Unraveling the structure and electrochemical supercapacitive performance of novel tungsten bronze synthesized by facile template-free hydrothermal method

Abstract

In this study, we report a facile hydrothermal method for preparing novel tungsten oxide bronze (HxWO3) and nanostructured tungsten oxide (WO3) using different structure-directing agents. The effects of phase and morphology of the hydrothermally fabricated nanostructured WO3 on the charging/discharging performance of electrochemical pseudocapacitors are thoroughly investigated. Most of the as-prepared samples exhibit WO3•0•33H2O orthorhombic structure, whereas post annealing at 350 °C promotes phase transformation to a phase-pure hexagonal structure (h-WO3). Scanning electron microscope (SEM) images show a variation of the prepared samples’ morphology between one-, two-, and three dimensions. The difference in structure between the developed bronze and the known WO3 phases is confirmed by Raman spectroscopic investigations. The optical bandgap estimated from diffuse reflectance is the highest for novel HxWO3 (2.84 eV) and h-WO3 (2.73– 2.80 eV), whereas it is lowest for the monoclinic phase (2.66 eV). X-ray photoelectron spectroscopy (XPS) analysis confirms the formation of higher amount of oxygen vacancies in h-WO3 than HxWO3. An exceptional capacitance of 1280 F/g at 1 A/g is obtained for h-WO3 when tested in 0.5-M H2SO4 aqueous electrolyte, which is superior to HxWO3. The symmetrical supercapacitor device of HxWO3 exhibits an energy density of 8 W h/kg with high stability and good capacitive retention. The number and size of the various crystallographic tunnels in the fabricated h-WO3 are found to be the primary driving forces for improving the capacitive performance and stability.