Construction of electrochromic porous NiO/Ni(OH)₂ hybrid nanoarrays with ultra-large optical modulation

Electrochromic capacitor energy storage device is an important part of the renewable energy infrastructure. While light transmittance can be regulated by changing the brightness of the device, the clear color change of the device makes the remaining power visible. Herein, hierarchical porous NiO/Ni(OH)₂ hybrid nanoarrays are fabricated by a simple hydrothermal process with following electrodeposited treatment. Ni(OH)₂ nanosheets with very narrow widths are uniformly grown on the surface of underlying NiO nanosheets which are synthesized on FTO substrates. The nanostructure exhibits enhanced electrochromic performance with high optical modulation (87.4 % at 550 nm), fast switching time (5.8/7.2 s) and exceedingly good cycle stability (80.0 % contrast retention after 500 cycles). The morphology of the nanostructure results in increased areal capacitance (104.0 mF/cm²), good rate property and cycle property of the film. The obtained improved electrochemical performances are mainly contributed to the structure design and the synergism between NiO and Ni(OH)₂ which increase structural robustness whilst more exposing more surface area to facilitate ion and electron diffusion. This work provides a practical pathway to design multi-component electrochromic energy storage devices with outstanding performance.