Studies On The Synthesis Of Pt/Pd-based Metastable Nanocrystals And Their Catalytic Properties

Among noble metals,nanomaterials of Pt and Pd have incomparable functions in fuel cell catalysts compared with other noble metals.However,considering high cost and low reserves of Pt and Pd,it is very desirable to improve the catalytic activity and utilization efficiency of Pt and Pd based catalysts.Of note,among various types of nanostructures,nanocrystals with dendritic or penta-cyclic-twined(PCT)structure are thermodynaically metastable and often exhibit better performances than those stable nanocrystals with exposed low surface energy surface.Therefore,in this paper,Pt and Pd based metastable nanocrystals with dendritic or PCT structure are taken as the research objects,with focus on the controlled synthesis,growth mechanism and their structure-activity relationship of fuel cell reactions.The main research progress is summarized as follows:1.A facile and efficient synthetic method of fabricating fractal PtAg nanodendrites without using any surfacants were proposed,and their performance in the electrooxidation of methonal under the acid medium was preliminarily investigated.The resulting dendritic PtAg NCs possess a large specific surface area and open achitecture,and thus exhibited much higher electrochemical surface areas(ECSAs)(29.2 m2 gPt-1 for Pt79Ag21,and 35.1 m2 gPt-1 for Pt60Ag40)compared to those spherical nanoparticles(21.9 m2 gPt-1 for PtAg-7:3-NaBH4,and 17.1 m2 gPt-1 for the commercial Pt black).Due to their unique characteristics in morphology and alloying structure,the resulting dendritic PtAg NCs exhibited the superior catalytic activity in MOR(methanol oxidation reaction,specific activity is 1.98 mA cm-2,maximum mass current density is 578.2 mA mgPt-1)and ORR(oxygen reduction reaction,specific activity is 1.05 mA cm2,mass activity is 362 mA mgPt-1)in acidic conditions.2.A kind of unique Pd NSs with a concave PCT structure(c-PCT Pd NSs)were synthesized by a simple wet chemistry method,which was for the first time reported to the best of our knowledge.Through the analysis of its morphology and structure,it was found that the c-PCT Pd NSs were spliced by five triangle plates with exposed {110}high-energy crystal planes by sharing the same vertex.This unique structure was formed by the growth mechanism of "nucleation at high temperature and growth at low temperature";specifically,it is formed by the preferential growth of initially formed PCT decahedron crystal nucleus along its[100]directions with the assistance of selective adsorption of the surfactant CTAC.Owing to the unique structure,c-PCT Pd NSs exhibited excellent performance in the electrocatalytic oxidation of formic acid.Its maximum mass current density was 945.5 mA mg-1 measured at 0.182 V(vs.SCE),which was 1.32 and 1.97 times that of Pd NCs and commercial Pd black exposed to{111} measured at the same potential,respectively.In addition,it was found that the electrocatalytic performance of c-PCT Pd NSs could be significantly enhanced by doping minute quantities of Ag into the concave PCT alloy nanosheet.3.The Pd@Pt PCT icosahedra with tunable composition were synthesized by a simple one-step method,and its electrocatalytic property of hydrogen oxidation reaction(HOR)were tested.It was found that the catalytic performance of Pd@Pt icosahedra was in a volcanic curve with the composition ratio of Pd:Pt.When the feeding ratio of metal salt precursor was 1:1,the synthesized Pd76Pt24 icosahedra exhibited the optimal performance.As the ratio of Pd:Pt increased or decreased,the catalytic performance of the catalysts decreased gradually.When the alloy composition was kept the same,the Pd76Pt24 icosahedra exhibited better properties than the singlecrystalline counterpart,indicating that the existence of lattice strain and twin boundary in the PCT icosahedra improved the HOR performance.