Capacitive deionization with asymmetric electrodes: Electrode capacitance vs electrode surface area

Asymmetry of electrodes on the equilibrium salt adsorption capacity in a capacitive configuration was studied. Experiments were carried out by using activated carbon cloth (ACC) with a specific surface area and specific capacitance of ∼1000 m2/g and 44 F/g as the anode and ACC coated with zinc oxide nanorods (ZnO NR) with a specific surface area and specific capacitance of 637 m2/g and 57 F/g as the cathode. The electrodes were characterized electrically and their salt adsorption capacities measured for various anode-cathode configurations to conclude that for multimodal electrodes, specific capacitance and not specific surface area regulates the salt adsorption capacity. The adsorption trends were analyzed and equated to an electrical model to qualitatively predict the equilibrium salt adsorption capacity, where the smaller capacitance was observed to be the limiting factor. The results in this work are especially useful for practical CDI units, where anode-cathode capacitance should be matched to achieve maximum salt removal efficiency.