis, D. L. Morse, and A. Linz, Proc. Nat. Acad. Sci., 72, 1518-1522 (1975). Zhang, D. Tryk, H. Irie, A. Fujishima, Editors, Handbook of Self-Cleaning Surfaces and Materials: From Fundamentals to Applications, Wiley-VCH (2023). Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, et al., Nature, 388, 431-433 (1997).

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

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.

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.

The intrinsic electrochemistry of platinum and other platinumgroup metals and noble metals has been under intense investigation for over forty years but is still not fully understood. Various in situ spectroscopic techniques, particularly vibrational spectroscopies, have provided and continue to provide many insights, but challenges remain. The intrinsic electrochemistry is capable of being elucidated through the combination of electrochemistry, vibrational spectroscopy and theory and is then further able to clarify the catalytic reactions involved in H2-O2 fuel cells and water electrolysis.

Recently, a new concept was developed for the design of polymer electrolyte fuel cells, based on a flat, solid separator and a porous GDL with interdigitated gas-flow channels. This newly designed cell has demonstrated the ability to overcome the principal issues of the conventional design, in which the interdigitated flow channels are formed on the separator; the latter can experience low performance under high and low humidity conditions. In the present study, we have sought to reveal the mechanism of the performance stability improvement. The temperature and gas flow distributions are calculated by numerical simulation, and the water distribution is visualized by X-ray imaging. From these results, the porous ribs in the newly designed cell are found to play several important roles as follows: under conditions of excess water, the porous ribs 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; and, under conditions of water shortage, the porous ribs alleviate the dry-out of the GDL by withdrawing water from the reservoir and also by decreasing the rate of gas flow forced through the GDL.

Anion electrolyte membrane water electrolysis (AEM WE) using non-precious metal catalyst is one of the most prospective systems for pure hydrogen generation. The Ni based oxide shows relatively lower overpotential of oxygen evolution reaction (OER) by adding the transition metals of Co, Fe and Mn, and approaches that of precious metal of Ir based oxide. Our previous study reported that the Ni-Co based catalyst was highest OER activity at operating temperature. The crystallized Ni-Co metal-based core particles with amorphous Ni oxyhydrates of top surface (shell) was confirmed to be preferable to obtain the higher OER activity by rotating disk electrode method, transmission electron microscopy and DFT calculation. Moreover, the fused aggregated network microstructure of Ni-Co based catalyst assisted to show a metallically electronic conductivity. In this study, we evaluated both OER and hydrogen evolution reaction (HER) activity of Ni based catalysts to confirm the prospective non-precious metal catalysts for AEM WE. Each Ni based catalysts with additive of transition metals were synthesized by the flame oxide-synthesis method. 1 The OER and HER activities of these catalysts were evaluated in 1 M KOH at 20 to 80 o C by use of the RDE. The Ni-Co based catalyst was highest OER activity in these Ni based catalysts obtained above. The amorphous top surface layer was correlated with a negative shift in the oxyhydroxide formation peak potential from the results of DFT calculations. 2 The HER activity of the Ni-Fe based catalyst also showed the higher OER activity. Especially, the Ni-Fe based catalyst had highest HER activity in these Ni cased catalysts above. The HER activity enhanced by the construction of well crystallized top surface in comparison with its amorphous or poorly crystalline ones. DFT calculation indicated that the defective or disordered surface was not as active for the HER. These results will provide a strategy of Ni based catalysts optimization for AEM. Acknowledgement This work was partially supported by funds for the JSPS KAKENHI (20H02839), and the project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta , 56 , 2881 (2011). Shi, T. Tano, D. A. Tryk, A. Iiyama, M. Uchida, and K. Kakinuma, ACS Catal. , 11 , 5222 (2021). Figure 1

With the increasing installation of renewable energy generation systems such as solar and wind power, efficient energy storage devices are required to overcome the problem of fluctuations in the availability of electric energy. In particular, the splitting of water by electrolysis to produce storable hydrogen and oxygen appears to be one of the most promising approaches for large-scale energy storage. Proton exchange membrane water electrolysis (PEMWE) has received a great deal of attention due to its ability to respond quickly to renewable energy fluctuations and to operate at high current densities. However, a key limitation of PEMWE is the high cost associated with the use of high loadings of noble metal Ir catalysts at the anode electrode, which is necessary due to the slow kinetics of the oxygen evolution reaction (OER). Therefore, reducing the Ir loading at the OER electrode is essential to realize large-scale applications of PEMWE. This requires the development of highly active OER catalysts to accelerate the reaction or reduce the overpotential. Herein, we report a remarkably active OER catalyst of Ir oxide with nanorod-like structure supported on antimony-doped tin oxide (Sb-SnO2), showing an order of magnitude increase in Ir mass-specific activity than a commercial IrOx catalyst at 1.5 V vs. RHE. We also investigated the temperature dependence of the OER activity, as a way to elucidate the mechanism of enhanced catalysis, from both experimental and theoretical perspectives. The Sb-SnO2 support material (Sb dopant content 4 at.%) with fused-aggregate network structure was prepared by flame pyrolysis method.1 The as-prepared support powder was annealed at 700 oC for 2 h in air using a rotary kiln furnace. Sb-SnO2 supported IrOx nanorods (NRs) catalyst was synthesized by a colloidal method.2 The obtained IrOx NRs/Sb-SnO2 powder was heat-treated at 350 °C in N2 atmosphere for 2 h and at 150 °C in 1% H2 (N2 balance) for 2 h. The Ir loading was determined to be 49.3 wt.% by inductively coupled plasma optical emission spectrometry (ICP-OES). The OER activity was evaluated in 0.1 M HClO4 at 25 to 80 oC using the rotating disk electrode (RDE) technique.2,3 From Figure 1a, it can be seen that the Sb-SnO2 support particles present an interconnected network structure, which is considered to be beneficial to enhance the electron conduction and gas diffusion in the electrocatalytic reaction. On the support, the majority of IrOx nanoparticles are shown to have coalesced, forming rodlike structures. Figure 1b shows OER polarization curves (iR-corrected and normalized to the Ir mass) of the IrOx NRs/Sb-SnO2 catalyst compared to commercial IrOx catalyst (TANAKA Precious Metals). It is clear that the IrOx NRs/Sb-SnO2 catalyst significantly outperforms the IrOx reference catalyst over the entire potential range, and the OER onset potential of the IrOx NRs/Sb-SnO2 catalyst is lower. At 1.5 V (Figure 1c), the IrOx NRs/Sb-SnO2 catalyst exhibits a mass activity (MA) of 0.40 A mg-1 Ir, which is about 10 times higher than that of the IrOx reference catalyst. This dramatically enhanced activity makes it possible to significantly reduce the Ir loading on the anode of PEMWE. Experimental observations and density functional theory (DFT) calculations suggest that the nanorod geometry and strong metal-support interactions may play an important role in lowering the energy barrier for the crucial first step of the OER, thereby enhancing the OER activity. Acknowledgement This work was partly supported by the JSPS KAKENHI (23H02059) and by the project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] Kakinuma, K.; Uchida, M.; Kamino, T.; Uchida, H.; Watanabe, M. Electrochim. Acta 2011, 56, 2881-2887. [2] Shi, G.; Tano, T.; Tryk, D. A.; Iiyama, A.; Uchida, M.; Kakinuma, K. ACS Catal. 2021, 11, 5222-5230. [3] Shi, G.; Tano, T.; Tryk, D. A.; Yamaguchi, M.; Iiyama, A.; Uchida, M.; Iida, K.; Arata, C.; Watanabe, S.; Kakinuma, K. ACS Catal. 2022, 12, 14209-14219. Figure 1

Here, we demonstrate high-performance and durable anion-exchange membrane (AEM) water electrolysis using a NiCoMo-based anode catalyst structured with a crystalline metal core and an amorphous oxide surface that enables high activity for oxygen evolution reaction. An in-house-developed quaternized poly(aryleneperfluoroalkylene) membrane is used as the AEM. The resulting electrolyzer exhibits a current density of 1 A cm(-2) at 1.63 V (voltage efficiency of 91%) and 80( degrees)C with a demonstrated stability of more than 500 h. Such a high performance even exceeds state-of-the-art proton-exchange membrane electrolysis using noble-metal catalysts.