Two birds with one stone: A novel thermochromic cellulose hydrogel as electrolyte for fabricating electric-/thermal-dual-responsive smart windows

Electric-/thermal-dual-responsive smart windows, which can change color by heat and electricity simultaneously or separately, show excellent modulation ability of solar radiation. In this work, H+ or Li+ doped HPC-PAA hydrogel, which undergoes a thermochromism transition from a highly transparent state to a milky white state in the UV–vis-NIR range, was used as a novel electrolyte for smart windows with a three-layer sandwiched structure (ITO glass, H+/Li+ doped HPC-PAA thermochromic hydrogel electrolyte, and WO3-coated ITO glass). The assembled smart windows allow four switches, namely highly transparent state, thermochromic state (milky white state), electrochromic state (blue color state), and double-colored state (fully nontransparent state), realized by the synergy between thermochromism due to the hydrogel layer and electrochromism due to the WO3 layer. In such smart windows, zero-transmittance (absolute private state) can be achieved for application in private circumstances. More importantly, these electric-/thermal-dual-responsive smart windows also show excellent heat regulation ability under colored states, a highly desirable property for their applications in building facades, energy harvesting, privacy protection, and color display.