Study On Heterostucture PtNi Supported By Pd Nanosheets Of High-performance Catalyst For Oxygen Reduction Reaction

Fuel cells have broad application prospects in portable power supply,aerospace,maritime transportation,new energy electric vehicles and other fields.A large number of research data have confirmed that platinum-based（Pt-based）catalysts are the best catalysts for oxygen reduction of fuel cell membrane electrode cathodes.However,due to the limitation of Pt reserves,high production cost,poor electrochemical activity and stability of pure Pt,low utilization rate of Pt atoms,and inevitable intermediate poisoning in the reaction process,the large-scale application of Pt catalysts is severely limited.It has become the focus of current research to improve the utilization rate of Pt atoms,accurately control the composition and morphology of nanostructure catalysts,and overcome the problems of slow kinetics and poisoning.Compared with single atom system,rational design of multiple Pt based binary,ternary or multivariate electrocatalysts provides more possibilities for improving the utilization rate of Pt atom and its electrocatalytic performance.A large number of research data have proved that the active electron interaction between Pt and transition metal after the introduction of transition metal can significantly improve the catalytic efficiency of Pt-based alloy catalyst.Electrocatalysis with special structures such as nanosheet,nanowires,nanoring and nano-frames can effectively adjust the electronic properties and lattice structure of the catalyst,thus improving the catalytic performance of the catalyst.Compared with the zero and one-dimensional nanostructured catalysts,the contact area between the two-dimensional nanostructured catalysts and the carbon support is larger,which can effectively enhance the surface charge transfer of the catalyst,promote the combination of catalyst and the support,and effectively enhance the catalytic performance of the catalyst.However,the controllable preparation of two-dimensional nanostructured catalysts is still a key problem that needs to be solved.Thus,in this paper,the electrocatalytic performance of two-dimensional nanostructure oxygen reduction reaction（ORR）catalyst was explored,and its growth process and reaction conditions were studied,and efficient and stable electrocatalyst was obtained.The specific research work is as follows:（1）Palladium nanosheets（PdNSs）catalyst was synthesized by one-pot hot-melting method.The effects of reducing agent molybdenum hexamcarbonyl（Mo（CO）₆）concentration and reaction time on catalytic morphology were investigated under the condition that the concentration of Pd precursor was basically unchanged.The electrocatalytic activity of Pd nanoparticles（PdNPs）and PdNSs catalysts for ORR was studied.PdNSs catalyst has an excellent mass activity(1.05 A mg_pd^-1),which is 1.96 times higher than that of commercial PdNPs catalyst(0.437 A mg_Pt^-1).Compared with TKK–Pt/C catalyst(0.131 A mg^-1_Pd),the mass activity of the catalyst increased by 8.01 times.PdNSs catalyst has high ORR activity,mainly due to its ultrathin two-dimensional nanostructures,which leads to the exposure of atomic active sites on the catalyst surface.（2）Palladium/platinum nickel nanosheets（Pd/PtNi NSs）catalyst was prepared by deposition method.The Pd/PtNi NSs catalysts was prepared by using the synthesis strategy of platinum-nickel（Pt-Ni）precursors deposited on PdNSs substrates.The effects of different Pt-Ni precursors concentration on the surface morphology of the catalyst,the thickness of nanosheets and the ORR performance were investigated.The results show that the optimum concentration of Pt-Ni precursor deposited on PdNSs can effectively improve the surface structure and electrocatalytic performance of the catalyst.Pd/PtNi NSs–2 catalyst has excellent ORR performance.At 0.9 V（vs.RHE）,the mass activity of Pd/PtNi NSs–2(1.038A mg^-1_Pt+Pd)was 6.25 times that of the commercial TKK–Pt/C catalyst.The excellent properties of Pd/PtNi NSs–2 is mainly attributed to its ultrathin nanostructure,ternary alloy structure,maximum Pd core coverage area and maximum surface atomic diffusion alloying.（3）Palladium rhodium/platinum nickel nanosheets（Pd Rh/PtNi NSs）quaternary alloy nanostructured catalyst was synthesized by two-step method.Firstly,palladium rhodium nanosheets（Pd Rh NSs）was prepared by one-pot hot melt method.Pd Rh/PtNi NSs quaternary alloy catalysts were prepared by synthesing Pd Rh NSs with different Rh concentration and depositing Pt-Ni precursor with the same concentration on Pd Rh NSs.A series of characterization and electrochemical performance tests showed that the ORR activity and durability of the catalyst increased with the increase of Rh precursor concentration under the condition that the structure of the nanosheet remained unchanged.The mass activity of Pd Rh₂/PtNi NSs(1.42 A mg^-1_Pt+Pd)catalyst with moderate Concentration of Rh precursor was 9.4 times higher than that of commercial TKK–Pt/C(0.151 A mg_Pt^-1)catalyst.It was 1.37 times higher than Pd/PtNi NSs(1.038 A mg^-1_Pt+Pd).More importantly,the mass activity of Pd Rh₂/PtNi NSs(1.239A mg^-1_Pt+Pd)was 8.21 times that of commercial TKK–Pt/C(0.151 A mg_Pt^-1)after10 k cycles durability test.These results indicate that Rh can effectively improve the activity and stability of catalyst.The excellent ORR electrocatalytic performance of Pd Rh₂/PtNi NSs was mainly derived from the two-dimensional nanosheet structure,the four-element alloy structures and the introduction of the oxygen philic element Rh.