Pseudocapacitive desalination via valence engineering with spindle- like manganese oxide/carbon composites

Manganese tetravalent oxide (MnO₂), a superstar Faradic electrode material, has been investigated extensively for capacitive desalination, enabling higher salt adsorption capacity compared to the great majority of carbonous electrodes. However, few works paid attention on the relationship between the valences of manganese oxide and their desalination performance. For the first time, we prepared the spindle-like manganese oxides/carbon composites with divalent (MnO@C), trivalent (Mn₂O₃@C) and divalent/ trivalent (Mn₃O₄@C) manganese by pyrolysis of manganese carbonate precursor under different condition, respectively. The electrochemical behavior in three-electrode system and electrosorption performance obtained in hybrid membrane capacitive deionization (HMCDI) cells assembled with capacitive carbon electrodes were systematically evaluated, respectively. High salt adsorption capacity (as large as 31.3, 22.2, and 18.9 mg·g^–1) and corresponding average salt adsorption rates (0.83, 0.53, and 1.71 mg·g^–1·min^–1) were achieved in 500 mg·L^–1 NaCl solution for MnO@C, Mn₂O₃@C, and Mn₃O₄@C, respectively. During fifteen electrosorption-desorption cycles, ex-situ water contact angle and morphology comparison analysis demonstrated the superior cycling durability of the manganese oxide electrodes and subtle difference between their surface redox. Furthermore, density functional theory (DFT) was also conducted to elaborate the disparity among the valence states of manganese (+2, +3 and +2/+3) for in-depth understanding. This work introduced manganese oxide with various valences to blaze new trails for developing novel Faradic electrode materials with high-efficiency desalination performance by valence engineering.