Based on ligand-mediated strategies, Changchun Institute of Chemistry, Chinese Academy of Sciences synthesized Ni diatomic catalysts

On August 18, 2022, Zhang Xinbo’s team of researchers from the Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences published a research report entitled “Nickel dual-atom sites for electrochemical carbon dioxide reduction” at Nature Synthesis.

The results report a Ni diatomic catalyst based on ligand-mediated strategy synthesis, which can be extended to the synthesis of other transition metal (Pd, Mn, Zn) diatomic catalysts. The research group clarified the formation mechanism of the diatomic site by combining in situ environmental electron microscopy and first-principles calculations. The catalyst exhibits ultra-high electrocatalytic activity through the promotion of the local microenvironment in electrocatalytic CO2 reduction.

Electrocatalytic carbon dioxide reduction (CO2RR) to produce high value-added fuels and chemicals offers a promising way to address global warming, mitigate the energy crisis, and achieve carbon neutrality. Electrochemical synthesis of carbon monoxide (CO) is currently a more feasible way to achieve efficient CO2 value, as CO can further synthesize higher-value products (methane, methanol, diesel, etc.) in industrial Fischer-Tropsch synthesis. Although single-atom catalysts (SACs) have made considerable progress in electrocatalytic CO2-CO, the problem of slow CO2RR kinetics remains unresolved. Diatomic catalysts (DACs) act as bridges between SACs and traditional bulk/nanoparticle/cluster catalysts, and can overcome the limitations of a single isolated site by synergistically modulating the activation of carbon dioxide and the formation/desorption of intermediate products, enabling rapid reaction kinetics. However, the limitations of synthetic methods and the lack of deep basic research on diatomic loci hinder the development of DACs in efficient CO2RR.

Recently, a team of researchers Zhang Xinbo from the Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences synthesized a Ni DAC with a Ni2N6 coordination structure. Ni atoms form the Ni2N6 site by combining Austewald maturation and atomization. In this work, Ni DAC exhibits nearly 100% CO selectivity in electrocatalytic CO2-CO as well as ampere-level diffar current density. The research group used molecular dynamics simulation to demonstrate the great role of the local microenvironment regulated by OH- adsorbed by the diatomic site in the catalytic process in improving the reaction kinetics, which provided strong support for the experimental results.

Figure 1: Morphological representation of ni DAC.

Figure 2: Coordination structure analysis of Ni DAC.

Uniformly distributed diatomic loci.HAADF-STEM shows that there are a large number of diatomic loci in the Ni DAC, and that the diatomic loci are evenly distributed on the carbon carrier. XAFS confirmed that the main coordination structure of ni atoms in Ni DACs is Ni2N6 diatomic structure, demonstrating the successful construction of diatomic loci.

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Figure 3: Dynamic processes of Ni species transitions and kinetic energy barriers formed at NiN4 and Ni2N6 sites.

Study on the formation mechanism of diatomic loci.During pyrolysis, ni species first form nanoparticles through the maturation process in Ostwald, and as the temperature continues to rise, the nanoparticles begin to dissolve, atomize, and finally disappear at 800 °C. CI-NEB calculations show that the formation of diatomic sites in the bi-pore defects of ni atoms embedded in carbon carriers during atomization has a lower kinetic energy barrier, and the surface preferentially forms diatomic sites in nickel species under high temperature conditions.

Figure 4: Electrocatalytic CO2 reduction performance.

Ultra-high alkaline CO2RR activity and selectivity.Compared with Ni SAC, Ni DAC exhibits a lower starting potential and higher current and catalytic selectivity in alkaline CO2RR (1M KOH). Faraday efficiencies greater than 95% are exhibited over a wide potential range (-0.15 to -0.79 V vs. RHE) and ampere-class CO current-sharing densities are achieved at -0.79 V vs. RHE potentials. In addition, continuous stable electrolysis of 30 mA at a current density that meets the requirements of industrialization (≥ 250 mA cm-2) is achieved.

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Figure 5: Environmental analysis of Ni atom coordination during electrocatalysis.

Figure 6: Molecular dynamics calculations.

The promoting role of the microenvironment.In situ XAFS showed that during electrocatalytic CO2 reduction, the Ni2N6 site adsorbed OH- (OHad) in the solution, forming a unique Ni2N6OH activity center. In order to study the effect of OHad on CO2RR, we used the de novo molecular dynamics simulation to study the kinetic process of CO2RR, and the calculation results showed that OHad can effectively regulate the charge around the Ni atom, so that the active center maintains the electron-rich center, and the construction of this electron-rich center effectively improves the adsorption of the reaction key intermediate *COOH and reduces the kinetic energy barrier of the entire reaction.

Figure 7: Characterization of the Pd/Mn/Zn DAC.

A certain universality of synthesis.Using ligand-mediated strategies, the research group also synthesized Pd, Mn and Zn diatomic catalysts with M2N6 coordination structures. This shows that the synthesis method has a certain universality and has a driving effect on the development of other transition metal diatomic catalysts and other electrocatalytic fields. (Source: Science Network)

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