Environment Technology

Molecular ‘Leaf’ can Collect and Store Solar Power without Solar Panels

sun as fuel

A new breakthrough in the search to recycle carbon dioxide in the Earth’s atmosphere into carbon neutral fuels and others materials has been achieved. The international team of chemists responsible was led by Liang-shi Li from the University of Indiana.

The researchers have designed a molecule that uses electricity or light to convert carbon dioxide into carbon monoxide more efficiently than any other method of reducing carbon can. Carbon dioxide is a greenhouse gas, while carbon monoxide is a carbon neutral fuel source.

Li, an associate professor in the IU Bloomington College of Arts and Sciences’ Department of Chemistry, noted that if an efficient enough molecule for this reaction can be created, it would produce energy that is free and can be stored in the form of fuels. This study is a major advance in that direction.

When fuel such as carbon monoxide is burned, it produces carbon dioxide and releases energy. Converting carbon dioxide back into fuel requires at least the same quantity of energy. Scientists have been trying to find ways to decrease the excess energy needed.

Li’s molecule achieves exactly that. It requires the smallest amount of energy reported thus far to drive the creation of carbon monoxide. The molecule is a nanographene rhenium complex connected through an organic compound known as bipyridine and it activates a highly efficient reaction that changes carbon dioxide into carbon monoxide.

The ability to create carbon monoxide exclusively and efficiently is significant due to the molecule’s versatility.

molecular leaf
The new molecule occupies a nanographene complex (on left) to absorb light and drive the conversion of carbon dioxide (upper center) to carbon monoxide (on right). (Image Credit: Ben Noffke and Richard Schaugaard)

Li explained that carbon monoxide is an important raw material in many industrial processes. It can also be used to store energy as a carbon neutral fuel, since no more carbon is released back into the atmosphere than what had been removed. The solar power that was used to make it is simply re-released.

Nanographene is a nanometer scale piece of graphite, a common form of carbon, and this is the secret to the molecule’s efficiency. As the material has a dark color, it absorbs a large amount of sunlight.

Li added that bipyridine metal complexes have been studied to reduce carbon dioxide to carbon monoxide with sunlight for a long time. These molecules can however use only a tiny shard of the light in sunlight, mainly in the ultraviolet range. This portion is invisible to the naked eye. The molecule developed at IU however utilizes the light absorbing power of nanographene to create a reaction that uses sunlight in the wavelength up to 600 nanometers. This means that a large portion of the visible light spectrum is used.

The molecule essentially acts as a two-part system comprising an atomic rhenium “engine” that produces carbon monoxide and a nanographene “energy collector” that absorbs energy from sunlight. The energy collector directs a flow of electrons to the rhenium atom, which then repeatedly binds and converts the carbon dioxide that is normally stable, to carbon monoxide.

Li’s earlier efforts to create a more efficient solar cell with the carbon based material led to the idea to link nanographene to the metal when they explored whether they could cut out the solar cells and use the light-absorbing quality of nanographene alone to drive the reaction.

Li plans to make the molecule more powerful next.  This would include making it last longer and able to survive in a non-liquid form, as in the real world, solid catalysts are easier to use. As rhenium is a rare element and therefore expensive, Li is working on replacing it with manganese, a more common and less expensive metal.

This study is published in the Journal of the American Chemical Society.