Understanding degradation of capacitive deionization cells: Full–cell simulations with anode corrosion

Abstract

Capacitive deionization (CDI) is notable for its ability to perform low–energy desalination of brackish water and selective separation/recovery of salt and metal ions. However, redox side–reactions at porous carbon electrodes, such as the formation of weakly–acidic, oxygen–containing surface groups, limit ion electrosorption into double layers and electrode lifespan. In this study, we employ porous electrode theory to investigate the coupled effects of transport, electrosorption, redox reactions, and acid/base surface chemistry in practical flow through electrode (FTE) CDI cells, and we develop an efficient numerical solver that allows us to perform full–cell simulations over numerous charging/discharging cycles. We verify that our high fidelity model's predictions agree with analytical solutions in the limit of small voltage perturbations, and we demonstrate the significance of accurately capturing interplay between pH dynamics and surface chemistry. Our simulations demonstrate that anodic oxidation in FTE CDI is substantially inhomogeneous in space and in time, exhibiting cyclic protonation/deprotonation of surface groups and traveling, shock–like pH fronts. Moreover, we uncover connections between microscopic behavior and macroscopic performance parameters, demonstrating that this theoretical and numerical approach may substantially benefit the practical design optimization of CDI cells.