Optimization of the voltage window for long-term capacitive deionization stability

•Both symmetric and asymmetric CDI were studied.•Long-term stability tests were conducted.•The optimization of the voltage window for steady and efficient CDI is presented.

Capacitive deionization (CDI) is an energy efficient desalination technology. In this study, we conducted CDI experiments under different conditions and observed severe performance degradation from 7.3 to 0.5 mg/g after 70 cycles in symmetric 1.2/0 V operation because of anode oxidation under polarization. To eliminate the degradation, different charging and discharging voltages were tested in an asymmetric cell and 0.8/−0.4 V was selected as the optimized voltage window. Under the optimized voltage window, a capacity of 6.3 mg/g was obtained, only somewhat lower than that of the 1.2/0 V experiment, and the stability and charge efficiency were much higher by greatly mitigating the electrode corrosion. In addition, the process had a lower energy consumption. The stable performance in air- and oxygen-saturated conditions indicates that the optimized CDI has superior capacity and better performance than conventional CDI.