The new material can store energy from the sun for months or even years

If we’re going to get better at feeding the planet with renewable energy, we must get better at finding ways to efficiently store that energy until it’s needed, and scientists have identified a particular material that could provide us with exactly that.

The material is known as an organometallic structure (MOF), in which carbon-based molecules form structures by binding metal ions. Fundamentally, MOFs are porous, so they can form composites with other small molecules.

That’s what the team did here, adding molecules of the light-absorbing compound azobenzene. The finished composite was able to store energy from ultraviolet light for at least four months at room temperature before releasing it again, a vast improvement over the days or weeks that most light-sensitive materials can handle.

“The material works a bit like phase change materials, which are used to supply heat in hand warmers,” says materials chemist John Griffin of Lancaster University in the UK.

“However, although hand warmers have to be heated to recharge them, the great thing about this material is that it captures free energy directly from the sun.”

Azobenzene acts as a photoelectric switch, a molecular machine that responds to an external stimulus such as light or heat. Under UV light, the molecules change shape while remaining within the MOF pore frame, effectively storing energy.

The application of heat to the MOF composite material triggers a rapid release of energy which in turn emits heat, which can then potentially be used to heat other materials or devices.

While the material still needs some work to make it commercially viable, it could eventually be used to de-ice car windshields, or provide additional heating for homes and offices, or as a heating source for off-grid locations. Photographic switches like this also have applications in data storage and drug delivery.

“It also has no moving or electronic parts, so there are no losses involved in storing and releasing solar energy,” says Griffin. “We hope that with further development we can make other materials that store even more energy.”

While previous research has also examined solar energy storage in photographic switches, they generally need to be kept in liquids. The change to a solid MOF compound means that the system is easier to contain and also has greater chemical stability.

At this time, more work is needed to prepare this MOF material for widespread use. While tests showed that it could retain energy for months at a time, the material’s energy density is relatively low, which is an area the researchers hope to improve.

The good news is that there are many things about the settings used in this research that can be tweaked and tweaked to try and improve the results, which will hopefully lead to another cost-effective and reliable way of storing energy that we can trust.

“Our approach means that there are a number of ways to try to optimize these materials, be it by changing the photo switch itself or the porous host frame,” says X-ray technician Nathan Halcovitch of Lancaster University.

The research has been published in Materials Chemistry.