Scalable Carbon Nanofoams for Faradaic Desalination of Brackish Water

Abstract

Water scarcity is becoming an increasing exigent humanitarian crisis which can be ameliorated with development of economical and practical water treatment systems. Present reverse-osmosis systems effectively desalinate seawater, but are energy and time-intensive, require regular maintenance due to membrane fouling, and are less adaptable to small scale uses. In contrast, capacitive deionization (CDI) technology shows promise for scalable, energy-efficient desalination of brackish waters, but its application has been limited by reliance on double-layer capacitance ion storage at carbon-only electrodes. Recent advances in electrochemical desalination have focused on exploiting Faradaic (redox-active) materials to increase ion-storage capacity. We design hybrid capacitive deionization (HCDI) flow-cells utilizing scalable, NRL-pioneered porous carbon nanofoam (CNF) architectures that also incorporate environmentally benign, faradaic active materials. Electrolessly deposited nanometric MnO2 on CNFs supports a 6-fold increase in sodium-ion adsorption capacity compared to bare-carbon CNFs, while solvothermally deposited BiOCl in CNFs renders a reversible, high-capacity chloride-ion adsorption electrode. We systematically explore architectural parameters such as the pore size distribution and electrode thickness of the CNF, and faradaic material loading with respect to their optimization for desalination performance in prototype flow-cells. Additionally, the performance of such electrode architectures may be further improved by constructing graded-pore, multilayer CNFs that optimize and balance ion transport to the electrode interior while maintaining high capacity. Continued progress in bench-top level HCDI flow-cells with faradaic materials will validate the promise of this technology en route to demonstrating larger-scale desalination devices.