Nanostructructured Engineering of Pt-based Bimetallic Alloy and Core-Shell Electrocatalysts for DMFCs Applications

Autor: Fadlilatul Taufany, 陶凡
Rok vydání: 2011
Druh dokumentu: 學位論文 ; thesis
Popis: 99
This dissertation summarizes the author’s research efforts to synthesize Pt-based bimetallic alloy and core-shell nanostructured electrocatalysts for applications in direct methanol fuel cells (DMFCs) that can be applied to both their anode and cathode sides. The behavior of bimetallic Pt-based catalysts are fundamentally different to their counter-monometallic catalysts, i.e. Pt and M (M is the second metal, e.g. Pd, Cr, Co, Fe, Sn, Ru, etc.). The addition of the second metal to Pt significantly enhances the rate of electrochemical oxidation and/or reduction in fuel cells; thereby achieving the maximum activity, together with exceptional selectivity and stability. The state-of-the-art of bimetallic Pt-based catalysts is highly dependent on the composition, structure, morphology, particle size, degree of alloying, and other properties. Overall, Pt-based alloy and core-shell nanostructured catalysts are emerging as one of the most promising solutions to address the existing challenges in DMFCs, namely the cost of the catalysts (Pt as the ‘common catalyst’, but it is scarce and extremely expensive) and the sluggish nature of the Pt-kinetic activity with respect to both the anode and cathode reactions. In this dissertation, three synthesis methods have been developed and used to prepare three different Pt-based electrocatalysts with well-controlled sizes, structures, and compositions. Structurally these Pt-based alloyed electrocatalysts are carbon-supported Pt-Cr (Pt-Cr/C), and Pt-Ru (Pt-Ru/C) catalysts; while for the core-shell structure, a carbon-supported Pt-monolayered shell on a Pd-core (PdcorePtmonoshell/C) was developed. Since these catalysts have their own characteristics in the functionalities in the anode or cathode reactions, each of them will be discussed in detail as a separate topic. In the search for an alternative to core-shell structured nanoparticle catalysts, with an active Pt-monolayer, supported on the surface of a dimensionally-stable (PdcorePtmonoshell), here we report a kinetically-controlled autocatalytic chemical process, in which a sacrificial Cu-monolayer on Pd nanoparticles (NPs) is autocatalytically deposited under kinetically-controlled conditions and which is later displaced to form a Pt-monolayer via redox-transmetallation. This process is also adaptable for use as a general protocol for the fabrication of bimetallic core@shell structured NPs, some examples being: Pt@Pd, Ir@Pt, and Ir@Pd. Unlike the thermodynamically-controlled under-potential deposition (UPD) process, the method presented here allows for the scaling-up of production of well-defined core@monolayered shell Pd@Pt nanoparticles without the need for any additional reducing agents and/or electrochemical treatments. Having immediate and obvious commercial potential PdcorePtmonoshell/C NPs have been systematically characterized by in situ X-ray absorption (XAS), electrochemical-FTIR, transmission electron microscopy (TEM), and electrochemical techniques; both during synthesis, and subsequent testing in one particularly important catalytic reaction, namely the oxygen reduction reaction (ORR) which is pivotal in fuel cell operation. It was found that the bimetallic Pd@Pt NPs exhibited a significantly enhanced electrocatalytic activity, with respect to this reaction, in comparison with their monometallic counterparts. In the development of Pt-Cr/C catalysts, two different methods were used to prepare bimetallic Pt3Cr1/C nanocatalysts from similar compositions where the resulting materials exhibit different alloying extents (structure). We investigated how these variations in alloying extent impact the catalytic activity, stability, and selectivity in the ORR. One method, based on the slow thermal decomposition of the Cr precursor at a rate that matches the chemical reduction of the Pt precursor, allows fine control of the Pt3Cr1/C alloy’s composition; whereas the second approach, using the conventional ethylene glycol method results in considerable deviation (> 25%) from the projected composition. Consequently these two methods give variations in the alloying extent that has a strong influence on the Pt d-band vacancy and the Pt-electroactive surface area (Pt-ECSA). This relationship was systematically evaluated using TEM, XAS, and electrochemical analyses. The ORR activity depends on two effects that nullify each other, namely the number of active Pt-sites and their activity. Here the Pt-site activity dominates in governing the ORR activity. The nanocatalyst’s selectivity towards the ORR and the competitive methanol oxidation reaction (MOR) depend on these two effects acting in cooperation to give enhanced ORR activity with suppressed MOR. The number of active Pt-sites is associated with the Pt-ECSA value, while the Pt-site activity is associated with the alloying extent and Pt d-band vacancy (electronic) effects. The presence of Cr atoms in Pt3Cr1/C enhances stability during electrochemical treatment. Overall, the Pt3Cr1/C catalyst prepared by the controlled composition synthesis was shown to be superior in ORR activity, selectivity, and stability owing to its favorable alloying extent, Pt d-band vacancy, and Pt-ECSA. While for the development of Pt-Ru/C catalysts, a controlled composition–based method, i.e. the microwave-assisted ethylene glycol (MEG) method was successfully developed to prepare bimetallic PtxRu100–x/C nanocatalysts with different alloy compositions. This study highlights the impact of the variation in alloy composition of PtxRu100–x/C catalysts on their alloying extent (structure) and subsequently their catalytic activity toward the MOR. The alloying extent of these PtxRu100–x/C catalysts has a strong influence on their Pt d–band vacancy and Pt–ECSA: this relationship was systematically evaluated using XAS, scanning electron microscopy-coupled with energy dispersive X–ray spectroscopy, TEM, density functional theory calculations, and electrochemical analyses. The MOR activity depends the on the number and activity of the Pt-sites. Here the number of active Pt-sites is associated with the Pt–ECSA value, while the Pt–site activity is associated with the alloying extent and Pt d–band vacancy (electronic) effects. Among the PtxRu100–x/C nanocatalyts with various Pt:Ru atomic ratios (x= 25, 50, and 75), the Pt75Ru25/C nanocatalyst was shown to be superior with respect to the MOR activity owing to its favorable alloying extent, Pt d–band vacancy, and Pt–ECSA.
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