3D Silver Sponge Electrodes for High-Rate Faradaic Deionization of High-Salinity Waters

Abstract

The availability of potable water is a growing global challenge that requires implementing water-purification technologies across a range of production scales from municipal systems to personal, distributed use. Reverse osmosis effectively desalinates high-salinity waters (e.g., undiluted seawater) at large scale, yet may be less suited to small-scale use due to the high-pressure pumps and regular maintenance required for operation. Capacitive deionization (CDI) utilizes high-surface-area carbon electrodes that can offer a more down-scalable desalination approach, but the relatively low capacity of double-layer storage mechanisms limits their use to low-salinity input (brackish water). Alternatively, faradaic deionization (FDI) materials that exhibit redox mechanisms for ion storage present an opportunity to displace reverse-osmosis systems in small-scale desalination systems. As one example, the redox reaction, Ag(s) + Cl–(aq) ↔ AgCl(s) + e–, can be exploited for Cl– capture in desalination cells that pair Ag/AgCl electrodes separated by a cation exchange membrane.1,2 We use macroporous silver “sponges” developed at the U.S. Naval Research Laboratory as millimeters-thick flow-through electrodes that show practical salt absorption capacities >100 mg/g while maintaining high salt removal rate courtesy of this kinetically facile conversion reaction. We investigate different silver sponge architectures and their effect on desalination throughput (L/m2/h) and energy consumption (Wh/L) for inputs up to seawater-level salinity using a custom computer-controlled batch-testing system. These readily fabricated, scalable Ag sponges serve as an attractive archetype electrode for high-performance faradaic desalination. 1 P. Srimuk, S. Husmann, and V. Presser, RSC Adv., 2019, 9, 14849–14858. 2 J. Ahn, J. Lee, S. Kim, C. Kim, J. Lee, P.M. Biesheuvel, and J. Yoon, Desalination, 2020, 476, 114216.