How to achieve the optimal performance of capacitive deionization and inverted-capacitive deionization

Abstract

Here we develop a novel but fundamental strategy for optimizing the ion-removal capacity of electrode materials in capacitive deionization (CDI) and inverted-capacitive deionization (i-CDI) systems without sacrificing their coulombic efficiency. The most efficient working cell voltages of different systems can be precisely determined by our newly designed cell voltage-programming method. We employ a palladium hydride reference electrode to monitor the electrode potentials of positive and negative electrodes during the double-layer charging-discharging processes. Accordingly, charge balance and maximum working cell voltages of CDI and i-CDI systems in brackish water can be achieved simultaneously to optimize the desalination efficiencies. Hence, the ion removal capability of an activated carbon//activated carbon (AC//AC) cell in 0.3–1.9 V reaches 12.2 mg g−1 with minor pH change in deionized solutions. The asymmetric CDI cell consisting of a positive graphene oxide-CNT composite and a negative AC electrode (GO@CNT//AC) is the most efficient CDI system with ion removal capacity of 21.3 mg g−1 in 0.2–1.45 V. The asymmetric AC//reduced GO-CNT composite (AC//rGO@CNT) cell is an exceptional i-CDI system which exhibits ion removal ability of 25.2 mg g−1 in 1.0–1.7 V. The above concepts in optimizing CDI/i-CDI performances are applicable to all types of electrode materials in various cell designs.