Platinum metal-based materials have been mostly used as the catalysts for polymer electrolyte fuel cells and extensively studied to improve their performance by alloying and shape-controlling. Although these advanced technologies are pursued mainly for enhancing the catalytic activities and showed significant improvement in deed, their durabilities under realistic conditions do not often accompany. For wider distribution of fuel cell vehicles, however, both high activity and high durability should be compatibly achieved.

Recently we have focused on metal-platinum oxide bronzes as a durable cathode catalyst and reported a study on their fundamental properties [1,2], in which a platinum bronze containing cobalt (Co-Pt bronze) was synthesized by solid state reaction using platinum dioxide and cobalt nitrate hexahydrates as the raw materials. Its oxygen reduction reaction (ORR) was enhanced after potential cycles down to the hydrogen region, which formed a thin Pt metallic layer over the oxide. While its specific activity was as high as that of pure platinum nanoparticles, its durability against the potential cycles was much higher. However, its mass-specific electrochemical surface area (ECSA) and mass activity were much lower than those of pure platinum nanoparticles because of its large particle size (20-50 nm). In the present study, a novel preparation process was applied to obtain smaller Co-Pt bronze nanoparticles and their characteristics and electrochemical performances for PEFC application were examined.

Co-Pt bronze nano powder was prepared by a template method using a mesoporous silica, TMPS-4R (pore diameter: 3.8 nm, Taiyo Kagaku Co., Ltd.). Hexachloroplatinate hexahydrates and cobalt nitrate hexahydrates were dissolved in ultrapure water and then mixed with the silica template powder. The mixed powder was heat-treated under O₂ flow (200 mL min^-1) at 650 ºC for 1 hour. The obtained powder was dispersed in NaOH to dissolve the silica template and subsequently washed with ultrapure water. Co-Pt bronze nano power was collected from the supernatant of the washed suspension after centrifuged at 5,000 g (large Co-Pt bronze and metallic platinum particles were precipitated.). The obtained powder was characterized by XRD, SEM, TEM and the nitrogen sorption method. The electrochemical behaviors including ORR activity of the obtained Co-Pt bronze nano particles were evaluated by the thin-film RDE method in 0.1 M HClO₄ at 30 ºC.

XRD measurement confirmed that the heat-treated powder is comprised of platinum bronze and platinum metal phases and that the latter was removed by the centrifuge process. The nanoparticles collected from the supernatant was comprised of the platinum bronze only and showed a much larger N₂-sorption surface area than the precipitate (58 and 7.3 m² g^-1 for the supernatant and precipitation, respectively). The SEM observation revealed that the platinum bronze particles collected from supernatant are cylindrically shaped with ca. 4 nm in diameter (Fig. 1A). This shape reflects the mesopores of the templated silica. Electrochemical measurement showed that the surface of the prepared nanoparticles was gradually metallized over potential cycling. The ECSA and ORR mass activity after potential cycling were respectively larger than those of large particles made by the solid state reaction.

This work was financially supported by NEDO, Japan.

References

[1] Y. Kamitaka and Y. Morimoto, 232th ECS Meeting Abstract, #1552 (2017)

[2] Y. Kamitaka, N. Taguchi and Y. Morimoto, Catalysts, 8, 258 (2018)