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.
Highlights
Anion exchange membranes (AEM [1,2]) are a crucial component of devices for various applications, including direct ethanol fuel cells [3], direct urea fuel cells [4], water purification devices [5], water electrolyzers [6], CO2 electrolyzers [7] and photoelectrochemical CO2 reduction cells (PEC-CRC) [8,9,10,11,12]
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 CO2 reduction products (OFm− and OAc−)
In the case of Nafion® 117 and PEGDA-AMPS (a crosslinked cation exchange membranes (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]
Summary
Anion exchange membranes (AEM [1,2]) are a crucial component of devices for various applications, including direct ethanol fuel cells [3], direct urea fuel cells [4], water purification devices [5], water electrolyzers [6], CO2 electrolyzers [7] and photoelectrochemical CO2 reduction cells (PEC-CRC) [8,9,10,11,12]. 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 CO2 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 CO2 reduction products (alcohols) can act to impact and, in some cases, mitigate the permeation of CO2 reduction products (carboxylates) [27,28,29,30,31,32]); see Figure 1A,D. Those of PEGDAAMPS/PEGMA were the same [28] or slightly decreased in co-diffusion with MeOH, where we conjectured a potential charge screening by long pendant chains
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