Scientists have found a way to convert carbon dioxide into a usable energy source with the help of sunlight, just like plants convert the greenhouse gas from the atmosphere into sugars that store energy. One of the chief challenges of sequestering carbon dioxide is that it is relatively chemically unreactive.
"On its own, it is quite difficult to convert carbon dioxide into something else," said Larry Curtiss, from the US Department of Energy's Argonne National Laboratory.
Researchers used catalysts to convert carbon dioxide to carbon monoxide. Although carbon monoxide is also a greenhouse gas, it is much more reactive than carbon dioxide and scientists already have ways of converting carbon monoxide 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,'" said Peter Zapol, from the Argonne National Laboratory. Although the reaction to transform carbon dioxide into carbon monoxide is different from anything found in nature, it requires the same basic 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 setup for the reaction is sufficiently similar to nature that the research team was able to construct an "artificial leaf" that could complete the entire three-step reaction pathway.
In the first step, incoming photons, packets of light, are converted to pairs of negatively-charged electrons and corresponding positively-charged "holes" that then separate from each other. Then, the holes react with water molecules, creating protons and oxygen molecules. Finally, the protons, electrons and carbon dioxide all react together to create carbon monoxide and water.
"We burn so many different kinds of hydrocarbons, like coal or 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 said.
The study also showed that the reaction occurs with minimal lost energy; the reaction is very 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 quite durable, lasting for more than 100 hours, a high bar for catalysts to meet. The study appears in the journal Science.