The morphologic dependence of MnO2 electrodes in capacitive deionization process

Manganese dioxide (MnO2) materials are one of promising cathode candidates for capacitive deionization (CDI) applications, with their morphology significantly impacting the performance of the electrode material itself. Therefore, this study systematically investigates the structure-performance relationships and the micro-interface ion storage mechanisms of four morphologies of MnO2 (1D nanorods (NNMO), nanowires (NWMO), 3D microspheres (MMO), and hollow urchin-like spheres (HUMO)) in relation to their capacitive deionization performance with typical heavy metal ions (Cd2+) through experimental and theoretical calculations. In terms of pore morphology, capacitance significantly increases with increasing surface curvature of materials, demonstrating a clear pore structure-dependent characteristic. NNMO, with the smallest pore size, has much lower surface capacitance (similar to 0.15 mu F cm(- 2)) than other materials (similar to 0.33 mu F cm(- 2)). As pore size increases, the capacitance distribution difference driven by pore structure size gradually disappears. Regarding surface and interface characteristics, high-energy crystal facets facilitate electron transfer ({310}>{200}approximate to{211}>{100}) and increase the proportion of surface active sites (Osur), thus promoting CDI absorption kinetics. During the capacitive deionization process, the pore structure (similar to 77 %) and surface-interface characteristics (similar to 23 %) exhibit highly coupled features and the driving force for interfacial ion storage among the four materials is in the order of HUMO>MMO>NWMO>NNMO. This work elucidates the morphology-dependent capacitive processes of MnO2 nanomaterials, enhancing the understanding of structure- electrochemical process at the nanoscale but also providing effective guidance for the design and development of practical, high-performance CDI electrode materials.