| A fuel cell is an electrochemical device that converts the chemical energy of a small molecules(e.g.,hydrogen and ammonia)and an oxidizing agent(often oxygen)into electricity through a pair of redox reactions.Fuel cells have many advantages such as high energy conversion efficiency,wide availability,clean and pollution-free,which would be one of the key technologies that can realize the energy and environmental sustainability in future.Currently,low-temperature hydrogen fuel cells are the most widely and extensively studied fuel cell,while low-temperature direct ammonia fuel cells(DAFCs)have recently received increasing attentions due to their mature industrial chain of ammoia production,storage and transportation.The main bottleneck restricting low-temperature hydrogen FCs is the lack of highly efficient and low-cost cathodic oxygen reduction reaction(ORR)catalysts,while the absence of high-performance ammonia oxidation reaction(AOR)catalysts that can overcome the sluggish anodic ammonia oxidation kinetics restricts the development of low-temperature DAFCs.Noble metal nanomaterials often play important roles in the field of energy electrocatalysis,while graphene has excellent physical and chemical properties along with high electrical conductivity,holding great prospects in preparation and application of energy storage and conversion materials.In this dissertation,we mainly focus on the design and synthesis of doped graphene and low-platinum(low-Pt)alloy nanomaterials,and their applications in low-temperature hydrogen FC cathode catalysts and low-temperature DAFC anode catalysts.In the cathodic ORR catalyst part,we aim to design and prepare novel doped graphene with high catalytic activities,find high-efficiency and high-quality stratety to prepare new catalysts,realize efficient and controllable heteroatom doping into graphene carbon matrix,investigate how to control the microstructures of graphene by heteroatom doping,and study the effects of mictrostructures of catalysts in the electrocatalytic performances.In the anodic AOR catalyst part,we focus on developing low-Pt catalysts of Pt M alloy nanoparticles supported on oxide-carbon composite support,studying their electrocatalytic AOR performances,discussing their catalytic mechanism by the combination of theoretical studies,and evaluating some optimal catalysts in DAFC devices.The main points have been summarized as follows:(1)The melamine formaldehyde resin generated in the hydrothermal process is used to realize the pore formation and N-doping of graphene by pyrolysis treatment.Further co-doping with S would induce more defects and catalytic active sites into the graphenre carbon matrix,resulting in an enhanced ORR activity.Additionally,we have carefully investigated how thermal activation temperature affect the morphology,structure,composition,and defects of the N,S-co doped three-dimensional reduced graphene oxide(NS-3Dr GO)catalysts,and further studied their structure-property relations.Among them,the NS-3Dr GO-950 catalyst has the highest percent of active N(pyridinic N and graphitic N,74.8%)and active S(thiophene-S,79.8%),showing superior ORR activity and stability.Furthermore,its half-wave potential is measured to be 0.732 V,which is close to that of the commercial Pt/C catalyst.(2)The F,N,and S tri-doped graphene(FNSG)cataltsts were synthesized by a novel tertiary N-precursors inspired strategy.In this work,we have synthesized single N-doped graphene,F,N co-doped graphene,as well as F,N,and S tri-doped graphene by simply adjusting different precursors.Significantly,the relations between ORR catalytic activity and surface area,active N content,active F content,as well as the S doping have been constructed to unveil the activity trend for different catalysts.Results indicated that the most critical factor affecting the ORR activity of one catalyst lies in its intrinsic activity and density of active sites,while its defect and porous architecture also provide great contributions on activity enhancement.Among the studied catalysts,the FN3SG catalyst with surface area of 115.5 m2g–1,F content of 1.09 at.%,N content of 6.39 at.%,and S content of 0.95 at.%has shown the best ORR activity,as can be confirmed by its half-wave potential of 0.803 V,which is comparable to that of the commercial Pt/C catalyst.(3)3D honeycomb-like structured graphene(HSG)is rich of defects and showing high initial surface area.Herein,3D porous graphene doped with Fe/N/S and supported with Fe3O4nanoparticles(Fe3O4/Fe NSG)has been prepared by using HSG as a support.The morphology,structure,and composition of the doped graphene can be well controlled by adjusting the N and S precursor ratios.Additionally,introducing Fe into the catalyst can induce the formation of highly active Fe Nx active sites,further enriching the types of active sites in the doped graphene.Among the studied catalysts,the Fe3O4/Fe NSG-3 has a large surface area of 530.5m2 g–1,Fe content of 0.31 at.%,N content of 1.71 at.%,and S content of 0.30 at.%,exhibiting optimal ORR activity with a half-wave potential of 0.810 V,better than commercial Pt/C catalyst.(4)After introducing Co into the precursors for NS-3Dr GO synthesis,Co9S8nanoparticles embedded in N,S co-doped 3Dr GO with a core-shell structure(Co9S8@NS-3Dr GO)have been prepared.During the process,we have realized controllable synthesis of the crystal structure,micromorphology,size and dispersion of Co9S8 nanoparticles,and heteroatom doping content in graphene by changing thermal activation temperatures.The unique core-shell structure and a synergetic effect between Co9S8 and doped graphene have resulted in the catalyst a dramatically enhanced ORR activity.At the same time,the obtained catalysts also showed superior oxygen evolution reaction(OER)performance due to the introduced transition Co metal.Among the studied catalysts,the Co9S8@NS-3Dr GO-850 has showed the optimal catalytic activity with an ORR half-wave potential of 0.826 V and an OER overpotential of 317 m V at 10 m A cm–2,and the potential difference between ORR and OER in 0.1 M KOH electrolyte is approximately 0.90 V.Moreover,the Co9S8@NS-3Dr GO-850catalyst showed a good performance in rechargeable Zn-air battery with a peak power density of 83.5 m W cm–2,which is comparable to that of the commercial Pt/C+Ru O2 catalyst.(5)We have prepared a novel efficient AOR catalyst,in which ternary Pt Ir Ni alloy nanoparticles were well dispersed on a binary composite support consisting of porous silicon dioxide(Si O2)and carboxyl-functionalized carbon nanotube(Pt Ir Ni/Si O2-CNT-COOH)through a sonochemical-assisted synthesis method.The Pt Ir Ni alloy nanoparticles,with the aid of abundant OHad provided by porous Si O2 and the improved electrical conductivity by CNTs,exhibit remarkable catalytic activity for the AOR in alkaline media.The introduction of Ni raises the center energy of the density of states projected onto the group d-orbitals of surface sites and thus lowers the theoretical onset potential for*NH2 dehydrogenation to*NH compared to Pt Ir alloy.We found that the support and composition of the catalyst along with ammonia concentration and operating temperature showed great influence on the AOR activity.The optimal Pt Ir Ni1/Si O2-CNT-COOH catalyst showed a reduced onset potential of?0.32 vs.RHE and increased peak current density(1382.7 A g-1)at 80°C,which are 80 m V negative shift and?11 times higher than at room temperature.Additionally,this catalyst with an activation energy of 50.0 k J mol–1 showed a high electrocatalytic ammonia oxidation activity and much reduced cost compared to the commercial Pt Ir/C catalyst.(6)A sonochemical-assisted synthesis method was used to prepare an efficient AOR catalyst,in which ternary Pt Ir Zn nanoparticles were highly dispersed on a binary composite support comprising of cerium oxide(Ce O2)and zeolitic imidazolate framework-8(ZIF-8)derived carbon(Pt Ir Zn/Ce O2-ZIF-8).During the synthesis process,deionized water and ethanol were used as the solvents to well disperse the composite support and then adsorbing the metal precursors to the support,while porous ZIF-8 derived carbon(surface area:?600 m2g–1)and Zn2+ions are beneficial for absorbing and dispersing PGM precursors(Pt Cl42–and Ir Cl62–).After the reduction by strong reducing agent of sodium borohydride,the ternary Pt Ir Zn nanoparticles with an average particle size of 2.3±0.2 nm were uniformly dispersed on the composite support.The alloying of Zn into Pt Ir could suppress the hydrogen adsorption and improve the ammonia dehydrogenation.The activation energy of the selected Pt Ir Zn2/Ce O2-ZIF-8 catalyst is 14.1 k J mol–1 lower than commercial Pt Ir/C catalyst,suggesting its higher intrinsic activity towards AOR.Using the Pt Ir Zn2/Ce O2-ZIF-8 as anodic catalyst,the DAFC reaches a peak power density of 91 m W cm–2.However,this catalyst exhibited a large charge transfer resistance and low mass transport relative to commercial Pt Ir/C.Employing an alternative composite support of Si O2 and CNT-COOH,we have achieved an enhanced peak power density of 282 m W cm–2on Pt Ir/Si O2-CNT-COOH and 314 m W cm–2on Pt Ir Zn2/Si O2-CNT-COOH anodic catalysts,respectively,superior to that of commercial Pt Ir/C catalyst.In addition,the higher DAFC performance of Pt Ir Zn2/Si O2-CNT-COOH than Pt Ir/Si O2-CNT-COOH further indicates the significance of alloying Zn with Pt Ir. |
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