The ions storage mechanism of capacitive-faradic coupling effect for pseudo-intercalation electrode MnO2

As one of the most promising faradic intercalation electrode materials, manganese dioxide (MnO2) has broad prospects in capacitive deionization for heavy metal removal. However, there are differences in capacitive deionization kinetics of MnO2 with different crystal structures, and the capacitive and faradic multi-process coupling mechanism are still unclear, which limits the optimization and development of these pseudo-intercalation electrodes. Here, we propose the potential ions storage routes involved into the capacitive and faradic coupling behavior and quantitatively analyze the evolution laws of heavy metal Cd2+ with respect to different crystal-structured MnO2 (α, β and δ). Mediated by the released electron from the cathode, Mn4+/Mn3+ faraday reaction on the MnO2 generates a large amount of surface faradic chemical charge (Mn-OH/Mn-O-), thereby driving the surface faradic storage of Cd2+ ions (28 %∼47 %). This finding breaks the traditional recognition that the pseudocapacitive deionization of MnO2 is dominated only tunnel pseudo-intercalation (14 ∼ 54 % in this study) and surface capacitive progress (12 ∼ 34 % in this study). Additionally, the fixed chemical-charge-controlled storage derived from crystal surface defect also contributed 4 ∼ 8 % to the total. Mn-O bond length is key to determine the tunnel intercalation capacity and the electron transfer ability along Mn4+-O-Mn3+ path to generates Mn4+/Mn3+ faradic charge, thus the surface faradic chemical-charge-controlled storage (β > α > δ) and pseudo-intercalation storage (δ > α > β) showed a potential mutual restriction effect. In this study, the electrochemical behavior of heavy metal ions and charge transfer mechanism at the interface of the MnO2 electrode were effectively clarified by combining the theoretical model and electrochemical analysis, which provided a new idea for expanding the understanding of the multiple ion storage processes coupling mechanism of intercalation materials and forming the regulation strategy for efficient capacitive coupled with faradic electrochemical deionization electrode materials.