Advances in the performance improvement strategies of tungsten oxide-based electrochromic smart windows

The national strategic goal of “carbon peak and carbon neutrality” can be achieved without lowering living standards by the immediate development of energy saving devices. Where and how to use energy saving devices also must be considered. Energy consumption for building operations occupies a very large proportion of the total energy consumption, with over half of the building operation energy consumption being used for heating and cooling. Electrochromic smart windows can adjust the transmittance of solar radiation into a building by regulating them according to people’s preferences or weather conditions, thereby reducing energy usage. Because electrochromic smart windows use the dual injection of ions and electrons to cause polarization absorption of the material and optical modulation to block solar radiation, they do not require a continuous energy supply to maintain the state, thereby reducing the energy consumption for lighting and cooling while ensuring the building’s aesthetics. Electrochromic materials are the most important part of electrochromic smart windows. Tungsten oxide is a popular electrochromic material and is considered a promising material for electrochromic applications. Tungsten oxide has a large optical modulation range and good stability. After nearly half a century of development, tungsten oxide-based electrochromic smart windows are gradually moving from the laboratory to practical applications. This review will introduce some performance evaluation standards of electrochromic smart windows, including optical modulation range, response time, coloration efficiency, and stability. Based on the performance evaluation standard of electrochromic smart windows, this review provides a summary of several strategies to improve the electrochromic performance of tungsten oxide and presents evaluations of the strategies’ advantages and shortcomings, including the fabrication of oxygen vacancies, doping of heterogeneous metal elements, morphology and size regulation, electrolyte ion screening, and the use of solid electrolytes. Introducing oxygen vacancies in tungsten oxide can improve the optical modulation range; however, it may affect the stability of tungsten oxide. Doping of heterogeneous metal elements can enhance the coloration efficiency at the cost of prolonging the response time. Adjusting morphology and size can shorten the time of electrochromic response; however, it is difficult to control both the morphology and the size of materials. Replacing the electrolyte ion can improve all properties if a suitable ion can be found. Using a solid electrolyte will broaden the scope of tungsten oxide application at the cost of degraded electrochromic properties. Finally, based on the existing problems in the development of electrochromic smart windows and the recently reported promising technologies, this review presents a projection of the development of tungsten oxide-based electrochromic smart windows.