Research on the electrochemistry of oxygen ion conductors in the former Soviet Union. II. Perovskite-related oxides

The review is devoted to the analysis of experimental results on electrochemical and physicochemical properties of the perovskite-related oxide phases obtained at scientific centers of the former Soviet Union. The main attention is focused on oxides with high electronic conductivity, which are potentially useful as electrodes for high-temperature electrochemical cells with oxygen-ion conducting solid electrolytes and interconnectors of solid oxide fuel cells, and on mixed ionic-electronic conductors for oxygen separation membranes. Along with perovskite-like solid solutions based on LnMO_3−δ (Ln is a rare-earth element, M = Cr, Mn, Fe, Co, Ni) and SrCoO_3−δ, properties of the oxide phases Ln₂MO_4±δ (M = Cu, Ni, Co) with the K₂NiF₄-type structure are briefly reviewed. Received: 5 November 1998 / Accepted: 26 November 1998     

Research on the electrochemistry of oxygen ion conductors in the former Soviet Union

Following previous reviews of research results on oxygen ion-conducting materials obtained in the former USSR, this article addresses the case of Bi₂O₃-based compositions. Phase formation in oxide systems with Bi₂O₃, thermal expansion, stability, bulk transport properties and oxygen exchange of bismuth oxide solid electrolytes are briefly discussed. Primary attention is focused on oxides with high ionic and mixed conductivity, including stabilized fluorite-type (δ) and sillenite (γ) phases of Bi₂O₃, γ-Bi₄V₂O₁₁ and other compounds of the aurivillius series. Another major point being addressed is on the applicability of these materials in high-temperature electrochemical cells, which is limited by numerous specific disadvantages of Bi₂O₃-based ceramics. The electrochemical properties of various electrode systems with bismuth oxide electrolytes are also briefly analyzed. Electronic Publication     

Thermodynamics of oxygen in dilute liquid silver–tellurium alloys

Abstract  

The activity coefficient of oxygen in liquid Ag and binary Ag–Te dilute alloys were determined between 1,285 and 1,485 K by coulometric titration using the electrochemical cell (Ir, [O] in liquid metal or alloy | yttria stabilized zirconia | air, Pt). The experimental and evaluation procedures described in the literature were adopted. The oxygen activity coefficient was determined in pure liquid silver to be . Next, the oxygen activity coefficient in dilute Ag–(Te)–O alloys for variable X _Te content (from 0.01 to 0.06) was measured. From the obtained results, Wagner’s interaction parameter as a function of temperature was derived in the form . The electrochemical coulometric titration method seems to be very useful to study the thermodynamics of oxygen interaction in liquid silver and its alloys.     

Evaluation of the proton transfer kinetics of potential electrolytes in non-aqueous solutions using electrochemical techniques Part 1. Kinetic analysis of the general CE mechanism at stationary and rotating electrodes

The transport and kinetics of potential electrolytes (such as weak organic acids) at stationary and rotating electrodes have been examined in detail. A coherent mathematical analysis enabling the normalised current response to be evaluated has been developed, and various rate limiting scenarios have been identified and examined. Received: 13 March 1998 / Accepted: 31 July 1998     

Oxide nitrides: From oxides to solids with mobile nitrogen ions

The possibility of fast nitrogen ion conduction in solids is reviewed. Promising electrolytes based on three different base compounds are in the focus of this contribution: Zirconium oxide nitrides, tantalum oxide nitrides and mayenite-based materials. All aspects ranging from preparation methods, crystal structures (ideal and defect structure, also at elevated temperatures), transport properties (ionic and electronic conductivity, transference numbers, diffusion) and correlations between structure and physical properties are presented and discussed, in part also in relation to theoretical calculations. Fluorite-type quaternary oxide nitrides of zirconium are proven to be the first known materials with high nitrogen ion mobility. They can be described as fast mixed oxygen/nitrogen conductors but are limited due to the low maximum nitrogen/oxygen ratio achievable. Corresponding phases based on stabilized tantalum oxide nitrides have a superior N/O ratio but show poor thermal stability. For the development of a pure nitrogen ion conductor a different approach has also been investigated: Some cage compounds, in particular mayenite, allow the substitution of oxygen anions not tightly bound in the framework by nitrogen ions. Some of the obtained N-containing phases exhibit an outstanding electrical conductivity at low temperatures. Possible devices and applications such as a new type of a nitrogen sensor and an ammonia-producing fuel cell are introduced and discussed.     

Immobilized metal-ion affinity chromatography of peptides on metalloporphyrin stationary phases

The retention of selected dipeptides and tripeptides containing tyrosine was examined. As stationary phase an aminopropylated silica gel loaded with covalently linked tetraphenylporphyrin was used. The effect of metalization of porphyrin with Cu(II) and Zn(II) on retention was investigated. The observed separation is based on a mixed mechanism involving π-π and hydrophobic interactions as well as complex formation between immobilized metal ions and peptides. A satisfactory separation was demonstrated for C-peptide and bovine insulin. The possibility of separation of various insulins was also investigated. Received: 10 August 1998 / Revised: 21 December 1998 / Accepted: 28 December 1998     

Electrochemical promotion of catalytic reactions

The electrochemical promotion of heterogeneously catalyzed reactions (EPOC) became feasible through the use of porous metal electrodes interfaced to a solid electrolyte. With the O^2? conducting yttrium stabilized zirconia (YSZ), the Na⁺ conducting β″-Al₂O₃ (β-alumina), and several other types of solid electrolytes the EPOC effect has been demonstrated for about 100 reaction systems in studies conducted mainly in the mbar range. Surface science investigations showed that the physical basis for the EPOC effect lies in the electrochemically induced spillover of oxygen and alkali metal, respectively, onto the surface of the metal electrodes. For the catalytic promotion effect general concepts and mechanistic schemes were proposed but these concepts and schemes are largely speculative. Applying surface analytical tools to EPOC systems the proposed mechanistic schemes can be verified or invalidated. This report summarizes the progress which has been achieved in the mechanistic understanding of the EPOC effect.     

生物质气为燃料的燃料电池理论电动势计算

The electromotive force (e.m.f.) of solid oxide fuel cells using biomass produced gas (BPG) as the fuels is calculated at 700-1,200 K using an in-house computer program, based on thermodynamic equilibrium analysis. Tour program also predicts the concentration of oxygen in the fuel chamber as well as the concentration of equilibrium species such as H2, CO, CO2 and CH4. Compared with using hydrogen as a fuel, the e.m.f.for cells using BPG as the fuels is relative low and strongly influenced by carbon deposition. To remove carbon deposition, the optimum amount of H2O to add is determined at various operating temperatures.Further the e.m.f, for cells based on yttria stabilized zirconia and doped ceria as electrolytes are compared.The study reveals that when using BPG as fuel, the depression of e.m.f, for a SOFC using doped ceria as electrolyte is relatively small when compared with that using Yttria stabilized zirconia.     

Specific heat capacity and Debye temperature of zirconia and its solid solution

Specific heat C_P of zirconia and yttria stabilized zirconia doped or not with erbia and ceria was measured from 128 to 823 K and of yttria stabilized zirconia doped with erbia and plutonia from 443 to 1573 K. The new determined data were modelled using Debye theory. Data for the tetravalent oxide and for the studied solid solutions show that the extended Dulong and Petit law in Neumann-Kopp rule is verified for zirconia and the quaternary compounds. The Debye temperature of zirconia (590 K) and its yttria, erbia and ceria doped solid solutions (575-625 K) derived from these C_P measurements between 150 and 823 K is discussed and compared with that reported for other tetravalent metal oxides.     

Application of abrasive stripping voltammetry in corrosion science. I. determination of the corrosion potentials of metals

The application of abrasive stripping voltammetry for the determination of the corrosion potential of metal powders is described. Micro amounts of a metal are fixed on a paraffin-impregnated graphite electrode immersed in suitable electrolyte solutions and polarization curves are recorded. The value of the corrosion potential is determined by Tafel analysis. The precision of the method is ±0.5% (mean rel. dev.) of the final potential value. Received: 24 March 1998 / Accepted: 27 August 1998     

Electrochemical oxidation of a carbon black loaded polymer electrode in aqueous electrolytes

The suitability of a polymeric composite material for use as part of an anode structure in a cathodic protection system has been examined. The composite material was a conductive blend (volume resistivity typically 1.5 Ω cm) of carbon black in a polyethylene binder. A long operational lifetime for the material demands that the rate of carbon loss must be low. In the work reported here, electrochemical and in situ analytical techniques were employed to characterise the performance of the material over a wide range of anodic current densities in a variety of aqueous electrolytes. The predominant anodic electrochemical reaction on the polymeric material is CO₂ formation in acid and neutral solutions, which causes loss of carbon from the surface and the development of a non-conducting layer of polyethylene. The characteristics of the reaction suggest that it occurs via the discharge of H₂O. In alkaline pH, however, the anodic reactions are more complex. A high OH⁻ concentration (pH 12 or higher) favours the formation of oxygen rather than CO₂, particularly at low anodic potentials. The presence of CO₃ ²⁻ in the electrolyte catalyses the evolution of oxygen at pH values as low as 9. The electrochemical formation of oxygen always occurs in parallel with the generation of some humic acid in the solution. Received: 23 July 1998 / Accepted: 17 November 1998     

Ion exchange as a simple and effective tool for screening possible cation conductors

Although kinetics of the low-temperature cation exchange in mixed oxide materials (aluminates, gallates, titanates, niobates, tantalates, antimonates, phosphates etc.) cannot provide quantitative information on self-diffusion and ionic conductivity in the starting material due to the mixed cation effect, it is the most direct and simple qualitative indication of the cation mobility in the solid state. It does not need using ceramics and single crystals and thus represents a useful tool for rapid selection of prospective cation conductors for subsequent detailed studies of dense samples with electrical methods. Examples of solid electrolytes discovered owing to their ion-exchange properties are reviewed, and rational principles of the ion-exchange testing are discussed. Laws of ion-exchange equilibria are based on ionic size compliance and the principle of hard and soft acids and bases. The former is most important for alkali/alkali exchange and the latter for exchanging cations of similar size but having different electronic structures: those of the rare-gas type and those having 18- or 18 + 2-electron shells, like Na⁺ and Ag⁺ or K⁺ and Tl⁺. Ion-exchange testing is especially useful for structures with non-intersecting conduction paths. It is shown that the resistivity of crystals with non-parallel and non-intersecting conduction paths cannot be described by the classical tensor formalism. Significant differences between isotope exchange, chemical ion exchange and ion conduction, quasi-one-dimensional and true one-dimensional conductors and single- and multiple-barrelled non-intersecting channels are disclosed and discussed.     

高分子固体电解质研究的新进展

本文简要总结了近年来国内外高分子固体电解质的研究成就,着重介绍其最新发展动向.     

Determination of carbaryl in foods by solid-phase room-temperature phosphorimetry

A phosphorimetric solid phase assay is proposed for the determination of the pesticide carbaryl (CBL) at room temperature. CBL was spotted on filter paper together with Tl(I) as heavy metal, and dried for 3 min, after which the diffuse transmitted phosphorescence was measured using two quartz plates to avoid the quenching effect produced by atmospheric oxygen. The linear dynamic range was 0.5–4.0 μg/mL and the detection and quantification limits were 0.09 and 0.31 μg/mL, respectively. The precision of the method, expressed as relative standard deviation, was 2.3% for a sample containing 2.0 μg/mL of CBL. The method was applied to the determination of CBL residues in cereals, potatoes and waters, obtaining recoveries ranging between 92 and 105%. Received: 10 October 1997 / Revised: 2 February 1998 / Accepted: 7 February 1998     

Study of electrochemical palladium behavior by the quartz crystal microbalance. I. Acidic Solutions

Sorption of hydrogen in a palladium electrode with a limited volume has been studied using the quartz crystal microbalance (QCM). During the hydrogen sorption process in the palladium electrode, strains are generated inside the metal which result in changes in the frequency of the crystal. These stresses change in a non-linear manner during electrode saturation, with α- and β-phases. This effect creates significant problems with the objective estimation of the amount of sorbed hydrogen inside the palladium electrode using the QCM method. This method is more accurate for the study of electrode surface processes, i.e. specific anion adsorption on the electrode surface or electrode dissolution. Received: 6 August 1998 / Accepted: 1 December 1998     

Unusual exchange of functional groups at the silicon and metal atoms

Reactions of α,ω-dichlorooligosilanes with metal hydroxides in the presence of pyridine results in the exchange of halogen and oxygen atoms and the formation of cyclic siloxyoligosilanes and pyridine complexes of metal halides. The metallosiloxane groups are not formed. A scheme for the transformations observed is proposed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1658–1659, August, 1998.     

Determination of metal traces in wine by argon stabilized d.c. arc

A spetroscopic method for the determination of metal traces (Mn, Cu, Fe) in wine has been developed based on argon stabilized d.c. arc plasma at atmospheric pressure. The experimental conditions were optimized using lateral distributions of spectral line intensities of the trace elements in aqueous and ethanol-aqueous solutions. The method was applied to the analysis of 6 wines from 3 Serbian wine-growing regions. Direct and standard addition methods were tested. The precision of the method is characterized by a relative standard deviation of 0.50– 3.00%. The accuracy of the method was assessed by flame AAS. Received: 8 October 1998 / Revised: 17 March 1999 / Accepted: 20 March 1999     

Stability and functional properties of Sr0.7Ce0.3MnO3 − δ as cathode material for solid oxide fuel cells

Studies of oxygen diffusion, interphase exchange, specific electric conductivity, and thermal expansion showed that perovskite-like Sr_0.7Ce_0.3MnO_{3 ? δ} (SCMO) as a potential cathode material for solid oxide fuel cells (SOFCs) has considerable advantages over the conventional materials based on lanthanum-strontium manganites. To prevent the interactions of SCMO with solid electrolyte membranes of stabilized zirconia and lanthanum gallate, it is necessary to deposit protective layers of solid solutions based on cerium oxide, which do not form new phases in contact with SCMO and electrolytes. The trials of model SOFCs with porous SCMO-based cathodes demonstrated satisfactory electrochemical and endurance characteristics of these electrodes.     

Solid Ion Conductors in Heterogeneous Catalysis

The electrochemical measurement of oxygen activity using ion-conducting solid electrolytes (λ-sensors) has become widely known, at least since the application of three-way catalysts in the postcombustion of exhaust gases from spark-ignition engines. However, the use of solid ion conductors is not limited to control devices. There are various other potential applications and numerous problems which can be studied: the formation of oxides in the course of catalytic reactions on metal surfaces, the improvement of selectivity and yield of catalytic reactions, such as the epoxidation of ethylene on silver catalysts and, finally, the cogeneration of electrical energy during oxidation reactions, such as the partial oxidation of methanol to formaldehyde.     

Stabilized Zirconia-Based Nanostructured Powders for Solid-Oxide Fuel Cells

High-purity uniform powders of zirconia-based solid electrolytes stabilized with yttria (4 and 8 mol %) are synthesized by co-precipitation with subsequent annealing at different temperatures. The obtained powders were studied using X-ray analysis and transmission electron microscopy; the specific surface area was measured by nitrogen adsorption. The stabilized zirconia powder sintering was studied over temperature range from 1000 to 1600°C. The ionic conductivity of samples containing 8 mol % of yttria was 0.06–0.07 S/сm, that is comparable with that obtained with commercial solid electrolytes.