Engineering Ionomer Materials for Addressing Ohmic Resistances in Electrochemical Desalination and Waste Heat Recovery

Abstract

Water scarcity and energy availability present important challenges that need to be addressed in the coming centuries. In the front of water technologies, desalting brackish water is of extreme importance for thermal electric power plants, chemical manufacturing plants, and other industrial operations that treat and reuse their water utilities. Membrane capacitive deionization (MCDI) is an energy efficient desalination technique that has drawn attention from commercial entities. Most material research studies on MCDI focus on enhancing electrode performance while little emphasis is given to rationale design of ion-exchange membranes (IEMs). In this work, the ionic conductivity, permselectivity, and thickness for three different IEM chemistries (polyaliphatic, poly (arylene ether), and perfluorinated) were correlated to MCDI performance attributes. A 5-10-fold reduction in area specific resistance (ASR), with unconventional perfluorinated and poly (arylene ether) IEMs reduced the energy expended per ion removed in MCDI by a factor of two, compared to conventional electrodialysis IEMs. For further advancement in energy efficiency of operation, ohmic resistance of the spacer channel needs to be addressed for which, ion-exchange resins bound by a polymeric binder termed resin wafers were explored. A new class of ion-exchange resin wafers (RWs) fabricated with ion-conductive binders were developed that exhibit exceptional ionic conductivities - a 3-5-fold improvement over conventional RWs containing a non-ionic polyethylene binder. Incorporation into a resin-wafer electrodeionization stack (RW-EDI) resulted in an increased desalination rate and reduced energy expenditure. Overall, this work demonstrates that ohmic resistances can be substantially curtailed with ionomer binder RWs at dilute salt concentrations. With respect to energy, thermally regenerative ammonia flow batteries (TRBs) are an emerging platform for extracting electrical energy from low-grade waste heat (T < 130 °C). Previous TRB demonstrations suffered from poor heat to electrical energy conversion efficiency when benchmarked against state-of-the-art thermoelectric generators. This work reports the highest power density to date for a TRB (280 W m-2 at 55 °C) with a 5.7× improvement in power density over conventional designs and thermal efficiency (ηth) values as high as 2.99 % and 37.9 % relative to the Carnot efficiency (ηth/C). The gains made in TRB performance was ascribed to the zero gap design and deploying a low-resistant, inexpensive anion exchange membrane (AEM) separator and implementing a copper ion selective ionomer coating on the copper mesh electrodes. The improved TRB power density and the use of a low-cost materials represent significant milestones in low-grade waste heat recovery using electrochemical platforms.