Abstract:

With the development of human society and economy, the demand for energy resources has also increased rapidly. However, the use of traditional fossil energy leads to high amounts of carbon dioxide emissions, causing severe greenhouse effects. This, in turn, triggers a series of environmental problems. Harnessing renewable energy such as solar energy, wind energy, and hydropower to replace the traditional energy sources is very urgent. Conversion CO₂ into value-added fuels and chemicals could be a useful strategy to mitigate the current energy and environmental crisis. It is well known that Cu-based materials are good electrocatalysts for the electrochemical reduction of CO₂ (ECR-CO₂). However, they suffer from some disadvantages such as high overpotential and poor selectivity and durability. Therefore, the development of copper based electrocatalysts with high activity and selectivity is essential.

Metal-organic frameworks (MOFs) materials that have the advantages of large specific surface area, tunable pore size and porosity, and highly dispersed unsaturated metal centers can be used as electrocatalysts for CO₂ reduction or as precursors for further preparation of catalysts with excellent performance. Through thermal decomposition in an inert atmosphere, metal ions in MOF can be transformed into metal clusters, metal oxides, or even metal mono-atoms. Meanwhile, organic ligands are carbonized into porous carbon materials. The addition of some heteroatoms such as B, N, P, and S to carbon materials has also been shown to be effective in changing the electron state and coordination structure of the catalysts. These heteroatoms combine with carbon atoms to form a new active site, denoted as M-X-C (M is the central metal ion and X is the mixed heteroatom) to enhance the catalytic activity of the ECR-CO₂.

Herein, pre-synthesized Cu-NBDC MOF (a Cu-based MOF synthesized by using 2-aminoterephthalic acid (NBDC) as ligand) is used as a precursor to anchor Cu₂O/Cu on nitrogen doped porous carbon (Cu₂O/Cu@NC) by annealing at different temperatures. XPS analysis shows that the Cu-N content in Cu₂O/Cu@NC decreases with increasing annealing temperature. Investigation of the ECR-CO₂ reveals that Cu₂O/Cu@NC can inhibit the HER more effectively compared to Cu₂O/Cu@C, thereby improving the overall catalytic activity and multi-electron product selectivity of the ECR-CO₂. While the Faradic efficiency of formate (FE_formate) increases with increasing temperature, those of ethylene and methane (FE_C2H4 and FE_CH4, respectively) decreases with increasing temperature. Specifically, upon annealing at 400 ℃, the CO₂ Faradic efficiency of Cu₂O/Cu@NC-400 is higher than 86% (−1.4 to −1.6 V vs. RHE), including 20.4% of FE_C2H4 (−1.4 V vs. RHE) and 23.9% of FE_CH4 (−1.6 V vs. RHE). By contrast, FE_CH4 (−1.6 V vs. RHE) in the presence of Cu₂O/Cu@C-400 without nitrogen doping is only 2.33%, and no C₂H₄ is detected. These significant differences in the catalytic behavior can be attributed to the fact that Cu-N is conducive for the stable adsorption of the *CH₂ intermediate during the ECR-CO₂, thus inhibiting the evolution of H₂. These results indicate that the pathway of the ECR-CO₂ and its performance can be effectivel regulated by complexing nitrogen with Cu motifs.

Key words: Copper-based MOFs material, Nitrogen doping, Cu₂O/Cu, CO₂ reduction