The Ratio between the Surface Charge and Electrode's Capacitance as a Fast Tool for Assessing the Charge Efficiency in Capacitive Deionization Processes

This paper suggests an effective approach to evaluate carbon electrodes that can be effective in capacitive de-ionization processes of salty aqueous solutions. The first assessment of any electrode material, before its assembly in a CDI cell, includes its specific capacitance and its electric conductivity among other physical properties. These properties may bring a sense whether the electrode can be suitable for electro-adsorption processes, but can hardly tell about the efficiency of the process which is related to the charge utilization. A new term is discussed - the ratio between the specific capacity of an electrode and the specific charge related to the electrode's surface groups. A close inspection of the amphoteric modified Donnan model of the electric double layer shows that the ratio between the surface charge density and the integral specific capacitance, namely, the overall charge density, of the carbon electrodes is a useful term for predicting the CDI process efficiency. Such a ratio is obtained by regular electrochemical analysis of carbon electrodes and a simple titration of the functional surface groups. For demonstration, electrodes comprising commercial activated carbon cloth were tested in CDI cells. The theoretical calculations were correlated to the performance of their performance in CDI processes.