In situ electrochromic efficiency of a nickel oxide thin film: Origin of electrochemical process and electrochromic degradation

Electrochromic nickel oxide (NiO_x) thin films are one of the most promising anodic colored materials. However, there is lack of accurate description of their electrochemical process and degradation mechanism. In this study, a novel approach involving in situ electrochromic efficiency is proposed to reveal the electrochemical origin of an electrochromic NiO_x thin film cycled in a Li⁺-ion electrolyte. The results indicate that the coloring process of the NiO_x thin film refers to the oxidation reactions of Ni²⁺ to Ni³⁺ and Ni²⁺ to Ni⁴⁺ (in two forms of Ni₃O₄ and Ni₂O₃), and the bleaching process is associated with the reduction reactions of Ni⁴⁺ to Ni²⁺, Ni⁴⁺ to Ni³⁺, and Ni³⁺ to Ni²⁺. The irreversible reduction of Ni⁴⁺ to Ni³⁺ plays a dominant role in the activation procedure of NiO_x. It is deduced that the Li⁺-ion trapping in the bleaching process along with the reduction reactions of Ni⁴⁺ to Ni³⁺ and Ni³⁺ to Ni²⁺ causes the degradation of the electrochromic properties. This study provides a further insight into the electrochromic mechanism and is conducive to the improvement of the long-term cyclic durability for Li⁺-based electrochromic NiO_x. Moreover, the study significantly establishes a direct connection between an electrochemical process and a variation in the optical absorbance of materials.