MOFs open window on new electrochromic material

A team of researchers at Massachusetts Institute of Technology (MIT) has developed a new way for making windows that can switch from transparent to opaque, potentially saving energy by blocking sunlight on hot days and thus reducing air-conditioning costs. While other systems for causing glass to darken do exist, the new method offers significant advantages by combining rapid response times with low power requirements.

Once the glass is switched from clear to dark, or vice versa, the new system requires little or no power to maintain its new state; unlike other materials, it only needs electricity when it's time to switch back again. The results are reported a paper in Chem by MIT professor of chemistry Mircea Dinca, doctoral student Khalid Al-Kaabi and former postdoc Casey Wade, now an assistant professor at Brandeis University.

The new discovery uses electrochromic materials, which change their color and transparency in response to an applied voltage, Dinca explains. These are quite different from photochromic materials, such as those found in some eyeglasses that become darker as the light gets brighter. Such materials tend to have much slower response times and to undergo a smaller change in their levels of opacity.

Existing electrochromic materials suffer from similar limitations and have so far only found niche applications. For example, Boeing 787 aircraft have electrochromic windows that get darker to prevent bright sunlight from glaring through the cabin. The windows can be darkened by turning on a voltage, Dinca says, but "when you flip the switch, it actually takes a few minutes for the window to turn dark. Obviously, you want that to be faster."

The reason for the slowness is that the changes within the material rely on the movement of electrons – an electric current – that gives the whole window a negative charge. Positive ions then move through the material to restore the electrical balance, creating the color-changing effect. But while electrons flow rapidly through materials, ions move much more slowly, limiting the overall reaction speed.

The MIT team overcame this problem by using sponge-like materials called metal-organic frameworks (MOFs), which can conduct both electrons and ions at very high speeds. MOFs are made by combining two chemical compounds, an organic material and a metal salt, which self-assemble into a thin porous film. Up to now, such materials have mainly been investigated for their ability to store gases within their structure; the MIT team was the first to harness them for their electrical and optical properties.

The other problem with existing versions of self-shading materials, Dinca says, is that "it's hard to get a material that changes from completely transparent to, let's say, completely black." Even the windows in the 787 can only change to a dark shade of green, rather than becoming opaque.

In previous research on MOFs, Dinca and his students had made materials that could turn from clear to shades of blue or green. Now, in this new work, they have achieved the long-sought goal of producing a coating that can go all the way from perfectly clear to nearly black (achieved by blending two complementary colors, green and red).

"It's this combination of these two, of a relatively fast switching time and a nearly black color, that has really got people excited," Dinca says.

According to Dinca, the new windows have the potential to do much more than just prevent glare. "These could lead to pretty significant energy savings," he says, by drastically reducing the need for air conditioning in buildings with many windows in hot climates. "You could just flip a switch when the sun shines through the window, and turn it dark," or even automatically make that whole side of the building go dark all at once, he says.

While the properties of the material have now been demonstrated in a laboratory setting, the team's next step is to make a small-scale device, about one inch square, for further testing. This will help demonstrate the principle in action for potential investors in the technology, as well as help determine what the manufacturing costs for such windows would be.

Further testing is also needed, Dinca says, to demonstrate what they have determined from preliminary testing: that once the switch is flipped and the material changes color, it requires no further power to maintain its new state. No extra power is needed until the switch is flipped to turn the material back to its former state, whether clear or opaque. Many existing electrochromic materials, by contrast, require a continuous voltage supply.

In addition to smart windows, Dinca says, the material could also be used for some kinds of low-power displays, similar to displays like electronic ink (used in devices such as the Kindle and also based on MIT-developed technology) but based on a completely different approach.

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.