Showing papers in "Journal of The Electrochemical Society in 1999"
TL;DR: In this article, an enhancing mechanism for the ORR based on an increased d−electron vacancy of the thin Pt surface layer caused by underlying alloy was proposed. But this mechanism was only applied to the perchloric acid solution.
Abstract: Electrocatalytic activity of Pt alloys with Ni, Co, and Fe, formed by sputtering, was investigated with regard to the oxygen reduction reaction (ORR) in perchloric acid solution. Hydrodynamic voltammograms with rotated electrodes were used to measure the electrocatalytic activity. Maximum activity was observed at ca. 30, 40, and 50% content of Ni, Co, and Fe, respectively, by which 10, 15, and 20 times larger kinetic current densities than that of pure Pt were attained. X‐ray photoelectron spectroscopy analysis of the surfaces after the reaction indicated that the active surfaces were covered by a few monolayers of Pt. We present an enhancing mechanism for the ORR based on an increased d‐electron vacancy of the thin Pt surface layer caused by underlying alloy. Results of this work contribute in development of new active cathode catalysts for fuel cells used as power sources of electric vehicles, etc. © 1999 The Electrochemical Society. All rights reserved.
1,250 citations
TL;DR: In this paper, the specific ionic conductivity, dynamic viscosity, and electrochemical stability of several imidazolium salts are reported as neationic liquids and their solutions in several organic solvents.
Abstract: The specific ionic conductivity, dynamic viscosity, and electrochemical stability of several imidazolium salts are reported as neationic liquids and their solutions in several organic solvents. The temperature dependence of conductivity and viscosity are analyzed for 1‐ethyl‐3‐methylimidazolium and 1,2‐dimethyl‐3‐n‐propylimidazolium salts, and the influence of theanions bis(trifluoromethylsulfonyl)imide , bis(perfluoroethylsulfonyl)imide , hexafluoroarsenate , hexafluorophosphate , and tetrafluoroborate on these properties are discussed. These imidazolium salts make possible electrolytes with high concentration (>3 M), high room temperature conductivity (up to 60 mS/cm), and a wide window of stability . Differential scanning calorimetric results confirm a large glass phase for the ionic liquids, with substantial (>80°C) supercooling. Thermal gravimetric results indicate the imidazolium salts with and anions to be thermally more stable than the lithium salt analogs. The Vogel‐Tammann‐Fulcher interpretation accurately describes the conductivity temperature dependence. © 1999 The Electrochemical Society. All rights reserved.
810 citations
TL;DR: In this article, the authors conducted direct observations, at the particle scale, of damage and cation disorder induced in LiCoO{sub 2} cathodes by electrochemical cycling.
Abstract: Among lithium transition metal oxides used as intercalation electrodes for rechargeable lithium batteries, LiCoO{sub 2} is considered to be the most stable in the {alpha}-NaFeO{sub 2} structure type. It has previously been believed that cation ordering is unaffected by repeated electrochemical removal and insertion. The authors have conducted direct observations, at the particle scale, of damage and cation disorder induced in LiCoO{sub 2} cathodes by electrochemical cycling. Using transmission electron microscopy imaging and electron diffraction, it was found that (1) individual LiCoO{sub 2} particles in a cathode cycled from 1.5 to 4.35 V against a Li anode are subject to widely varying degrees of damage; (2) cycling induces severe strain, high defect densities, and occasional fracture of particles; and (3) severely strained particles exhibit two types of cation disorder, defects on octahedral site layers (including cation substitutions and vacancies) as well as a partial transformation to spinel tetrahedral site ordering. The damage and cation disorder are localized and have not been detected by conventional bulk characterization techniques such as X-ray or neutron diffraction. Cumulative damage of this nature may be responsible for property degradation during overcharging or in long-term cycling of LiCoO{sub 2}-based rechargeable lithium batteries.
649 citations
TL;DR: In this paper, the electroanalytical behavior of thin electrodes is elucidated by the simultaneous application of three electro analytical techniques: slow scan-rate cyclic voltammetry (SSCV), potentiostatic intermittent titration technique, and electrochemical impedance spectroscopy.
Abstract: The electroanalytical behavior of thin electrodes is elucidated by the simultaneous application of three electroanalytical techniques: slow‐scan‐rate cyclic voltammetry (SSCV), potentiostatic intermittent titration technique, and electrochemical impedance spectroscopy. The data were treated within the framework of a simple model expressed by a Frumkin‐type sorption isotherm. The experimental SSCV curves were well described by an equation combining such an isotherm with the Butler‐Volmer equation for slow interfacial Li‐ion transfer. The apparent attraction constant was −4.2, which is characteristic of a quasi‐equilibrium, first‐order phase transition. Impedance spectra reflected a process with the following steps: ion migration in solution, ion migration through surface films, strongly potential‐dependent charge‐transfer resistance, solid‐state diffusion, and accumulation of the intercalants into the host materials. An excellent fit was found between these spectra and an equivalent circuit, including a Voigt‐type analog ( migration through multilayer surface films and charge transfer) in series with a finite‐length Warburg‐type element ( solid‐state diffusion), and a capacitor (Li accumulation). In this paper, we compare the solid‐state diffusion time constants and the differential intercalation capacities obtained by the three electroanalytical techniques. © 1999 The Electrochemical Society. All rights reserved.
594 citations
TL;DR: Anode-supported sold oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolyte, Sr-doped LaMnO{sub 3} (LSM) + YSZ cathode, and Ni + ySZ anode were fabricated and their performance was evaluated between 650 and 800 C as discussed by the authors.
Abstract: Anode-supported sold oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolyte, Sr-doped LaMnO{sub 3} (LSM) + YSZ cathode, and Ni + YSZ anode were fabricated and their performance was evaluated between 650 and 800 C with humidified hydrogen as the fuel and air as the oxidant. Maximum power densities measured were {approximately} 1.8 W/cm{sup 2} at 800 C and {approximately} 0.82 W/cm{sup 2} at 650 C. Voltage (V) vs. current density (i) traces were nonlinear; V vs. i exhibited a concave-up curvature [d{sup 2}V/di{sup 2} {ge} 0] at low values of i and a convex-up curvature [d{sup 2}V/di{sup 2} {le} 0] at higher values of i, typical of many low temperature fuel cells. Analysis of concentration polarization based on transport of gaseous species through porous electrodes, in part, is used to explain nonlinear V vs. i traces. The effects of activation polarization in the Tafel limit are also included. It is shown that in anode-supported cells, the initial concave-up curvature can be due either to activation or concentration polarization, or both. By contrast, in cathode-supported cells, the initial concave-up curvature is entirely due to activation polarization. From the experimentally observed V vs. i traces for anode-supported cells, effective binary diffusivity of more » gaseous species on the anodic side was estimated to be between {approximately} 0.1 cm{sup 2}/s at 650 C and {approximately} 0.2 cm{sup 2}/s at 800 C. The area specific resistance of the cell (ohmic part), varied between {approximately} 0.18 {Omega} cm{sup 2} at 650 C and {approximately} 0.07 {Omega} cm{sup 2} at 800 C with an activation energy of {approximately} 65 kJ/mol. « less
590 citations
TL;DR: In this article, the authors used high angle X-ray scans of several samples inserted with different amounts of lithium to identify two distinct phases which are mutually interconvertible upon lithium exchange.
Abstract: Li{sub 1.33}Ti{sub 1.6}7O{sub 4} synthesized from Li{sub 2}CO{sub 3} and TiO{sub 2} was electrochemically inserted with lithium at room temperature. The defect spinel Li{sub 1.33}Ti{sub 1.67}O{sub 4} and the fully lithiated Li{sub 2.33}Ti{sub 1.67}O{sub 4} with ordered rock-salt-type structure show nearly identical X-ray diffractions. Due to similar lattice constants of both phases and the low scattering factor of the lithium ions, the identification of the insertion mechanism using X-ray powder diffraction is possible only by using high angle X-ray scans of several samples inserted with different amounts of lithium. Precise analysis of the obtained data supplies the evidence for the presence of two distinct phases which are mutually interconvertible upon lithium exchange. This result is in good agreement with electrochemical data postulating a two-phase mechanism from the constant electrical potential found during cycling.
513 citations
TL;DR: In this paper, the authors report the experimentally obtained response surfaces of silicon etching rate, aspect ratio dependent etching (ARDE), photoresist etch rate, and anisotropy parameter in a time multiplexed inductively coupled plasma etcher.
Abstract: We report the experimentally obtained response surfaces of silicon etching rate, aspect ratio dependent etching (ARDE), photoresist etching rate, and anisotropy parameter in a time multiplexed inductively coupled plasma etcher. The data were collected whi le varying eight etching variables. The relevance of electrode power, pressure, and gas flow rates is presented and has been found t o agree with observations reported in the literature. The observed behavior presented in this report serves as a tool to locate a nd optimize operating conditions to etch high aspect ratio structures. The performance of this deep reactive ion etcher allows the tai loring of silicon etching rates in excess of 4 mm/min with anisotropic profiles, nonuniformities of less than 4% across the wafer, and ARDE control with a depth variation of less than 1 mm for trenches of dissimilar width. Furthermore it is possible to prescribe the slope of etched trenches from positive to reentrant. © 1999 The Electrochemical Society. S0013-4651(98)01-009-X. All rights reserved.
476 citations
TL;DR: In this paper, an accelerating rate calorimeter (ARC) was used to measure the thermal stability of a lithiated mesocarbon microbead (MCMB) material in electrolyte under adiabatic conditions.
Abstract: An accelerating rate calorimeter (ARC) was used to measure the thermal stability of a lithiated mesocarbon microbead (MCMB) material in electrolyte under adiabatic conditions. Measurements were carried out to determine the effects of the lithium content and surface area of the electrode as well as the effects of the electrolyte type and the initial heating temperature on thermal stability. MCMB electrodes with both high and low surface area were reacted electrochemically to three compositions: , , and in ethylene carbonate/diethyl carbonate (EC:DEC) (33:67) electrolyte. The low‐surface‐area MCMB samples were also lithiated in EC:DEC (50:50) and EC:DEC (50:50) electrolytes The results showed that self‐heating of the MCMB samples depends on (i) the initial lithium content of the material; (ii) the electrolyte used; (iii) the surface area, and (iv) the initial heating temperature of the sample. Measurable self‐heating in the EC:DEC (33:67) samples was detected at 80°C, at 70°C for MCMB in EC:DEC (1:1), and at 50°C for MCMB in EC:DEC (1:1). The initial self‐heating rate for samples containing EC:DEC (33:67) electrolyte could be fit by an Arrhenius relation with an activation energy of 1.4 eV. The initial form of the self‐heating rate profile was a result of the conversion of metastable solid electrolyte interface (SEI) components to stable SEI components. © 1999 The Electrochemical Society. All rights reserved.
446 citations
TL;DR: In this paper, a mathematical model is presented to predict the conditions for the lithium deposition overcharge reaction on the negative electrode (graphite and coke) and investigate the effect of various operating conditions, cell designs and charging protocols on lithium deposition side reaction.
Abstract: Two major issues facing lithium-ion battery technology are safety and capacity grade during cycling. A significant amount of work has been done to improve the cycle life and to reduce the safety problems associated with these cells. This includes newer and better electrode materials, lower-temperature shutdown separators, nonflammable or self-extinguishing electrolytes, and improved cell designs. The goal of this work is to predict the conditions for the lithium deposition overcharge reaction on the negative electrode (graphite and coke) and to investigate the effect of various operating conditions, cell designs and charging protocols on the lithium deposition side reaction. The processes that lead to capacity fading affect severely the cycle life and rate behavior of lithium-ion cells. One such process is the overcharge of the negative electrode causing lithium deposition, which can lead to capacity losses including a loss of active lithium and electrolyte and represents a potential safety hazard. A mathematical model is presented to predict lithium deposition on the negative electrode under a variety of operating conditions. The Li{sub x}C{sub 6} {vert{underscore}bar} 1 M LiPF{sub 6}, 2:1 ethylene carbonate/dimethyl carbonate, poly(vinylidene fluoride-hexafluoropropylene) {vert{underscore}bar} LiMn{sub 2}O{sub 4} cell is simulated to investigate the influence of lithium deposition on the chargingmore » behavior of intercalation electrodes. The model is used to study the effect of key design parameters (particle size, electrode thickness, and mass ratio) on the lithium deposition overcharge reaction. The model predictions are compared for coke and graphite-based negative electrodes. The cycling behavior of these cells is simulated before and after overcharge to understand the hazards and capacity fade problems, inherent in these cells, can be minimized.« less
425 citations
TL;DR: In this paper, the intermetallic phases in the binary Sn-Fe system, Sn{sub 2}Fe, SnFe, snFe,snfe, Snfe, snfe, and snfe were prepared by mechanical alloying methods or by direct melting.
Abstract: The authors have prepared intermetallic phases and mixtures of such phases in the Sn-Fe-C Gibbs triangle by mechanical alloying methods or by direct melting. This second paper in a three-part series focuses on the intermetallic phases in the binary Sn-Fe system, Sn{sub 2}Fe, SnFe, Sn{sub 2}Fe{sub 3}, and Sn{sub 3}Fe{sub 5}. Using in situ X-ray diffraction and electrochemical methods, the authors study the reversible reaction of Li with these materials. Li/Sn-Fe cells made from annealed powders have reversible capacities of 600, 50, 20 mAh/g, respectively, for Sn{sub 2}Fe, SnFe, Sn{sub 2}Fe{sub 3}, and Sn{sub 3}Fe{sub 5}. Li/Sn-Fe cells made from the same materials, but after high-impact ballmilling, show reversible capacities of 650, 320, 200, and 150 mAh/g. Specific capacities of 804, 676, 582, and 557 mAh/g are expected for Sn{sub 2}Fe, SnFe, Sn{sub 2}Fe{sub 3}, and Sn{sub 3}Fe{sub 5} if all compounds react fully with Li to form Li{sub 4.4}Sn and Fe. In situ X-ray diffraction experiments on the ballmilled materials confirm the formation of /Li{sub 4}Sn during discharge but also show that in the cases of SnFe, Sn{sub 2}Fe{sub 3}, and Sn{sub 3}Fe{sub 5} at least 50% of the starting phase remains unreacted. Structural considerations suggest that as themore » Fe:Sn ratio increases, Fe atoms may form a impenetrable skin on the surface of particles or grains, as Li reacts with the Sn-Fe compounds. This skin prevents the full reaction of the intermetallic with Li, leading to an observed capacity which is lower than expected. High-impact ballmilling reduces particle and grain size, so the effect of the skin is less than for the annealed powders and higher capacities are obtained. As the Fe content in the Sn-Fe intermetallics increases, the cycle life of the materials improves, presumably because there is more Fe per Sn and because the formed Fe and residual starting material act as a matrix to hold the Sn and Li-Sn alloys together during cycling. The authors give an example of a material with a volumetric capacity of 1200 mAh/cm{sup 3} showing stable cycling for over 80 cycles.« less
401 citations
TL;DR: In this paper, a solid-oxide fuel cell based on doped ceria electrolytes and operating at 500°C was shown to be feasible and showed that the ionic conductivity of ceria-based fuel cells is sufficiently high for operation with hydrogen fuel at low temperatures.
Abstract: Solid‐oxide fuel cells based on doped ceria electrolytes and operating at 500°C are shown to be feasible. The operating regime of doped ceria electrolytes is discussed. It is shown that the ionic conductivity of ceria‐based fuel cells is sufficiently high for operation with hydrogen fuel at low temperatures. The major challenges of fabricating a thin electrolyte by a conventional method and the development of high‐performance cathodes capable of operating at 500–600°C are addressed. Cells based on thin‐film ceria electrolytes also exhibited good open‐circuit voltages between 0.97 and 1 V. Cathode materials with high performance have been developed from pyrochlores, perovskites, and cermets of silver and doped bismuth oxide. The advantages and disadvantages of different cathode materials are discussed. The maximum power density obtained at 500°C was . © 1999 The Electrochemical Society. All rights reserved.
TL;DR: In this article, the authors investigated the thermal stability of prismatic Li-ion cells and their components and concluded that the heat generation from the decomposition of PE material and reaction of that with electrolyte initiates thermal runaway in a Li−ion cell, under thermally or abusive conditions.
Abstract: Thermal stability of fully charged 550 mAh prismatic Li‐ion cells (Sn‐doped carbon) and their components are investigated. Accelerating rate calorimetry (ARC) is used to determine the onset temperature of exothermic chemical reactions that force the cell into thermal runaway. Differential scanning calorimetry (DSC) and thermogravimetry analysis are used to determine the thermal stability of the cell's positive electrode (PE) and negative electrode (NE) materials from 35 to 400°C. The cell self‐heating exothermic reactions start at 123°C, and thermal runaway occurs near 167°C. The total exothermic heat generation of the NE and PE materials are 697 and 407 J/g, respectively. Heat generations of the NE and PE materials, washed in diethyl carbonate (DEC) and dried at ≈65°C under vacuum, are significantly lower than unwashed samples. Lithium plating increases the heat generation of the NE material at temperatures near the lithium melting point. Comparison of the heat generation profiles from DSC and ARC tests indicates that thermal runaway of this cell is close to the decomposition temperature range of the unwashed PE material. We conclude that the heat generation from the decomposition of PE material and reaction of that with electrolyte initiates thermal runaway in a Li‐ion cell, under thermally or abusive conditions. © 1999 The Electrochemical Society. All rights reserved.
TL;DR: In this article, a complex impedance model for spherical particles was used to determine the lithium ion diffusion coefficient in graphite as a function of the state of charge (SOC) and temperature.
Abstract: A complex impedance model for spherical particles was used to determine the lithium ion diffusion coefficient in graphite as a function of the state of charge (SOC) and temperature. The values obtained range from 1.12 {times} 10{sup {minus}10} to 6.51 {times} 10{sup {minus}11} cm{sup 2}/s at 25 C for 0 and 30% SOC, respectively, and for 0% SOC, the value at 55 C was 1.35 {times} 10{sup {minus}10} cm{sup 2}/s. The conventional potentiostatic intermittent titration technique (PITT) and Warburg impedance approaches were also evaluated, and the advantages and disadvantages of these techniques were exposed.
TL;DR: In this paper, the formation of a solid electrolyte interphase (SEI) on HOPG samples that were cycled in ethylene carbonate:diethyl carbonate (EC:DEC 1:2) solutions containing 1 M was studied.
Abstract: The mechanisms of oxidation of the basal plane and of the cross‐sectional face of highly oriented pyrolytic graphite (HOPG) and the formation of a solid electrolyte interphase (SEI) on HOPG samples that were cycled in ethylene carbonate:diethyl carbonate (EC:DEC 1:2) solutions containing 1 M were studied. X‐ray photoelectron spectroscopy, energy dispersive spectrometry, and scanning electron microscope techniques were used for the analysis of the surface layer formed on the basal plane and cross section of HOPG. The analysis indicates that the oxidation mechanisms of the basal plane and the cross section are entirely different. The SEI formed in the solution is thinner on the basal plane than on the cross section and its composition is different. The SEI formed on the cross section is rich in inorganic compounds whereas the SEI formed on the basal plane is rich in organic compounds. Thus it can be concluded that on the basal plane, the greatest contribution to SEI formation is solvent reduction (EC and DEC), whereas on the cross‐sectional face, it is electrolyte salt reduction. © 1999 The Electrochemical Society. All rights reserved.
TL;DR: In this article, the authors used a steady-state multicomponent transport model to investigate the hydrodynamics of gases in the cathode of a proton exchange membrane fuel cell that is contacted to an interdigitated gas distributor.
Abstract: Hydrodynamics of gases in the cathode of a proton exchange membrane fuel cell that is contacted to an interdigitated gas distributor are investigated using a steady‐state multicomponent transport model. The model describes the two‐dimensional flow patterns and the distributions of the gaseous species in the porous electrode and predicts the current density generated at the electrode and membrane interface as a function of various operating conditions and design parameters. Results from the model show that, with the forced flow‐through condition created by the interdigitated gas distributor design, the diffusion layer is greatly reduced. However, even with a much thinner diffusion layer, diffusion still plays a significant role in the transport of oxygen to the reaction surface. The results also show that the average current density generated at an air cathode increases with higher gas flow‐through rates, thinner electrodes, and narrower shoulder widths between the inlet and outlet channels of the interdigitated gas distributor. © 1999 The Electrochemical Society. All rights reserved.
TL;DR: In this article, experimental thermal properties of the Sony US-18650 lithium-ion battery and components are presented, as well as thermal property measuring techniques, including specific heat capacity (C{sub p}), thermal diffusivity (α), and thermal conductivity (kappa), in the presence and absence of electrolyte.
Abstract: Experimental thermal property data of the Sony US-18650 lithium-ion battery and components are presented, as well as thermal property measuring techniques. The properties in question are specific heat capacity (C{sub p}), thermal diffusivity ({alpha}), and thermal conductivity ({kappa}), in the presence and absence of electrolyte [1 M LiPF{sub 6} in ethylene carbonate-dimethyl carbonate (EC:DMC, 1:1 wt %)]. The heat capacity of the battery, C{sub p}, is 0.96 {+-} 0.02 J/g K at an open-circuit voltage (OCV) of 2.75 V, and 1.04 {+-} 0.02 J/g K at 3.75 V. The thermal conductivity, {kappa}, was calculated from {kappa} {identical_to} {alpha}{rho}C{sub p} where {alpha} was measured by a xenon-flash technique. In the absence of electrolyte, {kappa} increases with OCV, for both the negative electrode (NE) and the positive electrode (PE). For the NE, the increase is 26% as the OCV increases from 2.75 to 3.75 V, whereas for the PE the increase is only 5 to 6%. The dependence of both C{sub p} and {kappa} on OCV is explained qualitatively by considering the effect of lithiation and delithiation on the electron carrier density, which leads to n-type semiconduction in the graphitic NE material, but a change from semiconducting to metallic character in Li{submore » x}CoO{sub 2} PE material. The overall effect is an increase of C{sub p} and {kappa} with OCV. For {kappa} this dependence is eliminated by electrolyte addition, which, however, greatly increases the effective {kappa} of the layered battery components by lowering the thermal contact resistance. For both NE and PE, the in-plane {kappa} value (measured along layers) is nearly one order of magnitude higher than the cross-plane {kappa}. This is ascribed mostly to the high thermal conductivity of the current collectors and to a lesser extent to the orientation of particles in the layers of electrodes.« less
TL;DR: In this article, the reaction mechanism in Li/[SnO:(B 2 O 3 ) x :(P 2 O 5 ) y glass (0.1≤x,y≤0.5) was shown to be inversely proportional to the spectator:Sn atom ratio.
Abstract: We show that the reaction mechanism in Li/[SnO:(B 2 O 3 ) x :(P 2 O 5 ) y glass (0.1≤x,y≤0.5)], Li/[SnO: B 2 O 3 ) 0.5 :(P 2 O 5 ) 0.5 :(K 2 CO 3 ) 0.04 glass] and Li/SnO cells is common. During the first discharge, the oxygen bonded to tin (as SnO) reacts with lithium to give metallic tin (which can be present as clusters of a few atoms) and lithia. The tin reacts with further lithium to the composition limit of Li 4.4 Sn. During charge the Li is removed from the lithium-tin alloy. The other components of the glass (e.g., B 2 O 3 . P 2 O 5 , Li 2 O, etc, ) are inert with respect to lithium, and we call the atoms which make up these phases spectator atoms, Using X-ray diffraction (XRD) and electrochemical methods, we show that size of the initial tin regions which form is inversely proportional to the spectator:Sn atom ratio. However, during cycling, all of these materials show the subsequent aggregation of the tin atoms into clusters which grow with cycle number until they reach a saturated size. The final cluster size is larger for materials with smaller X:Sn ratio. We propose a speculative model, which predicts the saturated Sn cluster size, as a function of the spectator:Sn ratio.
TL;DR: In this paper, a liquid organic electrolyte system for lithium-ion cells with graphitic anodes containing the solvents ethylene sulfite (ES) and propylene carbonate (PC) has been studied.
Abstract: A liquid organic electrolyte system for lithium-ion cells with graphitic anodes containing the solvents ethylene sulfite (ES) and propylene carbonate (PC) has been studied. Even in additive amounts (5 vol %) ES is suppressing cointercalation of PC into graphite. The PC-ES electrolytes are characterized by a high oxidation stability allowing the cycling of a LiMn{sub 2}O{sub 4} cathode with good reversibility. Moreover, the good low temperature performance compared to ethylene carbonate-dimethyl carbonate electrolytes may favor PC-ES electrolytes for special applications.
TL;DR: In this article, an impedance arc with a characteristic frequency of about 10 to 100 Hz is related to finite diffusion limitation by considering the dependence on water partial pressure, apparent thermal activation, and effect of the inert gas component.
Abstract: Impedance spectra obtained on high‐performance Ni/yttria stabilized zirconia (YSZ) cermet anodes at 1000°C in gas mixtures are considered. An impedance arc with a characteristic frequency of about 10 to 100 Hz is related to finite diffusion limitation by considering the dependence on water partial pressure, apparent thermal activation, and effect of the inert gas component. By experiments in different test geometries the diffusion zone is demonstrated to be a stagnant gas layer outside the anode structure. An estimated stagnant gas layer thickness of about 1 mm is arrived at. The diffusion impedance is demonstrated to be avoidable by using a setup geometry with two coupled electrodes in direct electrical contact through opposing sides of a contacting mesh. © 1999 The Electrochemical Society. All rights reserved.
TL;DR: In this article, a survey of the chemical stability of high surface area LiMn{sub 2}O{sub 4} in various Li-based electrolytes was performed as a function of temperature.
Abstract: A survey of the chemical stability of high-surface area LiMn{sub 2}O{sub 4} in various Li-based electrolytes was performed as a function of temperature. The evidence for an acidic-induced Mn dissolution was confirmed, but more importantly the authors identified, by means of combined infrared spectroscopy, thermogravimetric analysis, and X-ray diffraction measurements, the growth, upon storage of LiMn{sub 2}O{sub 4} in the electrolyte at 100 C, of a protonated {lambda}-MnO{sub 2} phase partially inactive with respect to lithium intercalation. This results sheds light on how the mechanism of high temperature irreversible capacity loss proceeds. Mn dissolution first occurs, leading to a deficient spinel having all the Mn in the +4 oxidation state. Once this composition is reached, Mn cannot be oxidized further, and a protonic ion-exchange reaction takes place at the expense of the delithiation reaction. The resulting protonated {lambda}-Mn{sub 2{minus}y}O{sub 4} phase has a reduced capacity with respect to lithium, thereby accounting for some of the irreversible capacity loss experienced at 55 C for such a material.
TL;DR: In this article, a simple analytical approximation is found which applies in a very broad materials parameter window (arbitrary carrier mobilities and concentrations) as long as linear response is guaranteed and the sample thickness is great compared to the Debye length.
Abstract: The electrochemical impedance of a mixed conductor with two charge carriers sandwiched between two equal electrodes is modeled phenomenologically. A simple analytical approximation is found which applies in a very broad materials parameter window (arbitrary carrier mobilities and concentrations) as long as linear response is guaranteed and the sample thickness is great compared to the Debye length. The approximation is applicable to reversible, selectively blocking as well as to partially blocking electrodes. The validity of the approximation is justified by comparison with the exact solution of Nernst‐Planck‐Poisson set of equations as given, e.g., by Macdonald. The analytical approximation describes the typical Warburg response as well as the single‐carrier behavior as limiting cases. Finite interfacial resistances (not perfectly blocking or not perfectly reversible electrodes) lead to additional semicircles and/or to distorted Warburg responses. © 1999 The Electrochemical Society. All rights reserved.
TL;DR: In this paper, the synthesis and characterization of poly(lauryl methacrylate)-b-poly[oligo(oxyethylene) methacarylate]-based block copolymer electrolytes (BCEs) are reported.
Abstract: For nearly 20 years, poly(ethylene oxide)-based materials have been researched for use as electrolytes in solid-state rechargeab le lithium batteries. Technical obstacles to commercialization derive from the inability to satisfy simultaneously the electrical and mechanical performance requirements: high ionic conductivity along with resistance to flow. Herein, the synthesis and characterization of a series of poly(lauryl methacrylate)- b-poly[oligo(oxyethylene) methacrylate]-based block copolymer electrolytes (BCEs) are reported. With both blocks in the rubbery state (i.e., having glass transition temperatures well below room temperatu re) these materials exhibit improved conductivities over those of glassy-rubbery block copolymer systems. Dynamic rheological testing verifies that these materials are dimensionally stable, whereas cyclic voltammetry shows them to be electrochemically stable over a wide potential window, i.e., up to 5 V at 55 8C. A solid-state rechargeable lithium battery was constructed by laminating lithium metal, BCE, and a composite cathode composed of particles of LiAl0.25Mn0.75O2 (monoclinic), carbon black, and graphite in a BCE binder. Cycle testing showed the Li/BCE/LiAl0.25Mn0.75O2 battery to have a high reversible capacity and good capacity
TL;DR: In this article, mild steel corrosion inhibitors in carbonated brine media were fitted to the Temkin adsorption isotherm, i.e., molecular interaction and adaption equilibrium constants, to glean important information about the behavior of adsorbed corrosion inhibitors.
Abstract: Corrosion rate data for mild steel corrosion inhibitors in carbonated brine media were fitted to the Temkin adsorption isotherm. The fundamental constants of the Temkin adsorption isotherm, i.e., molecular interaction and adsorption equilibrium constants, were used to glean important information about the behavior of adsorbed corrosion inhibitors. Adsorption data were also used to calculate , , and , and the results demonstrated that some compounds are chemisorbed onto the electrode. Significantly, it has been found that chemisorption of corrosion inhibitors can yield very good film persistency (i.e.. corrosion protection even when the inhibitor is no longer present). Molecular modeling using the quantum mechanical program PCSpartan plus, along with a comparative analysis of adsorption data for a broad range of inhibitor molecules, has been used to derive a tentative structure/activity relationship for some oil field corrosion inhibitors. © 1999 The Electrochemical Society. All rights reserved.
TL;DR: The success of rechargeable lithium ion batteries may be largely attributed to the surface films on the carbon anodes, which provide the anodes with adequate protection against continued reaction with electrolyte while permitting charge transfer as discussed by the authors.
Abstract: The success of rechargeable lithium ion batteries may be largely attributed to the surface films on the carbon anodes, which provide the anodes with adequate protection against continued reaction with electrolyte while permitting charge transfer.
TL;DR: In this article, the authors scrutinized the conventional practice of measuring an electrolyte stability window and recommended that to obtain a real stability window, the working electrode material should simulate the electrodes used in a real device.
Abstract: Electrolyte is an indispensable component of any electrochemical cell. Among the properties of an electrolyte, its inertness toward electrochemically induced electronation (reduction at negatively charged electrode) and de-electronation (oxidation at positively charged electrode) has the most direct influence over the operation of the cell. As an ionic conductor in contact with the electrodes at different potentials, electrolyte in most cases is expected to maintain this inertness against both oxidation and reduction while the desired electrochemical process is in progress at the electrodes. The authors scrutinized the conventional practice of measuring an electrolyte stability window. It is shown that misleading values might be generated by this practice. Thus, the authors recommend that to obtain a real stability window, the working electrode material should simulate the electrodes used in a real device. Further, in applications that have a high-surface-area electrode, a new quantification of a stability window is proposed. The electrochemical stability values of various nonaqueous electrolytes that are derived this way should reflect the actual operation limits of these electrolytes in real-life devices.
TL;DR: In this paper, the authors used neutron radiography to measure water gradient profiles within Nafion{reg_sign} in an operating polymer electrolyte fuel cell (PEFC) in order to optimize the PEFC performance.
Abstract: Water transport is an important consideration in the optimization of polymer electrolyte fuel cell (PEFC) performance, affecting both internal resistance and cathode polarization losses. Novel experiments are described using neutron radiography to measure water gradient profiles within Nafion{reg_sign} in an operating PEFC. Preliminary neutron intensity gradients show qualitative agreement with the expected response of membrane water content to changes in feed gas humidification and fuel cell current. Previous experimental measurements of similar water gradients have generally relied on integral measurements such as ac impedance spectroscopy which cannot probe details of the gradient within the membrane. This is one of the first differential measurements of water gradients within Nafion.
TL;DR: In this article, the application of the scanning reference electrode technique (SRET) to the study of PANI coatings on carbon steel was described, and it was shown that conductive PANI passivates pinhole defects in coatings applied to carbon steel and is more effective for corrosion protection than sulfonic acid salts.
Abstract: Growing environmental concerns regarding the use of heavy metals in coating formulations have lead to a new coating strategy employing inherently conducting polymers (ICPs), such as polyaniline (PANI), as a key component. The principal potential advantage offered by the ICP coating technology is toleration of pinholes and minor scratches. This paper describes the application of the scanning reference electrode technique (SRET) to the study of PANI coatings on carbon steel. SRET results demonstrate that conductive PANI passivates pinhole defects in coatings on carbon steel. In addition, it is shown that phosphonic acid salts of PANI are more effective for corrosion protection than sulfonic acid salts. A model is proposed which entails passivation of the metal surface through anodization of the metal by PANI and formation of an insoluble iron-dopant salt at the metal surface.
TL;DR: In this paper, several equivalent circuits for the faradaic electrode process involving n state variables besides electrode potential are established in terms of the corresponding FAR equation in the case of ignoring mass transport, and the correspondence between element values of the circuits and electrochemical parameters of the FAR equation has also been determined by comparing the admittance or impedance of the equivalent circuit and the FAR equations.
Abstract: Several different types of equivalent circuits for the faradaic electrode process involving n state variables besides electrode potential are established in terms of the corresponding faradaic admittance equation in the case of ignoring mass transport. These circuits can develop into general equivalent circuits by introducing negative resistive and inductive and capacitive elements, each of which can be used to analyze any electrochemical impedance spectroscopy of the faradaic electrode process under electrochemical reaction control. The correspondence between element values of the circuits and electrochemical parameters of the faradaic admittance equation has also been determined by comparing the admittance or impedance of the equivalent circuit and the faradaic admittance or impedance equation. The use of general equivalent circuits, together with the faradaic admittance equation, greatly simplifies the analysis of electrochemical impedance spectroscopy for the faradaic electrode processes. © 1999 The Electrochemical Society. All rights reserved.
TL;DR: The performance of Cu cermets as anodes for the direct oxidation of in solid oxide fuel cells was examined in this article, and the results demonstrate that direct, electrocatalytic oxidation of dry methane is possible, with reasonable performance.
Abstract: The performance of Cu cermets as anodes for the direct oxidation of in solid oxide fuel cells was examined. Mixtures of Cu and yttria‐stabilized zirconia (YSZ) were found to give similar performance to Ni‐YSZ cermets when was used as the fuel, but did not deactivate in dry . While Cu‐YSZ was essentially inert to methane, the addition of ceria to the anode gave rise to reasonable power densities and stable operation over a period of at least 3 days. Proof of direct oxidation of came from chemical analysis of the products leaving the cell. The major carbon‐containing product was , with only traces of CO observed, and there was excellent agreement between the actual cell current and that predicted by the methane conversion. These results demonstrate that direct, electrocatalytic oxidation of dry methane is possible, with reasonable performance. © 1999 The Electrochemical Society. All rights reserved.
TL;DR: In this article, the roles of bis(3-sulfopropyl disulfide (SPS) and Janus Green B (JGB) additive agents in the deposition of copper from an acid-copper sulfate electrolyte containing polyethylene glycol (PEG) were studied by deposition on microprofiled electrodes, scanning electron microscopy (SEM), and transmission electron microscope (TEM) and was found that leveling occurs only when all four additive agents are present.
Abstract: The roles of two model additives, bis(3‐sulfopropyl) disulfide (SPS) and Janus Green B (JGB), in the deposition of copper from an acid‐copper sulfate electrolyte containing polyethylene glycol (PEG) and were studied by deposition on microprofiled electrodes, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) It was found that leveling occurs only when all four additives are present, suggesting that additive‐additive interactions are important to the leveling mechanism Moreover, an optimal flux of the active leveling agent exists, an effect that may be explained by the classical diffusion‐adsorption theory of leveling SEM and TEM micrographs show that the additive SPS removes the columnar structure of the deposit and effects micron‐sized, unoriented grains provided PEG and are present; the subsequent addition of JGB decreases the grain size of the film significantly © 1999 The Electrochemical Society All rights reserved