Electrochemical Desalination Using Intercalating Electrode Materials: A Comparison of Energy Demands

One approach for desalinating brackish water is to use electrode materials that electrochemically remove salt ions from water. Recent studies found that sodium-intercalating electrode materials (i.e., materials that reversibly insert Na⁺ ions into their structures) have higher specific salt storage capacities (mg_salt/g_material) than carbon-based electrode materials over smaller or similar voltage windows. These observations have led to the hypothesis that energy demands of electrochemical desalination systems can be decreased by replacing carbon-based electrodes with intercalating electrodes. To test this hypothesis and directly compare intercalation materials, we examined nine electrode materials thought to be capable of sodium intercalation in an electrochemical flow cell with respect to volumetric energy demands (W·h·L^-1) and thermodynamic efficiencies as a function of productivity (i.e., the rate of water desalination, L·m^-2·h^-1). We also examined how the materials' charge-storage capacities changed over 50 cycles. Intercalation materials desalinated brackish water more efficiently than carbon-based electrodes when we assumed that no energy recovery occurred (i.e., no energy was recovered when the cell produced electrical power during cycling) and exhibited similar efficiencies when we assumed complete energy recovery. Nickel hexacyanoferrate exhibited the lowest energy demand among all of the materials and exhibited the highest stability over 50 cycles.