Sol-Gel Process for Making Pt-Ru Fuel-Cell Catalysts

A sol-gel process has been developed as a superior alternative to a prior process for making platinum-ruthenium alloy catalysts for electro-oxidation of methanol in fuel cells. The starting materials in the prior process are chloride salts of platinum and ruthenium. The process involves multiple steps, is time-consuming, and yields a Pt-Ru product that has relatively low specific surface area and contains some chloride residue. Low specific surface area translates to incomplete utilization of the catalytic activity that might otherwise be available, while chloride residue further reduces catalytic activity ("poisons" the catalyst). In contrast, the sol-gel process involves fewer steps and less time, does not leave chloride residue, and yields a product of greater specific area and, hence, greater catalytic activity. In this sol-gel process (see figure), the starting materials are platinum(II) acetylacetonate [Pt(C5H7O2)2, also denoted Pt-acac] and ruthenium(III) acetylacetonate [Ru(C5H7O2)3, also denoted Ru-acac]. First, Pt-acac and Ru-acac are dissolved in acetone at the desired concentrations (typically, 0.00338 moles of each salt per 100 mL of acetone) at a temperature of 50 C. A solution of 25 percent tetramethylammonium hydroxide [(CH3)4NOH, also denoted TMAH] in methanol is added to the Pt-acac/Ruacac/ acetone solution to act as a high-molecular-weight hydrolyzing agent. The addition of the TMAH counteracts the undesired tendency of Pt-acac and Ru-acac to precipitate as separate phases during the subsequent evaporation of the solvent, thereby helping to yield a desired homogeneous amorphous gel. The solution is stirred for 10 minutes, then the solvent is evaporated until the solution becomes viscous, eventually transforming into a gel. The viscous gel is dried in air at a temperature of 170 C for about 10 hours. The dried gel is crushed to make a powder that is the immediate precursor of the final catalytic product. The precursor powder is converted to the final product in a controlled-atmosphere heat treatment. Desirably, the final product is a phase-pure (Pt phase only) Pt-Ru powder with a high specific surface area. The conditions of the controlled- atmosphere heat are critical for obtaining the aforementioned desired properties. A typical heat treatment that yields best results for a catalytic alloy of equimolar amounts of Pt and Ru consists of at least two cycles of heating to a temperature of 300 C and holding at 300 C for several hours, all carried out in an atmosphere of 1 percent O2 and 99 percent N2. The resulting powder consists of crystallites with typical linear dimensions of <10 nm. Tests have shown that the powder is highly effective in catalyzing the electro-oxidation of methanol

Fundamental study of the development and evaluation of biodegradable Mg-Y-Ca-Zr based alloys as novel implant materials

Degradable metals hold considerable promise as materials which exhibit higher mechanical properties than degradable polymers while corroding over time to alleviate complications such as stress-shielding and infection that is inherent to permanent, bioinert metallic biomaterials. Specifically, degradable magnesium (Mg) alloys have emerged as a promising alternative for orthopedic and craniofacial applications due to their positive bone remodeling behavior, good biocompatibility, and relatively high strength compared to polymers while exhibiting similar stiffness to natural bone. Increasing the strength to maintain device integrity during degradation while simultaneously controlling the rapid corrosion of Mg to reduce the risk of hydrogen gas accumulation and toxicity are ongoing paramount goals for optimizing Mg alloys for musculoskeletal applications. In order to address these goals, novel Mg-Y-Ca-Zr based alloys were developed with alloying elements judiciously selected to impart favorable properties. Processing techniques including solution heat treatment combined with hot extrusion were employed to further enhance the desired properties of the material namely, controlled corrosion, high strength and ductility, and minimal toxic response. Increasing the Y content contributed to improved corrosion resistance yielding corrosion rates similar to commercial Mg alloys. Hot extrusion was employed to reduce the grain size, thereby improving mechanical properties through the Hall-Petch relation. Extrusion yielded extremely high strength relative to other Mg alloys, values approaching that of iron-based alloys, due to the presence of Mg12YZn, a long period stacking order phase that served to impede dislocation propagation. Both as-cast and extruded Mg-Y-Ca-Zr alloys demonstrated excellent in vitro cytocompatibility eliciting high viability and proliferation of MC3T3 pre-osteoblast cells and human mesenchymal stem cells. Alloying elements Y and Zr were specifically shown to improve cell proliferation. Finally, implantation of Mg-Y-Ca-Zr based alloys into the mouse subcutaneous tissue and intramedullary cavities of fractured rat femurs resulted in a normal host response and fracture healing, without eliciting any local or systemic toxicity. Thus, the alloys investigated in this work demonstrated great potential for applications as orthopedic and craniofacial implant biomaterials, warranting additional pre-clinical safety and efficacy trials that will be conducted in the near future

Single-Wall Carbon Nanotube Anodes for Lithium Cells

In recent experiments, highly purified batches of single-wall carbon nanotubes (SWCNTs) have shown promise as superior alternatives to the graphitic carbon-black anode materials heretofore used in rechargeable thin-film lithium power cells. The basic idea underlying the experiments is that relative to a given mass of graphitic carbon-black anode material, an equal mass of SWCNTs can be expected to have greater lithium-storage and charge/discharge capacities. The reason for this expectation is that whereas the microstructure and nanostructure of a graphitic carbon black is such as to make most of the interior of the material inaccessible for intercalation of lithium, a batch of SWCNTs can be made to have a much more open microstructure and nanostructure, such that most of the interior of the material is accessible for intercalation of lithium. Moreover, the greater accessibility of SWCNT structures can be expected to translate to greater mobilities for ion-exchange processes and, hence, an ability to sustain greater charge and discharge current densities

Pulsed Current Electrodeposition of Silicon Thin Films Anodes for Lithium Ion Battery Applications

Electrodeposition of amorphous silicon thin films on Cu substrate from organic ionic electrolyte using pulsed electrodeposition conditions has been studied. Scanning electron microscopy analysis shows a drastic change in the morphology of these electrodeposited silicon thin films at different frequencies of 0, 500, 1000, and 5000 Hz studied due to the change in nucleation and the growth mechanisms. These electrodeposited films, when tested in a lithium ion battery configuration, showed improvement in stability and performance with an increase in pulse current frequency during deposition. XPS analysis showed variation in the content of Si and oxygen with the change in frequency of deposition and with the change in depth of these thin films. The presence of oxygen largely due to electrolyte decomposition during Si electrodeposition and the structural instability of these films during the first discharge–charge cycle are the primary reasons contributing to the first cycle irreversible (FIR) loss observed in the pulse electrodeposited Si–O–C thin films. Nevertheless, the silicon thin films electrodeposited at a pulse current frequency of 5000 Hz show a stable capacity of ~805 mAh·g−1 with a fade in capacity of ~0.056% capacity loss per cycle (a total loss of capacity ~246 mAh·g−1) at the end of 500 cycles

A Complexed Sol-Gel (CSG) Approach to High Surface Area (HSA) Durable Ultra Active Platinum-Ruthenium Electro-Catalysts for Direct Methanol Fuel Cells

Direct Methanol Fuel Cell (DMFC) is a promising power source for continuous generation of energy without evolution of any toxic by-products and greenhouse gases. Pt-Ru has been the accepted gold standard anode electro-catalyst for DMFC, but significant advances are required to enhance its performance and stability. A complexed sol-gel (CSG) approach has been used to develop nanostructured powder materials. Herein we report a novel CSG process to synthesize nanoparticulate high specific surface area (HSA), completely unsupported Pt(Ru) based electro-catalyst exhibiting three fold higher electrochemically active surface area (ECSA) and ultra high electrochemical performance compared to commercially available Johnson Matthey Pt-Ru black catalyst, the currently accepted gold standard. Furthermore, in identical single full cell DMFC configuration tests for methanol oxidation, current and power densities ∼40% higher than that displayed by Johnson Matthey catalyst is achieved. Proton exchange membrane fuel cells (PEMFCs) are ubiquitously known for efficient continuous energy and power generation with reduced greenhouse emissions. Commercialization of low cost PEMFCs have however been thwarted by inferior catalyst activity and stability hence limited by loading constraints. • C), has been considered for applications that require faster start-up times, and frequent starts and stops such as automotive applications, material handling equipment, and auxiliary backup power systems. 1-5 On the other hand, methanol-powered direct methanol fuel cell (DMFC) are well suited for portable power applications in consumer electronic devices wherein the power requirements are low. 3-7 A significant fraction of the cost of DMFC and PEMFC arises from the use of precious metal catalysts hitherto platinum group-metal (PGM) catalysts currently used to accelerate electrochemical reactions at the electrodes. 3-8 For widespread commercialization of PEMFC and DMFCs, there is a critical need for continued advancements to minimize PGM loading or the development of equally performing non-PGM catalysts alternatives to reduce the cost. 3-10 The durability of catalysts is also a major issue under conditions of load-cycling in harsh corrosive environment. Mitigation of catalyst dissolution/degradation during operation of low and high-temperature fuel cells will certainly translate alone to higher performance leading to reduce costs. 3-11 Moreover, while addressing cost and durability, fuel cell performance and efficiency must also meet or exceed that of competing technologies (e.g. battery) to allow for market penetration and the benefits of this technology. In light of the current situation, it is of paramount importance to design and synthesize effective electro-catalysts with improved electrochemical activity, improved durability/stability, and significantly reduced precious metal loading to ultra-low levels exhibiting enhanced tolerance to air, fuel and system-derived impurities. To meet these tremendous constraints on performance and efficiency, durability and * Electrochemical Society Student Member. * * Electrochemical Society Active Member. z E-mail: [email protected] cost requirements of fuel cells, significant research has been conducted over the years focusing largely on identifying new materials, and developing novel design and fabrication methods for catalysts and supports. 3-13 Carbon monoxide generated during methanol oxidation reaction (MOR) as it is the surface bound intermediate at low temperatures during DMFC operation further inevitably leads to poisoning of the platinum catalyst surface which reduces the catalytic activity of platinum thus causing it to become inactive if used by itself. In this context, a novel complexed sol-gel process (CSG) has been developed by our group to synthesize unsupported nanocrystalline PtRu based binary, ternary and quaternary solid solutions having high ESCA with excellent electrochemical activity and durable microstructure for the methanol oxidation reaction

Synthesis and structural investigations of infrared transmitting materials in rare earth chalcogenide systems.

γ La₂S₃ (lanthanum sulfide) shows potential as a candidate window material for far IR transmission in the 8-14 μm spectral region. Novel low temperature routes using metalorganic solutions have been developed for the synthesis of γ and β La₂S₃. Two different amorphous precursors (20 nm size and SSA of 74 m²/gm) were synthesized at room temperature which undergo transformation in a sulfur atmosphere to generate sulfide particles (1-3 μm size). A sulfur content of 8.50 wt% in the precursor is critical for the transformation to the cubic (γ) form of La₂S₃. The transformation controlled by the extent of hydrolysis of the alkoxide as determined by thermal stability and chemical analysis of the precursor. Detailed microstructural characterization of the precursors and phase evolution mechanisms were studied. Hot pressed disks (9mm thick) of the cubic sulfide ceramic show a far IR cut-off at 13 μm. Structural analysis by convergent beam electron diffraction (CBED) confirmed the BCC cell structure and the space group of I43d for the powders and ceramic. Amorphous IR materials in the La₂S₃-GeS₂ system were also studied with an emphasis on bulk glass formation and structural characteristics. Glasses containing 92.5 mol% GeS₂ show evidence of primary (60-800 Å) and secondary (30-130 Å) phase separation at the molecular level. The effect of composition on the microstructure and thermal stability of these glasses were also investigated. The Molecular Dynamics simulation technique was applied to study the structure of IR transmitting materials in liquid and glassy forms. An empirical model was developed which successfully describes the liquid and glass structures of a representative IR material (ZnCl₂) and studies were performed to assess the effect of the mass of the borders (W) of the simulation cell on the glass transition temperature. An extension of the model to simulate crystalline La₂S₃ was also attempted

RAPID SOLIDIFICATION PROCESSING OF INDIUM GALLIUM ANTIMONIDE ALLOYS

Solidification from the melt is an essential step in nearly all conventional processes to produce bulk materials for industrial applications. Rapid quenching from the liquid state at cooling rates of 102 to 106K/s or higher has developed into a new technology for processing novel materials. InxGa1 - xSb a ternary III-V compound semiconductor was synthesized by using the rapid spinning cup (RSC) technique. Several compositions of these alloys were batched and cast into ingots in evacuated sealed quartz tubes. These ingots were then melted and ejected onto a rapidly rotating copper disk. This resulted in the generation of flakes or powders depending on the rpm of the disk. Microstructural characterization of the flakes and powders was performed using XRD, SEM and TEM. Efforts were also made to measure the bulk resistivity of the annealed flakes to see the effect of annealing on ordering of the phases