Self-limiting electrodeposition of hierarchical MnO₂ and M(OH)₂/MnO₂ nanofibril/nanowires: mechanism and supercapacitor properties.

Hierarchical nanostructures have generated great interest in the energy, materials, and chemical sciences due to the synergic properties of their composite architectures. Herein, a hierarchical MnO₂ nanofibril/nanowire array is successfully synthesized. The structure consists of a conformal layer of MnO₂ nanofibrils evenly distributed on the surface of the individual MnO₂ nanowires. The synthetic mechanism of this hierarchical structure is characterized by electrochemical measurements, Raman spectroscopy, EELS, and electron microscopy. This material was then investigated at slow scan rates for its charge storage mechanisms in different solvents. In aqueous electrolyte, the nanofibrils show a capacitance almost purely dedicated to double-layer and surface adsorption processes, while in an acetonitrile electrolyte, the nanofibrils' capacitance comes mainly from a cation insertion process. This material was also tested at high scan rates in aqueous solution for its practical supercapacitor capabilities. The material shows a large capacitance of 298 F/g at 50 mV/s and 174 F/g at 250 mV/s. It also maintains 85.2% of its capacitance after 1000 cycles. The material also displays easily controllable parameters such as nanowire length, nanowire diameter, and amount of nanofibril material which is shown here to affect the capacitance dramatically.