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Abstract In capacitive deionization (CDI) systems, more Faradaic reactions generally occur at a higher cell voltage, resulting in the poor ion-removal efficiency and high energy consumption, but a low cell voltage may lead to a low deionization capacity. In this study, an integrated strategy, consisting of (1) selecting the acceptable charge/discharge time ratio, (2) finding the potential windows of both positive and negative electrodes, and (3) using the charge-balanced method for determining the optimal mass ratio between the positive and negative electrodes (denoted as (m+)/(m−)), is proposed to find the optimal cell voltage of a CDI system with a high desalination efficiency and low energy consumption. According to this strategy, two newly designed cells using two types of activated carbon (AC) show excellent CDI performances; cell 1 with AC = ACS679, (m+)/(m−) = 1:1.4, and cell voltage = 1.4 V: salt adsorption capacity of 12 mg g−1, charge efficiency of 62%, and energy consumption of 109 kJ mol−1 and cell 2 with AC = ACS25, (m+)/(m−) = 1:1, and cell voltage = 1.2 V: salt adsorption capacity of 13 mg g−1, charge efficiency of 81%, and energy consumption of 72 kJ mol−1 although the potential window of water decomposition for such AC-coated electrodes in 8 mM NaCl is about 2 V. This universal strategy is applicable to all CDI systems utilizing various electrode materials. Graphical abstract Image 1 Highlights • We demonstrate a universal strategy for constructing the AC//AC cell with high CDI performances. • The same charge/discharge time ratio with high reversibility on both electrodes is the key. • Influences of two-electrode mass ratio on the desalination performances are studied. • The pH change is affected by the Faradaic reactions from the potential windows of electrodes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]