Photosynthetic Solar Cell Converts Atmospheric Carbon Dioxide, Sunlight into Fuel

An international team of researchers and engineers from South Korea and the United States has developed a solar cell that efficiently converts carbon dioxide directly into synthesis gas, using only sunlight for energy.

Photosynthetic Solar Cell Converts Atmospheric Carbon Dioxide, Sunlight into Fuel

Simulated sunlight powers a solar cell that converts atmospheric carbon dioxide directly into syngas. Image credit: University of Illinois at Chicago.
This new work, published in the journal Science , was led by Dr. Larry Curtiss from the Argonne National Laboratory and Dr. Amin Salehi-Khojin from the University of Illinois at Chicago.
Unlike conventional solar cells, the new device essentially does the work of plants, converting atmospheric carbon dioxide into fuel, solving two crucial problems at once.
A solar farm of such cells could remove significant amounts of carbon from the atmosphere and produce energy-dense fuel efficiently.
According to the team, the device delivers synthesis gas (syngas), a mixture of hydrogen gas and carbon monoxide.
Syngas can be burned directly, or converted into diesel or other hydrocarbon fuels.
To make carbon dioxide into something that could be a usable fuel, the researchers needed to find a catalyst – a particular compound that could make carbon dioxide react more readily.
“Chemical reactions that convert carbon dioxide into burnable forms of carbon are called reduction reactions, the opposite of oxidation or combustion,” Dr. Salehi-Khojin said.
“Engineers have been exploring different catalysts to drive carbon dioxide reduction, but so far such reactions have been inefficient and rely on expensive precious metals such as silver.”
The team focused on a family of nano-structured compounds called transition metal dichalcogenides as catalysts, pairing them with an unconventional ionic liquid as the electrolyte inside a two-compartment, three-electrode electrochemical cell.
The best of several catalysts the scientists studied turned out to be tungsten diselenide (WSe2), which they fashioned into nanosized flakes to maximize the surface area and to expose its reactive edges.
“The new catalyst is more active; more able to break carbon dioxide’s chemical bonds,” said Dr. Mohammad Asadi, from the University of Illinois at Chicago.
“In fact, the new catalyst is 1,000 times faster than noble-metal catalysts – and about 20 times cheaper.”
According to the team, the catalyst is also quite durable, lasting for more than 100 hours – a high bar for catalysts to meet.
The team’s device consists of two silicon triple-junction photovoltaic cells of 18 cm2 to harvest light; the tungsten diselenide and ionic liquid co-catalyst system on the cathode side; and cobalt oxide in potassium phosphate electrolyte on the anode side.
When light of 100 W/meter2 – about the average intensity reaching the Earth’s surface – energizes the cell, hydrogen and carbon monoxide gas bubble up from the cathode, while free oxygen and hydrogen ions are produced at the anode.
“The hydrogen ions diffuse through a membrane to the cathode side, to participate in the carbon dioxide reduction reaction,” Dr. Asadi said.
“The technology should be adaptable not only to large-scale use, like solar farms, but also to small-scale applications,” Dr. Salehi-Khojin added.
“In the future, it may prove useful on Mars, whose atmosphere is mostly carbon dioxide, if the planet is also found to have water.”