Effect of the Catalyst Metal Content on PEMFC Durability

Abstract

The sluggish oxygen reduction reaction (ORR) continues to limit proton exchange membrane fuel cell (PEMFC) performance, while degradation of ORR catalysts contributes to poor durability. So far, carbon supported Pt (Pt/C) and Pt based alloy catalysts are the most effective for electrocatalysis of ORR. In addition to the metal particle properties, the metal to carbon ratio and type of carbon support also affect the performance and durability. Therefore, it is necessary to single out these factors and evaluate each of them while keeping other parameters consistent. In this work, we systematically investigated the effect of cathode catalysts on the PEMFC activity and durability using a single cell membrane electrode assembly (MEA). By screening commercial Pt/C catalysts supplied by Tanaka, we were able to establish that both metal to carbon ratio (from 5% to 50% wt.) and the type of carbon support (Vulcan XC-72 and high surface area) influence the MEA performance and durability. The Vulcan XC-72 carbon catalysts (VC) showed inferior initial mass activity at 0.90 V and lower electrochemically active surface area (ECSA) but similar power density at 0.67 V compared with the high surface area carbon catalyst (HSC). Moreover, the degradation of Pt/VC was more severe than that of Pt/HSC after accelerated stress testing (AST, 30000 cycles between 0.60 V and 0.95 V). This may be related to Pt particles distribution on the surface of carbon as opposed to those in the pores. The Pt particle distribution on the HSC was also probed by the carbon monoxide (CO) stripping under different relative humidity, which further indicated that Pt particles became more accessible after AST. On the other hand, the HSC with lower metal carbon ratio (10%, 20% wt.) demonstrate better durability, which may be attributed to the higher inter-particle distance. All the data will be presented and discussed for implication in practical PEMFC operation.