Showing papers in "Journal of The Electrochemical Society in 1994"

Simulation and Optimization of the Dual Lithium Ion Insertion Cell

Abstract: The galvanostatic charge and discharge of a dual lithium ion insertion (rocking‐chair) cell are modeled. Transport in the electrolyte is described with concentrated solution theory. Insertion of lithium into and out of the active electrode material is simulated using superposition, greatly simplifying the numerical calculations. Simulation results are presented for the cell, and these results are compared with experimental data from the literature. Criteria are established to assess the importance of diffusion in the solid matrix and of transport in the electrolytic solution. Various procedures to optimize the utilization of active material are considered. Simulation results for the dual lithium ion insertion cell are compared with those for a cell with a solid lithium negative electrode.

Solid‐State Redox Reactions of LiCoO2 (R3̅m) for 4 Volt Secondary Lithium Cells

Abstract: LiCoO[sub 2] (R[bar 3]m; a = 2.82 [angstrom], c = 14.1 [angstrom] in hexagonal setting) was prepared and examined in nonaqueous lithium cells using 1M LiClO[sub 4] propylene carbonate solution at 30 C. The oxidation of LiCoO[sub 2] and the reduction of Li[sub 1[minus]x]CoO[sub 2] proceeded reversibly in the voltage region above 3.9 V. X-ray diffraction (XRD) examinations indicated the reaction proceeded in a topotactic manner, i.e., two-phase reactions (0 < x < 1/4 and 3/4 < x < 1 in Li[sub 1[minus]x]CoO[sub 2]) and a single-phase reaction (1/4 < x < 3/4). A monoclinic phase was observed in 3/4 < x < 1 in addition to that at about x = 0.45. Detailed open-circuit voltage measurements were carried out. The open-circuit voltage are invariable at 3.92 V for 0 < x < 1/4 and at 4.50 V for 3/4 < x < 1. The two straight lines are connected smoothly by a composition-dependent curve for 1/4 < x < 3/4, which was consistent with the XRD observations. The differences and similarities between the solid-state redox reactions of LiCoO[sub 2] and LiNiO[sub 2] were discussed by comparing the structural and electrochemical data. Possible lithium ordering at x = 1/4 andmore » 3/4 for this type of material is described in terms of a [2 x 2] superlattice in a triangular lattice of sites.« less

Synthesis Conditions and Oxygen Stoichiometry Effects on Li Insertion into the Spinel LiMn2O4

Abstract: Using a new electrolyte composition which is stable against oxidation up to 5 V, the full electrochemical deintercalation of lithium from the spinel LiMn[sub 2]O[sub 4], is studied. The origin of two new reversible oxidation-reduction peaks near 4.5 and 4.9 V are examined. The capacity associated with these peaks depends on both the nominal composition x in Li[sub x]Mn[sub 2]O[sub 4] and the synthesis conditions (annealing temperatures and cooling rates), and thereby can be used as an indicator for electrochemically optimized LiMn[sub 2]O[sub 4] powders. The authors present evidence that these peaks are related to local structural defects. Thermogravimetric measurements (TGA) on Li[sub x]Mn[sub 2]O[sub 4] powders show a reversible loss of oxygen that can reach 5% at 1,000 C. The authors find that some of this weight loss is associated with the conversion of cubic LiMn[sub 2]O[sub 4] to a new tetragonal spinel phase and then to the decomposition of this phase into the orthorhombic LiMnO[sub 2] phase plus other products. This new tetragonal LiMn[sub 2]O[sub 4] spinel is prepared as a single phase, and its electrochemical properties are reported.

Spinel Anodes for Lithium‐Ion Batteries

Abstract: Anodes of Li4MnsO12, Li4Ti5012, and Li2Mn409 with a spinel-type structure have been evaluated in room-temperature lithium cells. The cathodes that were selected for this study were the stabilized spinels, Li1.03Mnl.g704 and LiZnoo25Mn1.9504, and layered LiCoO2. The electrochemical data demonstrated that Li + ions will shuttle between two transition-metal host structures (anode and cathode) at a reasonably high voltage with a concomitant change in the oxidation state of the transition metal cations so that the Li § ions do not reduce to the metallic state at the anode during charge. These cells reduce the safety hazards associated with cells containing metallic-lithium, lithium-alloy, and lithium-carbon anodes.

Temperature‐Dependent Methanol Electro‐Oxidation on Well‐Characterized Pt‐Ru Alloys

Abstract: The kinetics of methanol electro-oxidation on well-characterized Pt-Ru alloy surfaces were measured in sulfuric acid solution as a function of temperature. The alloy surfaces were prepared in ultrahigh vacuum with the surface composition determined by low energy ion scattering. It was found that the activity of Ru towards the dissociative adsorption of methanol is a strong function of temperature. This change in the adsorptive nature of the Ru sites with temperature produced a variation in the optimum surface composition with temperature. The optimum surface had an Ru content which increased with increasing temperature, from close to [approximately]10 atomic percent (a/o) Ru at 25 C to a value in the vicinity of [approximately]30 a/o at 60 C. The shift in optimum composition with temperature was attributed to a shift in the rate-determining step from methanol adsorption/dehydrogenation at low temperature to the surface reaction between the dehydrogenated intermediate and surface oxygen at high temperature. The apparent activation energies were consistent with this change in the rate-determining step.

Water Uptake of Perfluorosulfonic Acid Membranes from Liquid Water and Water Vapor

Abstract: The water uptake of several perfluorosulfonic acid membranes from liquid water over the temperature range 25 to 130°C and from water vapor at 80°C was determined. For water uptake from liquid water, the water uptake depended on the immersion temperature, the ion exchange capacity of the membrane, and the pretreatment of the membrane. The effects of pretreatment were not significant at immersion temperatures higher than 100 to 110°C. For water uptake from water vapor at 80°C, the sorption isotherms were similar in shape to those reported by previous investigators for 18.5 to 30°C, although the water uptake at 80°C was less than that reported for the lower temperatures. The water uptake from water vapor of some membranes that have been found to give relatively good performance when used in polymer electrolyte fuel cells was higher then that observed with, e.g., Nafion®117.

Mobility and Ionic Association of Lithium and Quaternary Ammonium Salts in Propylene Carbonate and γ‐Butyrolactone

Abstract: Propylene carbonate (PC) and [gamma]-butyrolactone (GBL) are important dipolar aprotic solvents used for nonaqueous electrolytes in high energy lithium batteries, aluminum electrolytic capacitors, and electrical double-layer capacitors. The conductivities of lithium and quaternary ammonium salts have been measured in propylene carbonate and [gamma]-butyrolactone at 25 C over the concentration range of 10[sup [minus]2] to 10[sup [minus]3] mol dm[sup [minus]3]. The limiting molar conductivities and the ionic association constants were calculated by the expanded Fuoss-Hsia equation. The mobility of anions in both solvents decreased in the following order: BF[sub 4][sup [minus]] > ClO[sub 4][sup [minus]] > PF[sub 6][sup [minus]] > CF[sub 3]SO[sub 3][sup [minus]] > (CF[sub 3]SO[sub 2])[sub 2]N[sup [minus]] > C[sub 4]F[sub 9]SO[sub 3][sup [minus]] > BPh[sub 4][sup [minus]]. The association constants increased in the following order: Li(CF[sub 3]SO[sub 2])[sub 2]N, LiAsF[sub 6] < LiAsF[sub 6] < LiClO[sub 4] < LiBF[sub 4] < LiC[sub 4]F[sub 9]SO[sub 3] < LiCF[sub 3]SO[sub 3], while those of the corresponding tetraethylammonium salts were leveled by electrostatic shielding effect of alkyl groups. These results were correlated with ionic radii obtained by MM2 calculation.

The Correlation Between the Surface Chemistry and the Performance of Li‐Carbon Intercalation Anodes for Rechargeable ‘Rocking‐Chair’ Type Batteries

Abstract: The correlation between the electrochemical properties of Li carbon intercalation electrodes and their surface chemistry in solutions was investigated. The carbons investigated were primarily graphite and petroleum coke, and the solvent systems included methyl formate (MF), propylene and ethylene carbonates, ethers and their mixtures. The surface chemistry of the electrodes was studied using mainly diffuse reflectance Fourier transform infrared spectroscopy. The following aspects were studied: (i) the effect of temperature on the buildup of the surface films; (ii) the effect of additives (e.g., , crown ethers), (iii) the behavior when the passive layer is built in one solution followed by cycling in another; and (iv) the effect of cosolvent in MF solutions. The results obtained further prove that the electrochemical behavior of these systems is surface film controlled. An understanding of the surface chemistry of these electrodes enables judicious optimization of carbon‐solution systems for use in rechargeable Li batteries.

Relaxation Phenomena in Lithium‐Ion‐Insertion Cells

Abstract: Relaxation phenomena in lithium-ion-insertion cells are modeled. Simulation results are presented for a dual lithium-ion-insertion cell and for a cell using a lithium-foil negative electrode. A period of relaxation after a charge or discharge can cause appreciable changes in the distribution of material in the insertion electrodes. Local concentration cells in the solution phase and an open-circuit potential that depends on state of charge for the solid phase drive the redistribution of material. Concentration profiles in solid and solution phases during relaxation are analyzed, and the consequences for cell performance are discussed. The model predicts the effects of relaxation time on multiple charge-discharge cycles and on peak power. Galvanostatic and potentiostatic charging are simulated; the results are compared to experimental data for a commercial battery.

Activity and Stability of Ordered and Disordered Co‐Pt Alloys for Phosphoric Acid Fuel Cells

Abstract: Co-Pt alloys were studied in detail by a corrosion test under phosphoric acid fuel cell (PAFC) conditions on the well-defined crystallographic structures for a typical combination of the alloy catalysts used in PAFCs, examining the long-life stabilities of the structures and the catalytic activities for O[sub 2] electroreduction. The ordered (O) and disordered (D) alloys at the same particle sizes can be obtained by heat-treating the mother alloy in different temperature sequences. The O-alloy exhibits a specific activity 1.35 times higher than the D-alloy before the corrosion test, but shows less activity after the corrosion test, due to a higher degradation in the O-alloy activity as compared with that of the D-alloy. It was found that the Co atoms on particle surfaces of both alloys dissolved easily in the acid. This is followed by a second slow dissolution from inside the alloy particles probably due to the protective action by a monolayer thickness of Pt remaining on the alloy surfaces, but the loss of Co in the second stage dissolution for the O-alloy is higher by several percentage points compared to that of the D-alloy. It was also found that the Pt content does not change on the catalyst supportmore » even after 50 h of corrosion test, but the pure Pt phase is formed in the corrosion product, where the phase for the O-alloy grows faster than that for the D-alloy with corrosion time. Based on these results, obtained by chemical, x-ray diffraction, and transmission electron microscopy with energy dispersive spectroscopy analyses, the corrosion for Pt alloy catalysts is clearly explained: after the dissolution of Co atoms in the first surface layer of alloys, both Co and Pt dissolve out simultaneously from small-size alloy particles and the Pt redeposits on the surfaces of large-size alloy particles. It is concluded that the D-alloy is preferable to the O-alloy from the viewpoint of the stabilities in the structure and the electrocatalytic activity.« less

Electrochemical Properties of Organic Liquid Electrolytes Based on Quaternary Onium Salts for Electrical Double‐Layer Capacitors

Abstract: The electrolytic conductivities and limiting reduction and oxidation potentials for various organic liquid electrolytes based on quaternary onium salts have been measured to find better electrolytes for electrical double‐layer capacitors. An electrolyte composed of tetraethylammonium cation, tetrafluoroborate anion, and propylene carbonate solvent showed well‐balanced performance of high electrolytic conductivity, a wide stable potential window and resistance to hydrolysis. Among quaternary onium salts, triethylmethylammonium, ethylmethylpyrrolidinium, and tetramethylenepyrrolidinium tetrafluoroborate salts exhibited higher electrolytic conductivity than the conventional tetraethylammonium salt due to their much greater solubility.

Dual Intercalating Molten Electrolyte Batteries

Abstract: The reductive and oxidative intercalation of ions into graphite from room-temperature and low temperature molten salts is demonstrated For this investigation, the molten salts use 1-ethyl-3-methylimidazolium (EMI[sup +]) or 1,2-dimethyl-3-propylimidazolium (DMPI[sup +]) as the cation and AlCl[sup [minus]][sub 4], BF[sup [minus]][sub 4], PF[sup [minus]][sub 6], CF[sub 3]SO[sup [minus]][sub 3], or C[sub 6]H[sub 5]CO[sup [minus]][sub 2] as the anion In a two-electrode battery configuration, the molten salt electrolyte provides both the cation and anion which are intercalated into the graphite anode and cathode, respectively A cell employing a (DMPI)(AlCl[sub 4]) electrolyte and two graphite rod electrodes achieved an open-circuit voltage of 35 V and a cycling efficiency of 85%

Physical Properties of Mixed Conductor Solid Oxide Fuel Cell Anodes of Doped CeO2

Abstract: Samples of CeO2 doped with oxides such as CaO and Gd203 were prepared. Their conductivities and expansions on reduction were measured at 1000~ and the thermal expansion coefficients in the range 50 to 1000~ were determined. The ionic and electronic conductivity were derived from curves of total conductivity vs. oxygen partial pressure. For both types of conductivity a dependence on dopant valency was observed. The electronic conductivity was independent of dopant radius in contrast to the ionic which was highly dependent. These measured physical properties are compared with the ideal requirements for solid oxide fuel cell anodes. Not all requirements are fulfilled. Measures to compensate for this are discussed.

The Surface Chemistry of Lithium Electrodes in Alkyl Carbonate Solutions

Abstract: The chemical composition of the surface films formed on lithium in alkyl carbonate solutions was explored using surface sensitive Fourier transform infrared spectroscopy (external reflectance mode). The solvents included propylene carbonate, ethylene carbonate, and dimethyl carbonate. The salts included LiAsF{sub 6}, LiClO{sub 4}, LiBF{sub 4}, and LiPF{sub 6}. The advantages of this work over previous studies are that highly reflective Li surfaces were prepared fresh in solution and that the aging processes of the surface films initially formed could be rigorously investigated. Furthermore these three important solvents were investigated in a single study. This work further proves that the films initially formed on Li surfaces in these solvents consist of ROCO{sub 2}Li as the major constituents. Upon storage, the films initially formed react with trace water to form Li{sub 2}CO{sub 3}, which gradually also becomes a major surface species. It was found that these aging processes also depend on the salts used (for example ROCO{sub 2}Li or Li{sub 2}CO{sub 3} films are not stable in LiPF{sub 6} or LiBF{sub 4} solutions).

Metastable Pitting of Aluminum and Criteria for the Transition to Stable Pit Growth

Abstract: Analysis of current spikes associated with pitting events was performed on high purity Al loop wire electrodes over a range of Cl[sup [minus]] concentrations (10[sup [minus]4] to 1M) and potentiostatically applied potentials. A distribution of pitting and repassivation potentials was observed at each Cl[sup [minus]] concentration. Factors controlling the transition from metastable to stable pitting were identified by comparing the electrochemical behaviors of stable pits at elapsed times equal to the mean lifetime of metastable pits. This comparison also provided insight on the origins of statistical distributions of pitting potentials. The key differences were that (1) stable pits had a faster rate of rise in pit current which implies a faster growth rate and subsequently, larger pit radii at times equal to metastable pit lifetimes, and (2) stable pits satisfied the criterion I[sub pit]/r[sub pit] > 10[sup [minus]2] A/cm, at all times during growth, indicating that a concentrated AlCl[sub 3] solution must be maintained for pit survival. However, the pit growth rates were ohmically limited near the pitting potential. Chromate inhibitor decreased the metastable pit nucleation rate and minimized pit growth rates so that the 10[sup [minus]2] A/cm criterion was difficult to achieve; hence the chance for pit stabilization wasmore » reduced.« less

A Polymer Electrolyte for Operation at Temperatures up to 200°C

Abstract: In developing advanced fuel cells and other electrochemical reactors, it is desirable to combine the advantages of solid polymer electrolytes with the enhanced catalytic activity associated with temperatures above 100 C. This will require polymer electrolytes which retain high ionic conductivity at temperatures above the boiling point of water. One possibility is to equilibrate standard perfluorosulfonic acid polymer electrolytes such as Nafion, with a high boiling point Bronsted base such as phosphoric acid. The Nafion/H[sub 3]PO[sub 4] electrolyte has been evaluated with respect to water content, ionic conductivity and transport of oxygen, and methanol vapor. The results show that at elevated temperatures reasonably high conductivity (>0.05 [Omega][sup [minus]1] cm[sup [minus]1]) can be obtained. Methanol permeability is shown to be proportional to the methanol vapor activity and thus decreases with increasing temperature for a given partial pressure. Comparisons and distinctions between this electrolyte and pure phosphoric acid are also considered.

Stabilized α ‐ Ni ( OH ) 2 as Electrode Material for Alkaline Secondary Cells

Abstract: Hydrotalcite-like compounds of formula Ni1-xAl(x)(OH)2(CO3)x/2 . nH2O (x = 0.1 to 0.25), having the same structure as that of alpha-Ni(OH)2, have been synthesized by substituting nickel hydroxide with aluminum. Of these, the compounds of compositions x greater-than-or-equal-to 0.2 are found to have prolonged stability in strong alkaline medium. The electrodes comprising stabilized alpha-Ni(OH)2 of x = 0.2 composition are rechargeable with discharge-capacity values of 240 (+/- 15) mAh-g-1 and are attractive for applications in various alkaline secondary cells employing nickel-positive electrodes.

Tribology Analysis of Chemical‐Mechanical Polishing

Abstract: To better understand the variation of material removal rate on a wafer during chemical-mechanical polishing (CMP), knowledge of the stress distribution on the wafer surface is required. The difference in wafer-surface stress distributions could be considerable depending on whether or not the wafer hydroplanes during polishing. This study analyzes the fluid film between the wafer and pad and demonstrates that hydroplaning is possible for standard CMP processes. The importance of wafer curvature, slurry viscosity, and rotation speed on the thickness of the fluid film is also demonstrated.

Study of Electrodeposited Nickel‐Molybdenum, Nickel‐Tungsten, Cobalt‐Molybdenum, and Cobalt‐Tungsten as Hydrogen Electrodes in Alkaline Water Electrolysis

Abstract: Electrodeposited nickel-molybdenum, nickel-tungsten, cobalt-molybdenum, and cobalt-tungsten were characterized for the hydrogen evolution reaction (HER) in the electrolysis of 30 w/o KOH alkaline water at 25 C. The rate-determining step (rds) of the HER was suggested based on the Tafel slope of polarization and the capacitance of electrode-solution interface determined by ac impedance measurement. The HER on the nickel- and cobalt-based codeposits was enhanced significantly compared with that o the electrolytic nickel and cobalt with comparable deposit loadings. The decrease in the HER overpotential was more pronounced on the molybdenum-containing codeposits, particularly on cobalt-molybdenum which also showed a high stability. The enhancement of the HER was attributed to both the synergetic composition and the increased active surface of the codeposits. The real electrocatalytic activity of te electrodes and the effect of their and the increased active surface of the codeposits. The real electrocatalytic activity of the electrodes and the effect of their surface increase were distinguished quantitatively. The linear relations between HER overpotential and surface roughness factor of the electrodes on a Y-log(X) plot were obtained experimentally and interpreted based on the Tafel law.

Mn2+ Concentration Effect on the Optical Properties of Zn2SiO4 : Mn Phosphors

Abstract: The optical properties of manganese‐activated zinc silicate are investigated using pulse selective photon excitation. The light response due to the transition is directly linked to the relative concentration of isolated Mn2+ and Mn2+ pairs in this structure. The decay curves can be fitted by a diffusion‐limited relaxation mechanism by considering two types of centers with radiative lifetimes equal to 15 and 1.75 ms, respectively.

The Susceptibility to Surface Corrosion in Acidic Fluoride Media: A Comparison of Diamond, HOPG, and Glassy Carbon Electrodes

Abstract: The chemical inertness and corrosion resistance of a boron‐doped diamond thin film electrode, grown by chemical vapor deposition (CVD), have been studied during potential cycling (PC) for 2 h in a solution of at 50°C. Similar experiments were performed on highly ordered pyrolytic graphite (HOPG) and glassy carbon (GC), for comparison. The physicochemical properties of the electrode surface were characterized before and after PC by cyclic voltammetry, optical and scanning electron microscopy, Raman spectroscopy, and ac impedance spectroscopy. The results indicated that the diamond electrode possesses a superior degree of chemical inertness and corrosion resistance, has no microstructural damage nor was surface oxidation observed after PC. HOPG and GC surfaces, on the other hand, exhibited severe corrosion in the form of surface cavitation, pitting, and oxidation. The relative degree of microstructural damage and surface oxidation increased in the order of diamond . This work represents some of the initial efforts at systematically characterizing how the physical, chemical, and electronic properties of conductive diamond thin films are affected during exposure to electrochemical conditions (i.e., solvent, electrolyte, and applied potential).

Grain‐Size Effects in Tungsten Oxide‐Based Sensor for Nitrogen Oxides

Abstract: The effects of the grain size of WO[sub 3] on WO[sub 3]-based sensor for NO[sub x] were investigated. By hydrolyzing ammonium paratungstate with hot nitric acid in solution and calcining the resulting precipitate in air at prescribed temperatures in the range of 300 to 600 C, the mean crystallite sizes (D) of WO[sub 3] were varied in the range of 16 to 57 nm. The sensitivities of the WO[sub 3] sensor elements to NO[sub 2] (10 ppm) as well as to NO (200 ppm) remained independent of D down to D = 33 nm, below which the sensitivities increased steeply with a decrease in D, reaching at D = 25 nm the sensitivity values three to four times as large as those for D > 33 nm.

Anodic Oxidation of Methanol on Pt / WO 3 in Acidic Media

Abstract: The details of the preparation and characterization of co-deposited Pt/WO[sub 3] electrodes are presented. X-ray, scanning, and transmission electron microscopic studies revealed that WO[sub 3] is amorphous and that [approximately]40 [angstrom] Pt crystallites are uniformly dispersed in the deposit. The influence of the deposition conditions and the effect of the solution acidity on the activity of such electrodes for methanol oxidation have been studied. The results demonstrated that the Pt/WO[sub 3] electrodes are much more active and more resistant to poisoning than Pt or Pt/Ru alloy catalysts. The reaction mechanism was studied by various electrochemical and surface analysis techniques. A reaction scheme that involves successive stepwise dehydrogenation of methanol, formation/oxidation of hydrogen tungsten bronze, and oxidation of organic intermediates and CO at the Pt/WO[sub 3] interface is proposed.

Polyoxometallate‐Based Layered Composite Films on Electrodes Preparation Through Alternate Immersions in Modification Solutions

Abstract: An approach for the modification of electrode surfaces with thin films composed of polyoxometallate anions and large water-soluble cationic species is described. In the procedure, a ca. monolayer of the iso- or heteropolyanion is first adsorbed onto the electrode surface. By immersing the resulting system into a solution containing a large monovalent, multivalent, or polyvalent cation, a composite layer is formed due to the interaction between the adsorbed polyanion and the solution cation. After rinsing, this electrode is reimmersed into the solution of the polyanion, and immobilization of an additional quantity of the polyanion takes place. By the repeated and alternate immersions into the anionic and cationic modification solutions, the amount of material on the electrode can be increased systematically in a controlled fashion leading to stable three-dimensional multilayered molecular assemblies. The immobilized polyanions (isopolymolybdate, phosphotungstate, or silicotungstate) have redox characteristics similar to those of their solution counterparts. The precipitate-forming cationic species include tetrabutylammonium and tris(1,10-phenanthroline)-iron(II) ions, as well as protonated poly(4-vinylpyridine). Composite films of heteropoly-12-tungstate anions with protonated poly(4-vinylpyridine) are the most robust. The approach permits introduction of multiple redox centers into the thin films on electrodes, formation of bilayer-type systems, and, in some cases, even insulating coatings. Detailsmore » of the preparation and physicochemical, particularly electrochemical, properties of the produced systems are described.« less

Covalent Binding of Pd Catalysts to Ligating Self‐Assembled Monolayer Films for Selective Electroless Metal Deposition

Abstract: A new approach for the selective electroless (EL) metallization of surfaces is described. Surfaces are modified with a chemisorbed ligand‐bearing organosilane film, and then catalyzed with an aqueous Pd(II) catalyst solution. The catalyzed substrate is then immersed in an EL metal deposition bath to complete the metallization process. The ligating surfaces are produced by molecular self‐assembly of 2‐(trimethoxysilyl)ethyl‐2‐pyridine (PYR) on silicon or silica substrates. The catalyst consists of chloride‐containing aqueous Pd(II) solutions buffered at pH 5.0 to 6.4; oligomeric chloro and/or hydroxo‐bridged Pd(II) complexes act as the catalytic species at the surface. The activity of the catalyst has been characterized and modeled as a function of solution pH, [Cl−], and time from preparation. Adhesion of the Pd(II) EL catalyst to the substrate involves covalent bond formation with the surface ligand. An average minimum Pd(II) level on the surface of ~1015 Pd atom cm2 is shown to be necessary to initiate EL metallization of the substrate with an EL Co bath. This process involves fewer steps and displays improved selectivity compared to processes that involve a conventional Pd/Sn catalyst. Fabrication of high resolution metal patterns using the new metallization chemistry in conjunction with deep UV patterning of PYR films is demonstrated.

Lithium Ion Conductivity of A‐Site Deficient Perovskite Solid Solution La0.67 − x Li3x TiO3

Abstract: La[sub 0.67[minus]x]Li[sub 3x]TiO[sub 3] solid solution forms in the range [approximately]0.15 > x > [approximately]0.04 by the substitution mechanism La[r equilibrium]3Li. Superstructure reflections observed in this range are consistent with a superlattice consisting of the stacking of two perovskite subcells. A line-splitting phenomenon observed for x [approximately]0.04. A maximum conductivity of 1.1 [times] 10[sup [minus]3] S cm[sup [minus]1] is found at x [approx] 0.1. The dome-shaped composition dependence of conductivity indicates that the conduction mechanism involves the movement of lithium ions through the A-site vacancies.

Solid‐State Redox Reactions of LiNi1 / 2Co1 / 2 O 2 ( R 3̄m ) for 4 Volt Secondary Lithium Cells

Abstract: LiNi[sub 1/2]Co[sub 1/2]O[sub 2] (R[bar 3]m; a = 2.84 [angstrom], c = 14.1 [angstrom] in hexagonal setting) whose parent structure was a solid solution of LiNiO[sub 2] and LiCoO[sub 2] was prepared and examined in a nonaqueous lithium cell. The oxidation of LiNi[sub 1/2]Co[sub 1/2]O[sub 2] and reduction of [open square]Ni[sub 1/2]Co[sub 1/2]O[sub 2] proceeded reversibly in the voltage range above 3.5 V. X-ray diffractional examinations indicated the reaction proceeded in a topotactic manner. During the oxidation of LiNi[sub 1/2]Co[sub 1/2]O[sub 2], the c-axis dimension elongated from 14.1 to 14.5 [angstrom] and the a-axis shortened from 2.84 to 2.80 [angstrom] almost linearly as a function of x in Li[sub 1[minus]x]Ni[sub 1/2]Co[sub 1/2]O[sub 2] until x reached about 0.5. Consequently, the lattice volume remained almost invariable at 98.5 A[sup 3]. Further oxidation resulted in the shrinkage of the c-axis dimension and the elongation of the a-axis. Detained open-circuit voltage measurements also were carried out. The open-circuit voltages varied as a function of x in Li[sub 1[minus]x]Ni[sub 1/2]Co[sub 1/2]O[sub 2] following a hyperbolic tangent curve, not an S-shaped Nernstian curve. The composition-dependent open-circuit voltage curve together with the XRD observations indicated that the reaction proceeded in a homogeneous phase. Differences and similaritiesmore » of the solid-state redox reactions among LiNiO[sub 2], LiNi[sub 1/2]Co[sub 1/2]O[sub 2], and LiCoO[sub 2] were discussed.« less

Lithium Intercalation from Aqueous Solutions

Abstract: Lithium can be intercalated into a wide variety of materials using nonaqueous electrochemical cells. The use of aqueous methods is less common because of the reactivity of many lithium intercalation compounds with water. Here the authors show that lithium can be intercalated into host compounds from aqueous LiOH solution, provided the chemical potential of the intercalated lithium is sufficiently lower than the chemical potential of lithium in lithium metal. Using LiMn[sub 2]O[sub 4] as the host, the authors formed Li[sub 2]Mn[sub 2]O[sub 4] by intercalating Li from LiOH solution in an aqueous cell. This method may prove to be an economical way of preparing lithium transition metal oxides with high lithium contents for lithium-ion cell cathodes.

Mass transport phenomena in proton exchange membrane fuel cells using O[sub 2]/He, O[sub 2]/Ar, and O[sub 2]/N[sub 2] mixtures; 1: Experimental analysis

Abstract: This investigation on proton exchange membrane fuel cells (PEMFCs) consists of two parts: (1) an experimental analysis of the oxygen reduction reaction, using an O[sub 2]/He, O[sub 2]/Ar, or O[sub 2]/N[sub 2] gas mixtures as the cathodic reactant, to determine the effects of oxygen partial pressure, temperature, and pressure on the contributions to activation and mass-transport overpotential; and (2) a theoretical analysis of oxygen reduction to: first, interpret the experimental results with O[sub 2]/He, O[sub 2]/Ar, and O[sub 2]/N[sub 2] gas mixtures as the cathodic reactant, and, second, elucidate the effects of the structure of the electrode and of the physicochemical parameters of the reactant gas, as well as of temperature and pressure, on the oxygen electrode potential vs. current density behavior. For the experimental analysis, which is presented in this Part of the paper, the performance evaluation of PEMFCs (in single cells with the geometric area of the electrodes being 50 cm[sup 2]) was carried out using O[sub 2]/He, O[sub 2]/Ar, and O[sub 2]/N[sub 2] gas mixtures as the cathodic reactants. Experiments were also carried out using air and oxygen. Cell potential vs. current density measurements were made over the temperature range of 50 to 90 C and pressuremore » range of 1 to 5 atm. These studies showed that mass-transfer limitations are less in PEMFCs with O[sub 2]/He, than with O[sub 2]/Ar or O[sub 2]/N[sub 2] gas mixtures. Furthermore, at an oxygen composition greater than 40% of O[sub 2] in the gas mixture, the pseudo-linear region of the potential vs. current density plot is extended at least up to 1 A/cm[sup 2]. With air as the cathodic reactant, mass-transport effects are minimal at pressures of 3 atm and higher. Temperature effects are less significant than pressure effects, because in the former case there is a compensating effect of higher temperature and lower partial pressure of oxygen on the electrode kinetics of oxygen reduction.« less

Pitting Corrosion of Titanium

Abstract: The breakdown of native and anodically grown oxide films on Ti electrodes is investigated by scanning electrochemical microscopy (SECM), video microscopy, transmission electron microscopy, and voltammetry. SECM is used to demonstrated that the oxidation of Br[sup [minus]] on Ti occurs at microscopic surface sites (10 to 50 [mu]m diam, 30 sites/cm[sup 2]) that are randomly positioned across the oxide surface. After determining the position of the active sites for Br[sup [minus]] oxidation, breakdown of the oxide is initiated by increasing the electrode potential to more positive values. Direct correspondence is observed between the location of the electroactive sites and corrosion pits, indicating that oxide breakdown is associated with a localized test of high electrical conductivity. The potential at which pitting is observed in voltammetric experiments is found to be proportional to the average oxide thickness, for values ranging between 20 and 100 [angstrom], indicating that breakdown is determined either by the magnitude of the electric field within the oxide or by the interfacial potential at the oxide/Br[sup [minus]] solution interface. Pitting occurs at significantly lower potentials in Br[sup [minus]] solutions than in Cl[sup [minus]] solutions, suggesting a strong chemical interaction between the TiO[sub 2] surface and Br[sup [minus]]. A mechanismmore » of oxide breakdown is proposed that is based on the potential-dependent chemical dissolution of the oxide at microscopic surface sites.« less