Effect of electrode thickness variation on operation of capacitive deionization

In capacitive deionization (CDI) water is desalinated by applying an electrical field between two porous electrodes placed on either side of a spacer channel that transports the aqueous solution. In this work we investigate the equilibrium salt adsorption and the dynamic development of the effluent salt concentration in time, both as function of spacer and electrode thicknesses. The electrode thickness will be varied in a symmetric manner (doubling both electrodes) and in an asymmetric manner, by doubling and tripling one electrode but not the other. To describe the structure of the electrostatic double layer (EDL) which determines the salt adsorption in the micropores of activated carbons, a modified Donnan-model is set up which successfully describes the data, also for situations of very significant electrode thickness ratios. We develop a generalized CDI transport model accounting for thickness variations, which compares favorably with experimental data for the change of the effluent salt concentration in time. These experiments are aimed at further testing our equilibrium and transport models, specifically the assumption therein that in first approximation, for electrodes made of chemically unmodified activated carbon particles, the EDL structure is independent of the sign of the electronic charge. To investigate the relevance of chemical surface charge we also varied pH of the salt solution flowing into the cell.