While solar panels are very useful at converting the sun's rays into electricity for immediate use, the storage of that energy for later use is ... well, it's still being figured out. The energy can be used to charge batteries, for instance, but that charge will wear off over time. Instead, scientists have been looking at thermo-chemical storage of solar energy. Last year, researchers from MIT discovered that the chemical fulvalene diruthenium was quite an effective storage medium. Unfortunately, the ruthenium element that it contains is rare and expensive. Now, however, one of those same scientists has created a new storage material that is cheaper, and is able to store much more energy.

The advantage of thermo-chemical storage in general is that the chemicals can be stored for long periods, without experiencing any energy loss. Suitable chemicals that don't contain the pricey ruthenium, however, have tended to degrade within just a few storage cycles.

MIT associate professor Jeffrey Grossman, who led the research last year, has now developed something better. He and postdoc Alexie Kolpak combined carbon nanotubes with the compound azobenzene, the result being a chemical that is less expensive than fulvalene diruthenium, and that has about 10,000 times the volumetric energy density - in other words, it can store more energy in less space.

Kolpak claims that its energy density is similar to that of a lithium-ion battery. By utilizing different methods of nanofabrication, it is also possible to independently control both how much energy can be stored, and how long it can be stored for.

The system works thanks to the azobenzene-functionalized carbon nanotube molecules, that change in structure when exposed to sunlight, and are capable of staying in that state indefinitely. When a stimulus such as a catalyst substance or a temperature change is applied, however, they revert to their previous form, releasing their stored energy as heat. That heat can be used directly in heating systems, or can be used to generate electricity. The molecules, meanwhile, are ready to be charged again.

"You've got a material that both converts and stores energy," said Grossman. "It's robust, it doesn't degrade, and it's cheap."

The MIT research was recently published in the journal Nano Letters.