In-Situ De-Aeration Towards Performance Stability of Capacitive Deionization Cells

Abstract

Carbon-based Capacitive Deionization (CDI) is an emerging treatment method for saline waters with ion concentration at or below brackish levels. This technology works similarly to a parallel plate aqueous capacitor in that the dissolved ions are removed from solution through an applied electric potential and adsorbed onto the active surface of the electrodes. Upon removal or reversal of the applied potential, the ions re-dissolve, creating a concentrated stream (1). One setback in the commercialization of CDI technology is performance stability, which has mainly been attributed to a combination of carbon oxidation at the anode (positive electrode) and oxygen reduction at the cathode (negative electrode)(2-4). This work explores an in-situ method for evacuating trapped air from the electrode’s pores towards mitigating Faradaic reactions and increasing the long-term stability of CDI systems. The flow through CDI cell used in this study utilized symmetric, pristine, carbon electrodes obtained from Kynol®. The CDI system, fitted with a pressure regulator and gauge, allows for operation at elevated pressures, up to 60 psi, within the cell. Figure 1, depicts the CDI system components with the pressurized and non-pressurized regions highlighted. The natural pressure gradient that occurs across the cell due to flow resistance remains constant because the entire cell lies within the pressurized portion of the system. This operating method takes advantage of Henry’s Law (where more dissolved oxygen is found at higher pressure) removing oxygen from the cell. Initial data, shown in Figure 2, demonstrates the response of dissolved oxygen to changes in system pressure for a ~5mM aerated sodium chloride solution. A CDI system, treating the same concentration sodium chloride solution, operating at increased pressure demonstrates a similar increase in dissolved oxygen, which decays back once the trapped air is removed. References M. E. Suss, S. Porada, X. Sun, P. M. Biesheuvel, J. Yoon and V. Presser, Energy & Environmental Science, 8, 2296 (2015).I. Cohen, E. Avraham, Y. Bouhadana, A. Soffer and D. Aurbach, Electrochimica Acta, 153, 106 (2015).C. Zhang, D. He, J. Ma, W. Tang and T. D. Waite, Water research, 128, 314 (2017).A. Omosebi, X. Gao, N. Holubowitch, Z. Li, J. Landon and K. Liu, Journal of The Electrochemical Society, 164, E242 (2017). Figure 1