Functional anode of membrane capacitive deionization enabled by Cu2+/Cu+ redox couple for simultaneous anions removal and anodic oxidation mitigation

Abstract

Activated carbon (AC) based conventional capacitive deionization has shown extraordinary promise for energy efficient water treatment, but its capacity and cycle stability are limited by the mechanism of electric double-layer and the oxidation of anode, respectively. Redox electrolytes are proposed as one of the most promising strategies to improve the charge storage capacitance. Here, functional anode enabled by Cu2+/Cu+ redox couple is constructed by adding 10 mM CuCl2 into 100 mM NaCl supporting electrolyte. A membrane capacitive deionization (MCDI) cell with the functional anode and anion exchange membrane, employing a two channel structure is proposed to remove the anions (F−, Cl−) from wastewater. The reverse constant current discharge mode with 1.2 V/−1.2 V cut-off voltage can realize Cu2+/Cu+ redox reaction. The water production rate of the MCDI cell is above 50 % and the Cl− removal capacity is increased by 163 % due to the additional faradaic process. The cell capacity can be maintained above 83 % after 40 cycles. In particular, fast charge balance and redox couple reaction processes of this functional anode could significantly alleviate the oxidation of the anode AC electrode. This work provides new opportunities for developing cost-effective technologies to remove the anions from wastewater.