Ion co-transport with alcohol in cation exchange membranes*

The co-transport of ionic species and alcohols in ion exchange membranes is of interest for applications such as photoelectrochemical CO₂ reduction cells (PEC-CRC). In PEC-CRC, CO₂ is reduced to valuable ions (e.g. formate and acetate) and alcohols (e.g. methanol and ethanol) and a lack of catalyst specificity leads to mixtures of these products. One of the major challenges in PEC-CRC is to design ion exchange membranes that minimize crossover of CO₂ reduction products as upon crossover they readily oxidize back to CO₂ diminishing device performance. Understanding the multicomponent transport behavior of these species in ion exchange membranes is thereby necessary to advance the design of new membranes for this and other applications. Here, we investigate the transport behavior of Nafion® 117 and PEGDA-AMPS cation exchange membranes to co-ion (formate or acetate) and alcohol (methanol and ethanol), where we observe an increase in both formate and acetate permeability in co-permeation with either methanol or ethanol. We attribute this behavior a shielding of electrostatic repulsion, in which the alcohol interferes with electrostatic repulsion between transporting co-ions and bound sulfonate anions.