Transport and co‐transport of carboxylate ions and alcohols in cation 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 (PEC‐CRC), it is important to understand the multi‐component transport behavior of CO2 electrochemical reduction products including mobile carboxylates (formate and acetate) and alcohols (methanol and ethanol) in the ion exchange membranes as one role of the membrane in these devices is to minimize the permeation of these CO2 reduction products to the anolyte as they often oxidize back to CO2. Cation exchange membranes (CEM) are promising candidates for such devices as they act to minimize the permeation of mobile anions, such as carboxylates. However, the design of new CEMs is necessary as the permeation of carboxylates often increases in co‐permeation with alcohols. Here, we investigate the transport behavior of carboxylates and alcohols in two types of CEMs (1) a crosslinked CEM was prepared by free‐radical copolymerization of a sulfonated monomer (AMPS) with a crosslinker (PEGDA), and (2) Nafion® 117. We observe an increase in both PEGDA‐AMPS and Nafion® 117 diffusivities to carboxylates in co‐diffusion with alcohols. We attribute this behavior to charge screening by co‐diffusing alcohol that reduces the electrostatic repulsion between bound sulfonates and mobile carboxylates.

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

“…Generally, EtOH solubilities are higher than those of salt solubilities by 1.7 times, on average, indicating that EtOH uptake is preferred in these films (φ w : 0.2-0.5) over the uptake of carboxylate salts. We observed similar behavior in a previous investigation of cation exchange membranes (CEM) [29,32,50], where the alcohol (MeOH and EtOH) solubilities were higher than the carboxylate (NaOFm and NaOAc) solubilities. However, contrary to our previous investigations of CEMs, the EtOH concentrations in the AEMs here (A0, A8, A12 and AMVN) after sorption in the external solution (1 M EtOH) were less than those of the external solution, such that EtOH is less preferred in these films over the external solution; see Supporting Information Table S8.…”

“…To rationalize this behavior, we conjecture the permeation of carboxylate salts to be dependent on the polyatomic carboxylate anions over the cations. Consequently, electrostatic attraction (i.e., counterion condensation [18]) between the bound quaternary ammonium (QA + ) and mobile carboxylate anions (OFm − and OAc − ) can be suppressed by co-permeation with EtOH (i.e., charge screening [29,30,32]); see Figure 1. As a result, the overall salt permeabilities of QA + -containing A8 and A12 are decreased in co-permeation with EtOH.…”

“…While the majority of ion exchange membranes (IEM [17][18][19][20]) designed for PEC-CRCs have focused on AEMs, one of the major drawbacks of AEMs for such devices is their high diffusibility for negatively charged CO 2 reduction products (OFm − and OAc − ). To avoid this issue, our group has performed a series of investigations to gain a fundamental understanding of cation exchange membranes (CEM [21][22][23][24]) for PEC-CRCs, such as how the presence of a series of charge-neutral comonomers (acrylic acid, hydroxyethyl methacrylate and poly(ethylene glycol) methacrylate (PEGMA) [25,26]) or how the presence of co-diffusing neutral CO 2 reduction products (alcohols) can act to impact and, in some cases, mitigate the permeation of CO 2 reduction products (carboxylates) [27][28][29][30][31][32]); see Figure 1A,D. In the case of Nafion ® 117 (a commercial CEM) and PEGDA-AMPS (a crosslinked CEM that we prepared by incorporating 2-acrylamido-2methyl-1-propanesulfonic acid (AMPS, sulfonate-containing ionomer) with a crosslinker, poly(ethylene glycol) diacrylate (PEGDA)), both OFm − and OAc − diffusivities were increased in co-diffusion with either MeOH or EtOH, where we conjectured a potential charge screening by co-diffusing alcohol [32].…”

“…To avoid this issue, our group has performed a series of investigations to gain a fundamental understanding of cation exchange membranes (CEM [21][22][23][24]) for PEC-CRCs, such as how the presence of a series of charge-neutral comonomers (acrylic acid, hydroxyethyl methacrylate and poly(ethylene glycol) methacrylate (PEGMA) [25,26]) or how the presence of co-diffusing neutral CO 2 reduction products (alcohols) can act to impact and, in some cases, mitigate the permeation of CO 2 reduction products (carboxylates) [27][28][29][30][31][32]); see Figure 1A,D. In the case of Nafion ® 117 (a commercial CEM) and PEGDA-AMPS (a crosslinked CEM that we prepared by incorporating 2-acrylamido-2methyl-1-propanesulfonic acid (AMPS, sulfonate-containing ionomer) with a crosslinker, poly(ethylene glycol) diacrylate (PEGDA)), both OFm − and OAc − diffusivities were increased in co-diffusion with either MeOH or EtOH, where we conjectured a potential charge screening by co-diffusing alcohol [32]. In the case of PEGDA-AMPS CEMs, which also incorporated a charge-neutral comonomer (PEGDA-AMPS/AA/HEMA, and/PEGMA) into the structure, the OAc − diffusivities in PEGDA-AMPS/PEGMA and/HEMA were increased (as in comonomer-free PEGDA-AMPS films) [28].…”

“…However, those of PEGDA-AMPS/PEGMA were the same [28] or slightly decreased in co-diffusion with MeOH, where we conjectured a potential charge screening by long pendant chains. [28,30,32] with permission from Elsevier and Wiley.…”