New System Uses Renewable Electricity to Transform CO2 into CO

Everyday, over 2 million pounds of carbon dioxide are released into the atmosphere from factories, the burning of natural gas for the generation of electricity, and emissions from trucks and cars.

Even though it is a cause for environmental concern for many, Haotian Wang, a Fellow at the Rowland Institute at Harvard University, finds it the perfect raw material.

Wang, along with his research team, has developed a system that makes use of renewable electricity to convert carbon dioxide (CO2) into carbon monoxide (CO), which is a primary product used in various industrial processes, through an electrochemical process. The efficiency of converting energy from sunlight to CO can be as high as 12.7%, which is more than one order of magnitude higher than natural photosynthesis. The description of this device is given in a recent paper published in Chem.

Basically, what this is is a form of artificial photosynthesis, in a plant, sunlight, CO2 and water become sugar and oxygen. In our system, the input is sunlight, CO2 and water, and we produce CO and oxygen.

Haotian Wang, a Fellow at the Rowland Institute at Harvard University

The aforementioned reaction occurs in a modest-looking device, just about the size of a smartphone, containing two chambers filled with electrolyte and separated by an ion exchange membrane.

On one site, water molecules are oxidized into oxygen gas and protons are released by an electrode powered by renewable energy. The protons, thus produced, move to the other chamber where - by means of a carefully designed metal single atom catalyst – they combine to form carbon dioxide molecules, producing carbon monoxide and water.

The challenge is that most catalysts that are known tend to produce hydrogen gas, so it's difficult, when you split water, to prevent those protons from combining together to form hydrogen gas. What we needed was a catalyst that can prevent hydrogen evolution and instead can efficiently inject those protons into CO2, therefore achieving a high selectivity for CO2 reduction.

Haotian Wang, a Fellow at the Rowland Institute at Harvard University

However, gold and silver are two of the best-known catalysts - costly precious metals which make it difficult to make the reaction cost-effective on a large scale.

"So we began by looking at low-cost materials like nickel, iron and cobalt, which are all Earth-abundant," Kun Jiang said, the first author of this work and a postdoctoral fellow in Wang group. "But the problem is that they are all very good hydrogen catalysts, so they want to produce hydrogen gas.

In addition, they can all very easily be poisoned by carbon monoxide," he added. "Even if you manage to use them to reduce CO2, the resulting CO bonds very strongly to the surface, preventing any further reactions from taking place."

To resolve those issues, Wang and his Stanford collaborators, Prof. Jens Nørskov and Prof. Yi Cui, began working to "tune" the metals’ electronic properties. Dr. Samira Siahrostami, a staff scientist from Prof. Nørskov group, then rationalized the nature of active sites through atomic scale modeling and found that when nickel metals are dispersed into isolated single atoms, which are confined in graphene vacancies, a material is produced that was highly reactive with carbon dioxide and capable of releasing the resulting carbon monoxide.

According to Wang, that carbon monoxide can then be used in a wide range of industrial processes.

"Carbon monoxide is a very important industry product," Wang said. "It can be used in plastics production, to make hydrocarbon products or can be burned as a fuel itself. It's widely used in industry."

Eventually, it is hoped that going forward this system could possibly be scaled up to remove carbon dioxide from the atmosphere in an effort to fight global climate change.

"The basic idea was if we can capture existing CO2 and use renewable electricity, from solar or wind power, to reduce it into useful chemicals," Wang said, "then we can possibly form a carbon loop."

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