Investigation Of The Phase Transformation In FePtM Nanoparticles (M=Ag,Cu) At Atomic Scale For Oxygen Reduction Catalysis

Recently,proton–exchange–membrane fuel cells（PEMFCs）have attracted considerable attention as a modern energy conversion device,due to their advantages of high efficiency,low emission and plenty sources of fuels.They are predicted to be constitute ultimate solutions to issues around novel electric vehicles and stationaries.The main challenge for PEMFC large–scale application is the problem surrounding the platinum catalysts used in their cathode,namely low activity and stability,and thus results in high Pt loading and high costs.Therefore,developing highly efficient catalysts for this cathodic oxygen reduction reaction（ORR）is the key towards the fabrication of commercial fuel cell devices.In this case,chemical ordered Fe Pt nanoparticles exhibit a high potential because of enhanced activity and durability simultaneously in acid operation environment.However,there are still challenges related to the synthesis of ordered Fe Pt nanoparticles,and a fundamental understanding on their structure/performance relationship is still lacking.In this work,we carried out one–pot synthesis of these Fe Pt nanoparticles by Ag/Cu additive with the objective to investigate their ordering structure and electrochemical catalysis performance,as well as performance enhancement.State–of–the–art spherical-aberration corrected transmission electron microscopes（TEM）,with sub–angstrom scale spatial resolution,have been widely used to characterize the microstructure of Fe Pt.The main results are summarized as follows:1.Uniform and chemical ordered Fe Pt Ag nanoparticles have been successfully synthesized by a one–pot method with octadecylamine solvent.Ag was adopted to in-situ promote phase transformation of Fe Pt in high boiling temperature（320～345?C）solvents.Magnetic measurements showed that Fe Pt Ag nanoparticles with a high coercivity of 5.23 k Oe has been achieved,which is significantly higher than the nanoparticles synthesized using hexadecylamine（2.84 k Oe）and trioctylamine（2.81 k Oe）solvents.Structural characterization demonstrated that the Fe Pt Ag particles synthesized in octadecylamine,with a diameter of 3.5?0.5 nm,are smaller in size and more uniform than those synthesized in hexadecylamine and trioctylamine solvents.The octadecylamine,hexadecylamine and trioctylamine high boiling-temperature solvents play significant roles in the morphology,structure and magnetism of Fe Pt Ag nanoparticles,and the solvent effects on the nucleation and growth of ordered Fe Pt Ag nanoparticles have been determined.2.Ordered Fe Pt Cu nanoparticles are prepared by one–pot method,with controllable composition of 10%～40%for Cu,30%～50%for Fe and Pt.The Cu alloying effect is found to be the driving force for Fe Pt ordering in this synthesis method,which is effective up to a Cu content of 40%.The optimized core–shell Cu@Fe Pt catalyst with Pt-enriched surface exhibits 0.5 A/mg _Ptmass activity and a factor of 4 better than commercial Pt/C(0.13 A/mg _Pt).The current density of Cu@Fe Pt drops only 3.0%after 1000 cycling measurement,demonstrating enhanced durability than Pt/C（34.2%decay）.The morphology and structure evolutions between solid solution Fe Pt Cu alloy and core–shell Cu@Fe Pt could clearly be explained.The composition effects of Fe,Pt and Cu on structure of Fe Pt Cu nanoparticles has been demonstrated that core–shell Cu@Fe Pt formed for 20%atom ratio of Cu and more than40%of Fe.The particle size increases with the Fe or Cu ratio,and the particles are sintered for Fe（50%）and Cu（>20%）.This study demonstrated that high performance fct–Fe Pt ORR catalyst are not only ordered but also have a core–shell structure.3.To investigate the crystalline behavior of ordered Fe Pt Cu,i.e.,their morphology and elemental evolution during one–pot synthesis,quasi-in-situ and in-situ-heating work was carried out in TEM.It was found that core–shell Cu@Fe Pt nanoparticles were initially formed with six branches shape at reaction time of 20 min.The particles size increased at 40 min but kept the same structure,which means that the system was still in anisotropic growth.For 1 h particles,they evolved into core–shell Cu@Fe Pt with truncated–octahedron shape,due to atoms of these branches degraded and diffused onto 111 bulk surfaces.Finally,Cu diffused out from core and uniformly distributed in the mother particle at 2 h,forming solid solution Fe Pt Cu alloy particles with sphere shape.Anisotropic growth,surface atoms diffusion and bulk atoms diffusion complete along Fe Pt Cu crystalline growth and dominate its growth in sequence.The truncated–octahedron Cu@Fe Pt nanoparticles（1 h）have a mass activity of11.7 times higher than Pt/C,and exhibit 0.5%half–wave potential loss after 5000 potential cycles.The branched Cu@Fe Pt nanoparticles（20 min）have a mass activity of 12.1 times higher than Pt/C,and show 3.1%half–wave potential loss after cycling.While the solid solution Fe Pt Cu nanoparticles（2 h）have a mass activity of 5.3 times higher than Pt/C and show 2.9%mass activity loss after 5000 potential cycles.This study reveal importance of rational synthesis for high performance low–platinum nanostructures of oxygen reduction reaction.