Design,Synthesis And Structure Regulation Of Cobalt-Based Single-Atom Catalysts For Electrocatalytic Hydrogen Evolution

With the increase of global energy demand and the aggravation of environmental pollution,it is imperative to search for clean alternative energy and optimize the energy structure.Hydrogen energy,as a kind of secondary energy with wide sources,clean and efficient and zero emission,is regarded as the most promising clean energy in the 21st century.The methods of producing hydrogen in large quantities mainly include hydrogen from fossil fuels and hydrogen from decomposing water.The technology of hydrogen from fossil fuels is relatively mature,but it does not conform to the concept of green and sustainable development.Using electric energy or solar energy to decompose water into hydrogen is a major research direction.The key to decompose water into hydrogen and oxygen under the action of electric energy or light is to find a suitable catalyst.The Pt noble metal catalyst has excellent performance and has been used in a certain range.However,the high price and use cost of Pt,as well as its low abundance in nature,seriously limit its wide application.Therefore,it is urgent to develop low cost,high activity and sustainable low platinum or even non-platinum based electrocatalytic materials.In recent years,single atom catalysts have attracted wide attention in many catalytic fields because of their maximum atomic utilization efficiency,uniform active sites and excellent catalytic activity.Among many non-platinum-based materials,cobalt-based catalysts have attracted much attention because of their advantages such as abundant reserves,low price,strong corrosion resistance and good electrocatalytic activity.However,at present,the cobalt single atom catalyst still has the problems of low intrinsic activity and low loading capacity,and the hydrogen evolution performance of cobalt single atom catalyst is far from that of noble metal Pt.Therefore,in view of the above problems,this paper focuses on the controllable synthesis,structure regulation and electrocatalytic hydrogen evolution reaction（HER）performance of cobalt-based single atom catalyst,and explores the structure-activity relationship between the microstructure of single atom cobalt and the electrocatalytic HER performance.Firstly,in order to increase the number of active sites,cobalt single atom catalyst supported on nitrogen-doped carbon framework was synthesized by doping-adsorption-pyrolysis strategy using zeolite imidazole framework（ZIF-8）as precursor and cobalt nitrate and cobalt tetraphenyl porphyrin as cobalt sources.The effect of the amount of single atom cobalt on the electrocatalytic HER performance was investigated by changing the addition amount of cobalt nitrate and cobalt tetraphenylporphyrin.The results show that the single atom cobalt catalyst prepared by doping-adsorption-pyrolysis method can not only significantly increase the load of single atom cobalt,but also increase the specific surface area of the catalyst material and expose more active sites.In the electrochemical test,the catalyst showed excellent electrocatalytic performance for HER in both acidic and alkaline electrolyte.Secondly,in order to improve the intrinsic activity of cobalt single atom catalyst,a cobalt-tellurium diatomic site catalyst（Co-TeDASs/N-C）with large specific surface area（1079 m²·g-1）was synthesized by encapsulation-adsorption-pyrolysis strategy using ZIF-8 as precursor,cobalt tetraphenylporphyrin as cobalt source and tellurium powder as tellurium source.By doping tellurium with semimetallic properties to improve the electrocatalytic performance of cobalt based single atom catalyst,the doping regulation of semimetallic tellurium element is understood.It was found that the synthesized catalyst had stable coordination structure of CoN₃C₁-TeN₁C₃,and the doping of Te changed the electronic structure of Co site,which significantly improved the electrocatalytic HER activity and stability of cobalt-based single atom catalyst.The higher HER performance could be achieved at a lower cobalt load（0.3 wt%）,and the economy of cobalt atom was improved.It provides an efficient electronic control strategy to improve the performance of transition metal single atom electrocatalyst.Finally,in order to further improve the alkaline electrocatalytic HER efficiency of cobalt-based electrocatalytic materials,the doping-adsorption-pyrolysis method was used to construct a cobalt-based catalyst coupled with single atom and nanocrystals,and the effect of the coupling effect of single atom/nanocrystals on the electrocatalytic HER performance was investigated.The results show that there is a strong synergistic effect between the cobalt single atom and nanocrystals,which can effectively improve the activation and dissociation ability of water molecules.The water molecules are first adsorbed on the cobalt nanocrystals for activation and dissociation,and the resulting H^* is adsorbed on the cobalt single atom for further reaction to generate hydrogen,thus improving the performance of alkaline electrocatalytic water.In conclusion,this paper realizes the optimization of the performance of electrocatalyst through precise control synthesis and structure regulation of cobalt-based single atom catalyst,which lays a foundation for the preparation of high-performance cobalt-based single atom electrolysis catalyst for HER and provides an effective synthesis strategy for the design and synthesis of efficient single atom catalytic materials.