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Advances in electrochromic coatings may bring us closer to environmentally friendly ways to keep inside spaces cool.

Hongbo Xu and colleagues used MOFs in their electrochromic film because of the crystalline substances' abilities to form thin films with pore sizes that can be customized by changing the length of the organic ligand that binds to the metal ion.

In demonstrations, Xu's MOF electrochromic film took 2 seconds to switch from colorless to green with an electric potential of 0.8 volts, and 2 seconds to switch to dark red with 1.6 V. The film maintained the green or red color for 40 hours when the potential dropped, unless a reverse voltage was applied to return the film to its transparent state.

In addition to Xu's MOF-based electrochromic film, several other research groups have reported electrochromic coating designs, including a UV-blocking but visually transparent radiative cooling film, a colorful plant-based film that gets cooler when exposed to sunlight, and a temperature-responsive film that turns darker in cold weather and lighter when it's hot.

Inside, the iX is opulent and luxurious, complete with crystal knobs and buttons, a simple and elegant wood panel where the iDrive controller lives, push-button door handles, and an optional vast sunroof with electrochromic shading that turns it from opaque to clear with the push of a button.