Tunnel-dependent supercapacitance of MnO₂: effects of crystal structure

A proportional relationship between the specific capacitance of MnO₂ and the percentage of effective Mn centers that act as active sites in the Faradaic charge storage has been established on the basis of a tunnel structure–crystallization behavior correlation. A quantitative relationship between the effective Mn centers at the surfaces and in the tunnels can distinguish the specific capacitance values that arise from the adsorption/desorption and insertion/extraction processes, respectively, of different MnO₂ crystallographic forms in the Faradaic charge storage. The different specific capacitance values between the MnO₂ crystallographic forms are mainly attributed to the different effective utilizations of Mn centers in the size-limited tunnels. The present model demonstrates that increasing the percentage of effective Mn centers via decreasing the crystal size can facilitate obtaining MnO₂-based electrode materials with higher specific capacitance values.