Preparation And Characterization Of Platinum-based Catalysts For Oxygen Reduction Reaction

It is urgent and of vital importance to further reduce energy and fossil fuel consumption and to make more efforts in low-carbon usage for sustainable development.Hydrogen is likely to play an important role in reducing emissions in some sectors such as steelmaking,heavy-duty truck transporation,shiping and cement production.China recently announced carbon neutrality pledge,encouraging major emitting sectors such as transport,power plant and related industry to take serious action.It is expected that hydrogen will become the mainstream in decarbonizing long-haul,heavy-payload trucks,which could be cheaper to operate than diesel engines.Proton exchange membrane fuel cells（PEMFCs）have widely been considered as one of the most efficient fuel cells with the advantages of high power density,low operating temperature and zero carbon dioxide emissions,as compared with conventional internal combustion engines.Currently,world-wide development of PEMFCs is primarily limited by the prohibitive cost of platinum（Pt）,which is used as catalysts in both anode and cathode.Chemical reactions at both anode and cathode,particularly the oxygen reduction reaction（ORR）at the cathode,has sluggish kinetics,which has triggered extensive research to improve its catalytic activity and to lower the costs in the past decade.One desirable strategy to reduce the amount of Pt while maintaining its sufficient activity is to alloy Pt with transition metals（e.g.Cu,Fe,Ni,Co and so forth）.In this thesis,carbon-supported Pt Cu and Pt Cu Fe catalysts have been prepared via a simple polyol method followed by a voltammetric cycling treatment for improving the ORR performance.The specific contents of this thesis are as follows:1.We report the synthesis of de-alloyed Pt Cu/C catalysts with high catalytic performance and low Pt-loading.The de-alloyed Pt Cu/C-10 catalyst delivers 5.0-fold higher mass activity as compared with that of commercial Pt/C-10 catalyst under identical de-alloying conditions.For Pt Cu/C catalysts,the compositional and morphologic evolution was determined after volvammetric cycling.Pt-rich shell is formed and supported on Pt Cu alloy core that leads to a changing lattice strain due to the continuous dissolution of Cu.In the X-ray powder diffraction patterns,it was found that the de-alloying treatment results in changes in crystal structure,by observing the shift of the diffraction peak.2.A promising type of low Pt-loading catalyst,ternary Pt Cu Fe nanoparticles supported on carbon,has been prepared using a simple polyol synthesis strategy,followed by a post heat-treatment.Pt-rich shell is exposed on the surface of Pt Cu Fe nanoparticles via a voltammetric dealloying treatment.The high-resolution transmission electron microscopy（HRTEM）and X-ray photoelectron spectroscopy（XPS）reveal respectively the elemental composition and surface chemical states of Pt Cu Fe nanoparticles.We found that the condition of electrochemical dealloying affects electrocatalytic activity in oxygen reduction reaction（ORR）.With the dissolution of non-noble metals,the Pt Cu Fe catalyst exhibits a significant improvement in ORR activity,as well as,an improvement in their durability.D-Pt₂Cu Fe/C-150 catalyst shows 2.9times higher mass activity(0.99 A·mg_Pt^-1)at 0.90 V（vs.RHE）compared with that of commercial TKK catalyst（Pt₃Co/C）.Furthermore,the mass activity loss for D-Pt₁Cu Fe/C-150is only 7.6%after 5,000 durability cycles,whereas TKK catalyst suffers from a 29.4%loss under the same conditions.This study provide a visual and valuable insight for the rational design of de-alloyed Pt-based electrocatalysts with a modified electronic structure.