Investigation On Three Dimensional Graphitic Carbon-based Catalyst For Direct Methanol Fuel Cells

Direct methanol fuel cells?DMFCs?have attracted widely attention due to their favorable features including high energy density,compact in size,enviromental friendliness and great potential applications in portable or mobile power and electric vehicles.To resolve the key technical issues of high cost and short life in the commercialization of DMFCs,the research has been focused on the three dimensional graphene supports and novel graphitic carbon catalyst with special structure in this dissertation.Three-dimensional?3D?structured Pt/graphene aerogel?GA?was synthesized by a facile one-pot solvothermal process.Pt/GA catalyst exhibits a well-developed 3D interconnected porous graphene framework with dendritic Pt nanoparticles?NPs?uniformly decorated on the surface of graphene aerogel.It's worth noting that the as-made Pt/GA catalyst exhibits a much higher electrocatalytic activity and stability than the Pt/graphene for methanol electrooxidation.Pt/C/GA hybrid electrocatalyst is synthesized through a facile and green hydrothermal process.Thanks to the unique 3D graphene framework structure,Pt/C/GA hybrid catalyst demonstrat s enhanced stability towards methanol electrooxidation with no decrease of electrocatalytic activity.Pt/C catalyst lost nearly 40%of its initial activity after1000 cyclic voltammetry cycles,by contrast,only 16% for Pt/C/GA.Importantly,our synthetic strategy can be easily applied to other supported Pt-based catalyst to improve its durability.Three dimensional hierarchical nitrogen-doped graphene?3D-NG? networks were successfully fabricated through a feasible solution dip-coating method with commercially available polyurethane sponges as template,graphene oxide?GO?as building block and urea as nitrogen source.A spongy template can help hinder the graphene plates restacking in the period of the annealing process.The Pt/3D-NG catalyst was synthesized employing a polyol reduction process.The resultant Pt/3D-NG catalyst exhibits 2.3 times higher activity for methanol electrooxidation along with the 12% increase in stability as compared with Pt/G owing to their favorable features including large specific surface area,high pore volume,high N doping level,and the homogeneous dispersion of Pt nanoparticles.The mass ratio of GO and urea has a great impact on the catalytic performance of as-prepared Pt/3D-NG catalyst towards methanol electrooxidation.Electrochemical measurements indicate that the Pt/3D-NG catalyst prepared at the mass ratio of 1:10 exhibits the best activity and durability towards methanol electrooxidation.Three dimensional porous nitrogen-doped graphene aerogel?3D-NGA? was successfully fabricated via a combined hydrothermal self-assembly,thermal treatment and template-removing process with poly-pyrrole as N source and Fe₂O₃ as hard template.Fe Cl₃ acts not only as the oxidant for polymerization of pyrrole,but also as the precursor of Fe₂O₃ nanoparticles,which using as"spacers" could efficiently prevent the agglomerate of graphene during the annealing process.Pt/3D-NGA holds a high electrochemical active surface area?ESA?of 90.7 m²·g^-1,and exhibits 4.39 times higher activity for methanol electrooxidation as compared with Pt/G.After celerated potential cycling test,only 21.3%of the initial ESA of the Pt/G remain,while,the Pt/3D-NGA preserves 55.9%of its initial value after the same condition.The excellent electrocatalytic performance of Pt/3D-NGA for MOR may be derived from the unique structure of 3D-NGA.First,the interconnected 3D porous graphene framework will maximize accessibility for fuel to the catalysts to ensure an effective mass transfer.Second,3DN-doped graphene framework could form a multidi-mensional conductive network to provide sufficient electronic conductive channels.Finally,the existence of large quantities of doped N atoms can improve dispersion of Pt NPs and provide strong interaction between the Pt NPs and the support,thus inhibiting particle agglomeration and particle detachment from the support.Three dimensional hierarchical porous nitrogen-doped graphene aerogel?NGA?was synthesized through a simple supramolecular polymerization-assisted approach by pyrolysis of a supermolecular aggregate of self-assembled melamine with cyanuric acid and graphene oxide.Melamine-cyanurate supermolecular aggregates work as the structure-directing agent playing a vital role in guiding the formation of unique 3D architecture.In addition,supermolecular aggregates act as not only a "spacer" to suppress the re-stacking of graphene nanosheets but also as a self-sacrificial pore-forming agent as well as a nitrogen source for in-situ N-doping.The resulting metal-free NGA products exhibit the enhanced ORR performance in terms of positive half-wave potential which is only⁴3 m V lower than that of a commercial Pt/C,four-electron-transfer process,good durability and outstanding methanol poisoning tolerance.Besides,it also performs as a good support for Pt particles.Consequently,the resultant Pt/NGA exhibits 1.5 times higher activity for methanol electrooxidation along with the 15%increase in stability as compared with commercial Pt/C.Hollow nitrogen-doped graphitic carbon tube?h-NGCT?was fabricated via a combined hydrothermal and thermal treatment process with melamine-cyanurate supramolecular aggregates as both the template and nitrogen source.The h-NGCT exhibits large-diameter hollow tubular structure.The carbonization temperature greatly affects the structural properties and catalytic performance of h-NGCT.The wall thickness of the tube will progressively become thinner with increasing carbonization temperature,while the nitrogen content is obviously declining.Electrochemical measurements indicate that the h-NGCT-900 catalyst prepared at 900? exhibits the best ORR performance including positive on-set and half-wave potential,low Tafel slope and high diffusion-limiting current.The metal-free h-NGCT-900 catalyst delivers impressive catalytic performance for oxygen reduction closely comparable with Pt/C of the similar quantity in terms of similar on-set potential and Tafel slope as well as positive half-wave potential which is even 5 m V higher than that of a commercial Pt/C.The h-NGCT-900 catalyst also exhibits superior ORR performance to a commercial Pt/C catalyst in both acid and alkaline medium,including better long-term stability and better methanol tolerance.Importantly,h-NGCT-900 also displays outstanding catalytic activity and stability for methanol electrooxidation when using a support for the Pt nanoparticle s:Pt/h-NGCT exhibits 1.4 times higher activity along with the 15% increase in stability compared to commercial Pt/C.