In situ efficient growth of Rubik nanocube WO3<middle dot>0.33H2O array films for high-performance electrochromic energy storage devices

Tungsten trioxide (WO3)-based electrochromic devices have attracted considerable attention due to their promising applications in smart windows, energy-efficient displays, and sunglasses. However, it is still challenging to fabricate high-performance WO3 electrochromic films using an efficient and facile wet method. Here, we report a one-step strategy for the in situ growth of WO3<middle dot>0.33H(2)O Rubik nanocube films on fluorine-doped tin oxide (FTO) conducting substrates in hydrogen peroxide (H2O2) and an ethylene glycol (EG)-containing hydrothermal system that is seed layer-free. The complexing effect of H2O2 results in the highly efficient formation of WO3<middle dot>0.33H(2)O films with good interface stability and a porous structure formed by the stacked Rubik nanocubes. Benefiting from the porous structure, the obtained WO3<middle dot>0.33H(2)O film delivers outstanding electrochromic and electrochemical energy storage performance, such as large optical modulation over the wide-band visible-near-infrared (Vis-NIR) range (up to 80% at 633 nm and 90% at 1000 nm), high coloration efficiency (65.6 cm2 C-1), high rate capability as well as good cycling stability (with 80% transmittance retention over 1000 cycles). Furthermore, as a demonstration, a complementary electrochromic device was assembled based on the WO3<middle dot>0.33H(2)O film, which offers a high coloration efficiency (92.6 cm2 C-1), optical modulation in the visible range (up to 70% at 633 nm), and large energy storage of 27.4 mA h m(-2) at 0.07 mA cm(-2). We believe that this efficient and facile wet method for preparing electrochromic films will provide a fresh approach to investigating high-performance electrochromic systems.