Effects of ionomer chemistry on nanostructure and reactant transport properties in high oxygen permeability ionomers: Molecular dynamics simulation study

Abstract

This work explores how the chemical structure of ionomers affects the nanostructure of ionomer systems and reactant transport properties under various hydration conditions utilizing full atomistic molecular dynamics simulations. We utilize ionomers incorporating the perfluoro-(2-methylene-4-methyl-1,3-dioxolane) structure (PFMMD2; EW 930, PFMMD3; EW 1170) and the perfluoro-(2,2-dimethyl-1,3-dioxole) structure (PDD2; EW 930, PDD3; EW 1170), all of which feature the same side chain pendant as Nafion. The chain packing based on the chemical structure of the model is analyzed, and it is proposed how these differences may influence the development of water channels and free volume within the ionomer. Subsequently, the transport properties of key reactants in the cathode catalyst layer, namely protons and oxygen, are examined. In this study, it is sug-gested that within the high oxygen permeability ionomer models, PFMMD-based ionomers, with their relatively more flexible structure, offer advantages in water channel connectivity and proton/water diffusion. Conversely, PDD-based ionomers, characterized by their more rigid chain structure, enhance free volume formation and thus improve oxygen diffusion. Our work offers valuable insights for designing ionomers that overcome mass transport limitations, potentially paving the way for improved fuel cell performance.