Multi-dimensional simulation tool development for the performance evaluation of a CDI electrode and its geometry parameter optimization

Abstract

A steep increase in water demand and desertification has brought about much interest in desalination. Among existing desalination methods, capacitive deionization (CDI) gets attention due to its lower cost and simpleness. The most influential factor on CDI lies on the quality of electrodes. Though various investigations and developments of electrodes have been aroused recently, no distinct standard and systematic comparison of those electrodes has been made. By solving Poisson–Nernst–Planck equations simultaneously, a simulation tool for a CDI electrode is developed. The simulation domain of the tool involves geometrical factors of electrodes, such as pore depth and pore width, in a two-dimensional manner. Using the tool, the most effective design parameters of CDI electrodes are obtained: pore depth, pore radius, capacitance, and pore diffusion coefficient. Since the capacitance and pore diffusion coefficient are not predictable in a theoretical way, those values are experimentally obtained with the support of the simulation tool. The capacitance and diffusion coefficient are calculated by our developed tool. The carbon aerogel electrode is found to have its average capacitance of 7.5 μF/cm2 and diffusion coefficient on the order of one hundredth of bulk diffusion coefficient. Also, we found out that the diffusion coefficient at the electrode recovery phase is by 3–6 times higher than that at the salt removal phase. The diffusion coefficients increase gradually as potential decreases because of the decreased concentration of ions inside pores.