Width and Clustering of Ion-Conducting Channels in Fuel Cell Membranes Are Insensitive to the Length of Ion Tethers
Adam Barnett, Jibao Lu, Valeria Molinero
Abstract
Ion transport is the main function of fuel cell membranes. Maintaining high ion conductivity, however, is challenging under the low water contents established by electro-osmotic drag under operando conditions. Separating the bound ion from the polymer backbone via tethers up to 7 carbons long enhances the conductivity of both anion- and cation-exchange membranes. This effect has been attributed to increased phase separation of hydrophilic and hydrophobic domains in the membranes. However, to date, there have not been systematic studies of the effect of tether length on the structure of ion-exchange membranes. Here, we use large-scale molecular simulations to investigate the structure of polyphenylene oxide trimethylalkylammonium anion-exchange membranes with the cations tethered by alkyl chains from 1 to 12 carbons long, as a model for fuel cell membranes with variable tether length. The diffusion of the anions in the simulations increases with tether length and saturates for chains with seven carbons or more. Nevertheless, the simulation reveals that the width and clustering of hydrophilic channels, as well as the local environment of the anions, are insensitive to tether length. Further comparison of the width of the hydrophilic channels along a wide range of zwitterion-, anion-, and cation-exchange membranes indicates that the width of hydrophilic channels is universal across all low-water-content ionic membranes. We conclude that the improved conductivity of fuel cell membranes with ion tethers does not have a structural origin.