Asymmetrical removal of sodium and chloride in flow-through capacitive deionization

Capacitive deionization (CDI) is an electrochemical method of removing salt ions from brackish water. A common assumption in CDI is that monovalent ions (e.g., Na⁺, Cl^-) are removed in a 1:1 symmetry on the electrodes. Validation of this assumption with techniques such as ion chromatography is not commonly performed, but is important to better understand how parasitic process, such as faradaic reactions, affect ion removals. In this study, we quantified the removals of Na⁺ and Cl^- as a function of electrode orientation in flow-through CDI. When the cathode was positioned upstream, Na⁺ and Cl^- removals approached a 1:1 symmetry, but when the anode was located upstream, we observed a significant drop in Na⁺, but not Cl^-, removals. We attributed this drop to oxygen reduction reactions at the cathode that competed with Na⁺ adsorption. Oxidation of carbon in the upstream anode yielded H⁺ that enhanced the reduction of oxygen to H₂O₂ at the downstream cathode, which in turn diverted electrons from Na⁺ adsorption. In the absence of oxygen, Na⁺ removals increased in the upstream anode orientation and were comparable to Cl^- removals, confirming that competition with oxygen reduction reactions was the primary reason for decreased Na⁺ removal. In the upstream cathode orientation, we show that H₂O₂ generated at the cathode can be oxidized at the downstream anode, possibly enhancing Na⁺ removals via internal electron recycling. Salt adsorption capacities calculated using actual ion removals did not always agree with those estimated using changes in solution conductivity, with the largest disagreement observed when conductivity data were corrected for pH changes. Our results highlight that faradaic reactions, particularly oxygen reduction reactions, can contribute to asymmetrical removals of monovalent ions in flow-through CDI.