Preparation And Study On Electrocatalytic Performance Of Palladium-copper-based Hollow Medium Entropy/high Entropy Alloy

The development of human society is accompanied by the increasing demand for energy.On the one hand,the large-scale exploitation and utilization of traditional fossil fuels will lead to the energy crisis.On the other hand,it will also produce greenhouse gases and a large number of pollutants,which will seriously threaten the ecological environment of the earth.With the aim to solve these problems,green energy conversion and storage devices and technologies,such as metal-air battery,water decomposition devices,and fuel cell,have been developed,which can not only convert chemical energy into electrical energy and store it,but also generate high value-added chemicals.Many electrochemical processes are involved in the above devices,including oxygen reduction reaction（ORR）,nitrate reduction reaction（NO₃RR）,and formic acid oxidation reaction（FAO）,etc.The catalytic performance of the catalyst plays a decisive role for the high energy conversion/storage efficiency of the devices,and noble metal alloy catalysts have attracted extensive attention because of their excellent catalytic performance.The traditional binary alloy has limited component control,and non-precious metal components in the alloy are easy to be dealloyed under harsh electrocatalytic conditions,which limit the activity and stability of the catalyst.Compared with the traditional binary alloy,the middle entropy alloy（MEA）and high entropy alloy（HEA）formed by alloying highly active precious metals with transition metals could reduce the amount of precious metals,improve the utilization rate of precious metals,and provide the possibility for optimizing electrocatalytic activity and stability.MEA and HEA have multiple combinations of elements（three or more）,which provides more possibilities for adjusting the electronic structure of catalysts and the adsorption energy of intermediates in the reaction process to enhance their electrocatalytic activity.More importantly,the increased mixing entropy of the system can further improve the structural stability of MEA and HEA catalysts.Due to the complex components of MEA and HEA,the active sites and performance sources in the reaction process are not clear.Moreover,the structure of catalyst is closely related to its catalytic performance,and fine morphological structure regulation can control the density and distribution of active sites.However,the research on the structural regulation of MEA and HEA is limited at present.In view of the above problems,Pd Cu-based MEA and HEA were prepared by solvothermal method,and hollow MEA and HEA with clear active sites,high activity and stability were finally obtained by adjusting the composition and microstructure of metal elements in alloy materials,and their catalytic properties were studied.The main research contents are as follows:1.In NO₃RR,the optimization of active sites is very important for catalytic activity,selectivity,and stability.In this study,MEA was first applied to the electrocatalysis of NO₃RR,and it was found that hollow Pd Cu Co MEA had excellent activity and stability for the reduction of NO₃^-to NH₃.The three elements of hollow Pd Cu Co MEA play different roles in the process of NO₃RR.The experimental results show that the reduction of NO₃^-to NH₃is mainly divided into two steps:the adsorbed NO₃^-is reduced to NO₂^-by single Cu sites,and when Cu and Co is alloyed,the adjacent Co sites further reduce NO₂^-to NH₃.The mental Co as the active site makes the process of synthesizing NH₃with high activity and selectivity.The alloying between Pd,Cu,and Co prevented the precipitation of transition metals in the test process,which significantly enhanced the stability of the catalyst.The synergistic effect between the three metals ensures the smooth operation of NO₃RR.Moreover,compared with solid nanoparticles,the hollow structure significantly improves the yield and Faraday efficiency of electrocatalytic NO₃^-to NH₃by Pd Cu Co MEA.2.By adjusting the alloy structure,composition,and the type of carbon support,nano-hollow Pd Cu Mo Ni Co HEA was prepared on the carbon hybrid of reduced graphene oxide（RGO）and carbon nanotubes（CNT）with a mass ratio of 3:1.In this paper,HEA（hollow Pd Cu Mo Ni Co HEA）was first used as an excellent bifunctional electrocatalyst for catalyzing acidic ORR and FAO,and maintained high catalytic activity and stability.The ligand effect,strain effect between alloy components,high entropy effect,and sluggish diffusion effect of HEA make hollow Pd Cu Mo Ni Co HEA have outstanding catalytic performance compared with other hollow alloys（binary,ternary,and quaternary）.CNT is used as support to enhance the conductivity of the catalyst.The abundant functional groups on the surface of RGO may provide favorable sites for anchoring metal precursors,and the large surface area of 2D structure serves as a support for uniform dispersion and facilitates the formation of individual hollow alloy.The results show that Pd is the active center for the catalytic reaction,while other elements act as co-catalysts to adjust the adsorption energy of intermediates and accelerate their further conversion.Alloying with transition metals will induce a change of intrinsic strain of Pd,while the hollow structure can alleviate the influence of strain change caused by heteroatoms,which makes the active center Pd in Pd Cu Mo Ni Co HEA has a moderate strain change,thus having the best catalytic activity.