As scientists and policymakers around
the world try to combat the increasing rate of climate change, they have
focused on the chief culprit: carbon dioxide.
Produced by the burning of fossil fuels in power plants and car
engines, carbon dioxide continues to accumulate in the atmosphere,
warming the planet. But trees and other plants do slowly capture carbon
dioxide from the atmosphere, converting it to sugars that store energy.
In a new study from the U.S. Department of Energy’s Argonne National
Laboratory and the University of Illinois at Chicago, researchers have
found a similar way to convert carbon dioxide into a usable energy
source using sunlight.
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 Argonne chemist
Larry Curtiss, an author of the study.
To make carbon dioxide into something that could be a usable fuel,
Curtiss and his colleagues needed to find a catalyst – a particular
compound that could make carbon dioxide react more readily. When
converting carbon dioxide from the atmosphere into a sugar, plants use
an organic catalyst called an enzyme; the researchers used a metal
compound called tungsten diselenide, which they fashioned into nanosized
flakes to maximize the surface area and to expose its reactive edges.
While plants use their catalysts to make sugar, the Argonne
researchers used theirs 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 travelling ‘downhill’ energetically,
while trying to create it directly from carbon dioxide means needing to
go ‘uphill,'” said Argonne physicist Peter Zapol, another author of the
study.
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. In the second step, 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, 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.
Towards this goal, 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, “Nanostructured transition metal dichalcogenide
electrocatalysts for CO2 reduction in ionic liquid,” is published in
today’s issue of
Science. Much of the experimental work was
performed at the University of Illinois at Chicago, while the
computational work was performed at Argonne.
