(161q) Transport Behavior of Polyether-Based Cation Exchange Membranes to Acetate in Co-Permeation with Methanol

Multicomponent transport through hydrated dense membranes is utilized in many applications, such as photoelectrochemical CO₂ reduction cells and pervaporation cells. One of the crucial challenges in photoelectrochemical CO₂ reduction cells is to design an ion exchange membrane that minimizes crossover of CO₂ reduction products, such as methanol and acetate, with decent ionic conductivity. Previously, the transport behavior of a sulfonated cation exchange membrane, Nafion® 117, to methanol and sodium acetate was investigated and an increase in permeability to sodium acetate was noticed in co-permeation with methanol. To further investigate this transport behavior, a charge-neutral membrane (PEGDA, n = 13) and a series of sulfonated cation exchange membranes (PEGDA-AMPS) were prepared by varying poly(ethylene glycol) diacrylate to 2-acrylamido-2-methylpropanesulfonic acid ratio, where AMPS is a comonomer with a negatively charged sulfonate end that higher AMPS content in the polymer matrix results higher ionic conductivity and water volume fraction. As in Nafion® 117, a distinct increase in PEGDA-AMPS permeability to sodium acetate in co-permeation with methanol was observed for these PEGDA-AMPS membranes. We attribute this transport behavior to the shielding of electrostatic repulsion, in which charge-neutral methanol interferes with electrostatic repulsion between acetate and membrane-bound sulfonate anions. The shielding of electrostatic repulsion was further investigated under varied sulfonate content by modifying membrane structure upon partially or completely replacing AMPS with three different charge-neutral comonomers, acrylic acid (AA, n = 0), 2-hydroxylethyl methacrylate (HEMA, n = 1), and poly(ethylene glycol) methacrylate (PEGMA, n = 5). While the permeability to sodium acetate in co-permeation with methanol was increased for membranes with shorter comonomers, AA and HEMA, it was consistent for membranes with PEGMA. From these experiments, we deduce that charge-neutral molecules, both moving (e.g. methanol) and membrane-bound (e.g. ethylene glycol), can interfere with the electrostatic repulsion during ionic transport in ion exchange membranes.