Visualize if carbon dioxide (CO2) could easily be transformed into usable energy. Every time one drives a motor vehicle or breathes, a key component for producing fuels will be produced. Similar to photosynthesis in plants, CO2 could be converted into molecules that are vital for day-to-day life. Now, researchers are one step closer.
A team of researchers at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory are part of a scientific partnership that has identified a new electrocatalyst that efficiently changes CO2 to carbon monoxide (CO), an extremely energetic molecule. Their findings were reported in Energy & Environmental Science on February 1.
There are many ways to use CO. 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.
Eli Stavitski, Author & Scientist at Brookhaven
For a long time, researchers have pursued a way to convert CO2 to CO, but traditional electrocatalysts cannot successfully initiate the reaction. That is due to a contending reaction, called the hydrogen evolution reaction (HER) or “water splitting,” takes precedence over the CO2 conversion reaction.
Some noble metals, such as platinum and gold, can avoid HER and change CO2 to CO; however, these metals are comparatively rare and very expensive to serve as cost-efficient catalysts. Therefore, to turn CO2 to CO in a cost-effective manner, researchers used a totally new form of catalyst. Rather than noble metal nanoparticles, they made use of single atoms of nickel.
Nickel metal, in bulk, has rarely been selected as a promising candidate for converting CO2 to CO. 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.
Haotian Wang, Author & Rowland Fellow at Harvard University
Single atoms of nickel, however, deliver 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.”
Besides the unique energetic properties of single atoms, the CO2 conversation reaction was enabled by the interaction of the nickel atoms with a surrounding sheet of graphene. Anchoring the atoms to graphene allowed the researchers to adjust the catalyst and suppress HER.
To obtain a closer look at the single nickel atoms within the atomically thin graphene sheet, the researchers 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 team was able to picture discrete nickel atoms on the graphene.
“Our state-of-art transmission electron microscope is a unique tool to see extremely tiny features, such as single atoms,” said Sooyeon Hwang, a scientist at CFN and a co-author on the paper.
“Single atoms are usually unstable and tend to aggregate on the support,” added 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.”
To examine the chemical complexity of the material, the researchers used beamline 8-ID at the National Synchrotron Light Source II (NSLS-II)—also a DOE Office of Science User Facility at Brookhaven Lab. The ultra-bright X-ray light at NSLS-II enabled the researchers to “see” a complete view of the material’s inner structure.
Photons, or particles of light, interact with the electrons in the nickel atoms to do two things. 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.
Eli Stavitski, Author & Scientist at Brookhaven
This provided the team with an image of the nickel atoms’ local structure.
Based on the findings from the studies at Harvard, NSLS-II, CFN, and other institutions, the team discovered single nickel atoms catalyzed the CO2 conversion reaction with a maximal of 97% efficacy. The researchers say this is a key 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,” said Wang.
This research was supported partly by the Rowland Institute at Harvard University. Operations at CFN and NSLS-II are supported by DOE’s Office of Science.