Abstract
Herein we illustrate the functionality as pseudocapacitive material of tungsten trioxide (WO3) nanochannel layers fabricated by electrochemical anodization of W metal in pure hot ortho-phosphoric acid (o-H3PO4). These layers are characterized by a defined nanochannel morphology and show remarkable pseudocapacitive behaviour in the negative potential (-0.8-0.5 V) in neutral aqueous electrolyte (1 M Na2SO4). The maximum volumetric capacitance of 397 F cm (3) is obtained at 2 A cm (3). The WO3 nanochannel layers display full capacitance retention (up to 114%) after 3500 charge-discharge cycles performed at 10 A cm (3). The relatively high capacitance and retention ability are attributed to the high surface area provided by the regular and defined nanochannel morphology. Kinetic analysis of the electrochemical results for the best performing WO3 structures, i.e., grown by 2 h-long anodization, reveals the occurrence of pseudocapacitance and diffusional controlled processes. Electrochemical impedance spectroscopy measurements show for the same structures a relatively low electrical resistance, which is the plausible cause for the superior electrochemical behaviour compared to the other structures. (C) 2017 Elsevier Ltd. All rights reserved.