pH-Regulated Cation-Dependent Electrochromism of Electrodeposited WO3·2H2O Films in Aqueous Electrolytes

Aqueous electrochromic systems based on tungsten oxide (WO3) have attracted increasing attention because of their high ionic conductivity, low cost, and improved safety compared with organic systems. However, the role of electrolyte pH in regulating the electrochromic behavior of hydrated WO3 films remains insufficiently understood, particularly across cation systems with different valences. In this work, amorphous WO3·2H2O films were electrodeposited on ITO substrates and systematically evaluated in LiCl and ZnCl2 aqueous electrolytes with different pH values, with acidic AlCl3 used as a supplementary trivalent system. The results reveal pronounced pH-dependent electrochromic behavior in both the monovalent and divalent systems. In LiCl, acidic conditions, especially pH 2.0, gave the best overall performance, including high optical modulation and improved cycling stability, while the dominant pseudocapacitive charge-storage behavior was largely preserved. In ZnCl2, films tested at pH 1.5–2.0 showed significantly better electrochromic performance than those at higher pH values, indicating a much stronger kinetic sensitivity to pH. Combined experimental and first-principles results show that electrolyte pH influences not only proton availability, but also the cation-dependent interfacial charge-compensation environment in hydrated WO3 films.