The current density can be scaled up to more than 50 mA cm−2 with a CO evolution turnover frequency of 2.1 × 10⁵ h⁻¹ while maintaining 97% CO selectivity using an anion membrane electrode assembly.

The anion membrane electrode assembly prevented the direct contact between the catalyst and liquid water, maximally suppressing the H₂ evolution side reaction and facilitating the CO₂ mass transport.

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, a scientist at Brookhaven and co-author

Scientists have long sought a way to convert CO₂ to CO, but traditional electrocatalysts cannot effectively initiate the reaction. 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. To convert CO₂ to CO in a cost-effective way, the researchers used single atoms of nickel instead of noble metal nanoparticles.

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, a Rowland Fellow at Harvard University and the corresponding author

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 explained.

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.

—Klaus Attenkofer, Brookhaven scientist and co-author

In addition to the unique energetic properties of single atoms, the O₂ 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. However, we found the individual nickel atoms were distributed uniformly, which accounted for the excellent performance of the conversion reaction.

—Dong Su, CFN scientist and co-author

To analyze the chemical complexity of the material, the scientists 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 scientists to see a detailed view of the material’s inner 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.

—Haotian 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. For a full list of collaborating institutions and facilities, please see the scientific paper.

Resources

• Kun Jiang, Samira Siahrostami, Tingting Zheng, Yongfeng Hu, Sooyeon Hwang, Eli Stavitski, Yande Peng, James Dynes, Mehash Gangisetty, Dong Su, Klaus Attenkofer and Haotian Wang (2018) “Isolated Ni single atoms in graphene nanosheets for high-performance CO₂ reduction” Energy Environ. Sci., doi: 10.1039/C7EE03245E