Showing papers in "Journal of The Electrochemical Society in 1997"

Phospho‐olivines as Positive‐Electrode Materials for Rechargeable Lithium Batteries

Abstract: Reversible extraction of lithium from LiFePO 4 (triphylite) and insertion of lithium into FePO 4 at 3.5 V vs. lithium at 0.05 mA/cm 2 shows this material to be an excellent candidate for the cathode of a low-power, rechargeable lithium battery that is inexpensive, nontoxic, and environmentally benign. Electrochemical extraction was limited to ∼0.6 Li/formula unit; but even with this restriction the specific capacity is 100 to 110 mAh/g. Complete extraction of lithium was performed chemically; it gave a new phase, FePO 4 , isostructural with heterosite, Fe 0.65 Mn 0.35 PO 4 . The FePO 4 framework of the ordered olivine LiFePO 4 is retained with minor displacive adjustments. Nevertheless the insertion/extraction reaction proceeds via a two-phase process, and a reversible loss in capacity with increasing current density appears to be associated with a diffusion-limited transfer of lithium across the two-phase interface. Electrochemical extraction of lithium from isostructural LiMPO 4 (M = Mn, Co, or Ni) with an LiClO 4 electrolyte was not possible; but successful extraction of lithium from LiFe 1-x Mn x PO 4 was accomplished with maximum oxidation of the Mn 3+ /Mn 2+ occurring at x = 0.5. The Fe 3+ /Fe 2+ couple was oxidized first at 3.5 V followed by oxidation of the Mn 3+ /Mn 2+ couple at 4.1 V vs. lithium. The Fe 3+ -O-Mn 2+ interactions appear to destabilize the Mn 2+ level and stabilize the Fe 2+ level so as to make the Mn 3+ /Mn 2+ energy accessible.

Electrochemical and In Situ X‐Ray Diffraction Studies of the Reaction of Lithium with Tin Oxide Composites

Abstract: We report our electrochemical and in situ x‐ray diffraction experiments on a variety of tin oxide based compounds; SnO, , , and glass, as cathodes opposite lithium metal in a rechargeable Li‐ion coin cell. These materials demonstrate discharge capacities on the order of 1000 mAh/(g Sn), which is consistent with the alloying capacity limit of 4.4 Li atoms per Sn atom, or 991 mAh/(g Sn). These materials also demonstrate significant irreversible capacities ranging from 200 mAh/(g active) to 700 mAh/(g active). In situ x‐ray diffraction experiments on these materials show that by introducing lithium, lithium oxide and tin form first, which is then followed by the formation of the various Li‐Sn alloy phases. When lithium is removed the original material does not reform. The ending composition is metallic tin, presumably mixed with amorphous lithium oxide. The oxygen from the tin oxide in the starting material bonds irreversibly with lithium to form an amorphous matrix. The Li‐Sn alloying process is quite reversible; perhaps due to the formation of this lithia "matrix" which helps to keep the electrode particles mechanically connected together.

Effect of Structure on the Fe3 + / Fe2 + Redox Couple in Iron Phosphates

Abstract: To understand the role of structure on the position of the octahedral redox couple in compounds having the same polyanions, four iron phosphates: , and were investigated. They vary in structure as well as in the manner in which the octahedral iron atoms are linked to each other. The redox couple in the above compounds lies at 2.8, 2.9, 3.1, and 3.5 eV, respectively, below the Fermi level of lithium. The reason for the difference in the position of the redox couples is related to changes in the P‒O bond lengths as well as to changes in the crystalline electric field at the iron sites.

Synthesis and Electrochemistry of LiNi x Mn2 − x O 4

Abstract: has been synthesized using sol‐gel and solid‐state methods for 0 < x < 0.5. The electrochemical behavior of the samples was studied in coin‐type cells. When x = 0, the capacity of cells appears at 4.1 V. As x increases, the capacity of the 4.1 V plateau decreases as 1−2x Li per formula unit, and a new plateau at 4.7 V appears. The capacity of the 4.7 V plateau increases as 2x Li per formula unit, so that the total capacity of the samples (both the 4.1 and 4.7 V plateaus) is constant. This is taken as evidence that the oxidation state of Ni in these samples is +2, and therefore they can be written as . The 4.1 V plateau is related to the oxidation of to and the 4.7 V plateau to the oxidation of to . The effect of synthesis temperature, atmosphere, and cooling rate on the structure and electrochemical properties of is also studied on samples made by the sol‐gel method. samples made by heating gels at temperatures below 600°C in air are generally oxygen deficient, leading to Mn oxidation states significantly less than 4. samples heated above 650°C suffer due to disproportionation into with x < 0.5 and with z ≈ 0.2, which occurs above about 650°C. Pure materials can be made by extended heatings near 600°C or by slowly cooling materials heated at higher temperatures. made at 600°C has demonstrated good reversible capacity at 4.7 V in excess of 100 mAh/g for tens of cycles.

Self‐Ordering of Cell Arrangement of Anodic Porous Alumina Formed in Sulfuric Acid Solution

Abstract: Self-ordering of the cell arrangement of the porous structure of anodic alumina has been studied in a sulfuric acid solution. Ordering of the cell arrangement was dependent on the applied potential, and a highly ordered structure was obtained under anodization at a constant potential of 25 to 27 V. Self-ordering of the porous structure proceeded with the growth of the oxide layer under anodization at an appropriate potential, and a porous film with an almost ideal hexagonal honeycomb structure was formed over an area of several micrometers after a long period of anodization.

A Stable Thin‐Film Lithium Electrolyte: Lithium Phosphorus Oxynitride

Abstract: The electrochemical and optical properties of lithium phosphorus oxynitride (Lipon) thin films have been studied with an emphasis on the stability window vs. lithium metal and the behavior of the Li/Lipon interface. Impedance measurements made between {minus}26 and 140 C show that Lipon exhibits a single, Li{sup +}-ion conducting phase with an average conductivity of 2.3 ({+-}0.7) {times} 10{sup {minus}6} S/cm at 25 C and an average activation energy of E{sub a} = 0.55 {+-} 0.02 eV. No detectable reaction or degradation was evident at the Li/Lipon interface, and linear sweep voltammetry measurements on three-electrode cells indicated that Lipon is stable from 0 to about 5.5 V with respect to a Li{sup +}/Li reference. The complex refractive index of Lipon was measured by spectroscopic ellipsometry. Optical bandgaps of 3.45 and 3.75 eV were obtained from the ellipsometry data and from optical absorption measurements, respectively.

Advanced Model for Solid Electrolyte Interphase Electrodes in Liquid and Polymer Electrolytes

Abstract: Recent studies show that the SEI on lithium and on anodes in liquid nonaqueous solutions consists of many different materials including , LiF, LiCl, , alkoxides, and nonconducting polymers. The equivalent circuit for such a mosaic‐type SEI electrode is extremely complex. It is shown that near room temperature the grain‐boundary resistance (Rgb) for polyparticle solid electrolytes is larger than the bulk ionic resistance. Up to now, all models of SEI electrodes ignored the contribution of Rgb to the overall SEI resistance. We show here that this neglect has no justification. On the basis of recent results, we propose here for SEI electrodes equivalent circuits which take into account the contribution of grain‐boundary and other interfacial impedance terms. This model accounts for a variety of different types of Nyquist plots reported for lithium and electrodes in liquid nonaqueous and polymer electrolytes.

An Iodine/Triiodide Reduction Electrocatalyst for Aqueous and Organic Media

Abstract: Reference LPI-ARTICLE-1997-026View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Local Dissolution Phenomena Associated with S Phase ( Al2CuMg ) Particles in Aluminum Alloy 2024‐T3

Abstract: Second-phase particles in Al-4.4Cu-15Mg-0.6Mn (2024-T3) were characterized by size and chemistry using scanning electron microscopy and associated electron-beam microanalysis methods. It was found that approximately 60% of particles greater than about 0.5 to 0.7 {micro}m were Al{sub 2}CuMg (the S phase). This fraction corresponded to 2.7% of the total surface area. S phase particles appeared to be active with respect to the matrix phase, consistent with open-circuit potentials reported in the literature for Al{sub 2}CuMg. The compound exhibited severe dealloying which resulted in the formation of Cu-rich particle remnants. Some particle remnants remained largely intact and induced pitting at their periphery once ennobled by dealloying. Other particle remnants decomposed into 10 to 100 nm Cu clusters that became detached from the alloy surface and were dispersed by mechanical action of growing corrosion product or solution movement. This observation suggests that nonfaradaic liberation of Cu from corroding 2024-T3 surfaces is possible, and provides one plausible explanation for how Cu can be redistributed across the surface by a pitting process which occurs at potentials that are hundreds of millivolts negative of the reduction potential for Cu.

Capacity Fading on Cycling of 4 V Li / LiMn2 O 4 Cells

Abstract: The cycle-life behavior of a Li/1 M-LiPF 6 + EC/DMC(1:2 by volume)/LiMn 2 O 4 cell was investigated at various temperatures (0, 25, and 50°C). The capacity fades faster on cycling at high rather than low temperatures. The mechanisms responsible for the capacity fading of the spinel LiMn 2 O 4 during cycling were extensively investigated by chemical analysis of the dissolved Mn in combination with in situ x-ray diffraction, Rietveld analysis, and ac impedance techniques. Chemical analytical results indicated that the capacity loss caused by the simple dissolution of Mn 3+ accounted for only 23 and 34% of the overall capacity losses cycling at room temperature and 50°C, respectively. In situ x-ray diffraction results showed that the two-phase structure coexisting in the high-voltage region persists during lithium-ion insertion/extraction at low temperatures during cycling. By contrast, this two-phase structure was effectively transformed to a more stable, one-phase structure, accompanied by the dissolution of Mn and the loss of oxygen (e.g., Mn 2 O 3 .MnO) at the high temperature; this dominated the overall capacity-loss process. AC impedance spectra revealed that the capacity loss at the high temperature was also due in part to the decomposition of electrolyte solution at the electrode.

Key Factors Controlling the Reversibility of the Reaction of Lithium with SnO2 and Sn2 BPO 6 Glass

Abstract: Tin oxide composite glasses represent a new class of material for the anode of Li-ion cells. Using results of experiments on Li/Sn{sub 2}BPO{sub 6} and Li/SnO{sub 2} cells, the authors identify those factors which are responsible for good charge-discharge capacity retention. First, the grains (those regions which diffract coherently) which make up the particles of the material should be as small as possible. Then, regions of tin which form are kept small and two-phase coexistence regions between bulk Li-Sn alloys of different composition do not occur. The Sn{sub 2}BPO{sub 6} glass represents the smallest grains possible. Second, the particles themselves should be small so that they can each be well contacted by carbon black during electrode manufacture. Third, the voltage range of cycling must be selected so that the tin atoms do not aggregate into large regions which grow in size. This aggregation is evidenced by the growth of peaks in the differential capacity vs. voltage as a function of cycle number. The peaks represent the coexistence between bulk Li-Sn alloy phases which have substantially different volumes. The coexistence is thought to cause fracturing and loss of contact between the grains. Therefore, materials with small particles, small grains, and smoothmore » sloping voltage profiles which do not change with cycle number (as indicated by a stable differential capacity) give the best cycling performance. The selection of the voltage limits for cycling strongly influences the stability of the voltage profile (as illustrated here), so this must be done with much care.« less

The room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate : Electrochemical couples and physical properties

Abstract: Room temperature molten salts composed of the 1-ethyl-3-methylimidazolium cation and a chloroaluminate anion have received much attention for use in a variety of commercial applications such as batteries, photovoltaics, metal deposition, and capacitors. The room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF{sub 4}) was demonstrated as a versatile electrolyte by examining three representative electrochemical couples: ferrocene and tetrathiafulvalene oxidations and lithium ion reduction. Square-wave voltammetric data for ferrocene oxidation were fit to a reversible one-electron process using the COOL algorithm to give a half-wave potential of 0.490 V vs. Al/Al(III) and a diffusion coefficient of 5.1 {times} 10{sup {minus}7} cm{sup 2}/s. The two-electron oxidation of tetrathiafulvalene was reversible and proceeded through two consecutive one-electron steps; although data collected at lower square-wave frequencies indicated a slow precipitation of the TTF{sup +} species. Lithium ion was reduced to lithium metal at a Pt electrode following the addition of water to the EMIBF{sub 4} electrolyte, whereas lithium ion reduction at an Al wire produced the {beta}-LiAl alloy. Conductivities and kinematic viscosities of EMIBF{sub 4} were measured from 20 to 100 C and had values of 14 mS/cm and 0.275 cm{sup 2}/s, respectively, at 25 C.

Kinetics of oxygen reduction on Pt(hkl) electrodes : Implications for the crystallite size effect with supported Pt electrocatalysts

Abstract: The kinetics of the oxygen reduction reaction (ORR) on Pt(hkl) surfaces is found to vary with crystal face in a different manner depending on the electrolyte. In perchloric acid, the variation in activity at 0.8 to 0.9 V is relatively small between the three low index faces, with activity increasing in the order (100) < (110) ≈ (111). A similar structure sensitivity was observed in KOH, increasing in the order (100) < (110) < (111), but with larger differences. In sulfuric acid, the variations in activity with crystal face were much larger, with the difference between the most active and the least active being about two orders of magnitude, and increased in the opposite order (111) << (100) < (110). The variations in activity with crystal face in perchloric acid and KOH arise from the structure sensitive inhibiting effect of , i.e., a strongly inhibiting effect on (100) and smaller effects on (110) and (111). The variations in activity with crystal face in sulfuric acid arise from highly structure specific adsorption of sulfate/bisulfate anions in this electrolyte, which has a strongly inhibiting effect on the (111) surface. The crystallite size effect for the ORR reported for supported Pt catalysts in sulfuric acid at ambient temperature is fully explained by applying our single crystal results to classical models of the variation in particle shape with size.

The Effect of Porous Composite Electrode Structure on Solid Oxide Fuel Cell Performance I. Theoretical Analysis

Abstract: The effect of porous composite electrodes on the overall charge-transfer process in solid-state devices, such as solid oxide fuel cells, is theoretically examined by taking into account various parameters such as electrolyte thickness, intrinsic charge-transfer resistance, electrode thickness, and porosity. A model is presented that accounts for ionic transport within the electrolyte, electronic conduction through electrocatalyst, and charge-transfer at the electrolyte-electrocatalyst interface. Diffusion of gaseous species in porous electrodes is assumed to be rapid so as not to be rate limiting. The conduction of electrons in the electrocatalyst is assumed to introduce negligible resistance. The activation overpotential as a function of current density is assumed to be ohmic, and an effective charge-transfer resistance is defined. The transport equations are solved numerically in two dimensions using a finite difference technique and analytically in one dimension. The analysis predicts that the use of composite electrodes in devices employing solid electrolytes can significantly increase performance under conditions where the intrinsic charge-transfer resistance is high in comparison to the area-specific resistance of the electrolyte. The results indicate a low effective charge-transfer resistance is obtained for relatively thick electrodes with a fine microstructure as long as the porosity is sufficient to ensure negligible concentration polarization.

Operating Proton Exchange Membrane Fuel Cells Without External Humidification of the Reactant Gases Fundamental Aspects

Abstract: Operation of proton exchange membrane fuel cells (PEMFC) without external humidification of the reactant gases is advantageous for the PEMFC system, because it eliminates the need of a gas-humidification subsystem. The gas-humidification subsystem is a burden in the fuel cell system with respect to weight, complexity, cost, and parasitic power. A model for the operation of PEMFC with internal humidification of the gases is presented and the range of operating conditions for a PEMFC using dry H 2 /air was investigated. The model predicts that dry air, entering at the cathode, can be fully internally humidified by the water produced by the electrochemical reaction at temperatures up to 70°C. This model was experimentally verified for cell temperatures up to 60°C by long-term operation of a PEMFC with dry gases for up to 1800 h. The current densities, obtained at 0.6 V, were 20 to 40% lower than those measured when both gases were humidified. The water distribution in the cell, while operating with dry gases, was investigated by measuring the amount of product water on the anode and cathode sides. It was found that the back-diffusion of product water to the anode is the dominant process for water management in the cell over a wide range of operating conditions. The dominating water back-diffusion also allows internal humidification of the hydrogen reactant and prevents drying out of the anode.

Electronic Conductivity of LiCoO2 and Its Enhancement by Magnesium Doping

Abstract: LiCoO{sub 2} the active cathode material in commercial rechargeable lithium batteries, is shown to be a p-type semiconductor, associated with the presence of a small concentration of CO{sup 4+} ions. Its conductivity at room temperature can be increased by over two orders of magnitude, to {approximately}0.5 S/cm, by partial substitution of CO{sup 3+} by Mg{sup 2+} and compensating hole creation. The electrochemical performance of LiMg{sub 0.05} Co{sub 0.95}O{sub 2} is comparable to that of LiCoO{sub 2}; a small reduction in capacity, associated with a reduction in Co{sup 3+} content, occurs but good reversibility is retained and, in contrast to LiCoO{sub 2}, the Mg-doped material is single phase throughout the charge/discharge cycle.

Mapping of Transition Metal Redox Energies in Phosphates with NASICON Structure by Lithium Intercalation

Abstract: The position of several transition metal redox energies with respect to the Fermi energy of lithium in phosphates with sodium super ionic conductor (NASICON) framework were determined electrochemically upon lithium intercalation. The materials studied were , and all having NASICON framework. The positions of the redox couples were found centered at the following energies: at 3.8 eV, at 2.8 eV, at 2.5 eV, at 2.2 eV, at 1.8 eV, and at 1.7 eV below the Fermi energy of lithium. Several of these redox couples were found to overlap each other. Some of these materials look promising as positive electrode materials for ‐ion batteries.

Low Cost Electrodes for Proton Exchange Membrane Fuel Cells: Performance in Single Cells and Ballard Stacks

Abstract: For widespread exploitation of proton exchange membrane fuel cells (PEMFCs) the cost of the stack must be reduced, and the performance per unit volume increased. Significant cost reduction has been achieved by the development of a high-volume, low cost, electrode manufacturing process and from reductions in the electrode precious metal catalyst loadings. The performance of membrane electrode assemblies (MEAs) employing printed cathodes ({le}0.6 mg Pt/cm{sup 2}) and anodes ({le}0.25 mg Pt/cm{sup 2}, 0.12 mg Ru/cm{sup 2}) in Ballard Mark V single-cell and advanced-stack hardware are at least comparable to current stack MEAs comprising high loading unsupported platinum black electrodes containing 4.0 mg Pt/cm{sup 2}. Optimum cell performance has provided high power densities of 0.42 W/cm{sup 2} at 0.7 V. Furthermore, under motive and utility test conditions, the low-cost electrodes show minimal loss in performance after over 3,000 h of stack operation and, in short and full sized stacks, the cell-to-cell reproducibility is excellent, highlighting the high consistency of product available from the electrode manufacturing process. Incorporation of the low cost electrodes in commercial PEMFC stacks is anticipated in the near future.

An Electrochemical Route for Making Porous Nickel Oxide Electrochemical Capacitors

Abstract: Porous nickel oxide films were prepared by electrochemically precipitating nickel hydroxide and heating the hydroxide in air at 300 C. The resulting nickel oxide films behave as an electrochemical capacitor with a specific capacitance of 59 F/g electrode material. These nickel oxide films maintain high utilization at high rates of discharge (i.e., high power density) and have excellent cycle life. Porous cobalt oxide films were also synthesized. Although the specific capacitances of these films are approximately one-fifth that of the nickel oxide films, the results demonstrate the versatility of fabricating a wide range of porous metal oxide films using this electrochemical route for use in capacitor applications. Electrochemical capacitors have generated wide interest in recent years for use in high power applications (e.g., in a hybrid electric vehicle, where they are expected to work in conjunction with a conventional battery).

Ionic Liquid‐Polymer Gel Electrolytes

Abstract: New rubbery gel electrolytes have been prepared from room temperature ionic liquids and poly(vinylidene fluoride)‐hexafluoropropylene copolymer [PVdF(HFP)]. The ionic liquids employed in these preparations were 1‐ethyl‐3‐methylimidazolium salts of triflate and . When properly processed, the ionic liquid‐PVdF(HFP) gels are freestanding, flexible films with room temperature conductivities ranging from 1.1 to . Because both the ionic liquids and the PVdF(HFP) are nonvolatile and are thermally stable, the gels can be operated at elevated temperatures without performance degradation. An ionic conductivity of was measured for a triflate ionic liquid‐PVdF(HFP) gel at 205°C.

Heat‐Generation Rate and General Energy Balance for Insertion Battery Systems

Abstract: A general energy balance has been developed for insertion battery systems by using enthalpy potentials. This leads to a new calculation method for the heat‐generation rate. The same result is also derived from an alternative model based on local heat generation in an electrochemical cell. A new concept, the effective open‐circuit potential of an insertion battery, was proposed to characterize the open‐circuit state of the battery during galvanostatic discharge. Simulation results are presented for heat generation in a lithium cell under galvanostatic discharge. The analysis of these results focuses on effects of the shape of the open‐circuit potential and ohmic losses in the electrolyte in the porous cathode. It is shown that a single reaction may look like two reactions due to the presence of two plateaus in the open‐circuit potential.

Oxidation of Ti3SiC2 in air

Abstract: Polycrystalline samples of Ti{sub 3}SiC{sub 2} were oxidized in air in the 900 to 1,400 C temperature range. The oxidation was parabolic with parabolic rate constants, k{sub p}, that increased from 1 {times} 10{sup {minus}9} to 1 {times} 10{sup {minus}4} kg{sup 2}/m{sup 4}s as the temperature increased from 900 to 1,400 C, respectively, which yielded an activation energy of 370 {+-} 20 kJ/mol. The scale that forms was dense, adhesive, resistant to thermal cyclings and layered. The outer layer was pure TiO{sub 2} (rutile), and the inner layer consisted of mixture of SiO{sub 2} and TiO{sub 2}. The results are consistent with a model in which growth of the oxide layer occurs by the inward diffusion of oxygen and the simultaneous outward diffusion of titanium and carbon. The presence of small volume fractions ({approx} 2%) of TiC{sub x} in Ti{sub 3}SiC{sub 2} were found to have a deleterious effect on the oxidation kinetics.

Oxidation of hydrogen on Ni/yttria-stabilized zirconia cermet anodes

Abstract: The oxidation of hydrogen on Ni/yttria-stabilized zirconia (Ni/YSZ) is studied by impedance spectroscopy. The active thickness obtained is 20 {micro}m or less. Conditions such as temperature, anodic overvoltage, electrode potential, H{sub 2} and H{sub 2}O partial pressure are varied. Three distinct arcs are identified in impedance spectra, representing at least three rate-limiting processes. One equivalent circuit of the type LR(RQ)(RQ)(RQ), where Q = Y{sub o}(j{omega}){sup n}, is used to describe all recorded impedance in the temperature range 850 to 1,000 C. The n-values are held constant, allowing a direct comparison of R and Y{sub o} values for different structures and conditions. The high-frequency arc (1 to 50 kHz) is sensitive to the cermet structure (particle size) and relatively insensitive to atmospheric composition and overvoltage. The related imperfect capacitance is suggested to be interpreted as a double-layer capacitance in the Ni/YSZ interface. The medium- (10 Hz to 1 kHz) and low-frequency arc (0.1 to 10 Hz) are sensitive to atmospheric composition and overvoltage. Both reaction resistances change their dependency on H{sub 2} partial pressure around 0.5 atm. The perfect capacitance related to the low-frequency arc is in the order of 0.5 to 2.5 F/cm{sup 2}, indicating an absorbed charged species rathermore » than surface adsorption.« less

Differential Capacitance Measurements in Solvent-Free Ionic Liquids at Hg and C Interfaces

Abstract: Series-stacked, double-layer carbon capacitors are slated to be used in electric vehicles for power management as well as in consumer electronics for memory backup and burst power. Nonaqueous electrolytes are preferred over aqueous electrolytes, since a wider voltage window can be accessed in the former electrolytes, thereby requiring fewer cells in the series stack. However, it has historically been difficult to assess whether the organic solvent and/or the supporting electrolyte determine the anodic limit. We have eliminated this ambiguity by using solvent-free ionic liquids where the source of anodic oxidation may be ascribed to the anion alone. Even though the new ionic liquids manifested high oxidation limits, we found that when used in practical capacitors comprising high-surface-area carbon cloth electrodes, a much lower capacitance (compared to smooth electrodes) was achieved. To understand whether the observed decrease in capacitance might be due to the microporosity of the carbon cloth electrode or to practical limitation of the device itself. we first measured the differential capacitance (C dl ) at a Hg/1-ethyl-3-methyl imidazolium imide. The integral capacitance at the Hg interface was then calculated and compared with that of a smooth glassy carbon electrode, a carbon yarn, and a cloth electrode. In addition, the effect of (CF 3 SO 2 ) 3 C - , (CF 3 SO 2 ) 2 N - , CF 3 SO 3 - , an BF 4 - on C dl were interpreted based on existing theories of double-layer structure.

Reduced-Temperature Solid Oxide Fuel Cell Based on YSZ Thin-Film Electrolyte

Abstract: A planar thin-film solid oxide fuel cell has been fabricated with an inexpensive, scalable, technique involving colloidal deposition of yttria-stabilized zirconia (YSZ) films on porous NiO-YSZ substrates, yielding solid oxide fuel cells capable of exceptional power density at operating temperatures of 700 to 800°C. The thickness of the YSZ film deposited onto the porous substrate is approximately 10 Rim after sintering, and is well bonded to the NiO/YSZ substrate. Ni-YSZ/YSZ/LSM cells built with this technique have exhibited theoretical open-circuit potentials (OCPs), high current densities, and exceptionally good power densities of over 1900 mW/cm 2 at 800°C. Electrochemical characterization of the cells indicates negligible losses across the Ni-YSZ/YSZ interface and minor polarization of the fuel electrode. Thinfilm cells have been tested for long periods of time (over 700 h) and have been thermally cycled from 650 to 800°C while demonstrating excellent stability over time.

The Limitations of Energy Density for Electrochemical Capacitors

Abstract: Electrochemical capacitors can be divided into two types depending on whether the salt concentration in the electrolyte changes during charging and discharging. In the first type of capacitor, such as double-layer capacitors, the salt concentration in the electrolyte reduces during the charging of the capacitor. The maximum energy density of this type of capacitor will depend not only on the specific capacitance and the operating voltage, but also on the salt concentration of the electrolyte. In this paper, a formula describing the dependence of energy density on specific capacitance, operating voltage, and salt concentration is given based on the optimized weight (or volume) ratio of the electrode material and the electrolyte. It shows that for electrochemical capacitors using nonaqueous electrolytes, the maximum energy density of the capacitor will be limited mainly by the low salt concentrations of the electrolyte. The relationship between the energy density and the mass density of the electrode is also given. The optimum mass density of the electrode can be obtained based on the value of the theoretical energy density for capacitors with different electrolytes. In the second type of capacitor, such as pseudocapacitors with metal oxide electrodes, the salt concentration in the electrolyte remains constant during charging and discharging. The maximum energy density of this type of capacitor will be limited mainly by specific capacitance and operating voltage.

Revised Pourbaix Diagrams for Copper at 25 to 300°C

Abstract: The Pourbaix diagrams (potential-pH diagrams) for copper at 25 to 300°C have been revised. Extrapolation of thermochemical data to elevated temperatures has been performed with the revised model of Helgeson-Kirkham-Flowers, which also allows uncharged aqueous complexes, such as CuOH(aq) and Cu(OH) 2 (aq), to be handled. Calculated high temperature thermodynamic data have been fitted against experimental data at elevated temperature. The hydrolysis of copper(I) and (II) is included with two and four hydroxide complexes, respectively. The Pourbaix diagrams show that the oxides (Cu 2 O(cr) and CuO(cr)) are stable at 25 to 200°C at 10 -6 mol kg -1 of dissoived species, and at 300°C only CuO is stable. The oxides are stable at 25°C at 10 -6 mol kg -1 , but at 100 to 300°C no solid compound is stable.

Electrolyte Effects on Spinel Dissolution and Cathodic Capacity Losses in 4 V Li / Li x Mn2 O 4 Rechargeable Cells

Abstract: Spinel dissolution and cathodic capacity losses in 4 V Li/Li{sub x}Mn{sub 2}O{sub 4} secondary cells were examined in various electrolyte solutions comprising different solvents and Li salts. It was found that spinel dissolution is induced by acids that are generated as a result of electrochemical oxidation of solvent molecules on composite cathodes. Among various organic solvents, ethers such as tetrahydrofuran and dimethoxyethane were readily oxidized to produce acids whereas carbonates (ethylene carbonate, propylene carbonate, diethylcarbonate) were relatively inert. Consequently, when a spinel-loaded composite cathode was charge/discharge cycled in the potential range of 3.6 to 4.3 V (vs Li/Li{sup +}), both the acid concentration and the extent of spinel dissolution was much higher in the ether-containing electrolytes as compared to the carbonates. The results, obtained from the chemical analysis on acid-attacked spinel powders and from the open-circuit potential measurement of composite cathodes, indicated that Li and Mn ion extraction is dominant in the earlier stage of acid attack. As the spinel dissolution further continues, however, oxygen losses from the lattice become more important. The combined feature of solvent oxidation and spinel dissolution was also affected by the nature of lithium salts added. Generally, the solvent-derived acid generation was not significant inmore » those electrolytes containing fluorinated salts (LiPF{sub 6}, LiBF{sub 4}, and LiAsF{sub 6}), yet the spinel dissolution in these electrolytes was still appreciable because acids were generated via another pathway; a reaction between the f-containing anions and impurity water.« less

Oxygen Exchange and Diffusion Coefficients of Strontium‐Doped Lanthanum Ferrites by Electrical Conductivity Relaxation

Abstract: Electrical conductivity relaxation experiments were performed on thin specimens of La1–xSrxFeO3–delta (x = 0.1, 0.4) at oxygen partial pressures pO2 = 10–5 – 1 bar in the temperature range 923 to 1223 K. The transient response of the electrical conductivity after a sudden change of the ambient oxygen partial pressure was analyzed in the frequency domain. The latter procedure allowed diffusion-limited and surface exchange-limited kinetics of re-equilibration to be distinguished. The response of specimens with thicknesses of 350 to 460 µm indicated diffusion-controlled kinetics at pO2 > 0.03 bar. The chemical diffusion coefficients, D-tilde , were found invariant with oxygen pressure. At 1073 K the absolute values were D-tilde = 6.5 × 10–6 cm2 s–1 for x = 0.1 and D-tilde = 1.1 × 10–5 cm2 s–1 for x = 0.4, with activation energies of about 80 kJ/mol. The equilibration process was governed by surface exchange at pO2 O[sub 2] n , where n = 0.65 to 0.85. This pressure dependency was interpreted in terms of a slow surface process involving an oxygen molecule and a surface oxygen vacancy, and causes the observed sharp transition from diffusion- to exchange-controlled kinetics. The activation energy of kO was estimated at 110 to 135 kJ/mol.

Selective and Blanket Electroless Copper Deposition for Ultralarge Scale Integration

Abstract: Electroless Cu thermodynamics, electrochemistry, mechanism, kinetics, and mass transport are reviewed. Electroless Cu deposition is a thermodynamically favorable and kinetically inhibited process, with two electrochemical reactions including anodic oxidation of a reducing agent and cathodic reduction of metal ions occurring simultaneously, with a multistep catalytic redox mechanism, an Arrhenius type of rate equation, and mass-transport limited reaction in narrow and deep features such as subhalf micron trenches and vias of high aspect ratios (>3). Selective and blanket electroless Cu deposition from formaldehyde-based solutions with ethylenediaminetetraacetic acid as a complexing agent was investigated for trench/via filling applications. A single-wafer electroless Cu deposition system with up to 8 in. wafer capability has been designed and manufactured. An electroless Cu deposition solution and operation conditions have been optimized to obtain electroless Cu films at high deposition rate (∼75 to 120 nm/min), with low resistivity (p < 2 μΩ cm), low surface roughness (R a ∼ 10 to 15 nm for ∼1.5 μm thick deposits) and good electrical uniformity (std dev <3% for 6 in. wafers and 5 to 7% for 8 in. wafers). A novel dry seeding method on sputtered Cu/Al bilayers has been developed to provide protection of Cu catalytic properties from passivation by using an Al sacrificial layer and to obtain uniform initiation and blanket growth of electroless Cu by in situ Al dissolution in the plating bath. Electroless Cu films blanket deposited on sputtered Cu/Al seed layer were conformal with 100% step coverage. A novel wet seeding method has been developed with Cu contact displacement deposition on a TiN diffusion barrier to provide selective and blanket electroless copper plating. Subhalf micron (down to 0.3 μm) trenches and vias of high aspect ratios (up to 5:1) were completely filled for both blanket and selective electroless deposition modes.