Design and optimization of electrode materials plays the pivotal role on the performance of capacitive deionization (CDI). Activated carbon (AC) has been a workhorse material for electrode fabrication in capacitive technologies. Several modification methods have been reported with enhanced activity and versatility attributes. Undeniably, tuning and tailoring AC properties have opened avenues for broadening the scope of applications, by meeting necessary features of electrodes for a given CDI cell configuration. This review traces the beneficial and also detrimental effects from various modifiers on AC electrodes with respect to CDI performance. Furthermore, a comprehensive classification of CDI cells based on different architectural aspects with a comparative performance is presented. On this basis, the tradeoff between physical, chemical, electrochemical properties in the course of electrode modification and the interdependence between electrode design and CDI cell configuration are discussed with disclosing some prospective guidelines on AC electrode design. It is important to evaluate the electrode materials and modifications in the way of practical including not only the electrode design, but also the cell architecture and operational parameters. This review aims to raise the attention on the rational electrode design by taking into account all necessary features of electrode in a given cell configuration.

Activated carbon (AC) modification has been intensively studied in order to design carbon electrodes with enhanced electrochemical performance. Hexadecyltrimethylammonium bromide (HDTMA) and Tween 80 were employed for enhancing the surface activity of AC via synergism. The synergistic effects of the mixed surfactants on AC surface activity in the light of interface behaviors were studied. Both field emission scanning electron micrographs and FTIR spectra indicated a successful adsorption of loaded surfactants. AC gained a good wettability originated from the surfactants, especially in the binary mixture (T80-HDTMA). The zeta potential results unveiled the positive charge density enhancement in the mixed surfactants system. Isoelectric point and point of zero charge implicate heterogeneous distribution of charges and the extent of surfactants treatment. Tween 80 displayed a significant size control dependence on AC particles. Electrochemical characterization revealed a higher specific capacitance and a decaying resistance of specific capacitance in AC-T80-HDTMA than AC-HDTMA at high concentration. In 5 g L⁻¹ of NaCl, AC-T80-HDTMA (0.01 : 0.01 mM) exhibits the specific capacitance of 209.79 F g⁻¹, at 0.8 V whereas AC-HDTMA (0.01 mM) and AC exhibited 186.5 F g⁻¹, 178.9 F g⁻¹, respectively. Moreover, the stability testing reveals a strong attachment of HDTMA in AC-T80-HDTMA than AC-HDTMA with the loss of 0.32% and 1.32%, respectively. The hypothetical synergistic mechanism of surfactants adsorption on the surface of AC was depicted as hydrophobic interaction and steric stabilization being the main keys for the synergy between cationic and nonionic surfactants. This study demonstrates the beneficial effects of mixed surfactants on AC electrode properties and discloses the impact on electrochemical performance.