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.


Hydrogen Catalysts Efficient After Twenty-Thousand Cycles

Rice University scientists who want to gain an edge in energy production and storage report they have found it in molybdenum disulfide. The Rice lab of chemist James Tour has turned molybdenum disulfide’s two-dimensional form into a nanoporous film that can catalyze the production of hydrogen or be used for energy storage. The versatile chemical compound classified as a dichalcogenide is inert along its flat sides, but previous studies determined the material’s edges are highly efficient catalysts for hydrogen evolution reaction (HER), a process used in fuel cells to pull hydrogen from water.
Tour and his colleagues have found a cost-effective way to create flexible films of the material that maximize the amount of exposed edge and have potential for a variety of energy-oriented applications. Molybdenum disulfide isn’t quite as flat as graphene, the atom-thick form of pure carbon, because it contains both molybdenum and sulfur atoms. When viewed from above, it looks like graphene, with rows of ordered hexagons.

thin filmThe Rice lab built supercapacitors with thin films; in tests, they retained 90 percent of their capacity after 10,000 charge-discharge cycles and 83 percent after 20,000 cycles.

So much of chemistry occurs at the edges of materials,” said Tour. “A two-dimensional material is like a sheet of paper: a large plain with very little edge. But our material is highly porous. What we see in the images are short, 5- to 6-nanometer planes and a lot of edge, as though the material had bore holes drilled all the way through.”

The Rice research appears in the journal Advanced Materials.