Nanodendrites Of Pt Alloys And Their Electrocatalytic Properties For Oxygen Reduction Reaction

As a clean and efficient energy conversion technology,the large-scale application of fuel cells is limited by the slow kinetics of the oxygen reduction reaction（ORR）and the high price of Pt as a cathode catalyst.Recent studies on a wide range of Pt single crystal surfaces have shown that some high-index-surfaces（HISs）of Pt,especially the n（111）×（110）and n（111）×（100）stepped Pt surfaces such as Pt（332）,Pt（331）,Pt（554）and so on,can exhibit enhanced ORR activity over Pt（111）/（110）surface in acidic electrolytes.This thesis is dedicated to combining the two strategies of alloying and building HISs to develop a catalyst with excellent oxygen reduction catalytic performance.DFT calculations were performed to further understand the origins of the enhanced ORR activity of the Pt-Cu catalysts with HISs.The main results of the paper are summarized as follows:（1）PtCu₃ nanodendrites with high-index-surfaces（HISs）synthesized by the strategy of modulating the reaction atmosphere.Using oleylamine（OAm）as both solvent and reducing agent,without additional protective agent,through the control strategy of atmosphere modulating,PtCu₃ alloy nanodendrites with HISs is prepared.In contrast,the whole process of growth in an inert atmosphere produces multipods structures（MPs）,while the whole process of growth in an oxidizing atmosphere produces nano-concave cubes（CBs）.By adopting this method,we also explored the effect of the concentration,temperature,protective agent and other conditions on the morphology of PtCu₃ nanodendrites.（2）Synthesis and characterization of PtCu₃@Pt₃Cu@Pt/C nanodendrites catalyst with gradient-composition and high-index-surfaces.Electrochemical dealloying approach was used to obtain PtCu₃@Pt₃Cu@Pt/C from the as-prepared PtCu₃ nanodendrites.The catalyst exhibit mass and area specific activities of Pt for ORR in 0.1 M HCl O₄ solution which are 15 and 24 times higher than that of commercial Pt/C respectively.Moreover,the HISs of the PtCu₃@Pt₃Cu@Pt/C nanodendritic catalyst is confirmed via CO-stripping.The difference in activity of PtCu₃@Pt₃Cu@Pt/C nano-dendritic catalyst in acid and alkaline electrolyte further verify the contribution of HISs to its oxygen reduction catalytic activity.（3）Synthesis and characterization of PtCu_xNi_3-x ternary nanodendrites oxygen reduction catalyst.To explore the effect of the ratio of various atom components on the electrochemical performance,we prepared PtCu_xNi_3-x nanodendrites with different compositions by adjusting the ratio of Ni:Cu in the precursor.It is found that their morphology was greatly affected by the composition of the precursor.Through the characterization results of XRD,XPS and electrochemistry,we believe that the PtCu_xNi_3-x nanodendrites have formed a relatively Pt-rich structure during synthesis.Through CV characterization,we believe that the adsorption strength of the dealloyed PtCux Ni3-x/C nano-dendritic surface catalyst for oxygen-containing species gradually weakens with the increase of Ni content.Through the characterization of CO-stripping,we believe that the dealloyed PtCu_1.5Ni_1.5/C nanodendritic catalyst has relatively uniform catalytic sites and large ECSA,and thus exhibits excellent ORR activity.（4）Theoretical calculation study of high-index-surfaces Pt-Cu alloy oxygen reduction catalyst.The adsorption strength of oxygen atoms at different fcc adsorption sites on the PtCu₃@Pt₃Cu@Pt（332）and other high-index-surfaces were calculated by DFT.The results show that for the step site with（111）platform,the adsorption capacity of the fcc adsorption site for oxygen atoms shows a gradually weakening trend from the edge of the platform to the bottom of the step.It has strong adsorption at the edge of the platform and weak adsorption strength under the steps.At the same time,due to the strong lattice compression effect,the adsorption capacity of PtCu₃@Pt₃Cu@Pt（111）plane for oxygen is weaker than the ideal adsorption strength.Under the combined action of the two,the edge of the PtCu₃@Pt₃Cu@Pt（332）platform has an oxygen atom adsorption strength close to the top of the"volcanic relationship".