Investigating the Effect of Iridium Based Water Electrolysis Catalysts on the Performance and Durability of the Catalyst Layer in PEMFCs

Abstract

Hydrogen-based polymer electrolyte fuel cells (PEMFCs), are becoming very popular, especially for automobile applications, given their high power density, high efficiency and zero emissions characteristics. However, they still face durability issues of the catalyst layer. Catalysts supported on carbon based materials show high electrochemically active surface area and performance [1] but are, at the same time, susceptible to carbon based degradation. To avoid carbon corrosion, it has recently become common strategy to include a water electrolysis catalyst like Ir or IrO2 to preferentially promote water electrolysis reaction since it is electrochemically more favourable than the harmful carbon corrosion reaction, thus improving the durability of the catalyst layer [2,3].In our group, a series of Ir(IrOx)-Pt/Vulcan catalysts (Fig. 1) were developed by varying Ir/Pt atomic ratio, 0.21 to 1.20. It was used on the cathode side, and the durability test simulating the stop/start cycle of FCVs was evaluated. Ir(IrOx)-Pt/Vulcan catalysts showed a slight decrease in the initial performance relative to Pt/Vulcan due to the presence of Ir, but there was a point after which a further increase in Iridium content did not noticeably affect the performance any longer. In contrast to the initial performance, post durability-test performance was expected to be higher for catalysts with higher Ir content, but our results showed the worst performance in the most Ir-rich catalyst after 10000 cycles. This was attributed to water electrolysis products trapped in small pores of the catalyst layer thereby affecting mass transport. We confirmed this by studying the various losses in each catalyst type, and found the highest concentration overvoltage in the Ir-rich catalyst. On repeating durability experiments for Ir-rich catalyst, but this time purging it with N2 gas overnight, results showed much higher performance relative to other samples for the same number of durability test cycles, supporting our hypothesis.We also consider water electrolysis catalysts on the anode side as well, since fuel starvation is commonly encountered in the anode under extreme operating conditions. Specifically, we explore Iridium-based corrosion tolerant anode catalysts and outline several strategies to reduce degradation at high humidity and loads where carbon corrosion becomes more preferable to water electrolysis. Our findings in understanding and addressing these durability issues could greatly aid in better catalyst design.