Synthesis Of Nano Alloy Materials And Performance Investigation Of Methanol And Formic Acid Electro-oxidation

Energy is one of the biggest challenges for twenty-first century. As interest in hydrogen economy grows, research on hydrogen storage and fuel cells is expanding. Among different types of fuel cells, proton exchange membrane fuel-cell (PEMFC) are of great interest because of low pollutant emission, high energy density and ease of liquid fuel handling, and are promising candidates for portable power sources. Based on the type of fuels used, PEMFC can be further categorized into direct hydrogen fuel cell (DHFC), direct methanol fuel cell (DMFC), direct formic acid fuel cell (DFAFC) and so on. Platinum-based nanoparticles are used as electro-catalysts in electrode layers in almost all these PEMFC devices. However, there are still some critical obstacles inhibiting broad applications of proton exchange membrane fuel-cell, including low electrocatalytic activity of anode catalysts for methanol oxidation reaction and the high cost of noble metal platinum (Pt)-based catalysts. The Pt-based nanocrystals (NCs) are being explored as the alternative to single Pt with addition of a second or third metal to Platinum to form Pt-based alloy catalysts for electrocatalysis. Improving the catalytic activity of anode catalyst is an important task in direct methanol and formic acid fuel cell development. We design and synthesis Pt-based and Pd-based catalysts, and which were characterized by power X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), and transmission electron microscope (TEM), et.al. And the catalytic activity were analyzed by cyclic voltammetry (CV), chronoamperometric (CA) and CO stripping voltammograms. There are three main parts in this thesis:(1) Exceptional methanol electro-oxidation activity by bimetallic concave and dendritic Pt-Cu nanocrystals catalystsPt-Cu bimetallic nanocrystal is one of the most promising candidates due to its high stability against catalyst poisoning and abundant sources of copper. This work presented a simple one-pot synthesis of concave dendritic PtCu3, PtCu2, PtCu nanocrystals and Pt3Cu nanosphere through a galvanic replacement mechanism between preformed Cu nanocrystals and Pt species and a comparative investigation of the electrocatalytic activity and stability in the methanol oxidation process. XRD exhibited that Pt and the fringe of Cu formed alloy, and with the increasing of Pt:Cu, the degree of alloy was increased. HAADF-STEM showed the Pt-Cu nanoparticles reforming core-shell structures. The core and shell were mainly Cu and Pt, and they were formed via direct nucleation on the Cu core rather than random aggregation of Pt particles. The as-prepared concave dendritic PtCu2/C nanocrystals exhibit enhanced electrocatalytic activity in the methanol oxidation reaction compared with other concave PtCu/C,PtCu3/C, Pt3Cu/C, and commercial Pt/C electrocatalyst. Such superior overall electrocatalytic performance, and especially the higher CO tolerance, suggests that the PtCu2/C catalyst could be regarded as a promising anode catalyst in the fuel cell. This enhanced performance toward the methanol oxidation could be attributed to the concave structure and possible synergetic effect of Pt and Cu components.(2) A high dispersed Pt0.35Pd0.35Co0.30/C as superior catalyst for methanol and formic acid electro-oxidationPEMFC are devices that directly convert chemical energy stored in fuel molecules into electric energy. During operations, oxygen gas is fed from cathode and electrochemically reduced while fuel molecules with low standard redox potential are electrochemically oxidized at anode. The first-row transition metals (FRTM) in nanoscale have been widely investigated as the catalytic materials because of their potential activities and relatively low costs. However, nano-FRTM was easily etched by acidic solution. When FRTM were alloyed with noble metals, their stabilities under acidic condition could be enhanced. Pt:Pd:Co ternary alloy nanoparticles were synthesized by sodium borohydride reduction under nitrogen, and were supported on carbon black as catalysts for methanol and formic acid electro-oxidation. Compared with Pt0.65Co0.35/C, Pt/C, Pd0.65Co0.35/C, and Pd/C catalyst, Pt0.35Pd0.35Co0.30/C exhibited relatively high durability and strong poisoning resistance. XRD and XPS proved PtPdCo formed ternary alloy. In the typical TEM image of the Pt0.35Pd0.35Co0.30/C, it could be seen that Pto.35Pd0.35Co0.30NCs were well-dispersed on the XC-72support, with an average particle size of about3nm. Meanwhile, the Pt0.35Pd0.35Co0.30/C exhibited excellent activity with higher current density and higher CO tolerance than that of Pt0.65Co0.35/C, Pt/C, Pd0.65sCo0.35/C, and Pd/C in formic acid electro-oxidation. The Pt0.35Pd0.35Co0.30/C was suit to catalysts of PEMFC.(3) PdAu/graphene as highly durable, efficient electrocatalysts for formic acid oxidationWe reported a facile, efficient and controllable route to disperse PdAu NPs on graphene with the aid of poly-(diallyldimethyl-ammonium chloride)(PAC). Adsorption of PAC on GO not leaded to a highly dense and homogeneous distribution of positive charge on the GO surface, but also preserved the intrinsic properties of the GO without any chemical oxidation treatment. With adding of Na2PdCl4and HAuCl4, Pd-Au nano-braches formed. XRD and HAADF-STEM proved PdAu formed stable alloy, and was supported on graphene successfully. The electrocatalytic activity of the PdAu/graphene toward formic acid oxidation was examined with electrochemical voltammetry (CV) and chronoamperometry (CA); Compared with commercial Pd/C catalyst, PdAu/graphene exhibited enhanced electrocatalytic activity with higher oxidation current density, higher CO tolerance and more negative onset potential. At the same potentials, PdAu/graphene showed higher stability than Pd/C catalysts in the CA analyses. Through the catalyst performance test, the ECSA of PdAu/graphene incresed, and the activity of formic acid electro-oxidation was enhanced. The activity of PdAu/graphene was3.2and5.1times than Pd/graphene and Pd/C. The activity of methanol electro-oxidation on PdAu/graphene was increased in alkaline. At last, the CO stripping experiments on PdAu/graphene, Pd/graphene and Pd/C was carried out. The oxidation and onset current of PdAu/graphene were shifted negatively and accelerate the speed of CO oxidation. The improved performance of PdAu/graphene has been attributed to the PdAu alloy structure and increased electrical conductivity of graphene support.