Electrochromic films, window sunglasses

Advances in electrochromic coatings can bring us closer to environmentally friendly ways to keep indoor spaces cool. Like glasses that darken to provide protection from the sun, the optical properties of these transparent films can be adjusted electrically to block solar heat and light.

Now, a team of scientists led by Xueying Fan of the Harbin Institute of Technology in China has come up with a new electrochromic film design based on metal-organic frameworks (MOFs) that quickly and reliably switches from transparent to transparent. to a green tone that reduces glare and, if desired, additionally to a red one that insulates heat.

Fan and his colleagues used MOFs in an electrochromic film because of the ability of crystalline substances to form thin films with pores whose size can be tailored by changing the length of the organic ligand that binds to the metal ion.

These features allow for better current flow, more precise color control, and greater durability.

In demonstrations, the new MOF electrochromic film took 2 seconds to turn from colorless to green at an electrical potential of 0.8 volts and 2 seconds to turn dark red at 1.6 volts.

The film maintains the green or red color for 40 hours when the potential is lowered, unless a reverse voltage is applied to return it to its transparent state.

The color and optical properties of this electrochromic film change when the electrical potential goes from 0 to 0.8 to 1.6 volts: Green helps reduce glare and red improves thermal insulation. The photos show only the change from 0 volts to 0.8. (Photos: adapted from ACS Energy Letters 2024, DOI: 10.1021/acsenergylett.4c00492)

In tests, the film also performed effectively for 4,500 color-to-clear cycles.

The researchers believe that, with further optimization, adjustable coatings could be used in smart windows that would regulate the interior temperature. They could also be used in smart optical devices and sensors.

Fan and his colleagues present the technical details of their electrochromic film in the academic journal ACS Energy Letters, under the title “Biphenyl Dicarboxylic-Based Ni-IRMOF-74 Film for Fast-Switching and High-Stability Electrochromism.” (Source: American Chemical Society)