Voltage-Based Stabilization of Microporous Carbon Electrodes for Inverted Capacitive Deionization

Inverted capacitive deionization (i-CDI) is examined using microporous Spectracarb carbon electrodes in 10 mmol L –1 NaCl solution without deaeration. i-CDI testing shows that using conventional operational methods, i.e., V ch = 0.8 V and V dis = 0 V (0.8/0 V), cannot stabilize salt separation after approximately 409 h with an averaged salt adsorption capacity (SAC) of 6.0 ± 0.8 mg g –1 . The cycled anode possesses a collapsed cyclic voltammogram due to an increase in the sheet resistance by the formation of a surface oxide layer. This layer eventually suppresses electronic charge utilization in the i-CDI cell causing degraded salt separation. By analysis of potential distributions incorporated with the modified Donnan model, an improved i-CDI operational method is proposed by reducing V ch to 0.4 V and V dis to −0.4 V (0.4/–0.4 V) while maintaining a voltage window ( V ch – V dis ) of 0.8 V. The improved i-CDI testing demonstrates that not only is the separation process stabilized up to approximately 420 h but the SAC also increases to 7.2 ± 0.3 mg g –1 . Additionally, operation at 0.4/–0.4 V possesses more stable pH and dissolved oxygen (DO) responses than that at 0.8/0 V. We believe that such improved performance stems from a reduced V ch mitigating carbon oxidation at the anode and DO reduction at the cathode while the reduced V dis compensates for salt removal capacity.