Showing papers in "Journal of The Electrochemical Society in 2000"

High Capacity Anode Materials for Rechargeable Sodium‐Ion Batteries

Abstract: Electrochemical techniques have been used to study the reversible insertion of sodium into hard‐carbon host structures at room temperature. In this paper we compare these results with those for lithium insertion in the same materials and demonstrate the presence of similar alkali metal insertion mechanisms in both cases. Despite the gravimetric capacities being lower for sodium than lithium insertion, we have achieved a reversible sodium capacity of 300 mAh/g, close to that for lithium insertion in graphitic carbon anode materials. Such materials may therefore be useful as anodes in rechargeable sodium‐ion batteries. © 2000 The Electrochemical Society. All rights reserved.

Novel Electrode Materials for Thin‐Film Ultracapacitors: Comparison of Electrochemical Properties of Sol‐Gel‐Derived and Electrodeposited Manganese Dioxide

Abstract: Thin films of manganese dioxide were formed on nickel foils by electrodeposition and by both dip‐coating and drop‐coating with manganese dioxide suspensions (sols) and their subsequent gelation and calcination. The performance of these films as ultracapacitors was studied by cyclic voltammetry in the range 0.0–0.9 V (SCE) and by chronopotentiometry in unbuffered solution. The cyclic voltammograms of ultrathin, dip‐coated sol‐gel‐derived films indicated better capacitive behavior and gave differential specific capacitance values as high as 698 F/g compared to values half to two‐thirds as great for the electrodeposited films. Multilayer drop‐coated sol‐gel films were prepared to attain film thicknesses comparable to the electrodeposited films, and these were found to provide charge‐storage capacity as high as , more than three times greater than that of the electrodeposited films. All films, except electrodeposited films that were not thermally cured, exhibited good cycling stability, losing not much more than 10% of capacity after 1500 cycles. © 2000 The Electrochemical Society. All rights reserved.

Computational Fluid Dynamics Modeling of Proton Exchange Membrane Fuel Cells

Abstract: A transient, multi-dimensional model has been developed to simulate proton exchange membrane (PEM) fuel cells. The model accounts simultaneously for electrochemical kinetics, current distribution, hydrodynamics and multi-component transport. A single set of conservation equations valid for flow channels, gas-diffusion electrodes, catalyst layers and the membrane region are developed and numerically solved using a finite-volume-based computational fluid dynamics (CFD) technique. The numerical model is validated against published experimental data with good agreement. Subsequently, the model is applied to explore hydrogen dilution effects in the anode feed. The predicted polarization cubes under hydrogen dilution conditions are found to be in qualitative agreement with recent experiments reported in the literature. The detailed two-dimensional electrochemical and flow/transport simulations further reveal that in the presence of hydrogen dilution in the fuel stream, hydrogen is depleted at the reaction surface resulting in substantial kinetic polarization and hence a lower current density that is limited by hydrogen transport from the fuel stream to the reaction site.

Thermal‐Electrochemical Modeling of Battery Systems

Abstract: As a follow-up of previous work, 1,2 the present work is intended to develop a thermal and electrochemical coupled model capable of predicting the spatial distribution and temporal evolution of temperature inside a battery. It is known that temperature variations inside a battery may greatly affect its performance, life, and reliability. Battery physicochemical properties are generally strong functions of temperature. For example, the equilibrium pressure of hydrogen absorption-desorption, which significantly affects the open-circuit potential of the metal hydride electrode and hence the performance of nickel‐metal hydride batteries, is strongly dependent on temperature. 3 Capacity losses occur at low temperatures due to high internal resistances and at high temperatures due to rapid self-discharge. 4 Therefore, a proper operating temperature range is essential for a battery to achieve optimal performance. In order to prolong the battery cycle life, balanced utilization of active materials is desired, which requires a highly uniform temperature profile inside the battery to avoid localized degradation. More important, the battery temperature may increase significantly due to the self-accelerating characteristics of exothermic side reactions such as oxygen reactions in aqueous batteries, eventually causing thermal runaway. 5-8 An optimal operating range and a high uniformity in the internal temperature distribution constitute two thermal requirements for a battery to operate safely. These two are particularly important for advanced electric-vehicle batteries because of their high energy and power densities, large size, and high charge and discharge rates. Although experimental testing and microcalorimetric measurement 9-11 are necessary to obtain battery thermal data for design and optimization, a mathematical model based on first principles is capable of providing valuable internal information to help optimize the battery system in a cost-effective manner. In general, a battery thermal model is formulated based on the thermal energy balance over a representative elementary volume (REV) in a battery. The differential equation that describes the temperature distribution in the battery takes the following conservation form 12,13

Carbon Electrodes for Double‐Layer Capacitors I. Relations Between Ion and Pore Dimensions

Abstract: We characterized activated carbon electrodes for electrical double-layer capacitor (EDLC) systems. High-surface-area carbons were prepared by carbonization of cotton cloth at elevated temperatures (up to 1050°C), followed by activation at 900°C by oxidation with CO 2 during different time periods. Specific surface areas and characteristic pore sizes obtained from gas adsorption isotherms were correlated with those obtained from ion electroadsorption at the electrical double layer. Electrolytes studied included aqueous LiCI, NaCI, and KCl solutions and nonaqueous propylene carbonate solutions with LiBF 4 and (C 2 H 5 ) 4 NBF 4 salts. We found clear evidence that the porous carbons thus formed exhibit ion sieving properties, and that increasing activation time systematically increases the average pore sizes of these carbons. The electric double layer (EDL) capacity of these samples (calculated from voltammetric measurements) depends strongly on the adsorption interaction of the ions in the pores, and hence the relationship between the average pore size and the effective ion size determines the specific EDL capacitance of these samples. The following order of dimension of adsorbed species was found, based on the ion sieving of the various synthesized carbons of different average pore size N 2 ; Na + (aq); Cl - (3.6 A) < BF 4 - < TEA + (PC) < Li + (PC).

The Study of Surface Phenomena Related to Electrochemical Lithium Intercalation into Li x MO y Host Materials (M = Ni, Mn)

Abstract: The authors report herein on the comparative study of LiNiO{sub 2} and LiMn{sub 2}O{sub 4} electrodes in three salt solutions, namely, LiAsF{sub 6}, LiPF{sub 6}, and LiC(SO{sub 2}CF{sub 3}){sub 3} in a mixture of the commonly used ethylene and dimethyl carbonates. The surface chemistry of the electrodes in these solutions was studied by surface-sensitive Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray analysis, and their electrochemical behavior was studied by variable-scan-rate voltammetry and impedance spectroscopy. It was found that the electrochemical behavior of these electrodes is strongly dependent on their surface chemistry. Complicated reactions between the active mass and solution components, which include the solvents, the salt anions, and unavoidable contaminants such as HF and perhaps, HSO{sub 3}CF{sub 3}, lead to the precipitation of surface films through which the Li ion has to migrate in order to reach the active mass. The impedance spectroscopy of these electrodes clearly reflects their surface chemistry. It demonstrates the serial nature of the Li insertion-deinsertion processes, which includes, in addition to solid-state diffusion and accumulation, Li-ion migration through surface films and their charge transfer across the surface film/active mass interface, which strongly depends on the chemical composition of the surface films andmore » hence, the solution chosen. LiNiO{sub 2} is considerably more reactive with these solutions than LiMn{sub 2}O{sub 4}, probably due to its stronger nucleophilic nature. In addition, in LiPF{sub 6} solutions, the electrodes' impedance is higher due to precipitation of films comprising LiF, which is highly relative to Li ion transport (probably produced by reactions of the Li{sub x}Mo{sub y} active mass with trace HF).« less

Methanol transport through Nafion membranes : Electro-osmotic drag effects on potential step measurements

Abstract: This paper describes methanol flux measurements across Nafion, 1100 equivalent weight membranes under conditions of a direct methanol fuel cell but in which methanol is completely electro-oxidized on the opposite side in an inert atmosphere at sufficiently high electrode potential. Both the diffusion coefficient and the methanol concentration in the membrane were determined from the measured transient limiting current density following a potential step. Corrections for electro-osmotic drag effects are developed and found necessary even for low MeOH concentrations. The results agree well with those obtained from nuclear magnetic resonance measurements. The partition coefficient [{rho} = [MeOH]{sub membrane}/[MeOH]{sub solution}] was approximately constant for the membranes in contact with methanol solutions of various concentration and from room temperature to 90 C. The activation energy of methanol diffusion in a fully hydrated Nafion membrane between 30 and 130 C is 4.8 kcal/mol, and that for protonic conduction under the same conditions is 2.3 kcal/mol. For a membrane dried in vacuum at above 100 C, lower values of methanol permeation rate and protonic conductance were found.

Manganese Oxide Thin Film Preparation by Potentiostatic Electrolyses and Electrochromism

Abstract: Manganese oxide thin films were deposited on transparent conducting tin oxide glass substrates by potentiostatic anodic electrolysis of alkaline solution of a manganese ammine complex at 298 K. The effects of varying deposition potentials on the microstructure and the electrochromic (EC) properties of the films were investigated. Characterization of films by X‐ray diffraction revealed that two distinct potential regions (lower and higher than 0.3 V vs. Ag/AgCl) were available for the film deposition; the crystal structure of the film deposited at lower and higher regions were and/or and , respectively. X‐ray photoelectron spectroscopy (XPS) analyses of the films featuring exchange splitting effect on Mn 3s spectra indicated that the valence of manganese in the films prepared at lower and higher potential regions are mixtures of divalence‐trivalence and of trivalence‐tetravalence, respectively. The XPS analysis also revealed that terminal chemical bonding species of the films are a mixture of hydroxide (Mn‐O‐H) and oxide (Mn‐O‐Mn). The mechanism of the EC process, by which the color change between brown and light yellow occurs, could be explained in terms of the transformation between these two oxygen groups in Mn‐O‐H and Mn‐O‐Mn, accompanied by the change in valence of Mn. The EC durability of the films in switching performance was also assessed. © 2000 The Electrochemical Society. All rights reserved.

Photoelectrochemical Studies of Oriented Nanorod Thin Films of Hematite

Abstract: Thin films consisting of nanorods of hematite (α-Fe2O3) with controlled orientation onto transparent conductive glass substrates have been tested as photoelectrochem. cells. These films allow a more efficient transport and collection of photogenerated electrons through a designed path compared to films constituted of sintered spherical particles. Expts. have been carried out taking into account the effect of morphol., orientation, film thickness, electrolyte compn., and dye sensitization. The results from a three-electrode system, with 0.1 M KI in water (pH 6.8) as electrolyte, illuminated either through the electrolyte/electrode interface or through the substrate (F:SnO2)/electrode interface, show an improvement of the IPCE (incident photon-to-current conversion efficiency) of 100 and 7 times, resp., compared to work done earlier on thin films with spherical particles. Increasing the pH in the electrolyte from 6.8 to 12.0 also increases the IPCE by a factor two. For a sandwich-type cell, with 0.5 M LiI and 0.5 mM I2 in ethylene carbonate/propylene carbonate (50:50% by wt.) electrolyte, the IPCE reaches 56% at 340 nm.

“Lithium‐Free” Thin‐Film Battery with In Situ Plated Li Anode

Abstract: The "Li‐free" thin‐film battery with the cell configuration Li diffusion blocking overlayer/Cu/solid lithium electrolyte is activated by in situ plating of metallic Li at the Cu anode current collector during the initial charge. Electrochemical cycling between 4.2 and 3.0 V is demonstrated over 1000 cycles at or over 500 cycles at . As corroborated by scanning electron microscopy during electrochemical cycling, the overlayer is imperative for a high cycle stability; otherwise the plated Li rapidly develops a detrimental morphology, and the battery loses most of its capacity within a few cycles. The Li‐free thin‐film battery retains the high potential of a Li cell while permitting its fabrication in air without the complications of a metallic Li anode. Thus, the Li‐free thin‐film battery survives solder reflow conditions, simulated by a rapid heating to 250°C for 10 min in air followed by quenching to room temperature, without any signs of degradation. © 2000 The Electrochemical Society. All rights reserved.

Diffusion of water in Nafion 115 membranes

Abstract: In this paper, experimental and simulated data for the diffusion of water across Nafion membranes as a function of the water activity gradient are presented. The gradient in the activity of water across the membrane was varied by changing the flow rate and pressure of nitrogen gas on one side of the membrane. The other side of the membrane was equilibrated with liquid water. It was found that the model predictions are very sensitive to the value of the diffusion coefficient of water in Nafion. Using the Fickian diffusion coefficient extracted from self-diffusion measurements reported in the literature, the model simulations matched experimental data with less than 5% error over a wide range of operating conditions.

A novel thermal management system for electric vehicle batteries using phase-change material

Abstract: A novel thermal‐management system that incorporates phase‐change material (PCM) is proposed and investigated for electric vehicle (EV) applications. A commercial finite‐element (FE) software, PDEase2D™, was used to simulate the thermal behavior of EV battery modules with a PCM thermal management system. Simulation results show that the temperature profile of the cells integrated in the module design was substantially more uniform during discharge at different rates than without PCM. The PCM system is effective in thermally sensitive batteries such as Li‐ion and most Li‐polymer batteries with a significant reversible heat effect. The heat generated during discharge and stored as latent heat is then largely utilized during charge, and a smaller part of it is transferred to the surroundings. The stored heat will be rejected to the module when the battery is left to relax or when its temperature drops below the melting point of the PCM. This is an important advantage for EV operation under cold conditions or in space applications where the battery temperature drops significantly when an orbiting satellite moves from the light to the dark side of the earth. © 2000 The Electrochemical Society. All rights reserved.

Superconformal Electrodeposition of Copper in 500-90 nm Features

Abstract: Superconformal electrodeposition of copper in 500 nm deep trenches ranging from 500 to 90 nm in width has been demonstrated using an acid cupric sulfate electrolyte containing chloride (Cl), polyethylene glycol (PEG), and 3‐mercapto‐l‐propanesulfonate (MPSA). In contrast, similar experiments using either an additive‐free electrolyte, or an electrolyte containing the binary combinations Cl‐PEG, Cl‐MPSA, or simply benzotriazole (BTAH), resulted in the formation of a continuous void within the center of the trench. Void formation in the latter electrolytes is shown to be reduced through the geometrical leveling effect associated with conformal deposition in trenches or vias with sloping sidewalls. The slanted sidewalls also counterbalance the influence of the differential cupric ion concentration that develops within the trenches. Examination of the i-E deposition characteristics of the electrolytes reveals a hysteretic response associated with the Cl‐PEG‐MPSA electrolyte that can be usefully employed to monitor and explore additive efficacy and consumption. Likewise, resistivity measurements performed on corresponding blanket films can be used to quantify the extent of additive incorporation and its influence on microstructural evolution. The films deposited from the Cl‐PEG‐MPSA electrolyte exhibit spontaneous recrystallization at room temperature that results in a 23% drop in resistivity within a few hours of deposition. © 2000 The Electrochemical Society. All rights reserved.

Electric double-layer capacitor performance of a new mesoporous carbon

Abstract: A new mesoporous carbon (NMC) was prepared, and its performance in an electric double‐layer capacitor (EDLC) was compared to that of a conventional carbon (a molecular‐sieving carbon, MSC25). The effect of pore size and pore connection pattern on EDLC performance was demonstrated. To prepare NMC, phenol resin was synthesized inside the pores of an inorganic template, Mobile Composite Material 48 (MCM48), and the resulting resin‐template composite was carbonized at 700°C under Ar atmosphere. A coke‐like carbonaceous material was obtained after removing the inorganic template by HF treatment. The surface area of NMC was which is smaller than that of MSC25 . NMC had three‐dimensionally interconnected mesopores (2.3 nm average diam), but randomly connected cage‐like micropores (<2,0 nm) were dominant in MSC25. The difference in the pore size and pore connection pattern between the two carbons gave rise to a remarkable difference in their EDLC performances. NMC exhibited a smaller specific capacitance than MSC25 as a result of its smaller surface area, but it showed a higher critical scan rate than the MSC25 electrode due to a smaller resistance-capacitance (RC) time constant. The specific charging capacity of the NMC electrode was about and was largely invariant vs. the charge‐discharge rate. This was contrasted by MSC25 which showed a steadily decreasing capacity with an increase in rate. As a result, the NMC electrode outperformed the MSC25 based on rate capability. The smaller RC time constant and better rate capability of the NMC electrode apparently arises from the lower electrolyte resistance in pores, which in turn stems from the faster ionic motion in larger pores. © 2000 The Electrochemical Society. All rights reserved.

Impedance Spectroscopy Study of PEO‐Based Nanocomposite Polymer Electrolytes

Abstract: The addition of nanometric fillers (e.g., , ) to polymer electrolytes induces consistent improvement in the transport properties. The increase in conductivity and in the cation transference number is attributed to the enhancement of the degree of the amorphous phase in the polymer matrix, as well as to some acid‐base Lewis type, ceramic‐electrolyte interactions. This model is confirmed by results obtained from a detailed impedance spectroscopy study carried out on poly(ethylene oxide) [P(EO)]‐based polymer electrolyte samples with and without ceramic fillers. © 2000 The Electrochemical Society. All rights reserved.

Water and Methanol Uptakes in Nafion Membranes and Membrane Effects on Direct Methanol Cell Performance

Abstract: This paper compares direct methanol fuel cells (DMFCs) employing two types of Nafion{reg{underscore}sign} (E.I.DuPont de Nemours and Company) membranes of different equivalent weight (EW). Methanol and water uptakes in 1,100 and 1,200 EW Nafion membranes were determined by weighing P{sub 2}O{sub 5}-dried and methanol solution-equilibrated membranes. Both methanol and water uptakes in the 1,200 EW membrane were about 70--74% of those in the 1,100 EW membrane. The methanol crossover rate corresponding to that in a DMFC at open circuit was measured using a voltammetric method in the DMFC configuration and under the same cell operating conditions. After accounting for the thickness difference between the membrane samples, the methanol crossover rate through a 1,200 EW membrane was 52% of that through an 1,100 EW membrane. To resolve the cathode and anode performances in an operating DMFC, a dynamic hydrogen electrode was used as a reference electrode. Results show that in an operating DMFC the cathode can be easily flooded, as shown in a DMFC using 1,100 EW membrane. An increase in methanol crossover rate decreases the DMFC cathode potential at open circuit. At a high cell current density, the DMFC cathode potential can approach that of a H{sub 2}/air cell.

Electrochemical Intercalation of PF 6 into Graphite

Abstract: Nonaqueous electrochemical cells using carbon as both the negative and positive electrodes have been proposed in the literature. In such "dual graphite" cells, lithium intercalates into the negative electrode, and the anion intercalates into the positive electrode when the cells are charged, depleting the electrolyte of salt. Here, the origin of the cell potential is considered first. Then, using Li/graphite test cells, intercalation into graphite is studied using electrochemical methods and in situ X‐ray diffraction. We prove that intercalated graphite exists in a series of staged phases. Differential capacity vs. voltage measurements are used to determine the voltages of the staging transitions and the compositions of the staged phases. The cell potential during anion intercalation rises to over 5 V, and we have learned that cells with electrolytes using ethyl methyl sulfone solvent can give reliable results up to 5.5 V under these highly oxidizing conditions. By contrast, ethylene carbonate based electrolytes are strongly oxidized above about 5.2 V, preventing complete loading of graphite with . © 2000 The Electrochemical Society. All rights reserved.

The Impedance Response of a Porous Electrode Composed of Intercalation Particles

Abstract: A mathematical model is developed to describe the impedance response of a porous electrode composed of spherical intercalation particles. The model considers a porous electrode without solution‐phase diffusion limitations. The model is developed by first deriving the impedance response of a single intercalation particle, obtained by solving a set of governing equations which describe charge‐transfer and double‐layer charging at the surface, solid‐phase diffusion inside the particle, and an open‐circuit potential which varies as a function of intercalant concentration. The model also considers the effect of an insulating film surrounding the particle. The governing equations are linearized to take advantage of the small amplitude of the perturbing current in impedance analysis. Once the impedance of a single particle is determined, this result is incorporated into a model which describes a porous electrode limited by ohmic drop in the solution and solid phases, and by the impedance of the particles of which the porous electrode is composed. The model can be used to examine the effect of physical properties and particle‐size distributions in the porous electrode, and the usefulness of impedance analysis to measure solid‐phase diffusion coefficients is scrutinized. © 2000 The Electrochemical Society. All rights reserved.

Preferred Orientation of Polycrystalline LiCoO2 Films

Abstract: Polycrystalline films of deposited by radio frequency magnetron sputtering exhibited a strong preferred orientation or texturing after annealing at 700°C. For films thicker than about 1 μm, more than 90% of the grains were oriented with their (101) and (104) planes parallel to the substrate and less than 10% with their (003) planes parallel to the substrate. As the film thickness decreased below 1 μm, the percentage of (003)‐oriented grains increased until at a thickness of about 0.05 μm, 100% of the grains were (003) oriented. These extremes in texturing were caused by the tendency to minimize volume strain energy for the thicker films or the surface energy for the very thin films. Films were deposited using different process gas mixtures and pressures, deposition rates, substrate temperatures, and substrate bias. Of these variables, only changes in substrate temperature could cause large changes in texturing of thick films from predominately (101)–(104) to (003). Although lithium ion diffusion should be much faster through cathodes with a high percentage of (101)‐ and (104)‐oriented grains than through cathodes with predominately (003)‐oriented grains, it was not possible to verify this expectation because the resistance of most cells was dominated by the electrolyte and electrolyte‐cathode interface. Nonetheless, cells with cathodes thicker than about 2 μm could deliver more than 50% of their maximum energies at discharge rates of or higher. © 2000 The Electrochemical Society. All rights reserved.

Structure/Performance Relations for Ni/Yttria‐Stabilized Zirconia Anodes for Solid Oxide Fuel Cells

Abstract: The relations between morphology and electrochemical performance of Ni/yttria-stabilized zirconia (YSZ) anodes are investigated. Four types of anodes are prepared on YSZ electrolyte three-electrode pellets. A fine cermet of 0.5--1 {micro}m particles, a coarse cermet of 2--3 {micro}m particles, a porous Ni-paste anode, and a Ni-felt anode. The anodes are characterized by impedance spectroscopy at open-circuit potential, and the electrode relevant part (polarization resistance, R{sub p}) of the spectra is identified and investigated. The active thickness of the fine cermet anode is demonstrated to be about 10 {micro}m and is believed to relate to the conductivity of the YSZ network. In the temperature range 850--1,000 C, R{sub p} exhibits an apparent activation energy which increases with coarseness of the anode. No significant dependence on {rho}{sub H{sub 2}} (0.01--0.97 atm) at 1,000 C is observed. A dependence on {rho}{sub H{sub 2}O} of about R{sub p} {proportional_to} {rho}{sub H{sub 2}O}{minus}1/2 is found. Physical transport limitations are suggested as possible causes for the observed anode polarization.

Photoresist‐Derived Carbon for Microelectromechanical Systems and Electrochemical Applications

Abstract: Photopatterned resists pyrolyzed at different temperatures and different ambient atmospheres can be used as a carbonaceous material for microelectromechanical systems. Carbon films were prepared by pyrolysis of photoresists at temperatures ranging from 600 to 1100°C. The carbon films were characterized by several analytical techniques, viz., profilometry, thermogravimetric analysis. four-point probe measurements, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. In addition, cyclic voltammetry was performed on the carbon film electrodes, and the carbon films were compared to glassy carbon (GC for their electrochemical behavior. Electron-transfer rate constants for the benchmark Fe(CN) 3-/4- 6 and Ru(NH 3 ) 3+/2+ 6 redox systems increased with increasing heat-treatment temperature and approached those observed on GC following-treatment at 1100°C. The pyrolyzed films have low capacitance and background current, approximately one-fourth of that observed on GC. The oxygenicarbon atomic ratio determined from XPS was low (∼1% for I 100°C pretreatment), and increased more slowly upon exposure to air than that for GC treated under identical conditions. Pyrolysis of photoresist films permits photolithographic fabrication of carbon electrode devices, and also appears to yield a carbon film with a smooth surface and unusual surface chemistry.

Effect of Carbon Coating on Electrochemical Performance of Treated Natural Graphite as Lithium‐Ion Battery Anode Material

Abstract: Carbon‐coated natural graphite has been prepared by thermal vapor decomposition treatment of natural graphite at 1000°C. Natural graphite coated with carbon showed much better electrochemical performance as an anode material in both propylene carbonate‐based and ethylene carbonate‐based electrolytes than "bare" natural graphite. The effect of carbon coating on the electrochemical performance was investigated by solid‐state in conjunction with standard electrochemical techniques. © 2000 The Electrochemical Society. All rights reserved.

Solid‐State Electrochemistry of the Li Single Wall Carbon Nanotube System

Abstract: Electrochemistry has proven to be very useful for the study of guest-host systems, particularly, carbon intercalation compounds. Not only does electrochemistry provide essential information about the thermodynamics and kinetics of these systems, but it also offers accurate control of guest stoichiometry which is difficult to achieve by other doping methods. Therefore, electrochemical doping has been used extensively to study the properties of carbon guest-host systems. In situ X-ray diffraction and electrochemical doping were used to study the phase diagram of Li xC6 graphite, 1 phase transitions in Li-doped polyacetylene 2 and the structure of Li-doped solid C 60. 3 In situ resistivity measurements were used to study the electronic transport properties of K- and Na-doped polyacetylene. 4,5 In this work, electrochemistry was used to study a new carbon guest-host system: Li/carbon nanotubes. Two types of carbon nanotubes can be distinguished according to their structural properties: multiwall (MWNT) and single wall (SWNT). 6 MWNT consist of graphitic sheets rolled into closed concentric cylinders, with a structure similar to that of Russian dolls. The concentric tubes are separated by Van der Waals gaps of ,3.4 A, a typical interlayer spacing in turbostratically disordered graphite. External diameters can be as large as 50 nm, and lengths are of micrometer scale. SWNT can be envisioned as a single graphene sheet rolled into a cylinder, with diameters in the range 1-2 nm and lengths of several micrometer. SWNT of nearly uniform diameters self-organize into long crystalline “ropes” in which parallel nanotubes are bound by Van der Waals forces. 7 The diameter of a rope is typically 10-50 nm corresponding to 30-600 tubes per rope. Ropes containing as few as 2-3 tubes or as many as several thousand are occasionally found. Figure 1 presents a high resolution transmission electron microscope (HRTEM) image of purified and annealed SWNT, in which several entangled ropes with different diameters can be observed. The parallel fringes within each rope are due to the constructive scattering from the parallel planes of SWNT. The fact that the fringe spacings differ among ropes does not arise from a wide distribution in nanotube diameters, but rather from the different orientation of each rope zone axis with respect to the electron beam. Figure 2 shows an X-ray profile from purified and annealed SWNT. The well

The Limits of Low‐Temperature Performance of Li‐Ion Cells

Abstract: The results of electrode and electrolyte studies reveal that the poor low-temperature (<-30 degrees C) performance of Li-ion cells is mainly caused by the carbon electrodes and not the organic electrolytes and solid electrolyte interphase, as previously suggested. It is suggested that the main causes for the poor performance in the carbon electrodes are (i) the low value and concentration depedence of the Li diffusivity and (ii) limited Li capacity.

High‐Temperature Proton Conducting Membranes Based on Perfluorinated Ionomer Membrane‐Ionic Liquid Composites

Abstract: Composite membranes that exhibit fast proton transport at elevated temperatures are needed for proton‐exchange‐membrane fuel cells and other electrochemical devices operating in the 100 to 200°C range. Traditional water‐swollen proton conducting membranes such as the Nafion membrane suffer from the volatility of water in this temperature range leading to a subsequent drop in conductivity. Here we demonstrate that perfluorinated ionomer membranes such as the Nafion membrane can be swollen with ionic liquids giving composite free‐standing membranes with excellent stability and proton conductivity in this temperature range while retaining the low volatility of the ionic liquid. Ionic conductivities in excess of 0.1 S/cm at 180°C have been demonstrated using the ionic liquid 1-butyl, 3-methyl imidazolium trifluoromethane sulfonate. Comparisons between the ionic‐liquid‐swollen membrane and the neat liquid itself indicate substantial proton mobility in these composites. © 2000 The Electrochemical Society. All rights reserved.

Factors Which Limit the Cycle Life of Rechargeable Lithium (Metal) Batteries

Abstract: Failure mechanisms due to high charging rates of rechargeable lithium batteries comprised of Li metal anodes, cathodes (tunneled structure), and electrolyte solutions based on the combination of 1,3‐dioxolane (DN), , and tributylamine (antipolymerization stabilizer) were explored with the aid of postmortem analysis. It was found that at high charging rates, lithium deposition produces small grains, which are too reactive toward the electrolyte solution, in spite of the excellent passivation of lithium in this solution. In practical batteries such as AA cells with spirally wound configurations, the amount of solution is relatively small, and the solution is spread throughout the battery in a thin layer. Therefore, upon cycling, the Li‐solution reactions deplete the amount of the solution below a critical value, so that only part of the active materials continues to function. This leads to a pronounced increase in the internal resistance of these batteries, which fail as a result of their high impedance and the decrease in the effective working electrodes area. Another failure mechanism relates to the extremely high charge‐discharge current densities developed as the active electrode area decreases. These high currents, developed after prolonged cycling, lead to the formation of dendrites that short‐circuit the battery, thus terminating its life. © 2000 The Electrochemical Society. All rights reserved.

Electrical and Ionic Conductivity of Gd‐Doped Ceria

Abstract: Electrical conductivity of , as a function of temperature and oxygen partial pressure, is measured with a complex impedance method. The increase of electrical conductivity with reducing oxygen partial pressure can be described well by a model that assumes constant mobility of both oxygen vacancies and electrons, based on an ideal‐solution model of non‐stoichiometry of Gd‐doped ceria. Ionic conductivity is calculated, and its activation energy is discussed. Electronic conductivity is discussed also. © 2000 The Electrochemical Society. All rights reserved.

Studies on the Capacitance of Nickel Oxide Films: Effect of Heating Temperature and Electrolyte Concentration

Abstract: Nickel oxide films were prepared by electrochemically precipitating the hydroxide and heating it in air to form the oxide. The resulting oxide films behave as a capacitor. The capacitance of the oxide depends on the heating temperature, showing a maximum at 300°C. The mechanism of charge storage was studied by measuring the capacitance and surface area as a function of heating temperature, and the capacitance in different electrolytes and potential windows. The charge‐storage mechanism is believed to be a surface redox reaction involving adsorbed hydroxyl ions. © 2000 The Electrochemical Society. All rights reserved.

An In Situ Small‐Angle X‐Ray Scattering Study of Sodium Insertion into a Nanoporous Carbon Anode Material within an Operating Electrochemical Cell

Abstract: An electrochemical cell with beryllium X‐ray windows has been designed and used for in situ small‐angle X‐ray‐scattering studies of operating electrodes for the first time. This cell is ideally suited to the study of the filling of nanoscopic pores in solids by electrochemically transported atoms. The mechanism of electrochemical lithium and sodium insertion in nanoporous carbonaceous materials has been the subject of some recent controversy, which is resolved by the studies reported here. We show indisputable evidence that the filling of the pores accounts for all the sodium (and lithium) inserted into these carbons at a chemical potential near that of metallic sodium (or lithium). At lower chemical potential (higher cell voltage), sodium (or lithium) is inserted between graphene layers in an intercalation mechanism. © 2000 The Electrochemical Society. All rights reserved.

Modeling of Conductive Transport in Proton-Exchange Membranes for Fuel Cells

Abstract: The proton-exchange membrane (PEM) fuel cell has lately emerged as a highly promising power source for a wide range of applications. The solid polymer electrolyte utilized in these fuel cells is typically a polyperfluorosulfonic acid (PFSA) membrane ( e.g., Nafion ® , manufactured by DuPont), that provides excellent performance in the presence of water by virtue of its strong acidity, low permeability of hydrogen and oxygen, and good electrochemical stability in the presence of electrocatalysts. This has allowed the development of low-temperature PEM fuel cells with impressive current densities. These membranes have also been widely utilized in the chlor-alkali industry. However, an understanding and modeling of the transport of ionic species through these ion-exchange membranes is not yet adequately developed, especially for proton transport, which is the focus of this paper. There are numerous studies on the nanostructural aspects of the