Graphene sheets and nickel turn CO2 into usable energy

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.

There are many ways to use CO, says 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. Indeed, scientists have been looking for a way to do just that, but traditional electrocatalysts can't 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, But these 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, says 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 prefer to produce CO, rather than performing the competing HER, because the surface of a bulk metal is very different from individual atoms, Stavitski says. Another Brookhaven scientist and a co-author on the paper, adds, 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 conversation 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.

To get a closer look at the individual nickel atoms within the atomically thin graphene sheet, the scientists used scanning transmission electron microscopy (STEM) at Brookhaven’s Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility. By scanning an electron probe over the sample, the scientists were able to visualize discrete nickel atoms on the graphene.

Single atoms are usually unstable and tend to aggregate on the support, adds Dong Su, also a CFN scientist and a co-author on the paper. However, we found the individual nickel atoms were distributed uniformly, which accounted for the excellent performance of the conversion reaction.

Photons, or particles of light, interact with the electrons in the nickel atoms to do two things, Stavitski says. They send the electrons to higher energy states and, by mapping those energy states, we can understand the electronic configuration and the chemical state of the material. As we increase the energy of the photons, they kick the electrons off the atoms and interact with the neighboring elements. In essence, this provided the scientists with an image of the nickel atoms’ local structure.

Based on the results from the studies at Harvard, NSLS-II, CFN, and additional institutions, the scientists discovered single nickel atoms catalyzed the CO₂ conversion reaction with a maximal of 97% efficiency. The scientists say this is a major step toward recycling CO₂ for usable energy and chemicals.

To apply this technology to real applications in the future, we are currently aimed at producing this single atom catalyst in a cheap and large-scale way, while improving its performance and maintaining its efficiency, says Wang.

This study was supported in part by the Rowland Institute at Harvard University. Operations at CFN and NSLS-II are supported by DOE’s Office of Science.