Preparation And Electrocatalytic Properties Of Carbon Based Single-Atom Catalyst

Proton exchange membrane fuel cells（PEMFC）and dye-sensitized solar cells（DSSCs）,as clean energy conversion and storage devices,can efficiently convert chemical energy and solar energy into electrical energy,and the preparation process is simple,environmentally friendly and other advantages,which have attracted widespread attention in the field of new energy,and are considered to be an effective ways to solve the serious environmental pollution and energy crisis.However,the key problems restricting the practical application of the above two types of batteries in the energy field are the stability and cost of the catalysts used for their cathodic oxygen reduction（ORR）and iodine reduction reaction（IRR）.Pt based catalysts show excellent catalytic activity in ORR and IRR,but their high price,low abundance and insufficient stability limit the large-scale use of PEMFC and DSSCs.Although the traditional non noble metal carbon based catalysts have outstanding electrocatalytic activity,their low atom utilization efficiency and the sudden drop of stability in harsh chemical environment still hinder the practical application of batteries.In recent years,carbon based single atom catalysts（C-SACs）,benefiting from the maximum atomic efficiency,high activity,high stability and excellent conductivity,are considered to be one of the most potential catalytic materials for large-scale use of PEMFC and DSSCs.So far,a variety of synthesis strategies have been proved to be effective methods for the synthesis of C-SACs,especially high temperature pyrolysis.However,the high temperature migration and easy aggregation of metal atoms result in low load of SACs.Meanwhile,some heteroatom precursors are easy to decompose at high temperature,which makes the yield of catalyst lower.These greatly hinder the large-scale preparation of SACs and limit their application in the field of electrocatalysis.Molten salts,as a strong polar solvent,can effectively inhibit the agglomeration of metal atoms,well dissolve the precursor,and improve the yield of catalyst.Consequently,the aim of this study is to synthesize C-SACs with high stability,high load and excellent electrocatalytic properties through molten salts assisted pyrolysis,and apply them to PEMFC and DSSCs to achieve the practicability of these two types of batteries.Firstly,KCl and ZnCl₂ are used as molten salts system.According to the phase diagram of the salts system and the pyrolysis characteristics of small molecular precursor1-butyl-3-methylimidazole（[Bminm]Br）,KCl and ZnCl₂ salt system with molar ratio of0.44:0.66 are selected to promote the high-temperature pyrolysis of[Bminm]Br into porous graphitized carbon materials.Based on this,1.71%Fe-N/C-SAC is successfully prepared by pyrolysis of[Bmim]Br and FeCl₃ in molten salts system.When this SAC is used to catalyze ORR in both acid and alkaline media,this SAC exhibits electrocatalytic activity andstability comparable to that of Pt/C,and has excellent methanol resistance.Furthermore,the ORR activity of 1.71%Fe-N/C in acid-base medium is still very good after 48 h and 85℃perchloric acid solution heat treatment,indicating that it has excellent chemical stability.A series of characterizations show that the excellent ORR performance of 1.71%Fe-N/C-SAC is ascribed to its porous carbon matrix with large specific surface area,which makes the Fe active site completely exposed,promoting the transfer of reaction intermediates and the rapid reaction.Additionly,O₂-Fe-N₄ active sites are firmly anchored in the carbon skeleton,which improves the corrosion resistance of this SAC,it therefore shows very excellent stability in the acid and alkaline ORR.S-doped carbon supported Co catalyst（Co-S/C-SAC）is successfully prepared by pyrolysis of Co-MOF-74,Na₂S₂O₃ and glucose in the molten salt system of KCl and NaCl.ICP test result shows that the load of this SAC is 7.35 wt%.The results of morphology and pore structure show that the Co-S/C-SAC has a sheet structure with a larger pore volume of7.50 nm.The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy（AC-HAADF-STEM）and Elementselective X-ray absorption fine structure（XAFS）characterization exhibit that the high-density single atom Co was embedded in the carbon skeleton with the structure of Co-S₄.The high active sites,large pore lamellar structure and stable Co-S₄ structure make the counter electrode material and DSSCs prepared by Co-S/C-SAC have IRR activity,8.18%photoelectric conversion efficiency and excellent stability compared with pyrolytic Pt.