Investigation Of Low Platinum Ordered Catalyst For Proton Exchange Membrane Fuel Cells

In recent years,rapid population growth and social economic development have increased the supply burden of carbon-based fuels on human society.And the accompanying global warming and environmental pollution have prompted mankind's eagerness to change the existing energy structure.Hydrogen is widely recognized because it is an energy-efficient,clean,and fuel-flexible secondary energy carrier.Through fuel cells,hydrogen can be the perfect bridge between many renewable and fossil energy sources.Proton Exchange Membrane Fuel Cells(PEMFCs)have received widespread attention due to their high energy conversion efficiency,high energy density,environmental friendliness,and low operating temperature.However,the expensive platinum catalysts used in the cathode have seriously hindered their large-scale commercialization.The development of high activity and low-cost fuel cell cathode catalysts is imperative for the“green hydrogen”economy.In this paper,four kinds of catalysts with excellent oxygen reduction reaction(ORR)performance are designed and synthesized step by step based on the basic knowledge of the synthesis theory of intermetallic compounds,the basic mechanism of ORR catalysis by platinum-based alloys,and the unique application theory of intermetallic compounds in the ORR.With the aim of synthesizing intermetallic compounds with small particle size,the following explorations are carried out.1.Considering the fact that the structure of Pt-Fe intermetallic compound is easy to obtain and the crystal surface of Pt Ni(111)has the best ORR specific activity,the ternary Pt-Fe-Ni/C catalyst is designed and synthesized.The intermetallic structure is used to ensure the stability and the Ni is introduced to enhance the activity.The introduction of high ratio of Ni and Fe elements further reduces the cost of Pt-Fe-Ni/C catalysts.The Pt-Fe-Ni/C catalysts with different Fe and Ni ratios are synthesized by microwave-assisted polyol method,and then the synthesis temperature of the ternary ordered structure in Pt-Fe-Ni system is optimized by exploring the ordering conditions at different temperatures.Finally,the most active catalyst Pt₂Fe₃Ni₃/C-675 is selected by electrochemical measurement.The mass activity of Pt₂Fe₃Ni₃/C-675 is 0.73 A/mg _Pt,which is about 7 times higher than that of commercial Pt/C catalyst.The uniformly dispersed nanoparticles(5 nm)and face-centered tetragonal ordered phase improve the activity and stability of Pt-Fe-Ni/C catalysts.In addition,the surface segregation of Pt elements improves the utilization of Pt in Pt-Fe-Ni/C catalysts under high M ratio conditions,which is one of the reasons for their high mass activity.2.Due to the damage to proton exchange membrane caused by the dissolution of Fe element and Fenton reagent formed by ORR intermediate(H₂O₂)in the PEMFC operating conditions,the carbon-supported Pt-Ni intermetallic nanocrystals are designed and synthesized.It is proved for the first time that the Pt-Ni intermetallic compound can be synthesized effectively under the annealing condition from 850?to 400?.The results of X-ray diffraction(XRD)and high resolution transmission electron microscopy(HRTEM)show that two of the Pt-Ni/C catalysts have ordered structure,which are Pt Ni/C and Pt Ni₂/C.The intermetallic structure is responsible for good performance of the two catalysts,with both activity and stability meeting the 2020 U.S.Department of Energy standards.Pt Ni₂/C has better activity compared to Pt Ni/C,which is attributed to the strain effect caused by lattice mismatch of face-centered tetragonal structure in Pt Ni₂/C and the ligand effect of the relatively high content of Ni atoms on the subsurface of Pt Ni₂/C.These two reasons synergistically reduce the d-band center on the catalyst surface,which leads to the weak adsorption of oxygen-containing intermediates on the catalyst surface during the ORR process.3.Given the large particle size(7-8 nm)of the above Pt-Ni intermetallic compound,its activity can be further optimized.Therefore,Au-doped Pt-Ni/C catalysts with ultra-small particle size modified by Au elements are designed and synthesized.The introduction of Au elements ensures that the catalyst has both high activity and stability.Pt-Ni/C catalysts with an average particle size of 2.8 nm are successfully prepared by the impregnation reduction method using polyvinyl alcohol(PVA)in combination with activated carbon support for the first time.The introduction of PVA has the following three main purposes.1.particle size control,the hydrogen bonding between PVA and the oxygen-containing functional groups in the activated carbon support guarantees the successful synthesis of small nanoparticles;2.thickening agent,which enhances the dispersion of precursors on the support and ensures that the support does not precipitate during impregnation;3.protective effect,synergistically protecting Pt-Ni nanoparticles from agglomeration during the high temperature reduction process within the carbon support.The mass activity of the Au-doped Pt-Ni/C catalyst obtained by surface galvanic replacement still maintain 0.6 A/mg _Pt after accelerated durability test(ADT),only 14%decrease in activity.4.The synthesis of Pt-Ni intermetallic nanocrystals with ultra-small particle size is difficult due to the limitation of thermodynamic and kinetic factors.Therefore,small particle size Pt-Co/C intermetallic compounds are designed and synthesized.The size of Pt-Co nanoparticles can remain at 2-3 nm at higher ordered temperatures by modified impregnation reduction with PVA.A multi-structure with compact Pt as shell and ordered Pt-Co as core is formed through dealloying and subsequent low temperature heating.The unique local atomic arrangement gives the Pt-Co/C catalyst excellent activity and stability.The activity of O-Pt Co/C is superior to O-Pt₃Co/C due to the unusual biaxial strain of the face-centered tetragonal structure relative to the face-centered cubic structure,while the stability of O-Pt₃Co/C is greater than that of O-Pt Co/C due to the ligand effect of more Pt atoms around Co in the O-Pt₃Co/C catalyst.