Study On The Preparation And Application Of Non-Traditional Electrocatalysts For Fuel Cells

With the development of global economy and society,the demand of fossil energy resources such as petroleum is steadily increasing.However,the combustion of fossil energy resources caused severe environmental pollutions,hence it is urgent to develop or explore some novel efficient energy conversion technologies and devices.Fuel cells can convert chemical energy in fuels into electrical energy directly through oxidizing fuel molecules at anode and reducing oxygen at cathode.Because of the advantages such as the high energy conversion efficiency,environment friendly and high potential application,fuel cell technology has been considered as one of the hottest research topics.Traditionally,platinum is considered as the best catalyst based on its electrocatalytic performance for ORR.However,the bottle neck that the limited resource and high cost of platinum greatly impede the development of fuel cells.Furthermore,platinum catalysts also faced many issues such as the low durability,poor tolerance to methanol and carbon monoxide.Therefore it needs an easily prepared alternative with high catalytic performance and low cost to conventional platinum.Our studies mainly focused on the development of methodology for synthesis of highly efficient catalysts with low cost,easily scale up and high durability.At the same time,we also discussed about the relationship among materials composition,structure and electrocatalytic performance.These studies will provide a novel and basic knowledge for exploring a series of outstanding catalysts.Our studies are summarized as follow:?1?We have developed a sandwich-like graphene sheets/carbon nanospheres/graphene sheets substrate supported Pt₃ Ni nanoparticles through a one-pot solvothermal process in N,N-dimethylformide without the addition of reducing agents and surfactants.Transmission electron microscopic?TEM?measurements showed that a sheet-like morphology of graphene matrix with large numbers of carbon nanospheres homogeneously implanted in the layers,and Pt₃ Ni nanoparticles were also observed highly dispersed on tgraphene surfaces without apparent agglomeration,where the average core size was estimated to be 12.6 ± 2.4 nm.X-ray photoelectron spectroscopic?XPS?studies demonstrated that electron transfer likely occurred between the Pt₃ Ni nanoparticles and the graphene sheets.Electrochemical measurements showed that the activity of the Pt₃Ni-C/rGO catalysts for oxygen reduction reaction?ORR?was higher than that of Pt/C?20 wt%?,and its mass activity for methanol oxidation reaction?MOR?was even 1.7-times higher than that of graphene supported Pt₃ Ni nanoparticles?Pt₃Ni/rGO?,and 1.3-times higher than that of Pt/C?20 wt%?.Additionally,the CO tolerance and durability of Pt₃ Ni nanoparticles were also remarkablyenhanced by sandwich substrate.These superior electrocatalytic activities of Pt₃Ni-C/rGO catalysts were mostly attributed to the following factors:?i?the implantion of carbon nanospheres into the graphene layers prevented restacking/refolding of the graphene sheets,leading to an increasing number of accessible active sites as well as transport channels for mass and charges;and?ii?the synergetic effect between Ni alloying with Pt and rGO weakened the bonding interactions with reactant species,as manifested by the enhanced kinetics of ORR,MOR and CO oxidative desorption.?2?Thermally removable nanoparticle templates were used for the fabrication of self-supported N-doped mesoporous carbons with a trace amount of Fe?denoted to Fe-N/C-T?.Experimentally Fe-N/C-T was prepared by pyrolysis of poly?2-fluoroaniline??P2FANI?containing a number of FeO?OH?nanorods that were prepared by a one-pot hydrothermal synthesis and homogeneously distributed within the polymer matrix.The FeO?OH?nanorods acted as rigid templates to preventthe collapse of P2 FANI during the carbonization process,where a mesoporous skeleton was formed with a medium surface area of about 400 m2/g.Subsequent thermal treatment at elevated temperatures led to the decomposition and evaporation of the FeO?OH?nanorods and the formation of mesoporous carbons with the surface area markedly enhanced to 934.8 m2/g.Electrochemical measurements revealed that the resulting nitrogen doped mesoporous carbons exhibited apparent electrocatalytic activity for ORR,and the sample prepared at 800 °C?Fe-N/C-800?was the best among the series,with a more positive onset potential?+ 0.98 V vs RHE?,higher diffusion-limited current,higher selectivity?number of electron transfer n > 3.95 at + 0.75 V vs RHE?,much higher stability,and stronger tolerance against methanol crossover than commercial Pt/C catalysts in a 0.1 M KOH solution.The remarkable ORR performance was attributed to the high surface area and sufficient exposure of electrocatalytically active sites that arose primarily from N-doped carbons with minor contributions from Fe-containing species.?3?Graphene-supported mesoporous carbons with rich nitrogen self-doped activesites?N-MC/rGO-T?were prepared by directly pyrolysis of a graphene supported polymer matrix with highly distributed amorphous Fe OOH that serve as efficient thermally removable templates.The resulting N-MC/rGO catalysts exhibit high surface areas and apparent electrocatalytic activity for ORR in alkaline media.Among the series,the sample prepared at 800? exhibited the best performance with a more positive onset potential,higher limiting currents,much higher stability,and stronger poison resistance than commercial Pt/C.The super-activity of N-MC/rGO-800 was likely ascribed to the synergetic functions of the highly conductive graphene substrate and the mesoporous N-doped carbons that effectively impede the restacking of the graphene sheets and enhance the exposure of the rich nitrogen self-doped active sites.