Asymmetric oxygen vacancies in Cu - Ov - Mn units boost charge transfer in MnO2 for enhanced hybrid capacitive deionization efficiency

Manganese dioxide (MnO₂), despite its high theoretical capacitance, has been limited in hybrid capacitive deionization (HCDI) applications due to its inherently low conductivity, which impairs salt removal efficiency. Here, we introduce a strategy involving the integration of an asymmetric oxygen vacancy structure, specifically a Cu-O_v-Mn unit into MnO₂, significantly enhancing their electronic structure and HCDI performance. This structural innovation promotes a redistribution of electrons at the oxygen vacancy, facilitating enhanced electron flow towards Mn atoms and altering their electronic states. Density functional theory (DFT) calculations reveal that Cu doping reduces the band gap of MnO₂, situating more electrons near the Fermi level and thereby improving electron transfer dynamics. These calculations also show an increase in the electron occupancy of lower energy t_2g orbitals in Cu-MnO₂/O_v, accompanied by a notable elevation in their energy levels. This adjustment not only enhances the probability of electron transitions but also significantly lowers the migration barrier for Na⁺ ions, thus improving ion mobility. In a 500 mg L⁻¹ NaCl solution, the Cu-MnO₂/O_v electrodes demonstrated a superior salt adsorption capacity (SAC) of 75.5 mg g⁻¹ and a salt adsorption rate (SAR) of 3.33 mg g⁻¹ min⁻¹ at 1.2 V. These findings underscore the potential of asymmetric oxygen vacancies to modulate the properties of transition metal oxides, providing a pathway to both improved HCDI performance and a broader application spectrum for MnO₂-based materials.