A bifunctional self-powered electrochromic and thermochromic smart window with enhanced privacy protection ability†

Abstract

In recent years, smart windows have increasingly focused on adjusting optical transmissive properties and providing efficient solutions for energy-saving buildings and vehicles. Electrochromic smart windows are considered a viable option for green buildings. However, conventional electrochromic devices require external voltage for operation, resulting in additional energy consumption. Furthermore, they cannot achieve zero transmission across the entire visible spectrum in the colored state, failing to provide high privacy protection. In this study, we present a self-powered electrochromic and thermochromic dual-responsive smart window that functions as a self-rechargeable battery and can achieve a highly private state across the entire visible range. We utilized the thermally stimulated phase transition behavior of hydroxypropyl methylcellulose to develop a thermochromic hydrogel for use in the electrolyte of electrochromic devices without compromising their electrochromic performance. This approach can be universally applied to electrochromic devices, and the phase transition temperature can be easily adjusted by varying the cationic species. In addition to its ion-conductive function, the electrolyte also exhibits thermochromic properties, enabling electrochromic devices to achieve four distinct states: a highly transparent (bleached) state, a milky white state (thermochromic), a blue state (electrochromic), and a fully nontransparent state (the dual-color private state). Notably, the electrochromic state can self-recover, regaining its transparent appearance simply by disconnecting the Zn and ITO electrodes. This recovery occurs due to the spontaneous oxidation of W⁵⁺ to W⁶⁺ facilitated by dissolved oxygen in the electrolyte. Consequently, the device we present is bifunctional, serving both as a self-powered electrochromic window and a thermochromic window. This innovative chromatic engineering significantly expands the industrial market for electrochromic smart window applications, catering to both public and private contexts, and offers enhanced flexibility in the design of building façades, paving the way for further industrial applications.