Oxygen vacancy enhancing intrinsic conductivity of rGO@MnO2−x electrode for efficient hybrid capacitive deionization

The poor intrinsic conductivity and serious aggregation of MnO2 has limited its further application in capacitive deionization (CDI). In the present work, GO was introduced as the growth substrate to alleviate the aggregation behavior of MnO2, while oxygen vacancies were created in the MnO2 crystal by calcination to improve the intrinsic conductivity of MnO2. The uniform dispersion and the expanded layer spacing of δ-MnO2 provide larger specific surface area and more active sites for ion adsorption and storage, while the abundant oxygen vacancies in the crystal enhance the intrinsic conductivity and provide convenient ion transport channels for ion movement and transport. The electrochemical tests show that the rGO@MnO2−x electrode presents higher specific capacitance (284 F g−1) and lower equivalent series resistance (1.11 Ω) than those of MnO2 electrode (128 F g−1 and 1.25 Ω). Meanwhile, the desalination tests display that the hybrid GO||rGO@MnO2−x cell demonstrates prominent desalination capacity (58.75 mg g−1) and ultrahigh charge efficiency (95.01%) compared to GO||MnO2 cell (38.25 mg g−1 and 83.81%) in 500 mg L−1 NaCl solution at 1.2 V. The proposed design offers an optional strategy to improve the intrinsic conductivity of MnO2 and enables synthesis of a novel composite to achieve efficient desalination.