Synthesis And Study Of Core/Shell Nanoparticles Coated By Amorphous Platinum

The fuel of the direct methanol fuel cell (DMFC)——methanol, which has wide source, low price and high energy density to be stored easily, is considered to be an ideal portable power. Although its superiority is obvious, some disadvantages make it difficult to commercialize, such as the methanol electrochemical activity is slow at room temperature, the crystalline precious metals (Pt et. al.) are easily poisioned by CO, the specific activity is low, the performance of resistance on alcohol is poor, and methanol permeability is high with the low energy convertion efficiencyUp to now, the catalysts are the critical material in DMFC, and platinum catalysts are main materials for DMFC. However, the platinum is limit that its cost is high, so other metals were added to form binary or multicomponent alloy or core-shell catalysts to solve the above problem.The catalysts with amorphous platinum, which wrapped the nano-nickel core for methanol electro-oxidation not only have the advantages of amorphous metals but also have the advantages of core-shell catalysts. The amorphous metals have high degree of coordination unsaturation and high concentration of sites and abounds with reactivity, which lead to superior catalytic activity and tolerance on CO compared to its crystalline counterparts. The core-shell catalysts have high atomic ratios on the surface and high active surface area, which increase the utilization of platinum.In this paper, Nicore-Ptshell core-shell nano-particles with different atomic ratios were synthesized in two-step reduction method, and their composition and microstructure were characterized through XRD, XPS, EDS, aberration-corrected HRTEM, and electrochemical properties through CV, in situ FTIR and EQCM. The results are as follows:Nicore-Ptshell core-shell nano-particles coated by amorphous platinum with different atomic ratio of Ni and Pt were synthesized in two-step reduction method, and by means of XRD, XPS, EDS, aberration-corrected HRTEM, it could be confirmed that the diameter was about12nm, the shell thickness was about1-2nm, and Ni core was face-centered cubic (fcc) crystals in most of which were Ni (111) crystals, whose diameter was about10nm. When the atomic ratio of Ni and Pt is less than10and the shell thickness is more than2nm, the platinum on the shell is crystalline.The CV curves in H2SO4solution showed that the corresponding voltage to the peak current density of oxygen species on different catalysts occured positive shift, the order of positive shift range was:Ni15Pt1/C>Ni1oPt1/C> Ni5Pt1/C>Ni2Pt1/C, which illustrated that the adsorption energy of oxygen species on catalysts weakened with the increasing of Pt content, and by contrast, the adsorption energy of nano-metals coated by amorphous platinum was high.The CV curves in H2SO4and CH3OH solution showed that the specific activity of methanol electro-oxidation on the nanoparticles coated by amorphous platinum was higher than the nanoparticles coated by crystalline platinum, whose trends first increased then decreased with the increasing of the shell thickness, which was similar to the trends of the nanoparticles all coated by amorphous platinum.The values of tafel slope b calculated by Ep and lnV were small, which showed that the activation overpotential of the nanoparticals was low. And the values of tafel slope b of the nanoparticals whose ratios of Ni and Pt were10:1 and15:1were the least, which proved that the activation overpotential was the least.In situ FTIR spectra of methanol electro-oxidation showed that the adsorption intensity of CO on Ni15Pt1/C and Ni10Pt1/C nanoparticles with amorphous platinum was less than that on pure platinum, which illustrated that the amorphous metals had higher tolerance on CO than crystalline metals.In experiment of EQCM, the methanol electrooxidation would consumed oxygen species which adsorbed on the surface of amorphous platinum, and the relationship between methanol oxidation and oxygen species was relatively close. At the same time, the crossover phenomenon in CV curves illustrated that the reaction mechanism of methanol oxidation on amorphous platinum is also dual path. The quality changes in the forward scan from0.2V to0.8V showed that the Ni15Pt1nanoparticles had stronger oxidative activity for intermediate species than Ni1oPt1nanoparticles.