Synthesis and characterization of carbon supported nano-catalysts for direct oxidation fuel cells

A direct oxidation fuel cell (DOFC) is an electrochemical energy-conversion device that converts chemical energy of liquid fuel into electrical energy directly. Because of its unique advantages, such as higher energy densities, facile liquid fuel storage, and simpler system structures, the DOFC has been identified as one of the most promising power sources for portable and mobile applications. Although DOFCs look appealing, the commercialization of DOFC technology is hindered by several critical technical problems. The most serious problem is the low activity of the state-of-the-art electrocatalysts for oxidation of liquid fuels, which severely reduces the cell voltage and decreases the efficiency of fuel cell system. Another critical issue in DOFCs is the so-called ‘fuel crossover’ through the state-of-the-art proton exchange membrane, which is crucial in direct methanol fuel cells (DMFCs). This thesis focuses on the synthesis of high catalytic activity and stability electrocatalysts for DOFCs. Studies reveal that alloying of Au with Pt can enhance significantly the electrocatalytic activities and poison tolerance of the formic acid electrocatalyst in direct formic acid fuel cells (DFAFCs) on the basis of an electronic effect, or an ensemble effect. However, the synthesis of single-phase Au-based alloy nanoparticles is complicated due to the different reduction kinetics of metal ions, especially for the co-reduction of Pt and Au ions. We develop a novel co-reduction method for the synthesis of the single phase AuPt alloy nanoparticles using dimethylformamide coordinated Au-Pt complex as a precursor. This method offers the possible nucleation of AuPt alloy with the atomic-level mixing at co-reduction. The catalytic activity of the prepared AuPt alloy nanoparticles were characterized by formic acid oxidation reaction. The results demonstrated that the AuPt alloy catalyst exhibited a higher activity for the formic acid oxidation reaction than did the commercial Pt catalyst, reflected by its lower onset potential and higher peak current. As a pure Pt cathode catalyst in DMFCs is not only favored for oxygen reduction but also for the unwanted oxidation of methanol that permeates from the anode. Based on the idea that alloying another metal can alter the surface structure of Pt and hence reduce the active sites for methanol adsorption, we investigate the effect of the surface composition of the AuPt alloy cathode catalysts on the performance of the DMFCs. The carbon supported Au-Pt nanoparticles with different surface compositions was prepared with DMF co-reduction method by adjusting the pH value from 14 to 12. Then the prepared catalysts were characterized towards methanol oxidation and oxygen reduction reactions. The electrochemical characterizations indicate that the alloyed Au-Pt catalyst prepared under pH=13 exhibits lower catalytic activity to methanol oxidation but retains the oxygen-reduction activity similar to that of the Pt/C. The cell performance tests show that the PtAu/C can almost double the peak power density of the cell with the Pt/C cathode due to its high methanol tolerance. Theoretical analysis indicates that Ag-Pt alloy nanoparticles might be a promising anode catalyst for the formic acid oxidation reaction with the similar mechanism to AuPt based catalysts on the ensemble effect. However, the synthesis of single-phase Ag-Pt alloy nanoparticles is complicated due to the different reduction kinetics of Pt and Ag ions with the situation as that of the preparation of the AuPt alloy nanoparticles. We introduce the DMF co-reduction method to prepare the Ag-Pt nano-catalyst. Bimetallic Ag-Pt alloy nanoparticles supported on carbon powder are successfully prepared by co-reduction method using DMF as a three-functional solvent, ligand and reductant in an alkaline medium at room temperature. The alloy-dependent catalytic properties of the PtAg/C alloy nanoparticles are analyzed through the formic acid electro-oxidation reaction. The result demonstrated that the PtAg/C synthesized in this work exhibited higher catalytic activity than did the commercial carbon-supported Pt catalyst by giving lower onset potential, lower peak potential and higher peak current for formic acid oxidation reaction. With the emergence of alkaline membranes that conduct hydroxide ions (OH-), alkaline direct ethanol fuel cells (DEFCs) have received increasing attention. The most significant advantage associated with the change in the electrolyte membrane from acid to base is that the reaction kinetics of both the ethanol oxidation reaction (EOR) and oxygen reduction reaction (ORR) in alkaline media become faster than in acidic media, making it possible to use Pd-based metal catalysts. While the stability problems of the Pd-based catalysts is a shortcoming. Motivated by the idea that a mono-metal catalyst can become more stabilized with the incorporation of gold due to its unique electron-withdrawing effect to neighboring primary metal atoms, we prepared Pd-Au alloy electrocatalysts for the EOR in an alkaline medium. The atomic ratio of Pd to Au was varied from 7:1 to 3:1 and 1:1 on the basis of a crystal cell of the face-centered cubic. Electrochemical characterizations indicate that the Pd3Au/C catalyst can exhibit an enhanced catalytic stability while maintain the comparable catalytic activity for the EOR as compared with the monometallic Pd/C catalyst. The cell performance tests demonstrated that the use of the Pd3Au/C anode could improve the catalyst stability and get a higher performance for long term test than that the monometallic Pd/C catalyst did. Theoretical analysis indicates that bimetallic Au-Pd catalyst might a promising cathode catalyst for alkaline DEFCs while the studies of the ORR on the Au-Pd catalyst in alkaline solution are limited and no cell performance data presented. We prepare the carbon nanotubles supported Au-Pd nanoparticles with the physically-mixed, core-shell and alloy structures and analyzed the catalytic activities for both the ORR and EOR in an alkaline solution. The studies indicated that the ORR catalytic activity of the prepared catalysts is sensitive to the surface active sites of palladium, and these Pd active sites can be modified by the incorporation of Au in the bimetallic Au-Pd catalysts. As a result, the Au-Pd catalyst with physically-mixed structure exhibits an increased ORR activity and decreased EOR activity than the original monometallic Pd catalyst does. The cell performance test shown that the alkaline DEFC with the Au-Pd as the cathode catalyst yielded a peak power density of 185 mW cm-2, which is 1.4 times higher than that with the Pd/CNT and 1.8 times higher than that with the Au/CNT cathode catalyst. Keywords: Fuel cell, direct oxidation fuel cells, gold, silver, platinum, palladium, dimethylformamide, co-reduction, bimetallic alloy, nano-catalys