Impact of PEGMA on transport and co-transport of methanol and acetate in PEGDA-AMPS cation exchange membranes

Understanding multi-component transport behavior in membranes with varied internal structures and functionalities is critical for the rational design of membranes that will be challenged with the separation of, or come in contact with, multi-component solutions. One interesting application is ion exchange membranes for CO2 reduction cells as these membranes interact with multiple CO2 reduction products (e.g. methanol and acetate) whose permeation must be controlled, as they readily oxidize back to CO2 and by-products in the anode chamber. Previously, the co-transport behavior of methanol and acetate in membranes with various pendant groups, such as sulfonate (-SO3-), carboxyl (-COOH), ethylene oxide (-CH2CH2OH), and poly(ethylene oxide) (-(CH2CH2O)5H, PEO), was investigated, where permeabilities to acetate was suppressed in PEO-containing films in co-permeation with methanol. Here, we further examine this co-transport behavior in pendant PEO-containing films by preparing three chemically different crosslinked films with a constant crosslinker, poly(ethylene glycol) diacrylate (PEGDA), content and varied the remaining between a PEO-containing comonomer, poly(ethylene glycol) methacrylate (PEGMA), and a sulfonate-containing comonomer, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) (i.e. PEGDA-PEGMA, PEGDA-AMPS/PEGMA, and PEGDA-AMPS). For each chemistry, three structurally distinct films are prepared by varying pre-polymerization water content, leading to differences in water volume fraction (and thereby free volume). We observe the diffusivities of PEGMA-free films (PEGDA-AMPS) to acetate are increased in co-diffusion, while those of PEGMA-containing films (PEGDA-PEGMA and PEGDA-AMPS/PEGMA) to acetate are decreased. These results suggest the strategic addition of a charge-neutral pendant group in a charged IEMs is a valid approach to suppress the crossover of undesired molecules.