Ohmic charging in capacitive deionization : efficient water desalination using capacitive spacers

Capacitive deionization (CDI) is a promising desalination technology based on electrosorption on the surfaces ofnanoporous electrodes. However, low total dissolved solids (TDS) in the water restricts its efficacy. In this work, we develop the theory of capacitive spacers in CDI. The investigations reveal a mechanism that we call ohmic charging; that is, the resistive losses in the spacer region drive adsorption in the capacitive spacer. As a consequence, the obtained results show that such spacers can improve desalination energy efficiency, especially at ion-starved conditions. The spacers also enhance the charging rate of the electrodes because the overall resistance is lower when the current can pass the spacer material instead of the solution, through the adsorption of anions on one side and cations on the other. Going deeper, the investigations reveal a major challenge; the spacer naturally discharges on the same timescale as the electrode charging timescale. However, only the fast timescale matters with low ionic content solutions, and under these conditions the capacitive spacers are found to be superior. Put together, capacitive spacers can make a significant difference, especially when the ion concentration is low or the cycle times are short. QC 20221201