Preparation And Catalytic Properties Of Au-based Bimetallic Nanoalloys

Alkaline anion exchange membrane fuel cells?AAEMFCs?,as new-style energy conversion devices,has a great potential to be used in future automobile and portable electronic devices due to their high energy density,wide fuel sources and zero emission.In order to maximize the energy efficiency and power output of AAEMFCs,it is highly critical to develop high-performance cathode and anode electrocatalysts to accelerate the oxygen reduction reaction and fuels oxidation reactions.Despite intensive research on the electrocatalysts for AAEMFCs,the highly active and stable electrocatalysts are still heavily dependent on the rare noble metal Pt and carbon support materials.The high cost,low earth reserve of Pt and the serious corrosion issue of carbon support materials under operating conditions have hindered the large-scale commercialization of AAEMFCs.Therefore,developing a multi-functional electrocatalyst with high activity and stability for cathode and anode reactions to substitute Pt is currently a research difficult and hot spot in the field of fuel cells.This work is carried out without the use of any carbon support materials,and studied the electrocatalytic properties and improvement mechanism of unique Au-based nanodendrite and nanosponge electrocatalysts prepared by well-designed synthetic strategies for oxygen reduction reaction,borohydride oxidation,ethylene glycol oxidation and glucose oxidation reactions.The detailed contents are listed as follows.In this work,a series of bimetallic Au Ni nanodendrites were synthesized under different conditions via the potentiostatic electrodeposition.The dealloyed Au Ni nanodendrites could be successfully achieved after the electrochemical dealloying processes in acid solution,in which metal Ni was selectively etched.Compared with the as-synthesized Au₁Ni₂nanodendrites,the dealloyed Au₁Ni₂ nanodendrites catalyst exhibits sharpened surface and developed secondary dendritic architecture,which induces the large electrochemical surface area?ECSA?.The ECSA of dealloyed Au₁Ni₂ nanodendrites catalyst is about 1.3 and 3.4times larger than that of as-synthesized Au₁Ni₂ and pure Au catalysts.The dealloyed Au₁Ni₂nanodendrites catalyst shows the excellent electrocatalytic ORR activity.The half-wave potential of the dealloyed Au₁Ni₂ nanodendrites catalyst is 86,67 and 27 m V more positive than that of pure Au,commercial Pt/C and as-synthesized Au₁Ni₂ catalysts,respectively.At the same time,the dealloyed Au₁Ni₂ nanodendrites catalyst exhibits more enhanced ORR stability than the commercial Pt/C,which is ascribed to the stable dendrite structure and negligible composition change.After 5000 potential cycles,the dealloyed Au₁Ni₂ catalyst only shows 38 m V negative shift in the half-wave potential.A relationship between the ORR activity of Au Ni nanodendrites and the morphology,crystalline structure,composition and electronic structure is well established,it is concluded that the excellent catalytic performance of dealloyed Au₁Ni₂ nanodendrites originate from the combination of large ECSA and effective electronic effect,Based on the two-step method to prepare the dealloyed catalysts,herein,a general one-pot electrochemical strategy is for the first time reported to synthesize the dealloyed Au Ni nanodendrites?NDs?catalyst.Through adjusting the periodic three-potential-step program,the simultaneous electrochemical deposition and dealloying process were successfully realized in the same metal precursor solution,which effectively simplifies the traditional dealloying route and avoids the use of corrosive solutions such as strong acids and bases,fully developing the application of dealloying method in electrocatalysis.Significantly,the dealloyed Au Ni NDs catalyst by the one-pot strategy shows the remarkably improved activity and stability towards ORR and BOR compared to the commercial Pt/C catalyst.The half-wave potential of dealloyed Au Ni NDs catalyst is 0.911 V,which is 81 and 121 m V more positive than that of the commercial Pt/C and pure Au catalysts,respectively,and even shows a 15 m V positive shift relative to the Au Ni DD catalyst via the combined electrodeposition and dealloying method.The specific activity of the dealloyed Au Ni NDs catalyst is 0.46 m A cm^-2 at 0.85 V,which is about 3.1 times higher than that of the Pt/C catalyst.The long-term i-t curves reveal that the dealloyed Au Ni NDs catalyst exhibit the more excellent stability and methanol tolerance than the Pt/C catalyst.At the same time,the dealloyed Au Ni NDs catalyst also shows exceptional BOR activity,the peak current density of the dealloyed Au Ni NDs catalyst towards the direct BH₄^-oxidation is around 7 times higher than that of the commercial Pt/C.Moreover,it exhibits the unique electrocatalytic selectivity for BH₄^-oxidation instead of the hydrolysis of BH₄^-.In addition,the dealloyed Au Cu catalyst was also successfully synthesized via the suggested one-pot strategy and exhibited excellent activity towards ORR and BOR,further proving the generality and validity of the one-pot strategy.In order to introduce the advanced porous nanostructures into catalysts,in this work,the kinetically-controlled chemical reduction approach was used to effectively synthesize self-supporting Au Cu nanosponge catalysts at room temperature.In this method,sodium borohydride,as a strong reductant,can rapidly reduce metal cations to produce massive fine zero-dimensional nanoparticles,which are assemled into three-dimensional open porous nanonetworks.Like metal aerogels,the as-synthesized Au Cu nanosponges are characterized with hierarchical pore system,providing abundant electrochemically active sites and pores or channels for mass diffusion during the electrocatalysis.Moreover,the Au Cu nanosponge prepared by the suggested method in this work has a fully alloyed structure,and their composition can be precisely controlled across the entire alloy composition range.Compared with the commercial Pt/C catalyst,the Au₅₂Cu₄₈ nanosponge catalyst shows the markedly enhanced activity and stability towards ORR and BOR.Through exploring the the electrocatalytic activity of Au Cu nanosponges with different Cu content and the d-band center,the volcano relationship between the ORR/BOR activity of Au Cu alloys with the d-band center or Cu content was well established,the Au Cu alloy system with an upshift in the d-band center by approximately 0.24 e V results in the highest electrocatalytic performance.At the peak of the volcano plots,the adsorption energy of the Au₅₂Cu₄₈ nanosponge is neither too strong nor too weak,showing the excellent catalytic activity,selectivity and stability for ORR and BOR.Furthermore,a series of Au Ag nanosponge catalysts were successfully prepared by the rapid and direct reduction of HAu Cl₄ and Ag NO₃ precursor with the assistance of excessive Na BH₄.Transmission electron microscopy and cyclic voltammetry certify the abundant unsaturated active sites exposed on the Au Ag_3.2 nanosponge surface,which is beneficial to obtain higher catalytic activity.Most importantly,different from the previous studies on the Au-Ag alloy system,XPS results exhibit the obvious electron transfer between Au and Ag after alloying,which effectively modified the electronic structure of Au Ag_3.2 alloy.Benefiting from the favorable surface structure and the obvious electronic regulation,the Au Ag_3.2nanosponge catalyst shows a remarkably positive shift of 32 m V in the half-wave potential relative to the commercial Pt/C catalyst.The mass activity of the Au Ag_3.2 nanosponge catalyst is 1.26 A mg_Au^-1 at 0.85 V,showing an improvement factor of 8.2 versus Pt/C.After 10000potential cycles,the ECSA,specific activity and mass activity of the Au Ag_3.2 nanosponge catalyst undergoes negligible decay.Moreover,the Au Ag_3.2 nanosponge catalyst also exhibit exceptional activity and stability for ethylene glycol and glucose oxidation reactions,it mass activity is 3 and 18.5 times higher than that of the commercial Pt/C catalyst,respectively,showing the promising multi-functional catalytic performance.