Phase transformation in tungsten oxide nanoplates as a function of post-annealing temperature and its electrochemical influence on energy storage

The morphology and crystal structure of electrode materials have an enormous impact on their electrochemical properties to employ the supercapacitors for various energy storage applications. In this study, the transformations of crystal structure phases of WO3 nanoplates have been prepared by post-annealing at various temperatures. The WO3 nanoplates are synthesized by a single-step template-free chemical method. The morphological and structural evolution of the electrodes is studied via FEG- SEM, HRTEM, FTIR, XRD, and Raman spectroscopy. The phase transition and enhanced degree of crystallinity are illustrated with the increasing temperature. The hydrate orthorhombic WO3.H2O structure (W80), mixed-phase mesoporous WO3.0.33H2O structure (W200), and monoclinic WO3 structures (W400) are achieved at annealed temperatures 80oC, 200oC, and 400oC respectively. The electrochemical analysis of WO3 nanosheets demonstrates the highest specific capacitance of 606 Fg-1 of mixed-phase WO3.0.33H2O structure ((W200)) than the electrode W80 (361 Fg-1), which is a two-fold time greater than the electrode W400 (302 Fg-1) at a current density 1 Ag-1. Moreover, the electrode W200 exhibits the excellent cyclic stability of 89% at an ultrahigh scan rate of 100 mVs-1 after 4000 cycles. The results highlight that the mixed-phase WO3 nanoplates can be applied as a suitable electrode material for supercapacitors with desired electrochemical features.