Simultaneously enabling dynamic transparency control and electrical energy storage via electrochromism

Transparency-switchable electrochromic devices (ECDs) offer promising applications, including variable optical attenuators, optical shutters, optical filters, and smart windows for energy-efficient buildings. However, the operation of conventional ECDs requires external voltages to trigger coloration/de-coloration processes, which makes them far from being an optimal energy-efficient technology. Electrochromic batteries that incorporate electro-optical modulation and electrical energy storage functionalities in a single platform, are highly-promising in the realization of energy-efficient ECDs. Herein, we report a novel Zn-Prussian blue (PB) system for aqueous electrochromic batteries. By utilizing different dual-ion electrolytes with various cations (e.g. Zn²⁺-K⁺ and Zn²⁺-Al³⁺), the Zn-PB electrochromic batteries demonstrate excellent performance. We show that the K⁺-Zn²⁺ dual-ion electrolyte in the Zn-PB configuration endows a rapid self-bleaching time (2.8 s), a high optical contrast (83% at 632.8 nm), and fast switching times (8.4 s/3 s for the bleaching/coloration processes). Remarkably, the aqueous electrochromic battery exhibits a compelling energy retrieval of 35.7 mW h m^-2, where only 47.5 mW h m^-2 is consumed during the round-trip coloration-bleaching process. These findings may open a new direction for developing advanced net-zero energy-consumption ECDs.