The Oxygen Reduction Reaction Activity of Pt-Co Catalysts Supported on Ta-Doped SnO2 for Polymer Electrolyte Fuel Cells

Abstract

Pt and Pt alloy nanoparticles supported on carbon were applied as cathode catalysts for polymer electrolyte fuel cells (PEFCs). The carbon support is appropriate for the support due to its high electrical conductivity and high surface area, although it corrodes under high potential conditions. Carbon corrosion leads to aggregation and detachment of the catalyst and reduces the performance of the PEFC. In our group, a highly durable Pt/Ta-SnO2 catalyst was developed for the cathode of PEFCs1). In this study, we synthesized Pt-Co alloy particles supported on Ta-SnO2 (PtCo/Ta-SnO2) in order to improve the activity while maintaining high durability. The Ta-SnO2 support was synthesized by the flame spray synthesis method. Pt67Co33, Pt75Co25 and Pt80Co20 catalysts were loaded on the Ta-SnO2 by the colloidal method. After heat treatment, the crystal phases, Pt/Co content, and microstructure were evaluated by X-ray diffractometry (XRD), inductively coupled plasma mass spectrometry (ICP-MS), and scanning transmission electron microscope (STEM), respectively. The electrochemical measurements were carried out by the half-cell method using the rotating disk electrode (RDE). The electrochemically active surface area (ECA) of each catalyst was estimated by cyclic voltammetry (CV). The kinetically controlled current density (j k) and mass activity (MA k ) at 0.85 V were also estimated from linear sweep voltammetry (LSV) with Koutecky-Levich plots. The Pt:Co composition ratios (mol%) of the catalysts were 67.1:32.9(Pt67Co33), 74.6:25.4(Pt75Co25), and 80.9:19.1(Pt80Co20), which were well controlled to the desired ratios. The metal loading amounts (wt%) of the catalysts were 17.9, 17.4 and 15.4, respectively. The particle diameters (nm) were 2.7 ± 0.9, 2.8 ± 0.9, and 3.1 ± 0.8, respectively. The catalysts were highly dispersed on the support with hemispherical shape (Fig. 1 (a)). Based on the HAADF-STEM image, it was confirmed that the lattice planes of the catalyst particles were oriented parallel to those of the support. This is expected to suppress the agglomeration of the catalyst particles during startup/shutdown. The Ta-SnO2 support particles formed a fused aggregated network structure, which is able to function as a pathway for both gas diffusion and electrical conduction2). The ECA values of the catalysts, estimated from the CV, were 56.2, 70.6, and 71.3 m2g-1. The LSV values of these catalysts (1750 rpm, Fig. 1 (a)) showed the same limiting current density of that of commercial Pt/carbon black (Pt/CB, TEC10E50E). The jk values at 0.85 V (Fig. 1 (b)) showed that the Pt75Co25/Ta-SnO2 catalyst had the highest oxygen reduction reaction (ORR) activity, which reached 2.5 times larger than that for commercial Pt/CB. Acknowledgement This work was supported by funds for the SPer-FC project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References 1) Y. Senoo, K. Taniguchi, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, M. Watanabe, Electrochem. Commun. 51, 37 (2015). 2) K. Kakinuma, Y. Chino, Y. Senoo, M. Uchida, T. Kamino, H. Uchida, S. Deki, M. Watanabe, Electrochemica Acta 110, 316 (2013). Figure 1