Showing papers in "Journal of The Electrochemical Society in 2003"

Electrochemical Characteristics and Impedance Spectroscopy Studies of Carbon-Carbon Supercapacitors

Abstract: This paper presents the results obtained on the electrochemical behavior of electrochemical capacitors assembled in nonaqueous electrolyte. The first part is devoted to the electrochemical characterization of carbon-carbon 4 cm2 cells systems in terms of capacitance, resistance, and cyclability. The second part is focused on the electrochemical impedance spectroscopy study of the cells. Nyquist plots are presented and the impedance of the supercapacitors is discussed in terms of complex capacitance and complex power. This allows the determination of a relaxation time constant of the systems, and the real and the imaginary part of the complex power vs. the frequency plots give information on the supercapacitor cells frequency behavior. The complex impedance plots for both a supercapacitor and a tantalum dielectric capacitor cells are compared. © 2003 The Electrochemical Society. All rights reserved.

Electrogeneration of Hydroxyl Radicals on Boron-Doped Diamond Electrodes

Abstract: The electrogeneration of hydroxyl radicals was studied at a synthetic B-doped diamond (BDD) thin film electrode. Spin trapping was used for detection of hydroxyl radicals with 5,5-dimethyl-1-pyrroline-N-oxide and with salicylic acid using ESR and liq. chromatog. measurements, resp. The prodn. of H2O2 and competitive oxidn. of formic and oxalic acids were also studied using bulk electrolysis. Oxidn. of salicylic acid gives hydroxylated products (2,3- and 2,5-dihydroxybenzoic acids). The oxidn. process on BDD electrodes involves hydroxyl radicals as electrogenerated intermediates. [on SciFinder (R)]

Dendrite Growth in Lithium/Polymer Systems A Propagation Model for Liquid Electrolytes under Galvanostatic Conditions

Abstract: Dendrite growth in a parallel-electrode lithium/polymer cell during galvanostatic charging has been modeled. The growth model is surface-energy controlled, incorporating the effect of dendrite tip curvature into its dendrite growth kinetics. Using data representative of the oxymethylene-linked poly(ethylene oxide)/LiTFSI system, it is shown that dendrites accelerate across cells under all conditions, and that growth is always slowed by lowering the current density. Cell shorting occurs during typical charges at current densities above 75% of the limiting current. Increased interelectrode distance slows failure, but the advantages decrease as distance lengthens. A factor of 1000 increase in surface forces delays cell failure by only 6%. While larger diffusion coefficients usually extend the time to cell failure, this trend is not consistent at high transference numbers. © 2003 The Electrochemical Society. All rights reserved.

Effect of Particle Size on Lithium Intercalation into α ­ Fe2 O 3

Abstract: The electrochemical reaction of lithium with crystallized -Fe2O3 (hematite) has been studied by means of in situ X-ray diffraction. When reacting large particles (~0.5 µm), we observed the well-known transformation of the close-packed anionic array from hexagonal (hc) to cubic (ccp) stacking. At the early stage of the reduction, a very small amount of lithium (xc<0.1 Li/Fe2O3) can be inserted before this structural transformation occurs. Nanosize -Fe2O3 made of fine monolithic particles (200 A) behaves very different, since up to one Li per formula unit (-Li1Fe2O3,xc = 1) can be inserted in the corundum structure without phase transformation. To our knowledge, this is the first time this phase is maintained for such large xc values. This cationic insertion was found to come with a small cell volume expansion evaluated to 1%. Unsuccessful attempts to increase the xc values on large particles by decreasing the applied discharge current density suggest that the particle size is the only parameter involved. The better structural reversibility of this monophasic process compared to the biphasic one was confirmed by electrochemical cycling tests conducted with hematite samples of various particle sizes. Therefore, by using nanosize particles, we can drastically increase the critical Li concentration required to observe the hcccp transition. This work demonstrates that a careful control of the texture/particle size of electrochemically active oxide particles is likely an important variable that has been largely disregarded for such properties. ©2002 The Electrochemical Society. All rights reserved.

Kinetic Model of Platinum Dissolution in PEMFCs

Abstract: This paper presents a mathematical model of oxidation and dissolution of supported platinum catalysts in polymer electrolyte membrane fuel cells (PEMFCs). Kinetic expressions for the oxidation and dissolution reactions are developed and compared to available experimental data. The model is used to investigate the influences of electrode potential and particle size on catalyst stability. © 2003 The Electrochemical Society. All rights reserved.

Reaction of Li with Alloy Thin Films Studied by In Situ AFM

Abstract: Many intermetallic materials deliver poor capacity retention when cycled vs Li Many authors have attributed this poor capacity retention to large volume expansions of the active material Here we report the volume changes of continuous and patterned films of crystalline Al, crystalline Sn, amorphous Si (a-Si), and a-Si 0 6 4 Sn 0 3 6 as they reversibly react with Li measured by in situ atomic force microscopy (AFM) Although these materials all undergo large volume expansions, the amorphous phases undergo reversible shape and volume changes The crystalline materials do not We attribute this difference to the homogeneous expansion and contraction that occurs in the amorphous materials Inhomogeneous expansion occurs in the crystalline materials due to the presence of coexisting phases with different Li concentrations Thin films of a-Si and a-Si 0 6 4 Sn 0 3 6 show good capacity retention with cycle number

The CO Poisoning Effect in PEMFCs Operational at Temperatures up to 200°C

Abstract: The CO poisoning effect on carbon-supported platinum catalysts (at a loading of 0.5 mg Pt/cm 2 per electrode) in polymer electrolyte membrane fuel cells (PEMFCs) has been investigated in a temperature range from 125 to 200°C with the phosphoric acid-doped polybenzimidazole membranes as electrolyte. The effect is very temperature-dependent and can be sufficiently suppressed at elevated temperature. By defining the CO tolerance as a voltage loss less than 10 mV, it is evaluated that 3% CO in hydrogen can be tolerated at current densities up to 0.8 A/cm 2 at 200°C, while at 125°C 0.1% CO in hydrogen can be tolerated at current densities lower than 0.3 A/cm 2 . For comparison, the tolerance is only 0.0025% CO (25 ppm) at 80°C at current densities up to 0.2 A/cm 2 . The relative anode activity for hydrogen oxidation was calculated as a function of the CO concentration and temperature. The effect of CO 2 in hydrogen was also studied. At 175°C, 25% CO 2 in the fuel stream showed only the dilution effect.

Plastic-Compatible Low Resistance Printable Gold Nanoparticle Conductors for Flexible Electronics

Abstract: Low resistance conductors are crucial for the development of ultra-low-cost electronic systems such as radio frequency identification tags. Low resistance conductors are required to enable the fabrication of high- Q inductors, capacitors, tuned circuits, and interconnects. The fabrication of these circuits by printing will enable a dramatic reduction in cost, through the elimination of lithography, vacuum processing, and the need for high-cost substrates. Solutions of organic-encapsulated gold nanoparticles many be printed and subsequently annealed to form low resistance conductor patterns. We describe a process to form the same, and discuss the optimization of the process to demonstrate plastic-compatible gold conductors for the first time. By optimizing both the size of the nanoparticle and the length of the alkanethiol encapsulant, it is possible to produce particles that anneal at low temperatures (,150°C) to form continuous gold films having low resistivity. We demonstrate the printing of these materials using

Rechargeable Lithium Sulfur Battery I. Structural Change of Sulfur Cathode During Discharge and Charge

Abstract: In this paper, the structural change of the sulfur cathode during the electrochemical reaction of a lithium sulfur battery employing 0.5 M -tetra(ethylene glycol) dimethyl ether (TEGDME) was studied by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), and wave dispersive spectroscopy (WDS). The discharge process of the lithium sulfur cell could be divided in the first discharge region (2.4-2.1 V) where the reduction of elemental sulfur to form soluble polysulfides and further reduction of the soluble polysulfide occur, and the second discharge region (2.1-1.5 V) where the soluble polysulfides are reduced to form a nonuniform solid film covered over the carbon matrix. It was also found that the charge of lithium sulfur cell leads to the conversion from to the soluble polysulfide, resulting in the removal of layer formed on carbon matrix. However, the oxidization of the soluble polysulfide to solid sulfur hardly occurs and few are left on carbon matrix even at 100% depth of charge. © 2003 The Electrochemical Society. All rights reserved.

Oxygen Transport Properties of Organic Electrolytes and Performance of Lithium/Oxygen Battery

Abstract: The oxygen transport properties of several organic electrolytes were characterized through measurements of oxygen solubility and electrolyte viscosity. Oxygen diffusion coefficients were calculated from electrolyte viscosities using the Stokes-Einstein relation. Oxygen solubility, electrolyte viscosity, and oxygen partial pressure were all directly correlated to discharge capacity and rate capability. Substantial improvement in cell performance was achieved through electrolyte optimization and increased oxygen partial pressure. The concentration of oxygen in the electrode under discharge was calculated using a semi-infinite medium model with simultaneous diffusion and reaction. The model was used to explain the dependence of cell performance on oxygen transport in organic electrolyte. © 2003 The Electrochemical Society. All rights reserved.

Selection and Evaluation of Heat-Resistant Alloys for SOFC Interconnect Applications

Abstract: Over the past several years, the steady reduction in SOFC operating temperatures to the intermediate range of 700~850oC [1] has made it feasible for lanthanum chromite to be supplanted by metals or alloys as the interconnect materials. Compared to doped lanthanum chromite, metals or alloys offer significantly lower raw material and fabrication costs. However, to be a durable and reliable, a metal or alloy has to satisfy several functional requirements specific to the interconnect under SOFC operating conditions. Specifically, the interconnect metal or alloy should possess the following properties: (i) Good surface stability (resistance to oxidation, hot corrosion, and carburization) in both cathodic (air) and anodic (fuel) atmospheres; (ii) Thermal expansion matching to the ceramic PEN (positive cathode-electrolyte-negative anode) and seal materials (as least for a rigid seal design); (iii) High electrical conductivity through both the bulk material and in-situ formed oxide scales; (iv) Bulk and interfacial thermal mechanical reliability and durability at the operating temperature; (v) Compatibility with other materials in contact with interconnects such as seals and electrical contact materials.

Carbon Metal Fluoride Nanocomposites High-Capacity Reversible Metal Fluoride Conversion Materials as Rechargeable Positive Electrodes for Li Batteries

Abstract: The structure and electrochemistry of FeF 3 :C-based carbon metal fluoride nanocomposites (CMFNCs) was investigated in detail from 4.5 to 1.5 V, revealing a reversible metal fluoride conversion process. These are the first reported examples of a high-capacity reversible conversion process for positive electrodes. A reversible specific capacity of approximately 600 mAh/g of CMFNCs was realized at 70°C. Approximately one-third of the capacity evolved in a reaction between 3.5 and 2.8 V related to the cathodic reduction reaction of Fe 3+ to Fe 2+ . The remainder of the specific capacity occurred in a two-phase conversion reaction at 2 V resulting in the formation of a finer Fe:LiF nanocomposite. Upon oxidation, selective area electron diffraction characterization revealed the reformation of a metal fluoride. Evidence presented suggested that the metal fluoride is related to FeF 2 in structure. A pseudocapacitive reaction is proposed as a possible mechanism for the subsequent Fe 2+ → Fe 3+ oxidation reaction. Preliminary results of FeF 2 , NiF 2 , and CoF 2 CMFNCs were used in the discussion of the electrochemical properties of the reconverted metal fluoride.

Rechargeable Lithium Sulfur Battery II. Rate Capability and Cycle Characteristics

Abstract: This paper reports on the investigation of rate capability and cycle characteristics of a lithium sulfur battery. The second discharge region where solid Li 2 S is formed on the surface of the carbon matrix in the cathode was highly sensitive to cathode thickness and discharge rate. The scanning electron microscope (SEM) observation suggests that thick Li 2 S layer formed at the surface of the cathode causes the diminution of the second discharge region at high discharge rate. Upon repeated cycle, the delocalization of the surface Li 2 S layer happened, however, the irreversible Li 2 S gradually increased with cycle as evidence by (SEM) and wave dispersive spectroscopy measurements, causing capacity fade. The formation of the irreversible Li 2 S was more significant for higher rates of discharge. It is believed that the destruction of the carbon matrix by stress generated during the localized deposition of Li 2 S is responsible for the formation of irreversible Li 2 S.

Carbon-Metal Fluoride Nanocomposites Structure and Electrochemistry of FeF3: C

Abstract: The practical electroactivity of electrically insulating iron fluoride was enabled through the use of carbon-metal fluoride nanocomposites (CMFNCs). The nanocomposites were fabricated through the use of high energy mechanical milling and resulted in nanodomains of FeF, on the order of 1-20 nm encompassed in a matrix of carbon as characterized by transmission electron microscopy and X-ray diffraction (XRD) Electrochemical characterization of CMFNCs composed of 85/15 wt % FeF 3 /C resulted in a nanocomposite specific capacity as high as 200 mAh/g (235 mAh/(g of FeF 3 ) with the electrochemical activity associated with the Fe 3+ → Fe 2+ occurring in the region of 2.8-3.5 V. The CMFNCs revealed encouraging rate capability and cycle life with <10% fade after 50 cycles. Structural evolution during the first lithiation reaction was investigated with the use of ex situ and in situ XRD. Initial results suggest that x from 0 to 0.5 in Li x FeF 3 proceeds in a two-phase reaction resulting in a phase with significant redistribution of the Fe atoms within a structure very similar to the base FeF 3 . FeF 3 -based CMFNCs also exhibited a very high specific capacity of 600 mAh/g at 70°C due to a reversible reaction at approximately 2 V.

Analysis of Electrochemical and Thermal Behavior of Li-Ion Cells

Abstract: This paper seeks to gain a better understanding of the thermal behavior of Li-ion cells using a previously developed twodimensional, first principles-based thermal-electrochemical modeling approach. The model incorporates the reversible, irreversible, and ohmic heats in the matrix and solution phases, and the temperature dependence of the various transport, kinetic, and mass-transfer parameters based on Arrhenius expressions. Experimental data on the entropic contribution for the manganese oxide spinal and carbon electrodes, recently published in the literature, are also incorporated into the model in order to gauge the importance of this term in the overall heat generation. Simulations were used to estimate the thermal and electrical energy and the active material utilization at various rates in order to understand the effect of temperature on the electrochemistry and vice versa. In addition, the methodology of using experimental data, instead of an electrochemical model, to determine the heat-generation rate is examined by considering the differences between the local and lumped thermal models, and the assumption of using heat generation rate determined at a particular thermal environment under other conditions. Model simulations are used to gain insight

A 3D, Multiphase, Multicomponent Model of the Cathode and Anode of a PEM Fuel Cell

Abstract: A computational fluid dynamics multiphase model of a proton-exchange membrane ~PEM! fuel cell is presented. The model accounts for three-dimensional transport processes including phase change and heat transfer, and includes the gas-diffusion layers ~GDL! and gas flow channels for both anode and cathode, as well as a cooling channel. Transport of liquid water inside the gas-diffusion layers is modeled using viscous forces and capillary pressure terms. The physics of phase change is accounted for by prescribing local evaporation as a function of the undersaturation and liquid water concentration. Simulations have been performed for fully humidified gases entering the cell. The results show that different competing mechanisms lead to phase change at both anode and cathode sides of the fuel cell. The predicted amount of liquid water depends strongly on the prescribed material properties, particularly the hydraulic permeability of the GDL. Analysis of the simulations at a current density of 1.2 A/cm 2 show that both condensation and evaporation take place within the cathode GDL, whereas condensation prevails throughout the anode, except near the inlet. The three-dimensional distribution of the reactants and products is evident, particularly under the land areas. For the conditions investigated in this paper, the liquid water saturation does not exceed 10% at either anode or cathode side, and increases nonlinearly with current density. The operation of proton-exchange membrane ~PEM! fuel cells depends not only on the effective distribution of air and hydrogen, but also on the maintenance of an adequate cell operating temperature and fully humidified conditions in the membrane. The fully humidified state of the membrane is crucial to ensuring good ionic conductivity and is achieved by judicious water management. Water content is determined by the balance between various water transport mechanisms and water production. The water transport mechanisms are electro-osmotic drag of water ~i.e., motion of water molecules attaching to protons migrating through the membrane from anode to cathode!; back diffusion from the cathode ~due to nonuniform concentration!; and diffusion and convection to/from the air and hydrogen gas streams. Water production depends on the electric current density and phase change. Without control, an imbalance between production and removal rates of water can occur. This can result in either dehydration of the membrane, or flooding of the electrodes, which are both detrimental to performance. A common water management technique relies on the humidification of the air and hydrogen gas streams. At higher current densities, the excess product water is removed by convection via the air stream, and the rate of removal is controlled by adjusting moisture content in concert with pressure drop and temperature in the flow channels. Thermal management is also required to remove the heat produced by the electrochemical reaction in order to prevent drying out of the membrane, which in turn can result not only in reduced performance but also in eventual rupture of the membrane. Thermal management, which is performed via forced convection cooling in larger stacks, is also essential for the control of the water evaporation or condensation rates. The operation of a fuel cell and the resulting water and heat distributions depend on numerous transport phenomena including charge-transport and multicomponent, multiphase flow, and heat transfer in porous media. The complexity and interaction of these processes and the difficulty in making detailed in situ measurements have prompted the development of a number of numerical models. The theoretical framework was laid out in early one-dimensional numerical models of the membrane-electrode. 1-3 A quasi-twodimensional model based on concentrated solution theory was also proposed by Newman and Fuller, 4 and a full two-dimensional model including flow channels but no electrodes was also presented by Nguyen and White. 5 This model was refined in a number of subsequent studies to account for the porous electrodes and interdigitated

Formation of Titania Nanotubes and Applications for Dye-Sensitized Solar Cells

Abstract: Highly efficient dye-sensitized solar cells were produced using single-crystalline TiO 2 nanotubes as a thin-film semiconductor because of the very high electron transfer through single-crystalline TiO 2 nanotubes when compared to that through nanoporous TiO 2 films composed of nanoparticles. The dye-sensitized solar cells with single-crystalline TiO 2 nanotubes showed more than double the short-circuit current density than those made of titania nanoparticles Degussa P-25 in the thin-film thickness region. Titania nanotubes were synthesized using molecular assemblies composed of surfactant molecules, i.e., laurylamine hydrochloride, and titanium alkoxide, i.e., tetraisopropylorthotitanate modified with acetylacetone, as a template. They have outer and inner diameters of about 10 and 5 nm, respectively, a length in the range from 30 nm to several hundred nanometers, and have a single-crystalline structure of anatase, as confirmed on lattice images observed by high-resolution transmission electron microscopy. The light to electricity conversion of the titania nanotube cells was around 5%. They also showed the highest photocatalytic activity when compared to the commercially available nanocrystalline titania.

Rigorous 3-d mathematical modeling of PEM fuel cells - II. Model predictions with liquid water transport

Abstract: In this part of the paper, we present a model to treat formation and transport of liquid water in proton exchange membrane ~PEM! fuel cells ~FCs! in three-dimensional ~3-D! geometry. The performance of modern-day PEM FCs at high current density are largely dictated by the effective management of liquid water. In the first part of this paper, a rigorous model was presented to model PEM FCs using a computational fluid dynamic technique. It was found that under the assumption of no liquid water formation, the model consistently overpredicted measured polarization behavior. In the model presented here, the phase change process is modeled as an equilibrium process, while the transport of liquid water is governed by pressure, surface tension, gravity and electro-osmotic drag. Results show that the inclusion of liquid water transport greatly enhances the predictive capability of the model and is necessary to match experimental data at high current density.

Combined XRD, EXAFS, and Mössbauer Studies of the Reduction by Lithium of α ­ Fe2 O 3 with Various Particle Sizes

Abstract: The electrochemical reduction of hematite with various particle sizes by metallic lithium has been studied by means of X-ray diffraction (XRD) Mossbauer and extended X-ray absorption fine structure (EXAFS) spectroscopy. Previous in situ XRD analysis coupled with electrochemical data showed that lithium can be inserted in the nanosized sample up to 1 Li per Fe2O3 whereas bulk material undergoes an irreversible Li-driven transformation from an hexagonal anionic packing to a close cubic packed framework as soon as 0.03 Li is inserted in the corundum structure. The present data show that only 0.6 Li per formula unit are actually inserted in the structure of small particles. The remaining lithium (0.4) is engaged in irreversible reduction of surface groups, or capacitive behavior. Beyond the solid solution domains, both samples are multiphase, and consist of Li2Fe2O3, Fe0 clusters (10-15 A) and inserted -Fe2O3, which proportions are used to calculate the mean iron oxidation state in the electrode as the reaction proceeds. From these data, we found that electrolyte decomposition can occur at very different steps of the reduction depending on the texture of the active materials. In addition, during the reduction process, we evidenced a reaction of disproportionation (3Fe2+2Fe3+ + Fe0), an intense electrochemical grinding of the hematite particles and the formation of extremely fine metallic surface clusters. For the first time, the EXAFS/X-ray absorption near-edge structure signature of the divalent intermediate Li2Fe2O3 phase is obtained.

Mathematical Modeling of Liquid-Feed Direct Methanol Fuel Cells

Abstract: A two-phase, multicomponent model has been developed for liquid-feed direct methanol fuel cells ~DMFC!. In addition to the anode and cathode electrochemical reactions, the model considers diffusion and convection of both gas and liquid phases in the backing layers and flow channels. In particular, the model fully accounts for the mixed potential effect of methanol oxidation at the cathode as a result of methanol crossover caused by diffusion, convection, and electro-osmosis. This comprehensive model is solved numerically using computational fluid dynamics. The transport phenomena and electrochemical kinetics in a liquid-feed DMFC are delineated and the effects of the methanol feed concentration on cell performance are explored. The model is validated against DMFC experimental data with reasonable agreement. The void fraction at the anode outlet is found to be as high as 95% at a cell current density of 0.45 A/cm 2 for a7c m longchannel. Increase in methanol feed concentration leads to a slight decrease in cell voltage and a proportional increase in the mass-transport limiting current density for a methanol concentration below 1 M. However, when the methanol feed concentration is larger than 2 M, the cell voltage is greatly reduced by excessive methanol crossover and the maximum current density begins to be limited by the oxygen supply at the cathode. The oxygen depletion results from excessive parasitic oxygen consumption by methanol crossing over.

Fuel Composition and Diluent Effect on Gas Transport and Performance of Anode-Supported SOFCs

Abstract: Anode-supported solid oxide fuel cells (SOFCs) with Ni+yttria-stabilized zirconia (YSZ) anode, YSZ-samaria-doped ceria (SDC) bilayer electrolyte, and Sr-doped LaCoO 3 (LSC)+SDC cathode were fabricated. Fuel used consistedof H 2 diluted with He. N 2 , H 2 O, or CO 2 , mixtures of H 2 and CO, and mixtures of CO and CO 2 . Cell performance was measured at 800°C with the above-mentioned fuel gas mixtures and air as oxidant. For a given concentration of the diluent, cell performance was higher with He as the diluent than with N 2 as the diluent. Mass transport through porous Ni-YSZ anode for H 2 -H 2 O, CO-CO 2 binary systems. and H 2 -H 2 O-diluent gas ternary systems was analyzed using multicomponent gas diffusion theory. At high concentrations of diluent, the maximum achievable current density was limited by the anodic concentration polarization. From this measured limiting current density, the corresponding effective gas diffusivity was estimated. Highest effective diffusivity was estimated for fuel gas mixtures containing H 2 -H 2 O-He mixtures (∼0.55 cm 2 /s), and the lowest for CO-CO 2 mixtures (∼0.07 cm 2 /s). The lowest performance was observed with CO-CO 2 mixture as a fuel, which in part was attributed to the lowest effective diffusivity of the fuels tested and higher activation polarization.

Application of Low-Viscosity Ionic Liquid to the Electrolyte of Double-Layer Capacitors

Abstract: The performance of a double-layer capacitor (DLC) composed of activated carbon electrodes and 1--ethyl-3-methylimidazolium fluoride (EMIF).2.3HF, which has extremely high conductivity with low viscosity, was examined and compared with those using the popular ionic liquid EMIBF 4 , conventional aqueous electrolyte 35 wt % H 2 SO 4 , and nonaqueous electrolyte 1 M Et 3 MeNBF 4 /propylene carbonate. The DLC using EMIF.2.3HF showed an intermediate capacitance and internal resistance between the aqueous and nonaqueous electrolyte systems due to its intermediate double-layer capacitance and electrolytic conductivity. EMIF.2.3HF afforded much higher capacitance than EMIBF 4 even at low temperatures, however, it had a lower decomposition voltage (∼2 V), resulting in lower energy density. The capacitance of EMIF.2.3HF was extremely dependent on the applied voltage.

Thermal modeling of porous insertion electrodes

Abstract: Isothermal calorimetry was performed on Li|LiPF 6 in ethylene carbonate:dimethyl carbonate|LiAl 0.2 Mn 1.8 O 4-δ F 0.2 cells. The measured rate of heat generation varied substantially with time. To understand why, we investigated the entropy, irreversible resistance, and heats of mixing. Two methods for computing the heat of mixing, one computational and one analytic, are derived. We demonstrate how the energy balance of Rao and Newman accounts for heat of mixing across electrodes, but neglects heat of mixing within particles and in the electrolyte, which may be of equal magnitude. In general, the magnitude of the heat of mixing, which is the amount of heat released during relaxation after interruption of the current, will be small in materials with transport properties sufficiently high to provide acceptable battery performance, with the possible exception of heat of mixing within the insertion particles if the particle radius is large. Comparing simulations of heat generation to calorimetry measurements reseals that the entropic heat is significant and accounts for much of the variation of the rate of heat generation. The rate of irreversible heat generation is larger when the open-circuit potential varies steeply with lithium concentration, because of diffusion limitations within the solid.

Liquid and Polymer Gel Electrolytes for Lithium Batteries Composed of Room-Temperature Molten Salt Doped by Lithium Salt

Abstract: The lithium ion binary room-temperature molten salt (i.e., ionic liquid), LiEMIBF 4 was prepared by mixing 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF 4 ) with LiBF 4 . The ionic conductivity of LiEMIBF 4 was 7.4 mS cm -1 at 20°C and lower than that of EMIBF 4 . A solidified LiEMIBF 4 , named GLiEMIBF 4 , was prepared by in situ polymerization of poly (ethyleneglycol) diacrylate with LiEMIBF 4 . The ionic conductivity of the homogeneous transparent membrane obtained was smaller than that of LiEMIBF 4 . The thermal decomposition temperatures of these ionic media measured by thermogravimetry-differential thermal analysis showed that LiEMIBF 4 and GLiEMIBF 4 have high thermal stability around 300°C. The cathodic limit of EMIBF 4 was ca. 1.1 V vs. Li/Li + measured by linear sweep voltammetry. To test the possibility of use of these ionic media for lithium-ion batteries, demonstration cells of Li[Li 1/3 Ti 5/3 ]O 4 /LiEMIBF 4 or GLiEMIBF 4 /LiCoO 2 were assembled. The capacity retention after 50 cycles was 93.8% of the initial capacity in the LiEMIBF 4 cell. Discharge potential profile of the GLiEMIBF 4 cell showed decline probably due to the concentration polarization in the gelled electrolyte. Liquid and gelled electrolytes composed of lithium ion coexisting room-temperature molten salt are shown to function as nonflammable electrolytes in the lithium-ion batteries.

Electrochemical Insertion Properties of the Novel Lithium Vanadium Fluorophosphate, LiVPO4 F

Abstract: The novel fluorophosphate compound, LiVPO 4 F, PI, a = 5.173(8) A, b = 5.309(6) A, c = 7.250(3) A, a = 72.479(4)°, β = 107.767(7)°, γ = 81.375(7)°, has been synthesized by a novel two-step reaction method based on a carbothermal reduction (CTR) process. In the initial CTR step, vanadium pentoxide, V 2 O 5 , ammonium dihydrogen phosphate, and a high surface area carbon are reacted under an inert atmosphere to yield the trivalent vanadium phosphate, VPO 4 . The transition-metal reduction is facilitated by the high temperature carbothermal reaction based on the C → CO transition. These CTR conditions favor stabilization of the vanadium as V 3+ as well as leaving residual carbon, which is useful in the subsequent electrode processing. In the second incorporation step, the CTR VPO 4 is reacted with LiF in an argon atmosphere to yield the single phase LiVPO 4 F product. Preliminary electrochemical evaluation of the LiVPO 4 F carried out at 23°C indicates a reversible specific capacity of around 115 mAh/g, a performance roughly equivalent to cycling of x = 0.74 in Li 1-x VPO 4 F. Elevated temperature testing suggests that the extraction process may yield the novel delithiated phase, VPO 4 F. High resolution measurements reveal a structured voltage response for the lithium extraction process characterized by two well-defined peaks in the differential capacity data. The corresponding discharge process, centered at around 4.19 V vs. Li, indicates a two-phase reaction mechanism coupled to phase nucleation behavior. The insertion properties of the LiVPO 4 F are compared with the other vanadium-based polyanion materials, namely Li 3 V 2 (PO 4 ) 3 and VOPO 4 . The demonstrated performance suggests that the LiVPO 4 F insertion system may offer some properties favorable for commercial application.

Synthesis and Characterization of MnO2-Based Mixed Oxides as Supercapacitors

Abstract: Mn/Pb and Mn/Ni mixed oxide were prepared at ambient temperature by reduction of KMnO4 with Mn, Pb, and Ni salts. This low-temperature approach provides amorphous structure of the active material. The specific capacitance of pure MnO 2 was estimated to be 166 F/g and increased to 210 and 185 F/g for Mn/Ni and Mn/Pb oxides, respectively. The carbon loading was optimized at 20 wt %. Based on a single electrode, the Mn/Ni mixed oxide showed a high rate capability of 3.12 Wh/kg at constant power discharge of 1 kW/kg.

Oxidation of ZrB2-Based Ceramics in Dry Air

Abstract: The oxidation behavior of three different ZrB 2 -based ceramics, a monolithic ZrB 2 and two ZrB 2 -SiC composites, was studied up to 1350°C and under isothermal conditions at 1120°C for 20 h. Oxidation kinetics and microstructural changes in the oxidized specimens indicate that the monolithic zrB 2 has low thermal stability. On the contrary, the introduction of SiC particles markedly improves oxidation resistance due to the formation of an adherent and protective borosilicate glass layer that coats the sample surface, effectively limiting the inward diffusion of oxygen toward the reaction interface. The influence of the secondary grain boundary phases on oxidation is discussed.

Li Insertion into Li4Ti5 O 12 (Spinel) Charge Capability vs. Particle Size in Thin-Film Electrodes

Abstract: Reference LPI-ARTICLE-2003-015doi:10.1149/1.1581262View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Ionic Conductivity of PEMFC Electrodes Effect of Nafion Loading

Abstract: The effect of Nafion loading in the cathode catalyst layer of proton exchange membrane fuel cell (PEMFC) electrodes was studied by impedance spectroscopy, cyclic voltammetry, and polarization experiments. Catalyst utilization. determined by cyclic voltammetry, peaked at 76% for a Nafion loading of ca. 30 mass %, and this coincides with the optimum performance obtained in H 2 /O 2 fuel cells. However, the small range of utilizations observed (55-76%) cannot explain the wide range of performances. The impedance results show that the ionic conductivity of the cathode increased greatly with increasing Nafion content, and this is the main factor responsible for the increase in performance up to 30% Nafion. The loss of performance at higher Nafion loadings must have been due to an increasing oxygen transport resistance, because the electronic resistance did not increase significantly. In fact, the highest electronic resistances were observed at low Nafion loadings, indicating that Nafion played a significant role as a binder.

Material characterization and electrochemical performance of hydrous manganese oxide electrodes for use in electrochemical pseudocapacitors

Abstract: Hydrous manganese oxide with promising pseudocapacitive behavior was deposited on a carbon substrate at anodic potentials of 0.5-0.95 V vs. saturated calomel electrode ~SCE! in 0.25 M Mn(CH3COO)2 solution at 25°C. The effects of the deposition potential on the material characteristics and electrochemical performances of the hydrous manganese oxide prepared were investigated. Porous manganese oxide with higher crystallinity was formed at a lower deposition potential. When the deposition potential was 0.8 VSCE or higher, the deposited oxide consisted of an inner layer with a laminated structure and a rough outer layer with nodules on the surface. X-ray photoelectron spectroscopy was also carried out to examine the chemical state of the deposited oxide. Analytical results indicated that the oxide was composed of both trivalent and tetravalent manganese oxides at a deposition potential of 0.5 VSCE. However, the tetravalent manganese oxide became the dominant species in the film deposited at above 0.65 VSCE. The manganese oxide formed at 0.5 VSCE exhibited a specific capacitance as high as 240 F/g, as evaluated by cyclic voltammetry ~CV! with a potential scan rate of 5 mV/s in 2 M KCl at 25°C. Increasing the CV scan rate reduced the specific capacitance. Only about 70% of the capacitance at 5 mV/s could be maintained when the CV scan rate was increased to 100 mV/s, for all the manganese oxide electrodes prepared. Moreover, a high deposition potential gave rise to a low specific capacitance of the manganese oxide formed.