Color-neutral WO3-I2 complementary electrochromic device based on reversible iodine electrodeposition

Complementary electrochromic devices can enhance optical modulation and cyclic stability. However, the optical modulation of complementary electrochromic devices based on WO3, one of the most outstanding inorganic electrochromic materials, remains limited and does not meet the standards for color neutrality. This study innovatively proposes a complementary electrochromic device that utilizes nonmetal iodine electrodeposition, eliminating the need for specifically prepared anodic electrochromic electrodes. In this device, I- is dissolved in the ZnCl2 water-in-salt electrolyte. During the coloring process of WO3, the I- in the solution can be oxidized to I2, which stably adheres to FTO glass and contributes to the coloring function. This allows the I2 to overlay with the color of WO3, achieving an almost completely opaque state and neutral color modulation. During the bleaching process, I2 is reduced back to I- and re-dissolved in the electrolyte. By eliminating the optical obstruction caused by the anodic electrochromic material, the device achieves an average optical modulation of 70.7 % in the visible range, with an optical stability of 88.5 % after 100 coloring/bleaching cycles. This work combines nonmetal electrodeposition with traditional electrochromic materials, providing a new option for constructing high-performance complementary electrochromic devices.