Development of PEFC Alloy Electrocatalysts Supported on SnO2

Abstract

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 PtxMy (M = Co, Ni) alloy catalyst (PtxMy/Sn(Nb)O2/VGCF-H). Co and Ni are relatively stable in the PEFC electrocatalyst environment and PtxMy alloy catalysts are known to be capable of exhibiting higher activity over Pt catalysts [4]. The aim of this study is thus to develop PtxMy alloy electrocatalysts on doped SnO2support. Sn0.98Nb0.02O2 was prepared on the VGCF-H (Sn0.98Nb0.02O2/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 PtxMy alloy, various heat-treatment conditions were applied to realize PtxMy 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 VRHE 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 VRHE. 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.