Preparation And Electrocatalytic Oxygen Reduction Reaction Performance Of Metal-supported Porous Carbon Materials

Nowadays,the energy and environmental problems are becoming increasingly serious.The combustion of fossil fuels contributes main form of energy,which generate a large amount of dust and harmful gases,threatening the living environment seriously.Thus,the developing appropriate and sustainable energy is the most important task in current scientific research.Fuel cell,an efficient and environmentally friendly energy conversion device,works without the limitation of Cano cycle.Fuel cell has potential applications in domestic electricity or automobile power field.Therefore,developing the technology of fuel cell should be an important strategy and goal.Oxygen reduction reaction（ORR）which occurred in the cathode of fuel cell,shows the slow reaction kinetics and requires efficient catalyst to accelerate the reaction.Pt-based catalysts have excellent performance towards ORR,but Pt is expensive.So the commercialization process of fuel cell has been seriously restricted by this factor.To date,researchers focus on the development of low-cost electrocatalysts with high ORR activity,and seek to replace Pt-based catalysts in fuel cell.In this thesis,three kinds of metal-supported porous carbon electrocatalysts were prepared,and their electrocatalytic ORR performances under alkaline conditions were investigated.（1）Synthesis and electrocatalytic ORR performance of Fe-N-C catalysts.In this work,N-doped porous Fe/Fe₃C@C electrocatalysts were synthesized by the pyrolysis of the hexamethylenetetramine（HMT）-incorporated MIL-100-Fe（HMT/MIL-100-Fe）at different temperatures（700-1000℃）under N₂ atmosphere.HMT moleculers could enter into the pores of MIL-100-Fe via wetness impregnation method.After removing excess water by rotary evaporation,the HMT/MIL-100-Fe was pyrolyzed at different temperatures（700-1000℃）under N₂ atmosphere.The results show that all pyrolyzed samples have porous textures with middle specific surface areas.XPS results demonstrate the successful introduction of N atoms into carbon framework.Sample Fe-N₂-800 prepared by annealing the precursors with the HMT/MIL-100-Fe weight ratio of 2 at 800℃ exhibits the best electrocatalytic activity towards ORR in terms of onset potential and current density due to high graphitic N and pyridinic N content.The embedded Fe/Fe₃C nanoparticles and Fe-N_x active sites in these samples could also boost the ORR activity synergistically.Moreover,sample Fe-N₂-800 demonstrates a dominant four electron reduction process,as well as excellent long-term operation stability and methanol crossover resistance.（2）Synthesis and electrocatalytic ORR performance of Co-N-C catalysts.In this work,the carbon source and cobalt source are aniline/pyrrole and cobalt nitride,respectively.Co₂N nanoparticles embedded in mesoporous carbon materials（Co₂N-x-T@NC）were synthesized via in situ polymerization and pyrolysis at 600-800℃ under NH₃ atmosphere.More nitrogen atoms could be doped into carbon framework due to the NH₃ atmosphere.The ORR performance of these samples showed an increase and then decrease with the increase of heat-treatment temperature,which might be due to low nitrogen content in the samples synthesized at high pyrolysis temperature.In addition,with the increase of cobalt doping,the ORR performance also firstly increase and then decrease,it might be that excessive amount of cobalt-doping could lead to the formation of metal cobalt that hinders the generation of Co₂N（Co-N）active sites.Sample Co₂N-2-700@NC perpared by the addition of 2 mmol Co source and pyrolysis at 700℃ exhibits the excellent electrocatalytic activity towards ORR in terms of onset potential（-0.038 V vs.SCE）,have-wave potential（-0.126 V vs.SCE）and current density,which might be attributed to the high total nitrogen content（11.86at.%）,graphitic N/pyridinic N content,Co-N typed nitrogen content and relative high BET surface(362 m² g^-1).Furthermore,sample Co₂N-2-700@NC shows better long-term stability and tolerance to methanol crossover than Pt/C.（3）Synthesis and electrocatalytic ORR performance of Fe-Co disulfide-supported mesoporous carbon catalysts.In this work,the carbon source and metal source are aniline/pyrrole and different ratio of cobalt nitride/iron nitride,respectively.Iron-cobalt sulfides nanoparticles embedded in N,S-doped mesoporous carbon(Fe_xCo_1-xS-T@NS-MC)were synthesized via in situ polymerization and pyrolysis at 1000 ℃ under N₂ atmosphere.The results show that all of samples have relative high specific surface area and contain a certain amount of iron-cobalt metal sulfide as the ORR active site.The ORR performance of the Fe_xCo_1-xS-T@NS-MC electrocatalysts were dependent on the proportion of metal sulfide.On the other hand,the ORR performance of the Fe_xCo_1-xS-T@NS-MC samples increase gradually with the increase of heat-treatment temperature,which is mainly due to the better crystallinity of metal sulfides and higher specific surface area.The sample Fe_0.5Co_0.5S-1000@NS-MC pyrolyzed at 1000 ℃ with an optimal iron/cobalt molar ratio（x=0.5）exhibits the best ORR performance,which has the positive onset potential of 0.947 V vs.RHE,half-wave potential of 0.842 V vs.RHE and a large limiting current density of 5.63mA·cm^-2.Moreover,the results obtained from RRDE measurements indicate a four-electron process towards ORR for Fe_0.5Co_0.5S-1000@NS-MC catalyst.The combination and synergy of the controllable active centers（Fe-adjusted Co₉S₈/CoS with appropriate ratios）and hierarchical porous structures with large specific surface area render the outstanding ORR activity and electrochemical stability.Therefore,the results indicate that bimetallic sulfides-based materials are promising as ORR electrocatalysts.