Capacitance Improvement and Electrochemical Characteristics of Silica-Coated Carbon Electrodes for Capacitive Deionization Application

Silicon oxides were coated on the surface of activated carbon powders (ACPs) by the sol-gel reaction in order to improve the capacitance of ACP electrodes for capacitive deionization application. The electrochemical characteristics of SiO2-coated carbon electrodes were studied by using cyclic voltammogram (CV), electrochemical impedance spectroscopy (EIS), and zeta potential measurement. The coating of SiO2 on the surface of ACPs was effective to enhance the specific capacitance due to the reduction in the diffusion resistance of ions and to the improvement in the propagation of the ac signal into the electrode pores. The zeta potential of ACPs was decreased with increasing the SiO2 content, which indicated that more adsorption sites for ions are formed. Consequently, the specific capacitance of SiO2-coated carbon electrodes showed a linear relation with the zeta potential value. In terms of the capacitance, the optimized SiO2 content was about 1.0 mmol SiO2 per unit carbon mass, and then the specific capacitance was improved about 34% of the pure ACP electrode. From a CDI unit cell test, it was confirmed that the SiO2 modification could improve the desalination efficiency about 10%.