N,O-Mixed-Coordinated Zn-MOFs For Electrocatalytic CO₂ Reduction To Formate

Electrochemical CO₂ reduction（ECR）to high value-added products will promote the"double carbon"work.Its reduction products mainly include CO,formate,hydrocarbons and oxygen-containing polycohol product.Among them,formate is not only the hydrogen carrier of fuel cell and common industrial raw material,but also the most economic benefit in terms of market value and energy input.However,due to the chemical inertia of CO₂,it is difficult to overcome the thermodynamic and kinetic energy barrier,and its various reduction pathways lead to poor selectivity of reduction products and difficult to explain the mechanism.Metal-organic frameworks（MOFs）are considered as a promising catalyst for ECR reaction because of their clear and adjustable structure and abundant ligands.Zn-MOFs have attracted the attention of researchers due to the rich reserves of metal Zn.However,the reported coordination mode of Zn in Zn based MOFs is only a single N-or O-coordination,and the main products of reduction are usually CO and CH₄,not formate.Considering that metal as an active site,its coordination environment has a large effect on the product selectivity of ECR.Therefore,it is expected to achieve its electrocatalytic ECR product as formate by change the coordination environment of metal.In this paper,a kind of N,O-mixed coordination Zn-MOFs were designed and synthesized as ECR catalysts to explore the effect of different functional groups functionalized ligands and structures.The main contents are following:Firstly,a new Zn-MOF（FJU-40-CH₃）was synthesized by 1,2,4-triazole and2-methylterephthalic acid with hydrophobic functional groups as mixed ligands and Zn as metal nodes.Single crystal diffraction results show that FJU-40-CH₃ is isomorphic with FJU-40-NH₂ and FJU-40-H.FJU-40-R（R=CH₃,NH₂,H）was applied to ECR.It was found that N,O-mixed coordination Zn-MOFs were conducive to the electroreduction of CO₂ to formate.Among them,FJU-40-CH₃ has the best catalysis,its faraday efficiency（FE）can reach 76.49%at-1.37 V,and the corresponding partial current density（j）of formate is-12.09 m A cm^-2.However,the crystallinity of these materials can not be maintained under electrolytic conditions.Therefore,under the same metal node conditions,3,5-diamino-triazole was selected as ligand 1 instead of triazole,and methylterephthalic acid and tetrafluoroterephthalic acid were used as ligand 2respectively to construct two new Zn-MOFs（FJU-126-4F/CH₃）.The results of structural analysis and ECR analysis showed that the introduction of amino functional groups formed intramolecular hydrogen bonds between different ligand,which improved the stability of FJU-126-4F/CH₃ under electrolytic conditions,but the selectivity of formate became worse(the maximum FE_formate of FJU-126-4F was 60.48%,and the FE_formate of FJU-126-CH₃ was 50.18%).In order to meet the requirements of good catalytic performance and stability at the same time,the experiment further selected3-amino-triazole instead of 3,5-diamino-triazole as ligand 1 and methylterephthalic acid as ligand 2.A new Zn-MOF（FJU-127-CH₃）catalyst with hydrophobic group and self-penetration structure was designed and synthesized for ECR to formate.Its FE_formatewas up to 90.24%at-1.57 V.Compared with other Zn-MOFs electrocatalysts FJU-127-NH₂/H&FJU-126-4F/CH₃&FJU-40-R with only self-penetration structure or only hydrophobic group or simple column structure,the coexistence of self-penetration structure and hydrophobic functional group can improve the selectivity of ECR to formate.In this work,Zn-MOFs with high selectivity for the conversion CO₂ to formate were constructed,which provided a new idea for the study of the structure property relationship of ECR catalyst.