Enhanced charge efficiency in capacitive deionization achieved by surface-treated electrodes and by means of a third electrode

In this paper, we report on attempts to improve the charge efficiency of electrochemical capacitive deionization (CDI) processes without limiting the range of applied potentials, by using surface-treated (oxidized or reduced) activated carbon fiber (ACF) electrodes, and by means of a third, auxiliary electrode. For oxidizing the ACF electrodes, we etched ACFs for different periods of time with a concentrated nitric acid solution. For reduction of the ACFs, several surface treatments were considered: reaction with hydrogen at high temperatures, removal of oxygen surface groups by heating under vacuum at high temperatures, reaction with a concentrated aqueous solution of a sodium borohydride solution, and reaction with a concentrated sodium borohydride solution after oxidation with a concentrated nitric acid solution. To examine the charge efficiency, we elaborated a special flow-through cell (where the solution flows through the ACF electrode) with a silver/silver chloride mesh reference electrode. The feasibility of using surface-treated carbon electrodes and/or of using a third, auxiliary electrode (with which the potential applied to each electrode can be controlled) for enhancing the charge efficiency is discussed and examined. We were able to demonstrate an increase in the charge efficiency of the CDI process by 30% without the need to reduce the potential range of operation.