|Abstract:||Cell-cycling performance in capacitive deionization (CDI) can suffer from various charge-efficiency loss mechanisms. In conventional CDI, we show that salt residue within electrodes introduces a temporal lag between charge and desalination stages of a CDI cycle. Without accounting for this effect in the collection of effluent, significant performance degradation occurs as current density increases. To overcome this we use pulse-flow operation to control fresh- and brine-water concentrations. The charge and energy efficiency performance between the two flow-modes is compared using a porous electrode model that is calibrated and validated with experimental data. To quantify specific contributions to charge efficiency losses, the model captures local salt variations resulting from a combination of electrosorption, leakage current, and immobile surface charge. Compared to traditional continuous-flow operation, simulation results show that charge efficiency increases up to 23% in the pulse-flow operation at a current density up to 20 A/m2, which leads to a 73% decrease of specific energy consumption (SEC). In addition, the SEC predicted by the pulse-flow operation model closely aligns with the predictions of the continuous-flow model after accounting for temporal lag in effluent salinity. Both simulations and experimental results suggest that pulse-flow operation closely approximates the performance in continuous-flow operation.
We further apply the pulse-flow model to simulate two different CDI architectures (membrane capacitive deionization and flow-through CDI) and focus on the identification energy losses specific to each system component. The model was used to quantify the effects of ohmic resistance, parasitic faradaic reactions, co-ion repulsion, and incomplete utilization of electrode capacitance on salt specific energy consumption (kJ g-NaCl-1) across a range of current densities and charging voltage limits. We show that significant irreversible energy loss is observed at low and high current density, which is mainly contributed by the parasitic reactions and resistive charge transport dissipation, respectively. However, the greatest source of energy loss can be linked to reverse diffusional flux at the beginning of a charging stage, caused by retention of salt from the brine discharge stage. From this analysis, we show how target effluent concentration and cell architecture can be controlled to reduce energy consumption by greater than one order of magnitude.