Structure–property correlations of high oxygen permeability ionomers for optimized design of ionomer performance, catalytic activity, and mass transport in PEFC cathodes

In polymer electrolyte fuel cell (PEFC) cathodes, oxygen transport through the ionomer, in addition to proton conduction, strongly influences performance. High oxygen permeability ionomers (HOPIs) based on cyclic monomers are therefore of great interest, but clear molecular design guidelines remain limited. Here, we compare two newly designed ionomers, one with a cyclic sulfonic acid monomer in a linear backbone and the other a fully cyclic ionomer with a short-side-chain sulfonic acid monomer, along with Nafion and a previously reported fully cyclic long-side-chain ionomer. At fixed ion exchange capacity, enhanced oxygen permeability required a cyclic hydrophobic monomer, whereas shortening the sulfonic acid side chain markedly increased the softening temperature. In membrane–electrode assemblies (MEAs), the use of fully cyclic ionomers led to reduced activation and concentration overvoltages, whereas the use of an ionomer containing only a cyclic sulfonic acid monomer produced only a modest improvement. Molecular dynamics simulations suggest that the higher catalytic activity of the fully cyclic ionomers is associated with a less dense ionomer layer at the Pt interface. Appropriate ink formulation and coating conditions were also essential to suppress macropore blockage and maximize the performance benefits of HOPIs in PEFC cathodes.