Platinum-based Nanocrystals On Carbon Materials As Superior Electrocatalysts Toward Methanol Oxidation Reaction

Not only will fossil fuels be exhausted,but also they can produce serious environmental pollution.Thus,the development of renewable clean energy is extremely urgent.Fuel cells have attracted much attention due to their high energy conversion efficiency,high energy density,environmental friendliness and capability of using renewable fuels.In comparison with traditional hydrogen-oxygen fuel cell,direct methanol fuel cell(DMFC)can use liquid methanol,which holds the merit of high volumetric energy,low price and a broad variety of sources.In addition,liquied methanol is convenient for storage and transportation,suitable for the applications in automotive and portable devices.However,the commercialization of DMFC is greatly limited by the slow electrode kinetics of methanol oxidation,expensive noble metal platinum(Pt)-based catalysts and their poor stability.Furthermore,the poisoning of Pt-based catalysts via adsorption of intermediates could make the catalysts lose their catalytic abilities and prevent their practical application in DMFC.Therefore,it attracts extensive interests to develop the methanol electrooxidation catalysts with high catalytic activity,low cost,good stability and good resistance to CO poisoning.In this thesis,we prepare the Pt-based electrocatalyst with different compositions,structures and carbon supports,and investigate their catalytic performances for methanol oxidation reaction.The bimetallic Pt4Ir1 alloy is grown on the reduced graphene oxide(rGO),and the resultant composite serves as the methanol electrooxidation catalyst.The improved electrocatalytic performance of Pt4Ir1/rGO may be attributed to the formation of Ir-Pt alloyed atomic bridges,which could oxidize CO absorbed on Pt surface.The synergistic effect between rGO and Pt4Ir1 alloy may also promote the mass transfer process.On the other hand,Pt nanoparticles are deposited on the surface of a composite substrate prepared from 20% oxidized multi-walled carbon nanotubes(o-MWCNTs)and rGO.The as-prepared Pt/rGO-o-MWCNTs catalyst exhibits excellent catalytic activity for methanol oxidation reaction.The enhanced catalytic performance may be arising from the integrated o-MWCNTs promote the mass transfer process and prevent the agglomeration of graphene during methanol oxidation reaction.This thesis includes the following five parts:1.Introduction.The working principle,classification and characteristics of fuel cells are briefly introduced.The anode catalyst and commonly-used catalyst supporting material of DMFC are described in detail.The challenges and bottlenecks of DMFC catalysts are also discussed.In addition,the main content of this thesis is also presented.2.Experiment section.The experimental apparatus,reagents and main methods used in this project are introduced.3.Bimetallic platinum-iridium alloy construct atomic scale bridges for synergistically boosting performance of electrooxidation of methanol.Ultra-small bimetallic PtIr alloy nanocrystals are directly and uniformly grown on reduced graphene oxide(rGO)by a one-pot solvothermal approach,and further used as a catalyst toward electrooxidation of methanol for the first time,in which the optimal Pt in Pt Ir/rGO can be as low as 80% while delivering extraordinarily higher catalytic current density than Pt/rGO and commercial Pt/C catalysts by 1.5 times and 2.6 times,respectively,and much better durability.The remarkable improved electrocatalytic performance of Pt4Ir1/rGO with a much lower loading of Pt is mainly attributed to Ir-Pt alloyed atomic bridges,in which-OH can easily form on Ir surface via the alloyed atomic scale bridge to oxidize CO absorbed on Pt surface for greatly boosting the catalytic performance of methanol electrooxidation.The highly conductive graphene support may have positive impact on the excellent catalytic performance.This work provides a facile approach to fabricate low-loaded Pt but highly active anode catalyst for direct methanol fuel cells.4.O-MWCNTs and rGO were used as composite support materials to grow Pt nanoparticles as high performance catalysts towards methanol electrocatalytic oxidation.Pt nanoparticles are deposited on the surface of a composite substrate prepared from the o-MWCNTs and rGO mixture.By optimizing the rGO to o-MWCNTs weight ratios,the best catalytic activity for methanol oxidation reaction is achieved on the catalyst with 20% o-MWCNTs in the composite.The enhanced catalytic activity of the as-prepared catalyst toward methanol oxidation is mainly attributed to the presence of oMWCNTs,facilitating the charge transfer between the graphene layers.In addition,the embedded o-MWCNTs within graphene sheets can prevent the agglomeration of graphene,keeping a porous structure of the composite.The porous structure can provide excellent transport paths for both electrons and reactants in electrolyte,enhancing the reaction kinetics of methanol oxidation and the electrocatalytic activity.In comparison to the commercial Pt-based catalyst,the Pt/rGO-o-MWCNTs catalyst shows better electrocatalytic stability.This phenomenon may be due to the defects on the rGO-oMWCNTs composite can serve as nucleation sites for the dissolved Pt ion and prevent the agglomeration of Pt nanoparticles into larger particles.5.Conclusions and outlook.Conclusions are delivered and future works are also discussed.