Scientists at the Massachusetts Institute of Technology (MIT) have created a novel type of material that stores heat. The solid but thin and transparent film assumes a charged state when exposed to sunlight and can maintain it over an extended period of time. Such solar thermal fuels (STFs) have thus far been largely unavailable in solid states. The research could therefore represent an important leap toward solid-state energy storage and heat-release materials.

“Our new solid film is a polymer where the backbone is composed of an alkyl chain, while the side chains are made up of azobenzene,” explains MIT graduate student Eugene Cho. “The STF material was then dissolved in toluene and spin-coated onto a quartz substrate, resulting in a solar thermal energy capacitor.” When the material is triggered using a small burst of heat, it returns to its original chemical composition, releasing a much larger amount of energy in the form of heat in the process.

Simple mass-production

Manufacturing at scale could be accomplished with spray-coating as well as roll-to-roll processing. The technology is unique in that the STF material can be processed to be in solid state while retaining its advantageous properties. “In the solid state, the STF material can be charged using light and discharged via a catalytic amount of heat, with minimal degradation, which has been demonstrated for the first time for such a solid-state thin-film approach,” Cho says.

Once the material is charged via light, the expert says it maintains its charged state because of an energy barrier between the charged and uncharged states, which then can be overcome via an additional input of energy — the “trigger”. Alternative triggers could be light of a different wavelength or an electric field.

All-in-one energy harvest and storage

This research could be an important advance toward practical solid-state materials that can both store energy and later release it on demand. Cho agrees that his team’s innovation offers an entirely new angle on solar technologies: “Current solar technologies require a two-part system, with the solar panel as the solar energy harvester and a battery as the energy storage mechanism,” he says. “However, solar thermal fuel enables solar harvesting and storage all in a single system.”

The innovation coming out of MIT could open avenues for consumer-oriented applications. Because the film is thin and transparent, it could be a useful solution for deicing windshields, heating homes through the windows and even a new kind of warm clothes for winter.

The prototype can increase the ambient temperature by about 10 degrees Celsius or 18 degrees Fahrenheit — enough to break up ice on a windshield, for instance. The material’s half life for is about two to three days, with the goal to eventually push this to weeks or months. Furthermore, the film’s tint still needs to be optimized to be less yellow, and the team aims to at least double the heat yield.

“The next step is figuring out what the requirements are for various applications and designing similar polymer materials around them,” Cho projects. “Some applications may require fast charging, increased transparency, longer storage lifetimes or higher temperatures. These are all parameters that are accessible by a joint computational and experimental effort that involves modifying the chemistry of the molecule at the heart of this solar harvesting platform.”

The research is detailed in the Advanced Energy Materials paper “Solid-State Solar Thermal Fuels for Heat Release Applications,” written by main author David Zhitomirsky, Cho himself and Cho’s advisor, Jeffrey Grossman.

Written by Sandra Henderson, research editor Solar Novus Today