Katsuyoshi Kakinuma

The oxygen reduction reaction (ORR) remains a major bottleneck for the efficiency of proton exchange membrane fuel cells. While alloying Pt with 3d transition metals can reduce Pt loading and improve catalytic activity of ORR catalysts, Pt-based solid solutions suffer from reduced activity and stability at Pt fractions below 75-80 at. %. Here, we investigate the role of atomic ordering in multimetallic Pt catalysts by comparing high-entropy solid solutions (HESOSO) and high-entropy intermetallics (HEI) with identical nominal composition, Pt5MnFeCoNiCu. Gas diffusion electrode (GDE) setup measurements at industrially relevant current densities (>1 A cm-2 geo) show that catalysts containing the intermetallic phase outperform solid solution counterparts in both specific and mass activity despite their larger particle sizes. Density functional theory calculations attribute this improvement to anisotropic strain arising from the tetragonal intermetallic lattice, which generates distinct adsorption environments with near-optimal *OH binding energies. These results identify strain anisotropy in ordered multimetallic intermetallics as a promising catalyst design principle for independently tuning adsorption energetics and Pt loading. On-line inductively coupled plasma mass spectrometry (ICP-MS) measurements reveal reduced 3d-metal dissolution in the HEI sample compared to HESOSO sample and binary PtNi reference. Operando X-ray total scattering and pair distribution function analysis further demonstrate that both crystallographic structure and local atomic arrangement are retained during accelerated stress testing, indicating that nanoparticle structural collapse is not the dominant degradation pathway. Instead, the observed activity losses are primarily associated with catalyst-layer degradation and mass transport limitations. Overall, the results identify atomic ordering and strain anisotropy as key parameters for designing low-Pt ORR catalysts with enhanced activity and structural stability under operating conditions.

Introducing hydrogen oxidation reaction (HOR) activity at the anode is considered a promising strategy for the oxidative removal of crossover hydrogen in proton exchange membrane water electrolysis (PEMWE), which may contribute to improved operational safety. In this study, Pt-incorporated IrOx catalysts supported on Nb-doped SnO2 (IrOx/Nb-SnO2) were investigated as bifunctional electrocatalysts with oxygen evolution reaction (OER) and HOR activities. Electrochemical evaluation using rotating disk electrode (RDE) measurements revealed that slight Pt incorporation (∼10 atom%) maintained high OER activity while introducing appreciable HOR activity. In contrast, excessive Pt incorporation enhanced HOR activity but significantly decreased OER performance, primarily due to the dilution of active Ir sites. These results demonstrate that trace Pt incorporation can effectively introduce HOR functionality while retaining high OER activity in IrOx-based catalysts, providing a potential strategy for the development of multifunctional anode electrocatalysts for PEMWE.

The oxygen reduction reaction (ORR) remains a major bottleneck for the efficiency of proton exchange membrane fuel cells. While alloying Pt with 3d transition metals can reduce Pt loading and improve catalytic activity of ORR catalysts, Pt-based solid solutions suffer from reduced activity and stability at Pt fractions below 75-80 at. %. Here, we investigate the role of atomic ordering in multimetallic Pt catalysts by comparing high-entropy solid solutions (HESOSO) and high-entropy intermetallics (HEI) with identical nominal composition, Pt5MnFeCoNiCu. Gas diffusion electrode (GDE) setup measurements at industrially relevant current densities (>1 A cm-2 geo) show that catalysts containing the intermetallic phase outperform solid solution counterparts in both specific and mass activity despite their larger particle sizes. Density functional theory calculations attribute this improvement to anisotropic strain arising from the tetragonal intermetallic lattice, which generates distinct adsorption environments with near-optimal *OH binding energies. These results identify strain anisotropy in ordered multimetallic intermetallics as a promising catalyst design principle for independently tuning adsorption energetics and Pt loading. On-line inductively coupled plasma mass spectrometry (ICP-MS) measurements reveal reduced 3d-metal dissolution in the HEI sample compared to HESOSO sample and binary PtNi reference. Operando X-ray total scattering and pair distribution function analysis further demonstrate that both crystallographic structure and local atomic arrangement are retained during accelerated stress testing, indicating that nanoparticle structural collapse is not the dominant degradation pathway. Instead, the observed activity losses are primarily associated with catalyst-layer degradation and mass transport limitations. Overall, the results identify atomic ordering and strain anisotropy as key parameters for designing low-Pt ORR catalysts with enhanced activity and structural stability under operating conditions.

The present work demonstrates an improvement in proton conductivity at 80 °C as a result of the addition of a hydrophilic filler (carboxymethyl cellulose nanofibers, CMCNFs) to Nafion composite membranes. The composite membranes (CMCNF/Nafion) prepared by die coating are translucent and flexible, without pores or cracks. The proton conductivity of the CMCNF/Nafion membrane reaches 0.165 S cm–1 at 80 °C and 80% RH. The water retention of the membranes, based on DSC measurements, indicates that the amount of nonfreezing water (water that does not freeze down to −55 °C) is independent of the addition of CMCNF, but the amount of capillary water (water that melts between −55 and 0 °C) increases with CMCNF addition. We considered that the presence of capillary water at the interface between CMCNFs and Nafion would be one of the essential factors that improve the proton conduction in the electrolyte. The small-angle X-ray scattering (SAXS) profiles prove that the size of the hydrophobic domain water clusters in Nafion becomes smaller with the addition of CMCNF, while the water content remains the same. The microstructure of the CMCNF/Nafion composite membrane and the behavior of the water content (capillary water) would be one possible contribution to show the improvement of proton conductivity.

The efficient energy storage devices are required to overcome the problem of fluctuations in the availability of electric energy. The water electrolysis to produce storable hydrogen and oxygen is one of the most expecting approaches for large-scale energy storage. The proton exchange membrane water electrolysis (PEMWE) has a potential to produce the high purified hydrogen under the operation at high current densities. However, the 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 is the key issues toward the widespread applications. Herein, we report a remarkably active OER catalyst of Ir oxide with nanorod-like structure supported on niobium-doped tin oxide (Nb-SnO₂), showing an order of magnitude increase in Ir mass-specific activity than a commercial IrO_x 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.

Hydrogen production plays a crucial role in achieving a carbon-neutral energy sector. In the current stage,^[1] anion exchange membrane water electrolyzer (AEMWE) offer the promising advantages of combining solid membranes with an alkaline environment. This allows for easier handling, operation at high current densities and lower material costs due to the less-corrosive alkaline conditions.^[2-3] However, the hydrogen evolution reaction (HER) activity of platinum-group metal (PGM) electrocatalysts is dramatically reduced with increasing pH values.^[4-5] Latest research works on HER catalysts suggest a positive influence of alloying Pt with transition metals. In particularly, Co, Ni and Ru^[6-8] have been found to show the most promising effect due to a lowering of the d-band center caused by lattice compression of Pt-Pt^[9-10] and a higher presence of adsorbed oxygen species on the more oxophilic transition metal.^[6-7]

The present transportation system based on the internal combustion engine is a major contributor to the issue of CO₂ emission, and a paradigm shift is required toward to electrical motorization based on batteries and polymer electrolyte fuel cells (PEFCs). The motorization of heavy-duty vehicles (HDVs) by PEFCs is one of the essential targets. HDVs will require higher operating temperatures, at least 120^oC, versus normal temperatures (60-80^oC), for optimal heat management. A commercial perfluorosulfonic acid-based electrolyte (PFSA, e.g., Nafion^Ⓡ) shows better proton conductivity with chemical stability over a wide temperature range, although the conductivity decreases under low back pressure at 100~120 ^oC. In order to improve the proton conductivity in the higher temperature range, a composite membrane of Nafion^Ⓡwith a hydrophilic filler of Ta-doped TiO₂ (Ta-TiO₂) was synthesized.

Proton exchange membrane water electrolysis (PEMWE) is a key technology for production of hydrogen at high current density and dynamic operation. However, long-term durability of PEMWE is limited by the sensitivity of perfluorosulfonic acid (PFSA) membranes to impurities in the water feed.[1,2] Even small amounts of metal cations alter membrane properties such as conductivity, water uptake, and transport phenomena due to proton-cation exchange processes on the sulfonic groups.[3,4] These cation-exchange processes not only reduce system efficiency but also influence water management, which is highly critical for membrane hydration. Despite their practical relevance, a fundamental relationship between cation contamination and water transport in PEMWE is not fully understood to date.