Researchers have reached another milestone in the search for ways to recycle atmospheric carbon dioxide into carbon-neutral fuel.

In a study published in the Journal of the American Chemical Society, researchers detailed the creation of a molecule that utilizes electricity or light to convert carbon dioxide and produce carbon monoxide, which is a carbon-neutral source of fuel. According to Liang-shi Li and colleagues, the molecule is more efficient compared with other carbon reduction methods available today.

"If you can create an efficient enough molecule for this reaction, it will produce energy that is free and storable in the form of fuels," said Li.

How The Carbon-Reducing Molecule Works

Burning fuel like carbon monoxide releases energy and creates carbon dioxide. To convert the greenhouse gas back into fuel will at least require the same energy amount so what researchers like Li and his team are focusing on is how to keep that energy requirement down as much as possible.

The molecule the researchers developed calls for the least energy amount so far to influence carbon monoxide formation. A complex of nanographene-rhenium connected through the organic compound bipyridine kickstarts a highly efficient reaction that turns carbon dioxide into carbon monoxide.

Essentially, the molecule functions as a two-part system: one, it has a nanographene "energy collector" tasked with absorbing energy from sunlight, and two, it has an atomic rhenium "engine" responsible for producing carbon monoxide. With the energy collector delivering a stream of electrons to it, the rhenium atom continuously binds and converts carbon dioxide into carbon monoxide.

The researchers got the idea to link nanographene and rhenium together from an earlier work by Li where he was focused on creating a more efficient solar cell using a carbon-based material. He said they thought what if the middle man — the solar cell — was cut out and the reaction was purely driven by nanographene's light-absorbing properties.

According to the researchers, the molecule is efficient thanks to its nanographene component, a piece of graphite sized at a nanometer scale. Nanographene is an effective material because its dark color allows for large amounts of sunlight to be absorbed.

Carbon Reduction Research

Complexes of bipyridine and metal, explained Li, have long been the focus of studies looking to convert carbon dioxide back to carbon monoxide using sunlight. However, earlier molecules can only use tiny slivers of light coming mostly from the ultraviolet range. The new molecule the researchers developed, in contrast, effectively harnesses the light-absorbing properties of nanographene, producing a reaction allowing the usage of sunlight in wavelengths of up to 600 nanometers. Light within that spectrum is generally visible.

For future research, Li is looking into powering up the molecule in different ways, such as making it longer-lasting and developing it to survive as non-liquid given catalysts that are solid are more practical to work with in the real world. He is also considering replacing rhenium with manganese as the latter is easily accessible and more affordable.

Supported by the National Science Foundation and Indiana University's Office of the Vice Provost for Research, the study also involved work from Xiaoxiao Qiao, Krishnan Raghavachari, Qiqi Li, Lu Liu, Richard Schaugaard, Dongping Li, Benjamin Noffke, and Yijun Liu.

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