The escalating atmospheric CO₂ levels pose significant environmental challenges, necessitating the development of efficient carbon capture and utilization technologies. Electrochemical CO₂ reduction offers a promising avenue for converting CO₂ into valuable chemical feedstocks, mitigating its environmental impact. However, the high energy barrier associated with the C=O bond (750 kJ·mol⁻¹) presents a substantial hurdle, requiring high negative potentials for activation. Copper-based electrodes have emerged as effective catalysts for CO₂ electroreduction, yielding various products such as CO, formic acid, and even C₂ products. Formic acid, a crucial C₁ product with broad industrial applications (leather, textiles, fuel cells), is particularly attractive. Its electrochemical production, however, often demands highly negative potentials, leading to catalyst degradation. While progress has been made in developing electrocatalysts for low-overpotential CO₂ reduction to formic acid, a comprehensive understanding of the underlying catalytic mechanisms remains crucial. Metal-organic frameworks (MOFs) have gained significant attention for their potential as electrocatalysts due to their high surface area, tunable pore structures, and the presence of potentially active metal centers. However, highly crystalline MOFs with fully coordinated metal centers often exhibit low charge-transfer ability, limiting their electrocatalytic efficiency. To enhance performance, MOFs with readily accessible metal sites and high charge-transfer capacity are highly desirable. This research focuses on addressing these challenges by introducing a novel approach to CO2 electroreduction.

Previous research has explored various catalytic approaches to electrochemical CO₂ reduction. Studies have demonstrated the efficacy of copper-based electrodes, highlighting their ability to produce a range of products including CO, formic acid, and higher-order hydrocarbons. The selectivity and efficiency of these processes are strongly influenced by factors such as electrode morphology, surface chemistry, and the electrolyte used. Formic acid, as a valuable C1 product, has been a target of significant research effort. Efforts have focused on achieving high Faradaic efficiency (FE) for formic acid production at lower overpotentials to enhance the overall efficiency and sustainability of the process. Metal-organic frameworks (MOFs), with their unique porous structures and tunable properties, have also been investigated as potential electrocatalysts for CO₂ reduction. However, challenges associated with charge transfer and the availability of active sites have limited their performance. This study addresses these limitations by integrating MOFs into a unique electrode design.

This study focuses on the electrochemical synthesis and characterization of a copper-based electrode decorated with a metal-organic framework (MOF) for enhanced CO₂ electroreduction to formic acid. The research employs a multi-pronged approach combining material synthesis, structural characterization, electrochemical analysis, and computational modeling. **Material Synthesis:** The ligand H₄L, 4,4',4'',4''-(1,4-phenylenebis(pyridine-4,2,6-triyl))tetrabenzoic acid, was synthesized using a three-step method. The MOF [Cu₂(L)] was then prepared via two methods: a conventional solvothermal synthesis to yield Cu₂(L)-t and an electro-synthesis method using an ionic liquid template to grow Cu₂(L) directly onto a copper foam electrode, creating Cu₂(L)-e/Cu. The HKUST-1 MOF was also electro-synthesized onto copper foam as a comparative material. **Structural Characterization:** Various techniques were employed to comprehensively characterize the synthesized MOF materials. Scanning Electron Microscopy (SEM) was used to image the morphology and size of the MOF crystals. Powder X-ray Diffraction (PXRD) confirmed the crystalline structure of the MOFs. Fourier Transform Infrared (FTIR) spectroscopy provided insights into the vibrational properties of the functional groups. Thermogravimetric Analysis (TGA) determined thermal stability, and Nitrogen adsorption/desorption isotherms (BET) measured surface area and porosity. Confocal fluorescence microscopy (CFM) using furfuryl alcohol oligomerization was used to visualize the distribution of open Cu(II) sites. Electron Paramagnetic Resonance (EPR) spectroscopy, at both X-band and Q-band, was instrumental in determining the concentration of uncoupled Cu(II) centers and their dynamics during CO₂ electroreduction. Elemental analysis determined the Cu:L ratio. **Electrochemical Measurements:** Electrochemical measurements were performed using an H-type cell with a three-electrode configuration. The Cu₂(L)-e/Cu and HKUST-1-e/Cu electrodes were used directly, while Cu₂(L)-t was loaded onto carbon paper. Linear sweep voltammetry (LSV) assessed the electrochemical response to CO₂. Chronoamperometry (i-t curves) monitored current density changes over time during CO₂ electrolysis. The Faradaic efficiency (FE) for formic acid production was calculated using gas chromatography (GC) and ¹H NMR spectroscopy. Electrochemical impedance spectroscopy (EIS) was used to determine the charge-transfer resistance (Rct), and cyclic voltammetry (CV) helped assess the double-layer capacitance (Cdl). **Computational Modeling:** Density Functional Theory (DFT) calculations using the Vienna ab initio simulation package (VASP) were performed to investigate the reaction mechanism. The Gibbs free energy of reaction steps were calculated for both pristine and defect Cu₂(L) structures, considering solvation effects, to understand the role of defect sites in the selective production of formic acid.

The electro-synthesized Cu₂(L)-e/Cu electrode, prepared by templated growth of MOF nanoparticles onto a copper foam, demonstrated remarkable performance in CO₂ electroreduction to formic acid. Key findings include: 1. **Low Onset Potential:** The Cu₂(L)-e/Cu electrode exhibited a low onset potential of -1.45 V vs. Ag/Ag⁺ for formic acid production, one of the lowest reported values for organic electrolytes. 2. **High Faradaic Efficiency:** A maximum Faradaic efficiency (FEHCOOH) of 90.5% was achieved at -1.8 V vs. Ag/Ag⁺, with a current density of 65.8 mA cm⁻². The FEHCOOH remained above 80% over a significant potential range (-1.75 V to -1.95 V vs. Ag/Ag⁺). 3. **Defect-Driven Catalysis:** EPR spectroscopy confirmed the presence of a significantly higher concentration of uncoupled Cu(II) centers in Cu₂(L)-e/Cu compared to the solvothermally synthesized Cu₂(L)-t. These uncoupled Cu(II) sites, created as structural defects, are identified as the primary active sites for CO₂ electroreduction. 4. **Enhanced Charge Transfer and Surface Area:** Electrochemical impedance spectroscopy (EIS) revealed a significantly lower charge transfer resistance (Rct) for Cu₂(L)-e/Cu compared to Cu₂(L)-t/CP and HKUST-1-e/Cu, indicating enhanced charge transfer. Similarly, Cu₂(L)-e/Cu showed a higher double-layer capacitance (Cdl), suggesting a larger electrochemically active surface area. 5. **DFT Confirmation:** DFT calculations supported the experimental findings by showing that the defect Cu(II) sites in Cu₂(L)-e facilitated the formation of O-bound HCOO⁻ intermediates, leading to preferential formic acid production. The calculations also indicated that the weaker hydrogen bonding in the defect structure helps in product release from the catalyst surface. 6. **Electrode Stability:** Unlike HKUST-1-e/Cu, the Cu₂(L)-e/Cu electrode showed good morphological and electrochemical stability during electrolysis. 7. **Comparison with other Materials:** Comparative analysis with Cu₂(L)-t/CP and HKUST-1-e/Cu clearly highlighted the superior performance of Cu₂(L)-e/Cu with respect to onset potential, FEHCOOH, current density, and stability.

The significantly improved performance of the Cu₂(L)-e/Cu electrode in CO₂ electroreduction can be attributed to the synergistic effects of its unique structure and the presence of defect sites. The electro-synthesis method, guided by the ionic liquid template, generated a thin, compact coating of small Cu₂(L) nanoparticles on the copper foam. This resulted in a high surface area, enhanced charge transfer, and effective utilization of the active Cu(II) sites. The high concentration of uncoupled Cu(II) centers, generated as structural defects, plays a pivotal role in catalyzing the reduction of CO₂ to formic acid. The DFT calculations support this mechanism, demonstrating that the defect sites facilitate the formation of O-bound HCOO⁻ intermediates, thus promoting the selectivity towards formic acid. The enhanced charge transfer, as evidenced by lower Rct and higher Cdl, also contributes to the improved performance. The stability of the electrode during prolonged electrolysis further underscores its potential for practical applications. The findings offer valuable insights into the design and optimization of MOF-based electrocatalysts for CO₂ reduction, highlighting the importance of defect engineering and electrode morphology.

This study demonstrates a novel approach to enhance CO₂ electroreduction to formic acid by integrating a copper-based MOF (Cu₂(L)) into a copper foam electrode using an electro-synthesis method. The resulting Cu₂(L)-e/Cu electrode exhibits superior performance, surpassing that of conventionally synthesized Cu₂(L)-t and HKUST-1-e/Cu, in terms of onset potential, Faradaic efficiency, and stability. The remarkable activity is attributed to the high concentration of uncoupled Cu(II) defect sites, confirmed by EPR spectroscopy and supported by DFT calculations. This work provides a valuable strategy for designing highly efficient MOF-based electrocatalysts for CO₂ reduction and opens avenues for further exploration of defect engineering in MOFs for improved catalytic performance. Future work could explore different MOF structures and synthesis strategies to further optimize the catalytic activity and selectivity.

While the study demonstrates the high efficiency of the Cu₂(L)-e/Cu electrode for CO₂ electroreduction to formic acid, there are some limitations to consider. The study was conducted in an organic electrolyte, which is not directly applicable to industrial-scale processes that typically use aqueous electrolytes. The long-term stability of the electrode was tested up to 5 hours, but longer-term stability tests under continuous operation are necessary. Furthermore, the study primarily focuses on formic acid production; a more detailed investigation into the formation of other possible byproducts might be beneficial for a comprehensive understanding of the catalytic mechanism. Finally, scaling up the electrode fabrication to industrial levels would require further optimization of the synthesis process.