Progress has been made in the study of post-modification of catalytic materials by the framework

Recently, Zeng Gao, a researcher at the Shanghai Institute for Advanced Study of the Chinese Academy of Sciences, and Xu Qing, an associate researcher, have made important progress in the use of post-covalent organic frameworks (COFs) modifications for electrocatalytic carbon dioxide reduction reactions. The research results are published in Nature Communications under the title Post-synthetic modification of covalent organic frameworks for CO2 electroreduction.

Electrocatalytic carbon dioxide reduction (CO2RR) can produce high-value products from greenhouse gases, providing strategies to address high CO2 emissions. Due to the designability of molecules, COFs are ideal templates for constructing CO2RR catalysts. The catalytic behavior of COFs to CO2RR depends on its structure and properties. Structurally, different catalytic COFs are assembled by the type of catalytic center, the electronic state of the linker molecule, and the diversity of the linker. By changing the building blocks or connecting bonds, the corresponding properties of COFs, such as the binding capacity of CO2 and electronic conductivity, can be adjusted. Amine bonding enhances the binding capacity of CO2 molecules, and ionic frames improve electronic conductivity and charge transfer along the frame. However, due to the limitations of electrostatic repulsion and ligation bond strength, it is difficult to achieve direct synthesis of covalent organic frameworks with amine bonds and ionic frames.

In this context, the team constructed COFs with amine bonds and ionic backbones through a double post-modification method, achieving high activity and selectivity of CO2RR. Among them, the CO Faraday efficiency can reach 97.32%, and the turnover rate value is 9922.68 h-1, which is higher than that of the original covalent organic framework and the single-modified covalent organic skeleton. In addition, theoretical calculations further show that higher activity is attributed to the easier direct generation of COOH*. This study provides new insights for the development of a covalent organic framework for CO2 reduction reactions.

The CO2RR performance of catalyzed COFs was investigated in CO2-saturated KHCO3 aqueous solution (0.5 M, pH 7.2). Through electrochemical tests, it was found that the FECO of N+-NH-COF was 82.56%, 88.76%, 92.51%, 97.32%, 90.12% and 82.78% in the range of -0.5 ~ -1.0 V, respectively, and the jCO was the highest in the -1.0 V range, 28.01 mA cm-2, which was higher than other COFs. In addition, N+-NH-COF exhibited higher TOF values and lower Tafel slope compared to other single-modified COFs, indicating that multilayer post-modification improved CO2RR activity and CO selectivity of COFs. In order to better understand the excellent catalytic behavior of COFs, the normalized current density was obtained by electrochemically active surface area. N+-COF and N+-NH-COF ionized COFs consistently exhibited higher normalized current densities than CoTAPP-PATA-COF and NH-COF, confirming that ionization helps to increase the activity of COF against CO2RR. The above studies proved that C-N amine bonds improve catalytic selectivity, and the ionic backbone helps to improve activity. In addition, theoretical calculations show that the *COOH free energy of M-I-N+-NH-COF has the smallest change, which is in line with its best performance from the thermodynamic point of view. TDOS showed that the contribution of Co in M-I-N+-COF and M-I-N+-NH-COF was greater than that in M-COTAPP-PATA-COF and M-NN-COF, indicating that the introduction of methyl groups enhanced the electron density on Co atoms.

In this study, the properties (porosity, crystallinity and electronic state) of COFs were adjusted through a multi-post modification strategy, which contributed to their tunable catalytic performance for CO2RR. Ionic backbone and amine bond-linked COFs have high activity and activity for CO2RR. This work has made more in-depth research progress on COFs and their application in electrochemical energy storage and conversion systems. At the same time, the work guides scientists to construct multi-level post-synthesis modified COFs to achieve customized activity and high stability.

The research work is supported by the National Natural Science Foundation of China, the Shanghai Municipal Science and Technology Commission, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and the Henan Provincial High-level Talent Internationalization Training Fund. Researchers from ShanghaiTech University and Henan Institute of Science and Technology participated in the research. (Source: Shanghai Advanced Research Institute, Chinese Academy of Sciences)

Related paper information:

Figure 1. Schematic diagram of post-multi-level synthesis modifications

Figure 2. Electrochemical performance of the four COFs

Figure 3.Theoretical calculation diagram

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