| Direct formic acid/ethanol fuel cells have received much attention in recent years due to their high energy density,low Nafion crossing rate,low toxicity,easy transportation and storage,simple battery structure,and low temperature operation.However,the commercialization of fuel cells is largely limited by the low abundance and high cost of the noble metal-based catalyst.Incorporation of one or more other cheap transition metals into the noble metal catalyst by making alloy or core-shell structure nanocatalyst,can significantly improve the mass activity and reduce the cost of the noble metal catalyst.However,metal alloys suffer from metal dissolution and the interface of core-shell NPs are still not clearly studied and show low anti-poisoning ability,limiting the further enhancement of the catalytic performance of the metal catalyst.In this paper,ruthenium nanoparticles?Ru?is prepared in-situ on carbon nanotubes by ethanol reduction method,and Ru@Pt and Ru@AuPt core-shell structures are prepared by growing Pt and AuPt alloy shells on top of Ru core,respectively.The catalysts are further characterized by X-ray diffraction?XRD?,transmission electron microscopy?TEM?and X-ray photoelectron spectroscopy?XPS?and the electrochemical catalytic activity towards formic acid oxidation?FOAR?and ethanol oxidation reaction?EOR?are discussed in terms of the composition,atomic and electronic structures of the prepared nanocatalysts.?1?First,we studied the impact of the ordering of Ru metal core on the Ru-Pt interface,the electronic structure and electrocatalytic performance.The crystalline and amorphous ruthenium nanoparticles with the same size are synthesized on the carbon nanotubes,and the Pt shell of different thicknesses are further grown on Ru nanoparticles to prepare Ru@Pt core-shell structure,labeled as Ru@Ptx and Ru?n?@Ptx,respectively.The x represents the elemental ratio of Pt to Ru.The Ru@Pt core-shell nanoparticles are characterized by XRD,XPS,high angle annular dark field scanning transition electron microspectroscopy?HAADF-STEM?and CO stripping voltammetry.The results show that the Ru@Ptx nanomaterials have a well-defined interface between Ru core and Pt shell,and Pt shell grow epitaxially on top of the crystalline Ru NPs.The lattice mismatch between Ru and Pt induces significant compressive stress on Pt shell.In contrast,the amorphous Ru?n?cores induce PtRu alloying at the Ru-Pt interface,on which the Pt shell grows subsequently,resulting in the fast release of the compressive stress of Pt shell.Therefore,the compressive strain of the Pt shell with the same thickness of Ru@Ptx acts stronger than that of Ru?n?@Ptx,and gradually decreases with the increase of the Pt shell.The catalytic activity of Ru@Ptx and Ru?n?@Ptx materials towards electrocatalytic oxidation of formic acid and ethanol showed a volcanic dependence on the thickness of the Pt shell.The lattice strain of Pt shell induces the down-shift of the d-band center,weakening the adsorption of COads and inhibiting the poisoning of Pt catalyst.The crystalline Ru cores induce more strain and thus more significant enhancement of the catalytic activity and stability of the catalysts than amorphous Ru cores.Among all these catalysts,Ru@Pt0.75 showed the best catalytic activity due to the optimal balance of the adsorption of COads and active intermediates on the surface,their specific activity towards electrocatalytic oxidation of formic acid and ethanol is25 and 6.5 times that of Pt/C and 3.4 and 1.8 times that of Ru?n?@Pt0.75,respectively.?2?The catalytic performance of Ru@Pt catalyst can be further enhanced by introducing Au element into Pt shell on Ru cores,as demonstrated on Ru@AuPt core-shell nanostructures.With constant thickness of AuPt alloy shell,the Pt composition on the surface of the catalyst gradually decreases with the ratio of Au to Pt.Due to the electronic effect,the d-band center of the catalyst gradually shifts upward closed to the Fermi level.With the elemental ratio of Au:Pt equal to 1:1,the compressive strain of AuPt alloy shell increases with the decreasing of AuPt shell.The catalytic activity of Ru@?Au5Pt5?n series nanomaterials towards electrocatalytic oxidation of formic acid displays much higher activity on the direct oxidation of formic acid than Ru?n?@Pt1.0 and Pt5Au5 alloy nanomaterials,with the low CO generation.Among these catalysts,Ru@?Au5Pt5?show the best catalytic performance toward formic acid direct oxidation due to the synergistic effect of the ensemble effect,the electronic effect and the strain effect,which show specific activity of 4.14 mA?cm–2 and mass activity of 1.2 A?mg–1,which are 52times and 17 times that of Pt/C,5.8 times and 3.5 times that of Ru?n?@Pt1.0,and 6.2 times and5.2 times that of Pt5Au5 alloy,respectively.Under alkaline conditions,the Ru@?Au5Pt5?n series nano-catalysts still demonstrate superior anti-poisoning ability.Among all the catalysts,Ru@?Au5Pt5?0.5 perform the best mass activity and specific activity,which is 12 times and 28times that of Pt/C,4.0 times and 5.1 times that of Ru?n?@Pt1.0 and 4.3 times and 3.9 times that of Pt5Au5 alloy,respectively.In addition,Ru@AuPt catalyst also exhibits catalytic and structural stability superior to other catalysts,due to Au component. |
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