Showing papers in "Journal of The Electrochemical Society in 1993"

Modeling of Galvanostatic Charge and Discharge of the Lithium/Polymer/Insertion Cell

Abstract: The galvanostatic charge and discharge of a lithium anode/solid polymer separator/insertion cathode cell is modeled using concentrated solution theory. The model is general enough to include a wide range of polymeric separator materials, lithium salts, and composite insertion cathodes. Insertion of lithium into the active cathode material is simulated using superposition, thus greatly simplifying the numerical calculations. Variable physical properties are permitted in the model. The results of a simulation of the charge/discharge behavior of the system are presented. Criteria are established to assess the importance of diffusion in the solid matrix and transport in the electrolyte. Consideration is also given to various procedures for optimization of the utilization of active cathode material.

Water Uptake by and Transport Through Nafion® 117 Membranes

Abstract: Water uptake and transport properties of Nafion[reg sign] 117 membranes at 30 C are reported here. Specifically, the authors have determined the amount of water taken up by membranes immersed in liquid water and by membranes exposed to water vapor of variable water activity. Transport parameters measured are the diffusion coefficient and relaxation time of water in the membrane and the protonic conductivity of the membrane as functions of membrane water content. The ratio of water molecules carried across the membrane per proton transported, the electro-osmotic drag coefficient, also was determined for a limited number of membrane water contents. The drag coefficient is contrasted with the experimentally determined net water transport across an operating PEM fuel cell.

Electrochemistry and Structural Chemistry of LiNiO2 (R3̅m) for 4 Volt Secondary Lithium Cells

Abstract: The synthesis and characterization of LiNiO[sub 2] for a 4 V secondary lithium cell was done. The LiNiO[sub 2] was prepared by ten different methods and characterized by X-ray diffraction and electrochemical methods. LiNiO[sub 2] prepared from LiNO[sub 3] and NiCO[sub 3] [or Ni(OH)[sub 2]] exhibited more than 150 mAh/g of rechargeable capacity in the voltage range between 2.5 and 4.2 V in 1M LiClO[sub 4] propylene carbonate solution. The reaction mechanism was also examined and explained in terms of topotactic reaction. Lithium nickelate (III) (R3m; a[equals]2.88 [angstrom], c[equals]14.18 [angstrom] in hexagonal setting) was oxidized to nickel dioxide (R3m; a[equals]2.81 [angstrom], c[equals]13.47 [angstrom]) via Li[sub 1[minus]x]NiO[sub 2] (0.25(2) [le]x[le]0.55(2)) having a monoclinic lattice (C2/m). The nickel dioxide could be reversibly reduced to lithium nickelate(III). Factors affecting the electrochemical reactivity of LiNiO[sub 2] are given and the possibility of using LiNiO[sub 2] for 4 V secondary lithium cells is described.

A water and heat management model for proton-exchange-membrane fuel cells

Abstract: Proper water and heat management are essential for obtaining high-power-density performance at high energy efficiency for proton-exchange-membrane fuel cells. A water and heat management model was developed and used to investigate the effectiveness of various humidification designs. The model accounts for water transport across the membrane by electro-osmosis and diffusion, heat transfer from the solid phase to the gas phase and latent heat associated with water evaporation and condensation in the flow channels. Results from the model showed that at high current (> 1A/cm[sup 2]) ohmic loss in the membrane accounts for a large fraction of the voltage loss in the cell and back diffusion of water from the cathode side of the membrane is insufficient to keep the membrane hydrated (i.e., conductive). Consequently, to minimize this ohmic loss the anode stream must be humidified, and when air is used instead of pure oxygen the cathode stream must also be humidified.

Formation of Lithium-Graphite Intercalation Compounds in Nonaqueous Electrolytes and Their Application as a Negative Electrode for a Lithium Ion (Shuttlecock) Cell

Abstract: Electrochemical reduction of natural graphite was carried out in 1M LiClO[sub 4] ethylene carbonate (EC)/1,2-dimethoxyethane (DME) solution at 30 C. Natural graphite was reduced stepwise to LiC[sub 6]. The staging phenomenon was observed by X-ray diffraction (XRD). The first stage and the second stage compounds were identified as a commensurate structure in which lithium atoms form a close-packed two-dimensional array. A second-stage compound (LiC[sub 18]) with a different in-plane lithium ordering based on a LiC[sub 9] two-dimensional packing in lithium intercalated sheets also was observed; also third, fourth-stage compounds were identified. The electrochemical oxidation of the first-stage compound (LiC[sub 6]) was examined and shown to reversible over the entire range, i.e., C[sub 6] + xLi [r reversible] Li[sub x]C[sub 6]. The reaction mechanism for the reduction of graphite and the oxidation of the first-stage compound are discussed in relation to the staging phenomenon from the detailed open-circuit voltage and XRD data. The chemical potential of LiC[sub 6] was estimated to be [minus]3.6 kcal mol from the observed reversible potential. The feasibility of using a lithium-graphite intercalation compound in lithium ion (shuttlecock) cells is described, and the innovative secondary systems, C[sub 6]/LiCoO[sub 2] and C[sub 6]/LiNiO[sub 2] fabricated in discharged states,more » are demonstrated.« less

The Physics of Macropore Formation in Low‐Doped p‐Type Silicon

Abstract: Macropore formation in n‐type silicon is a self‐adjusting phenomenon characterized by a specific current density at the pore tip. At this specific current density, the dissolution reaction changes from the charge‐transfer‐limited to the mass‐transfer‐limited regime. The passivation of the pore walls in hydrofluoric acid is caused by a depletion of holes due to the n‐type doping of the substrate. Equations based on these findings are presented and allow us to precalculate the dimensions of the pores. The validity of the model and its mathematical description is verified in experiments. Pores of a depth up to the wafer thickness and aspect ratios of 250 were etched using this method.

Water and Thermal Management in Solid‐Polymer‐Electrolyte Fuel Cells

Abstract: A mathematical model of transport in a solid‐polymer‐electrolyte fuel cell is presented. A two‐dimensional membrane‐electrode assembly is considered. Water management, thermal management, and utilization of fuel are examined in detail. Because the equilibrium sorption of water between the gas phase and the polymer‐electrolyte depends strongly on temperature, water and thermal management are interrelated. The rate of heat removal is shown to be a critical parameter in the operation of these fuel cells.

A Comparative Study of Water Uptake By and Transport Through Ionomeric Fuel Cell Membranes

Abstract: Water uptake and transport parameters measured at 30 C for several available perfluorosulfonic acid membranes are compared. The water sorption characteristics, diffusion coefficient of water, electroosmotic drag, and protonic conductivity were determined for Nafion 117, Membrane C, and Dow XUS 13204.10 developmental fuel cell membrane. The diffusion coefficient and conductivity of each of these membranes were determined as functions of membrane water content. Experimental determination of transport parameters, enables one to compare membranes without the skewing effects of extensive features such as membrane thickness which contributes in a nonlinear fashion to performance in polymer electrolyte fuel cells.

Modeling and Experimental Diagnostics in Polymer Electrolyte Fuel Cells

Abstract: This paper presents a fit between model and experiment for well-humidified polymer electrolyte fuel cells operated to maximum current density with a range of cathode gas compositions. The model considers, in detail, losses caused by: (1) interfacial kinetics at the Pt/ionomer interface, (2) gas-transport and ionic-conductivity limitations in the catalyst layer and (3) gas-transport limitations in the cathode backing. The authors` experimental data were collected with cells that utilized thin-film catalyst layers bonded directly to the membrane, and a separate catalyst-free hydrophobic backing layer. This structure allows a clearer resolution of the processes taking place in each of these distinguishable parts of the cathode. In their final comparison of model predictions with the experimental data, they stress the simultaneous fit of a family of complete polarization curves obtained for gas compositions ranging from 5 atm O{sub 2} to a mixture of 5% O{sub 2} in N{sub 2}, employing in each case the same model parameters for interfacial kinetics, catalyst-layer transport, and backing-layer transport. This approach allowed them to evaluate losses in the cathode backing and in the cathode catalyst layer, and thus identify the improvements required to enhance the performance of air cathodes in polymer electrolyte fuel cells. Finally, theymore » show that effects of graded depletion in oxygen along the gas flow channel can be accurately modeled using a uniform effective oxygen concentration in the flow channel, equal to the average of inlet and exit concentrations. This approach has enabled simplified and accurate consideration of oxygen utilization effects.« less

Electrochemical Insertion of Sodium into Carbon

Abstract: Electrochemical insertion of sodium ions into carbon using solid polymer electrolytes or organic liquid electrolytes is described. Cells with the configuration Na/P(EO)sNaCF3SOJCP(EO) = polyethylene oxide) or Na/liquid electrolyte/C were galvanostatically discharged, charged, and cycled. The extent of insertion into C (Le., x in Na§ was found to be a strong function of the type and particle size of the carbon used, and the reversibility of the process was highly dependent upon the type of electrolyte used. The possibility of designing a sodium ion rocking chair cell is discussed, and a first-generation example, using a petroleum coke anode, polymer electrolyte, and sodium cobalt bronze cathode is described. Rocking chair batteries, in which both the anode and cathode are intercalation materials, have recently been commercialized. Because the anodes are commonly inexpensive carbons such as petroleum coke or graphite, reductive intercalation of lithium into these materials is now the subject of intense scrutiny] Similar sodium insertion reactions into carbons have been observed, 2 but have not yet been exploited for use in batteries. We now describe a preliminary study of these insertion reactions and discuss the possibility of developing a sodium ion cell analogous to the wellknown lithium ion systems. Experimental Conoco petroleum coke, Shawinigan black, and JohnsonMatthey microcrystalline graphite were either ground in an attritor mill or used as received after heat-treatment. Polymer electrolytes of composition P(EO)sNaCF3SO3 (PEO = polyethylene oxide) and composite cathodes containing the carbon of interest, PEO, and NaCF3SO3 were made as described previously. 3 Electrodes for use in cells with liquid electrolytes consisted of carbon and ethylene propylene diene monomer (EPDM) binder (2% by weight) and were vacuum dried prior to use. Battery-grade solvents from Mitsubishi Petrochemical Company were stored in an inert atmosphere glove box (02 < 1 ppm) and used as supplied. Sodium was purified as described previously?

Surface Area Loss of Supported Platinum in Polymer Electrolyte Fuel Cells

Abstract: Life tests of polymer-electrolyte fuel cells using supported Pt catalyst in thin film catalyst layers are run for up to 4,000 h at maximum power. Particle ripening is readily evident using these types of electrodes in which the high catalyst utilization efficiency apparently subjects in majority of the platinum to conditions that sustain particle growth. X-ray diffraction analyses indicate that the initial platinum specific surface areas of 100 m[sup 2]/g Pt eventually stabilize to about 40 to 50 m[sup 2]/g in the cathode and 60 to 70 m[sup 2]/g in the anode. Interestingly, this loss in surface area does not affect the apparent catalytic activity of these fuel cell electrodes. A crystallite migration particle growth mechanism is suggested by the shape of the particle size distribution curves. Since the presence of liquids is known to lower the activation energy for particle growth, the particle size difference between the two electrodes may possibly be attributed to the different hydration levels at the anode and the cathode in operating polymer electrolyte fuel cells.

The Aerocapacitor: An Electrochemical Double‐Layer Energy‐Storage Device

Abstract: The authors have applied unique types of carbon foams developed at Lawrence Livermore National Laboratory (LLNL) to make an {open_quotes}aerocapacitor{close_quotes}. The aerocapacitor is a high power-density, high energy-density, electrochemical double-layer capacitor which uses carbon aerogels as electrodes. These electrodes possess very high surface area per unit volume and are electrically continuous in both the carbon and electrolyte phase on a 10 nm scale. Aerogel surface areas range from 100 to 700 m{sup 2}/cc (as measured by BET analysis), with bulk densities of 0.3 to 1.0 g/cc. This morphology permits stored energy to be released rapidly, resulting in high power densities (7.5 kW/kg). Materials parameterization has been performed, and device capacitances of several tens of Farads per gram and per cm{sup 3} of aerogel have been achieved.

Mechanism of Chemical Bath Deposition of Cadmium Sulfide Thin Films in the Ammonia‐Thiourea System In Situ Kinetic Study and Modelization

Abstract: The mechanism of chemical bath deposition of cadmium sulfide thin films from the ammonia‐thiourea system is studied in situ by means of quartz crystal microbalance technique (QCM). The influence of reaction parameters (concentration of reactants, pH, anions, temperature, stirring rate) is determined. The growth is thermally activated with an activation energy of about 85 kJ/mol, which probably corresponds to a chemical step related to the decomposition of thiourea. The results are well interpreted by assuming an atom‐by‐atom growth mechanism. A model is presented, which fits most of the experimental results quantitatively. It involves two or three rate‐limiting surface steps, the formation of taking place via a surface complex between thiourea and cadmium hydroxide. Analytical expressions are given, allowing prediction of the rate under various conditions in this system.

Electrochemical Intercalation of Lithium into Graphite

Abstract: The results of a study of the electrochemical intercalation of lithium into Lonza KS15 artificial graphite in 1M PC/EC (50:50) electrolyte are presented here. Electrolyte decomposition reactions occur during the first discharge at about 0.8 V vs. Li metal and their extent is greatly reduced by addition of crown ethers with 12 crown 4 being most effective. A mechanism is proposed for electrolyte decomposition reactions which includes at least two types of process, namely, propylene and ethylene evolution and formation on the surface of graphite of a solid electrolyte interphase film which includes lithium alkyl carbonates. The stoichiometry of lithium intercalation into graphite is found to be inversely dependent on the cycling rates and electrode thickness and proportional to the amount of carbon black added to the graphite.

Electrochromic Reactions in Manganese Oxides I . Raman Analysis

Abstract: Like nickel oxide, manganese oxide is a widely studied material in the primary batteries field. The reactions taking place during voltametric cycling of manganese oxides can be determined using in situ Raman spectroscopy. The main difficulty for the oxide identification is to obtain relevant Raman reference spectra because of the many possible compounds and, for some of these compounds, of their instability in the laser beam. As a consequence, several modifications of different tetra-, tri- and divalent manganese oxides and oxyhydroxides were carefully studied. The electrochromic behavior of three types of manganese oxides, two prepared by thermal oxidations and the other by electrochemical deposition, were then compared. The presence of nonstoichiometry in the pristine material was necessary to obtain a reversible electrochromic effect. The reaction during electrochromic cycling is more complicated than a simple passage from MnO[sub 2] to MnOOH.

Electrical Properties of Amorphous Silicon Transistors and MIS‐Devices: Comparative Study of Top Nitride and Bottom Nitride Configurations

Abstract: The electrical properties of silicon nitride/amorphous silicon structures were investigated using thin film transistors (TFTs) and metal insulator semiconductor (MIS) devices employing either a top nitride (TN) or bottom nitride (BN) as gate insulator. The density of states (DOS) deduced from the subthreshold transfer characteristic of the TFTs is one to two orders of magnitude higher than that obtained from quasistatic C(V) measurements on the MIS structures. This difference is discussed by considering the different thickness of the a‐Si:H layers of the two devices and the role of a fixed charge at the rear interface. Both techniques indicate a DOS in BN devices which is only slightly lower than in TN devices, by less than a factor of two. The measured field effect mobility of BN TFTs is about 70% higher. The differences in the measured field effect mobility for TN and BN configuration are discussed and ascribed to the source and drain parasitic resistances. The conclusion is verified by the fabrication of a TN TFT with a pure phosphine rear surface treatment, which exhibits performance comparable to BN TFTs.

Performance of Solid Oxide Fuel Cell Using Proton and Oxide Ion Mixed Conductors Based on BaCe1 − x Sm x O 3 − α

Abstract: ceramics were synthesized and their ionic conduction was investigated. These ceramics showed protonic and oxide ionic mixed conduction under fuel cell condition. While protonic conduction was predominant below 1027 K, oxide ionic conduction became significant as the temperature increased. Using these oxides as solid electrolyte, hydrogen‐air fuel cell could be constructed. exhibited the best cell performance among the electrolytes examined. The maximum short‐circuit current density was about 900 mA/cm2 at 1273 K. The polarization at each electrode was low. Porous nickel could be used as anode material instead of expensive platinum and as cathode material.

Copper transport in thermal SiO2

Abstract: The transport of copper in silicon dioxide thermally grown on single crystalline silicon was studied by capacitance techniques, secondary ion mass spectroscopy (SIMS) analysis, and Rutherford backscattering spectrometry (RBS). Metal/oxide/silicon (MOS) capacitors were used to study the penetration of copper into the oxide as a function of temperature and applied electric field. The role of a titanium layer between the copper and the oxide was also studied. Bias‐thermal stress (BTS) studies of MOS structures were conducted at 150°C to 300°C with an electric field of 1 MV/cm for times ranging between 10 min and 168 h. It is shown that without bias a relatively small amount of copper reaches the silicon/silicon dioxide interface, with a maximum surface concentration of about 1017 cm−3 that drops exponentially with depth in the oxide. The high‐frequency (100 kHz) capacitance vs. voltage (CV) characteristics of the MOS devices changed drastically when a positive bias was applied to the gate and copper reached the silicon/silicon‐dioxide interface. The penetration time for copper through the oxide was characterized as a function of the temperature. The copper drift velocity, mobility and diffusivity in the oxide were determined and the copper profiles in the capacitors were characterized by SIMS. The activation energy for the diffusivity and mobility models was found to be . Devices without a barrier layer, which were stressed under a positive electric field, showed high copper concentration in the oxide, up to 1021 cm−3. At high temperatures and long stress times a significant amount of copper was also found in the silicon substrate. A titanium layer thicker than 5 nm acted as effective barrier even after 30 h of BTS at 300°C.

Competitive adsorption effects in the electrodeposition of iron-nickel alloys

Abstract: Two-step reaction mechanisms involving adsorbed monovalent intermediate ions for the electrodeposition of iron and nickel as single metals can be combined to form a predictive model for the codeposition of iron-nickel alloys. Inhibition of the more noble nickel in the presence of iron is caused by preferential surface coverage of the adsorbed iron intermediate resulting from a difference between the two elements in Tafel constant for the electrosorption step. The role of hydrolyzed cations and surface pH is investigated and methods for evaluating the influence of pH are explored. The analysis shows that changes in surface pH with potential are not necessary for iron-rich (anomalous) deposits, but that variations in pH from one electrolyte to another may influence deposit composition. The tendency toward iron-rich deposits with increasing overpotential exists in all systems, however, and can be prevented only by decreasing the iron concentration of the bath. An extension of the analysis to account for transport limitations in baths with low iron concentration is developed and calculations with the model are presented to illustrate the effects of current density and electrolyte convection under conditions similar to those investigated experimentally in the literature.

Global Model of Plasma Chemistry in a High Density Oxygen Discharge

Abstract: the gas phase kinetics and plasma chemistry of high density oxygen discharges are studies. A self-consistent, spatially-averaged model is developed to determine positive ion, negative ion and electron densities, ground state and metastable free radical densities, and electron temperature as functions of gas pressure, microwave input power, and cylindrical source diameter and length. For an electron cyclotron resonance (ECR) discharge, the reduction in radial transport due to the confining magnetic field is also modeled. The kinetic scheme includes excitation, dissociation, and ionization of neutrals due to electron impact, electron attachment and detachment, and ion-ion neutralization. In addition, ion neutralization at the reactor walls is included. Model results show that for a low neutral pressure, high plasma density discharge, oxygen molecules are almost completely dissociated to form oxygen atoms, and the dominant positive ion is O+ rather than O2+. The metastable species are not important for the pressure range studies (0.5 - 100 mTorr), and the confining magnetic field significantly affects the plasma chemistry, the total positive ion density, and the electron temperature. Comparisons are made to experimental data, and qualitative agreement between experiment and model is observed.

Etching of Silicon in NaOH Solutions II . Electrochemical Studies of n‐Si(111) and (100) and Mechanism of the Dissolution

Abstract: In Part I of this work, the bias dependence of the etching of silicon (111) has been investigated by means of in situscanning tunneling microscopy observations. In this second part, current‐voltage curves and etch rate results derived from the loss of material and performed with n‐type Si samples of various orientations, show that electrochemical and chemical reactions coexist in the oxidation of Si. A model is presented for the oxidation of a Si atom in a kink site in different situations of polarization. The key feature of the description is the understanding of the persistent hydrogen termination of the surface in spite of the continuous oxidative removal of Si atoms from the surface. The model includes the hydrolytic splitting of Si—H and Si—Si bonds as the important chemical contributions to the etching process. At the rest potential, the chemical component is dominant. The sequence of reactions leaves the surface in the terminated state. The anodic current is due to the injection of electrons which are produced during the substitution of Si—H by Si—OH bonds. This results above a critical electrode potential in passivation. In this respect, (111) and (100) faces present quite different behaviors. At cathodic bias where the hydrogen evolution becomes fast, due to the accumulation of electrons at the surface, not only the anodic component of the etching reaction vanishes but also the chemical component decreases in rate and is eventually stopped.

A Study of the Oxidation of Phenol at Platinum and Preoxidized Platinum Surfaces

Abstract: The oxidation of phenol at the platinum electrode was studied in aqueous acidic solutions. The effects of electrode surface oxide on the oxidation reactions of phenol and on electrode passivation by reaction products were investigated using cyclic voltammetry and chronoamperometry. X‐ray photoelectron spectrometry was used to detect changes in the nature of the passive film. Phenol reacted at both the inner and outer Helmholtz layers at platinum metal electrodes. Phenol in the inner Helmholtz layer is adsorbed irreversibly and is conductive. Its oxidation involves ring cleavage with a greater than 18 eq/mol. The outer Helmholtz layer reactions are characterized by rapid simple oxidations involving minimal rearrangement of the reactant molecule. This implies that once stable oxidized products such as benzoquinone and polymers with quinone or ether structures are formed they must move from the outer to the inner Helmholtz layer to be oxidized further by ring‐cleavage reactions. We postulate that the bulk of the initial current flow during phenol oxidation is due to the simple fast outer Helmholtz reactions. This initial current continues until the buildup of unreactive products blocks further outer Helmholtz reactions and the slower inner layer reactions predominate. This electrode behavior changed if the electrode was preoxidized producing a platinum oxide coating. The inner layer reactions were greatly reduced at a platinum oxide coated electrode resulting in lower passivated electrode current flow. The onset of passivation however was delayed at the oxide coated electrode. This is attributed to a weaker adsorption of reaction products at the electrode surface requiring additional reaction to produce a stable passive film. The final resulting passive films at platinum and platinum oxide electrodes were chemically similar based on x‐ray photoelectron spectrometric analysis but differed in thickness indicating that the electrode passivation is not due simply to the thickness of the passive film.

Electrochemical Insertion of Magnesium in Metal Oxides and Sulfides from Aprotic Electrolytes

Abstract: The electrochemistry of , , , , and has been studied in several organic solvents containing in view of their application as positive electrodes in rechargeable magnesium batteries. Only showed promising coulombic capacity and reversibility. Mg2+ insertion into this oxide depends on the ratio between the amounts of and Mg2+ as well as on the absolute amount of in the electrolyte. Water molecules preferentially solvating Mg2+ions appear to facilitate the insertion process. The highest coulombic capacities of up to 170 Ah/kg were reached in acetonitrile solutions containing .

Etching of Silicon in NaOH Solutions I . In Situ Scanning Tunneling Microscopic Investigation of n‐Si(111)

Abstract: The etching of n-type silicon (111) has been investigated by means of in situ scanning tunneling microscopy (STM) observations performed over a wide range of bias of the sample A special procedure has been used to observe topography changes at potentials close and positive of the rest potential Irrespective of the bias, images show that the surface consists in atomically smooth terraces separated by 31 A high steps At cathodic bias, the etching occurs principally at terrace edges and (111) terraces are most probably H terminated, which prevents their reconstruction, as could be seen in atomically resolved pictures taken in situ Triangular etch pits nucleate when the potential approaches the rest potential

Rechargeable Li1+xMn2O4/carbon cells with a new electrolyte composition : potentiostatic studies and application to practical cells

Abstract: To improve the high temperature performance of Li[sub 1+x]Mn[sub 2]O[sub 4]/carbon rocking-chair secondary batteries the authors searched for and identified a new electrolyte composition whose range of stability extends up to 4.9 V vs Li at room temperature and 4.8 V vs Li at 55 C for the Li[sub x]Mn[sub 2]O[sub 4] material. The behavior of the M = LiMn[sub 2]O[sub 4] composite new electrolyte interface at high voltage (4.2 to 5.1 V vs Li) shows the superposition of two phenomena: (i) an irreversible behavior due to a very slow electrolyte oxidation caused by the large surface area of carbon black (mixed with the Li[sub x]Mn[sub 2]O[sub 4] active material to improve the conductivity) and (ii) two reversible Li deintercalation-intercalation processes in the Li[sub x]Mn[sub 2]O[sub 4] spinel structure. In order to evaluate the kinetics of the high voltage phenomena, the behavior of the LiMn[sub 2]O[sub 4]/new electrolyte interface was investigated as a function of time and temperature. The electrolyte oxidative degradation is a well-stabilized reaction with nontime evolving kinetics, and with an activation energy close to 8 kcal/mol. The self-discharge mechanism a local redox process involving electrolyte oxidation at the electrode surface and reversible intercalation of Li in themore » Li[sub x]Mn[sub 2]O[sub 4] spinel structure. The effective stability of this new electrolyte against oxidation allows for better performance of the rocking-chair cells, in terms of cycle-life and self-discharge, over a wider temperature range ([minus]20 to 55 C.)« less

Self‐Assembled Layers of Alkanethiols on Copper for Protection Against Corrosion

Abstract: Self-assembled monolayers of alkanethiols C[sub n]H[sub 2n+1]SH(n = 6 [approximately] 18) adsorbed on the surface of a polycrystalline bulk cu were constructed and characterized by x-ray photoelectron (XPS) and surface-enhanced Raman scattering (SERS) spectroscopy and contact angle and impedance measurements. The protection ability of the alkanethiol monolayers against Cu corrosion in an aerated 0.5 M Na[sub 2]SO[sub 4] solution was examined by impedance and polarization techniques. Results of XPS, SERS, and contact angle measurements showed that alkanethiols were chemisorbed on the Cu surface by the formation of strong bonds between Cu and S atoms following cleavage of a S-H bond and formed densely packed, water-repellent monolayers on the surface. The advancing contact angle of these monolayer films was comparable to that of alkanethiolate monolayers adsorbed at a vapor-deposited Cu on a Si wafer. However, sufficiently high protection abilities of the films against Cu corrosion were not obtained in 0.5 M Na[sub 2]SO[sub 4]. A preliminary experiment demonstrated the formation of a promising protective film which was prepared on the Cu surface by modification of self-assembled 11-mercapto-1undecanol monolayer with octyltrichlorosilane to form cross-linkages between the thiol molecules with siloxane bonds.

Analysis of Free Energy and Entropy Changes for Half‐Cell Reactions

Abstract: A comprehensive study of different entropy scales used in chemical thermodynamics is presented, and a semi‐absolute entropy scale is introduced, by which problems involving noncharged and charge species can be considered correctly and further, the heat generation of half‐cell reactions can be calculated. In this context, the entropy of an electron in a metal is derived. The entropy changes for electrode reactions are calculated, and the heat distribution among the electrodes of the cell is solved. The correlation of the zeros of the energy scales for , H+(aq), and (vacuum) is studied, and the value of chemical potential of electron in metal and gas phase is derived. An estimation for the free‐enthalpy change for the half‐cell reaction (Pt electrode) is presented. The Galvani potential differences for half cells are calculated, and tables of , , and are presented. An example of the use of half‐cell reaction entropies and free‐energy changes for a fuel cell is presented. With this method we can establish how the total heat generated in the fuel cell is distributed between the cathode and the anode of the cell. This method gives new basic information on electrochemical cells, which can be applied to mathematical models of single electrodes. The physical meaning of for half‐cell reactions is illustrated by Poyinting's vector.

Electrochemical Noise Analysis of Iron Exposed to NaCl Solutions of Different Corrosivity

Abstract: Potential and current noise data have been collected for pure iron foils which were exposed to 0.5N which was aerated, deaerated, or aerated with added as inhibitor. Data were obtained at the beginning of each hour over a 24 h period. The noise data were displayed as power spectral density plots or as the rms values of potential and current . The potential noise data seem to be affected by fluctuations of mass transport and did not agree with the known corrosion behavior of the systems under investigation. However, good agreement was observed for the current noise data. For the noise resistance similar values as for the polarization resistance determined with EIS at the end of the tests were obtained. The pit index defined by others as , where is the mean current in a noise measurement, did not provide meaningful information. The experimental data obtained in this investigation have been compared with those reported previously by Lumsden et al.

Synthesis and Electrochemical Studies of LiMnO2 Prepared at Low Temperatures

Abstract: A new form of LiMnO{sub 2}, which has recently been shown to reversibly intercalate lithium, has many advantages over other popular lithium-ion battery cathode materials. The authors describe improvements in the previously reported low temperature (300--450 C) synthesis and report a new synthesis route based on ion exchange which takes place at 100 C. Electrochemical performance is shown to be strongly dependent on synthesis temperature. In situ x-ray diffraction and long time cycling measurements indicate that an irreversible structural transformation to disordered spinel takes place when Li is removed from LiMnO{sub 2}. Nevertheless, the specific capacity of this material is over 190 mAh/g between 2.5 and 4.2 V.

Heat Transfer Phenomena in Lithium/Polymer‐Electrolyte Batteries for Electric Vehicle Application

Abstract: Mathematical modeling of heat generation and transport in lithium/polymer‐electrolyte batteries for electric vehicle applications has been conducted The results demonstrate that thermal management may not be a serious problem for batteries under low discharge rates However, under high discharge rates, the temperature of a battery may increase remarkably if the thickness of a cell stack exceeds a certain value Also, due to the low thermal conductivity of the polymer, the improvement of cooling conditions is not an effective means of improving heat removal for large‐stack systems For a required operational temperature range and a given discharge rate, model predictions can be used to design appropriate battery structures and to choose a suitable cooling scheme