Characterizing Transport of Crosslinked PEGDA Membranes to Carboxylate Ions of Varying Chain Lengths

The need for renewable energy sources has led to research in numerous technologies including photoelectrochemical CO₂ reduction cells (PEC-CRC) which utilize sunlight and water to reduce carbon dioxideinto carboxylates and alcohols that can be used as feedstock chemicals or fuels. One difficulty facing CO₂ reduction cells is the design of new ion exchange membrane (IEM) that minimize permeation of electrochemical reduction products formed (carboxylates and alcohols) while maintaining ionic conductivity (ion transport). Previously, the transport of the carboxylate product acetate has been investigated for IEMs composed of poly(ethylene glycol) diacrylate (PEGDA, n = 13), poly(ethylene glycol) methacrylate (PEGMA, n = 9), and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) but the transport behavior of other carboxylates of varying chain lengths (e.g. formate, propionate, butanoate) has not been previously examined. To address this knowledge gap and further our understanding of how carboxylate chain length and membrane chemistry impact carboxylate permeation behavior in IEMs, four IEMs of varied composition were synthesized (100%PEGDA, 68%PEGDA/32%PEGMA, 68%PEGDA/16%PEGMA/16%AMPS, 68%PEGDA/32%AMPS, all mol%) and characterized for their physiochemical and transport behavior. Overall, as the carboxylate chain length increases, the membrane permeability (P) to each carboxylate decreases across the four membrane compositions. Membranes with smaller crosslinker (PEGDA) content resulted in larger water uptake and presumably larger mesh size resulting in the higher observed permeability. To fully characterize the transport behavior, measurement of membrane solubility (S) to these solutes is ongoing, and the diffusivity (D) will be determined through the solution-diffusion model (P = S*D).