Preparation And Investigation Of Oxygen Reduction Performance Of Heteroatom-doped Carbon Materials Catalysts

Fuel cells are considered to be one of the most promising new energy technology in the future due to their pollution-free,high energy density and high conversion efficiency.However,the slow kinetic process of its cathode oxygen reduction reaction（ORR）greatly reduces the efficiency of the fuel cell.At present,the ORR catalysts with the best catalytic performance are still platinum-based catalysts,but the shortcomings of limited reserves,high cost,and poor stability limit their large-scale commercial application.Therefore,it is of great importance to develop non-precious metal catalysts with low-cost,excellent catalytic activity to replace the traditional Pt-based catalysts.Heteroatom doped carbon materials are regarded as the most promising metal-free ORR catalysts have encouraged intensive research,but the synthesis method,doping efficiency and active sites of catalysts are remains unclear.In this paper,we developed low-cost and green method to regulate the morphology and active sites of catalysts,to synthesis heteroatom（N/S）doped carbon nanomaterials for efficient ORR catalysts.The research results in the study are summaraized as follows:（1）The pomelo peel pyropolymer was used as precursor and actived by KOH at room temperature.And then mechanically mixed with thiourea（nitrogen source and sulfur source）followed by heat-treated at high temperature.Ultimately,a highly active nitrogen and sulfur co-doped porous carbon materials catalyst（PP350KOH800-S）was obtained.In alkaline media,PP350KOH800-S shows excellent ORR catalytic activity with the half-wave potential of is 0.87 V.After 5000 cycles of CV tests,the onset potential and half-wave potential of the catalyst negatively shifts by only 5 mV and 7 mV,and with a slightly descend of limiting diffusion current density,indicating that PP350KOH800-S exhibits a excellent long-term stability.The best ORR catalytic activity is mainly ascribed to the large specific surface area(983 m² g^-1),the porous structure,the high degree of defects,and the synergistic effect of nitrogen and sulfur atoms.（2）The walnut shell pyropolymer used as a precursor of carbon and nitrogen,and mechanically mixed with ZnCl₂ and FeCl₃.Then,a nitrogen doped carbon nanomaterials catalyst（WS350Z900@Fe）was obtained by high temperature pyrolysis.This catalyst has the characteristics of high specific surface area,high content of pyridinc-N and hierarchical porous structure.We found that the addition of ZnCl₂ could promote the formation of porous structure,and the introduction of FeCl₃ could facilitate the graphitization process of carbon materials.At the same time,it can also affect the total nitrogen content of catalysts and the ratio of diffirent nitrogen species.When the mass ratio of precursor/Zn Cl₂ is 1:2 and Fe is 5 wt.%,the catalyst shows the outstanding ORR performance:in 0.1 M KOH,its onset potential and half-wave potential is 0.98 V and0.86 V,and the ORR almost completely followed the 4 electron mechanism.In addition,the catalyst exhibits a better long-term stability and good methanol tolerance than Pt/C catalyst.Furthermore,density functional theory calculation indicates that the rate-determining step of the ORR of these catalysts is the protonation of*O₂.Also,the pyridinic-N structure is conductive to reduce the reaction barrier,and resulting in a better ORR activity for WS350Z900@Fe.（3）Using the mixture of glucose,thiourea and ammonium chloride as the raw materials,a nitrogen and sulfur codoped carbon（N,S-C-900）nanomaterials ORR catalyst with highly catalytic activity was prepared by high temperature pyrolysis in nitrogen atmosphere.The N,S-C-900 obtained at 900°C exhibts the outstanding catalytic activity with the half-wave of 0.86 V and the limited diffusion current density of-6.2 mA cm^-2,significantly better than Pt/C catalyst(0.86 V,-5.95 mA cm^-2).At the same time,the electron transfer number of the catalyst was 3.96,indicating that it catalytic ORR followed a 4 electron reaction process.Moreover,the catalyst towards a better long-term stability,as well as good methanol tolerance.The improved ORR catalytic activity of N,S-C-900 is attributed to its high BET surface areas which exposes more active sites,and the synergistic effect between nitrogen and sulfur atoms.