Preparation Of MWCNT-supported PdNi And PdNiCo Nano-composites For Electro-oxidation Of Ethanol

Due to the advantages of high efficiency,clean and environmental protection,fuel cells are regarded as the 21 st most promising power generation devices. The anode catalysts research focus has gradually shifted from the Pt-based catalysts to Pd-based catalysts. Non-noble metals Ni and Co have shown certain catalytic activity to ethanol oxidation in alkaline media. This thesis has prepared a series of Pd-based catalysts,by adding Ni and Co into Pd Cl2 solution during the preparation of nano-Pd catalyst. The physical characterization of these samples has been characterized by SEM,TEM,XRD and ICP,and their electro-activity has been investigated by some conventional electrochemical techniques.?1? Multi-wall carbon nano-tube?MWCNT?-supported palladium nano-catalysts?Pd-EDTA/MWCNT, Pd-Gly/MWCNT and Pd-Ls/MWCNT? were prepared by supporting Pd nano-particles on MWCNT. The Pd nano-particles were obtained from reduction of Pd²⁺ with Na BH4 as the reducing agent in the presence of EDTA,Glycine?Gly?,and sodium lignin sulfonate?Ls? as the ligand,separately. The catalysts were characterized by SEM,TEM and XRD. The electrochemical activity of the catalysts towards ethanol oxidation in alkaline media was examined by cyclic voltammetry?CV? and chronoamperometry?CA?. The characterization results show that the Pd-EDTA/MWCNT catalyst exhibits a smaller particle size and a better dispersity of Pd nanoaprticles compared to the Pd-Gly/MWCNT and Pd-Ls/MWCNT catalysts. As for the ethanol oxidation in alkaline media,the Pd-EDTA/MWCNT catalyst exhibits a lower onset potential,a higher current density,a smaller electron transfer resistance,a larger reaction rate,and more stable oxidation current,in comparison with the other two catalysts.?2? Pd-Ni/MWCNT and Pd-Ni-Co/MWCNT catalysts with various metals ratios were prepared by the traditional chemical reduction method,with Na BH4 as reducing agent and MWCNT as the carrier. Results of TEM images show that the particle size of these catalysts is around 5 nm. Cyclic voltammetry test in ethanol solution indicates that when the ratio of Pd:Ni/?NiCo? is 8:2,the catalyst has the highest activity for the electro-oxidation of ethanol. The maximum current density at the presence of Pd₈?NiCo?₂/MWCNT and Pd₈Ni₂/MWCNT is 159.2 m A·cm^-2 and 133.9 m A·cm^-2,respectively. In contrast to the chronoamperometry of Pd/MWCNT,Pd₈?NiCo?₂/MWCNT and Pd₈Ni₂/MWCNT, the current density at the Pd₈?NiCo?₂/MWCNT is significantly greater than that at the other two catalysts,and all these catalysts have relatively good electrocatalytic stability. According to the analysis of AC impedance,the impedance value of Pd₈?NiCo?₂/MWCNT forethanol oxidation is lower than that of Pd/MWCNT and Pd₈Ni₂/MWCNT,this illustrating Pd₈?NiCo?₂/MWCNT has a lower charge transfer impedance,higher electrochemical oxidation rate and a better catalytic activity for the catalytic oxidation of ethanol.?3? Palladium nanoparticles supported on multi-walled carbon nanotube?MWCNT?,with ultra-low Pd loadings?wt%? of 1.1%- 1.9%,have been prepared through an in situ reduction of Pd²⁺ by Ni nanoparticles immobilized on MWCNT. That is,Ni nanoparticles are firstly loaded on MWCNT by chemical reduction method to fabricate the catalyst nano Ni/MWCNT. Then,aqueous Pd Cl2 solution with various concentrations is added to the nano Ni/MWCNT catalyst stepwise,leading to the formation of Pd nanoparticles and subsequent in situ deposition on the nano Ni/MWCNT. The as-synthesized nano-catalysts?Pd4.1Ni1/MWCNT,Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT? have been characterized by SEM and XRD. Their electrocatalytic activity for ethanol oxidation in alkaline media has been investigated. Compared to the Pd/MWCNT catalyst prepared by the conventional Na BH4 reduction method, the as-prepared Pd-Ni/MWCNT catalysts present low Pd-loading and significantly high electroactivity for ethanol oxidation. According to the cyclic voltammetric data,the forward-scan anodic peak current density on the Pd3.7Ni1/MWCNT catalyst is 130.9 m A·cm^-2·?g?Pd?-1,which is over 11 times higher than on the Pd/MWCNT.?4? Ni/MWCNT and NiCo/MWCNT nanoparticles were firstly prepared by chemical reduction method with hydrazimum hydroxide as reductant ina mixture medium of water and alcohol using Multi-walled carbon nano-tube?MWCNT? as a support. Pd Ni/MWCNT and Pd NiCo/MWCNT catalysts were then prepared through an in-situ reduction by the reaction of the Ni/MWCNT and NiCo/MWCNT with the aqueous Pd Cl2 solution respectively. SEM and TEM images show that the large aggregates on the surface of MWCNT with the diameter of 30¹00 nm are composed of small particles with the diameter of 5¹0 nm. The diameter of the ternary metallic catalyst is smaller than bimetallic catalyst,and exhibitsa better dispersity on MWCNT. Combined with ICP and EDS tests, Pd nanoparticals are directly reduced and coated on the surface of Ni and NiCo nanoparticles. The electrochemical activity of the catalysts towards ethanol oxidation in alkaline media has been examined by cyclic voltammetry and chronoamperometry methods. Pd-Ni-Co/MWCNT catalysts present an anodic peak current density of 101.8 m A·cm^-2 for ethanol oxidation inalkaline medium. Long-term cyclic voltammetric tests show superior stability of Pd nanoparticles upon addition of Ni and NiCo to the support.