Design And Development Of Pt-Free Catalysts Based On Carbon Materials For Enhanced Catalytic Activity

Proton exchange membrane fuel cell(PEMFC)is considered to be among the most important technologies for the next generation of sustainable energy.It converts the chemical energy to electrical energy via an electrochemical process.However,the bottleneck of fuel cell is the sluggish kinetic that occurs on the cathodic side of the electrochemical oxygen reduction reaction(ORR).Platinum(Pt)and Pt-based materials are still commonly used as the efficient catalysts for the ORR,but the scarcity of Pt included the low reserve abundance,with the high cost,poor stability and methanol crossover impede the application and the commercialization of PEMFC.As a result,to bring drown the PEMFC barriers,this thesis aimed to design,study and develop Pt-free catalysts based on carbon materials with excellent ORR performance,by varying the synthesis conditions and precursors employed.This study includes:The development of a three-dimensional(3D)N-doped graphene as a new promising metal-free catalyst.N-doped graphene has attracted a considerable interest in the development of a new promising metal-free catalyst for ORR to replace Pt-based catalysts.Herein we reported a facile fabrication of three-dimensional N-doped graphene using graphitic carbon nitride(g-C₃N₄)and graphene oxide(GO)as raw materials.Urea was used as a precursor of synthesizing g-C₃N₄and N doping.Thermal treatment has been done at different temperatures(800?-1000?).As a result,a large surface area(442 m²g^-1)and high N content(4.9 at.%)with a unique porous structure were obtained for the sample U₃/3DNG-900.The obtained catalyst showed an outstanding catalytic activity with on-set potential(E₀)of 0.88 V vs.RHE and half wave potential(E_1/2)of 0.81 V vs.RHE,with superb stability and methanol tolerance.The structural characterizations of different materials demonstrated that the combination of an optimal heating temperature,a high N content and a porous structure was essential for high ORR electrocatalytic activities.Besides the investigation of the catalytic performance in the ORR,further researches were done on the 3D N-doped graphene obtained at 1000°C(U₃/3DNG-1000)as a cost-effective photocatalyst for hydrogen production.The characterizations identified that the porous 3D structure with hydrogen bonding and?-?interaction with a better charge transfer resulted from the nitrogen doping were the major reasons for the enhanced photocatalytic performance.Iron-nitrogen-carbon(Fe-N-C),a noble metal-free catalyst holds a great place as efficient electrocatalyst to Pt.However,the synthesis and the stabilizations of single iron atoms that tend to migrate and aggregate,without jeopardizing the catalytic activity remain challenging.Here we report a facile fabrication of Fe-N-C catalyst with single iron atoms dispersed on N-doped carbon as efficient noble metal-free electrocatalyst,g-C₃N₄nanosheet as a template and nitrogen source.Fe ions are absorbed on the surface of g-C₃N₄ and imprisoned after the addition of pyrrole to stop any Fe migration susceptible to aggregate and form nanoparticles.During the pyrolysis,Fe atoms coordinated with N provided by g-C₃N₄and pyrrole leading to the isolated Fe atoms on graphitic carbon.During the synthetization,no iron crystals or ferrous complex were formed at 800°C where the aberration-corrected high-angle annular dark-field-scanning transmission electron microscopy(AC HAADF-STEM)image revealed the existence of Fe as single atoms.Moreover,Fe-N species were introduced in the graphitic carbon as high active sites,where the coexistence of micro and mesopores were favorable to provide a high density of active sites.The electrocatalytic activity was investigated in both alkaline and acidic media.Fe-N-C-800 exhibited a good electrocatalytic performance with E₀of 0.9 V vs.RHE,E_1/2 of 0.82 V vs.RHE and a long stability of 85%with a better methanol tolerance than commercial Pt/C.The isolated Fe atoms coordinated with nitrogen(Fe Nx)in carbon matrix were widespread accepted as the real active centers for ORR.But more efforts still need to be taken to further improve their ORR performance in acidic solution.Here,we report a novel approach to synthesize the single-atom Fe catalyst supported on N,S co-doped graphitic carbon by the pyrolysis of Fe-doped thiourea/zeolitic imidazolate framework(NS/Fe/Z8),wherein thiourea is utilized as N and S source.AC HAADF-STEM analysis shows highly dispersive of single Fe atoms on graphitic carbon.X-ray photoelectron spectroscopy results indicate that the Fe Nx and N,S-co-doped structure are achieved in carbon matrix.The optimized NS/Fe/Z8-1 catalyst exhibits an excellent ORR activity in acidic solution,including on-set and half-wave potentials of 0.9 and 0.84 V(vs.RHE),respectively,which are comparable with those of commercial Pt/C(0.9 and 0.86 V)and most of the state-of-the-art noble metal-free electrocatalysts.In addition,NS/Fe/Z8-1 displays a long-term durability which is reflected by only a 10 m V negative shift after 40000 cycles,indicates a promising ORR catalyst in application of fuel cells.The above results,contributed to provide a deep understanding of the mechanism of the ORR process,with both metal-free and noble metal-free catalysts in alkaline and acidic media.It updated the important factors responsible of an enhanced electrocatalytic activity and provided a possibility to produce a non-precious catalyst to replace Pt as catalyst in fuel cells.