Preparation Of Single-atom Ni Catalysts And Their Application In Electrocatalytic Reduction Of CO₂

The excessive use of fossil energy has emitted a large amount of CO₂,causing serious ecological and environmental problems.Therefore,in order to reduce CO₂,the capture and utilization of CO₂has become an important technique.Electrocatalytic reduction of CO₂,which is proved to be a green and environmentally friendly method,can effectively convert CO₂into high value-added chemicals even under ambient conditions.However,CO₂has a very stable linear molecular structure,which causes lots of problems in its electrocatalytic reduction process,such as high overpotential and severe hydrogen evolution reaction.Therefore,it is necessary to design an efficient catalyst to improve the efficiency of the electrocatalytic reaction of CO₂.In this thesis,the porous hollow carbon spheres were used as supports,and the transition metal Ni was highly dispersed on the carbon spheres by heat treatment to form a single-atom catalyst for CO₂electrocatalytic reduction.In order to improve the activity of the catalyst and the combination ability of carbon spheres,nitrogen was used to functionalize the hollow carbon spheres to modify the coordination environment of metal species.The main research contents are as follows:（1）The nitrogen-containing carbon spheres N/C-x（x=1/4,2/3,3/2）with hollow structure were fabricated using resorcinol-formaldehyde（RF）and melamine-formaldehyde（MF）as carbon precursor through St?ber method.Then the Ni-N/C-x catalyst was prepared by pyrolysis of N/C-x and metal salt acetylacetone nickel.The nitrogen species of carbon hollow spheres can be tuned by changing the concentration of melamine in the synthesis system for the precursor of carbon hollow spheres.The results showed that Ni-N/C-x catalysts all maintain a uniformly spherical hollow structure.Moreover,the more melamine is used,the easier it was to form a atomic catalyst.The Ni in the Ni-N/C-1/4 catalyst mainly exists in the form of single-atom,while Ni-N/C-2/3 and Ni-N/C-3/2 contain more Ni metal nanoparticles.The XPS results showed that the higher the content of pyrrolic nitrogen,the easier it was to form a single atom catalyst.The content of pyrrolic nitrogen of Ni-N/C-1/4,Ni-N/C-2/3 and Ni-N/C-3/2 was 28.43%,24.74%and 21.69%,respectively.（2）The performance of the Ni-N/C-x catalyst in the electrocatalytic reduction of CO₂was investigated in detail.Compared with N/C-x,all Ni-N/C-x shows better catalytic activity.After Ni loading,the hydrogen evolution reaction was suppressed,and the CO selectivity was significantly improved.Ni-N/C-1/4,which contains more pyrrolic N and Ni ions,exhibits higher selectivity for CO and excellent stability during 4.5 h of CO₂reduction electrolysis at the potential of-0.78 V（versus.RHE）.Furthermore,the faraday efficiency of CO exceeds 70%in the potential range of-0.63V～-0.98 V（versus.RHE）.（3）Using SiO₂spheres as hard template,nitrogen-containing carbon spheres were formed by coating the template with 3-aminophenol-formaldehyde（APF）phenolic resin.At the same time,Ni salt was added in situ during the synthesis process,and the Ni-N-C-z（z=7,10,21 nm）catalysts with different shell thickness were prepared by adjusting the amount of APF.And,with the increase of APF amount the shell thickness increases gradually,while the etching process by HF becomes more difficult,resulting in the decrease of hollow structure.（4）The catalytic performance of Ni-N-C catalysts with different shell thickness were evaluated in electrocatalytic reduction of CO₂.The results show that Ni-N-C-10display the best catalytic activity:the CO faraday efficiency can reach more than 90%in the potential range of-0.7 V～-1.0 V（versus RHE）,and the CO partial current density can be achieved 17.3 m A cm^-2at-1.0 V（versus RHE）.At the same time,compared to Ni-N-C-7 and Ni-N-C-21,Ni-N-C-10 catalysts also have the highest TOF value of 2263 h^-1[at-1.0 V（versus RHE）potential].