Reducing carbon dioxide (CO₂) to carbon monoxide (CO) is useful for creating CO feedstocks and for lessening atmospheric CO₂, a potent greenhouse gas. This work used data obtained at several U.S. Department of Energy (DOE) user facilities including the Advanced Photon Source (APS) and published in the journal Energy & Environmental Science. It is the first to use nickel (Ni) as a catalyst for the reaction, an advance that will make the process less expensive and more widespread. The process was found to be highly efficient, with >90% CO generation at industry-level current densities.

Removing carbon dioxide from the atmosphere or from effluent from fossil fuel-burning entering the atmosphere is one way to limit climate change. One possible reaction to remove CO₂ is to convert it to CO in an electrochemical carbon dioxide reduction reaction (CO₂RR). In addition to using the CO as feedstocks for further conversion into liquid hydrocarbons, solar and wind energy can be stored in carbon-based fuels and chemicals using the electrochemical CO₂ reduction reaction.

However, it remains a grand challenge for the CO₂ to CO conversion at high selectivity (>90%) with large current densities. CO₂ molecules are very stable; reduction requires a tremendous amount of energy and a catalyst. While precious metals (e.g., gold and silver) currently are the most efficient catalysts known for this reaction, costs make this process impractical on a large scale. The need for catalysts that are stable, abundant, and inexpensive is high, but until recently, identifying practical possibilities has been elusive.

For this paper, the researchers designed a highly efficient CO₂ to CO reduction reaction using a high-performance, atomically dispersed and nitrogen (N) coordinated single Ni site catalyst embedded in partially graphitized carbon. Single Ni sites are highly efficient for CO₂ reduction to CO. The researchers carefully controlled factors that had previously hindered this work, including carbon particle size, Ni content, and Ni-N bond structures and coordination. In addition, previously unknown extrinsic and intrinsic factors governing the performance of catalytic CO₂ electrolysis and selectivity of single Ni site catalysts were studied experimentally and theoretically.

Energy-dispersive x-ray spectroscopy and the corresponding scanning transmission electron microscopy were conducted at the Center for Functional Nanomaterials at Brookhaven National Laboratory, which is a U.S. Department of Energy (DOE) Office of Science user facility. High-resolution scanning transmission electron microscopy and electron energy-loss spectroscopy were conducted at the Center for Nanophase Materials Sciences at at Oak Ridge National Laboratory, also a U.S. DOE Office of Science user facility. X-ray absorption spectroscopy measurements were performed at the APS X-ray Science Division Chemical & Materials Science Group’s beamline 12-BM at the APS, an Office of Science user facility at Argonne National Laboratory.

Compressively straining NiN₄ sites could be used to shorten Ni-N bonds to enhance the CO₂RR activity and the selectivity of the Ni-N-C catalyst (Fig. 1). However, NiN₃ sites formed at higher temperature were more active and CO-selective than NiN₄; this allowed researchers to design a catalyst using NiN₃ sites with increased density. The team also studied how morphological factors, such as how carbon host particle size and Ni loading alter the final catalyst structure and performance.

The intensive engineering of the Ni-N-C catalyst included Ni-N bond structures, extrinsic particle size, and metal content. The implementation of this catalyst in an industrial flow-cell electrolyzer was impressive for CO generation with more than 90% efficiency. This turned out to be one of the best CO₂ to CO catalysts, achieving a current density of CO up to 726 mA cm² with faradaic efficiency of CO above 90%,

This research elucidates how thermal activation temperatures can affect the activity and selectivity of active sites in the CO₂ reduction reaction. Advances in the field include a better understanding of the evolution of the atomically dispersed and nitrogen coordinated single Ni sites from small Ni-groupings during thermal activation, NiN₄ moieties, and NiN₃ behavior at high activation temperatures, including a lessening of N-coordination numbers and an increase in Ni-N bond strain. This new technology could generate CO from CO₂ at an industrial scale using materials that are abundant and inexpensive.   ― Dana Desonie

See: Yi Li^1,2, Nadia Mohd Adli¹, Weitao Shan³**, Maoyu Wang⁴, Michael J. Zachman⁵, Sooyeon Hwang⁶, Hassina Tabassum¹, Stavros Karakalos⁷, Zhenxing Feng⁴***, Guofeng Wang³, Yuguang C. Li¹****, and Gang Wu¹*, “Atomically dispersed single Ni site catalysts for high-efficiency CO2 electroreduction at industrial-level current densities,” Energy Environ. Sci. 15, 2108 (2022). DOI: 10.1039/d2ee00318j

Author affiiations: ¹State University of New York at Buffalo,  ²Jiangsu University, ³University of Pittsburgh, ⁴Oregon State University, ⁵Oak Ridge National Laboratory, ⁶Brookhaven National Laboratory, ⁷University of South Carolina

Correspondence: * [email protected], ** [email protected], *** [email protected], **** [email protected]

G. Wu and G. F. Wang acknowledge the support for a collaborative project from the U.S. National Science Foundation (NSF) (CBET-1804326 and 1804534). Z. Feng thanks the U.S. NSF (NNCI-2025489). Y. Li. thanks the Natural Science Foundation of Jiangsu Province (BK20210769). This research was conducted as part of a user project at the Center for Nanophase Materials Sciences, which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory. This research used resources of the Center for Functional Nanomaterials (CFN), which is a U.S. Department of Energy Office of Science User Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

The U.S. Department of Energy's APS at Argonne National Laboratory is one of the world’s most productive x-ray light source facilities. Each year, the APS provides high-brightness x-ray beams to a diverse community of more than 5,000 researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. Researchers using the APS produce over 2,000 publications each year detailing impactful discoveries, and solve more vital biological protein structures than users of any other x-ray light source research facility. APS x-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being.

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