Showing papers in "Journal of The Electrochemical Society in 1995"

Zero‐Strain Insertion Material of Li [ Li1 / 3Ti5 / 3 ] O 4 for Rechargeable Lithium Cells

Abstract: having a defect spinel‐framework structure was prepared and examined in non‐aqueous lithium cells. (white in color) was reduced to (dark blue) at a voltage of 1.55 V and the reaction was highly reversible. X‐ray diffraction measurements indicated that the lattice dimension did not change during the reactionSince the reaction consists of lithium ion and electron insertion into/extraction from the solid matrix without a noticeable change in lattice dimension, called a zero‐strain insertion reaction, capacity failure due to the damage to the solid matrix was not observed even after 100 cycles. Feasibility of zero‐strain insertion materials for advanced batteries is discussed based on the experimental results.

Hydrous Ruthenium Oxide as an Electrode Material for Electrochemical Capacitors

Abstract: The hydrous ruthenium oxide has been formed by a sol-gel process. The precursor was obtained by mixing aqueous solutions of RuCl{sub 3}{center_dot}xH{sub 2}O and alkalis. The hydrous ruthenium oxide powder was obtained by annealing the precursor at low temperatures. The crystalline structure and the electrochemical properties of the powder have been studied as a function of the annealing temperature. At lower annealing temperatures the powder is in an amorphous phase with a high specific capacitance. Specific capacitance as high as 720 F/g was measured for the powder formed at 150 C. when the annealing temperature exceeded 175 C, the crystalline phase was formed, and the specific capacitance dropped rapidly. The surface area of the powder and the resistivity of the pellet made from these powders have also been studied. The specific surface area and the resistivity decreased as the annealing temperature increased. A capacitor was made with electrodes comprised of hydrous ruthenium oxide and H{sub 2}SO{sub 4} electrolyte. The energy density of 96 J/g (or 26.7 Wh/kg), based on electrode material only, was measured for the cell using hydrous ruthenium oxide electrodes. It was also found that hydrous ruthenium oxide is stable in H{sub 2}SO{sub 4} electrolyte.

Acid-doped polybenzimidazoles : a new polymer electrolyte

Abstract: Polybenzimidazole films doped with phosphoric acid are being investigated as potential polymer electrolytes for use in hydrogen/air and direct methanol fuel cells. In this paper, we present experimental findings on the proton conductivity, water content, and methanol vapor permeability of this material, as well as preliminary fuel cell results. The low methanol vapor permeability of these electrolytes significantly reduces the adverse effects of methanol crossover typically observed in direct methanol polymer electrolyte membrane fuel cells.

Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction An In Situ XANES and EXAFS Investigation

Abstract: The electrocatalysis of the oxygen reduction reaction (ORR) on five binary Pi alloys (PtCr/C, PtMn/C, PtFe/C, PtCo/C, and PtNi/C) supported on high surface area carbon in a proton exchange membrane fuel cell was investigated. All the alloy electrocatalysts exhibited a high degree of crystallinity with the primary phase of the type Pt3M (LI2 structure with fcc type lattice) and a secondary phase (only minor contribution from this phase) being of the type PtM (LIo structure with tetragonal lattice) as evidenced from x-ray powder diffraction (XRD) analysis. The electrode kinetic studies on the Pt alloys at 95~ and 5 atm pressure showed a two- to threefold increase in the exchange current densities and the current density at 900 mV as well as a decrease in the overvoltage at i0 mA em -2 relative to Pt/C eleetrocatalyst. The PtCr/C alloy exhibited the best performance. In situ EXAFS and XANES analysis at potentials in the double-layer region [0.54 V vs. reversible hydrogen electrode (RHE)] revealed (i) all the alloys possess higher Pt d-band vacancies per atom (with the exception of PtMn/C alloy) relative to Pt/C electrocatalyst and (it) contractions in the Pt-Pt bond distances which confirmed the results from ex situ XRD analysis. A potential excursion to 0.84 V vs. RHE showed that, in contrast to the Pt alloys, the Pt/C electrocatalyst exhibits a significant increase in the Pt d-band vacancies per atom. This increase, in Pt/C has been rationalized as being due to adsorption of OH species from the electrolyte following a Temkin isotherm behavior, which does not occur on the Pt alloys. Correlation of the electronic (Pt d-band vacancies) and geometric (Pt-Pt bond distance) with the electrochemical performance characteristics exhibits a volcano type behavior with the PtCr/C alloy being at the top of the curve. The enhanced electrocatalysis by the alloys therefore can be rationalized on the basis of the interplay between the electronic and geometric factors on one hand and their effect on the chemisorption behavior of OH species from the electrolyte. The role of Pt/C and Pt alloys on the mechanism of the oxygen reduction reaction (ORR) has been investigated previously, 1-4 however the mechanism still remains elusive. One of the first investigations I of the ORR on Pt alloy electrocatalysts was in phosphoric acid; the effect of changes in the Pt-Pt interatomic distances, caused by alloying, was examined. The strength of the [M-HO2]aas bond, the intermediate formed in the rate-determining step of the molecular dioxygen reduction, was shown to depend on the Pt-Pt bond distance in the alloys. A plot of the electrocatalytic activity vs. adsorbate bond strength exhibited a volcano type behavior. 5 It was shown that the lattice contractions due to alloying resulted in a more favorable Pt-Pt distance (while maintaining the favorable Pt electronic properties) for dissociative adsorption of 02. This view was disputed by Glass et al. ~ in their investigation on bulk alloys of PtCr (the binary alloy at the top of the volcano plot) of different compositions. The latter investigation showed no activity enhancement for the ORR in phosphoric acid. This study therefore suggested the possibility of differences in electrochemical properties of bulk vs. supported alloy electrocatalysts (small particles of 35-85 A). A recent study on supported PtCo electrocatalysts ~ revealed the possibility that particle termination, primarily at the vicinal planes in the supported alloy electrocatalyst, is the reason for the enhanced ORR electrocatalysis (i.e., vicinal planes are more active than ). Paffett et al., 3 attributed higher activities for the ORR on bulk PtCr alloys in phosphoric acid to surface roughening, and hence increased Pt surface area, caused by the dissolution of the more oxidizable alloying component Cr. In contrast to these findings on bulk alloys, the supported alloy electrocatalysts have been reported to retain their nonnoble alloying element in the electrode during long periods (6000-9000 h) of operation in phosphoric acid fuel cells (PAFCs) 6 and proton exchange membrane fuel ceils (PEMFCs). 7 Based on these previous investigations and in the context of the ORR mechanisms, the principle explanations for the

A New Charge Storage Mechanism for Electrochemical Capacitors

Abstract: The hydrous form of ruthenium oxide (RuO[sub 2][center dot]xH[sub 2]O) has been demonstrated to be an excellent electrode material for electrochemical capacitors. This material, as prepared by a sol-gel process at low temperatures, is amorphous and electrically conductive. The specific capacitance is over 720 F/g. This value is at least two times higher than the highest value ever reported for such materials. The charge storage mechanism is believed to involve bulk electrochemical protonation of the oxide. This discovery opens a new avenue of research in the field of high energy density electrochemical capacitors.

Performance modeling of the Ballard Mark IV solid polymer electrolyte fuel cell. II: Empirical model development

Abstract: A parametric model predicting the performance of a solid polymer electrolyte, proton exchange membrane (PEM) fuel cell has been developed using a combination of mechanistic and empirical modeling techniques. This paper details the mechanistic model development. Mass transport properties are considered in the mechanistic development via Stefan‐Maxwell equations. Thermodynamic equilibrium potentials are defined using the Nernst equation. Activation overvoltages are defined via a Tafel equation, and internal resistances are defined via the Nernst‐Planck equation, leading to a definition of ohmic overvoltage via an Ohm's law equation. The mechanistic model cannot adequately model fuel cell performance, since several simplifying approximations have been used in order to facilitate model development. Additionally, certain properties likely to be observed in operational fuel cells, such as thermal gradients, have not been considered. Nonetheless, the insights gained from the mechanistic assessment of fuel cell processes were found to give the resulting empirical model a firmer theoretical basis than many of the models presently available in the literature. Correlation of the empirical model to actual experimental data was very good.

A Linear Kronig‐Kramers Transform Test for Immittance Data Validation

Abstract: A method is described with which immittance data can be tested for Kronig‐Kramers compliance. In contrast with other procedures, this method is linear in nature and is based on a predetermined set of relaxation times. The model contains as many parameters (or less) as there are data sets. Three modes of operation are described, the first two are based on a linear fit of the model function to the imaginary part or to the real part of the data set. With the fit parameters the corresponding real or imaginary dispersion can be calculated and compared with the actual measurement. In the third mode a complex model function is fitted to the complete data set. As the model function does comply with (a relaxed set of) the Kronig‐Kramers (K‐K) rules, it will not be able to reproduce the data set satisfactory in the case of nonK‐K behavior, as can be observed from the residuals plot. Due to its linear nature, no starting values are needed for the data validation. The main limitation of this procedure is the size of the matrix and the accuracy of the matrix inversion.

Modeling of Proton Exchange Membrane Fuel Cell Performance with an Empirical Equation

Abstract: An empirical equation [E = E{sub 0} {minus} b log i {minus} Ri {minus} m exp (in)] was shown to fit the experimental cell potential (E) vs. current density (i) data for proton exchange membrane fuel cells (PEMFCs), at several temperatures, pressures, and oxygen compositions in the cathode gas mixture. The exponential term compensates for the mass-transport regions of the E vs. i plot; i.e., the increase in slope of the pseudolinear region and the subsequent rapid fall-off of the cell potential with increasing current density. As has been previously shown, the terms E{sub 0} and b yield the electrode kinetic parameters for oxygen reduction in the PEMFC and R represents the resistance, predominantly ohmic and, to a small extent, the charge-transfer resistance of the electro-oxidation of hydrogen. The exponential term characterizes the mass-transport region of the E vs. i plot. The parameter n has more pronounced effects than the parameter m in this region. A physicochemical interpretation of these parameters is needed. The PEMFC is the most promising candidate fuel cell power source for a zero emission vehicle, because of its desirable characteristics, such as quick start capability, low operating temperature, high energy efficiency, and high power density.

Electrochemical Determination of the Diffusion Coefficient of Hydrogen Through an LaNi4.25Al0.75 Electrode in Alkaline Aqueous Solution

Abstract: Metal hydrides are being used as electrodes in nickel/metal-hydride batteries because of their ability to store large quantities of hydrogen and because of their many advantages over conventional lead-acid and nickel-cadmium batteries. The performance of a metal hydride electrode is determined by both the kinetics of the processes occurring at the metal/electrolyte interface and the rate of hydrogen diffusion within the bulk of the metal. The constant potential and constant current discharge techniques were used to determine the hydrogen diffusion coefficients in an LaNi{sub 4.25}Al{sub 0.75} electrode. The values obtained were 2.97 {times} 10{sup {minus}11} and 3.30 {times} 10{sup {minus}11} cm{sup 2}/s, respectively. The advantages and disadvantages of these two techniques are discussed.

Structural Considerations of Layered and Spinel Lithiated Oxides for Lithium Ion Batteries

Abstract: Rechargeable lithium batteries that can be assembled in the discharged state with lithiated metal oxide cathodes and carbon anodes are being developed to minimize the safety hazards associated with batteries that use pure metallic lithium anodes. This paper reviews crystallographic aspects of insertion electrodes with layered and spinel structures . The use of (spinel) instead of lithiated carbon as the anode is briefly discussed. Emphasis is placed on the structural properties of insertion electrodes that control their stability during electrochemical cycling.

Synthesis and Characterization of LiAl1 / 4Ni3 / 4 O 2 ( R 3̄m ) for Lithium‐Ion (Shuttlecock) Batteries

Abstract: The synthesis and characterization of LiAl{sub 1/4}Ni{sub 3/4}O{sub 2} were carried out in order to improve the electrochemical properties of LiNiO{sub 2} for lithium ion batteries. Single phase of LiAl{sub 1/4}Ni{sub 3/4}O{sub 2} (R{bar 3}m; a = 2.86 {angstrom} and c = 14.24 {angstrom} in hexagonal setting) was obtained by heating a reaction mixture of LiNO{sub 3}, NiCO{sub 3}, and Al(OH){sub 3} at 750 C under an oxygen stream for 20 h and examined in nonaqueous lithium cells. LiAl{sub 1/4}Ni{sub 3/4}O{sub 2} was oxidized to {open_square}{sub 3/4}Li{sub 1/4}Ni{sub 3/4}O{sub 2} in the voltage range of 3.5--4.8 V. {open_square} denotes the vacant octahedral sites in a cubic-close packed oxygen array. X-ray diffraction examinations of Li{sub 1{minus}x}Al{sub 1/4}Ni{sub 3/4}O{sub 2} indicated that the reaction proceeded in a single phase over the entire range. The rechargeable capacity was observed to be about 150 mAh/g based on the sample weight. DSC measurements on fully charged LiNiO{sub 2} and LiAl{sub 1/4}Ni{sub 3/4}O{sub 2} at 4.8 V were also done and the effects of substitution of aluminum for nickel in LiNiO{sub 2} upon the electrochemical properties were discussed in terms of capacity, failure, and safety coupled with overcharge.

Structural and Kinetic Characterization of Lithium Intercalation into Carbon Anodes for Secondary Lithium Batteries

Abstract: Electrochemical intercalation of lithium into carbons has been studied using mesophase‐pitch‐based carbon fibers with different heat‐treatment temperatures, coke, and graphites as anodes for secondary lithium batteries. The variations in the average layer spacing and the voltage profile for the carbons with intercalating depend on the degree of graphitization. The intercalation into a more disordered carbon fiber heated at 900°C has been characterized as intercalation into the layer structure for in , but additional lithium insertion into an unorganized carbon loses the layer structure. The polarization resistance estimated from the impedance spectrum decreased by increasing degree of graphitization. The variation in with intercalation revealed the intercalation processes in various disordered carbons to be single‐phase reactions with different stoichiometries of lithium intercalation. The chemical diffusion coefficient of lithium ions in carbons decreased by increasing the composition in up to . The chemical diffusion coefficient was considerably affected by the texture and the degree of graphitization of the carbons. The graphitized carbon fiber heated at 3000°C for in exhibited one order magnitude larger values of than those of graphites. The rapid diffusion in the graphitized carbon fiber has been attributed to the radial texture in the cross section. It has been found that the activation energy for the diffusion process decreased by increasing the degree of graphitization.

The Study of Electrolyte Solutions Based on Ethylene and Diethyl Carbonates for Rechargeable Li Batteries II . Graphite Electrodes

Abstract: The electrochemical behavior of Li-graphite intercalation anodes in ethylene and diethyl carbonates (EC-DEC) solutions was studied using surface sensitive Fourier transform infrared spectroscopy (FTIR) and impedance spectroscopy in conjunction with standard electrochemical techniques. Three different solvent combinations, four different salts: LiBF{sub 4}, LiPF{sub 6}, LiClO{sub 4}, and LiAsF{sub 6}, and the influence of the presence of CO{sub 2} were investigated. Graphite electrodes could be cycled hundreds of times obtaining a reasonable reversible capacity. The best electrolyte was found to be LiAsF{sub 6} and the presence of CO{sub 2} in solutions considerably increased the reversible capacity upon cycling. This improved performance is due to precipitation of the ethylene carbonate reduction product, (CH{sub 2}OCO{sub 2}Li){sub 2}, which is an excellent passivating agent, on the electrode surface. Aging processes of these surface films and their influence on the electrode properties are discussed.

Rocking Chair Lithium Battery Based on Nanocrystalline TiO2 (Anatase)

Abstract: Reference LPI-ARTICLE-1995-007View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Investigation of the Microstructure in the Catalyst Layer and Effects of Both Perfluorosulfonate Ionomer and PTFE‐Loaded Carbon on the Catalyst Layer of Polymer Electrolyte Fuel Cells

Abstract: Effects of a perfluorosulfonate ionomer (PFSI) and of a polytetrafluoroethylene (PTFE) loaded carbon (PTFE‐C) on the catalyst layer in the electrode of a polymer electrolyte fuel cell (PEFC) prepared by a new method based on the process of PFSI‐colloid formation were investigated by electrochemical techniques and a mercury pore sizer. The microstructure of the catalyst layer and its effect on the PEFC performance were affected by the contents of both PFSI and PTFE‐C. The catalyst layer has two distinctive pore distributions with a boundary of ca. 0.04 μm. The volume of larger pore (secondary pore) deceased with an increase of the PFSI content and increased with an increase of the PTFE‐C content. The volume of the smaller pore (primary pore) was independent of the content of both PFSI and PTFE‐C. The PFSI as well as the PTFE existed only in the secondary pore. The content of PFSI affected the performance of PEFC in the whole current density range. On the other hand, the content of PTFE‐C influenced it greatly at high current density due to its gas feeding faculty. In the PEFC, reaction sites were found to exist in the secondary pore coated with the macromolecule PFSI. The hydrophobic PTFE‐C works to supply the reaction gas to the reaction sites covered with the PFSI in the secondary pore, and to exhaust the product water from there. The high performance of PEFC at high current density was achieved with the best mixture of the PFSI and the PTFE‐C.

A Compilation of Corrosion Potentials Reported for Intermetallic Phases in Aluminum Alloys

Abstract: A compilation of corrosion potentials reported for Al-based intermetallic compounds is presented. The range of corrosion potentials for dilute aluminum binary alloys is also given. This compilation is intended to serve as an aid in establishing galvanic relationships among discrete microstructural elements in aluminum alloys. This compilation is based primarily on data reported in the corrosion literature with a focus on corrosion potentials for intermetallic compound particles found in commercial alloys.

Charge‐Discharge Characteristics of the Mesocarbon Miocrobeads Heat‐Treated at Different Temperatures

Abstract: Mesocarbon microbeads (MCMB) is one of the promising carbon materials as anodes for rechargeable lithium batteries among commercially available carbon materials. have examined the correlation between carbon structures and charge-discharge characteristics of the MCMBs prepared at different heat-treatment temperatures. It was found that the MCMB heat-treated at 700 C possesses a tremendously high charge-discharge capacity of 750 Ah/kg. This suggests that there is another mechanism for the charge-discharge reaction besides a graphite intercalation compound mechanism which is well known. Therefore, the authors propose a cavity mechanism in which intercrystallite spaces in MCMB are capable of storing lithium species.

The Study of Electrolyte Solutions Based on Ethylene and Diethyl Carbonates for Rechargeable Li Batteries: I . Li Metal Anodes

Abstract: The behavior of Li electrodes was studied in ethylene and diethyl carbonates (EC-DEC) solutions of LiAsF{sub 6}, LiClO{sub 4}, LiBF{sub 4}, and LiPF{sub 6}. The correlation of the surface chemistry to the interfacial properties, morphology, and Li cycling efficiency was investigated using surface sensitive Fourier transform infrared spectroscopy and impedance spectroscopy, scanning electron microscopy, X-ray energy dispersive microanalysis, and standard electrochemical techniques. The Li surface chemistry is initially dominated by EC reduction to an insoluble species, probably (CH{sub 2}OCO{sub 2}Li){sub 2}. Upon storage, several aging processes may take place, depending on the salt used. Their mechanisms are discussed. Although EC-DEC solutions were found to be adequate for Li ion rechargeable batteries, this work indicates that they are not suitable as electrolyte solutions for batteries with Li metal electrodes. This is mostly because Li electrodes cannot be considered stable in these systems and Li deposition is highly dendritic.

Lithium Insertion in Carbons Containing Nanodispersed Silicon

Abstract: Graphite and pregraphitic carbons are intercalation hosts commonly used in Li ion cells. Using chemical vapor deposition of benzene and of silicon-containing precursors, the authors have prepared carbons containing nanodispersed silicon. The silicon resides within the unorganized regions in the pregraphitic carbons. Materials with up to 11% atomic silicon have been prepared. These materials reversibly react with lithium in electrochemical cells and the reversible specific capacity increases from {approximately}300 mAh/g, in the absence of silicon, to near 500 mAh/g as silicon is added. For silicon content < 6 atomic percent, the reversible capacity increases linearly with a slope of approximately 30 mAh/g per percentage point silicon. This suggests that each silicon atom can reversibly bond with {approximately}1.5 lithium atoms. The increased capacity due to the silicon appears as a broad feature in the differential capacity between 0.1 and 0.6 V vs. Li metal. The large reversible capacities are maintained over many charge/discharge cycles. The carbonaceous matrix provides a pathway for diffusion of Li to the nanodispersed silicon atoms, while it can still intercalate a substantial amount of lithium. Nanodispersions of other lithium alloying atoms in carbon probably can be prepared.

Evaluation of Ethanol, 1‐Propanol, and 2‐Propanol in a Direct Oxidation Polymer‐Electrolyte Fuel Cell A Real‐Time Mass Spectrometry Study

Abstract: Ethanol, 1-propanol, and 2-propanol have been evaluated as alternative fuels for direct methanol/oxygen fuel cells. The relative product distributions for the electro-oxidation of these alcohols under fuel-cell conditions were determined using on-line mass spectrometry. For water/ethanol mole ratios between 5 and 2, ethanol is the main product, while CO{sub 2} is a minor product. However, an increase of the water/ethanol mole ratio increased the relative product distribution of CO{sub 2} slightly. Propanol was the main product of the electro-oxidation of 1-propanol with a similar percentage of CO{sub 2} being formed as for ethanol. In contrast, the electro-oxidation of 2-propanol yielded practically only acetone. Between 150 and 190 C, the product distributions for the electro-oxidation of ethanol, 1-propanol, and 2-propanol do not depend significantly on the temperature. No differences in the product selectivities of Pt-Ru and Pt-black were found. Ethanol is a promising alternative fuel for direct methanol fuel cells (DMFCs) with an electrochemical activity comparable to that of methanol. Conversely, 1-propanol and 2-propanol are not suitable as fuels for DMFCs due to their low electrochemical activity.

New Preparation Method for Polymer-Electrolyte Fuel-Cells

Abstract: A new preparation method for the catalyst layer in the electrodes of a polymer electrolyte fuel cell, based on the process of preparing perfluorosulfonate‐ionomer (PFSI) colloid, was developed. In this method, both a good network of PFSI and uniformity of PFSIs on Pt particles were achieved with colloid formation of PFSI chains in the specific organic solvents. The applicable organic solvents had dielectric constants ranging from 3 to 10. The PFSI colloids were selectively adsorbed onto the carbon agglomerates with highly dispersed Pt particles on the surface, and a catalyst paste‐formation followed. Then the catalyst paste was spread over a carbon paper representing the gas diffusion layer, and two sheets of carbon paper with catalyst paste on them were hot pressed onto an ion‐exchange membrane from both sides. This simple preparation method provided high performance in fuel cells at low temperature (50°C).

Lithium Insertion in High Capacity Carbonaceous Materials

Abstract: We report studies of lithium insertion in a variety of organic precursors pyrolyzed below 1000°C. These include two types of petroleum pitch, polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polyphenylene sulfide (PPS), and epoxy novolac resin (ENR). Each of these materials shows reversible specific capacities for lithium of between 550 and 900 mAh/g when treated near 700°C. The majority of this high capacity shows large hysteresis ; that is the lithium is inserted near 0 V (vs. lithium metal) and removed near 1 V. Furthermore, the amount of capacity showing large hysteresis is well correlated to the hydrogen to carbon atomic ratio of the samples. As the pitch, PVC and PVDF samples are heated above 700°C, the H/C ratio and the specific capacity decrease proportionally, suggesting that the hydrogen in these samples plays an important role. The PPS and ENR samples behave differently ; as they are heated above 700°C, the high capacity is maintained as H/C is reduced, although the voltage profile changes dramatically to one without significant hysteresis and with a plateau of several hundred mAh/g near 0 V. We believe that structural differences associated with the presence of single carbon layers in the ENR and PPS samples account for the difference between their behavior and that of the other samples. The materials made from ENR appear to be interesting candidates for high capacity anodes for lithium ion cells.

Thermal Modeling of the Lithium/Polymer Battery I . Discharge Behavior of a Single Cell

Abstract: Two models are presented to predict the thermal behavior of the lithium/polymer battery. Part 1 presents the one-cell model, a one-dimensional model for predicting the thermal behavior of the lithium negative electrode/solid polymer separator/insertion positive electrode cell. Part 2 presents the cell/stack model, a one-dimensional model that uses variable heat-generation rates calculated by the one-cell mode to predict temperature profiles in cell stacks. The one-cell model, presented in Part 1, is based on the model of Doyle et al. with the addition of an energy balance in the form given by Bernardi et al. Physical properties are allowed to vary with temperature. The model is general and can be used to simulate a wide rang of polymeric separator materials, lithium salts, and composite insertion electrodes. Simulation results for the Li/PEO{sub 15}-LiCF{sub 3}SO{sub 3}{vert_bar}TiS{sub 2} system are presented for isothermal operation at several temperatures and adiabatic operation at several discharge rates. Heat transfer to the surroundings is considered by defining a position-dependent heat-transfer coefficient for various cells in a cell stack.

The Investigation of Cerium as a Cathodic Inhibitor for Aluminum‐Copper Alloys

Abstract: In situ current density mapping, scanning electron microscopy, and energy dispersive spectroscopy were used to study the effects of cerium as a corrosion inhibitor for an aluminum copper alloy (Al 2024-T4) in chloride containing solutions. It was found that cerium inhibits corrosion of this alloy by reducing the rate of the cathodic reaction. This was due to the formation of cerium-rich films over copper containing intermetallics which act as local cathodic sites. Results from tests carried out on an aluminum/copper galvanic couple, which was used to simulate the electrochemical behavior of the copper containing intermetallics, showed that corrosion inhibition was associated with the formation of a Ce-rich film over the copper in agreement with that observed for the alloy.

In situ Raman study on electrochemical Li intercalation into graphite

Abstract: Electrochemical lithium intercalation into graphite materials has been extensively studied for use in negative electrodes of secondary lithium batteries. Electrochemical lithium intercalation into highly oriented pyrolytic graphite and natural graphite powder was investigated using in situ Raman spectroscopy. Three plateaus were observed on the charging curve for both samples. From the Raman spectral changes the first plateau was assigned to a phase transition from dilute stage 1 to stage 4, the second from a stage 2 phase to another stage 2 phase, and the third from stage 2 to stage 1, which are in good agreement with Dahn's results by in situ X-ray diffraction. The spectral changes associated with the phase transitions occurred reversibly during a charge and discharge cycle. It was shown from the Raman spectral changes of the HOPG electrode that the electrode potential during the electrochemical intercalation is determined by the surface stage of graphite intercalation compounds.

A Review of Power Generation in Aqueous Thermogalvanic Cells

Abstract: The literature on aqueous thermogalvanic cells has been reviewed Wherever possible, the power conversion efficiency, {phi}, relative to that of a Carnot engine operating between the same temperatures and the figure of merit, Z, of the cell have been extracted from the literature data in order to assess the cell`s possible use for solar energy conversion The determination of {phi} and Z in such a cell requires the temperature dependence of its open-circuit potential difference and the knowledge of the current delivery characteristics of the cell The current delivery characteristics depend on the effects of activation overpotential, ohmic overpotential, and mass transport overpotential on the cell`s current The determination of the cell performance is hindered because a number of researchers apply an external potential to their cells or use forced electrolyte stirring, both of which introduce unknown amounts of energy into the energy balance for the cell The best performance found for an aqueous thermogalvanic cell which does not have such external energy inputs, is {phi}=50% and Z-058{times}10{sup {minus}4}K{sup {minus}1} Estimates of the maximum {phi} likely to be obtained from an aqueous thermogalvanic cell suggest that it would be difficult to obtain values in excess of {phi}=12%, which corresponds tomore » Z=15{times}10{sup {minus}4}K{sup {minus}1} These values are somewhat lower than the typical values for metal and semiconductor thermocouples The main cause of the low efficiencies of aqueous thermogalvanic cells is the presence of high concentrations of water molecules which conduct heat from the hot to the cold electrode but which are not themselves charge carriers It is shown that the replacement of aqueous electrolytes by molten salts would not provide a sufficiently large increase in {phi} to justify the very high temperatures required for such systems« less

The Measurement of a Complete Set of Transport Properties for a Concentrated Solid Polymer Electrolyte Solution

Abstract: Polymer electrolytes based on alkali metal salts in poly(ethylene oxides) are important for possible use in rechargeable batteries for both electric vehicle and consumer electronics applications. The authors measure a complete set of transport properties for one particular binary salt solution: sodium trifluoromethanesulfonate in poly(ethylene oxide), over a wide range of salt concentrations (0.1 to 2.6M) at 85 C. The properties measured include the conductivity, the salt diffusion coefficient, and the Na ion transference number. The mean molar activity coefficient of the salt is also determined. The conductivity and diffusion coefficients of NaCF{sub 3}SO{sub 3} are similar in magnitude to those of LiCF{sub 3}SO{sub 3} in (polyethylene oxide). The transference number and thermodynamic factor are found by combining concentration cell data with the results of galvanostatic polarization experiments. A theoretical analysis of the experimental method based on concentrated-solution theory is given. The study verifies that the transference numbers derived from the experiments retain fundamental significance in applications involving both steady and transient processes and in systems coupling the polymer electrolyte with electrodes of all types (stoichiometries). The relevant transference numbers can be determined independently of any knowledge of speciation of the polymer electrolyte. The transference numbers found here for themore » sodium ion are much lower than those reported for the lithium ion, especially in the concentrated solutions. The transference number of the sodium ion is negative in the more concentrated solutions and levels off at its maximum value of 0.31 in the dilute concentration range. The transference number results are interpreted in terms of complexation of the sodium ion with the anionic species.« less

Electrochemical Destruction of Aniline and 4‐Chloroaniline for Wastewater Treatment Using a Carbon‐PTFE O 2 ‐ Fed Cathode

Abstract: The electrochemical degradation of 100 ppm aniline and 4-chloroaniline in basic aqueous solutions of pH ranging between 10.1 and 12.7 has been studied at constant current intensity using a Pb/PbO{sub 2} anode and a carbon-polytetrafluoroethylene O{sub 2}-fed cathode. Under these conditions, hydrogen peroxide was electrogenerated in the cell via a two-electron reduction of oxygen gas fed to the cathode. The current efficiency for H{sub 2}O{sub 2} generation depended on the applied current intensity and the background electrolyte used. The concentration decay for each contaminant with electrolysis time was followed by high pressure liquid chromatography (HPLC). Kinetic analysis of these data showed a pseudo first-order decomposition reaction for both substrates at anodic current densities of 30 mA cm{sup {minus}2} or higher, as well as a decrease, of their half-lifetimes when current intensity increased. A gradual decrease in total organic carbon (TOC) for 0.05 mol/dm{sup 3} NAOH solutions with electrolysis time was always found. Product analysis of these electrolyzed solutions by gas chromatography-mass spectroscopy (GC-MS) and HPLC allowed the detection of nitrobenzene and 1-chloro-4-nitrobenzene proceeding from the anodic decomposition of aniline and 4-chloroaniline, respectively. Maleic acid was also detected as intermediate and ammonia was found as final product. Destruction of initial pollutantsmore » and intermediates is explained by their oxidative reactions with OH and HO{sub 2} radicals at the anode reaction layer. The effect of HO{sub 2} upon decomposition pathways of anilines is discussed.« less

Atomic Layer Epitaxy Growth of Tantalum Oxide Thin Films from Ta ( OC 2 H 5 ) 5 and H 2 O

Abstract: Ta{sub 2}O{sub 5} thin films have been used in a number of applications such as dielectric materials for storage capacitors and thin film transistors, antireflection coatings for solar cells, optical waveguides, and filters chemical sensors, as well as corrosion resistant materials. The deposition of thin Ta{sub 2}O{sub 5} films by atomic layer epitaxy (ALE) was investigated in the temperature range of 150 to 450 C using Ta(OC{sub 2}H{sub 5}){sub 5} and water as precursors. Because of the thermal self-decomposition of Ta(OC{sub 2}H{sub 5}){sub 5} the self-limiting ALE growth was achieved only below 350 C. All the films grown were amorphous as examined by X-ray diffraction analysis. The films grown at 250 and 325 C were stoichiometric within the accuracy of Rutherford backscattering spectrometry and contained 4 and 0.6 atom percent (a/o) hydrogen as determined by nuclear reaction analysis, respectively. Except for the outermost surface, the content of carbon residues was below 3 a/o as analyzed by X-ray photoelectron spectroscopy. The films exhibited smooth surfaces as observed by scanning electron microscopy and relatively uniform thicknesses with 7% deviation in the gas flow direction. The refractive index of the films increased with deposition temperature stabilizing at 2.23 at temperatures higher than 300more » C. The permittivities for the films grown at 250 and 325 C were 21 and 25, respectively, and leakage current densities at 1 MV/cm electric field were 4.0 and 2.3 mA/cm{sup 2}, respectively.« less