Preparation of Pt and Pt-bimetallic catalysts via sonochemical and solvothermal processing routes in non-aqueous solvent systems for proton exchange membrane fuel cell applications

Date

2014-05

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Abstract

Fuel cells are the focus of many research efforts due to their potential to generate electricity through the direct chemical conversion of hydrogen and simple oxygenated compounds with greater efficiency than thermal combustion processes and their ability to operate on renewable hydrogen derived from captured solar energy. Platinum (Pt) and Pt based nanoparticles are of particular interest in the development of electrocatalysts for proton exchange membrane fuel cells (PEMFCs). This dissertation focuses on new methods for the preparation of Pt and Pt bimetallic nanoparticle catalysts that possess high activity and durability in PEMFC related electrochemical reactions. Two strategies were employed. In one approach, sonochemistry was used to promote mixing of precursor metal salts and the formation of bimetallic metal nanoparticles with uniform alloy properties. In a second approach, a surfactant-free solvothermal method employing N,N-dimethylformamide (DMF) was developed for the preparation of preferentially oriented nanoscale metal crystallites.
In applications of sonochemistry, metal precursor salts were reacted in a non-aqueous solution containing a strong reducing agent. The strategy was applied to prepare Pt-Ni and Pt-Cu catalysts. Metal nanoparticles consistent with the Pt3Ni stoichiometry were formed by sonication of a reaction mixture that contained a 3:1 mole ratio of Pt4+ and Ni2+ ions in tetrahydrofuran. Structural studies indicate the sample consisted mainly of particles in the 2 nm – 5 nm size range with characteristics of a uniform Pt3Ni alloy. Following electrochemical activation to form a Pt-rich surface layer, the sonochemically prepared Pt-Ni nanoparticles showed a three-fold enhancement in O2 reduction kinetics compared with a commercial Pt catalyst. Subsequently, a sonochemical synthesis of homogeneous PtCu3 nanoparticles was carried out. Ultra-sonication during reduction in a non-aqueous solution was compared with synthesis under identical conditions in the absence of sonication (to form a Rieke alloy). The sonochemical procedure produced an amorphous, uniformly alloyed nanomaterial having a composition consistent with the PtCu3 stoichiometry, while the Rieke alloy materials were polyphasic. The as-synthesized PtCu3 particles (2 nm – 3 nm) were activated using an electrochemical de-alloying procedure to prepare oxygen reduction electrocatalyst. De-alloyed catalyst consisted of a Pt-rich surface layer, over a core indicated as having a Pt3Cu composition. The de-alloyed sample exhibited ~ 3-6 fold enhancements in oxygen reduction reaction (ORR) activity as compared to commercial Pt catalysts. In applications of solvothermal methods, shape controlled Pt and Pt-Ni crystallites were prepared in a simple, one-step procedure from the metal acetylacetonate (acac) precursors in DMF and DMF-water mixtures. For Pt, conditions that give < 10 nm particles with primarily cubic, or truncated octahedral shapes were demonstrated. Effects of reaction time, DMF-water ratio, reaction temperature and metal precursor salt on the Pt properties were examined. A mechanism for Pt reduction and the growth of preferentially shaped Pt nanocrystals in the DMF-water solvent system is proposed. Additionally, Pt-Ni nanostructures prepared with defined particle geometries exhibited up to 12 fold enhancements in ORR activity and good durability relative to commercial Pt catalyst following activation to produce a Pt-rich surface layer.

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Keywords

Nanoparticles, Fuel cells, Electrocatalysis

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