Fig. 2: Structure and electrochemical characterizations of the composite.

a, b SEM, c TEM, and d high-resolution TEM images of the composite. e XRD pattern of the composite showing a hexagonal WO3 crystalline structure. f A cyclic voltammetry curve of the composite from −0.4 to 0.5 V vs. reversible hydrogen electrode (RHE). g Galvanostatic charge/discharge curves of the composite at different current densities. h Cycling stability of the composite electrode in 0.5 M H2SO4.