Micropatterned Ion-Exchange Membranes for Membrane Capacitive Deionization
Authors: Hasan Mahmudul , Kulkarni Tanmay , Shrimant Bharat , Arges Christopher G. 
Published: September 2023

US Marine missions require technologies that produce potable water from various water sources that include both seawater and ground water. Reverse osmosis (RO) is the most wide-spread and economical process for desalinating water from seawater. However, it is only cost-competitive and practical when deployed in large, centralized production facilities and is not conducive for marine squad units. Membrane capacitive deionization (MCDI) is an alternative water desalination platform that is enticing to military missions as it does not require high pressure piping and it does not generate significant acoustic, thermal, or electromagnetic signals. MCDI removes ions from the liquid solution using electrical energy, and the commercialized variant, flow-by MCDI, feature two porous electrodes covered by ion-exchange membranes(1). During deionization, the positively biased electrode has an anion exchange membrane (AEM) in front of it while the negatively biased electrode contains a cation exchange membrane (CEM) in front of it.
Our previous research improved the energy efficiency of MCDI by using ion-exchange membranes with lower area-specific resistance (ASR) values(2) and porous ionic conductors in the spacer channel that augment solution conductivity and curtail ohmic losses(3). Reducing these resistances enabled the MCDI to operate at higher current density – which is important for shrinking the size of the MCDI unit and lowering the overall capital costs.
This talk highlights our recent work examining ionic charge transport resistance at the membrane-electrode interface in MCDI. We studied the impact of this resistance on MCDI performance metrics (salt removal and energy normalized adsorbed salt (ENAS)) by systematically changing the interfacial area of the ion-exchange membrane-electrode interface via patterning of the ion-exchange membrane surfaces using soft-lithography. This strategy has been shown to be effective in reducing charge-transfer resistances in both proton exchange membrane (PEM)(4) and AEM(5) fuel cells and reducing the water dissociation kinetics resistance in bipolar membranes(6). Micropatterned AEMs and CEMs based upon poly(arylene ether) and all-carbon poly(arylene) backbones(7) were fabricated with periodic line and cylinder topographies and lateral feature sizes that vary from 2 to 20 μm. The patterned membranes were coated with graphitic carbon electrodes. It is posited that smaller lateral feature sizes, which gives rise to larger interfacial area values, reduces interfacial resistance values in MCDI and larger ENAS values.

Keywords: Desalination, Membrane capacitive deionization, Ion exchange membranes, Soft lithography, Energy normalized adsorbed salt, Membrane electrode assembly

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