Quantifying the Trade-Offs between Energy Consumption and Salt Removal in Membrane-Free Cation Intercalation Desalination

Electrochemical desalination devices that use redox-active cation intercalation electrodes show promise for desalination of salt-rich water resources with high water recovery and low energy consumption. While previous modeling and experiments used ion-exchange membranes to maximize charge efficiency, here a membrane-free alternative is evaluated to reduce capital cost by using a porous diaphragm to separate NaNiFe(CN)$_6$ electrodes. Two-dimensional porous-electrode modeling shows that, while charge efficiency losses are inherent to a diaphragm-based architecture, charge efficiency equal to the anion transference number (61$\%$ for NaCl) is achievable for diaphragms with sufficiently low salt conductance. Closed-form equations are thereby derived that relate the charge efficiency to the non-dimensional Peclet and Damkohler numbers that enable selection of current and flow velocity to produce a desired degree-of-desalination. Simulations using these conditions are used to quantify the tradeoffs between energy consumption and salt removal rate for diaphragm-based cells operated at a range of currents. The simulated distributions of reactions are shown to result from the local salt concentration variations within electrodes using diffusion-potential theory. We also simulate the cycling dynamics of various flow configurations and show that flow-through electrodes exceed the degree-of-desalination compared with flow-by and flow-behind configurations due to solution stagnation within electrodes.