Extending pore network models to include electrical double layer effects in micropores for studying capacitive deionization

Abstract

Capacitive Deionization (CDI) is an electro-driven water desalination device. It consists of oppositely charged porous electrodes on either side of a dielectric spacer. The electrodes may be either capacitive or pseudocapacitive electrodes. Capacitive electrodes work by storing ions in the Electrical Double Layers (EDLs) of micropores. To model the uptake of ions into EDLs, transport equations are coupled with appropriate EDL structure model. Previous research has shown that pore structure effects CDI performance. A Pore Network Model (PNM) is a unique tool capable of measuring the effects of pore structure. To the best of the author's knowledge, there is no pore network model of CDI or any PNM in general which applies EDL theory. It is in this work that discretized PDEs are derived for modelling the capacitive storage of ions in a pore network using EDL theory. The EDL theories studied were: Helmholtz, Gouy-Chapman-Stern and modified Donnan. Results from the PNM framework were compared to finite element solver results, and it was found that the error in concentration and potential never exceeded 10% or 15% respectively. Lastly, the new pore network framework was used to accurately predict salt content in effluent of a stop flow CDI cell found in literature.