(651c) Co-Transport of Methanol and Carboxylates in PEGDA-Spmak/Pegpea Cation Exchange Membranes

Artificial photosynthesis devices such as photoelectrochemical CO₂ reduction cells (PEC-CRC) reduce CO₂ into useable chemicals such as alcohols and carboxylates. However, these chemicals often diffuse through the ion exchange membrane, which decreases device efficiency. Moreover, the transport of a particular product behaves differently in the presence of other product molecules. Therefore, it is necessary to understand this multi-component transport to membranes of different compositions to decipher membrane structure-physiochemical-transport relationships. Previous observations by our group have shown that acetate permeabilities increase with fast-moving methanol, and the inclusion of neutral monoacrylate monomers suppresses this undesirable transport. To further extend this investigation, crosslinked membranes were prepared with poly(ethylene glycol) diacrylate (PEGDA) as a crosslinker, 3-sulfopropyl methacrylate potassium (SPMAK) as a sulfonated comonomer, and poly(ethylene glycol) phenyl ether acrylate (PEGPEA) as a neutral comonomer. Secondly, membrane compositions were tuned with two different crosslinker chain lengths (n = 10 and 13) and various ratios of SPMAK to PEGPEA. Membrane permeabilities to methanol and carboxylates (formate, acetate, and propionate) were carried out individually and in co-permeation (methanol-carboxylate). Membrane solubilities were carried out individually for carboxylates. Permeabilities vary as methanol > formate > acetate ≈ propionate, and solubilities vary as propionate > formate > acetate. Permeabilities, solubilities, water content, and ionic conductivities decrease with decreasing PEGDA chain length. Permeabilities and ionic conductivities decrease with increasing PEGPEA content. Solubilities to carboxylates remain almost the same with increasing PEGPEA content, which indicates carboxylate permeabilities are affected largely by diffusion than sorption. Therefore, further investigations should focus on understanding methanol solubilities in this series of membranes and decreasing crosslinking chain length and water content to reduce CO₂ product transport.