Potential Gradient‐Driven Power‐Free Films Fabrication Strategy for Electrochromic Window Achieving on‐Demand Indoor Photothermal Regulation

ABSTRACT Electrochromic windows capable of dynamically regulating indoor sunlight transmission are promising for building energy conservation. However, conventional film preparation techniques often require power consumption, and the final electrochromic windows encounter the dilemma of completely independent control of visible (VIS) and near‐infrared (NIR) transmission. Herein, we propose a power‐free and scalable strategy for growing polyaniline and WO 3 films in a galvanic cell through potential‐gradient‐driven spontaneous reactions. This strategy is shown to be applicable for preparing other films, including Prussian blue and MnO 2 . By sandwiching a Zn anode between the polyaniline and WO 3 film electrodes, an electrochromic window is constructed, which presents four distinct states, including “bright” (colorless, VIS and NIR transparent), “dark” (blue‐black, VIS and NIR opaque), “warm” (purple, allowing NIR while blocking VIS), and “cool” (green, blocking VIS while allowing VIS) states. Therefore, such a platform not only displays multi‐color four states but also achieves individual control of VIS and NIR transmission. Additionally, a upscaled window of 100 cm 2 is fabricated, enabling an indoor temperature reduction of 32.4°C in a model house compared to the flat glass window. Our strategy, upon power‐free film preparation and the electrochromic window design for selective VIS light and NIR heat modulation, provides new insights for future building energy savings.