Design And Performance Research On Dual-band Tungsten Oxide-based Electrochromic Smart Windows

Electrochromic technology refers to the phenomenon of reversible changes in the optical properties of a material under the influence of an applied electric field.Smart windows made of electrochromic materials have excellent ability to dynamically adjust the light and temperature,and have promising applications in commercial and civil buildings,automobiles and aerospace.The near-infrared（NIR）band of the solar spectrum accounts for about 50%of the total solar radiation,so selective modulation of NIR light by electrochromic smart windows have a significant impact on the thermal management,energy consumption and indoor occupant comfort of buildings.Among the inorganic electrochromic materials,tungsten oxide（WO₃）has been widely studied for its high contrast,good ion storage capacity and low cost.However,in the field of electrochromic smart windows,most of the studies have focused on the performance of WO₃ in the visible band.Therefore,this paper systematically investigated the electrochromic properties of WO₃ in the NIR band,such as increasing the optical modulation range in the NIR band,independent modulation of the visible-NIR dual band and modulation of the local surface plasmon resonance（LSPR）in the NIR region,and the research mainly contains the following aspects.1.Amorphous WO₃（a-WO₃）has high coloring efficiency and fast response time due to its disordered atomic arrangement and loose internal structure,but the strong scattering from its rough surface makes the optical modulation in the NIR region poor,and the poor long-term reversibility and weak adhesion to the conductive substrate still limit its application in electrochromic smart windows.In this part,Sn O₂ nanosheets grown by a hydrothermal method are used as composites to effectively enhance the visible electrochromic properties of a-WO₃ and broaden its optical modulation in the NIR region.WO₃/Sn O₂ structure exhibited the NIR light modulation reaches 65.7%and 66.5%at1200 nm and 1600 nm,respectively,and the visible light modulation range is 73.5%at 633 nm.2.For the independent regulation of visible and NIR bands,electrochromic materials for visible light regulation are usually combined with materials for NIR light regulation,resulting in smart windows with three modes:“Bright”mode,“Cool”mode,and“Dark”mode.However,the two electrochromic elements may not operate independently at different voltages due to inefficient electron conduction,ion movement,and interfacial penetration between electrolyte and element,and special design and fabrication are required to allow electrolyte contact with different components for electrochromic reaction.Therefore,in this part,a loosely porous amorphous/crystalline WO_3-xnanostructure is fabricated on the Sn O₂ nanosheet array,crystalline WO_3-x(c-WO_3-x)modulates visible as well as NIR light through phase transition as well as capacitive adsorption,and a-WO_3-xmodulates visible light through polariton absorption.The synergistic effect of a-WO_3-x and c-WO_3-xexhibits excellent modulation of light transmission（68.4%at 633 nm and 65.8%at 1600 nm）,high coloration efficiency and good cycling stability.3.Single-component electrochromic materials with a dual-band response are still preferred over complex structures made of a composite of complementary NIR modulation element and visible modulation element.Heterovalent metal atom substitution doping and the creation of oxygen vacancies can increase the free carrier density and produce stronger LSPR absorption in the NIR range.However,the doping strategy can make the radius size mismatch between the dopant ion and the host ion introduce lattice strain and lattice distortion,which adversely affects the structural stability of the host and the ion diffusion in the host lattice.Therefore,this work combines photodeposition and carbon thermal reduction methods to synthesize c-WO_3-x with abundant oxygen vacancies and tunable LSPR performance by controlling the content of PVP.The generation of oxygen vacancies can reduce the potential barrier for Li⁺diffusion and effectively alleviate the lattice strain during Li⁺insertion and detachment.The c-WO_3-x modified by oxygen vacancy doping provides high optical modulation range in the full solar spectrum with 73.0%and 79.5%NIR light modulation at 1200 nm and 1600 nm,respectively,and 72.4%visible light modulation at 633 nm.