A Proton-Based Tandem Design for a Quasi-Solid-State Polyoxometalate-Based Electrochromic Device with Solar-Spectral Modulation

Electrochromic (EC) glazing can dynamically regulate solar radiation through windows, improving indoor comfort and reducing cooling loads in buildings and vehicles. However, EC devices (ECDs) are often limited by sluggish ion diffusion and ion trapping, which leads to the slow switching speed and poor cycling stability. Here, we report a quasi-solid-state, proton-based tandem-structure ECD fabricated by scalable spray coating. In this design, a Preyssler-type polyoxometalate (POM) serves as the EC layer, while PEDOT:PSS functions as a solid-state proton source, enabling protons to act as the insertion/extraction ions during the EC process. Benefiting from the small ionic size of protons, ion-transport kinetics are improved, and ion trapping is suppressed, delivering fast switching (1.4 s for coloration and 2.5 s for bleaching) and stable cycling (93.5% optical-modulation retention after 3000 cycles). Moreover, densely packed POM nanoparticles enable broadband visible–NIR solar-spectral modulation (78.4% at 650 nm; 61.1% at 1200 nm). Large-area and flexible ECD further demonstrate the scalability of tandem design for energy-efficient buildings and vehicles.