Preparation Of Carbon-supported Pd-rich Nanoparticles And Their Electrocatalytic Properties For Oxygen Reduction Reaction

The direct methanol fuel cell (DMFC), because of high power density and system simplicity, has been considered as a promising energy converter for a variety of portable applications. However, several challenges need to be addressed before its practical application. In particular, several factors related to the oxygen reduction reaction (ORR) have hindered the commercialization of DMFC. Examples include the high usage of Pt, slow kinetics, and crossover of methanol from the anode to the cathode. The latter leads to a "mixed potential" effect on the cathode side and decreases fuel efficiency. To address these problems, one strategy is to develop the low-cost electrocatalyst with high ORR activity and good methanol-tolerance. Pd has similar lattice constant and electronic properties with Pt, being more abundant than Pt and inactive toward methanol oxidation under acid medium. However, Pd remains less active to ORR as compared to Pt electrode. The focus of this thesis is on novel Pd-Pt bimetallic catalysts, for which might be promising candidates as methanol-tolerant ORR catalysts. From the views of catalyst itself and the support, we extensively investigated the influence of synthesis route, atomic composition, microstructure and carbon support on the ORR activities, selectivities, and stabilities of Pd-Pt bimetallic catalysts.Carbon-supported Pd-Pt bimetallic nanoparticles of different atomic ratios (Pd-Pt/C) have been prepared by a simple procedure involving the complexing of Pd and Pt species with sodium citrate followed by ethylene glycol reduction. As-prepared Pd-Pt alloy nanoparticles were evenly deposited on carbon with a single phase fcc structure. The highest ORR activity of the Pd-Pt/C catalysts was found with a Pd/Pt atomic ratio of1:2, i.e. an atomic distance of0.2782nm. Moreover, all Pd-Pt alloy catalysts exhibited significantly enhanced methanol tolerance during the ORR than the Pt/C catalyst, ensuring a higher ORR performance while diminishing Pt utilization. In addition, the home-made PdiPti/C catalyst was heat-treated at a N2atmosphere. A moderate temperature of300℃was considered to enhance the ORR activity of catalyst, while a high temperature leaded to the growth and aggregation of nanoparticles, resulting in a decreased mass activity.Meanwhile, the Pt-decorated Pd/C catalysts were prepared by two consecutive steps, synthesizing Pd/C catalyst and then decoration of Pt via a polyol reduction. It was believed that Pt was deposited onto the Pd surface through conformal epitaxial growth. The CO stripping voltammograms of the Pt-decorated Pd/C catalysts exhibited two COad oxidation peaks, indicative of two active sites for COad oxidation. With respect to Pd/C catalysts, the Pt-decorated Pd/C catalysts exhibited enhanced Pt-normalized mass activity and good methanol-tolerance during the ORR.As we known, the development of new cost-effective cathode catalysts with high methanol tolerance and at a high catalyst loading is highly desirable for the direct methanol fuel cell. The Pd3Pt1bimetallic alloy nanoparticles highly loaded on different carbon supports, including Vulcan XC-72R carbon, single and multi-walled carbon nanotubes (SWCNTs/MWCNTs) and ordered mesoporous carbon (OMC), have been prepared by a modified polyol reduction route. The OMC-supported PdaPt1catalyst showed a highest ORR activity, which can be ascribed to the smallest particle size arising from the high surface area of OMC. Kinetic analysis revealed that the ORR on the Pd3Pt1/OMC catalyst predominantly undergoes a four-electron process, leading to water formation. Furthermore, the passive DMFC consisting of the Pd3Pt1/OMC as the cathode catalyst delivered the enhanced peak power density of25.3mW·cm-2, being superior to the DMFC with Pt/C cathode. Apart from the reasons of the high ORR activity and good methanol-tolerance of Pd3Pt1/OMC catalyst, the regular mesoporous structure of OMC enables the filling of the ionomer or polymer electrolyte to bring the catalyst particles close to the reactants, thus maximizing the triple-phase interface and facilitating the removal of water to avoid flooding.On graphene nanosheets, Pt and Pd-Pt nanoparticles were deposited with the aid of poly(diallyldimethylammonium chloride)(PDDA), where Pt and Pd ions were embedded first onto PPDA-modified graphene oxide sheets and then the encased metal ions and graphene oxide were reduced simultaneously by ethylene glycol. Metal nanoparticles, of small particle size even at a high metal loading, were found to be attached onto the reduced graphene oxide (RGO) with PDDA-functionzlization (PDDA-RGO). The as-prepared Pd-Pt nanoparticles have a single-phase fcc disordered structure and are principally alloys of Pd and Pt. Among the RGO-supported Pt and Pd-Pt catalysts, Pt nanoparticles chemically attached on PDDA-RGO exhibited the highest activity for the ORR, and the ORR activity of the Pd-Pt alloy electrocatalysts increases with Pt content. Importantly, all the catalysts demonstrated an enhanced ORR durability when PDDA was present; strongly suggesting that PDDA played a crucial role in the dispersion and stabilization of metal nanoparticles on RGO. Moreover, it was observed that the ORR activities for the Pd-Pt catalysts remain enhanced even after accelerated durability test. The formation of a Pt-rich shell, as confirmed by X-ray photoelectron spectroscopy and CO stripping voltammetry, may account for the activity enhancement.