Three-dimensional tubular arrays of MnO2-NiO nanoflakes with high areal pseudocapacitance

Transition metal oxide nanostructures are current research focus for energy storage applications. We herein report the synthesis of MnO2-NiO nanoflake-assembled tubular array on stainless steel substrate to function as pseudocapacitor electrode by programmed three-dimensional (3D) interfacial reactions, in which the ZnO nanowire array is employed as the low-cost in situ sacrificial template. In this 3D nanoelectrode, MnO2 and NiO nanoflakes share the same "root" and form an integrated hierarchical structure, which adheres robustly to the substrate. Importantly, both MnO2 and NiO contribute to the charge storage. The highly porous structure, which allows easy penetration of the electrolyte, gives additional merits. Detailed electrochemical characterization reveals that the assembled MnO2-NiO array exhibits good rate performance and cycle life. In particular, it displays an areal capacitance that is four orders of magnitude higher than that of carbonaceous materials and significantly superior to those of previous directly-grown pseudocapacitive nanostructure films.