Researchers Develop New Method to Transform Carbon Dioxide Back into Fuel

Scientists and policymakers across the world are working out ways to deal with the growing rate of climate change and have pinpointed the key cause to be carbon dioxide (CO₂).

CO₂ in the atmosphere keeps increasing as fossil fuels continue to be burnt in car engines and power plants. This, in turn, leads to the global warming.

Trees and other plants, however, do slowly absorb CO₂ from the atmosphere and convert it to sugars that store energy. In a new research conducted by the U.S. Department of Energy's Argonne National Laboratory and the University of Illinois at Chicago, the researchers discovered a similar method to convert CO₂ into an exploitable energy source with sunlight.

One of the major challenges of capturing CO₂ is that it is comparatively chemically unreactive. "On its own, it is quite difficult to convert carbon dioxide into something else," said Argonne chemist Larry Curtiss, an author of the research paper.

In order to convert CO₂ into something that can be used as a fuel, Curtiss, and his colleagues had to find a catalyst — a specific compound that would enable CO₂ to react more readily.

Plants use an organic catalyst referred to as an enzyme while converting CO₂ from the atmosphere into a sugar. The researchers used a metal compound known as tungsten diselenide, which they designed into nano-sized flakes to increase the surface area and to expose its reactive edges.

The Argonne team used their catalyst to convert CO₂ to carbon monoxide (CO). CO is also a greenhouse gas, but compared to CO₂ it is more reactive, and scientists already are aware of methods to convert CO into usable fuel, such as methanol.

Making fuel from carbon monoxide means traveling 'downhill' energetically while trying to create it directly from carbon dioxide means needing to go 'uphill,'

Argonne physicist Peter Zapol, another author of the study.

Though the reaction to convert CO₂ into CO varies from anything present in nature, it still needs the same fundamental inputs as photosynthesis.

"In photosynthesis, trees need energy from light, water and carbon dioxide in order to make their fuel; in our experiment, the ingredients are the same, but the product is different," said Curtiss.

The process for the reaction is quite similar to nature that the team was able to build an "artificial leaf" that could fulfill the total three-step reaction pathway. In the first step, incoming photons or packets of light are converted to pairs of negatively-charged electrons and equivalent positively charged "holes" that then split from each other.

In the following step, the holes react with water molecules, producing oxygen molecules and protons. In the final step, the CO₂, protons, and electrons all react together to generate CO and water.

We burn so many different kinds of hydrocarbons — like coal, oil or gasoline — that finding an economical way to make chemical fuels more reusable with the help of sunlight might have a big impact,

Zapol

The research also revealed that the reaction takes place with minimal lost energy — the reaction is highly efficient. "The less efficient a reaction is, the higher the energy cost to recycle carbon dioxide, so having an efficient reaction is crucial," Zapol said.

According to Curtiss, the tungsten diselenide catalyst is also fairly durable, and can last for over 100 hours — which is a high achievement for catalysts to meet.

The study, "Nanostructured transition metal dichalcogenide electrocatalysts for a CO₂ reduction in ionic liquid," has been published in Science. Most of the experimental work was conducted at the University of Illinois at Chicago, while the computational work was done at Argonne.

The U.S. Department of Energy's Office of Science and the National Science Foundation funded the research.