Better Electrochromics

A group of Swedish researchers claim they have developed the first electrochromic materials or foils on flexible plastic. The innovation, they say, overcomes obstacles in manufacturing that have hamstrung commercial application of other electrochromic products, which can change from dark to transparent and vice versa with electrical current (Mater. Sci. Eng. B 2005, 119, 214).

One of the inventors of the new foil, Claes G. Granqvist, an engineering sciences professor at Sweden’s Uppsala University, presented the group’s findings Nov. 29 at the Materials Research Society meeting in Boston. He is also affiliated with ChromoGenics, a company that is an offshoot of the research effort and which hopes to commercialize the technology.

For decades, Granqvist explains, scientists have been trying to capitalize on electrochromic materials. The ability of a material to lighten and darken on command could have wide application in a variety of consumer products. For example, the material would be ideal for energy-efficient windows or for applications such as eye shields on motorcycle helmets or ski goggles.

But electrochromic products have been slow to reach the market, primarily because of high manufacturing costs and low durability. Granqvist says his group’s material could be manufactured by inexpensive technology.

Granqvist’s group builds the new electrochromic foils by taking two transparent polyester foils and coating them with an electrically conductive indium tin oxide. Then they coat one of the foils with tungsten oxide and the other foil with nickel oxide. Finally, they sandwich an ion-conducting polymer between the two layers.

“It’s a little bit like a battery,” Granqvist explained. The oxide coatings are akin to a battery’s anode and cathode, and the polymer in between works like an electrolyte, shuttling ions between them.

Applying just 1.4 V to the device, through a switch or a solar cell, charges the tungsten oxide, discharges the nickel oxide, and darkens the foil. Reversing the voltage turns the material transparent. And the foils don’t just go from dark to light but can take on intermediate shades, Granqvist says.