Comprehensive Study on the Ion-Selective Behavior of MnO_x for Electrochemical Deionization

Manganese oxide is an effective active material in several electrochemical systems, including batteries, supercapacitors, and electrochemical deionization (ECDI). This work conducts a comprehensive study on the ion-selective behavior of MnO_x to fulfill the emptiness in the energy and environmental science field. Furthermore, it broadens the promising application of MnO_x in the ion-selective ECDI system. We propose a time-dependent multimechanism ion-selective behavior with the following guidelines by utilizing a microfluidic cell and the electrochemical quartz crystal microbalance (EQCM) analysis. (1) Hydrated radius is the most critical factor for ions with the same valence, and MnO_x tends to capture cations with a small hydrated radius. (2) The importance of charge density rises when comparing cations with different valences, and MnO_x prefers to capture divalent cations with a strong electrostatic attraction at prolonged times. Under this circumstance, ion swapping may occur where divalent cations replace monovalent cations. (3) NH₄⁺ triggers MnO_x dissolution, leading to performance and stability decay. The EQCM evidence has directly verified the proposed mechanisms, and these data provide a novel but simple method to judge ion selectivity preference. The overall ion selectivity sequence is Ca²⁺ > Mg²⁺ > K⁺ > NH₄⁺> Na⁺ > Li⁺ with the highest selectivity values of β_Ca//Li and β_Ca//Na around 3 at the deionization time = 10 min.