Multi-walled Carbon Nanotube Supported Pt-ni, Preparation And Electrochemical Properties Of Pd-ni Alloy Catalysts

Direct methanol fuel cells (DMFCs) have been recognized as the one of the ideal choice for dealing with the problem of energy in future, because its high efficient energy, rich in the source of methanol, the price of methanol is low, easy to store and transport, zero pollution, and so on.Up to now, acid electrolyte is widely used in DMFCs and the platinum is as the main anode catalysts. But the high price, low storing and easy to poison by CO limit the application of Pt. Moreover, the low kinetics of reaction of methanol and oxygen reaction in acid electrolyte is another key issue that limits the commercialization of acid DMFCs. Recently, with the development of anion exchange membrance, increasing attention has been paid to the search for alkaline DMFCs. Compared with the acide DMFCs, there are several significant advantage in alkaline DMFCs, such as the high reaction kinetics of methanol oxidation reaction and oxygen reduction, methanol crossover can be suppressed more effectively, lower prices of anion exchange membrane, the non-Pt metals could be employed as ctalysts, and so on.It is well known that the structure of nanoparticles significant impacts the catalytic performance. In recent years, alloy structure catalyst has gained great attention because it not only improves the utilization of shell metal, but also greatly enhances the catalytic performance and stability.In this paper, the Pt-Ni and Pd-Ni alloy nanoparticles with different Pt/Ni and Pd-Ni atomic ratios supported on functionalized multiwalled carbon nanotubes surface were synthesized via an impregnation-reduction method. The nanocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and electrochemical techniques.XRD demonstrated a contraction of the lattice upon substitution of Pt or Pd with Ni and the formation of Pt-Ni alloy or Pd-Ni alloy. TEM suggest that the Pt-Ni and Pd-Ni nanoparticles are uniformly dispersed on the MWCNTs surface and the mean particle size of Pt-Ni and Pd-Ni nanoparticles are about5nm. It can be concluded that appropriate amount of Ni in Pt-Ni or Pd-Ni alloy can facilitate the dispersion nanoparticles on the MWCNTs surface and reduce the mean particle size. EDX and XPS revealed that the Pt4f or Pd3d peak in Pt-Ni/MWCNTs or Pd-Ni/MWCNTs catalyst shifted to a lower binding energy compared with Pt/MWCNTs or Pd/MWCNTs catalyst, and Nickel oxides/hydroxides such as NiO, Ni(OH)2and NiOOH were on the surface of Pt-Ni and Pd-Ni nanoparticles. This negative binding energy may be caused by the electron transfer from a lower electronegativity of Ni (1.91) to a higher electronegativity of Pt (2.28) or Pd (2.20), which also lowers the density of states on the Fermilevel and reduces the Pt-CO or Pd-CO bond energy, indicating that the electronic structure of Pt in Pt-Ni/MWCNTs and Pd in Pd-Ni/MWCNTs catalyst are changed after the formation of Pt-Ni alloy and Pd-Ni alloy.Electrochemical data based on cyclic voltammetry and chronoamperometric curves indicated that Pt-Ni and Pd-Ni alloy nanoparticles exhibited distinctly higher activity and better stability than Pt/MWCNTs and Pd/MWCNTs toward methanol oxidation in alkaline media. The peak current density of Pt-Ni/MWCNTs (4:1) catalyst is972mA mg-1, and1.89and2.03times higher than that of Pt/MWCNTs and Pt/C (JM), respectively. The peak current density of Pd-Ni/MWCNTs (4:1) catalyst is828mAmg-1and1.32times higher than that of Pd/MWCNTs catalyst. The enhanced activity and stability of Pt-Ni/MWCNTs and Pd-Ni/MWCNTs catalyst for methanol oxidation in alkaline media can be attributed to the electronic effect and the synergistic effect.