In this study, the use of carbon spheres of 500 nm diameter as a catalyst support in polymer electrolyte fuel cells was investigated. The carbon spheres were expected to promote diffusion and improve platinum utilization. The carbon spheres' exterior surface consisted of graphene oxide, which supported highly dispersed platinum nanoparticles. This graphene oxide wall was reduced during the platinum-supporting process. Scanning electron microscopy images and the crystallite size showed that platinum particles of diameter less than 10 nm were supported on the surface of the carbon sphere. The electrochemical surface area (ECSA) of the carbon sphere-supported platinum (Pt/CS) was lower than that of commercial Ketjen black-supported platinum (Pt/KB). However, the approximate platinum utilization rate, which was calculated from the crystallite size and Pt/CS ECSA, was over 20% higher than that of Pt/KB. The oxygen reduction reaction (ORR) activity of Pt/CS was approximately twice that of Pt/KB at 0.9 V, which is a kinetically controlled region. The difference between the ORR activities of Pt/CS and Pt/KB increased with the increasing effect of material diffusion. Electrochemical measurements indicated that the platinum utilization rate and ORR activity were enhanced by changing the catalyst support to carbon spheres. The improved platinum utilization rate and ORR activity were attributed to the material diffusion achieved by the catalyst layer constructed from carbon spheres being superior to that achieved by Pt/KB. The results of this study showed that the use of a uniform spherical material such as carbon spheres as the catalyst support improved the platinum utilization rate and ORR activity because platinum nanoparticles were highly dispersed on the catalyst surface.

The quasi-steady-state current values and XANES spectra of Rh2O3　were investigated using the in-situ XAFS method to understand the adsorption state of oxygen molecules on the surface. It was suggested that the adsorption of oxygen molecules on the Rh2O3 surface was rapid in the low-potential region where the oxygen reduction reaction proceeds sufficiently. In addition, it was suggested that the adsorption state of oxygen molecules might be different from that of the high potential region where the oxygen reduction reaction did not proceed. XANES spectra also showed that the valence of Rh atoms in Rh2O3 remained high, independent of oxygen partial pressure.

Polymer electrolyte fuel cells (PEFCs) are attracting attention as a next-generation power source with low environmental impact. However, the low durability and high cost of Pt-based catalysts used as electrocatalysts have hindered the widespread use of PEFCs. Currently, many studies are being conducted to improve the durability of Pt-based catalysts, such as alloying, core-shelling, and the use of oxide supports. Although these studies have achieved certain results, the high cost of raw materials cannot be avoided as long as Pt, which is a noble metal, is used to some extent. Therefore, research on non-Pt catalysts that do not use Pt is attracting attention as the next generation of electrocatalysts. Non-Pt catalysts can be broadly classified into oxide-based and carbon-based catalysts, but considering the start-up and shutdown potentials of PEFCs, oxide-based non-Pt catalysts are expected to show higher durability. It is known that the introduction of oxygen-deficient sites in the oxides of Group 4 and Group 5 elements in the oxide-based non-Pt catalysts results in a high oxygen reduction reaction rate (ORR). Therefore, oxygen-deficient sites are considered to be active sites for ORR, and the introduction of oxygen-deficient sites in the crystal lattice is a guideline for catalyst design in non-Pt catalysts based on oxides. On the other hand, it has been reported that noble metal oxides show high ORR activity even without clear oxygen-deficient sites. It is easy to imagine the hypothesis that the ORR proceeds by the adsorption of oxygen molecules on the oxygen-deficient sites, but the same hypothesis holds true for low-coordinated metal atoms on the oxide surface without the introduction of oxygen-deficient sites. Furthermore, it has been reported that in alkaline solution, oxygen atoms in the perovskite structure are the active sites. These discrepancies are due to the fact that the ORR mechanism on oxides is still unknown. In this study, in-situ XAFS measurements were performed to identify the active sites of ORR on oxides, and changes in the chemical states of metal elements during the ORR were investigated from changes in XANES spectra. A noble metal oxide (Rh2O3), whose valence changes with potential, was used as the target oxide. The target oxide, Rh2O3, was prepared by the Adams method. The prepared sample was placed on a glassy carbon plate at 100 g cm-2 and placed in a tripolar electrolytic cell for in-situ XAFS. The in-situ XAFS cell was designed to detect X-ray fluorescence by irradiating X-rays from the back of the sample. Energy of the incident X-ray was set near the Rh K edge. In-situ XAFS measurements were performed at five points: 0.4 V, where the ORR is in progress; 0.6 V and 0.8 V, near the ORR starting potential; 1.0 V and 1.2 V, where the ORR is not in progress. To investigate the effect of oxygen partial pressure, the oxygen partial pressure varied between 0%, 25%, 50%, 75%, and 100%. Comparing the height of the white line in nitrogen at 0.4 V and at 1.2 V, which is the most reducing state, the height of the white line was lower when the applied voltage was 0.4 V. This is because the average valence of Rh was higher at 0.4 V than at 1.2 V, which is the most reducing state. This indicates that the average valence of Rh changes with applied voltage. In other words, at 0.4 V, when the ORR is fully advanced, the oxidation number of Rh is considered to be in a reduced state. This behavior is consistent with the electrochemical behavior and with previous reports. In contrast, when the oxygen partial pressure was set to 25%, this difference in the applied voltage was hardly observed. The difference in the white lines between the high and low potential sides became smaller as the oxygen partial pressure increased, and under 100% oxygen, the XANES spectra obtained at the applied voltage of 0.4 V and 1.2 V almost overlapped. Therefore, unlike under a nitrogen atmosphere, the oxygen gas adsorbs on the Rh2O3 surface at 0.4 V, where the ORR proceeds under oxygen distribution. As a result, it is considered that the oxidation number of Rh increased. From the above, we predict that in Rh2O3, the ORR progresses in the state where the valence of Rh is decreased.