Mechanistic Insights into the Coloration, Evolution, and Degradation of NiO_x Electrochromic Anodes

NiO_x is recognized as the leading candidate for smart window anodes that can dynamically modulate optical absorption, thereby achieving energy efficiency in construction buildings. However, the electrochromic mechanism in NiO_x is not yet clear, and the ionic species involved are sometimes ambiguous, particularly in aprotic electrolytes. We demonstrate herein that the "net coloration effect" originates from newly generated high-valence Ni³⁺/Ni⁴⁺ ions during anion-dependent anodization, and the Li⁺ intercalation/deintercalation only plays a role in modulating the oxidation state of Ni. Unambiguous evidences proving the occurrence of anodization reaction were obtained by both chronoamperometry and cyclic voltammetry. Benefiting from the irreversible polarization of Ni²⁺ to Ni³⁺/Ni⁴⁺, the quantity of voltammetric charge increases by ∼38% under the same test conditions, enhancing the corresponding electrochromic modulation by ∼8%. Strong linkages between the coloration, evolution, and degradation observed in this work provide in-depth insights into the electrocatalytic and electrochromic mechanisms.