Electrochromic Behavior Originating from the W⁶⁺/W⁵⁺ Redox in Aurivillius-type Tungsten-Based Layered Perovskites

Simple oxide materials, typically, WO₃, have been conventionally employed for electrochromic (EC) materials because of their high coloration efficiency; however, it is quite difficult to realize multiple coloration because they involve redox reactions due to single ions. On the other hand, multiple oxides are expected to show various colors when applying different voltages due to the diverse structures and combinations of ions; however, multiple oxide-type EC materials are still in the research stage, and the discovery of further EC materials is necessary. Toward the development of multiple oxide-type EC materials, tungsten-containing layered perovskites have been synthesized, and their optical properties have been evaluated. X-ray diffraction and X-ray absorption fine structure analyses revealed that the discovered tungsten-based layered perovskites Bi₂Na_0.5La_0.5TiWO₉ (BNLTW) and Bi₂LaTi_1.5W_0.5O₉ (BLTW) have an orthorhombic phase with an Aurivillius-type layered perovskite structure. EC devices fabricated with three kinds of perovskites, including well-known Aurivillius-type Bi₂W₂O₉ (BWO), have no absorption in the visible-light region when no voltage is applied, while they show absorption over the whole visible-light spectrum to black when a voltage of +4.5 V is applied. Furthermore, with an applied voltage of -4.5, the transmittance recovered to the same level as the initial state, meaning the EC function is reversible. In this reaction, only tungsten in the perovskite framework acted as a redox-active species (W⁶⁺/W⁵⁺ redox) without the redox of the other metal ions. From the electrochemical analysis of the EC materials using cyclic voltammetry, redox peaks could be observed at -0.2 to 0.4 V for reduction and +0.1 to +0.3 V for oxidation. Interestingly, the redox potentials are linearly related to the W content in the perovskite unit, indicating that the redox potentials can be tuned by controlling the chemical formula. The coloration efficiency of the BNLTW EC device was the best at 37.1 cm²/C in the prepared perovskite-based EC device, which is comparable to that of a typical WO₃ EC material.