Showing papers in "Journal of The Electrochemical Society in 1998"

Pitting Corrosion of Metals A Review of the Critical Factors

Abstract: Pitting corrosion is localized accelerated dissolution of metal that occurs as a result of a breakdown of the otherwise protective passive film on the metal surface. This paper provides an overview of the critical factors influencing the pitting corrosion of metals. The phenomenology of pitting corrosion is discussed, including the effects of alloy composition, environment, potential, and temperature. A summary is then given of studies that have focused on various stages of the pitting process, including breakdown of the passive film, metastable pitting, and pit growth.

Capacity Fade Mechanisms and Side Reactions in Lithium‐Ion Batteries

Abstract: The capacity of a lithium‐ion battery decreases during cycling. This capacity loss or fade occurs due to several different mechanisms which are due to or are associated with unwanted side reactions that occur in these batteries. These reactions occur during overcharge or overdischarge and cause electrolyte decomposition, passive film formation, active material dissolution, and other phenomena. These capacity loss mechanisms are not included in the present lithium‐ion battery mathematical models available in the open literature. Consequently, these models cannot be used to predict cell performance during cycling and under abuse conditions. This article presents a review of the current literature on capacity fade mechanisms and attempts to describe the information needed and the directions that may be taken to include these mechanisms in advanced lithium‐ion battery models.

Characterization of High‐Surface‐Area Electrocatalysts Using a Rotating Disk Electrode Configuration

Abstract: A newly developed method is presented which allows the characterization of the electrocatalytic properties of highly dispersed electrocatalysts in a true rotating disk electrode (RDE) configuration by attaching the catalyst powder on a glossy carbon electrode via a thin Nafion film. Complete utilization and high reproducibility of both the electrode preparation and the catalyst loading could be shown via voltammetry and CO stripping voltammetry. Furthermore RDE measurements on the electro‐oxidation of hydrogen on Pt/Vulcan showed that the effect of diffusion through the Nation film can be avoided by proper electrode preparation. Therefore, the electrode kinetics for fuel cell relevant reactions under continuous flow conditions can be measured directly without mathematical modeling.

Common Electroanalytical Behavior of Li Intercalation Processes into Graphite and Transition Metal Oxides

Abstract: This paper compares the electroanalytical behavior of lithiated graphite, , , and spinel electrodes. Slow scan rate cyclic voltammetry (SSCV), potentiostatic intermittent titration (PITT), and electrochemical impedance spectroscopy (EIS) were applied in order to study the potentiodynamic behavior, the variation of the solid‐state diffusion coefficient, and the impedance of these electrodes. In addition, X‐ray diffractometry and Fourier transform infrared (FTIR) spectroscopy were used in order to follow structural and surface chemical changes of these electrodes upon cycling. It was found that all four types of electrodes behave very similarly. Their SSCV are characterized by narrow peaks which may reflect phase transition between intercalation stages, and the potential‐dependent Li chemical diffusion coefficient is a function with sharp minima in the vicinity of the CV peak potentials, in which the differential electrode capacity is maximal. The impedance spectra of these electrodes reflect an overall process of various steps in series. These include ion migration through surface films, charge transfer which depends strongly on the potential, solid‐state diffusion and, finally, accumulation of the intercalants in their sites in the bulk of the active mass, which appears as a strongly potential‐dependent, low‐frequency capacitive element. It is demonstrated that the above electro‐analytical response, which can be considered as the electrochemical fingerprint of these electrodes, may serve as a good in situ tool for the study of capacity fading mechanisms.

Behavior of Molybdenum Nitrides as Materials for Electrochemical Capacitors Comparison with Ruthenium Oxide

Abstract: Ruthenium oxide (RuO{sub 2}), formed as a thin film on a Ru or Ti metal substrate, exhibits a large specific (cm{sup {minus}2}) and almost constant, electrochemical capacitance over a 1.35 V range in aqueous H{sub 2}SO{sub 4}. This behavior has led to its investigation and use as a material for fabrication of supercapacitor devices. However, its cost has encouraged search for other materials exhibiting similar behavior. Work reported in the present paper evaluates two nitrides of Mo, Mo{sub 2}N and MoN, as substitutes for RuO{sub 2}. It is shown that very similar capacitance behavior to that of RuO{sub 2} films arises, e.g., in cyclic voltammetry and dc charging curves; in the former, almost mirror-image anodic and cathodic current-response profiles, characteristic of a capacitor, arise. However, the nitride materials have a substantially smaller voltage operating range of only some 0.7 V due to electrochemical decomposition above ca. 0.7 V vs. RHE. This limits their usefulness as a substitute for RuO{sub 2}. Of interest is that the nitride films exhibit potential-decay and potential-recovery on open circuit after respective charge and forced discharge. The decay and recovery processes are logarithmic in time, indicating the role of internal faradaic charge redistribution processes.

Mechanistic Study of Cathodic Electrodeposition of Zinc Oxide and Zinc Hydroxychloride Films from Oxygenated Aqueous Zinc Chloride Solutions

Abstract: Films of zinc oxide and related compounds [ZnO, ZnO x (OH) y , Zn(OH) x Cl y ] are electrodeposited cathodically in aqueous zinc chloride solutions using dissolved oxygen as a precursor. The influence of the precursor concentrations, pH, and deposition temperature on the growth, composition, and properties of the films are investigated by means of in situ techniques : voltammetry, electrochemical quartz-crystal microgravimetry, surface pH, and ex situ techniques: X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy. The deposition mechanism is analyzed in terms of electrochemically induced surface precipitation due to an increase of local pH resulting from the oxygen reduction reaction. This approach allows us to explain the formation of either zinc hydroxychloride compounds or zinc oxide from their calculated solubility diagrams. In conditions of the formation of ZnO, a dramatic effect of temperature is observed, with a transition between amorphous insulating zinc oxyhydroxide to well-crystallized and conducting zinc oxide when the temperature increases (T transition 50 °C).

Characterization of Sol‐Gel‐Derived Cobalt Oxide Xerogels as Electrochemical Capacitors

Abstract: Very fine cobalt oxide xerogel powders were prepared using a unique solution chemistry associated with the sol-gel process The effect of thermal treatment on the surrace area, pore volume, crystallinity, particle structure, and corresponding electrochemical properties of the resulting xerogels was investigated and found to have significant effects on all of these properties The xerogel remained amorphous as Co(OH) 2 up to 160°C, and exhibited maxima in both the surface area and pore volume at this temperature With an increase in the temperature above 200°C, both the surface area and pore volume decreased sharply, because the amorphous Co(OH) 2 decomposed to form CoO that was subsequently oxidized to form crystalline Co 3 O 4 In addition, the changes in the surface area, pore volume, crystallinity, and particle structure all had significant but coupled effects on the electrochemical properties of the xerogels A maximum capacitance of 291 F/g was obtained for an electrode prepared with the CoO x xerogel calcined at 150°C, which was consistent with the maxima exhibited in both the surface area and pore volume; this capacitance was attributed solely to a surface redox mechanism The cycle life of this electrode was also very stable for many thousands of cycles

Self-discharge of LiMn2O4/C Li-ion cells in their discharged state: Understanding by means of three-electrode measurements

Abstract: The potential distribution through plastic Li-ion cells during electrochemical testing was monitored by means of three- or four-electrode measurements in order to determine the origin of the poor electrochemical performance (namely, premature cell failure, poor storage performance in the discharged state) of LiMn{sub 2}O{sub 4}/C Li-ion cells encountered at 55 C. Several approaches to insert reliably one or two reference electrodes that can be either metallic lithium or an insertion compound such as Li{sub 4}Ti{sub 5}O{sub 12} into plastic Li-ion batteries are reported. Using a reference electrode, information regarding the evolution of (1) the state of charge of each electrode within a Li-ion cell, (2) their polarization, and (3) their rate capability can be obtained. From these three-electrode electrochemical measurements, coupled with chemical analyses, X-ray diffraction, and microscopy studies, one unambiguously concludes that the poor 55 C performance is mainly due to the instability of the LiMn{sub 2}O{sub 4} phase toward Mn dissolution in LiPF{sub 6}-type electrolytes. A mechanism, based on Mn dissolution, is proposed to account for the poor storage performance of LiMn{sub 2}O{sub 4}/C Li-ion cells.

An Along‐the‐Channel Model for Proton Exchange Membrane Fuel Cells

Abstract: An along-the-channel model is developed for evaluating the effects of various design and operating parameters on the performance of a proton exchange membrane (PEM) fuel cell. The model, which is based on a previous one, has been extended to include the convective water transport across the membrane by a pressure gradient, temperature distribution in the solid phase along the flow channel, and heat removal by natural convection and coflow and counterflow heat exchangers. Results from the model show that the performance of a PEM fuel cell could be improved by anode humidification and positive differential pressure between the cathode and anode to increase the back transport rate of water across the membrane. Results also show that effective heat removal is necessary for preventing excessive temperature which could lead to local membrane dehydration. For heat removal and distribution, the counterflow heat exchanger is most effective.

Characterization of AA2024-T3 by Scanning Kelvin Probe Force Microscopy

Abstract: Volta potential mapping of AA2024-T3 on surfaces was performed with an atomic force microscope. A linear relation was found between the Volta potential measured in air and the corrosion potential in aqueous solution for a range of pure metal samples, indicating that this potential is a measurement of the practical nobility of the surface. Large differences in the Volta potential of intermetallic particles in AA2024-T3 and the matrix phase resulted in a potential map with high contrast that clearly identifies the location of the particles. All intermetallic particles, including the Mg-containing S-phase particles, had a Volta potential noble to that of the matrix. Surface films on the particles and the matrix were found to have strong effects on the potential, and probably explain the noble nature of the Mg-containing particles, which have been reported to be active to the matrix in solution. The effect of these surface films was examined by refreshing the sample surface using different techniques. Lateral heterogeneities in certain intermetallic particles were also revealed.

Micro-macroscopic coupled modeling of batteries and fuel cells: I. Model development

Abstract: A micro-macroscopic coupled model, aimed at incorporating solid-state physics of electrode materials and interface morphology and chemistry, has been developed for advanced batteries and fuel cells. Electrochemical cells considered consist of three phases: a solid matrix (electrode material or separator), an electrolyte (liquid or solid), and a gas phase. Macroscopic conservation equations are derived separately for each phase using the volume averaging technique and are shown to contain interfacial terms which allow for the incorporation of microscopic physical phenomena such as solid-state diffusion and ohmic drop, as well as interfacial phenomena such as phase transformation, precipitation, and passivation. Constitutive relations for these interfacial terms are developed and linked to the macroscopic conservation equations for species and charge transfer. A number of nonequilibrium effects encountered in high-energy-density and high-power-density power sources are assessed. Finally, conditions for interfacial chemical and electrical equilibrium are explored and their practical implications are discussed. Simplifications of the present model to previous macrohomogeneous models are examined. In a companion paper, illustrative calculations for nickel-cadmium and nickel-metal hydride batteries are carried out. The micro-macroscopic model can be used to explore material and interfacial properties for desired cell performance.

Differential Scanning Calorimetry Study of the Reactivity of Carbon Anodes in Plastic Li‐Ion Batteries

Abstract: Chemical reactions taking place at elevated temperatures in a polymer-bonded lithiated carbon anode were studied by differential scanning calorimetry. The influences of parameters such as degree of intercalation, number of cycles, specific surface area, and chemical nature of the binder were elucidated. It was clearly established that the first reaction taking place at ca. 120-140 °C was the transformation of the passivation layer products into lithium carbonate, and that lithiated carbon reacted with the molten binder via dehydrofluorination only at T > 300 °C. Both reactions strongly depend on the specific surface area of the electrodes and the degree of lithiation.

Electrochemical Behavior of Highly Conductive Boron‐Doped Diamond Electrodes for Oxygen Reduction in Alkaline Solution

Abstract: Highly conductive boron-doped polycrystalline diamond thin films (ρ ≃ 10 -3 Ω cm) reduction prepared via microwave plasma chemical vapor deposition (CVD). The electrochemical behavior for oxygen reduction was examined in 0.1 M KOH using linear sweep voltammetry. Oxygen reduction was found to be highly inhibited, the cathodic voltammetric peak being observed at ∼ -1.2 V vs. Ag/AgCl, compared with the standard potential for the two-electron reduction of oxygen (O 2 + H 2 O + 2e - = HO 2 - + OH - , E°' = -0.234 V vs. Ag/AgCl at pH 13). This demonstrates that, even in the presence of dissolved oxygen, diamond retains a relatively wide potential window, which could be advantageous in certain types of analytical applications. Possible interpretations for the high overpotential for oxygen reduction include a lack of adsorption sites for oxygen and/or reduced intermediates, a low density of states or a potential drop within a thin were nm) surface layer, all of which have also been proposed for highly ordered pyrolytic graphite. The experimental data were fitted using digital simulation, which showed that the reduction peak appearing at ca. -1.2 V also consistent with an nantly due to the reduction of oxygen to peroxide. Rotating disk electrode measurements were also consistent with an overall two-electron process. Experiments involving the addition of superoxide dismutase also supported this conclusion. The oxygen reduction reaction is proposed to occur on the sp 3 carbon component of the surface, with a very small contribution from sp 2 carbon impurities at smaller overpotentials.

Copper Deposition in the Presence of Polyethylene Glycol I. Quartz Crystal Microbalance Study

Abstract: The addition of polyethylene glycol (PEG) and Cl - to an acid copper electrolyte inhibits the deposition reaction for cathodic overpotentials of up to about 150 mV. Adding Cl - only promotes the deposition reaction, while adding PEG alone has a relatively small effect on electrode kinetics. Frequency shifts of an electrochemical quartz crystal microbalance suggest the adsorption of a monolayer of PEG molecules that are collapsed into spheres provided chloride ions are present, with little adsorption occurring when Cl - is absent. This behavior is the same for gold and copper surfaces. Transient current measurements suggest that chloride ions affect the PEG adsorption equilibrium rather than adsorption kinetics alone.

Modeling of Solid Oxide Heat Exchanger Integrated Stacks and Simulation at High Fuel Utilization

Abstract: This work provides an evaluation of the behavior of a planar circular solid oxide fuel cell stack with an integrated air preheater fed with hydrogen at the anode and air at the cathode under conditions of high fuel utilization. This evaluation is based on a simulation model, whose hypotheses and equations are presented and discussed, together with model validation. Indeed, it has been shown that the simulation results compare favorably with the experimental data of temperature distribution and I-V characteristics at low fuel utilization. Having ascertained the reliability of the model, the simulation results obtained at high fuel utilization are presented and discussed. I-V curves show an almost linear behavior up to fuel utilization factors above 80%. For higher utilizations, the slopes of the curves increase due to an increase of anodic polarization resistance, due to both activation effects and diffusion limitations in the electrode regions underneath the ribs of the gas distributor; on the contrary, diffusion limitations in the regions underneath the gas channels have been demonstrated to be negligible. The results for temperature distributions at high fuel utilization are discussed as well, indicating that the benefit of solid oxide fuel cells with an integrated air preheater, i.e., anmore » almost even distribution of solid temperature on the cell plane, is still valid at high hydrogen utilization factors.« less

Mass Transport in Gas‐Diffusion Electrodes: A Diagnostic Tool for Fuel‐Cell Cathodes

Abstract: Two mathematical models of gas-diffusion electrodes, one for liquid electrolytes and one for ion-exchange polymer electrolytes, are presented to investigate the effects of mass-transport limitations on the polarization characteristics of a reaction obeying Tafel kinetics. The focus is on low-temperature fuel-cell cathodes, and in particular, contrasting two limiting cases that may be encountered at high current densities: control by kinetics and dissolved oxygen mass transport vs. control by kinetics and ionic mass transport. It is shown that two distinct double Tafel slopes may arise from these two limiting cases. The former is first order, and the latter is half-order with respect to oxygen concentration. How the modeling results may be applied to diagnose the performance of fuel-cell cathodes is also presented. Since the ionic-mass-transport-limited case has generally been neglected in previous gas-diffusion electrode models, specific examples of fuel-cell cathode data from the literature which display the behavior predicted by the models in this case are given and briefly discussed.

Sr‐ and Ni‐Doped LaCoO3 and LaFeO3 Perovskites New Cathode Materials for Solid‐Oxide Fuel Cells

Abstract: An improved cathode material for a solid-oxide fuel cell would be a mixed electronic and oxide-ion conductor with a good catalytic activity for oxygen reduction at an operating temperature T{sub op} {le} 700 C and a thermal expansion matched to that of the electrolyte and interconnect. The authors report on the properties of Sr- and Ni-doped LaCoO{sub 3} and LaFeO{sub 3} perovskites that meet these criteria. Single-phase regions were determined by X-ray diffraction, and thermogravimetric analysis measurements were used to obtain the temperatures above which oxygen loss, and hence oxide-ion conductivity, occurs. The conductivity and Seebeck measurements indicate the coexistence of both p-type and n-type polaronic charge carriers resulting from an overlap of the Ni{sup III}/Ni{sup 2+} redox couple with the low-spin/intermediate-spin Co{sup IV}/Co{sup III} and high-spin Fe{sup 4+}/Fe{sup 3+} redox couples. Motional enthalpies {Delta}H{sub m} = 0.03, 0.02, and 0.08 eV, respectively, were estimated for Ni{sup 2+}, Co{sup IV}, and Fe{sup 4+} polarons. Optimal compositions have percolation pathways between dopants. Comparisons with transport data for the conventional cathode materials La{sub 1{minus}x}Sr{sub x}CoO{sub 3{minus}{delta}} and La{sub 1{minus}x}Sr{sub x}MnO{sub 3} indicate superior cathode performance can be expected.

Thin‐Film Crystalline SnO2‐Lithium Electrodes

Abstract: Crystalline SnO{sub 2} thin films have been investigated as possible negative electrodes for lithium-ion batteries. The films have been cycled electrochemically vs. lithium and shown reversible capacity as high as 500 mAh/g over more than 100 cycles. The substantial irreversibility during the first cycle can be explained by the formation of metallic tin and amorphous lithium oxide. This last phase probably plays an important role in allowing the thin-film electrode to contract and expand during the cycling process.

Graphites for lithium-ion cells : The correlation of the first-cycle charge loss with the Brunauer-Emmett-Teller surface area

Abstract: The authors have investigated the correlation of the irreversible charge loss during the first lithium intercalation into graphite electrodes with the Brunauer-Emmett-Teller (BET) specific surface area of Timrex graphites. The irreversible charge loss typically increases with a higher BET specific surface area but also depends strongly on the graphite type. The authors have found that not only the total external surface area but also the ratio between the prismatic and basal surfaces affects the irreversible charge loss. The latter effect occurs because particular reactions associated with charge losses such as solvated lithium intercalation or self-discharge of intercalated lithium (reaction of Li{sub x}C{sub 6} with electrolyte components) take place only via/on the prismatic surfaces. The influence of the prismatic and basal surfaces is not taken into account if the irreversible charge losses are correlated only with the entire BET specific surface area of the samples. In order to describe the surface-dependent irreversible charge loss behavior of graphite the authors developed simple models that, in addition to the BET specific surface area, also take into account particle size distribution and particle morphology.

Self-Organized Formation of Hexagonal Pore Structures in Anodic Alumina

Abstract: The morphology and formation conditions of ordered hexagonal pore arrays in anodic alumina are discussed. The ordered arrangement of pores is shown to form by a self-organized process starting from randomly distributed pore positions at the surface of the alumina. The influence of the pretreatment of the aluminum substrate and the anodizing conditions on the growth kinetics and the tendency to form hexagonal pore structures were investigated. Homogeneous etching conditions are required in order to obtain regular pore arrays. This observation corresponds to the finding that hexagonal pore arrays are related to a smooth etching front and a homogeneous depth of neighboring pores.

Phenomenological theory of electro-osmotic effect and water management in polymer electrolyte proton-conducting membranes

Abstract: Partial dehydration of the proton-conducting membrane under working conditions is one of the major problems in low-temperature fuel cell technology. In this paper a model, which accounts for the electro-osmotically induced drag of water from anode to cathode and the counterflow in a hydraulic pressure gradient is proposed. A balance of these flows determines a gradient of water content across the membrane, which causes a decline of the current-voltage performance. Phenomenological transport equations coupled with the capillary pressure isotherm are used, involving the conductivity, permeability, and electro-osmotic drag coefficients dependent on the local water content. The effects of membrane parameters on current-voltage performance are investigated. A universal feature of the obtained current-voltage plots is the existence of a critical current at which the potential drop across the membrane increases dramatically due to the dehydration of membrane layers close to the anode. For a membrane with zero residual conductivity in its dry parts, the critical current is a limiting current. Well below the critical current the effect of dehydration is negligible and the current-voltage plot obeys Ohm`s law. The shape of the capillary pressure isotherm determines the nonohmic corrections. A comparison of the results of this study to those of themore » pertinent diffusion-type models reveals qualitatively different features, the convection model is found to be closer to experimental observations.« less

Corrosion Study of AA2024‐T3 by Scanning Kelvin Probe Force Microscopy and In Situ Atomic Force Microscopy Scratching

Abstract: The localized corrosion of AA2024-T3, and the behavior of intermetallic particles in particular, were studied using different capabilities of the atomic force microscope (AFM). The role of intermetallic particles in determining the locations and rates of localized corrosion was determined using scanning Kelvin probe force microscopy in air after exposure to chloride solutions. Al-Cu-Mg particles, which have a noble Volta potential in air because of an altered surface film, are actively dissolved in chloride solution after a certain induction time. Al-Cu(Fe, Mn) particles are heterogeneous in nature and exhibit nonuniform dissolution in chloride solution as well as trenching of the matrix around the particles. Light scratching of the surface by rastering with the AFM tip in contact mode in chloride solution results in accelerated dissolution of both pure Al and alloy 2024-T3. The abrasion associated with contact AFM in situ resulted in the immediate dissolution of the Al-Cu-Mg particles because of a destabilization of the surface film.

Impedance Study on the Electrochemical Lithium Intercalation into Natural Graphite Powder

Abstract: Electrochemical lithium intercalation into natural graphite powder of different sizes was studied by alternating current impedance spectroscopy. Impedance spectra at various potentials were fitted with a modified Randles equivalent circuit including a pseudocapacitance to express the observed finite diffusional behavior. The variations of electrochemical parameters with electrode potential, such as the charge-transfer resistance, the pseudocapacitance, the Warburg prefactor, and, finally, the chemical diffusion coefficient of lithium ion within graphite, were evaluated and discussed. It was shown that the charge-transfer reaction takes place on the whole surface of graphite particles, whereas lithium ion is intercalated from the edge plane and diffuses to the interior. The kinetics of the charge-transfer reaction was independent of the structure of the host. In contrast, the diffusivity of lithium ion within graphite was strongly dependent on the host structure, and the dependence was explained in terms of differences in in-plane and stacking order of lithium-graphite intercalation compounds formed by the intercalation.

Gas Conversion Impedance: A Test Geometry Effect in Characterization of Solid Oxide Fuel Cell Anodes

Abstract: The appearance of an extra arc in impedance spectra obtained on high performance solid oxide fuel cell (SOFC) anodes is recognized when experiments are conducted in a test setup where the working and reference electrodes are placed in separate atmospheres. A simple continuously stirred tank reactor (CSTR) model is used to illustrate how anodes measured with the reference electrode in an atmosphere separate from the working electrode are subject to an impedance contribution from gas conversion. The gas conversion impedance is split into a resistive and a capacitive part, and the dependences of these parameters on gas composition, temperature, gas flow rate, and rig geometry are quantified. The fuel gas flow rate per unit of anode area is decisive for the resistivity, whereas the capacitance is proportional to the CSTR volume of gas over the anode. The model predictions are compared to actual measurements on Ni/yttria stabilized zirconia cermet anodes for SOFC. The contribution of the gas conversion overpotential to dc current-voltage characteristics is deduced for H{sub 2}/H{sub 2}O and shown to have a slope of RT/2F in a Tafel plot.

Semiconducting Properties of Passive Films Formed on Stainless Steels Influence of the Alloying Elements

Abstract: Passive films formed on stainless steels in a borate buffer solution (pH 9.2) have been investigated by capacitance measurements and photoelectrochemistry. The study was carried out on films formed on AISI type 304 and 316 stainless steels and high purity alloys with differing chromium, nickel, and molybdenum contents. Complementary research by Auger analysis shows that the passive films are composed essentially of an inner chromium region in contact with the metallic substrate and an outer iron oxide region developed at the film/electrolyte interface. The semiconducting properties of the passive films are determined by those of the constituent chromium and iron oxides which are of p-type and n-type, respectively. Thus the influence of the alloying elements on the semiconducting properties of the passive films is explained by changes in the electronic structure of each of these two oxide regions.

X‐Ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy Study of Passive Films Formed on (100) Fe‐18Cr‐13Ni Single‐Crystal Surfaces

Abstract: X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were combined to investigate the thickness, chemical composition, and structure of passive films formed in 0.5 M H 2 SO 4 on (100)Fe-18Cr-13Ni. The XPS measurements show that aging under polarization at +500 mV/SHE causes a dehydration reaction of the outer chromium hydroxide layer of the passive film. This reaction results in a thickening of the mixed Cr(III) and Fe(III) inner oxide layer and increases the Cr 2 O 3 enrichment. This reaction consumes, in addition to chromium hydroxide of the outer layer, chromium from the metallic phase underneath the passive film. Only traces of nickel (hydroxide) are detected in the passive film, whereas Ni enrichment is observed in the alloy underneath the passive film. High-resolution STM images reveal that aging under polarization causes a crystallization of the inner Cr 2 O 3 oxide layer in epitaxy with the substrate. The epitaxial relationship is (0001) α-Cr 2 O 3 ||(100) Fe-18Cr-13Ni with [2130]α-Cr 2 O 3 ||[011] Fe-18Cr-13Ni. The crystallization proceeds with a faster kinetics than on (110) Fe-22Cr in the same conditions. The crystallization rate is modified by the presence of Ni in the alloy, which is enriched in the metallic phase underneath the film and slows down the formation of Cr 2 O 3 in the inner part of the film. This favors a more complete process of crystallization. Aging under polarization is beneficial to the further stability of the passive film in air.

Intermediate Temperature Solid Oxide Fuel Cells Using a New LaGaO3 Based Oxide Ion Conductor I. Doped as a New Cathode Material

Abstract: ‐based perovskite oxides doped with Sr and Mg exhibit high ionic conductivity over a wide range of oxygen partial pressure. In this study, the stability of ‐based oxide was investigated. The ‐based oxide was found to be very stable in reducing, oxidizing, and atmospheres. Solid oxide fuel cells (SOFCs) using ‐based perovskite‐type oxide as the electrolyte were studied for use in intermediate‐temperature SOFCs. The power‐generation characteristics of cells were strongly affected by the electrodes. Both Ni and (Ln:rare earth) were suitable for use as anode and cathode, respectively. Rare‐earth cations in the Ln site of the Co‐based perovskite cathode also had a significant effect on the power‐generation characteristics. In particular, a high power density could be attained in the temperature range 973–1273 K by using a doped for the cathode. Among the examined alkaline earth cations, Sr‐doped exhibits the smallest cathodic overpotential resulting in the highest power density. The electrical conductivity of increased with increasing Sr doped into the Sm site and attained a maximum at . The cathodic overpotential and internal resistance of the cell exhibited almost the opposite dependence on the amount of doped Sr. Consequently, the power density of the cell was a maximum when was used as the cathode. For this cell, the maximum power density was as high as 0.58 W/cm2 at 1073 K, even though a 0.5 mm thick electrolyte was used. This study revealed that a ‐based oxide for electrolyte and a ‐based oxide for the cathode are promising components for SOFCs operating at intermediate temperature.

A Novel Method of CO 2 Capture from High Temperature Gases

Abstract: Environmental concerns have stirred up much interest in CO{sub 2} exhaust from energy power plants. A large part of CO{sub 2} emitted to the atmosphere originates from the combustion of fossil fuels, especially coal and petroleum. To reduce the CO{sub 2} exhaust from flue gases, several methods of CO{sub 2} separation have been proposed. Lithium zirconate powder reacts immediately with ambient CO{sub 2} in the range of 450 to 550 C. Moreover, the products react and return reversibly to lithium zirconate at temperatures above 650 C. Utilizing this reaction, the possibility of a CO{sub 2} separation system which operates at around 550 C is suggested. A CO{sub 2} separation process operable at temperatures beyond 500 C may have a special benefit, because the separation can be achieved directly during the fuel reforming process, where high CO{sub 2} concentration is expected.

Proton Conductivity in Nafion® 117 and in a Novel Bis[(perfluoroalkyl)sulfonyl]imide Ionomer Membrane

Abstract: A study of proton conductivity in a commercial sample of Nation® 117 and a structurally similar bis[(perfluoroalkyl)sulfonyl]imide ionomer membrane under variable temperature and humidity conditions is reported. The sulfonyl imide ionomer was synthesized using a novel redox‐initiated emulsion copolymerization method, and conductivities were measured using a galvanostatic four‐point‐probe electrochemical impedance spectroscopy technique. Both materials exhibited a strong dependence of conductivity on temperature and humidity, with conductivity in both cases being strongly diminished with decreasing humidity (at constant temperature) and increasing temperature (at constant water partial pressure). The observed behavior is consistent with a "liquid‐like" mechanism of proton conductivity whereby protons are transported as hydrated hydronium ions through water‐filled pores and channels in the ionomer.

Mechanistic Aspects of Phenol Electrochemical Degradation by Oxidation on a Ta / PbO2 Anode

Abstract: The electrochemical oxidation of phenol in an aqueous solution is a complex transformation involving several transfer steps of oxygen atoms and electrons. Transfer of the oxygen atom occurs through the intermediary of hydroxyl radicals adsorbed on the active sites of the anode. Galvanostatic electrolyses of phenol (10.5 to 105 mmol/dm{sup 3}) in aqueous solution at pH 2 on a Ta/PbO{sub 2} anode were followed by high-pressure liquid chromatography and by analysis of the total organic carbon. Hydroquinone, catechol, 1,4-benzoquinone (1,4-BQ), maleic and fumaric acids, and carbon dioxide are the main products. The nonidentified products consist mainly of polymers. Study of the influence of temperature shows that the rate consumption of phenol initially at 21 mmol/dm{sup 3} is mass transport limited. CO{sub 2} is immediately formed following the 1,4-BQ-maleic acid pathway involving 20 faradays and forming 4 mol of CO{sub 2} and/or the 1,4-BQ-intermediary in C2 pathway at 16 faradays with formation of 2 mol of CO{sub 2}. The faradaic yield values show that a phenol molecule adsorbed on a catalytic site undergoes a succession of oxidation steps involving, on average, five electrons without desorption of the intermediate products. This number of electrons varies according to the operating conditions (temperature, anodicmore » current density, initial phenol concentration, hydrodynamic conditions, etc.). The mean faradaic yield decreases during electrolysis; it can reach 70% at the beginning of electrolysis of a 21 mmol/dm{sup 3} phenol solution for an anodic current density of 100 mA/cm{sup 2}. The phenol conversion into insoluble polymers increases as a function of its initial concentration and the anodic current density but it does not exceed 10%.« less