Formation of hexagonal-molybdenum trioxide (h-MoO₃) nanostructures and their pseudocapacitive behavior

The crystallographic structure and morphology of redox active transition metal oxides have a pronounced effect on their electrochemical properties. In this work, h-MoO₃ nanostructures with three distinct morphologies, i.e., pyramidal nanorod, prismatic nanorod and hexagonal nanoplate, were synthesized by a facile solvothermal method. The morphologies of h-MoO₃ nanostructures were tailored by a controlled amount of hexamethylenetetramine. An enhanced specific capacitance about 230 F g^-1 at an applied current density of 0.25 A g^-1 was achieved in h-MoO₃ pyramidal nanorods. Electrochemical studies confirmed that the h-MoO₃ pyramidal nanorods exhibit superior charge-storage ability. This improved performance can be ascribed to the coexistence of its well exposed crystallographic planes with abundant active sites, i.e., hexagonal window (HW), trigonal cavity (TC) and four-coordinated square window (SW). The mechanism of charge-storage is likely facilitated by the vehicle mechanism of proton transportation due to the availability of the vehicles, i.e., NH₄⁺ and H₂O. The promising, distinct and unexploited features of h-MoO₃ nanostructures reveal a strong candidate for pseudocapacitive electrode materials.