Progress on Post-Synthetic Modification of Framework Catalysts at SARI, CAS
Source: Shanghai Advanced Research Institute, CAS | Jun 29, 2023
Recently, the team led by Prof. Gaofeng Zeng and Associate Prof. Qing Xu at the Shanghai Advanced Research Institute, Chinese Academy of Sciences, has made important progress in the post-synthetic modification of covalent organic frameworks (COFs) for electrocatalytic CO₂ reduction reaction (CO₂RR). The relevant work, entitled Post-synthetic modification of covalent organic frameworks for CO₂ electroreduction, was published in Nature Communications.
Electrocatalytic CO₂ reduction reaction (CO₂RR) can convert greenhouse gases into high-value products, offering a strategy to mitigate excessive CO₂ emission. Thanks to their tailorable molecular structures, COFs serve as ideal platforms for constructing CO₂RR catalysts. The catalytic performance of COFs for CO₂RR depends on their structures and intrinsic properties. Structurally, diverse catalytic COFs can be assembled by tuning the types of catalytic centres, electronic states of linkers and linker diversity. Modification of building blocks or linkages enables the regulation of COF properties such as CO₂ binding affinity and electronic conductivity. Amine linkages strengthen CO₂ adsorption, while ionic frameworks improve electronic conductivity and charge transfer along the skeleton. Nevertheless, direct synthesis of COFs simultaneously bearing amine linkages and ionic frameworks remains challenging, restricted by electrostatic repulsion and linkage stability.
Against this backdrop, the team fabricated COFs with amine linkages and ionic skeletons via a dual post-synthetic modification strategy, achieving high activity and selectivity toward CO₂RR. The maximum CO Faradaic efficiency reaches 97.32%, with a turnover frequency (TOF) of 9922.68 h⁻¹, outperforming the pristine COF and single-modified COFs. Furthermore, theoretical calculations reveal that the enhanced activity originates from the favourable formation of CO intermediate directly from COOH. This study provides new insights into the design of COFs for CO₂ reduction.
The CO₂RR performance of the catalytic COFs was evaluated in CO₂-saturated 0.5 M KHCO₃ aqueous solution (pH 7.2). Electrochemical tests show that the FE_CO of N⁺-NH-COF is 82.56%, 88.76%, 92.51%, 97.32%, 90.12% and 82.78% at potentials ranging from −0.5 V to −1.0 V. The j_CO reaches its maximum value of 28.01 mA cm⁻² at −1.0 V, superior to other COF materials. In addition, N⁺-NH-COF exhibits higher TOF and a lower Tafel slope compared with other singly modified COFs, demonstrating that multi-step post-modification boosts both CO₂RR activity and CO selectivity of COFs. To rationalize the excellent catalytic behaviour, current densities normalized by electrochemically active surface area were analysed. The ionic COFs (N⁺-COF and N⁺-NH-COF) consistently deliver higher normalized current densities than CoTAPP-PATA-COF and NH-COF, confirming that ionic functionalization promotes CO₂RR activity. These results verify that C–N amine linkages improve catalytic selectivity while ionic skeletons enhance catalytic activity. Theoretical calculations also show that M-I-N⁺-NH-COF has the lowest Gibbs free energy change for the *COOH intermediate, consistent with its optimal performance from a thermodynamic perspective. TDOS analysis indicates that Co contributes more significantly in M-I-N⁺-COF and M-I-N⁺-NH-COF than in M-CoTAPP-PATA-COF and M-NH-COF, suggesting that methyl functionalization increases the electron density on Co sites.
This work modulates COF properties (porosity, crystallinity and electronic states) via a multi-step post-synthetic modification strategy, enabling tunable CO₂RR performance. COFs with ionic skeletons and amine linkages exhibit high activity for CO₂RR. This research deepens the understanding of COFs and their applications in electrochemical energy storage and conversion. Meanwhile, it provides guidance for constructing hierarchically post-modified COFs with tailored activity and high stability.
This research was supported by the National Natural Science Foundation of China, Science and Technology Commission of Shanghai Municipality, Youth Innovation Promotion Association of CAS and Henan Provincial High-level Talent International Training Fund. Researchers from ShanghaiTech University and Henan Institute of Science and Technology participated in this study.
Figure 1. Schematic of hierarchical post-synthetic modification
Figure 2. Electrochemical performance of four COFs
Figure 3. Theoretical calculations