Pt/C is widely used as polymer electrolyte fuel cell (PEFC) electrocatalysts. However, under a high potential at the cathode, Pt detachment and aggregation due to carbon corrosion can occur. Therefore, Pt/C has a difficulty in durability. Electrocatalysts with tin oxide (SnO₂) support are thus developed where vapor grown carbon fiber (VGCF-H) acts as a conducting backbone, achieving both high activity and high durability ^[1-3]. However, their electrocatalytic activity should be further improved. For this reason, here, we are focusing on the improvement in catalytic activity by preparing Pt_xM_y (M = Co, Ni) alloy catalyst (Pt_xM_y/Sn(Nb)O₂/VGCF-H). Co and Ni are relatively stable in the PEFC electrocatalyst environment and Pt_xM_y alloy catalysts are known to be capable of exhibiting higher activity over Pt catalysts ^[4]. The aim of this study is thus to develop Pt_xM_y alloy electrocatalysts on doped SnO₂support.

Sn_0.98Nb_0.02O₂ was prepared on the VGCF-H (Sn_0.98Nb_0.02O₂/VGCF-H) by the ammonia coprecipitation method. Co-impregnation of Pt-M alloy (M = Co, Ni) nano-particles was made using acetylacetonate (acac) complexes ^[5]. In this procedure, a 2-step procedure was applied, i.e., after impregnating Pt, M was impregnated. The amount of SnO₂ on the VGCF-H was determined to set the SnO₂ loading to 50 wt.% to maintain a high specific surface area of SnO₂ without SnO₂ particle aggregation and to secure conductive passway among the VGCF-H fibers. In case of Pt_xM_y alloy, various heat-treatment conditions were applied to realize Pt_xM_y alloying without the reduction of SnO₂to Sn metal. In order to evaluate Pt and M loadings, ICP analysis was performed. The nanostructure of the electrocatalysts prepared was observed by using FESEM and STEM. We then made half-cell tests to evaluate electrochemical activity and durability of these electrocatalysts.

From the ICP results, the atomic Pt/Co ratio measured was 73.5/26.5, confirming that Pt₃Co alloy electrocatalyst has been successfully prepared. Figure 1 shows Pt-based catalyst particles with a diameter of 2 to 3 nm are selectively impregnated on the SnO₂particles rather than on the VGCF-H.

Figure 2 shows that ORR activity was improved, exceeding that of Pt/SnO₂/VGCF-H electrocatalyst. The mass activity improvement by the alloying has been confirmed. The highest mass activity of 273 Ag^-1 at 0.9 V_RHE has been obtained, for the Pt₃Co/Sn(Nb)O₂/VGCF-H electrocatalyst. The mass activity of this electrocatalyst has reached beyond 1800 Ag^-1 measured at 0.85 V_RHE. Especially, for Pt : Co = 3 : 1 and heat treatment from 210 ^oC (3 h) to 240 ^oC (3 h) and 270 ^oC (1 h), the mass activity reached the highest value both before and after the voltage cycling durability test, clearly indicating the positive effect of the alloying. In terms of durability, 38% of the initial MA remained even after the 60,000 voltage cycling test, which is larger than that of the Pt/C and is almost the same as that of the Pt/SnO₂/VGCF-H electrocatalyst (39%). From these results, we can conclude that SnO₂support is applicable also for Pt alloy catalysts, exhibiting high start-stop cycle durability.

References

[1] A. Masao, S. Noda, F. Takasaki, K. Ito, and K. Sasaki, Electrochem. Solid-State Lett., 12(9) B119-B122 (2009).

[2] F. Takasaki, S. Matsuie, Y. Takabatake, Z. Noda, A. Hayashi, Y. Shiratori, K. Ito, and K. Sasaki, J. Electrochem. Soc., 158(10) B1270-B1275 (2011).

[3] D. Horiguchi, T. Tsukatsune, Z. Noda, A. Hayashi, and K. Sasaki, ECS Trans., 64(3), 215-220 (2014).

[4] T. Toda, H. Igarashi, and M. Watanabe, J. Electrochem. Soc., 145, 4185 (1998).

[5] A. Hayashi, H. Notsu, K. Kimijima, J. Miyamoto, and I. Yagi, Electrochim Acta. 53, 6117–6125 (2008).