Microstructure of the LaSrCuO<Subscript>4 − δ</Subscript>|Ce<Subscript>0.9</Subscript>Gd<Subscript>0.1</Subscript>O<Subscript>2 − δ</Subscript> interface and its reversibility with respect to oxygen

The conditions of formation of electrode/electrolyte interfaces LaSrCuO_{4 − δ}|Ce_0.9Gd_0.1O_{2 − δ} are optimized. It is shown that electrode layers formed by the screen printing method have better developed surfaces and are more uniform and strong as compared with thick film layers applied by a brush. Symmetric LaSrCuO_{4 − δ}|Ce_0.9Gd_0.1O_{2 − δ}|LaSrCuO_{4 − δ} cells with porous electrodes are studied by impedance spectroscopy and cyclic voltammetry in the temperature range of 773–1173 K at the oxygen partial pressure of (28–2.1) × 10⁴ Pa. The oxygen process is shown to be limited by the charge transfer across the electrode/electrolyte interface. The exchange currents are calculated in the temperature range of 773–1173 K to amount from 1 × 10⁻³ to 3.5 × 10⁻² A/cm², which points to the high reversibility of the electrode/electrolyte interface with respect to oxygen.     

The effect of loading and particle size on the oxygen reaction in CGO impregnated Pt electrodes

Porous platinum electrodes impregnated with Gd _x Ce_1−x O_2−δ (CGO) are investigated to characterise how nano-sized CGO grains affect the oxygen reaction. Impedance measurements were performed at temperatures between 450 and 750 °C and at oxygen partial pressures of 0.2 and 5 × 10⁻⁵ bar for electrodes with various CGO loadings and electrodes annealed at various temperatures. The morphology was characterised by scanning electron microscopy and the CGO grain size was determined from X-ray diffraction peak broadening. The results showed that the polarisation resistance decreased with increasing CGO loading and increasing annealing temperature. CGO facilitates transport of oxygen ions thereby increasing the effective triple-phase boundary.     

Kinetics of the oxygen transfer reaction at the (U0.5Sc0.5)O2±x/YSZ interface by impedance spectroscopy

The complex impedance dispersion analysis technique was used to study the electrode kinetics of (U_0.5Sc_0.5)O_2±x, a fluorite type solid solution material potentially suitable as electrode for low temperature oxygen sensors. Variables included the temperature and oxygen partial pressure. The effect of heat treatment on the interfacial contact resistance and the electrode morphology was also investigated. A single are for the electrode reaction was observed over most of the experimental ranges of temperature and oxygen partial pressure. The angle of depression of the electrode are was small (8–18°) compared with platinum electrodes (20–45°). The activation energy for the overall electrode reaction was between 170 and 180 kJ mol^?1. The average value for the pressure exponent, determined from the oxygen partial pressure dependence of the electrode resistance, was 0.16. A mechanism for the oxygen transfer reaction is proposed. Materials of this type show promise for future use in low temperature oxygen sensors.     

Ionic conductivity and thermal expansion of anion-deficient Sr11Mo4O23 perovskite

Transport properties of perovskite-type Sr₁₁Mo₄O₂₃ and composite Sr₁₁Mo₄O₂₃ - 1 wt% Al₂O₃ were studied at 400–1300 K in the oxygen partial pressure range from 0.21 down to 10⁻¹⁹ atm. The electromotive force and faradaic efficiency measurements, in combination with the energy-dispersive spectroscopy of the fractured electrochemical cells, unambiguously showed prevailing role of the oxygen ionic conductivity under oxidizing conditions. At temperatures above 600 K, protonic and cationic transport can be neglected. The oxygen ion transference numbers vary in the range of 0.95–1.00 at 973–1223 K. At temperatures lower than 550 K, the total conductivity of Sr₁₁Mo₄O₂₃ - 1 wt% Al₂O₃ composite measured by impedance spectroscopy tends to increase in wet atmospheres, thus indicating that hydration and protonic transport become significant. Reducing oxygen partial pressure below 10⁻¹⁰–10⁻⁹ atm leads to a significant increase in the n-type electronic conduction. The average thermal expansion coefficients in oxidizing atmospheres are (14.3–15.0) × 10⁻⁶ K⁻¹ at 340–740 K and (18.3–19.2) × 10⁻⁶ K⁻¹ at 870–1370 K.

     

Electrical conduction nature and phase transition in CaTi1−x FexO3−δ (x = 0.1–0.5)

Transport numbers for oxygen ions and protons are measured by an emf method in the system CaTi_1?x Fe_xO_3?δ (x = 0.1–0.5) in the oxidizing and reducing atmospheres in the temperature interval 973–1173 K. It is shown that the compounds under study are mixed ion-electron conductors at small iron concentrations and electron conductors, at large iron contents. The proton conductivity in the compounds is very poor and does not exceed 0.5% in air. On the basis of the temperature dependences of transport numbers for ions and linear expansion, it is established that the CaTi_0.9Fe_0.1O_3?δ system has a phase transition of a second order in a reducing environment at 1020–1050 K. The total and partial electron conductivities of CaTi_0.9Fe_0.1O_3?δ are studied as a function of the partial pressure of oxygen at 1173 K. The nature of electroconduction in CaTi_1?x Fe_xO_3?δ is discussed.     

Electrochemical performance of strontium-doped neodymium nickelate mixed ionic–electronic conductor for intermediate temperature solid oxide fuel cells

The Nd_2???x Sr _x NiO_4?+?δ (x?=?0.1–0.5) solid solutions prepared by combustion synthesis are of submicron/superfine crystallite size. The crystal structure estimated by Rietveld analysis reveals increase in space in the rock salt layer on partial replacement of Nd³⁺ by Sr²⁺. The transition from negative temperature coefficient to positive temperature coefficient of conductivity is observed at 913 K. The maximum dc conductivity (σ?=?1.3?±?0.02 S?cm^?1 at 973 K) is obtained for x?=?0.2 in Nd_2???x Sr _x NiO_4?+?δ . The low dc conductivity compared with reported (≈100 S?cm^?1) is due to high porosity (low relative density) resulting from agglomeration of submicron crystallites. The variation in the conductivity with Sr content in Nd_2???x Sr _x NiO_4?+?δ is understood on the basis of defect chemistry. The electrochemical properties of the cathode materials are studied using electrochemical impedance spectroscopy at various temperatures and oxygen partial pressures. Nd_1.8Sr_0.2NiO_4?+?δ cathode exhibits lowest-area-specific resistance?=?0.52?±?0.015 Ohm?cm² at 973 K. At low $ {P_{{{{{\bf O}}_2}}}} $ (<1,000 Pa), oxygen ion transfer from Nd_1.8Sr_0.2NiO_4?+?δ cathode to gadolinium-doped ceria electrolyte is the rate-limiting step, whereas, charge-transfer reaction on the cathode becomes more important at high oxygen partial pressures and temperature (973 K).     

Improved electrochemical performance of Ca2Fe1.4Co0.6O5–Ce0.9Gd0.1O1.95 composite cathodes for intermediate-temperature solid oxide fuel cells

The performance of Ca₂Fe_1.4Co_0.6O₅–Ce_0.9Gd_0.1O_1.95 (CFC–CGO) composite cathode has been investigated for potential application in intermediate-temperature solid oxide fuel cells (IT-SOFCs). The composite cathodes are prepared and characterized by XRD and SEM, respectively. The electrochemical properties of the composite cathodes are investigated using AC impedance and DC polarization methods from 500 to 700 °C under different oxygen partial pressures. The polarization resistance (R _p) decreases with the increase of CGO content in the composite electrode. The addition of 40 wt.% CGO in CFC results in the lowest R _p of 0.48 Ω cm² at 700 °C in air. Oxygen partial pressure dependence study indicates that the charge-transfer process is the rate limiting step for oxygen reduction reaction. CFC-40CGO composite cathode exhibits the lowest overpotential of about 67 mV at a current density of 85 mA cm⁻² at 700 °C in air.     

Pr<Subscript>5</Subscript>Mo<Subscript>3</Subscript>O<Subscript>16 + δ</Subscript>: A New Anode Material for Solid Oxide Fuel Cells

The thermomechanical and electrical conductivity properties of praseodymium molybdate Pr₅Mo₃O_{16 + δ} prepared by a solid-phase method were studied. The electrical conductivity of praseodymium molybdate samples measured at temperatures in the range 373–1173 K with the oxygen partial pressure in the gas of 10^–3 to 0.21 atm was found to increase from ~10^–7 to ~10^–2 S/cm and to be almost independent of oxygen pressure. It is for the first time that electrical conductivity a reductive atmosphere (Ar/H₂ 5%) was found to increase from 0.1 to 1.2 S/cm in the same temperature range. Studies of the chemical stability of Pr₅Mo₃O_{16 + δ} with respect to solid electrolytes showed the absence of chemical reactions with GDC at 1273 K and with YSZ at 1223 K. The combination of these properties evidences for the potential of praseodymium molybdate for use as an anode material for solid oxide fuel cells (SOFCs).     

Experimental and computational studies of the thermal decomposition of halon 1211

Thermal pyrolysis of halon 1211 (CBrClF₂), diluted in nitrogen, in a tubular alumina reactor, has been studied over the temperature range of 773–1073 K at residence times from 0.3 to 2 s. At temperatures below 973 K, the major products were CCl₂F₂, CBr₂F₂, C₂Cl₂F₄, C₂BrClF₄, C₂F₄, and C₂Br₂F₄. Further increasing temperature resulted in the formation of CBrF₃, CClF₃, and many other species whose formation necessitated the rupture of C? F bonds. Coke formation was also observed on the surface of the reactor at high temperatures. A kinetic reaction scheme involving 16 species and 25 reaction steps was developed and applied to model the thermal pyrolysis of halon 1211 over the temperature range of 773–973 K. Sensitivity analysis suggests that the reaction CBrClF₂ + CClF₂→CCl₂F₂ + CBrF₂ constitutes the major pathway for the decomposition of halon 1211 under the conditions investigated. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 134–146, 2005     

The electrochemical behavior of the LaSrCuO<Subscript>4 − δ</Subscript>/Ce<Subscript>0.9</Subscript>Gd<Subscript>0.1</Subscript>O<Subscript>2 − δ</Subscript> interface

The electrochemical behavior of the LaSrCuO_{4 − δ}/Ce_0.9Gd_0.1O_{2 − δ} interface is studied by impedance spectroscopy and cyclic voltammetry methods. By analyzing the dependence of the impedance frequency spectra on the oxygen partial pressure, the rate-determining stages of oxygen exchange are determined in the temperature interval of 500–900°C. For temperatures above 700°C, the adsorption of oxygen molecules and their dissociation to oxygen atoms are shown to make a substantial contribution to the polarization resistance of the overall electrode process, besides the charge-transfer resistance.     

Conductivity of Lanthanum-Strontium Cuprate LaSrCuO<Subscript>4−δ</Subscript> and Ionic Reversibility of Relevant Electrodes

A single-phase sample of the LaSrCuO_3.58 composition is prepared by solid-state synthesis at 1473 K in air. The conductivity of LaSrCuO_3.58 is measured by a four-probe method at direct current in the temperature range from 298 to 1173 K at oxygen partial pressures from 28 to 2.1 × 10⁴ Pa. Heating samples above ∼670 K is shown to result in a changeover of the conduction type from semiconducting (p-type) to metallic. Exchange currents at the LSCuO/YSZ interface are measured by impedance spectroscopy. The measurements are carried out in high-density symmetrical cells LSCuO|YSZ|LSCuO, fabricated by hot pressing of powders under a pressure of 3.5 × 10⁹ Pa at 773 K. Experimental exchange currents i ₀ (varying from 10⁻³ to 10⁻⁴ A/cm² in the temperature interval from 800 to 1173 K) are comparable with such of materials based of lanthanum-strontium manganite.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 516–522.Original Russian Text Copyright © 2005 by Mazo, Savvin, Mychka, Dobrovol’skii, Leonova.     

Electrochemical behavior of uranium(III) in NaCl–KCl molten salt

The electro-redox behavior of uranium(III) on Mo electrode in NaCl–KCl molten salt in the temperature range 973–1073 K has been investigated using cyclic voltammetry electrochemical method and so on, such research will help to understand uranium behavior in pyro-reprocessing. The results showed that UCl₃ could be reduced into uranium metal in a quasi-reversible one-step process exchanging three electrons. The diffusion coefficients of U(III) ions were determined and the activation energy for diffusion was found to be 55.794 kJ mol⁻¹. The apparent standard potentials of U(III)/U(0) at several temperatures were calculated. The thermodynamic properties of UCl₃ have also been investigated.

     

The sequence of equilibrium phase states of the Tb-Mn-O system in the thermal dissociation of the TbMn<Subscript>2</Subscript>O<Subscript>5</Subscript> compound

Phase equilibria in the Tb-Mn-O system during the removal of oxygen from the TbMn₂O₅ compound in stages were studied by the static method on a vacuum circulation unit (973–1173 K) with subsequent X-ray analysis of quenched solid phases. The dissociation of TbMn₂O₅ was found to occur in three stages. The temperature dependences of equilibrium oxygen pressure were determined experimentally for the phase equilibria observed. The standard thermodynamic functions of the dissociation and formation from the elements of TbMn₂O₅ and TbMnO₃ were calculated.     

Kinetic study on preparation of substoichiometric tungsten oxide WO2.72 via hydrogen reduction process

Controlling the conditions of the oxygen partial pressure and temperature to prepare the WO_2.72 (W₁₈O₄₉) via reduction was possible through thermodynamic consideration. WO_2.72 was synthesized via heating to 1073 K in 5% H₂–95% Ar mixture gas flow from ammonium tungstate which was prepared by hydrothermal process. With the reducing prolonging time, the products were changed from WO_2.72 to WO₂ and then metal W. Thermogravimetric (TG) analysis showed ammonium tungstate decomposed completely to WO₃ at 773 K. Isothermal reductions using TG analysis were carried out at 905 K, 925 K, 945 K and 973 K in 5% H₂–95% Ar mixture gas flow, respectively. The whole reduction from WO₃ to WO_2.72 divided into three parts: initial nucleation and growth stage, final interfacial reaction stage and intermediate stage, was controlled jointly by both mechanisms. Fitting results showed that the initial stage obey the one-dimensional Avrami–Erofeev equation, the apparent activation energy was 132.7 ± 1.1 kJ mol⁻¹ and the pre-exponent factor was 4.82 × 10⁵ min⁻¹; the final stage expressed by 2-dimensional interfacial reaction, the apparent activation energy was 144.0 ± 2.1 kJ mol⁻¹ and the pre-exponent factor was 3.20 × 10⁵ min⁻¹.

     

Study on Gd3+ and Sm3+ co-doped ceria-based electrolytes

Doped ceria electrolytes of Ce_1-aGd_a-ySm_yO_2–0.5a, wherein a=0.15 or 0.2, and 0ya, were prepared with the citrate method, and characterized by inductively coupled plasma–atomic emission spectrometry, energy dispersive spectrometry, scanning electron microscopy, powder X-ray diffraction, and AC impedance spectroscopy. The effect of composition on the structure and conductivity was studied. All the samples were fluorite-type ceria-based solid solutions. For the singly doped samples, the optimal composition was Ce_0.85Gd_0.15O_1.925 for Gd³⁺-doped ceria (CGO), which showed higher ionic conductivity than the best Sm³⁺-doped ceria (CSO) at 773–973 K. For the co-doped samples, the ionic conductivities were higher than those of the singly doped ones in the temperature range 673–973 K when a=0.15, but only better in 673–773 K when a=0.2. For the samples of Ce_0.85Gd_0.15-ySm_yO_1.925, wherein 0.05y0.1, much higher ionic conductivity was observed than those of the singly doped ceria at 773K~973 K. Therefore, these co-doped samples would be better than CGO and CSO to be the electrolytes of intermediate-temperature solid oxide fuel cells.     

Electrocatalytic properties of La0.9Sr0.1MnO3-based electrodes for oxygen reduction

Electrochemical reduction of oxygen at the interface between a La_0.9Sr_0.1MnO₃ (LSM)-based electrode and an electrolyte, either yttria-stabilized-zirconia (YSZ) or La_0.8Sr_0.2Ga_0.9Mg_0.1O₃ (LSGM), has been investigated using DC polarization, impedance spectroscopy, and potential step methods at temperatures from 1053 to 1173 K. Results show that the mechanism of oxygen reduction at an LSM/electrolyte interface changes with the type of electrolyte. At an LSM/YSZ interface, the apparent cathodic charge transfer coefficient is about 1 at high temperatures, implying that the rate-determining step (r.d.s.) is the diffusion of partially reduced oxygen species, while at an LSM/LSGM interface the cathodic charge transfer coefficient is about 0.5, implying that the r.d.s. is the donation of electrons to atomic oxygen. The relaxation behavior of the LSM/electrolyte interfaces displays an even more dramatic dependence on the type of electrolyte. Under cathodic polarization, the current passing through an LSM/YSZ interface increases with time whereas that through an LSM/LSGM interface decreases with time, further confirming that it is the triple phase boundaries (TPBs), rather than the surface of the LSM or the LSM/gas interface, that dominate the electrode kinetics when LSM is used as an electrode. Electronic Publication     

La0.8Sr0.2Ga0.8Mg0.2O3与La0.8Sr0.2Ga0.8Mg0.15Co0.05O3电导的对比

采用固相合成法制备了La0.8Sr0.2Ga0.8Mg0.2O3(LSGM8282)和La0.8Sr0.2Ga0.8Mg0.15Co0.05O3 (LSGMC5), 利用四电极交流阻抗法和Hebb-Wagner 极化法对比研究了两种材料的总电导率和电子电导率. 实验结果表明, LSGM8282 的总电导率与氧分压无明显依赖关系, 而LSGMC5 的总电导率在高氧分压区随氧分压降低而增加,在中等氧分压区域基本保持不变. 在973-1173 K的温度范围内, LSGM8282的自由电子电导率以及电子空穴电导率的氧分压级数分别为-1/4和1/4.在1073-1173 K的温度范围内, LSGMC5的自由电子电导率以及电子空穴电导率的氧分压级数分别为-1/4和约为1/8, 表明LSGMC5的空穴产生机制可能与LSGM8282不同. LSGM8282 的氧离子电导率与氧分压无关, 而LSGMC5 的氧离子电导率在高氧分压区随氧分压的减小而增加.     

REDUCTION OF NO BY CO OVER NiWO4, NiO, AND WO3 CATALYSTS

We present a comparative study of NiWO₄, NiO, and WO₃ catalysts for simultaneous conversion of NO and CO. Samples were synthesized by reacting ammonium metatungstate and/or nickel nitrate at high temperature (773 K to 903 K) under an oxygen stream. Catalysts were characterized by X-ray diffraction, surface area measurements, energy dispersive spectroscopy and scanning electron microscopy. The catalytic reduction of NO by CO took place in the temperature range (523 to 973) K under highly reductive conditions (NO:CO= 1:5) over NiWO₄NiO, and WO₃, respectively. The 100 % NO conversion at GHSV of 11460 h^-1 was achieved at 773 K over NiWO₄ and at 848 K over NiO. The WO₃ was deactivated at 898 K. However, in the range (523 to 723) K NiO was more active than NiWO₄ and WO₃ catalysts.     

Redox Potentials of Samarium and Europium in Molten Cesium Chloride

The method of direct potentiometry was applied to measure redox potentials of Sm^{3 +}/Sm^{2 +} and Eu^{3 +}/Eu^{2 +} in molten cesium chloride relative to chlorine reference electrode in the temperature range 973-1173 K. Changes in the Gibbs energy of the redox reaction LnCl₂(l) + 1/2Cl₂(g) LnCl₃(l) were calculated.     

Electrically driven oxygen separation through gadolinia-doped ceria using PrBaCo2O5?+?x and NdBaCo2O5?+?x electrodes

Oxygen gas can be electrochemically separated from ambient air with very high purity and compressed by using a solid-electrolyte ion-transport membrane. An electrolyte with high ionic conductivity such as gadolinium-doped ceria (CGO) and mixed conducting electrodes are used to construct the electrochemical cell. To achieve high oxygen flux, the electrodes must exhibit very fast electrode kinetics. Here, we report the performances of mixed conducting PrBaCo₂O_5 + x and NdBaCo₂O_5 + x electrodes in oxygen separation in a planar CGO electrolyte-supported cell. The properties of the electrode materials were evaluated using potentiostatic and potentiodynamic measurements and alternating current impedance spectroscopy. The oxygen flux was also measured using gas chromatography to confirm the absence of gas leaks. The electrodes demonstrated very low polarization resistances as a result of very high cathodic and anodic reaction rates at temperatures of 600–800 °C. High oxygen gas flow rates were observed on applying potentials up to 1 V with an almost linear relationship between the applied potential and the molar flow rate of oxygen gas.