Ameliorating Ion-Transport Resistances in Membrane Capacitive Deionization Using Patterned Ion-Exchange Membranes and Ionomer Infiltrated Electrodes

Abstract

Currently, 2 billion people live in water scarce regions and 4 billion people experience water scarcity for at least 1 month per year [1]. Besides implications to the broad masses, the U.S. Marine Corps also face enormous challenges of generating potable water during squad level field missions. Reverse osmosis is the most reliable and economical process for desalinating water, but it requires piping that can tolerate large pressures and is not conducive for portable missions. Membrane capacitive deionization (MCDI) is a modular and electrified water desalination platform that does not generate significant acoustic, thermal, or electromagnetic signatures nor does it require high pressure piping. Flow-by MCDI, which is commercialized, uses electrical energy to remove ions from water. The system architecture feature two porous electrodes covered by ion-exchange membranes – the latter material is used to prevent co-ion adsorption and greater Coulombic efficiency than the variant that does not utilize ion-exchange membranes.Our previous research improved the energy efficiency of MCDI by using IEMs with lower area-specific resistance (ASR) values and porous ionic conductors in the spacer channel that augmented solution conductivity and curtailed ohmic losses. Reducing these resistances enabled the MCDI to operate at 700 mV lower cell voltage at 2 mA cm-2 current density. This is important for shrinking the size of the MCDI unit and lowering the overall capital costs.This talk presents our recent work examining ionic charge transport resistance at patterned and non-patterned ion-exchange membrane interfaces in MCDI. Membrane surface patterning increased the interfacial area between the membrane and aqueous stream promoting greater salt removal fluxes. Soft-lithography methods were used to micropattern poly(phenylene alkylene) cation exchange membranes and anion exchange membranes with systematically varied topographies (e.g., cylindrical well and line structures that vary from 9 to 20 mm). We can reduce the MCDI cell voltage by 1 V when utilizing micropatterned ion-exchange membranes and ionomer infiltrated electrodes (operating at 2 mA cm-2 with a 2000 ppm NaCl feed). This translated to a 2.4x greater energy normalized adsorbed salt (ENAS) value - a metric used to assess the energy consumption for MCDI. Our future work examines electrode materials deposited at pattern membrane interfaces to reduce interfacial transport resistances from the membrane to the electrode. This strategy has been shown to be effective in reducing charge-transfer resistances in both proton exchange membrane (PEM) and AEM fuel cells and reducing the water dissociation kinetics resistance in bipolar membranes [2-5]. References Somini Sengupta, Weiyi Cai, A Quarter of Humanity Faces Looming Water Crises, in The New York Times. 2019: United States of America.Kole, S., et al., Bipolar membrane polarization behavior with systematically varied interfacial areas in the junction region. Journal of Materials Chemistry A, 2021. 9(4): p. 2223-2238.Breitwieser, M., et al., Tailoring the Membrane-Electrode Interface in PEM Fuel Cells: A Review and Perspective on Novel Engineering Approaches. Advanced Energy Materials, 2018. 8(4).Hee-Tak Kim, T.V.R., Ho-Jin Kweon, Microstructured Membrane Electrode Assembly for direct methanol fuel cell. Journal of The Electrochemical Society, 2007. 154(10).Tomizawa, M., et al., Heterogeneous pore-scale model analysis of micro-patterned PEMFC cathodes. Journal of Power Sources, 2023. 556. Figure 1