Electrochemical CO₂ reduction reaction (ECO₂RR) has attracted extensive attention due to its minor environmental pollution, mild reaction condition, and adjustable product selectivity. Generally, it is preferred to selectively convert CO₂ to multi-carbon (C₂₊) products with high energy densities and economic values, such as ethanol (C₂H₅OH) and ethylene (C₂H₄). However, significant hurdles regarding the poor selectivity of ECO₂RR catalysts to a specific product and precise regulation of conversions between products still need to be overcome. Owing to their modest binding energy to the key intermediates in CO₂ reduction, copper (Cu)-based catalysts have been extensively studied to prepare highly selective electrocatalysts. Metal-organic frameworks (MOFs) with precise structural information have significant advantages in the study of the mechanism of the ECO₂RR. It can be demonstrated that establishing appropriate Cu-based MOF catalyst models may effectively regulate the selectivity of carbon-based reduction products as well as help in better understanding the mechanism of the catalytic reaction. In this thesis, we focus on the structural design of Cu-based MOF catalyst in nanoscale by adjusting the exposed facets, the coordination environment (e.g., ligand type and bonding mode) and crystallinity to achieve high ECO₂RR electrocatalytic activity.

The first part describes the controlled synthesis of MOF nanostructures with different exposed facets by simply tuning the solvent in reaction systems, without the assistance of any surfactants. Specifically, when water (H₂O) acts as the reaction solvent, Cu(I) 5-mercapto-1-methyltetrazole framework (Cu-MMT) nanoribbons with dominantly exposed (100) facets are obtained, which exhibit excellent catalytic selectivity in converting CO₂ to single-carbon (C1) products, especially methane (CH₄) with Faradaic efficiency (FE) up to 55.22% at −1.4 V (vs reversible hydrogen electrode (RHE)). In sharp contrast, when the reaction solvent is changed to isopropyl alcohol (IPA), Cu-MMT inter-crossed nanosheets with (001) facets are formed, which demonstrate superior selectivity to C₂₊ products, particularly C₂H₄ with FE of 50.98% at −1.15 V (vs RHE). Significantly, similar phenomena can be observed on the resultant Cu-MMT nanostructures when changing the solvents from H₂O and IPA to benzyl alcohol (BA) and ethylene glycol (EG), respectively. This work provides a feasible strategy of switching the ECO₂RR selectivity from C₁ to high-value C₂₊ products by facet engineering of MOFs.

In the second part, two Cu(I)-based tetrazole MOFs were synthesized by a solvothermal method, and the effects of ligands with different side chains on the selectivity of ECO₂RR from C₁ to C₂₊ products were studied. In order to ensure the unity of the ligand variables, the two structures can be strictly controlled and adjusted to the same coordination structure (Cu-2S-2N), morphology (nanoflakes), size (200 ~ 300 nm), and even major exposed crystal facets (100). Specifically, when MMT with methyl (-CH₃) as the side chain acts as the ligand, Cu-MMT nanoflakes exhibit excellent catalytic selectivity in converting CO₂ to C₁ products, especially CH₄ with FE up to 53.7% at −1.25 V (vs RHE). In sharp contrast, when the ligand is changed to 5-Mercapto-1-phenyl-1H-tetrazole (MPT), with benzene ring (-C₆H₆) as the side chain, Cu-MPT nanoflakes demonstrate superior selectivity to C₂₊ products, particularly C₂H₄ with FE of 50.08% at −1.25 V (vs RHE). This work demonstrates the usefulness and tenability of flexible electrocatalysts and provides a feasible strategy of switching the ECO₂RR selectivity from C₁ to high-value C₂₊ products by ligand engineering of MOFs.

In the last part, a series of well-defined Cu-MPT MOFs were established as electrocatalysts to study the effects of the morphology and crystallinity on the performance of electro-reducing CO₂ to C₂H₄. In terms of the morphology, compare with the microflakes (Cu-MPT-1), nanoflowers assembled by ultrathin nanosheets, show the higher FE_C2H4 (ca. 47.5% at −1.2 V vs RHE). More importantly, compared with polycrystalline nanostructures (Cu-MPT-2), nanoflowers assembled by single crystal nanoflakes (Cu-MPT-3) exhibit the highest activity and selectivity to C₂₊ products with good durability. Furthermore, Cu-MPT polycrystalline nanoflowers supported by copper oxide nanoparticles (Cu-MPT@CuO NPs) were first synthesized by a one-pot solvothermal method to further improve the catalytic activity and stability to C₂H₄. Interestingly, different amounts of CuO NPs can be facilely adjusted by tunning the different Cu:S in the raw materials. Among all proportions, Cu₃-MPT1 shows the highest selectivity to C₂₊ products (ca. 50% FEC₂H₄ and ca. 75.8% FE_C2+ at −1.1 V vs RHE) and higher stability (ca. 16 mA cm⁻² with negligible drop for 20,000 s at −1.1 V vs RHE).