Electrochromic coatings can become an ecological way of cooling rooms. In the same way that tinted glasses protect from the sun, the optical properties of these transparent films can be adjusted with electricity, blocking the sun's heat and light.
Now scientists have created a new electrochromic film based on metal-organic frameworks . It switches from transparent to blackout green and heat-insulating red in two seconds and stays that way for 40 hours. The material withstands 4500 cycles of switching from colored to transparent state.
The scientists used metal-organic frameworks (MOFs) in their electrochromic film because of the ability of these crystalline substances to form thin films with adjustable pore size. The size of the pores can be adjusted by changing the length of the organic ligands that bind the metal ions. These properties contribute to improved current flow, more accurate color control, and increase the durability of the film.
During demonstrations, the MOF-based electrochromic film switched from colorless to green in two seconds at an electric potential of 0.8 V, and to dark red – for the same time at a potential of 1.6 V. When the voltage was turned off, the film remained green or red for 40 hours, until the reverse voltage was applied to return it to a transparent state. In addition, the film functioned reliably for 4,500 cycles of switching from a colored to transparent state.
The researchers say that with further optimization, their adjustable coatings could be used in smart windows that control indoor temperatures, as well as in intelligent optical devices and sensors of a smaller size.
Other research groups also have developments in the field of electrochromic coatings. Among them – a radiant cooling film that blocks UV radiation but remains transparent to visible light; vegetable-based color film that cools under the influence of sunlight; thermosensitive film that becomes darker in cold weather and lighter in hot weather.