Capacitive desalination of ZnO/activated carbon asymmetric capacitor and mechanism analysis

Abstract

Zinc oxide/activated carbon composite electrode (ZnO/AC) was prepared by simply mixing ZnO nanoparticles with AC granules in the presence of Teflon emulsion. Scanning electron microscopy shows an even and seamless surface with effective filling of ZnO nanoparticles in between AC granules. Cyclic voltammetry and impedance analysis demonstrate the ideal double-layer capacitor behavior. The desalination behavior of the asymmetric capacitor with ZnO/AC as positive electrode and AC as negative electrode (+ZnO/AC‖AC), or ZnO/AC as negative electrode and AC as positive electrode (−ZnO/AC‖AC) was studied, respectively. As compared with pure AC‖AC capacitor, −ZnO/AC‖AC capacitor showed a very stable desalination behavior with high desalination amount of 9.4 mg/g and charge efficiency of 80.5%; while +ZnO/AC‖AC capacitor showed no obvious difference after several desalination cycles due to poor stability. The mechanism was analyzed based on zeta potential of ZnO particles and pH variation near the electrode surface during charging process. The different desalination properties on positive and negative electrodes due to zeta potential variation of ZnO with pH change at electrode surface were further confirmed by using other metal oxides like CuO, MnO2 and WO3. This study provides a particularly important guidance for screening electrode materials and optimizing operation parameters for capacitive desalination (also called capacitive deionization, CDI).