Transport and Co-Transport of Carboxylate Ions and Ethanol in Anion Exchange Membranes

Abstract

Understanding multi-component transport behavior through hydrated dense membranes is of interest for numerous applications. For the particular case of photoelectrochemical CO2 reduction cells, it is important to understand the multi-component transport behavior of CO2 electrochemical reduction products including mobile formate, acetate and ethanol in the ion exchange membranes as one role of the membrane in these devices is to minimize the permeation of these products. Anion exchange membranes (AEM) have been employed in these and other electrochemical devices as they act to facilitate the transport of common electrolytes (i.e., bicarbonates). However, as they act to facilitate the transport of carboxylates as well, thereby reducing the overall performance, the design of new AEMs is necessary to improve device performance through the selective transport of the desired ion(s) or electrolyte(s). Here, we investigate the transport behavior of formate and acetate and their co-transport with ethanol in two types of AEMs: (1) a crosslinked AEM prepared by free-radical copolymerization of a monomer with a quaternary ammonium (QA) group and a crosslinker, and (2) Selemion® AMVN. We observe a decrease in diffusivities to carboxylates in co-diffusion. We attribute this behavior to charge screening by the co-diffusing alcohol, which reduces the electrostatic attraction between QAs and carboxylates.

Advancing ion-exchange membranes to ion-selective membranes: principles, status, and opportunities

“…While diffusion of H 2(aq) is slower across thicker membranes, ion flux will also be proportionally depressed. The free volume model works reasonably well for neutral solutes with relatively high molecule weights and high swelling degree IEMs, suggesting that the permeants indeed transport through the solvated fraction of the membranes (Yasuda et al, 1969;Kato et al, 1992;Dischinger et al, 2020;Kim and Beckingham, 2021;Kim et al, 2021a;Kim et al, 2022a). However, for smaller solutes, such as H 2(aq) , and low swelling degree membranes, the permeant can plausibly be transported through the unsolvated free volumes within the polymer chains, akin to gas permeation across dense films (Baker, 2012).…”

Improved Structural Stability of Charged Hydrogels under Organic CO2 Reduction Products: Effect of Acrylate and Methacrylate Backbone Linkages

“…As a conjecture, mobile formate anions (OFm – ) may be more favorable in QA + -containing films (counterion condensation , ). In mixed solute sorption, OFm – mobility can be reduced in these films as MeOH softens the fixed charge interface and interferes with the counterion condensation. , …”

Controlling Fractional Free Volume, Transport, and Co-Transport of Alcohols and Carboxylate Salts in PEGDA Membranes

“…These relative permeabilities are attributed to the differences in solute size, kinetic diameter for the alcohols, and hydrated diameter for the carboxylate salts. The kinetic diameter of EtOH (4.5 Å [ 33 ]) is higher than the kinetic diameter of MeOH (3.6 Å [ 34 ]), and the hydrated diameter of OFm − (5.9 Å [ 35 ]) is smaller than the hydrated diameter of OAc − (7.44 Å [ 32 , 36 ]). The smaller solutes permeate faster than the larger solutes (diffusivity varies inversely proportional to the solute size).…”

“…However, at higher pre−polymerization water content, and thereby higher water volume fraction, EtOH permeability decreases in co-permeation with NaOAc. Recently, in a study by Kim et al (2021) , it was observed that EtOH permeability remained essentially the same in PEGDA-based films in single and co-transport with carboxylate ions, which was attributed to the larger observed differences in sorption behavior of EtOH for these mixtures (ethanol’s solubility was 0.298 whereas the solubility was 0.386 while co-transporting with NaOFm) [ 34 ]. Thus, in our current study, the consistency in permeability values for EtOH between single and co-permeation in PEGDA-PEGMA films with NaOFm is likely also a result of analogous sorption differences that might compensate for the difference in the diffusive behavior between two molecules.…”

“…As with MeOH and EtOH, the observed differences are smaller in absolute terms at lower water volume fraction, with the largest differences observed at the highest water volume fraction. While the underlying reason for this difference is unclear, two contributing factors are likely to be differences in sorption behavior as well as the differences in hydrated radius (NaOAc has a larger hydrated radius than NaOFm) [ 34 ].…”