Emergent Transport in Ion Exchange Membranes

Abstract

The ion exchange membrane (IEM) is a crucial part of various applications from water purification (i.e. electrodialysis) to energy conversion (i.e. photoelectrochemical CO2 reduction cells (PEC-CRC) and direct urea fuel cells (DUFC)). Theoretically, these approaches are more profitable and eco-friendly than their alternatives, such as distillation and fossil fuels. However, a major drawback of these applications is the selectivity of existing IEMs not being adequate (i.e. crossover of undesired solutes). Moreover, each application requires a different membrane specification. For instance, membranes for PEC-CRCs should be minimizing the crossover of CO2 reduction products (i.e. methanol (MeOH), ethanol (EtOH), formate (OFm-), acetate (OAc-)), while allowing the permeation of electrolytes (i.e. bicarbonate (HCO3-)). In the case of DUFC, membranes should minimize the crossover of urea to avoid catalyst sweeping effect. To design target-specific membranes, we took three series of investigations, which are (1) understanding alcohol-carboxylate co-transport behavior in IEMs, (2) analyzing the impact of charge-neutral comonomers in cation exchange membranes (CEM), and (3) introducing a new class of IEMs. From the first series, we conjectured a charge screening behavior based on the carboxylate diffusivity of CEMs being increased and that of anion exchange membranes (AEM) being decreased in co-diffusion with an alcohol. From the second series, we conjectured the interaction between different two dissimilar pendant groups on polymer network can offset the charge screening behavior in CEMs, where the carboxylate diffusivity in CEMs with sulfopropyl groups and poly(ethylene glycol) phenyl ether (PEGPE) groups being consistent in co-diffusion with alcohol. From the third series, we introduced a new class of crosslinked IEMs with phenyl acrylate (hydrophobic monomer). More findings from each series of investigations will be discussed in corresponding sections.