Architected Silver Sponges for Faradaic Deionization of Salt Water

Much of the contemporary research on advanced desalination materials strives for high gravimetric capacity but ignores their applicability to engineered devices. Practical faradaic deionization of saline and brackish waters requires electrodes that are scalable beyond micrometer-thickness in order to achieve high areal capacity. To break the status quo approach to materials research at nanometric scales, we design and test three-dimensional architectures capable of melding materials properties with performance. Monolithic sponges that facilitate ionic and electronic conductivity within the architected electrode offer a path to rapid and energy-efficient desalination. Salt-templated synthesis of porous Ag and AgCl@Ag sponges yields millimeters-thick architected electrodes with full-cell capacity surpassing 40 mAh cm–2 (200 mAh cm–3), making them suitable for compact and portable desalination devices. This form factor comprises interconnected networks of micrometer-sized Ag particles that electron-wire the electrode volume, which greatly increases the specific surface area (260 cm2 g–1) and the extent of Ag/AgCl conversion compared with bulk commercial forms (e.g., silver mesh). Desalination tests performed in a rocking-chair, flow-through cell demonstrate NaCl-removal capacities of 78 mg g–1 (66 mg cm–2) at rates up to 2 mg g–1 min–1 (1.7 mg cm–2 min–1), removing 25% salt from 35 g L–1 NaCl. The low-voltage reaction of the Ag/AgCl conversion cell consumes as little as 0.02 Wh g–1 at 5 mA cm–2. In a 5 g L–1 influent concentration of NaCl, the Ag sponge cell provides single-stage, fit-for-purpose desalination, achieving 90% salt removal at a throughput productivity of 6.21 L m–2 h–1 with only 0.55 Wh L–1 energy consumption. When combined with multistage batch desalination, the single-membrane device can reduce saline water to potable levels at 6.4 Wh L–1.