Recyclable quasi-solid-state dynamic windows via reversible dual-metal electrodeposition for building energy modulation

Reversible metal electrodeposition-based electrochromic devices (RMEDs) with broadband regulation manifest great promises as smart windows to increase building energy efficiency. However, manufacturing durable and scalable RMEDs at an agreeable cost is still challenged by the limited reversibility of interfacial plating (tinting)/stripping (bleaching). Here, we report a quasi-solid-state hydrogel electrolyte with well-tuned Cu:Zn ratio and pH values to achieve highly reversible CuZn-RMEDs. The dual-metal CuZn electrolyte ensures staged nucleation and homogeneous CuZn growth, and the appropriate pH guarantees effective stripping of CuZn metal layers, thus attaining reduced metal deposition activation energy (6.7 kJ mol^-1), increased coloration efficiency, large transmittance modulation (ΔT = 78% @550 nm), and excellent reversibility (ΔT retention over 90% after 3000 cycles). In addition, the quasi-solid-state hydrogel matrix with abundant molecular chains can inhibit strong hydrogen bonds in aqueous electrolyte, achieving anti-freezing (-20°C) and leakage-free CuZn-RMEDs with fast switching kinetics (coloration time t_c = 15.0 s, bleaching time t_b = 24.0 s), extended memory effect, efficient thermal insulation, and effective reusability (2000 cycles) of both the transparent conductor and hydrogel electrolyte. The as-assembled CuZn-RMEDs manifest 18% to 33% energy saving across different climatic regions. This quasi-solid-state CuZn-RMED is a successful embodiment of cost-effective and durable electrochromic smart windows towards increasing buildings' energy efficiency.