Showing papers in "Journal of The Electrochemical Society in 2001"

Optimized LiFePO4 for Lithium Battery Cathodes

Abstract: LiFePO 4 powders were synthesized under various conditions and the performance of the cathodes was evaluated using coin cells, The samples were characterized by X-ray diffraction, scanning electron microscope observations, Brunauer, Emmett, and Teller surface area measurements, particle-size distribution measurements, and Mossbauer spectroscopy. Ab initio calculation was used to confirm the experimental redox potentials and Mossbauer parameters. The choice of a moderate sintering temperature (500°C 95% of the 170 mAh/g theoretical capacity at room temperature. There are two main obstacles to achieving optimum charge/discharge performance of LiFePO 4 : (i) undesirable particle growth at T > 600°C and (ii) the presence of a noncrystalline residual Fe 3+ phase at T < 500°C.

The Mechanisms of Lithium and Sodium Insertion in Carbon Materials

Abstract: In this paper, we compare the interactions of lithium and sodium with a range of carbon materials in electrochemical cells. Through wide angle in situ X-ray scattering studies, we demonstrate that both lithium and sodium can be inserted into the interlayer space in disordered carbon materials. This insertion process is accompanied by an increase in the interlayer spacing in these materials. Small-angle in situ scattering studies are presented to clearly show the insertion of lithium and sodium into nanopores within disordered hard carbons. We also show that very little, if any sodium can he inserted into graphitic materials in contrast to the large capacity seen for lithium insertion.

An Asymmetric Hybrid Nonaqueous Energy Storage Cell

Abstract: A nonaqueous asymmetric electrochemical cell technology is presented where the positive electrode stores charge through a reversible nonfaradaic or pseudocapacitive reaction of anions on the surface of an activated carbon positive electrode. The negative electrode is a crystalline intercalation compound which supports the fast reversible intercalation of lithium ions. Using a positive electrode material of activated carbon and newly developed negative electrode material of nanostructured Li 4 Ti 5 O 12 we obtain a cell which exhibits a sloping voltage profile from 3 to 1.5 V, 90% capacity utilization at 10C charge/discharge rates, and 10-15% capacity loss after 5000 cycles. Electrolyte oxidation on the activated carbon positive electrode was characterized in a Li metal asymmetric hybrid cell by cyclic voltammetry. Oxidation during the anodic scan was found to decrease significantly after surface passivation at high voltage and elevated temperatures. We also introduce the asymmetric hybrid technology in a bonded flat plate plastic cell configuration where packaged energy densities were calculated to be in excess of 20 Wh/kg. In addition, a practical method for three-electrode analysis of Li cells by use of a Ag quasi-reference electrode wire is discussed.

Lithium Ionic Conductor Thio-LISICON: The Li2 S ­ GeS2 ­ P 2 S 5 System

Abstract: The new crystalline material family, lithium superionic conductor (thio-LISICON), was found in the Li 2 S-GeS 2 -P 2 S 5 system. The solid solution member x x 0.75 in Li 4-x Ge 1-x P x S 4 shows the highest conductivity of 2.2 x 10 -3 S cm -1 at 25°C of any sintered ceramic together with negligible electronic conductivity, high electrochemical stability, no reaction with lithium metal, and no phase transition up to 500°C. Its material design concepts of changing constituent ions with various ionic radii, valence, and polarizability are described.

Particle Size Effects on the Electrochemical Performance of Copper Oxides toward Lithium

Abstract: The electrochemical reactivity of tailor-made or CuO powders prepared according to the polyol process was tested in rechargeable Li cells. To our surprise, we demonstrated that CuO, a material well known for primary Li cells, and could reversibly react with 1.1 Li and 2 Li ions per formula unit, respectively, leading to reversible capacities as high as 400 mAh/g in the 3-0.02 V range. The ability of copper oxide-based Li cells to retain their capacity upon numerous cycles was found to be strongly dependent on the particle size, and the best results (100% of the total capacity up to 70 cycles) were obtained with 1 μm and CuO particles. Ex situ transmission electron microscopy data and in situ X-ray experiments show that the reduction mechanism of by Li first involved the formation of Cu nanograins dispersed into a lithia matrix, followed by the growth of an organic coating that partially dissolved upon the subsequent charge while Cu converted back to nanograins. We believe that the key to the reversible reactivity mechanism of copper oxides or other transition metal oxides toward Li is the electrochemically driven formation of highly reactive metallic nanograins during the first discharge, which enables the formation-decomposition of upon subsequent cycles. © 2001 The Electrochemical Society. All rights reserved.

In Situ X-Ray Diffraction Study of P 2 ­ Na2 / 3 [ Ni1 / 3Mn2 / 3 ] O 2

Abstract: The electrochemical extraction and insertion of Na in P2-Na 2/3 ][Ni 1/3 Mn 2/3 ]O 2 was studied by in situ X-ray diffraction. All the original Na can be extracted from P2-Na 2/3 ][Ni 1/3 Mn 2/3 ]O 2 and it can he reversibly inserted again. When x > 1/3 in Na x [Ni 1/3 Mn 2/3 ]O 2 . the compound remains in the P2 structure. For x 1/3, a small amount of 02-type stacking faults are introduced into the structure. When x < 1/3, the electrode exists as two coexisting phases. These are P2-Na 1/3 [Ni 1/3 Mn 2/3 ]O 2 (with some 02-type stacking faults) and [Ni 1/3 Mn 2/3 ]O 2 . [Ni 1/3 Mn 2/3 ]O 2 adopts the 02 structure with stacking faults. As sodium is reinserted in [Ni 1/3 Mn 2/3 ]O 2 to make Na 1/3 [Ni 1/3 Mn 2/3 ]O 2 the P2 structure forms again. The evolution of the lattice parameters of P2-Na x [Ni 1/3 Mn 2/3 ]O 2 (1/3 < x < 2/3) with x and the lattice constants of [Ni 1/3 Mn 2/3 ]O 2 are also reported.

Impedance of Solid Oxide Fuel Cell LSM/YSZ Composite Cathodes

Abstract: A number of lanthanum strontium manganate/yttria-stabilized zirconia (LSM/YSZ) composite electrodes are produced with varying composition and processing parameters. The composites are investigated using impedance spectroscopy. General trends related to the oxygen reduction process are extracted from the impedance data. Literature concerning kinetic studies of LSM/YSZ electrodes and related systems is reviewed and compared to new experimental data. From this it is found that at least five processes affect the impedance. Going from high to low frequency, these processes are (i), (ii) two geometry-related contributions interpreted as transport across LSM/YSZ interfaces and through the YSZ of the composite, (iii) a process reflecting competitive reaction steps such as bond breaking and surface diffusion, gas diffusion in a stagnant gas layer above the electrode structure, and an activation process (inductive) presumably located at the triple phase boundary of electrode, electrolyte, and gas phase. © 2001 The Electrochemical Society. All rights reserved.

Chemical Composition and Morphology of the Elevated Temperature SEI on Graphite

Abstract: The relationship between the elevated temperature performance of Li/graphite half-cells and the composition and morphology of the solid electrolyte interphase (SEI) formed on the graphite surface has been investigated for two electrolyte systems: I M LiPF 6 in ethylene carbonate/dimethyl carbonate EC/DMC (2:1) and 1 M LiBF 4 in EC/DMC (2:1). Precycled cells were stored at different temperatures up to 80°C, and the graphite electrodes were analyzed chemically (by X-ray photoelectron spectroscopy) and electrochemically under continued cycling. Loss of charge (for both salts) and of intercalation capacity (for LiBF 4 ) occurred after elevated temperature storage. The charge loss could be coupled to disappearance of the R-OCO 2 Li phase from the surface, with subsequent exposure of the graphite surface. The amount of LiF increased with increased storage temperature, but the LiF morphology differed between the two electrolyte systems. A model for the morphological changes of the SEI layer on storage at elevated temperature is proposed.

A Transmission Electron Microscopy Study of the Reactivity Mechanism of Tailor-Made CuO Particles toward Lithium

Abstract: The electrochemical reactivity of tailor-made CuO powders prepared according to a new low-temperature synthesis method was studied by a combination of transmission electron microscopy (TEM) and electrochemical techniques. All the processes involved during cycling were successfully identified. We show that the reduction mechanism of CuO by lithium involves the formation of a solid solution of Cu II 1x Cu I x O 1 1/2 :, 0 ≤ x ≤ 0.4, a phase transition into Cu 2 O, then the formation of Cu nanograins dispersed into a lithia matrix ( Li 2 O) followed by the growth of an organic-type coating. This one is responsible for the extra capacity observed on the voltage vs. composition curve. During the subsequent charge, the organic layer vanishes first, and then the Cu grains are partially or fully oxidized with a concomitant decomposition of Li 2 O. The formation of Li 2 O and Cu nanograins and then the one of Cu. CuO, and Cu 2 O nanograins on the first discharge and subsequent charge, respectively, were identified by high-resolution TEM studies. These results enabled a better understanding of the processes governing the reactivity of 3d metal oxides vs. lithium down to 0.02 V.

A Two-Dimensional, Two-Phase, Multicomponent, Transient Model for the Cathode of a Proton Exchange Membrane Fuel Cell Using Conventional Gas Distributors

Abstract: A two-dimensional, two-phase, multicomponent, transient model was developed for the cathode of the proton exchange membrane fuel cell. Gas transport was addressed by multicomponent diffusion equations while Darcy's law was adapted to account for the capillary flow of liquid water in the porous gas diffusion layer. The model was validated with experimental results and qualitative information on the effects of various operating conditions and design parameters and the transient phenomena upon imposing a cathodic overpotential were obtained. The performance of the cathode was found to be dominated by the dynamics of liquid water, especially in the high current density range. Conditions that promote faster liquid water removal such as temperature, dryness of the inlet gas stream, reduced diffusion layer thickness, and higher porosity improved the performance of the cathode. There seems to be an optimum in the diffusion layer thickness at the low current density range. The model results showed that for a fixed electrode width, a greater number of channels and shorter shoulder widths are preferred. The transient profiles clearly showed that liquid water transport is the slowest mass-transfer phenomenon in the cathode and is primarily responsible for mass-transfer restrictions especially over the shoulder.

Crystal Chemistry of the Olivine-Type Li ( Mn y Fe1 − y ) PO 4 and ( Mn y Fe1 − y ) PO 4 as Possible 4 V Cathode Materials for Lithium Batteries

Abstract: A potential 4 V cathode material for lithium batteries was investigated. The crystal chemistry of the olivine-type of Li(Mn y 2+ Fe 1-y 2+ )PO 4 (discharged state) and its delithiated form (Mn y 3+ Fe 1-y 3+ )PO 4 (charged state) were comparatively studied using X-ray diffraction, Mossbauer spectroscopy, and ab initio calculations. A strong oxidizer, nitronium tetrafluoroborate, NO 2 BF 4 , was used for chemical delithiation of Li(Mn y 2+ Fe 1-y 2+ )PO 4 to obtain (Mn y 3+ Fe 1-y 3+ )PO 4 . The strong electron/lattice interaction induced by the trivalent manganese (3d 4 ) in (Mn y 3+ Fe 1-y 3+ )PO 4 (charged state) is highlighted as the intrinsic obstacle to generating the full theoretical capacity (ca. 170 mAh/g) of the Mn-rich phase (y >0.8), followed by an efficient cathode performance of the optimized Li(Mn 0.6 Fe 0.4 )PO 4 .

Studies of Mg-substituted Li4-xMgxTi5O12 spinel electrodes (0 ≤ x ≤ 1) for lithium batteries

Abstract: Magnesium-substituted Li{sub 4-x}Mg{sub x}Ti{sub 5}O{sub 12} spinel electrodes (0

Oxidation of 4-chlorophenol at boron-doped diamond electrode for wastewater treatment

Abstract: The electrochem. behavior of synthetic B-doped diamond thin-film electrode (BDD) was studied in acid media contg. 4-chlorophenol (4-CP) by cyclic voltammetry, chronoamperometry, and bulk electrolysis. The results showed that in the potential region of supporting electrolyte stability occur reactions involving the oxidn. of 4-CP to phenoxy radical and 1,4-benzoquinone. Polymeric materials, which result in electrode fouling, are also formed in this potential region. Electrolysis at high anodic potentials, in the region of electrolyte decompn., complex oxidn. reactions can take place involving electro-generated hydroxyl radicals, leading to the complete incineration of 4-chlorophenol. Electrode fouling is inhibited under these conditions. The exptl. results were compared with a theor. model. This model is based on the assumption that the rate of the anodic oxidn. of 4-CP is a fast reaction. HPLC analyses revealed that the main intermediate products of 4-CP oxidn. were 1,4-benzoquinone, maleic acid, formic acid, and oxalic acid. [on SciFinder (R)]

Thermal Model of Cylindrical and Prismatic Lithium-Ion Cells

Abstract: Oven exposure testing is a standard benchmark that Li-ion cells must pass in order to he approved for sale by regulating bodies. In order to test the safety of new cell designs or electrode materials, manufacturers must make small test hatches of cells. This can be both costly and time consuming. Using reaction kinetics that have been developed for electrode materials with electrolyte exposed to high temperature, and thermal properties of cells from the literature, a predictive model for oven exposure testing has been developed. The model predictions are compared to oven exposure test results for E-One/Moli Energy, Canada, 18650 LiCoO 2 /graphite cells and shown to be in good agreement. The model can predict the response of new cell sizes and electrode materials to oven exposure testing without actually producing any cells. This is illustrated with a number of examples: (i) increasing the specific surface area of the graphite electrode; (ii) using LiMn 2 O 4 or other cathode substitutes instead of LiCoO 2 ; (iii) varying the diameter of cylindrical cells; and (iv) varying the thickness of prismatic cells.

Grain Boundary Blocking Effect in Zirconia: A Schottky Barrier Analysis

Abstract: The grain boundary electrical properties of high purity ZrO 2 ceramic materials doped with 2, 3, and 8 mol % Y 2 O 3 , and 8 mol % Y 2 O 3 co-doped with 0.4 mol % Al 2 O 3 were studied in the temperature range of 200 to 500°C by electrochemical techniques and were theoretically analyzed. Although the presence of a siliceous phase is shown to be a major cause for the grain boundary blocking effect, the grain boundary properties appear to he significantly influenced by space charges, particularly in materials of high purity. The oxygen vacancy distribution and the grain boundary resistivity were calculated for 8 mol % Y 2 O 3 doped ZrO 2 by assuming double Schottky barriers, and the results were compared with the experiment. It is shown that reasonable space charge potentials lead to grain boundary effects which are consistent with the experimental features. In contrary to the bulk in which defect associates prevail (at temperatures <560°C), in the boundary regions, association effects can be assumed to be much less pronounced due to the vacancy depletion.

Electrochemical and Infrared Studies of the Reduction of Organic Carbonates

Abstract: The reduction potentials of five organic carbonates commonly employed in lithium battery electrolytes, ethylene carbonate ~EC!, propylene carbonate ~PC!, diethyl carbonate ~DEC!, dimethyl carbonate ~DMC!, and vinylene carbonate ~VC! were determined by cyclic voltammetry using inert ~Au or glassy carbon! electrodes in tetrahydrofuran/LiClO4 supporting electrolyte. The reduction potentials for all five organic carbonates were above 1 V ~vs. Li/Li 1 !. PC reduction was observed to have a significant kinetic hindrance. The measured reduction potentials for EC, DEC, and PC were consistent with thermodynamic values calculated using density functional theory ~DFT! assuming one-electron reduction to the radical anion. The experimental values for VC and DMC were, however, much more positive than the calculated values, which we attribute to different reaction pathways. The role of VC as an additive in a PC-based electrolyte was investigated using conventional constant-current cycling combined with ex situ infrared spectroscopy and in situ atomic force microscopy ~AFM!. We confirmed stable cycling of a commercial li-ion battery carbon anode in a PC-based electrolyte with 5 mol % VC added. The preferential reduction of VC and the solid electrolyte interphase layer formation therefrom appears to inhibit PC cointercalation and subsequent graphite exfoliation.

Capacitance and Pore-Size Distribution in Aqueous and Nonaqueous Electrolytes Using Various Activated Carbon Electrodes

Abstract: Various kinds of activated carbon/activated carbon fibers were used in the evaluation of electrical double layer capacitors using the method of image analysis. The appropriate hydrated ion structures in an aqueous system of H 2 SO 4 /H 2 O and an organic system of LiClO 4 /polypropylene carbonate were calculated using the software Cerius 2 (ver. 3.8). The capacitance obtained varied with the electrolyte used, even though the capacitor material remained the same. The relationship between the pore size and the electrolyte ion diameter is discussed.

The Electrochemistry of Ni Pattern Anodes Used as Solid Oxide Fuel Cell Model Electrodes

Abstract: Ni pattern electrodes with well-defined triple phase boundary (TPB) lengths were prepared in order to investigate the reaction mechanisms at solid oxide fuel cell (SOFC) anodes The anode microstructures were stable during thermal treatment and electrochemical measurements Electrochemical impedance spectroscopy was used to study the influence of the overpotential of the gas atmosphere, of the temperature, and of the pattern geometry on the electrochemical behavior of SOFC anodes It is found that the reaction kinetics are dominated by one main process This process is thermally activated with an activation of energy of E A = 088 ± 004 eV At overpotentials higher than 300 mV, a second process becomes relevant The partial pressure of water in the fuel gas atmosphere has a catalytic effect on the anode performance, whereas variations of the patrial pressure of hydrogen in the fuel gas atmosphere have no significant influence on the electrode behavior A model was established in order to explain the catalytic effect of water The direct proportionality between the relaxation frequency and the TPB length suggests a TPB limitation of the anode kinetics

Nonflammable Trimethyl Phosphate Solvent-Containing Electrolytes for Lithium-Ion Batteries: I. Fundamental Properties

Abstract: To develop nonflammable electrolytes for lithium-ion batteries, the fundamental properties of trimethyl phosphate (TMP)-based electrolytes with LiPF 6 as solute were investigated for natural graphite anode and LiCoO 2 cathodes, It was found that the TMP solvent had good oxidation stability and poor reduction stability, which led to TMP reduction decomposition on the natural graphite electrode at the negative potential of 1.2 V. To solve this problem, ethylene carbonate (EC). propylene carbonate (PC), and diethyl carbonate (DEC) cosolvents were mixed with TMP solvent. As a result, the reduction decomposition of the TMP solvent was considerably suppressed in < 10% TMP containing EC + PC + TMP and <25% TMP containing EC + DEC + TMP electrolytes due to the formation of good solid electrolyte interphase film on natural graphite electrode in these two mixed electrolytes. The nonflammability of the TMP electrolyte declined with mixing flammable cosolvents, which was explained by a flame retarding mechanism involving a hydrogen radical trap in the gas phase. According to this mechanisms it was deduced that the cosolvents with high boiling point and fewer hydrogen atoms were promising for nonflammability of mixed electrolytes Furthermore, a thermal test disclosed that the thermal stability of lithium-ion cells may be improved by using TMP-containing electrolytes.

Electrochemical Characterization of Polyaniline in Nonaqueous Electrolyte and Its Evaluation as Electrode Material for Electrochemical Supercapacitors

Abstract: In this study, the performance of a polyaniline (PANI) based supercapacitor where electroactive PANI films were prepared on carbon paper electrodes from a nonaqueous solution with an organic acid (CF 3 COOH) as the proton source was investigated. The use of nonaqueous media as electrolyte led to an increase of the electroactivity window from 0.75 V in aqueous media up to 1.0 V. Low frequency capacitance, evaluated by electrochemical impedance spectroscopy, of about 150 F/g is reported. Scanning electron microscopy indicated a highly porous material for deposited charges greater than 1 C/cm 2 . Constant current charge/discharge cycling of a symmetric supercapacitor based on PANI in nonaqueous medium was performed in a two-electrode cell configuration and a loss of about 60% of the discharge capacity was demonstrated after 1000 cycles. Tetramethylammonium methanesulfonate (Me 4 NCF 3 SO 3 ) was also used instead of tetraethylammonium tetrafluoborate (Et 1 NBF 4 ) as supporting electrolyte in acetonitrile for the charge/discharge testing of the PANI-PANI capacitor. Energy and power densities of approximately 3.5 Wh/kg and 1300 W/kg, respectively, were developed by this supercapacitor for a cell voltage of I V and a discharge time of 20 s. On the other hand, an asymmetrical supercapacitor with polypyrrole and polyaniline used as positive and negative electrodes, respectively, displayed slightly improved performance. Indeed, an energy density of 5 Wh/kg and a power density of 1200 W/kg were reported for discharge time of about 20 s with 1 M Me 4 NCF 3 SO 3 /acetonitrile as electrolyte.

A High-Rate, High-Capacity, Nanostructured Sn-Based Anode Prepared Using Sol-Gel Template Synthesis

Abstract: Li-ion battery anodes derived from oxides of tin have recently received considerable interest because in principle they can store over twice as much Li as graphite. However, large volume changes occur when Li is inserted and removed from these Sn-based materials, and this causes internal damage to the electrode, resulting in loss of capacity and rechargability. We describe here a new nanostructured -based electrode that has extraordinary rate capabilities, can deliver very high capacities (e.g., >700 mAh at 8 C rate), and still retain the ability to be discharged and recharged through as many as 1400 cycles. These electrodes, prepared via the template method, consist of monodisperse 110 nm diam nanofibers protruding from a current-collector surface like the bristles of a brush. © 2001 The Electrochemical Society. All rights reserved.

Conditions for Fabrication of Ideally Ordered Anodic Porous Alumina Using Pretextured Al

Abstract: The conditions for the fabrication of ideally ordered anodic porous alumina with a high aspect ratio were examined using pretextured Al in oxalic acid solution. The obtained anodic porous alumina has a defect-free array of straight parallel channels perpendicular to the surface. The channel interval could be controlled by changing the interval of the pretextured pattern and the applied voltage. However, the depth at which perfect ordering could be maintained depended on the anodizing conditions, that is, the hole array with a high aspect ratio could be obtained only under the appropriate anodizing voltage, which corresponded to that of the long-range ordering conditions in the oxalic acid solution. Under the most appropriate condition, ideally ordered channels with an aspect ratio of over 500 could be obtained. From these results, it was concluded that the long-range ordering conditions significantly influenced the growth of channels in anodic porous alumina even in/on the pretextured Al.

Model for Polymer Electrolyte Fuel Cell Operation on Reformate Feed: Effects of CO, H;2 Dilution, and High Fuel Utilization

Abstract: We describe a polymer electrolyte fuel cell model emphasizing operation on hydrocarbon reformate, i.e., the anode feed stream consists of dry H 2 concentrations as low as 40%, inlet CO levels of 10-100 ppm, and hydrogen fuel utilization as high as 90%. Refinements of interfacial kinetics equations used in our previous work on CO effects in H 2 anodes have yielded a better quantitative fit to the measured dependence of voltage loss on inlet CO level [in Electrode Materials and Processes for Energy Conversion and Storage, J. McBreen, S. Mukerjee, and S. Srinivasan, Editors, PV 97-13, pp. 15-24, The Electrochemical Society Proceedings Series, Pennington, NJ (1997)]. We calculate anode potential losses by coupling such interfacial kinetic processes to reactant diffusion limitations and ionic resistance in the catalyst layer, and by accounting for the drop in local hydrogen concentration along the flow channel due to significant fuel utilization. As a result of internal readjustment of cell overpotentials when hydrogen concentration drops along the flow channel, we show that loss of current, or power, under the realistic condition of constant cell voltage is smaller than loss of current at constant anode potential. We show that voltage losses associated with CO poisoning are significantly amplified with diluted hydrogen feed streams and particularly so under high fuel utilization. We make projections on improvements required, qualitative and quantitative, in the physical parameters of the anode catalyst surface chemistry to significantly improve CO tolerance.

A Novel Method of Etching Copper Oxide Using Acetic Acid

Abstract: The removal of copper oxide using acetic acid at low temperatures was investigated. Acetic acid removes a variety of copper oxides, including cuprous oxide, cupric oxide, and cupric hydroxide without attacking the underlying copper film. The removal of these oxides was determined by X-ray photoelectron spectroscopy. Acetic acid can tolerate up to 4 vol % water dilution without hindering the oxide removal while producing an oxide-free surface. However, if a deionized water rinse is performed after an acetic acid treatment, a surface film of cupric hydroxide forms immediately. An acetic acid treatment at 35°C without a water rinse removes the native copper oxide and produces an oxide-free, streak-free copper surface.

LiCoO2 Cathode Material That Does Not Show a Phase Transition from Hexagonal to Monoclinic Phase

Abstract: Structural instability of LiCoO 2 can be improved by sol-gel coating of Al 2 O 3 and subsequent heat-treatments. While Al 2 O 3 phase does not exist after heat-treatments, solid solution LiCo 1-x Al x O 2 that has discretely higher Al concentration was formed at the surface up to ∼500 A inside the particle. However, heat-treatment to 700°C results in the presence of the solid solution beyond ∼500 A. The different Al concentration at the surface significantly affects the structural stability of the materials during cycling, and those prepared at 400°C do not show a phase transition from hexagonal to monoclinic phase. Disappearance of such a phase transition improves capacity retention of the cathode. Moreover, cathodes prepared at 400 and 500°C show improved layered characteristics with cation order.

AC Impedance Spectroscopy in Characterizing Time-Dependent Corrosion of AZ91 and AM50 Magnesium Alloys Characterization with Respect to Their Microstructures

Abstract: The corrosion behavior of as-cast magnesium alloys (AM50, AZ91, and AZ91Si) was investigated in a 0.1 M sodium sulfate solution at the corrosion potential (E corr ) using electrochemical impedance spectroscopy. Transmission electron microscopy was used to analyze the corrosion product layer, and phase shifting interferometric microscopy was carried out to characterize the reactivity of intermetallic particles. Due to its microstructure, the AM50 alloy presented uniform corrosion during immersion, whereas corrosion of the AZ91 alloys began in the grain body and progressively spread to the eutectic areas. For the AZ91 alloys, the dissolution of the α-eutectic phase led to a strong aluminum enrichment of the corrosion product layer and, when a threshold was reached in the level of Al 2 O 3 in the magnesium oxide (or hydroxide) layer a change of phenomenology occurred in the impedance diagrams. In addition, electrochemical results revealed that an increase of silicon concentration for the AZ91 alloys decreased the corrosion resistance, This was attributed to an increase of the number of Mg 2 Si particles, accelerating the dissolution n of eutectic areas.

Characterization of Corrosion Interfaces by the Scanning Kelvin Probe Force Microscopy Technique

Abstract: A variety of interfaces relevant to corrosion processes were examined by the scanning Kelvin probe force microscopy ~SKPFM! technique in order to study the influences of various parameters on the measured potential. SKPFM measurements performed on AA2024-T3 after solution exposure showed that surface composition is not the only parameter that controls the Volta potential difference, which is measured by SKPFM. The influence of surface oxide structure and adsorption at the oxide surface can be probed by SKPFM and lateral potential gradients can be observed in the absence of significant differences in oxide composition. The influence of tip-sample separation distance on the measured Volta potential difference was studied for different pure oxidecovered metals. SKPFM measurements were made in air on pure Ni and Pt samples withdrawn from solution at open circuit or under potential control. The Volta potential difference was found to be composed of a transient component that slowly discharged and a more permanent component associated with the charge of adsorbed species. The Volta potential difference transients measured on the samples emersed under potential control decayed much slower than the open-circuit potential transient measured in solution upon release of the potential control. These different measurements validate the use of SKPFM for the prediction of

Evaluation of a Sol-Gel Derived Nafion/Silica Hybrid Membrane for Polymer Electrolyte Membrane Fuel Cell Applications: II. Methanol Uptake and Methanol Permeability

Abstract: Sol-gel derived Nafion/silica hybrid membranes were investigated as a potential polymer electrolyte for direct methanol fuel cell applications. Methanol uptake and methanol permeability were measured in liquid and vapor phase as a function of temperature, methanol vapor activity, and silica content. Decreased methanol uptake from liquid methanol was observed in the hybrid membranes with silica contents of 10 and 21 wt %. The hybrid membrane with silica content of ≈20 wt % showed a significant lower methanol permeation rate when immersed in a liquid methanol-water mixture at 25 and 80°C. Methanol uptake from the vapor phase by the hybrid membranes appears similar to that of unmodified Nafion. Methanol diffusion coefficients, as determined from sorption experiments, were slightly lower in the hybrid membranes than in unmodified Nafion. However, in direct permeation experiments, significantly lower methanol vapor permeability was seen only in the hybrid membrane with silica content of ≈20 wt %. Based on these results, Nafion/silica hybrid membranes with high silica content have potential as electrolytes for direct methanol fuel cells operating either on liquid or vapor-feed fuels. © 2001 The Electrochemical Society. All rights reserved.

Evaluation of a Sol-Gel Derived Nafion/Silica Hybrid Membrane for Proton Electrolyte Membrane Fuel Cell Applications: I. Proton Conductivity and Water Content

Abstract: Sol-gel derived Nafion®/silica hybrid membranes were investigated as a potential polymer electrolyte for fuel cell applications. Membrane proton conductivity and water content were measured as a function of temperature, water vapor activity, and silica content. The hybrid membranes have a higher water content at 25 and 120°C, but not at 150 and 170°C. Despite the higher water content, the proton conductivities in the hybrid membranes are lower than, or equal to, that in unmodified Nafion membranes under all conditions investigated. The proton conductivity of the hybrid membrane decreases with increasing silica content under all conditions. © 2001 The Electrochemical Society. All rights reserved.