Design of cobalt phosphate/nickel phosphate films with improved electrochromic performance

Abstract

Electrochromic devices have gained great attention owing to their potential application in fields such as smart windows, glassware, and low-cost displays. Recent investigations have shown that the nickel phosphate films (NiHPO₄·3H₂O, NiHP films) are highly promising for the fabrication of large-area smart windows with large optical modulation. However, the performances such as the color switching time, coloration efficiency and cycling stability of such films are not ideal and need further investigations. In this study, we propose a layer-by-layer electrodeposition method for the fabrication of a composite metal phosphate film. A CoHPO₄ layer is first deposited on the ITO glass as the frame, and then NiHPO₄ is further electrodeposited on the CoHPO₄ frame as an electrochromic active layer. The resulting film with NiHPO₄ on CoHPO₄ (CoHP/NiHP film) exhibits an optical modulation of 65.55%, a fast color switching time (t_c/t_b 2.0/8.5 s, the coloring time is three times faster than that of the pure NiHPO₄ film), a high charge capacity (81.36 mF cm⁻²), excellent memory effect (10.53% attenuation after 5000 s), a high coloration efficiency (140.37 cm² C⁻¹, it is twice that of the pure NiHPO₄ film) and 5 times improved outstanding cycling stability compared with the pure NiHPO₄ film (1400 cycles with retained performance). The uniform nanosheet morphology of CoHPO₄ provides a large active area for the electrochromic response. Meanwhile, the electrochemical investigations indicate that, compared with the pure NiHPO₄ film, the NiHPO₄ on CoHPO₄ composite structure shows much faster electrical dynamics with diffusion coefficient values of OH⁻ increased by an order of magnitude. The larger active area and much faster electrical dynamics promote the electrochromic performances of the metal phosphate films. Our findings may help in the development of metal phosphate films with high electrochromic performances. The layer-by-layer electrodeposition strategy may help to develop new types of electrochromic materials.