The influence of tungsten oxide concentration on a carbon surface for capacitance improvement in energy storage devices: A combined experimental and theoretical study

Different concentrations of tungsten oxide (WO3) were incorporated in nanoporous carbon, and analyzed in supercapacitor cells. With only 1.5 wt% of WO3, the capacitance properties of the bare carbon were improved from 120 to 150 F/g (5 mV/s) in three-electrode cells, and from 30 to 43 F/g in supercapacitor cells, with excellent stability through cycling. The low oxide concentration in the electrodes was confirmed by scanning electron microscopy and X-ray diffraction, and had no significant effect on the bare carbon surface area. A theoretical study using Density Functional Theory at the Perdew–Burke–Ernzerhof level was performed to understand the electronic structure properties of the nanocomposite material based on WO3 and carbon, and the impact of the composite on the energy storage properties observed in the supercapacitor cells. Simplified W/carbon and WO3/carbon substrate models revealed directionality on the charge transfer at the interface. Moreover, the amount of WO3 dispersed on carbon appeared to be critical to the electron transport in accordance with non-equilibrium Green's functions. At low WO3 concentrations, the charge transfer was improved, enhancing the electronic charge retention. This electron transfer may be fundamental to elucidate the mechanism behind the energy storage in nanocomposite materials used in devices such as supercapacitors.