A newly designed class of carbon nanotubes can store solar power by changing shape, effectively becoming extremely stable batteries for solar energy. These tiny tubes combine the energy harvesting of photovoltaics and the electricity storage of chemical batteries in one exceptional material.
The result is a rechargeable battery with a long shelf life that squeezes energy harvesting and storage into one easy step. The new material can also release the stored energy as heat when prompted by a slight change in temperature or by a flash of light; its molecules simply go back to their original shape.
"You've got a material that both converts and stores energy," said Jeffrey Grossman, a materials scientist at MIT. "It's robust, it doesn't degrade, and it's cheap."
The new material can store about 10,000 times more energy per given amount of space compared to the older chemical storage that requires ruthenium. That puts its energy density on par with that of lithium-ion batteries powering laptops and smartphones.
Having the ability to directly store sunlight's heat in chemical form allows for a much longer storage period without energy loss, as opposed to converting the energy into electricity or storing the heat in an insulated container. But chemical storage up until now has either degraded quickly within a few recharge cycles or has required rare and expensive elements such as ruthenium.
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To get around those challenges, the MIT team combined the carbon nanotubes — tiny tubes made of pure carbon — with a compound called azobenzene. Their work on the nanoscale level allowed them to precisely figure out the "energy barrier" level at which the new material's molecule would switch between its two stable shapes.
"You can control [the molecules'] interactions, increasing the amount of energy they can store and the length of time for which they can store it — and most importantly, you can control both independently," said Alexie Kolpak, an MIT postdoc.
But the MIT team won't stop with this new material. It has already moved on to looking at more that can use nanoscale engineering to create new material combinations.
"I see this as the tip of the iceberg," Grossman said. "We’re pretty jazzed up about it."
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