The exploration and comparison of adsorption mechanisms in MnO₂ with different crystal structures for capacitive deionization

Among transition metal oxides, manganese dioxide (MnO2) has attracted much attention in Capacitive Deionization (CDI) due to their structural diversity. However, the mechanism by which MnO2 crystal types lead to different performances was rarely analyzed. In this paper, the method of successfully preparing alpha-MnO2, gamma-MnO2 and delta-MnO2 by adjusting the H+ concentration was developed. Meanwhile, the adsorption mechanisms of alpha-MnO2, gamma-MnO2 and delta-MnO2 were discussed by XPS. The results show that H+ concentration plays major role in the arrangement of {MnO6} octahedrons, and 2 x 2 cavity structure of alpha-MnO2 can enhance the stability of material. Importantly, the oxygen vacancy (O-v) with internal built-in electric field induced by active Mn3+ is the key factor for the performance of MnO2. And alpha-MnO2 with the highest O-v content is beneficial for improving conductivity and providing more active sites. The electrochemical results are as follows: alpha-MnO2(138.4 F/g)>delta-MnO2(105.8 F/g)>gamma-MnO2(96.1 F/g). In addition, C||alpha-MnO2 cell has maximum desalination capacity (19.64 mg/g) and better capacitance retention (91.01 %) in CDI tests. In summary, the experimental results are consistent with the mechanism analysis. This work provides a method for achieving high performance by constructing the optimal crystal form of materials for CDI.