Enhancing sodium removal in capacitive deionization via cathode asymmetry and faradaic reactions suppression

Capacitive deionization (CDI) is an emerging low-energy desalination technology that removes salt ions from brackish water using porous carbon electrodes under low-voltage operation. However, the occurrence of cathodic Faradaic reactions, particularly oxygen reduction reactions (ORRs), reduces charge efficiency and limits sodium (Na⁺) removal. This study presents a practical strategy to enhance Na⁺ removal efficiency in CDI by employing an asymmetric electrode mass configuration. Reducing the cathode-to-anode mass ratio to 0.5 significantly improved Na⁺ adsorption capacity and overall cell charge efficiency by 83% and 40%, respectively, relative to a symmetric cell. In the asymmetric configuration, the working potential windows of the electrodes were strategically shifted. Specifically, the cathode half-cell potential increased by up to 30%, resulting in a stronger electric driving force for Na⁺ electrosorption. Additionally, analysis of the anode potential profile and corresponding current-time behavior revealed a reduction of up to 67% in the charge fraction consumed for co-ion (Na⁺) desorption, particularly during the period when the anode operated below its potential of zero charge ( E pzc ). Moreover, effluent measurements of dissolved oxygen (DO) and hydrogen peroxide (H₂O₂) confirmed that ORRs were significantly suppressed under asymmetric conditions, likely due to DO depletion at the cathode, thereby minimizing competition with capacitive ion removal. This work demonstrates a simple and effective operational approach to improve CDI desalination performance by tuning electrode mass ratios, optimizing electrode potential distributions, and mitigating parasitic Faradaic losses. The findings offer valuable insights for the development of more efficient and sustainable CDI-based water treatment systems.