Controllable Preparation Of Graphene-based Catalyst And Its Performance For Direct Methanol Fuel Cells

Due to the low-pollution, abundant sources, high energy efficiency, the easystorage and transportation of the fuel, Direct Methanol Fuel Cells (DMFCs) have beenattracted more and more attention around the world. However, the big challenge forthe practical application of DMFCs is to improve its electrocatalyst activity at a lowcost. Developing novel catalyst support is an effective approach to improve theperformance of DMFCs catalyst and to promote the commercialization of DMFCs.Graphene, a single layer of carbon atoms arranged in a hexagonal lattice, has beenexplored as potential supporting materials owing to its high thermal conductivity,unique graphitized basal plane structure, excellent mechanical properties, highelectronic conductivity and large specific surface area. In this dissertation, we proposea one-step hydrothermal method to synthesize three-dimensional (3D) Pt/graphenemonolith (Pt/G) catalysts. The as-recevied Pt/G catalysts possess a3D macroscopicmorphology, crosslinked inner microstructure, and a porous system, which favorefficient fuel and product diffusion and improve the electrocatalytic activity. Themain points in this thesis are summarized as follows:(1) The nanoparticle sizes, structure and electrocatalytic performance of thecatalysts are intimately dependent on the Pt loadings, reaction temperature and pHvalue. We found that the catalyst with a Pt content of20wt%exhibits the highestelectrochemical surface area. This is attributed to the homogeneous distribution of PtNPs, whose average particle size is around3.5nm. The electrocatalytic activity ishighly sensitive to pH value. The catalyst synthesized at a pH value of10shows thehighst ECSA/GSA ratio. Moreover, the catalyst self-assembled at150oC exhibitsbetter electrocatalytic activity for methanol oxidation reaction and oxygen reductionreaction.(2) We have developed a facile and rapid hydrothermal self-assembly method toload Pt nanoparticles on the layer of3D graphene. The reduction of GO and Pt NPsdecorated3D graphene are realized in a single step. The Pt/G catalysts exhibitsuperior electrocatalytic activity and durability for methanol oxidation reaction withlower onset potential, higher If/Ibratio and methanol oxidation current compared withthe commercial Pt/C catalyst. The enhancement in the Pt/G catalyst may be mainly attributed to the porous structure formed by the linkage of graphene nanosheets. Thepores, especially macropores, are helpful for the fuel and product diffusion and canimprove tolerance to poisoning of the catalyst. Although further experiments andinvestigations are needed to illustrate the reason for the improved catalyticperformance,3D graphene is believed to be a promising catalyst support.(3) We further annealed the Pt/G at different temperatures under a gas mixture of95%Ar+5%H2for6h. The Pt/G catalyst annealed at350oC (Pt/G-350) has the highestMOR activity. And If/Ib(the ratio the forward oxidation peak current to the reversepeak current) of Pt/G-350is improved as compared to the commercial Pt/C catalyst,which may be ascribed to the enhanced interaction between Pt and graphene afterannealing.(4) Carbon nanotubes (CNTs) are used as additives in the preparation of Pt/Gcatalysts. However, the produced Pt/G/CNTs-350catalyst does not show enhancedcatalytic actvity as compared to Pt/G-350catalyst. And Pt/G-350catalyst has bettertolerance to poisoning species such as adsorbed CO intermediates. The Pt/G-350catalyst owns more defects and these defects have reactivity and easier to be oxidizedto oxygen-containing groups which may be responsible for better tolerance towardsCO poisoning.In conclusion, graphene, especially graphene with three-dimensional macroscopicarchitecture have provided a new way to improve electrocatalytic activity in directmethanol fuel cells.