Synthesis Of Pt/Pd And Ni-Based Nanocomposite Catalysts And Their Electrocatalytic Performance

Direct liquid fuel cells(DLFCs)are one of the best portable energy conversion devices which have the capability to convert chemical energy to electric energy.In DLFCs,methanol,ethanol,formic acid,urea or other organic small molecules are used as the fuels.The advantages of the organic small molecules including environmentally friendly,high energy density,and easy storage and transportation make DLFCs one of the most promising energy conversion devices for commercialization.The nano-sized noble metal platinum and palladium are considered to have the highest intrinsic activity for the electro-oxidation reaction of small molecular fuels.However,the anodic electrocatalysts have many disadvantages such as high cost,low CO-like intermediate tolerance and poor durability which seriously hinder the commercialization of DLFCs.Herein,based on the reaction mechanism in DLFCs,we adapt multiple strategies of nano-structural engineering in order to increase the activity and stability of our home-made Pt,Pd or Ni-based nanoarchitectures.Meanwhile,the structure-activity relationship of these electrocatalysts are also investigated.In this work,the bimetallic NiPd hollow nanocrystals(HNCs)are firstly fabricated by taking advantage of the galvanic replacement reaction.The home-made Ni nanospheres that prepared rapidly by Na BH₄ in an aqueous solution are treated as sacrificial template.We prove that the morphology of NiPd hollow nanocrystals are strongly depends on the surface oxidation degree of Ni nanotemplates,which NiPd-HNCs with dendritic morphology can be obtained from Ni nanotemplates with surface partial oxidation property.It is inferred from the electrochemical tests that NiPt-HNCs with dendritic morphology shows better EOR and FAOR activity and stability,which is 2.4-fold times higher than that of commercial Pd black catalyst.The main reason is that the dendritic morphology brings the exposure of Ni species,which is responsible for the electrostructural modification of Pt active sites.To further improve the performance of the hollow-type nanomaterials,trimetallic NiPtPd hollow nanocrystals with porous and dendric morphology are successfully fabricated by combining the templating method and co-reduction strategy.The different role of Pt,Pd andNi on MOR catalysis are fully investigated.It is find thatPt is the main active site for MOR catalysis.Due to the electric interaction between Ni and Pd on surface of NiPtPd,the electron-deficientPd sites are formed and play an important role in the OH_ads binding.DFT calculations prove that OH_ads binding energy of NiPtPd surface are significantly enhanced,accelerating the CO-like intermediates oxidation removal and improve the MOR activity and CO tolerance.Electrochemical tests show that trimetallic NiPtPd hollow nanocrystals deliver MOR specific mass activity of 10.68m A·cm^-2,which is 4.2 times higher than that of Pt/C catalyst.To clarify the effect of the transition metal oxides decorated on tne noble metal surface on their electrocatalytic performance,a two-step hydrothermal method is employed to prepare CuO_x decorated Pd nanowires catalysts.Their metal/oxide interfacial engineering is realized by introducing a fast air plasma treatment.During the process of air plasma treatment,the initial amorphous CuO_x layer is gradually turned into crystalline state.The as-prepared PdNW/crystalline CuO_x catalyst with 17.2 at.%of Cu on the PdNW surface exhibits better MOR and EOR activity,6.14 and 7.19m A·cm^-2,respectively,compared with that of PdNW/amorphous CuO_x and pristine PdNW catalysts.The detailed analysis of their chemical state reveals that the enhanced activity and anti-poison ability of PdNW/crystalline CuO_x catalyst originated from the electron-deficientPd^?+active sites which can be induced and stabilized by moderate crystalline CuO_x decorated on the surface of PdNW.A two-step hydrothermal sulfuration method is employed to prepare ultrafine Mn S decoratedNi₃S₂ nanosheet array,NF@Ni₃S₂-Mn S heterogeneous catalyst from the precursor of NF@Mn O nanosheet for highly efficient UOR catalysis.Electrochemical tests show that NF@Ni₃S₂-Mn S heterogeneous catalyst exhibits superior urea catalytic activities with a potential of 1.37 V vs.RHE to attain a 20 m A cm^-2 UOR current density.The enhanced UOR activity and promising stability can be attributed to the structural superiority of NF@Ni₃S₂-Mn S.The ultrafine semiconductor Mn S decoration makes NF@Ni₃S₂-Mn S a schottky heterojunction with modulated surface charge distribution that effectively activate the C-N bond in urea molecule,yielding more effective UOR kinetics on the NF@Ni₃S₂-Mn S heterogeneous surface.