Main-chain, statistically sulfonated proton exchange membranes: the relationships of acid concentration and proton mobility to water content and their effect upon proton conductivity

Abstract

An in-depth analysis has been developed for proton exchange membranes to examine the effect of acid concentration and effective proton mobility upon proton conductivity as well as their relationship to water content. The analysis was carried out on a series of main-chain, statistically sulfonated polymers with varying ion-exchange capacities. These polymer systems consisted of: sulfonated poly(ether ether ketone) (1), poly(ethylenetetrafluoroethylene-graft-polystyrenesulfonic acid) (2), sulfonated polyimide (3) and BAM^® membrane (4) with Nafion^® (5) as baseline. They represent membranes comprising polyaromatic polymers (1 and 3), one of which is also a rigid-rod polymer (3), vinylic polymers (4) and a vinylic polymer polymerized inside a polymer matrix (2). In order to remove the differences in acid strength for the membranes, proton mobility values at infinite dilution (X_v = 1.0) and 25 °C were calculated and found to be 3.2 (±0.4) × 10⁻³ cm² s⁻¹ V⁻¹ (1), 2.9 (±0.4) × 10⁻³ cm² s⁻¹ V⁻¹ (2), 1.6 (±0.7) × 10⁻³ cm² s⁻¹ V⁻¹ (3) and 2.1 (±0.2) × 10⁻³ cm² s⁻¹ V⁻¹ (4). These were then compared with the theoretical value for the mobility of a free proton at infinite dilution. Significant deviations from this value were theorized to be due to possible differences in tortuosity and proximity of acid groups.