New Solar Devices on the Horizon

Scientists have been working on solar energy projects for a long time. In fact, they have been working on a way to store solar energy that responds to changes in light since the 1970s. These molecules, if they could be created, would be the perfect solar fuel as they would be flexible, cost-effective and reusable. Despite the years of hard work, scientists have not seen much success on these fronts.

Perhaps the closest they have been was pretty recently when they began using tetracarbonly-diruthenium, but it proved to be too expensive. Plus, the compound turned out to have a volumetric energy density that was much, much smaller than a typical lithium-ion battery.

Alexie Kolpak and Jeffrey Grossman, both scientists from the Massachusetts Institute of Technology, have talked about a new kind of solar fuel that would be cost-effective, reusable and even more energy dense than standard lithium-ion batteries. The fuel that they talk about would be combine azobenzene with carbon nanotube.

How Photoactive Molecules Store Solar Energy

When a photoactive molecule takes in sunlight, it experiences a conformational change that lifts it from its ground energy state to a higher energy state. The higher energy state is temporarily stable but could move back down due to energy loss; any significant trigger like voltage, heat, etc. could cause the molecule to go back to its ground state. The difference in energy between the higher energy state and the ground state is then released. A good photoactive molecule can go through various cycles of charging and discharging.

What makes producing a solar thermal fuel so complicated is that one must find a material that has a large change in energy between its states and large activation energy. Making this even more difficult is the fact that these two things don’t always coincide. You also want to make sure that the higher energy state doesn’t have too much energy, less it be unstable. A molecule is considered instable when the fuel has a small activation energy and normally discharges its stored energy very easily.

Kolpak and Grossman have been able to find the perfect balance between the change in energy states and activation energy when they took a look at computational models of azobenzene tied together to carbon nanotubes in azo/CNT nanostructures. They have calculated that placing azobenzene on carbon nanotubes can stablize both the ground and higher up energy states. Plus, stabilizing the higher energy states means that the activation energy is significant enough to give the azo/CNT material a half life of more than a year.

The azo/CNT materials also outperform typical lithium-ion batteries in their energy storage capacity. The team estimates that the material will have a volumetric energy density of around 690 watt-hours per liter while regular lithium-ion batteries are somewhere between 200 and 600 watt-hours per liter.

Drawbacks

Though their results are promising, the team still faces a bit of adversity. Not only has their substance not actually been created yet, but the energy that it stores can only be released as heat. So in order to use the heat as electricity it must first be converted which will result in even more energy loss.

ars technica Photo by : National Science Foundation