How To Convert CO2 In Energy

Imagine if carbon dioxide (CO2) could easily be converted into usable energy. Every time you breathe or drive a motor vehicle, you would produce a key ingredient for generating fuels. Like photosynthesis in plants, we could turn CO2 into molecules that are essential for day-to-day life. Now, scientists are one step closer. Researchers at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory are part of a scientific collaboration that has identified a new electrocatalyst that efficiently converts CO2 to carbon monoxide (CO), a highly energetic molecule. Their findings have been published  in Energy & Environmental Science.

Brookhaven scientists are pictured at NSLS-II beamline 8-ID, where they used ultra-bright x-ray light to “see” the chemical complexity of a new catalytic material. 

There are many ways to use CO,” said Eli Stavitski, a scientist at Brookhaven and an author on the paper. “You can react it with water to produce energy-rich hydrogen gas, or with hydrogen to produce useful chemicals, such as hydrocarbons or alcohols. If there were a sustainable, cost-efficient route to transform CO2 to CO, it would benefit society greatly.

Scientists have long sought a way to convert CO2 to CO, but traditional electrocatalysts cannot effectively initiate the reaction. That’s because a competing reaction, called the hydrogen evolution reaction (HER) or “water splitting,” takes precedence over the CO2 conversion reaction. A few noble metals, such as gold and platinum, can avoid HER and convert CO2 to CO; however, these metals are relatively rare and too expensive to serve as cost-efficient catalysts. So, to convert CO2 to CO in a cost-effective way, scientists used an entirely new form of catalyst. Instead of noble metal nanoparticles, they used single atoms of nickel.

Nickel metal, in bulk, has rarely been selected as a promising candidate for converting CO2 to CO,” said Haotian Wang, a Rowland Fellow at Harvard University and the corresponding author on the paper. “One reason is that it performs HER very well, and brings down the CO2 reduction selectivity dramatically. Another reason is because its surface can be easily poisoned by CO molecules if any are produced.”

Single atoms of nickel, however, produce a different result. “Single atoms prefer to produce CO, rather than performing the competing HER, because the surface of a bulk metal is very different from individual atoms,” Stavitski said. Klaus Attenkofer, also a Brookhaven scientist and a co-author on the paper, added, “The surface of a metal has one energy potential—it is uniform. Whereas on a single atom, every place on the surface has a different kind of energy.”

In addition to the unique energetic properties of single atoms, the CO2 conversion reaction was facilitated by the interaction of the nickel atoms with a surrounding sheet of graphene. Anchoring the atoms to graphene enabled the scientists to tune the catalyst and suppress HER.


How To Convert 90% Of Water Into Hydrogen

Researchers from North Carolina State University (NC State) have significantly boosted the efficiency of two techniques, for splitting water to create hydrogen gas and splitting carbon dioxide (CO2) to create carbon monoxide (CO). The products are valuable feedstock for clean energy and chemical manufacturing applications. The water-splitting process successfully converts 90 percent of water into hydrogen gas, while the CO2-splitting process converts more than 98 percent of the CO2 into CO. In addition, the process also uses the resulting oxygen to convert methane into syngas, which is itself a feedstock used to make fuels and other products.

These advances are made possible by materials that we specifically designed to have the desired thermodynamic properties for each process,” says Fanxing Li, an associate professor of chemical and biomolecular engineering at NC State who is corresponding author of two papers on the work. “These properties had not been reported before unless you used rare earth materials.”

For the CO2-splitting process, researchers developed a nanocomposite of strontium ferrite dispersed in a chemically inert matrix of calcium oxide or manganese oxide. As CO2 is run over a packed bed of particles composed of the nanocomposite, the nanocomposite material splits the CO2 and captures one of the oxygen atoms. This reduces the CO2, leaving only CO behind.

Previous CO2 conversion techniques have not been very efficient, converting well below 90 percent of the CO2 into CO,” Li says. “We reached conversion rates as high as 99 percent. “And CO is valuable because it can be used to make a variety of chemical products, including everything from polymers to acetic acid,” Li adds.

Meanwhile, the oxygen captured during the CO2-splitting process is combined with methane and converted into syngas using solar energy.


How To Recycle Carbon Dioxide

An international team of scientists led by Liang-shi Li at Indiana University (IU) has achieved a new milestone in the quest to recycle carbon dioxide in the Earth’s atmosphere into carbon-neutral fuels and others materials.


The chemists have engineered a molecule that uses light or electricity to convert the greenhouse gas carbon dioxide into carbon monoxide — a carbon-neutral fuel source — more efficiently than any other method of “carbon reduction.”

molecular leaf

If you can create an efficient enough molecule for this reaction, it will produce energy that is free and storable in the form of fuels,” said Li, associate professor in the IU Bloomington College of Arts and Sciences‘ Department of Chemistry. “This study is a major leap in that direction.”

Burning fuel — such as carbon monoxide — produces carbon dioxide and releases energy. Turning carbon dioxide back into fuel requires at least the same amount of energy. A major goal among scientists has been decreasing the excess energy needed.

This is exactly what Li’s molecule achieves: requiring the least amount of energy reported thus far to drive the formation of carbon monoxide. The molecule — a nanographene-rhenium complex connected via an organic compound known as bipyridine — triggers a highly efficient reaction that converts carbon dioxide to carbon monoxide. The ability to efficiently and exclusively create carbon monoxide is significant due to the molecule’s versatility.

Carbon monoxide is an important raw material in a lot of industrial processes,” Li said. “It’s also a way to store energy as a carbon-neutral fuel since you’re not putting any more carbon back into the atmosphere than you already removed. You’re simply re-releasing the solar power you used to make it.

The secret to the molecule’s efficiency is nanographene — a nanometer-scale piece of graphite, a common form of carbon (i.e. the black “lead” in pencils) — because the material’s dark color absorbs a large amount of sunlight.

Li said that bipyridine-metal complexes have long been studied to reduce carbon dioxide to carbon monoxide with sunlight. But these molecules can use only a tiny sliver of the light in sunlight, primarily in the ultraviolet range, which is invisible to the naked eye. In contrast, the molecule developed at IU takes advantage of the light-absorbing power of nanographene to create a reaction that uses sunlight in the wavelength up to 600 nanometers — a large portion of the visible light spectrum.

Essentially, Li said, the molecule acts as a two-part system: a nanographeneenergy collector” that absorbs energy from sunlight and an atomic rheniumengine” that produces carbon monoxide. The energy collector drives a flow of electrons to the rhenium atom, which repeatedly binds and converts the normally stable carbon dioxide to carbon monoxide.

The idea to link nanographene to the metal arose from Li’s earlier efforts to create a more efficient solar cell with the carbon-based material. “We asked ourselves: Could we cut out the middle man — solar cells — and use the light-absorbing quality of nanographene alone to drive the reaction?” he said.

Next, Li plans to make the molecule more powerful, including making it last longer and survive in a non-liquid form, since solid catalysts are easier to use in the real world.

The process is reported in the Journal of the American Chemical Society.