Transport of Neutral and Charged Solutes in Imidazolium-Functionalized Poly(phenylene oxide) Membranes for Artificial Photosynthesis

Abstract

Anion exchange membranes (AEMs) play an essential role in artificial photosynthesis devices, which photoelectrochemically convert CO2 and water into useful products. AEMs allow the transport of charge carriers between electrodes while minimizing the transport of CO2 reduction products (e.g., ethanol). Fundamental transport studies in AEMs relevant to artificial photosynthesis are uncommon. Herein, we describe the preparation of an imidazoliumfunctionalized poly(phenylene oxide) membrane. Membrane transport properties were controlled by systematic variation of the degree of imidazolium functionalization, which induced changes in the membrane water volume fraction. Ethanol permeability and ionic conductivity increased with membrane water volume fraction. Consequently, membranes of relatively high ionic conductivity exhibited relatively high ethanol permeability, presenting a tradeoff in the transport properties desirable for artificial photosynthesis applications. This work seeks to enable optimization of AEMs for artificial photosynthesis through systematic study of membrane structure (water volume fraction) and its relevance to alcohol transport and electrolyte ion conductivity.

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

“…However, chlorine compositions were lower than the theoretical values by three times, on average. Complete counterion conversion (Cl − to HCO 3 − ) is presumed in all AEMs (A8, A12 and AMVN) as Cl − was not detected in EDS elemental analysis on any film after the conversion [ 16 ]; see Figure S2 .…”

“…The ionic conductivities of the AEMs in HCO 3 − form were lower than those in Cl − form by 1.4 times, on average [ 16 ]. The ionic conductivities of films in both Cl − and HCO 3 − form are plotted in a function of inverse water volume in Figure 4 along with the upper bound regression line for a series of Selemion ® AMV and ImPPO-χ AEMs [ 16 ].…”

“…The ionic conductivities of the AEMs in HCO 3 − form were lower than those in Cl − form by 1.4 times, on average [ 16 ]. The ionic conductivities of films in both Cl − and HCO 3 − form are plotted in a function of inverse water volume in Figure 4 along with the upper bound regression line for a series of Selemion ® AMV and ImPPO-χ AEMs [ 16 ]. While the conductivities of AMVN are within the range of other AEMs (Selemion ® AMV and ImPPO-χ [ 16 ]), the conductivities of both A8 and A12 are lower than their expected conductivity for their respective water volume fractions.…”

“…Then, 0.75-inch and 1-inch hole-punches (General Tools 1271 Arch Punches) were used to cut each AEM into 0.75-inch films for sorption–desorption experiments and 1-inch films for diffusion cell experiments. All films were then placed in 1 M sodium bicarbonate (NaHCO 3 ) for 2 days, where the solution was replaced daily and gently stirred [ 16 ]. Next, all films were washed with DI water and placed in DI water for 2 days to remove excess Cl − and NaHCO 3 , where the solution was gently stirred, and water was replaced daily.…”

“…Of particular interest here are PEC-CRCs, which utilize solar power to reduce CO 2 to various chemicals [ 13 , 14 ], such as methanol (MeOH), ethanol (EtOH), formate (OFm − ) and acetate (OAc − ). Major roles of the AEM in such devices are to provide preferential ion transport (i.e., hydroxide (OH − and anionic electrolytes (bicarbonates, HCO 3 − ) [ 15 , 16 ]) with membrane-bound, charged functional groups (i.e., quaternary ammonium (QA + ) for AEMs) and to minimize the permeation of CO 2 reduction products to the anode chamber, as they readily oxidize back to CO 2 and by-products [ 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 CO 2 reduction products (OFm − and OAc − ).…”