Synthesis And Electrochemical Performance Of Hierarchical Porous Cobalt-nitrogen-carbon Materials

In recent years,due to the excessive consumption of traditional fossil energy,many countries are taking active measures to cope subsequent environmental problems.Fuel cell as a kind of high energy density and cleaner sources of power,has received the close attention to scientific researchers,but its high cost has been the main obstacle to hinder its commercialization.In the production of the numbers in hundreds of time,catalysts cost the most among the fuel cell systems,so using cheap catalysts to replace traditional noble metal catalyst plays an important role.Transition metal-nitrogen-carbon materials with good oxygen reduction catalytic activity and stability is considered to be the most potential low cost materials to replace noble metal catalyst,but its catalytic activity,especially under the condition of acid catalytic activity is still need to further improve to satisfy commercial requirements.To this,this thesis puts forward two kinds of method and obtains the prepare hierarchical porous transition metal-nitrogen-carbon catalyst.In this thesis,the main research contents and conclusions are as follows:（1）Different Co/Zn proportion of bimetallic ZIF material was prepared using P123as structure-directing agent in aqueous solution,and bimetallic ZIF material had a high degree of crystallinity,the catalyst has larger specific surface area and the hierarchical pore structure of microporous and mesoporous.In the choice of the precursor,when Co（NO₃）₂·6H₂O:Zn（NO₃）₂·6H₂O equal to 5:95,the as-prepared HC-5Co95Zn showed the highest oxygen reduction and oxygen evolution reaction catalytic activity in alkaline condition,the excellent catalytic activity is caused by the appropriate proportion of Co content promoted the formation of the active site as well as a hierarchical pore structure of abundant micropores and mesopores caused by Zinc evaporation and P123elimination in the process of pyrolysis,which promoted mass transfer.In addition,the HC-5Co95Zn showed a superb methanol tolerance and excellent stability in alkaline conditions,the oxygen reduction reaction kinetics analysis responsed the process was a quasi-four-electron process.Primary Zn-air battery was assembled with HC-5Co95Zn which showed superior discharge capacity compared with the Pt/C,and it exhibited higher open circuit voltage,power density and specific capacity than Pt/C.HC-5Co95Zn also exhibited excellent oxygen reduction activity in acidic condition.Electron transfer number of HC-5Co95Zn showed as a quasi-four-electron ORR process in 0.1 M HClO4.PEMFC was assembled with HC-5Co95Zn which showed excellent power density and low impedance.（2）F127 and PMMA were used as soft templates and hard templates respectively.Phenolic resin,dicyandiamide and Co（NO3）2·6H2O were used as carbon source,nitrogen source and cobalt source to prepare hierarchical pore structure cobalt-nitrogen-carbon materials.N-doping,Co-doping and 3D macroporous mesoporous hierarchical pore structure slightly decrease the ordering of mesoporous catalysts after carbonization,but increase the graphitization degree of catalysts.A series of carbon materials was tested by electrochemical CV and LSV test.Based on the pure mesoporous,we found N-doping and Co-doping and the hierarchical pore structure is conducive to the oxygen reduction and oxygen evolution reaction,hierarchical pore structure lead to material transfer to the active site involved in the reaction process and the oxygen reduction and oxygen evolution reaction activity of Co-N-3DOM/mC is superior than Pt/C in alkaline medium.Through the calculation of oxygen reduction process,the electron transfer pathway of Co-N-3DOM/mC is a quasi-four-electron process.In addition,the durability of Co-N-3DOM/mC is better than Pt/C under alkaline conditions and its tolerance to methanol is much stronger than Pt/C.