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 the 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, serious 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.