“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.