Preparation Of Transition Metal Species/Carbon Catalysts And Study On Their Catalysis For Oxygen Reduction Reaction

The rapid increasing energy needs have inspired a widespread research on developing clean energy sources to replace the fossil fuels.Fuel cells have been widely regarded as alternative energy technologies in the future owe to their low cost,high efficiency and environmental benignity.However,due to the slow kinetics of oxygen reduction reaction(ORR),the development of efficient ORR catalysts becomes one of the key tasks in this renewable energy technology.As the most effective catalyst for oxygen reduction reaction in the fuel cell cathode,Pt is too expensive and scarce to support the massive manufacture of fuel cells.Therefore,the only way to slove the problem of slow kinetics towards ORR is exploring the alternatives of Pt.Doped carbons and transition metal species are the potential materials due to their good ORR activities,and the synergistic effect between these two components can enhance their own strengths and avoid each shortages.We aim to prepare the transition metal species/doped carbons via several simple,facile and eco-friendly methods.For different metal species,we discussed the growth,structures,and elements.For different types of carbon materials,such as graphene,enteromorpha algae or shaddock peel,we designed nitrogen doping or nitrogen and phosphorus dual-doping schemes.For different composites,we conducted various electrochemical tests.we successfully synthesizing high-performance and stable ORR catalysts by simple and eco-friendly methods,where cheap graphite oxide or waste biomass was used as raw materials,water or ethanol was applied as solvents.A magnetically separable hybrid of cobalt ferrite(CoFe₂O₄),anchored on nitrogen-doped reduced graphene oxide(CoFe₂O₄/NG)was successfully prepared via a facile solvothermal reaction followed by calcination at 500?.The magnetically separation is much more convenient and timesaving than ordinary centrifugalization.After analyzing the components,structures,morphologies and the ORR performances under alkaline condition,we get following conclusions:The GO in CoFe₂O₄/NG is well-reduced,the nitrogen atoms are successfully doped into the graphene sheets,and CoFe₂O₄ nanoparticles are well-dispersed on the graphene sheets;The XPS results indicate the presence of N-species active sites,such as pyridinic-N,Fe-N/C and Co-N/C;The ORR activities follow as CoFe₂O₄/NG>CoFe₂O₄/G>CoFe₂O₄,which suggests that CoFe₂O₄nanoparticles,the doped N species and graphene support are all functional to the catalytic enhancement.While the CoFe₂O₄/NG surpasses the contrastive Co₃O₄/NG and Fe₃O₄/NG due to its more metal active sites and the better,denser dispersion of nanoparticles.In addition,the CoFe₂O₄/NG also has better catalytic activity than unannealed-CoFe₂O₄/NG,which may owe to the deeper reduction of GO and the higher crystallinity of CoFe₂O₄;Finally and more notably,CoFe₂O₄/NG shows comparable ORR current and potential with respect to the commcial Pt/C,following 4e pathway,and it gives better methanol-tolerance,less CO poisoning and higher abundance than Pt/C does,which make it a promising alternative for costly Pt-based electrocatalysts in fuel cells.A new ORR catalyst named as Cu^?MnFe^?O/NG,which is suitable for both alkaline and neutral conditions,has been synthesized via a facile,clean and low-cost solvothermal method without any further treatments.The stable Cu^?MnFe^?O/NG catalyst successfully combined crystal Fe₂O₃,MnCO₃ and amorphous Cu^?MnO(Cu₂O&Mn_xO_y) with nitrogen-doped graphene(N/C atomic ratio up to 10.56%),to afford various active sites toward ORR.The growth mechanism is detailed discussed.It exhibits the outstanding ORR performance in both pH-situations with comparable half-wave potential and current density to commercial Pt/C,and it has a four electron pathway during ORR process.Furthermore,comparative results have suggested in this work that the combination of Fe₂O₃,Mn CO₃,Cu^?MnO and nitrogen doping exhibits different degrees of ORR effects in different media.The synergistic effect among these components can remarkably enhance the catalytic activity to ORR in both alkaline and neutral media.More notably,excellent methanol tolerance and high stability of Cu^?MnFe^?O/NG can make it a promising alternative for expensive commercial Pt-based electrocatalysts in both alkaline and neutral fuel cells.A novel ORR catalyst named as Ni-TiN/G is synthesized by only 2h-long hydrothermal reaction and a calcination treatment at 500?,the final product named as “h-Ni-TiN/G” is obtained.It is a hybrid of metal Ni,nano TiN and G,where Ni and TiN are separated.By analyzing the components,structures,morphologies and the ORR performances of as-prepared samples,it is found that:The nano metal-Ni particles,20-40nm TiN nanoparticles and G can all contribute to catalytic activity;After heat treatment,the GO was further reduced and then the ORR activity of h-Ni-TiN/G is slightly better than that of Ni-TiN/G under alkaline condition,where both of them follow 4e transfer pathway;h-Ni-TiN/G shows an half-wave potential,which is only negatively 25 mV compared with commercial Pt/C.Finally,h-Ni-TiN/G shows a certain ORR activity in the neutral condition,and it is much stable than commercial Pt/C under both pH-situations.A honeycomb-like nitrogen-doped carbon derived from enteromorpha algae(N-EA),using a one-step pyrolysis process at only 600? to simultaneously achieve doping,carbonization and activation.After anchoring CoFe₂O₄ nanoparticles on the N-EA by a facile hydrothermal growth,the hybrid,marked as CoFe₂O₄/N-EA,not only inherits the high surface area,porous structure,active nitrogen species and good conductivity from N-EA,but also benefits from the active sites from CoFe₂O₄.The synergistic effect makes CoFe₂O₄/N-EA an excellent catalyst for oxygen reduction reaction(ORR) with more positive half-wave potential and close current compared with commercial Pt/C.4e-transfer pathway and outstanding stability are confirmed with systematical tests.N,P dual-doped shaddock peel derived(NPSP)hierarchical porous carbons were simply prepared via one-step pyrolysis process under different temperatures,accomplishing carbonization,activation and dual-doping synchronously.During 600?900?,contrast results indicate the contents of pyridinic-N,graphitic-N and P-C species increase with the rising of temperatures,and the temperature also affects the degree of graphitization,surface area,morphologies,thus influences the ORR performance.More importantly,the NPSP-900demonstrates an outstanding ORR activity with a comparable half-wave potential(0.83V vs.RHE)and higher current density with respect to commercial Pt/C,following 4e transfer pathway.Simple preparation,waste utilization,excellent ORR performance,good methanol tolerance and superior stability of NPSP-900 could make it a promising cathode candidate for fuel cells.