Boosting WO3-Based Electrochromic Performance for Practical Smart Windows: Balancing Charge Density and Matching Color Based on Mn2+/MnO2 Counter-Electrode Reaction

Tungsten trioxide (WO₃)-based electrochromic devices (ECDs) often struggle to match suitable counter-electrode materials due to their characteristic of high charge density and bluish hue. Herein, a new strategy is reported to meet the required charge density and color by introducing an interconversion between solution-phase Mn²⁺ and film-phase MnO₂ as the counter-electrode reaction in WO₃ ECDs, and completing the full process from design and optimization to fabricating a practical smart window prototype. By optimizing the harmful byproduct Mn³⁺ in electrolytes with varying pH values, the typical ECD achieves overall modulation over 70% across the full visible spectrum, retains >90% modulation after 12 h, and maintains excellent stability after 10 000 cycles. Additionally, by gelling the electrolyte, the ECD retains ≈80% modulation retention after 4 h of power-off at 80 °C. This approach also enables the smart window prototype to nearly block all solar radiation, reducing indoor temperature by over 40 °C, and enhancing waste heat recovery. Simulation results suggest that this smart window can significantly reduce building energy consumption in tropical and subtropical regions, offering promising application potential.