Pore-scale study of capacitive charging and desalination process in porous electrodes and effects of porous structures

The capacitive deionization (CDI) involves processes of ion transport and storage within porous electrodes. A pore-scale model based on the lattice Boltzmann (LB) method is developed to study the dynamics of capacitive charging and desalination in CDI cells by considering microstructures of porous electrodes. The governing equations for mass transport and potential within the separator and pores of electrodes are derived for the case of thin electrical double layers. For the salt adsorption and charge transport between the solution and charged matrixes, the effective boundary conditions suitable for LB simulations are developed based on the Gouy-Chapman-Stern model. The transient processes of capacitive charging and desalination upon the application of a step cell voltage are revealed, and effects of potential, porosity, particle diameter and porous structure on the dynamic processes are investigated. Simulation results show that the salt removal of the separator is a diffusion limited and relatively slow process compared with the charging process of porous electrodes. The amount of equilibrium salt removal is proportional to the cell voltage and specific surface area of porous electrode. The salt removal rate is mainly affected by the effective diffusivity of porous electrode. Approaches of enhancing the effective diffusivity, such as increasing the porosity under the same particle size and enlarging the particle diameter under the same porosity, can improve the salt removal rate but reduce the salt adsorption capacity due to the decrease of specific surface area. Under the same specific surface area, the porous electrode with large pores introduced along the diffusion direction shows the highest salt removal rate without decreasing in salt adsorption capacity.