Nanostructured Gold and Alloys for Fuel Cell Catalysis
Chuan-Jian Zhong, Mathew Maye, Jin Luo, Li Han, Nancy Kariuki,
Our work is aimed at the preparation, characterization and application of gold and its alloy nanoparticles as fuel cell catalysts. Fuel cells operating by electrochemical oxidation of hydrogen or methanol fuels at the anode and reduction of oxygen at the cathode are attractive commercial power sources due to their high conversion efficiencies, low pollution, and high power density. The poor catalytic activity and the requirement of Pt-group metals that have limited global supply and suffer from the propensity of surface poisoning are currently some of the series barriers to commercial applications of fuel cells. Gold and its alloy with other metals in the nanosize range (<10 nm) present new alternatives . Such catalysts could generate unique bifunctional catalytic properties with hydrogenation/dehydrogenation sites and the ability to remove the poisonous CO under low temperature operations.
The ability to harness the large surface area-to-volume ratios and the unique binding surface sites of nanoparticles in <10 nm size range, especially in heterogeneous catalysis, constitutes a major driving force in fundamental research and practical applications of nanoparticle catalysts. It is this size range over which the metal particles undergo a transition from atomic to metallic properties. The understanding of factors controlling morphological and surface reconstitution associated with this size range is of fundamental importance to the ultimate exploration of the nanostructured catalysts. This presentation describes recent findings of an investigation of the interfacial structures and properties of gold and alloy nanoparticles in the electrocatalytic oxidation and reduction of methanol and oxygen, which are of fundamental interest to fuel cell technology. Built upon our initial findings of the preparation of nanoscale catalysts via core-shell assembled nanoparticles consisting of gold or alloy nanocrystal cores and organic monolayer shells with controllable size monodispersity, processibility, and stability towards size- and composition- tunable catalysts, we have expanded our investigation into the preparation of the nanoparticles on oxide and carbon support materials for the electrocatalytic oxidation of methanol and reduction of oxygen in fuel cells [2-8]. Gold and bimetallic (AuFe, AuPt, AuNi, etc) nanoparticles of 2-5 nm core size range on planar substrates and high surface area carbon materials are studied as a model system of the nanostructured catalyst.
Experimentally, the gold and alloy nanoparticles of controlled size and composition were prepared by a combination of the well-established two-phase synthesis protocol and the thermal processing protocol developed in our laboratory . The carbon-supported gold or alloy nanoparticles were cast quantitatively onto a glassy carbon electrode. A Nafion film will be applied to fix the catalysts. The comparison of voltammetric peak potentials and currents provided the initial catalyst screening for CO oxidation, CH3OH oxidation, and O2 reduction reaction (ORR). The relative composition was controlled by the synthetic feed ratio and thermal processing parameters. Instead of detecting two distinctive waves corresponding to the catalytic activity of Pt and Au sites, a single catalytic wave was detected for these reactions at Au catalyst of different alloy compositions, implying the operation of the alloy in the catalytic reaction. The catalytic activity for methanol oxidation in acidic electrolyte at AuPt catalysts demonstrated new opportunities of the binary nanoparticle system, because previous studies of the bulk form of gold alloys have not shown such catalytic activity. For the ORR activity, the rotating disk electrode data revealed a mixed kinetic-diffusion control current, which is qualitatively similar to those known for other Pt-based nanoparticle catalysts. While a further investigation is in progress to delineate the reaction mechanism, the result suggests that the 4-electron reduction process can be manipulated by alloy composition. The bifunctional activity is believed to be operative, in which the alloyed Au leads to an effective removal of Pt-CO for the anode reaction and of the -Pt-OH for the cathode reaction.
Furthermore, the study of the nanostructured catalyst using surface spectroscopy (IR and XPS) and atomic force microscopy has provided important insights into the structural and morphological reconstitution of the nanostructured catalysts in the catalytic processes. Implications of the findings to the design, preparation and processing of highly-active nanostructured gold and alloy catalysts will be discussed. Further research exploring the gold-based nanotechnology in fuel cells and industrial applications using refined preparative and activation routes will also be discussed.
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Alloy, nanoparticles, Methanol oxidation, Oxygen reduction, Nanostructure, Electrocatalysis, catalyst, Fuel cell, Gold, nanotechnology