Influence of Commercial Ionomers and Membranes on a PGM-Free Catalyst in the Alkaline Oxygen Reduction

Abstract

Hitherto, research into alkaline exchange membrane fuel cells lacked a commercial benchmark anionomer and membrane, analogous to Nafion in proton-exchange membrane fuel cells. Three commercial alkaline exchange ionomers (AEIs) have been scrutinized for that role in combination with a commercial platinum-group-metal-free Fe-N-C (Pajarito Powder) catalyst for the cathode. The initial rotating disc electrode benchmarking of the Fe-N-C catalyst’s oxygen reduction reaction activity using Nafion in an alkaline electrolyte seems to neglect the restricted oxygen diffusion in the AEIs and is recommended to be complemented by measurements with the same AEI as used in the alkaline exchange membrane fuel cell (AEMFC) testing. Evaluation of the catalyst layer in a gas-diffusion electrode setup offers a way to assess the performance in realistic operating conditions, without the additional complications of device-level water management. Blending of a porous Fe-N-C catalyst with different types of AEI yields catalyst layers with different pore size distributions. The catalyst layer with Piperion retains the highest proportion of the original BET surface area of the Fe-N-C catalyst. The water adsorption capacity is also influenced by the AEI, with Fumion FAA-3 and Piperion having equally high capabilities surpassing Sustainion. Finally, the choice of the membrane influences the ORR performance as well; particularly, the low hydroxide conductivity of Fumion FAA-3 in the room temperature experiments mitigates the ORR performance irrespective of the AEI in the catalyst layer. The best overall performance at high current densities is shown by the Piperion anion exchange ionomer matched with Sustainion X37-50 membrane.