(Invited) Lessons in Electrode Architecture for Faradaic Desalination

Abstract

Potable water is an essential supply for humanitarian missions worldwide, yet delivering pre-purified water to remote locations is challenging, dangerous, and expensive. Therefore, it is desirable to develop and deploy on-site and portable desalination/water-purification equipment that draws on local water sources. Present reverse-osmosis systems effectively desalinate seawater, but are energy and time-intensive, require regular maintenance due to membrane fouling, and suffer from poor scalability. Capacitive deionization (CDI) technology provides energy-efficient desalination of brackish waters, but its use remains limited by low-capacity carbon-only electrodes that rely on double-layer ion storage. Transitioning to electrodes that also incorporate Faradaic ion-capturing materials with substantively higher storage capacity expands the efficacy and applicability of CDI. As one example, NRL-developed porous carbon nanofoam (CNF) architectures infiltrated with electrolessly deposited nanometric MnO2 demonstrate 6-fold increase in sodium-ion adsorption capacity compared to bare-carbon CNFs, while solvothermally deposited BiOCl renders a high-capacity chloride-ion adsorption electrode. The tunability of CNFs as an electrode scaffolding affords the opportunity to balance ion-storage capacity and electrolyte transport for optimized desalination performance. Lessons of electrode architecture extend to NRL silver “sponge” electrodes that provide high capacity for chloride, fast uptake dynamics, and opportunities for various flow configurations. Continued progress in bench-top level flow-cells with high-capacity faradaic materials will demonstrate the promise of this technology en route to development of larger-scale prototype desalination devices.