(Nanowerk Spotlight) Electrochemical conversion of carbon dioxide to carbon monoxide represents an effective approach to both reduce greenhouse gas emissions and produce valuable chemical feedstocks. While researchers have pursued this technology for decades, they’ve faced a persistent challenge: finding catalysts that can both activate the stable carbon dioxide (CO2) molecule and readily release the resulting carbon monoxide (CO) product.
Noble metals like gold and silver perform this conversion well but remain too expensive for widespread use. Earth-abundant metals like nickel could offer cost-effective alternatives, but they typically bind CO too strongly, causing catalyst deactivation. This occurs because the same electronic properties that help nickel activate CO2 also create strong π-backbonding with CO, preventing efficient catalytic cycling.
Previous attempts to resolve this trade-off have used nitrogen-coordinated metal sites with limited success. Competing side reactions, particularly hydrogen evolution, further complicate the development of efficient CO2 conversion systems.
Researchers have now engineered a highly efficient nickel catalyst that overcomes these limitations. As reported in Advanced Energy Materials (“Low-Coordinated Ni Single Atom Catalyst with Carbon
Coordination for Efficient CO2 Electroreduction”), a team led by Wenli Sun and Shilong Liu has developed a low-coordinated nickel single atom catalyst (L-Ni-NC-C) that achieves remarkable CO2-to-CO conversion performance.
Schematic illustration of the synthetic route and model of L─Ni─NC─C. The Ni-ZIF-8 polyhedron was selected as the precursor, carbonized in N2 atmosphere, and then NH3 thermal treatment process for 1 h to prepare L─Ni─NC─C. The CO2RR process enables the conversion of CO2 to CO by providing a comprehensive elucidation of the reaction mechanism, wherein L─Ni─NC─C catalyst can effectively promote the formation and adsorption of *COOH intermediates, followed by the enhanced desorption of *CO products. (Image: Reprinted with permission by Wiley-VCH Verlag)
The catalyst features individual nickel atoms embedded in a carbon matrix with a unique coordination structure – each nickel atom bonds with only two carbon atoms and one nitrogen atom rather than the conventional four-atom coordination. This structure creates moderate electron depletion at the nickel sites, effectively balancing the competing electronic requirements of the reaction.
“High-valence nickel sites lack sufficient electrons for CO2 activation, while metallic nickel binds CO too strongly,” the researchers explain. “Our low-coordinated structure with carbon coordination engineers the electronic properties to overcome this limitation.”
The researchers created their catalyst through a two-step synthesis approach using nickel-doped ZIF-8 metal-organic frameworks. After controlled heat treatment under nitrogen, they performed ammonia thermal treatment, which selectively removed nitrogen atoms surrounding the nickel sites while establishing carbon coordination bonds.
Advanced spectroscopic characterization confirmed the atomic dispersion of nickel and its unique coordination environment. X-ray absorption spectroscopy revealed that nickel atoms existed in a lower oxidation state than typical nitrogen-coordinated sites, consistent with carbon coordination.
The catalyst demonstrated exceptional performance in electrochemical testing. In a standard H-cell configuration, it achieved 99.1% selectivity for CO production at -0.7 volts versus the reversible hydrogen electrode. (Selectivity, or Faradaic efficiency, refers to the proportion of electrical current used to produce the desired chemical product.) It maintained high selectivity (>97%) across a wide potential range from -0.5 to -1.1 V.
More impressively, when tested in a gas-diffusion flow cell at industrial-level current densities, the catalyst maintained CO selectivity exceeding 99% from 50 to 400 mA cm⁻². It operated stably at 250 mA cm⁻² for 20 hours with minimal degradation. In neutral electrolyte (1 M KHCO3), the catalyst achieved over 99% selectivity across this entire current density range, outperforming its performance in alkaline conditions.
To understand this performance, the researchers used in situ infrared spectroscopy to monitor reaction intermediates in real-time. This revealed that L-Ni-NC-C effectively stabilized the *COOH intermediate (critical for CO2 activation) while showing minimal binding to the CO product.
Complementary computational studies validated these findings. Density functional theory calculations showed that the unique coordination environment created optimal binding energies for both *COOH formation and CO release. The low-coordinated structure positioned the nickel d-band center at an energy level that allowed sufficient electron transfer for CO2 activation while preventing excessive backdonation that would strengthen CO binding.
This catalyst design marks a significant departure from conventional approaches. While nitrogen-coordinated metal sites have been standard, this work demonstrates that low-coordinated structures with carbon coordination offer a more effective strategy for balancing the competing requirements of CO2 electroreduction.
The researchers also conducted control experiments using nickel catalysts with conventional nitrogen coordination and nickel nanoparticles. These tests confirmed that the superior performance of L-Ni-NC-C stemmed from its unique low-coordinated structure rather than other factors such as nickel content.
By achieving nearly perfect selectivity at industrial-level current densities, this technology could enable more efficient carbon utilization systems. Carbon monoxide serves as a valuable feedstock in numerous industrial applications, including Fischer-Tropsch synthesis for producing liquid fuels. The ability to efficiently produce CO from CO2 using renewable electricity represents a promising approach to recycling carbon emissions from industrial processes.
The engineering of low-coordinated structures with carbon coordination may also find applications in other electrochemical reactions with similar electronic tuning challenges. This precise control of coordination environments demonstrates how atomic-level catalyst design can overcome previously intractable trade-offs in electrocatalysis.
Get our Nanotechnology Spotlight updates to your inbox!
Thank you!
You have successfully joined our subscriber list.
Become a Spotlight guest author! Join our large and growing group of guest contributors. Have you just published a scientific paper or have other exciting developments to share with the nanotechnology community? Here is how to publish on nanowerk.com.
