19 research outputs found
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The electrochemical properties of bundles of single-walled nanotubes
This is the final report of a Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The authors studied electrochemical properties of single-walled fullerene nanotube bundles. The materials exhibited a highly anisotropic conductivity. Electrochemical cycling in solutions of alkyl ammonium salts in propylene carbonate revealed that the nanotubes are stable to at least {+-}1.5 V and have a fairly high accessible surface area. Double-layer charging currents of approximately 30 farads per gram were observed. This is on the same order of magnitude, though somewhat lower, than state-of-the-art values for ultra-capacitor materials. Electrochemical insertion of lithium was attempted. Though several features were observed in a slow cyclic voltammetric scan, these features were not reversible, indicating little reversible insertion. Several possible reasons for this behavior are discussed
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A rapid method for the determination of lithium transference numbers
Lithium ion-conducting polymer electrolytes are of increasing interest for use in lithium-polymer batteries. Lithium transference numbers, the net fraction of current carried by lithium in a cell, are key figures of merit for potential lithium battery electrolytes. The authors describe the Electrophoretic NMR (ENMR) method for the determination of lithium ion transference numbers (T{sub Li}). The work presented is a proof-of-concept of the application of the ENMR method to lithium ion transference measurements for several different lithium salts in gelled electrolytes. The NMR method allows accurate determination of T{sub Li} values, as indicated by the similarity of T{sub Li} in the gelled electrolytes to those in aqueous electrolyte solutions at low salt concentration. Based on calculated tradeoffs of various experimental parameters, they also discuss some conclusions concerning the range of applicability of the method to other electrolytes with lower lithium mobility
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Molecular modeling of the pendant chain in Nafion{reg_sign}
Ion transport through perfluorosulfonic acid ionomers such as Nafion{reg_sign} is controlled by both the microstructure of the polymer and the charge and water distribution in the hydrated polymer. The authors present here the results of theoretical calculations on the side chain of Nafion{reg_sign}, establishing microscopic information for the modeling of water modeling of water modeling of water and proton transport in the membrane. Optimized geometries for the trifluoromethane sulfonic acid fragment (CF{sub 3}SO{sub 3}H), the di-trifluoromethane ether fragment (CF{sub 3}OCF{sub 3}), and the side chain (CF{sub 3}{single_bond}OCF{sub 2}CF(CF{sub 3})OCF{sub 2}CF{sub 2}SO{sub 3}H) were determined by means of both ab initio Hartree Fock theory with second order Moeller-Plesset electron correlation corrections, and density functional theory with Becke`s three parameter hybrid method. Several rotational potential energy surfaces were calculated to assess chain flexibility and proton accessibility. A probe water molecule was added to each of the fragments to characterize hydrophilic sites. These calculations confirmed that the sulfonic acid group is hydrophilic and the ethers are hydrophobic. Molecular dynamics simulations were then performed on the side chain to check the conditions required to stretch the pendant chain. Thermal averages of several structural parameters assessing the flexibility and stretch of the chain were computed from selected conformations produced in the simulation and these results indicate that although the sulfonate group is free to rotate, the chain stretches little. The construction of a potential energy surface for rotation about the second ether group suggests that the side chain exists in a folded or curled up conformation. A physical continuum dielectric solvent model was used to obtain free energies of electrostatic interaction of the fragments and the full chain with the solvent
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Theoretical Structures of Triflic Acid-Water Clusters and the Molecular Mechanism of Proton Dissociation
Structural and energetic information required for recently proposed quasi-chemical theories of solution chemistry have been obtained for clusters of water with triflic acid, CF{sub 3}SO{sub 3}H(H{sub 2}O){sub n} for n=1-6. Quantum mechanical calculations on the clusters indicate that the acid proton does not dissociate with n=1 or 2 hydrating water molecules, but does dissociate for n>=3 water molecule partners. The computed minimum energy structures indicate that both ''Eigen'' (H{sub 9}O{sub 4}{sup +}) (n=3,4,6) and ''Zundel'' (H{sub 5}O{sub 2}{sup +}) (n=5) structures are likely to play a role in the molecular mechanism of acid dissociation in Nafion{reg_sign}
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Modeling of polymer electrolyte fuel cell performance with reformate fuel feed streams
Carbon monoxide poisoning of polymer electrolyte fuel cell anodes is a key problem to be overcome when operating on reformed fuels. CO adsorbs on the precious metal surface leading to substantial performance losses. Much recent work has explored this problem, using various Pt alloys in attempts to overcome the performance loss. In their studies of hydrogen oxidation on Pt and Pt alloy (Pt/Sn, Pt/Ru) electrodes exposed to H2/CO mixtures, Gasteiger et al. (1, 2) showed that a small oxidation current is observed before the onset of CO oxidative stripping (ca. 0.4 V) on Pt/Ru. However, these workers concluded that the observed current wa
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Ion and water transport in a Nafion{reg_sign} membrane pore: A statistical mechanical model with molecular structure
With the well established importance of the coupling of water and protons through electroosmotic drag in operating PEFCs the authors present here a derivation of a mathematical model that focuses on the computation of the mobility of an hydronium ion through an arbitrary cylindrical pore of a PEM with a non-uniform charge distribution on the walls of the pore. The total Hamiltonian is derived for the hydronium ion as it moves through the hydrated pore and is effected by the net potential due to interaction with the solvent molecules and the pendant side chains. The corresponding probability density is derived through solution of the Liouville equation. This probability density is then used to compute the friction tensor for the hydronium ion. The authors find two types of contributions: (a) due to the solvent-ion interactions for which they adopt the conventional continuum model; (b) due to the interaction between the pendant charges and the hydronium ion. The latter is a new result and displays the role of the non-uniform nature of the charge distribution on the pore wall
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PEM fuel cells for transportation and stationary power generation applications
We describe recent activities at LANL devoted to polymer electrolyte fuel cells in the contexts of stationary power generation and transportation applications. A low cost/high performance hydrogen or reformate/air stack technology is being developed based on ultralow Pt loadings and on non-machined, inexpensive elements for flow-fields and bipolar plates. On board methanol reforming is compared to the option of direct methanol fuel cells because of recent significant power density increases demonstrated in the latter
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Solvation and Ionic Transport in Polymer Electrolyte Membranes
We developed a general theoretical framework to study the problem of proton solvation and transport in Nafion{reg_sign} and related materials
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Advanced composite polymer electrolyte fuel cell membranes
A new type of reinforced composite perfluorinated polymer electrolyte membrane, GORE-SELECT{trademark} (W.L. Gore & Assoc.), is characterized and tested for fuel cell applications. Very thin membranes (5-20 {mu}m thick) are available. The combination of reinforcement and thinness provides high membrane, conductances (80 S/cm{sup 2} for a 12 {mu}m thick membrane at 25{degrees}C) and improved water distribution in the operating fuel cell without sacrificing longevity or durability. In contrast to nonreinforced perfluorinated membranes, the x-y dimensions of the GORE-SELECT membranes are relatively unaffected by the hydration state. This feature may be important from the viewpoints of membrane/electrode interface stability and fuel cell manufacturability
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Tribology studies of organic thin films by scanning force microscopy
The use of organic thin films as lubricants on solid surfaces is important in many modern technologies including magnetic storage and micromachines. Langmuir-Blodgett (LB) films and self-assembled monolayers (SAMs) are attractive candidates for lubricant layers and for model studies of lubrication because of their strong adsorption to the surface. The recent interest on the properties of LB films and SAMs has been also motivated by their potential applications in sensors, non-linear optical devices, lithography and microelectronics. Using the micro-contact printing method the authors prepared patterned SAMs consisting of methyl-terminated alkanethiols of different chain lengths. The samples were characterized using lateral force microscopy (LFM) and the force modulation technique (FMT). In general, higher friction is observed over the short chain regions than over the long chain regions when a low or moderate load is applied to the SFM tip. For such cases the high friction (short chain) regions are also ``softer`` as measured by FMT. A high loads, a reversal of the image contrast is observed and the short chain regions show a lower friction than the long chain regions. This image contrast is reversible upon reduction of the applied load