Charge storage mechanisms of manganese dioxide-based supercapacitors: A review

Carbon-based materials, such as carbon nanotubes, graphene and mesoporous carbons, are typical electrochemical double-layer capacitive electrodes of supercapacitors (SCs). Although these carbon electrode materials have excellent electrochemical stability, they usually have a low capacitance. Therefore, pseudocapacitive materials are often combined with them to increase capacitance. Among these pseudocapacitive materials, manganese dioxide (MnO₂) has been widely used because of its high theoretical specific capacitance, low-cost, abundance, and environmentally friendly nature. However, the use of MnO₂ often produces rather low actual specific capacitances due to its poor electrical conductivity, phase transformation and large volumetric changes during repeated charge and discharge. To explore high-performance MnO₂/carbon composite electrode materials, it is necessary to understand the charge storage mechanisms of MnO₂. These are analyzed and classified into four types: surface chemisorption of cations, intercalation-deintercalation of cations, a tunnel storage mechanism and a charge compensation mechanism. Although the fourth involves pre-interaction of the cations in MnO₂, the essence of all these mechanisms is the valence transition of manganese atoms between +3 and +4, and many mechanisms are usually involved in MnO₂-based SCs because of the complicated charge storage process. Critical challenges and possible strategies for achieving high-performance MnO₂/carbon-based SCs are discussed and prospective solutions are presented.