New solar cell converts atmospheric CO2 into usable energy

A new type of solar cell developed by engineers at the University of Illinois at Chicago (UIC) not only converts sunlight into usable energy but can also recycle atmospheric carbon dioxide, changing it directly into usable hydrocarbon fuel, according to a newly-published study.

As Amin Salehi-Khojin, an assistant professor of mechanical and industrial engineering at UIC, and his colleagues reported in the July 29 issue of the journal Science, their solar cell is capable of turning CO2 into fuel at a cost relatively close to the price of a gallon of gasoline.

“The new solar cell is not photovoltaic – it’s photosynthetic. Instead of producing energy in an unsustainable one-way route from fossil fuels to greenhouse gas, we can now reverse the process and recycle atmospheric carbon into fuel using sunlight,” Salehi-Khojin said in a statement.

These so-called “artificial leaves” essentially due the work of power plants, and could lead to the creation of solar farms capable of efficiently producing fuel while removing significant levels of greenhouse gases from the atmosphere at the same time, the study authors noted. Salehi-Khojin’s team has filed a provisional patent application for their newly-developed technology.

So how does it work?

In traditional solar cells, sunlight is converted into electricity and then stored in batteries for later use. However, in the UIC engineers’ artificial leaf device, atmospheric CO2 is converted directly into a substance known as synthetic gas or syngas. Syngas, which is a mixture of hydrogen gases and carbon monoxide, can either be burned directly or converted into hydrocarbon fuels.

The engineers use a series of chemical reactions known as reduction reactions to convert carbon dioxide into burnable forms of carbon. This process could potentially make the use of fossil fuels obsolete, they explained, provided scientists could find an inexpensive catalyst for them. In most cases, reduction reactions require the use of precious metals and have been relatively inefficient.

Searching for “a new family of chemicals with extraordinary properties,” Salehi-Khojin and his fellow researchers turned their attention to a group of nanostructured compounds called TMDCs (transition metal dichalcogenides). By pairing them with an unconventional ionic liquid that served as the electrolyte inside a two-compartment, three-electrode electrochemical cell, one particular TMDC – nanoflake tungsten diselenide – proved to be a quality catalyst.

This new catalyst, first author and UIC postdoctoral researcher Mohammad Asadi explained, is “more active” and better at “[breaking] carbon dioxide’s chemical bonds” than traditional types of catalysts, such as silver. It is also 20 times less expensive and works up to 1,000 times faster, he added. While other scientists have used TMDC catalysts to produce hydrogen in other ways, this marks the first time they have survived reduction reactions involving CO2.

Building upon their discovery, the UIC team developed a cell made up of  two 18 square cm-long silicon triple-junction photovoltaic cells to capture light, the tungsten diselenide and ionic liquid co-catalyst system on the cathode side, and an anode side that consists of cobalt oxide in potassium phosphate electrolyte. When exposed to light of average intensity (100 watts/square meter), the cell becomes energized, producing hydrogen and carbon monoxide in the cathode while freeing hydrogen ions and oxygen in the anode.

“The results nicely meld experimental and computational studies to obtain new insight into the unique electronic properties of transition metal dichalcogenides,” said Robert McCabe, program director at the National Science Foundation (NSF), which helped fund the project. “The research team has combined this mechanistic insight with some clever electrochemical engineering to make significant progress in one of the grand-challenge areas of catalysis as related to energy conversion and the environment.”

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Image credit: University of Illinois at Chicago/Jenny Fontaine

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