The surface of commercially-available activated carbon (AC) was functionalized through the TiO2 coating and sulfonation, and the resulting powder was evaluated as a capacitive deionization (CDI) electrode. TiO2 coating on the AC surface was achieved by a sol-gel method and followed by the sulfonation using Tiron. From the cyclic voltammetry and the electrochemical impedance analysis, the TiO2 coating and sulfonation improved specific capacitance and reduced ion diffusion resistance. To achieve improvement in the desalination performance, it was fond that the AC-Ti or AC-Ti-S electrode was used as a negative electrode rather than a positive electrode. This result was well explained by shifting the potential of zero charge (E-PZC) from 0.15 V (pristine AC) to 0.25 (AC-Ti) or 0.35 V (AC-Ti-S). Finally, the specific adsorption capacity (SAC) of the AC-Ti-S electrode was improved about 1.5 times higher than that of the pristine AC electrode. Also, the charging efficiency was improved from 61% (pristine AC) to 84% via the TiO2 coating and sulfonation, which was due to the increase of wettability and the reduction of co-ion repulsion effect. (C) 2018 Published by Elsevier Ltd.