Preparation And Electrocatalysis Of Carbon-supported Pd And Pd-Pt Alloy Nanoparticle Catalysts

Direct alcohol fuel cell (DAFC) is regarded as a promising power source for portable applications, such as laptop computer and cellphones. The use of alcohol as a fuel has several advantages in comparison to the use of hydrogen: inexpensive liquid fuel; ease of handling, transport and storage; as well as high theoretical energy density. However, the commercial viability of a DAFC is still hindered by the high overpotential of both anode and cathode reactions and by the high cost of the platinum based catalysts and overpotential. For a direct formic acid fuel cell (DFAFC), the utilization of Pt based materials as anode catalyst would lead to the formation of the adsorbed CO species, thus decreasing the activity of Pt catalysts. For a direct methanol fuel cell (DMFC), even under open-circuit condition, the overpotential for the oxygen reduction reduction (ORR) is ca. 0.2 ~ 0.3 V, due to the irreversibility of the ORR and the"mixed potential"effect at the cathode. Therefore, it is very necessary to develop nevel electrocatalysts with high ORR activity and good methanol tolerance. In fact, there is great room for the improvement in catalytic activity and decrease in platinum content.In this dissertation, carbon-supported Pd (Pd/C) and Pd-Pt (Pd-Pt/C) alloy nanoparticle catalysts were prepared via a complexing-reduction route in aqueous solution, respectively. The effects of the heat-treatment on Pd nanoparticle size and electrocatalytic activity for formic acid oxidation were examined. Electrocatalytic ORR activity of carbon-supported Pd-Pt bimetallic nanoparticle catalysts with different Pd/Pt atomic ratios was evaluated and compared with that of commercial E-Tek Pt/C catalyst. The effects of composition, structure and particle size of the PdPd alloy catalysts on both ORR and methanol tolerant ORR were investigated. The obtained main results are shown as follows:1. A simple approach via the complexing of PdCl2 with EDTA, followed by NaBH4 reduction, has been used to prepare the Pd/C catalyst with a 20wt% Pd loading for the anode in a DFAFC. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results indicate that the mean partilce size of as-prepared Pd/C catalyst is 3.3±0.8nm with a relatively narrow particle size distribution. After the Pd/C catalyst was heat-treated at various temperatures, the Pd/C catalysts with different particle sizes could be easily obtained. The mean particle size of the Pd/C catalysts is found to increase from 3.3 to 9.2 nm with heat-treated temperature. A correlation between the electrocatalytic activity of formic acid oxidation and particle size of the Pd/C catalysts indicates that the highest activity of formic acid oxidation is found with a Pd mean particle size of ca. 4.7 nm, heat-treated at 120 oC. The increase in catalytic activity for two catalysts heat-treated from 70 to 120 oC is probably due to a change in Pd electronic structure, while the decrease in catalytic activity for the catalysts heat-treated from 120 to 200 oC could be ascribed to an increase in particle size. The Pd/C catalyst prepared with this method may be the more promising anode catalyst for a DFAFC.2. The Pd-Pt/C catalysts of different Pd/Pt atomic ratios were prepared via the complexing with EDTA, followed by NaBH4 reduction. XRD data show that the Pd-Pt/C catalysts are single-phase fcc disordered structure and that the mean particle sizes are ca. 2.0 ~ 3.2 nm. For synthesized Pd-Pt/C catalysts, the highest catalytic activity for the ORR was found for a Pd:Pt atomic ratio of 1:1 and at a heat treatment temperature of ca. 150oC, corresponding to a Pd-Pt mean inter atomic distance of 0.2756nm. Such a PdPt(1:1)/C catalyst exhibits comparative ORR activity in comparison with E-Tek Pt/C. Moreover, the PdPt(1:1)/C catalyst shows a high methanol tolerance during the ORR. The enhancement in the ORR activity of the PdPt(1:1)/C catalyst could be ascribed to to a suitable Pd/Pt atomic ratio, a favorable Pt-Pt interatomic distance, and to an improved alloying degree. Because Pd is inactive for the adsorption and oxidation of methanol, this may explain why the PdPt catalysts exhibit a high methanol tolerance during the ORR.