Properties of electrochromic windows enhanced with water

According to the team, the advance would allow building occupants to switch their windows between three modes: transparent, or “normal” windows; windows that block infrared light, helping to keep a building cool; and tinted windows that control glare while maintaining the view. Their work, carried out in collaboration with colleagues from University of Texas at Austin and Vanderbilt University, is detailed in ACS Photonics.

“Our work demonstrates that there are more options available,” Veronica Augustyn, co-corresponding author of a paper on the work said in a statement. “Specifically, we’ve shown that you can allow light to pass through the windows while still helping to keep buildings cooler and thus more energy efficient.”

Key to more dynamic window materials is water. When water is bound within the crystalline structure of a tungsten oxide – forming tungsten oxide hydrate – the material is said to exhibit a previously unknown behaviour.

Tungsten oxides have been used in dynamic windows due to them being transparent. By applying an electrical signal, and injecting lithium ions and electrons into the material, the material becomes dark and blocks light.

Researchers have now shown that it is possible to tune the wavelengths of light that are blocked when lithium ions and electrons are injected into tungsten oxide hydrate. When lithium ions and electrons are injected into the hydrate material, it first transitions into a ‘heat blocking’ phase, allowing visible wavelengths of light to pass through, but blocking infrared light. If more lithium ions and electrons are injected, the material then transitions into a dark phase, blocking visible and infrared wavelengths of light.

“The presence of water in the crystalline structure makes the structure less dense, so the structure is more resistant to deformation when lithium ions and electrons are injected into the material,” said Jenelle Fortunato, first author of the paper and a postdoctoral fellow at NC State. “Our hypothesis is that, because the tungsten oxide hydrate can accommodate more lithium ions than regular tungsten oxide before deforming, you get two modes. There’s a ‘cool’ mode – when injection of lithium ions and electrons affects the optical properties, but structural change hasn’t occurred yet – which absorbs infrared light. And then, after the structural change occurs, there’s a ‘dark’ mode that blocks visible and infrared light.”

“The discovery of dual-band light control in a single material that’s already well-known to the smart windows community may accelerate development of commercial products with enhanced features,” said Delia Milliron, co-corresponding author of the paper from the University of Texas at Austin. “More broadly speaking, the unforeseen role of structural water in producing distinctive electrochemical properties may inspire the research community beyond smart window developers, leading to innovation in energy storage and conversion materials.”