Materials Properties vs. Device Performance: Batch Testing of MnOx-Loaded Carbon Nanofoam Papers for Low-Energy Faradaic Deionization

Abstract

Electrochemical desalination is presently a popular topic in the scientific literature as a response to global challenges with production of potable water. Many of these reports focus on materials/electrode properties for ion capture yet fail to account for other factors that impact practical device performance. For example, gravimetric ion-storage capacity is commonly reported while areal and volumetric capacities may be more important for desalination cell performance. Furthermore, the dynamic fluid mechanics of flow-cell devices require deliberately engineered electrodes in contrast to their static-electrolyte analogs for energy-storage devices (batteries or supercapacitors). To break from the academic status quo, we have designed a computer-controlled batch-process system to investigate desalination performance of practical electrode materials and architectures. This system allows continuous desalination to high degrees of salt removal on a laboratory scale without the need to use large-footprint electrodes. We demonstrate the characteristics of materials-based vs. system-based desalination metrics using NRL-pioneered electrodes — MnOx-decorated carbon nanofoam paper (MnOx@CNFP)1 — relatives of which are effective for faradaic desalination via Na+ capture.2 Scalable and freestanding CNFP electrodes possess three-dimensionally interconnected pore structures with tunable porosities to facilitate ion transport,3 while the interior surfaces of the CNFP (>200 m2 g–1 in mesopores and macropores) are readily functionalized with nanoscale MnOx by self-limiting electroless deposition.1 We investigate the effect of MnOx@CNFP pore structure and electrode thickness on practical desalination performance, and demonstrate the importance of reporting throughput (L/m2/h) and energy consumption (Wh/L) to better validate new electrode materials and architectures in flow-cell desalination devices. E. Fischer, K. A. Pettigrew, D. R. Rolison, R. M. Stroud, and J. W. Long, Nano Lett., 2007, 7, 281–286.Hand and R. D. Cusick, Environ. Sci. Technol., 2017, 51, 20, 12027–12034.C. Lytle, J. M. Wallace, M. B. Sassin, A. J. Barrow, J. W. Long, J. L. Dysart, C. H Renninger, M. P. Saunders, N. L. Brandell, and D. R. Rolison, Energy Environ. Sci., 2011, 4, 1913–1925.