A dual-mode electrochromic and thermochromic smart window utilizing a polyzwitterionic hydrogel

Hydrogel electrolyte-based electrochromic devices (ECDs) have become a research frontier in the electrochromic community as hydrogels combine the advantages of both liquid and solid electrolytes. However, these ECDs still require external power to operate. In addition, the color change is induced by cations insertion/extraction, accompanying anions accumulation at electrode-electrolyte interface, which may reduce the lifespan of ECDs. Herein, we present a dual-mode smart window by integrating WO₃·xH₂O/Prussian blue (PB) complementary device and a temperature-sensitive polyzwitterionic sulfobetaine hydrogel. Without any external power, temperature-induced phase separation of the hydrogel enables the smart window a large optical modulation (65.6 % at 490 nm), and the indoor temperature can be reduced by 6.9 °C in a housing model. Upon applied voltages of -1.0 and 2.5 V, the smart window demonstrates a larger optical modulation (75.8 % at 665 nm) and fast responses (2.5 s for coloration and 2.2 s for bleaching). Moreover, the zwitterionic ion channels of the hydrogel facilitate both cations and anions migration, and anions accumulation at electrode-electrolyte interface can be eliminated. Therefore, an excellent cycling stability is also achieved for the smart window, retaining 99.5 % and 86.7 % of the original optical modulation after 4000 and 6000 cycles. These results present an effective approach for designing dual-mode smart windows towards more efficient energy saving.