PEFC Electrocatalysts Using Ta-Based Support on Mesoporous Carbon

Abstract

Introduction Cathode catalysts of polymer electrolyte fuel cells (PEFCs) are subjected to a severe environment with strong acidity and high potential, leading to catalyst degradation[1]. In particular, the degradation of electrocatalysts in fuel cell vehicles is often caused by oxidative corrosion of their carbon support during start-stop cycles, and by dissolution and re-deposition of platinum during load cycles[2]. Our research group has developed electrocatalysts using Nb-doped SnO2 on carbon support, achieving high start-stop potential durability[3]. The use of Ti and Sn as metal supports instead of carbon has also been considered as a fundamental solution against start-stop related degradation[4]. However, catalytic activity has to be improved, compared to the standard Pt/C catalysts. Here in this study, catalysts using mesoporous carbon (MC) support are prepared using metallic Ta with a high melting point. Parameters such as loading and heat treatment conditions are varied to tailor higher performance and durability of PEFC electrocatalysts. Experimental Ta/MC supports were prepared by steam hydrolysis of an ethoxide reagent on mesoporous carbon (CNovel®, TOYO TANSO) with mesopores of 10 nmΦ. The Pt/Ta/MC catalysts was then prepared by decorating with Pt using the acetylacetonate (acac) method[5] . The microstructure of the prepared Ta/MC supports and the Pt/Ta/MC catalysts was observed using scanning transmission electron microscopy (SU9000, Hitachi High-Tech Corp., Japan). Electrochemical characterization of the catalysts was performed by half-cells: Pt electrochemical surface area (ECSA) was evaluated by cyclic voltammetry (CV), and oxygen reduction reaction activity (ORR activity) was evaluated from the activation-dominant current value by rotating disk electrode (RDE) measurements. The catalysts durability was evaluated by applying potential cycles simulating start-stop and load cycles. Results and discussion As shown in Fig. 1, microstructural observation of the catalyst supports by STEM confirmed that small Ta particles were uniformly supported, and that the Ta particles were impregnated into the mesopores of MCs. EDS elemental analysis was performed to calculate the residual oxygen content. It was found that the reduction of Ta2O5 to Ta proceeded by high-temperature heat treatment in an H2 atmosphere. The initial activity and durability of the prepared Pt/Ta/MC catalysts were evaluated by half-cell electrochemical measurements. ECSA and mass activity were 78.5 m2/g and 179.2 A/g, respectively, showing initial activity approaching that of the standard Pt/C catalyst. Regarding durability, high load cycle durability was confirmed. In this presentation, the microstructure and electrochemical properties of the Pt/Ta/MC catalysts will be presented as alternative cathode catalysts. Acknowledgment This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).