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 started fundamental studies on mixed oxides of platinum (so called "platinum bronze") consisting of platinum, oxygen and other metal ions (such as Li, Na, Mg, Ca, Zn, Cd, Co, Ni [1]), which were reported by Shannon et al. as an ORR catalyst for fuel cells. Although their activities were not extremely high, they were reported not to be dissolved with hot aqua regia. This remarkable property made us expect that these materials can exhibit excellent durability under fuel cell conditions (high temperature, high potential and also chloride contaminant). Here, we present the preparation, characterization and electrochemical behavior of Co-Pt bronze for PEFC application.

Co-Pt bronze was prepared according to the literature [1] with some modifications. Platinum oxide and Co(NO₃)₂·6H₂O powders were mixed and heat-treated under air conditions at 650 ºC for 5 hours. The obtained powder was washed with heated (80 °C) aqua regia for 3 times and the remaining powder was then thoroughly rinsed with pure water. The electrochemical behaviors including ORR activity of the obtained Co-Pt bronze were evaluated by the thin-film RDE method in 0.1 M HClO₄ at 30 ºC. Then, the electrode was subjected with potential cycling (0.05-1.2 V, 500 mV s^-1, 100 cycles) before another electrochemical behavior measurement. The Co-Pt bronze powders were characterized as prepared and before and after electrochemical measurement by nitrogen sorption method, XRD, XPS, SEM etc.

XRD measurement confirmed that the as-prepared powder after the heat treatment contained a bronze phase and a platinum metal phase and the latter was completely removed by the heated aqua regia treatment. The particle size of the obtained Co-Pt bronze powder was 20-80 nm from SEM observation and the specific surface area evaluated by BET analysis was 18.8 m² g^-1. The elemental composition was Co_0.5Pt₃O₄ from EDX analysis. The ORR current for the Co-Pt bronze was low in the 0.6-1.0 V potential sweep (Figure 1a, before potential cycling). Cyclic voltammogram under anaerobic conditions showed no hydrogen sorption and platinum redox currents before the potential cycling but these currents gradually increased (Figure 1b) over the potential cycling. The ORR current of the Co-Pt bronze after potential cycling was comparable to that of Pt/Vulcan (Figure 1a, after potential cycling). While XRD measurement did not exhibit any change before and after the potential cycling, XPS spectrum indicated presence of metallic platinum after the potential cycling. These results clearly indicated that a thin metallic platinum phase was formed on the surface of the Co-Pt bronze by potential cycling and the metallic Pt was the main active site for ORR. Moreover, the Co-Pt bronze also exhibited an HOR activity due to the metallic site. Therefore, Co-Pt bronze can be used not only for the cathode but also for the anode. The durability of the Co-Pt bronze will be also discussed at the meeting.

This work supported by NEDO, Japan.

References

[1] R.D. Shannon, et al., Inorg. Chem., 21, 3372 (1982)