Mechanism of Facilitation of Ion Mobility in Low-Water-Content Fuel Cell Membranes
A Barnett and JB Lu and V Molinero, JOURNAL OF PHYSICAL CHEMISTRY C (2021).
Ion-exchange membranes for fuel cells have nanophase-segregated structures with percolated hydrophilic channels. Adding tethers up to seven carbons long between polymer ions and backbone has been shown to increase the conductivity of anion- and cation-exchange membranes at low water contents that are established under fuel cell operando conditions. This effect had been attributed to the development of better-clustered, wider ion-conducting channels. However, recent studies indicate that the width and clustering of the channels are insensitive to tether length. This poses the question of how do tethers increase ion mobility in membranes. Here, we use large-scale molecular simulations to elucidate the mechanisms of ion diffusion in polyphenylene oxide trimethylalkylammonium (PPO-TMA) anion-exchange membranes with cations tethered by alkyl chains from 1 to 12 carbons long, as a model for fuel cell membranes with variable tether length. We find that tethers increase the conductivity through a purely dynamical facilitation mechanism that originates in the synergism of strong cation-anion pairing and the restricted mobility of the polymer-bound cation. In the absence of a tether, the anion diffusion is dominated by similar to 0.3 nm jumps into positions of their solvation water. Local mobility of tethered cations removes the jamming for anions, increasing the contribution of the faster continuous diffusion mechanism. The facilitation mechanism saturates for approximately seven carbon long tethers, revealing why that tether length is used in the best-performing commercial membranes.
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