Did UIC just save the world?

Videography by Vibhu Rangavasan

Civilizations are powered by the burning of fossil fuels. Humans are adding tons of carbon dioxide to the Earth’s atmosphere, causing climate change and global warming.

But what if instead of drilling carbon out of the ground and releasing it into the air, we could recycle carbon out of the air and produce fuel that could be burned — again and again? That’s essentially what plants do, using the power of the sun.

Researchers in the College of Engineering have created a potentially game-changing solar cell that works like an artificial leaf, cheaply and efficiently converting carbon dioxide from the air directly into burnable hydrocarbon fuel, using only sunlight for energy. Their finding is reported in the July 29 issue of Science and was funded by the National Science Foundation and the U.S. Department of Energy. A provisional patent application has been filed.

Unlike conventional solar cells, which convert sunlight into electricity that must be stored in heavy batteries, the new device essentially does the work of plants, converting carbon dioxide from the air into burnable fuel, solving two crucial problems at once. A solar farm of such artificial leaves could remove carbon from the atmosphere and produce energy-dense fuel efficiently.

“The new solar cell is not photovoltaic — it’s photosynthetic,” said Amin Salehi-Khojin, assistant professor of mechanical and industrial engineering at UIC and senior author on the study. Like the photosynthesis that takes place inside the leaves of a plant, the artificial leaf captures the energy of the sun as burnable fuel and closes the recycling loop on atmospheric carbon.

“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.

While plants produce fuel in the form of sugar, the artificial leaf delivers syngas, or synthesis gas, a mixture of hydrogen gas and carbon monoxide. Syngas can be burned directly, or converted easily into diesel or other hydrocarbon fuels. The ability to turn CO2 from the air into fuel at a cost comparable to a gallon of gasoline would render fossil fuels obsolete.

The UIC artificial-leaf solar cell represents a breakthrough in chemistry.

Chemical reactions that convert CO2 into burnable forms of carbon are called reduction reactions, the reverse of oxidation or combustion. These chemical reactions are inefficient and have required a catalyst made of expensive precious metal, such as silver, Salehi-Khojin said.

“What we needed was a new family of chemicals with extraordinary properties,” he said.

He and his coworkers focused on a related group of nanomaterials to serve as catalysts. The best among several they studied turned out to be nanoflake tungsten diselenide.

“The new catalyst is more active; more able to break carbon dioxide’s chemical bonds,” said UIC postdoctoral researcher Mohammad Asadi, first author on the Science paper.

In fact, he said, the new catalyst is 1,000 times faster than precious-metal catalysts — and about 20 times cheaper.

Powering the catalyzed reduction reaction inside the UIC artificial leaf are two silicon photovoltaic cells of 18 square centimeters that harvest light energy. When light of 100 watts per square meter — about the average intensity reaching the Earth’s surface — energizes the cell, hydrogen and carbon monoxide gas bubble up from one electrode, while free oxygen and hydrogen ions are produced at the opposite electrode. The hydrogen ions then diffuse through a membrane to the other side, to join in the carbon dioxide reduction reaction, Asadi said.

The new technology should be adaptable not only to large-scale use, like solar farms, but also to small-scale applications, Salehi-Khojin said. In the future, he said, it may prove useful on Mars, whose atmosphere is mostly carbon dioxide, if the planet is also found to have water.

Co-authors from UIC’s mechanical and industrial engineering department, who prepared the catalyst and performed the solar cell experiments are Kibum Kim, Aditya Venkata Addepalli, Pedram Abbasi, Poya Yasaei, Amirhossein Behranginia, Bijandra Kumar and Jeremiah Abiade. Robert F. Klie and Patrick Phillips of UIC’s physics department performed electron microscopy and spectroscopy experiments. Other co-authors are Richard Haasch of the Urbana-Champaign campus; Larry A. Curtiss, Cong Liu and Peter Zapol of Argonne National Laboratory; and José M. Cerrato of the University of New Mexico.