Nitrogen Doped Carbon/Transition Metals Composite Materials:Preparation And Their CO₂ Electrochemical Reduction Performance

In recent years,the excessive emission of carbon dioxide has led to the rise of global average temperature.The"greenhouse effect"caused by carbon dioxide has worsened the global climate,therefore,the problem of carbon dioxide needs to be solved urgently.Nowadays,the electrocatalytic reduction of carbon dioxide CO₂RR has attracted wide attention due to its advantages of easy operation and environmental friendliness.At present,the focus of this electrochemical method is the preparation of electrode materials with high catalytic activity,selectivity and stability.Therefore,designing and synthesing suitable electrode catalyst materials has become the efficient way to solve this problem.Metal-organic-frameworks?MOFs?are a new kind of materials with large specific surface area and ordered pore structure.Carbon based materials prepared by MOFs as template/precursor have excellent structural properties and incomparable advantages compared with traditional materials.Their special structure has become a scientific research hotspot,and they have great potentials in the application of electrocatalytic reduction of carbon dioxide.In this thesis,ZIF-67 is synthesized by magnetic stirring method,and then Co@N-C composites are prepared by adjusting calcination temperature of ZIF-67.The crystal phase,morphology,and element content and their chemical states of the composites are detailedly studied by XRD,XPS,SEM and TEM.The catalytic performance of Co@N-C composites for CO₂RR is tested,and the effects of the structure of material skeleton and existing forms of its elements on the catalytic performance are investigated.It could be found that the different existing states of Co species have different catalytic properties.Specifically,the Co species exposed on the surface of the material mainly promoted HER to produce H₂,while the Co particles packaged by the C-N skeleton mainly catalyzed CO₂RR process.Therefore,in order to further improve the catalytic performance of Co@N-C composites,we remove the exposed Co nanoparticles on the surface of the composites by acid treatment.The results show that the composites treated by aqua regia have higher catalytic activity.Among all the catalysts,the obtained Co@N-C-700-A shows the highest CO₂ reduction activity and stability,achieving a specific current of 25 A·g_cat^-1 and a productivity of 0.266 mol_CO·h^-1·g_cat^-1 at a mild potential of-0.57 V vs.RHE.The ratio of CO/H₂ in products could be adjusted in the range of 1/4 to 1.5/1.In the above research work,we found that the size of Co particles in Co@N-C composites obtained by direct calcination of ZIF-67 was large,and some Co particles were exposed on the surface of the composites,hindering the electrocatalytic reduction of carbon dioxide.In order to control the size and distribution of Co nanoparticles,Zn is introduced to prepare bimetallic ZIFs.A series of bimetallic ZIFs composites with same topological structure are successfully synthesized by adjusting the molar ratio of Co²⁺/Zn²⁺.In the calcination process,the presence of Zn successfully inhibits the agglomeration of Co nanoparticles.Meanwhile,the imidazole skeleton connected with Zn effectively packages the Co particles and prevents them from moving to the surface of the material.In addition,the volatilization of Zn causes more vacancies in the composites and increases the amounts of active sites.The obtained Co-Zn@N-C composites have good catalytic performance for CO₂RR.The experimental results show that Co-Zn@N-C-25 prepared by calcining the samples with Co²⁺/Zn²⁺molar ratio of 1:3 at 800? exhibit the best catalytic performance,achieving a specific current of 37 A·g_cat^-1 and a productivity of 0.492 mol_CO·h^-1·g_cat^-1 at a mild potential of-0.57 V vs.RHE.In addition,the ratio of CO/H₂?1/4²/1?could be adjusted by changing the molar ratio of Co²⁺/Zn²⁺in the precursor,and the application range of the composite materials are broadened.