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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,261 records · Page 70
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

A Hybrid Catalyst-Bonded Membrane Device for Electrochemical Carbon Monoxide Reduction at Different Relative Humidities

A hybrid catalyst-bonded membrane device using gaseous reactants for a carbon monoxide reduction (COR) reaction in the cathode chamber, an aqueous electrolyte for an oxygen evolution reaction (OER) in the anode chamber, and an anion exchange membrane (AEM) for product separation as modeled, constructed, and tested. The Cu electrocatalyst was electrodeposited onto gas diffusion layers (GDLs) and was directly bonded to AEM by mechanical pressing in the hybrid device. The impacts of relative humidity at the cathode inlet on the selectivity and activity of COR were investigated by computational modeling and experimental methods. At a relative humidity of 30%, the Cu-based catalyst in the hybrid device exhibited a total operating current density of 87 mA cm -2 with a -2.0 V vs Ag/AgCl reference electrode, a Faradaic efficiency (FE) for C 2 H 4 generation of 32.6%, and an FE for a liquid-based carbon product of 42.6%. Significant improvements in the partial current densities for COR were observed in relation to planar electrodes or flooded gas diffusion electrodes (GDEs). Finally in addition, a custom test bed was constructed to characterize the oxidation states of the Cu catalysts in real time along with product analysis though the backside of the GDLs via operando X-ray absorption (XAS) measurements.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2012

Charge Transport and Glassy Dynamics in Ionic Liquids

Ionic liquids (ILs) exhibit unique features such as low melting points, low vapor pressures, wide liquidus temperature ranges, high thermal stability, high ionic conductivity, and wide electrochemical windows. As a result, they show promise for use in variety of applications: as reaction media, in batteries and supercapacitors, in solar and fuel cells, for electrochemical deposition of metals and semiconductors, for protein extraction and crystallization, and many others. Because of the ease with which they can be supercooled, ionic liquids offer new opportunities to investigate long-standing questions regarding the nature of the dynamic glass transition and its possible link to charge transport. Despite the significant steps achieved from experimental and theoretical studies, no generally accepted quantitative theory of dynamic glass transition to date has been capable of reproducing all the experimentally observed features. In this Account, we discuss recent studies of the interplay between charge transport and glassy dynamics in ionic liquids as investigated by a combination of several experimental techniques including broadband dielectric spectroscopy, pulsed field gradient nuclear magnetic resonance, dynamic mechanical spectroscopy, and differential scanning calorimetry. Based on EinsteinSmoluchowski relations, we use dielectric spectra of ionic liquids to determine diffusion coefficients in quantitative agreement with independent pulsed field gradient nuclear magnetic resonance measurements, but spanning a broader range of more than 10 orders of magnitude. This approach provides a novel opportunity to determine the electrical mobility and effective number density of charge carriers as well as their types of thermal activation from the measured dc conductivity separately. We also unravel the origin of the remarkable universality of charge transport in different classes of glass-forming ionic liquids.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2012

DEVELOPMENT AND SELECTION OF IONIC LIQUID ELECTROLYTES FOR HYDROXIDE CONDUCTING POLYBENZIMIDAZOLE MEMBRANES IN ALKALINE FUEL CELLS

Alkaline fuel cell (AFC) operation is currently limited to specialty applications such as low temperatures and pure HO due to the corrosive nature of the electrolyte and formation of carbonates. AFCs are the cheapest and potentially most efficient (approaching 70%) fuel cells. The fact that non-Pt catalysts can be used, makes them an ideal low cost alternative for power production. The anode and cathode are separated by and solid electrolyte or alkaline porous media saturated with KOH. However, CO from the atmosphere or fuel feed severely poisons the electrolyte by forming insoluble carbonates. The corrosivity of KOH (electrolyte) limits operating temperatures to no more than 80°C. This chapter examines the development of ionic liquids electrolytes that are less corrosive, have higher operating temperatures, do not chemically bond to CO and enable alternative fuels. Work is detailed on the IL selection and characterization as well as casting methods within the polybenzimidazole based solid membrane. This approach is novel as it targets the root of the problem (the electrolyte) unlike other current work in alkaline fuel cells which focus on making the fuel cell components more durable.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2011

Epitaxial Strain-Induced Chemical Ordering in La0.5Sr0.5CoO3-delta Films on SrTiO3

Fast ion conductors are at the foundation of a number of important technologies, ranging from fuel cells to batteries to gas separators. Recent results suggest that strained interfaces and thin films may offer new mechanisms for achieving enhanced ionic transport. In this work, we investigate strained 40-nm films of perovskite La{sub 0.5}Sr{sub 0.5}CoO{sub 3-{delta}}, which is an important material for solid oxide fuel cell cathodes and oxygen separation membranes. We demonstrate that a strained thin film of La{sub 0.5}Sr{sub 0.5}CoO{sub 3-{delta}} on SrTiO{sub 3} can have dramatically different anion and cation thermodynamics and kinetics than bulk La{sub 0.5}Sr{sub 0.5}CaO{sub 3-{delta}}. We use synchrotron X-ray diffraction to show that La{sub 0.5}Sr{sub 0.5}CoO{sub 3-{delta}} thin films form an ordered phase at 650 K. The ordered phase consists of La and Sr cations in planes parallel to the surface and is associated with coherent expansion in the c-direction of {approx}5%. This chemical ordering is not observed in the bulk material and is ascribed to the interplay between the epitaxial strain imposed by the substrate, changes in oxygen vacancy content and cation mobility, and the ordering of oxygen vacancies.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2011

Graphene-Based Non-Noble-Metal Catalysts for Oxygen Reduction Reaction in Acid

Non-noble-metal catalysts based on Fe-N-C moieties have shown promising oxygen reduction reaction (ORR) activity in proton exchange membrane fuel cells (PEMFCs). In this study, we report a facile method to prepare a Fe-N-C catalyst based on modified graphene (Fe-N-rGO) from heat treatment of a mixture of Fe salt, graphitic carbon nitride (g-C{sub 3}N{sub 4}), and chemically reduced graphene (rGO). The Fe-N-rGO catalyst was found to have pyridinic N-dominant heterocyclic N (40% atomic concentration among all N components) on the surface and have an average Fe coordination of {approx}3 N (Fe-N{sub 3,average}) in bulk. Rotating disk electrode measurements revealed that Fe-N-rGO had high mass activity in acid and exhibited high stability at 0.5 V at 80 C in acid over 70 h, which was correlated to low H{sub 2}O{sub 2} production shown from rotating ring disk electrode measurements.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2011

Synthesis of Pt{sub 3}Sn alloy nanoparticles and their catalysis for electro-oxidation of CO and methanol.

Monodisperse Pt{sub 3}Sn alloy nanoparticles (NPs) were synthesized by a controlled coreduction of Pt(II) acetylacetonate and Sn(II) acetylacetonate at 180-280 C in 1-octadecene. In the synthesis, oleylamine was used as a reducing agent, and oleylamine/oleic acid served as surfactants. The sizes of the Pt{sub 3}Sn NPs were tuned from 4 to 7 nm by controlling the metal salt injection temperatures from 180 to 240 C. These monodisperse Pt3Sn NPs were highly active for CO and methanol oxidation in 0.1 M HClO{sub 4} solutions, and their activity and stability could be further improved by a postsynthesis thermal treatment at 400 C in Ar + 5% H{sub 2} for 1 h. They are promising as a practical catalyst for CO and methanol oxidation reactions in polymer electrolyte membrane fuel cell conditions.

10 SYNTHETIC FUELS↗
DOE OSTI2015

Platinum-ruthenium nanotubes and platinum-ruthenium coated copper nanowires as efficient catalysts for electro-oxidation of methanol

The sluggish kinetics of methanol oxidation reaction (MOR) is a major barrier to the commercialization of direct methanol fuel cells (DMFCs). In this study, we report a facile synthesis of platinum–ruthenium nanotubes (PtRuNTs) and platinum–ruthenium-coated copper nanowires (PtRu/CuNWs) by galvanic displacement reaction using copper nanowires as a template. The PtRu compositional effect on MOR is investigated; the optimum Pt/Ru bulk atomic ratio is about 4 and surface atomic ratio about 1 for both PtRuNTs and PtRu/CuNWs. Enhanced specific MOR activities are observed on both PtRuNTs and PtRu/CuNWs compared with the benchmark commercial carbon-supported PtRu catalyst (PtRu/C, Hispec 12100). Finally, x-ray photoelectron spectroscopy (XPS) reveals a larger extent of electron transfer from Ru to Pt on PtRu/CuNWs, which may lead to a modification of the d-band center of Pt and consequently a weaker bonding of CO (the poisoning intermediate) on Pt and a higher MOR activity on PtRu/CuNWs.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2011

Chemical structures of low-pressure premixed methylcyclohexane flames as benchmarks for the development of a predictive combustion chemistry model

The chemical compositions of three low-pressure premixed flames of methylcyclohexane (MCH) are investigated with the emphasis on the chemistry of MCH decomposition and the formation of aromatic species, including benzene and toluene. The flames are stabilized on a flat-flame (McKenna type) burner at equivalence ratios of φ = 1.0, 1.75, and 1.9 and at low pressures between 15 Torr (= 20 mbar) and 30 Torr (= 40 mbar). The complex chemistry of MCH consumption is illustrated in the experimental identification of several C 7 H 12 , C 7 H 10 , C 6 H 12 , and C 6 H 10 isomers sampled from the flames as a function of distance from the burner. Three initiation steps for MCH consumption are discussed: ring-opening to heptenes and methyl-hexenes (isomerization), methyl radical loss yielding the cyclohexyl radical (dissociation), and H abstraction from MCH. Mole fraction profiles as a function of distance from the burner for the C 7 species supplemented by theoretical calculations are presented, indicating that flame structures resulting in steeper temperature gradients and/or greater peak temperatures can lead to a relative increase in MCH consumption through the dissociation and isomerization channels. Trends observed among the stable C 6 species as well as 1,3-pentadiene and isoprene also support this conclusion. Relatively large amounts of toluene and benzene are observed in the experiments, illustrating the importance of sequential H-abstraction steps from MCH to toluene and from cyclohexyl to benzene. Furthermore, modeled results using the detailed chemical model of Pitz et al. (Proc. Combust. Inst.2007, 31, 267–275) are also provided to illustrate the use of these data as a benchmark for the improvement or future development of a MCH mechanism.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2012

Detailed kinetic modeling study of n -pentanol oxidation

To help overcome the world’s dependence upon fossil fuels, suitable biofuels are promising alternatives that can be used in the transportation sector. Recent research on internal combustion engines shows that short alcoholic fuels (e.g., ethanol or n -butanol) have reduced pollutant emissions and increased knock resistance compared to fossil fuels. Although higher molecular weight alcohols (e.g., n -pentanol and n -hexanol) exhibit higher reactivity that lowers their knock resistance, they are suitable for diesel engines or advanced engine concepts, such as homogeneous charge compression ignition (HCCI), where higher reactivity at lower temperatures is necessary for engine operation. The present study presents a detailed kinetic model for n -pentanol based on modeling rules previously presented for n -butanol. This approach was initially validated using quantum chemistry calculations to verify the most stable n -pentanol conformation and to obtain C–H and C–C bond dissociation energies. In addition, the proposed model has been validated against ignition delay time data, speciation data from a jet-stirred reactor, and laminar flame velocity measurements. Overall, the model shows good agreement with the experiments and permits a detailed discussion of the differences between alcohols and alkanes.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2011

Kirkendall Effect and Lattice Contraction in Nanocatalysts: A New Strategy to Enhance Sustainable Activity

Core-shell nanoparticles increasingly are found to be effective in enhancing catalytic performance through the favorable influence of the core materials on the active components at the surface. Yet, sustaining high activities under operating conditions often has proven challenging. Here we explain how differences in the components diffusivity affect the formation and stability of the core-shell and hollow nanostructures, which we ascribe to the Kirkendall effect. Using Ni nanoparticles as the templates, we fabricated compact and smooth Pt hollow nanocrystals that exhibit a sustained enhancement in Pt mass activity for oxygen reduction in acid fuel cells. This is achieved by the hollow-induced lattice contraction, high surface area per mass, and oxidation-resistant surface morphology - a new route for enhancing both the catalysts activity and durability. The results indicate challenges and opportunities brought by the nanoscale Kirkendall effect for designing, at the atomic level, nanostructures with a wide range of novel properties.

30 DIRECT ENERGY CONVERSION↗