Controlled Synthesis Of Pt/Pd Based Intermetallic Compounds Catalysts And Their Catalytic Properties

Electrocatalysts are the bottleneck restricting the commercialization of proton exchange membrane fuel cells?PEMFC?.Due to the acidic environment of PEMFC,the electrode reactions of PEMFC are strongly dependent on the precious metal platinum.The current electrocatalysts suffer from high platinum loading and insufficient stability.Therefore,it is urgent to develop new,highly-efficient and long-life electrocatalysts.Intermetallic compounds have a long-range ordered crystal structure,so they have a definite surface composition and regular active site distribution,which are not afford by common alloys?solid solutions?.In addition,compared with disordered alloys,ordered intermetallic compounds have higher mixing enthalpy,thus higher chemical and structural stability.Because of these unique properties,intermetallic compounds have gradually become a hot research topic about fuel cell catalysts.This thesis mainly focuses on the design of platinum-based and palladium-based intermetallic compound catalysts for the cathode and anode reactions of fuel cells,and the investigation of their electrocatalytic performance and mechanism.The research content includes the following parts:?1?The controllable preparation of intermetallic compounds.It is a great challenge to synthesize the desired intermetallic catalyst?5 nm and smaller?with easily-operated route.To promote metal atom arrangement and form atomically ordered intermetallic phases,high temperature annealing is generally a necessary process,often resulting in serious particle agglomeration and non-uniform size distribution.To address this issue,we proposed several synthetic strategies including KCl matrices,carbon coating and hydrogel-freeze drying method.Among these three methods,the hydrogel-freeze drying is the most effective and economical route for the controlled synthesis of Pt-based intermetallic catalysts.The formation of hydrogel prevents the aggregation of graphene oxide and significantly promotes their excellent dispersion,while a freeze-drying can retain the hydrogel derived three-dimensionally?3D?porous structure and immobilize the metal precursors with defined atomic ratio on GO support during solvent sublimation,which is not afforded by traditional oven drying.The subsequent annealing process produces r GO supported ultra-small ordered Pt₃M intermetallic NPs??3 nm?due to confinement effect of 3D porous structure.Such Pt₃M?M=Mn,Fe,Co and Cr?intermetallic NPs exhibit the smallest particle size among the reported ordered Pt-based intermetallic catalysts.This study provides an economical and scalable route for the controlled synthesis of Pt-based intermetallic catalysts,which can pave a way for the commercialization of fuel cell technologies.?2?The synthesis of Pt₃Mn and Pt Mn₃intermetallic compounds and their electrocatalytic performance towards oxygen reduction reaction.For the oxygen reduction reaction at the fuel cell cathode,we designed and synthesized high-platinum content Pt₃Mn and low-platinum content Pt Mn₃ intermetallic catalysts by the hydrogel freeze-drying technology.We evaluated the activity and stability of the two catalysts by various electrochemical technologies and investagated the mechanism of the two catalysts for oxygen reduction reaction by the DFT theoretical calculation.Our studies clearly show that both electrocatalysts exhibit much better electrochemical performance than commercial Pt/C catalysts in terms of activity and stability.For the graphene-supported Pt₃Mn catalyst?Pt₃Mn/r GO?,its half-wave potential?0.90 V vs.REH?is 20 m V and 50 m V higher than those of the disordered alloy Pt₃Mn/r GO?0.88 V?and a commercial Pt/C?0.85 V?,respectively;After 5000 cycles,Pt₃Mn/r GO catalyst has a half-wave potential(?E_1/2)decay of 15 m V,while the?E_1/2 decay of disordered alloy Pt₃Mn/r GO and commercial Pt/C is about 52 m V and 25 m V.In case of carbon nanotube suported Pt Mn₃?Pt Mn₃/NCNT?,its half-wave potential?0.88 V?is 30 m V higher than that of commercial Pt/C?0.85 V?.At 0.9 V,the mass activity of Pt Mn₃/NCNT is 4.4 times that of commercial Pt/C;Pt Mn₃@Pt/NCNT catalyst only has a half-wave potential(?E_1/2)decay of 20 m V after 5000 cycles.Density functional theory calculation illustrates that the superior ORR performance of Pt₃Mn and Pt Mn₃ is due to the introduction of Mn atoms and the ordering of the atomic arrangement so that the outermost Pt atoms gain electrons and the Pt-Pt bond is compressed,leading to the d-band center of Pt atoms to be far away from the Fermi level and weaken the adsorption energy of the oxygenated species,which makes it much more closer to the optimal value of ORR activity descriptor.?3?The controllable preparation of antiperovskite structure Pd Fe₃N and its electrocatalytic performance towards formic acid oxidation.It is a great challenge to develop highly active and durable electrocatalyst for formic acid oxidation catalysts in acidic media.In this study,we demonstrated a new and highly efficient catalyst,antiperovskite structure Pd Fe₃N,which not only significantly enhanced the activity of electrocatalytic oxidation of formic acid,but also suppressed the leaching of non-noble metal Fe and thus improved the stability of the catalyst.Electrochemical tests show that the mass activity of Pd Fe₃N/r GO catalyst is 2.3 times higher than that of Pd/C catalyst;after 1000 cycles,the mass activity only loses 9.24%,which is much smaller than Pd Fe₃/r GO,suggesting its excellent stability.It can be proved by Density Functional Theory that compared with Pd Fe₃and Pd,Pd Fe₃N intermetallic compounds are more likely to perform HCOO*reaction path during FAOR and the introduction of Fe atoms is beneficial to weaken the adsorption energy of CO while enhancing the adsorption energy of OH.The orderly arrangement of atoms is more conducive to the reaction of CO*and OH*,and timely transfer of CO.Furthermore,the introduction of N atoms can form Fe-N covalent bonds and improve the stability of Fe atoms.In conclusion,the above calculation results are exactly consistent with the experimental phenomenon.This research provides a new direction for the development of highly active and stable intermetallic compound catalysts.