The Synthesis And Properties Of High Performance Fe/N/C Non-Precious Metal Catalysts Towards Oxygen Reduction Reaction

Proton exchange membrane fuel cell(PEMFC)is a device which can convert chemical energy to electricity directly.With the multiple virtues such as high efficiency,environmental friendly and extensive use,especially in vehicles,fuel cells are expected to solve the air pollution caused by the combustion of fossil fuels.To date,the main voltage loss of fuel cells is from oxygen reduction reaction(ORR)at the cathode due to the sluggish kinetics.ORR usually requires Pt electrocatalysts.However,the high cost,scarcity,poor durability and susceptibility to fuel crossover of Pt catalysts severely impede the large-scale applications of PEMFC.Therefore,researchers start exploring inexpensive non-precious metal(NPM)ORR catalysts with high performance to replace Pt-based catalysts.Great progresses have been made in the NMP field recently.However,the activity of NPM catalysts is still far away from satisfaction compared to Pt.In this thesis,we designed and synthesized several heteroatom-doped graphene materials with controlled structure as highly active and robust ORR electrocatalysts.Fuel cells with these catalysts as cathode exhibited high performance,indicating the promising potential for practical application.The main works include the following several parts:(1)The synthesis of highly active and stable Fe,N,S co-doped graphene nanosheets for ORR through the direct catalytic conversion of aminothiazole.N,S co-doped graphene usually exhibits better ORR activity compared to N-doped ones.We employed the bottom-up synthetic procedure using poly-2-aminothiazole(2-AT)as both nitrogen and sulphur sources,FeCl3 as an iron source and carbon black(KJ-600)as support.Graphene nanosheets were formed through catalytic conversion of ploy-2-AT by Fe catalysis during heat treatment,and supported on carbon black.As a result,Fe,N,S atoms were in-situ doped and distributed uniformly on the graphene nanosheets.The carbon black nanoparticles can prevent the graphene nanosheets from self-stacking,providing therefore high surface area and channels for the transport of ORR relevant species.This AT-Fe/N/C catalyst possesses a high ORR catalytic activity in both acid and alkaline medium.In alkaline medium,its mass activity at 1.0 V(RHE)is 0.51 A g-1,which can reach 37.8%of state-of-the-art Pt/C(20 wt%).More importantly,the catalyst exhibits the best durability so far:the catalytic activity only lost 9%after 100 h durability test at 0.80 V.The PEMFC and AEMFC test using the AT-Fe/N/C as cathode catalyst both demonstrates a high power density of 0.56 W cm-2 and 0.164 W cm-2.The AT-Fe/N/C has high tolerance to methanol crossover,which enables its application in direct methanol fuel cell(DMFC)composed of low-cost permeable fiber membrane instead of expensive ion-exchanged membrane,while Pt/C can not.(2)Remarkably increase the ORR active sites by preparing graphene nanoribbon/carbon nanotube composites with abundant carbon defects as support.Previous works proved that the edge carbon atoms exhibit higher activity compared to the basal plane ones towards ORR due to the higher charge density.Heteroatoms doping is another effective strategy to tailor the electronic structure of graphene and improve the ORR activity.We prepared partial longitudinally unzipped multi-walled CNTs(MWCNTs)by chemical oxidation method.The outer wall formed graphene nanoribbons,while the inner walls remained intact nanotubes.The prepared graphene nanoribbons/carbon nanotubes composite contained abundant edge carbon defects,which might improve the number of active sites.The partial unzipped MWCNTs were post-treated with melamine and FeCl3 by hydrothermal and high temperature heat-treatment.The best Fe,N-doped unzipped MWCNTs was achieved at 700 ?,labeled as FeN-uCNT-700.The mass activity at 1.0 V can reach up to 1.36 A g-1 equal to that of state-of-the-art Pt/C.The mass activity at 0.95 V can also reach 42%of Pt/C.The ORR current only reduced 19%after 30,000 s durability test at 0.9 V,thanks to the robust framework of graphene.Further comparison with pristine MWCNTs and KJ600 carbon black demonstrated that highly graphitization carbon defects were found to be easier to coordinate with foreign dopants.As a result,the doping level of FeN-uCNT-700 was improved and ORR activity was promoted.(3)Using CaCl2 derived from CaC03 as templates to prepare Fe,N,S co-doped porous graphitic carbon as highly active and stable ORR catalyst.The synthesis of meso-porous carbon ORR catalysts usually requires harsh conditions to remove the templates,while ORR active sites might be damaged.Herein,we chose CaCl2 derived from CaCO3 dissolution as template,which can be easily removed in facile conditions,melamine-formaldehyde resin with high N content as C and N source,and Fe(SCN)3 as Fe and S source,without adding any carbon supports.The Fe,N,S co-doped porous graphite carbon catalyst(FeNS-G)can be easily obtained by directly carbonization at 900“C.S,N co-doping can indeed further promote the activity.However,too much S would lead to more loss of Fe due to the formation of FeS and acid-leaching,and then reduce the ORR activity.The FeNS-G catalyst exhibited high ORR activity and durability in acid medium.The half-wave potential was only 78 mV negatively to that of Pt/C.The mass activity of FeNS-G at 0.8 V was as high as 10.2 A g-1.The activity only decreased 15.6%after 10,000 s chronoamperometry test at 0.75 V.The activity and durability of FeNS-G are even superior to Pt/C in alkaline medium.The power density of PEMFC utilizing FeNS-G as cathode can reach 0.49 W cm-2.Furthermore,the current of PEMFC can still remain 65%of initial current after 10 h at 0.6 V.The excellent performance of PEMFC demonstrated the potential of FeNS-G for practical application.In this thesis,we developed several Fe/N/C non-precious metal catalysts with high activity and durability towards ORR.New strategies were proposed such as preparing in situ heteroatom-doped graphene by simply pyrolyzing polymer,creating graphene carbon defects to improve ORR active site density,utilizing templates that can easily be removed to form porous graphitic carbon.The as-prepared catalysts exhibited excellent potential for practical application.The results are of importance for the exploration of novel NPM catalyst with high ORR performance.