CO Oxidation Behavior of Pt/RuO2-Sphere Catalyst in Acidic Electrolyte

Abstract

Polymer electrolyte fuel cells (PEFCs) are one of the candidates for the next-generation power source. PtRu alloy catalyst is used as an anode catalyst because it has high CO poisoning tolerance. It is well known that PtRu alloy has high CO poisoning tolerance because of the bifunctional mechanism and the ligand effects of the Ru atom working as the co-catalyst. However, the Ru atom dissolves during the operation of PEFCs, such as start-up/shut-down, and then the CO poisoning tolerance becomes low. The author reported using ruthenium oxide nanosheet (RuO2ns) acted as high stable co-catalyst [1]. The composition catalyst of traditional Pt/C and RuO2ns (RuO2-Pt/C) showed high CO poisoning tolerance and long life. But the high CO poisoning tolerance site in the RuO2ns-Pt/C catalyst was limited to the very close position of Pt nanoparticles and RuO2ns. Thus, the CO poisoning tolerance was uneven, even within the same catalyst. On the other hand, we reported that new catalyst support with the sphere shape composited by the reduced graphene oxide wall provided high substance diffusion and Pt utilization rate [2]. It is expected that the spherical catalyst support composed from the RuO2ns wall provides complete contact with Pt nanoparticles and not only exquisite CO poisoning tolerance but also high Pt utilization. In this study, the new spherical catalyst support with the RuO2ns wall was synthesized and supported Pt nanoparticles, and its CO oxidation ability was evaluated by CO stripping voltammetry. The spherical catalyst support (RuO2-sphere) was composed of a SiO2 bead as a core and the RuO2ns wall, and its synthesis method was similar to the previous work [2]. Concretely, this study changed the wall material from graphene oxide to RuO2ns. And the Pt complex adsorbing on a RuO2-sphere was reduced by heating reflux with hydrazine at 100°C and 24 hours in ethanol solvent. After water washing, Pt-supported RuO2-sphere (Pt/RuO2-sphere) was obtained. The morphology, crystal structure, and electron state of the Pt/RuO2-sphere were evaluated by SEM, XRD, and XPS, respectively.A three-electrode cell, which is composed of a working electrode, Pt counter, Ag/AgCl reference electrode, and 0.1 M HClO4 electrolyte, was used for the electrochemical measurements, and the measurement temperature was 60°C. This study converted the potentials from Ag/AgCl reference to RHE reference. The CO stripping voltammetry was conducted in the following steps. CO gas was introduced had flowed into the electrolyte for two minutes to adsorb on the metal surface while its potential was kept at 0.05 V. The inert gas purged out excess CO gas in the electrolyte for 30 minutes. And then, the potential was swept to 1.2 V at 10 mV s-1. SEM images of the Pt/RuO2-sphere showed about 500 nm sphere of RuO2-sphere and a few nanometer sizes of Pt particles. Results of XRD and XPS suggested that Pt particles were mainly metallic state and not alloying with Ru. In addition, Ru valence in Pt/RuO2-sphere was predicted at quadrivalent. The cyclic voltammogram of the Pt/RuO2-sphere had characteristic peaks attributed to both Pt and RuO2ns. Specifically, the redox peaks below 0.4 V and at 0.8V-1.2V could be assigned the hydrogen adsorption/desorption and redox of the Pt surface, respectively. In addition, the unique redox peaks of RuO2ns were also observed around 0.6 V. The estimated on-set potential of the CO oxidation peak in the Pt/RuO2-sphere was at 0.36 V, and this value was superior to that of traditional Pt/C and near that of PtRu/C. The CO oxidation peak of the Pt/RuO2-sphere was sharply compared to the composite catalyst of Pt/C and RuO2ns in previous work. This change in peak shape was thought to be attributed to the expansion of the contact sites between the Pt nanoparticles and the RuO2ns surface.Developed Pt/RuO2-sphere catalyst, which had the RuO2ns wall and spherical shape, showed high CO poisoning catalyst as well as PtRu/C.[1] T. Saida, et al., Electrochim. Acta, 55, 857–864 (2010).[2] T. Saida, et al., Energy & Fuels, 36, 1027-1033 (2022).