In situ grown hierarchical NiO nanosheet@nanowire arrays for high-performance electrochromic energy storage applications.

Electrodes with hierarchical nanoarchitectures could promote electrochemical properties due to their largely exposed active sites and quick charge transfer. Herein, in situ grown hierarchical NiO nanosheet@nanowire films are reported by a one-step hydrothermal process followed by heat treatment. The unique NiO hierarchical nanostructures, which are composed of NiO nanowires grown on the surface of a nanosheet array, show improved electrochromic properties such as large optical modulation in different light regions (95% at 550 nm and 50.6% at 1000 nm), fast color change (9.8/5.4 s) and better coloring efficiency (91.2 cm 2 C -1 ) with long-term cycling properties (82.2% after 700 cycles). Simultaneously, the hierarchical nanostructures possess optimal areal capacitance (117.2 mF cm -2 ), rate performance and cycling properties. The enhanced electrochemical properties are due to the pretreated seed layer and the synergistic effect between the unique in situ grown ultrathin nanowire and the underlying vertical nanosheet layer which can strengthen the mechanical adhesion of the nanoarray film to the substrate and make both nanosheets and nanowires more exposed to the electrolyte, enhancing charge transfer and mass diffusion. This work provides a promising pathway towards developing high quality electrochromic energy storage devices.