Effects of side-chain structure and spacing on the assemblies of perfluorosulfonic acid ionomers and local oxygen transport in PEM fuel cells

Molecular dynamics simulations were conducted to investigate the effects of side-chain structure and spacing on the self-assembly behaviors of perfluorosulfonic acid (PFSA) ionomers in both bulk proton exchange membrane (PEM) and nano-thin films within the catalyst layer (CL) for PEM fuel cells. Differences and interconnections between the two systems were highlighted; and the local oxygen transport properties at Pt/ionomer interfaces are analyzed. Results reveal that the side-chain length predominantly influence the size of primary sulfonate aggregates and the formation of the secondary aggregates, respectively, thereby playing distinct roles in the connectivity of proton-conducting hydrophilic domains. Specifically, in bulk system, the connectivity was primarily determined by the sizes of the secondary aggregate, making the side-chain spacing a critical factor; whereas in CL, combined effects of nanoscale confinement and ionomer-catalyst interactions restrict the formation of secondary sulfonate aggregates, rendering the size of primary aggregates and thus the side-chain length more important. Besides, although longer side chains with flexible ether groups enhance microphase separation, they also intensify high backbone aggregation, leading to inhomogeneous ionomer distribution and impeded proton transport in CL. A negative correlation is observed between the oxygen flux and the backbone aggregation on Pt. As a result, among the PFSA ionomers studied, the one with medium-length and closely spaced side chains (3M₈₇₇) exhibited the most favorable self-assembly characteristics for CL applications, balancing both the proton conduction and oxygen permeability. These findings provide crucial molecular-level insights into optimization of ionomer side chain structures toward PEM and CL.