Electrocatalytic Properties Of Modified MWCNTs Supported Precious Metal Nanoparticles Catalysts

Direct methanol fuel cells (DMFCs) have attracted much attention for their potential application as clean and mobile power sources with high energy density and simple structure. The anode catalysts of DMFCs are usually platinum or platinum based catalysts. In order to successfully realize the commercialization of DMFCs at low cost, the amount of electrocatalysts should be reduced remarkably while maintaining high mass specific current density. It is very important that precious metal nanoparticles can be uniformly deposited on support materials. In recent years, Multi-walled carbon nanotubes (MWCNTs) have been considered to be the most potential catalyst carrier material due to their good conductivity, large specific surface area, high strength and hardness. However, the inert surface of MWCNTs makes them difficult to disperse in polar solvent and in nonpolar solvent. Oxidation treatment has usually been proposed to solve this problem, but it will destroy the structure and reduce the conductive performance of MWCNTs. Therefore, MWCNTs must be properly functionalized to make metal nanoparticles uniform on the surface of MWCNTs and meanwhile to maintain the integrity of the structure of MWCNTs.In this paper, Pt, Au, Pd, Au@Pd nanoparticles were synthesized by method of ultraviolet irradiation. MWCNTs modified with methylene blue during ultraviolet irradiation (f1-MWCNTs) were as a new support of these nanoparticles. For comparison, these nanoparticles were also assembled to MWCNTs modified with methylene blue without ultraviolet irradiation (f1-MWCNTs). This paper systematically studied the conditions of modifying MWCNTs with methylene blue during ultraviolet irradiation as a new support of Pt nanoparticles, mainly including the functionalization time, methylene blue dosage and ultraviolet wavelength. Ultraviolet and Visible (UV-Vis) absorption spectroscopy was used to analyze the reaction process of precious metal precursor solution under ultraviolet irradiation. Fourier transform infrared (FT-IR) absorption spectroscopy was used to study the functionalized MWCNTs, finding that C-N (1093cm-1) and N-H (667cm-1) groups existed in f1-MWCNTs. In pure methylene blue, X-ray photoelectron spectroscopy (XPS) analysis showed that there were two state of N peak, that the corresponding binding energy value was respectively399.19eV and400.95eV, that the corresponding peak area ratio was about2:1and that the binding energy value of S was about164.41eV. However, as to f1-MWCNTs, N had three states and the corresponding binding energy value was396.69eV,400.11eV and401.91eV respectively. Besides, the binding energy value of S in f1-MWCNTs increased to165.34eV. TEM and XRD analyses showed that the prepared nanoparticles with the uniform distribution on the surface of f1-MWCNTs were about2-6nm in diameter, and had face-centered cubic crystal structure.Electrochemical experiments indicated that the synthesized catalysts showed excellent catalytic activity and stability for methanol in alkaline solution. The mass specific current density (MSCD) of the peak in the forward sweep for Pt/f1-MWCNT was about2035mA mg-1, and was1.63times higher than that for Pt/fo-MWCNTs and2.12times higher than that for Pt/C; The MSCD of Au/f1-MWCNT was about59.7mA mg-1, and was1.56times higher than that for Au/fo-MWCNTs; The MSCD of Pd/f1-MWCNT was about781mA mg-1, and was1.58times higher than that for Pd/fo-MWCNTs and1.94times higher than that for Pd/C; The MSCD of Au@Pd/f1-MWCNT was about785mA mg-1, and was1.34times higher than that for Au@Pd/fo-MWCNTs and1.95times higher than that for Pd/C. This is mainly related to the following three reasons.(1) The precious metal nanoparticle distribution. Through TEM photos it can be seen that these precious metal nanoparticles have uniform distribution on f1-MWCNTs, which improves the utilization ratio of precious metal atoms.(2) The influence of the functional groups. C-N and N-H groups in f1-MWCNTs may be able to change the surrounding electronic state of precious metal atoms, resulting in the improvement of the catalytic performance for methanol.(3) Carrier material carbon nanotubes. Carbon nanotubes have higher electronic transmission performance than carbon black, which reduces the electron transfer resistance in the process of methanol oxidation. At the same time, the mesopore structure of carbon nanotubes is in favor of the material transmission during methanol oxidation, so as to promote the electrocatalytic performance for methanol oxidation.