Browsing by Subject "Electrocatalysts"
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Item Addition of platinum to palladium-cobalt nanoalloy catalyst by direct alloying and galvanic displacement(2010-12) Wise, Brent; Manthiram, Arumugam; Ferreira, PauloDirect methanol fuel cells (DMFC) are being investigated as a portable energy conversion device for military and commercial applications. DMFCs offer the potential to efficiently extract electricity from a dense liquid fuel. However, improvements in materials properties and lowering the cost of the electrocatalysts used in a DMFC are necessary for commercialization of the technology. The cathode electrocatalyst is a critical issue in DMFC because the state-of-the-art catalyst, platinum, is very expensive and rare, and its performance is diminished by methanol that crosses over from the anode to the cathode through the Nafion membrane. This thesis investigates the addition of platinum to a palladium-cobalt nanoalloy electrocatalyst supported on carbon black in order to improve catalyst activity for the oxygen reduction reaction (ORR) and catalyst stability against dissolution in acidic environment without significantly reducing the methanol-tolerance of the catalyst. Platinum was added to the palladium-cobalt nanoalloy catalyst using two synthesis methods. In the first method, platinum was directly alloyed with palladium and cobalt using a polyol reduction method, followed by heat treatment in a reducing atmosphere to form catalysts with 11 and 22 atom % platinum. In the second method, platinum was added to a palladium-cobalt alloy by galvanic displacement reaction to form catalysts with 10 and 22 atom % platinum. The palladium cobalt alloy was synthesized using a polyol method, followed by heat treatment in a reducing atmosphere to alloy the nanoparticles before the Pt displacement. It was found that both methods significantly improve catalyst activity and stability, with the displaced catalysts showing a higher activity than the corresponding alloy catalyst. However the alloy catalysts showed similar resistance to dissolution as the displaced catalysts, and the alloyed catalysts were more tolerant to methanol. The displaced catalyst with 22 atom % platinum (8 wt. % Pt overall) performed similar to a 20 wt. % commercial platinum catalyst in both RDE and single cell DMFC tests. The 10 and 22 atom % Pt displaced catalysts and 22 atom % Pt alloyed all showed higher Pt mass specific activities than a commercial Pt catalyst.Item Development of anode catalysts for direct alcohol fuel cells(2010-08) Lee, Eungje; Manthiram, Arumugam; Goodenough, John B.; Bard, Allen J.; Ferreira, Paulo; Meyers, Jeremy P.Direct alcohol fuel cells (DAFC) are attracting considerable interest to meet a variety of energy needs as they offer higher efficiency with less pollution compared to other conventional energy-conversion devices. However, the sluggish alcohol oxidation reaction kinetics and durability problems of the conventional Pt-Ru anode catalyst hamper the commercialization of the DAFC systems. With an aim to overcome these problems, there have been intensive efforts to alloy Pt-Ru with other metals. Although such strategies have led to some enhancement in activity, the durability problem caused by the instability of Ru could still not be alleviated. In this regard, this dissertation focuses on the development of non-Ru electrocatalysts with high activity and durability for DAFC applications. First, Ru-free, Pt-based bimetallic electrocatalysts for methanol oxidation reaction (MOR) were studied. Particularly, Pt-Sn and Pt-CeO₂ catalysts were synthesized, respectively, by a polyol method and a one-step reverse microemulsion (RME) method. The prepared samples are investigated for phase and morphological evaluations by various material-characterization techniques. Cyclic voltammetry and accelerated durability tests revealed that these alternative catalysts have much higher stability with a catalytic activity for MOR comparable to that of Pt-Ru. In the case of Pt-CeO₂, an improved particle morphology is obtained by the RME synthesis, and the advantage of the RME method is reflected by a higher catalytic activity in comparison to that of Pt-CeO₂ synthesized by the conventional synthesis method. It has been known that Pt-Sn is better than Pt-Ru for ethanol oxidation reaction (EOR), and the direct ethanol fuel cells (DEFC) employing Pt-Sn as the anode catalyst have better durability than the DMFC system employing a Pt-Ru anode catalyst. Therefore, this dissertation then focused on the enhancement of the catalytic activity for EOR by incorporating a third metal M to the Pt-Sn catalyst. Following the synthesis and characterization of the Pt-Sn-M (M = Mo and Pd) alloys, the effect of M on the enhanced catalytic activity of Pt-Sn-M is presented. The activity enhancement of the above catalysts is based on the promoting effect of the second or third elements added to Pt. However, in the final chapter of this dissertation, the activity enhancement of Pt nanoparticle itself through the formation of low energy surfaces is investigated. Carbon-supported Pt nanoparticles are synthesized in mixed water-ethylene glycol solvent, and the positive effect of the mixed solvent on both the morphology and surface structure of the Pt nanoparticles for COad oxidation is discussed.Item Mixed metal oxide semiconductors and electrocatalyst materials for solar energy conversion(2013-12) Berglund, Sean Patrick; Mullins, C. B.The sun is a vast source of renewable energy, which can potentially be used to satisfy the world's increasing energy demand. Yet many material challenges need to be overcome before solar energy conversion can be implemented on a larger scale. This dissertation focuses on materials used for solar energy conversion through photo-electrochemical (PEC) processes. It discusses methods for improving PEC materials, namely mixed metal oxide semiconductors, by nanostructuring, incorporation of additional elements, and application surface electrocatalysts. In this dissertation several material synthesis techniques are detailed. A high vacuum synthesis process known as reactive ballistic deposition (RBD) is used to synthesize nanostructured bismuth vanadate (BiVO₄), which is studied for PEC water oxidation. Additionally, ballistic deposition (BD) is used to incorporate Mo and W into nanostructured BiVO₄ to improve the PEC activity. An array dispenser and scanner system is used to incorporate metals into copper oxide (CuO) and copper bismuth oxide (CuBi₂O₄) and over 3,000 unique material compositions are tested for cathodic photoactivity. The system is also used to test 35 elements as single component metal oxides, mixed metal oxides, and dopants for titanium dioxide (TiO₂) for use in dye-sensitized solar cells (DSCs). Lastly, RBD is used to deposit tungsten semicarbide (W₂C) onto p-type silicon (p-type) substrates as an electrocatalyst for PEC proton reduction. In many cases, the synthesis techniques and new material combinations presented in this dissertation result in improved PEC performance. The materials are thoroughly assessed and characterized to gain insights into their nanostructure, chemical composition, light absorption, charge transport properties, catalytic activity, and stability.Item Synthesis and characterization of nanostructured palladium-based alloy electrocatalysts(2009-08) Sarkar, Arindam; Manthiram, ArumugamLow temperature fuel cells like proton exchange membrane fuel cells (PEMFC) are expected to play a crucial role in the future hydrogen economy, especially for transportation applications. These electrochemical devices offer significantly higher efficiency compared to conventional heat engines. However, use of exotic and expensive platinum as the electrocatalyst poses serious problems for commercial viability. In this regard, there is an urgent need to develop low-platinum or non-platinum electrocatalysts with electrocatalytic activity for the oxygen reduction reaction (ORR) superior or comparable to that of platinum. This dissertation first investigates non-platinum, palladium-based alloy electrocatalysts for ORR. Particularly, Pd-M (M = Mo and W) alloys are synthesized by a novel thermal decomposition of organo-metallic precursors. The carbon-supported Pd-M (M = Mo, W) electrocatalyts are then heat treated up to 900 oC in H2 atmosphere and investigated for their phase behavior. Cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements reveal that the alloying of Pd with Mo or W significantly enhances the catalytic activity for ORR as well as the stability (durability) of the electrocatalysts. Additionally, both the alloy systems exhibit high tolerance to methanol, which is particularly advantageous for direct methanol fuel cells (DMFC). The dissertation then focuses on one-pot synthesis of carbon-supported multi-metallic Pt-Pd-Co nanoalloys by a rapid microwave-assisted solvothermal (MW-ST) method. The multi-metallic alloy compositions synthesized by the MW-ST method show much higher catalytic activity for ORR compared to their counterparts synthesized by the conventional borohydride reduction method. Additionally, a series of Pt encapsulated Pd-Co nanoparticle electrocatalysts are synthesized by the MW-ST method and characterized to understand their phase behavior, surface composition, and electrocatalytic activity for ORR. Finally, the dissertation focuses on carbon-supported binary Pt@Cu and ternary PtxPd1-x@Cu “core-shell” nanoparticles synthesized by a novel galvanic displacement of Cu by Pt4+ and Pd2+ at ambient conditions. Structural characterizations suggest that the Pt@Cu nanoparticles have a Pt-Cu alloy layer sandwiched between a copper core and a Pt shell. The electrochemical data clearly point to an enhancement in the activity for ORR for the Pt@Cu “core-shell” nanoparticle electrocatalysts compared to the commercial Pt electrocatalyst, both on per unit mass of Pt and per unit active surface area basis. The increase in activity for ORR is ascribed to electronic modification of the outer Pt shell by the Pt-Cu alloy core. However, incorporation of Pd to obtain PtxPd1-x@Cu deteriorates the activity for ORR.