Effect of Ion Volume on the Asymmetry of Two-Dimensional Planar Capacitive Deionization System: Determination and Manipulation

When analyzing a two-dimensional (2D) planar capacitive deionization (CDI) system, "symmetry" is a classic assumption: anode and cathode are of the same mass of the same material; the potential drop and the capacitance are evenly contributed by both electrodes and their electrical double layers (EDLs). However, this is unrealistic for nonmonovalent salt, or for a monovalent salt due to the ion size effect. In this work, the factors causing asymmetry to the potential and ion distribution in the CDI process were analyzed in detail. Our calculation revealed how the cation/anion ratio and ion size effect affected the asymmetry of the CDI process, and predicted and explained a special phenomenon, "balance shifting", of the CDI process under particular conditions. For each type of salt, due to its ion diameter and molecular formula, a particular curve of the anode/cathode ratio can be provided to control the potential distribution and thus EDL structure. This information can be used to optimize a CDI device, especially for those designed with Faradaic reaction (hybrid capacitive deionization, HCDI) whose capacitance can only be enhanced when occupying a certain potential difference. This conclusion was confirmed by our HCDI experiments which had successfully achieved an enhancement of 75% in the salt adsorption capacity by changing the electrode mass ratio between the cathode and anode.