Electrical transport and kinetics of electrode processes in the system of (H<Subscript>2</Subscript> + H<Subscript>2</Subscript>O + Ar), Pt|La<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>ScO<Subscript>3–δ</Subscript>

The method of impedance spectroscopy is used to study the electrochemical behavior of (H₂ + H₂O + Ar), Pt|La_0.9Sr_0.1ScO_3–δ system depending on the temperature (500–700°C) and gas phase composition in the range of hydrogen gas pressures of–1.3 < log(pH₂) <–0.01 and water vapor gas pressures of–1.3 < log(pH₂O) <–0.03. It is found that conductivity of electrolyte increases and polarization resistance decreases at an increase in water and hydrogen content in the gas phase. The possible causes of such behavior of conductivity are considered and the possible mechanism of the electrode reaction is suggested for the temperature of 700°C.     

Nature of conductivity of Perovskites La1 ? x Sr x ScO3 ? α (x = 0.01–0.15) under oxidative and reducing conditions

Transport numbers of ions and protons are measured on ceramic samples of La_{1 ? x} Sr _x ScO_{3 ? ??} (x = 0.01?C0.15); partial conductivities (hole, proton, and oxygen-ion) are determined in the temperature range of 500?C900°C at pH₂O = 0.04?C2.35 kPa and pO₂ from air to 10^?15 Pa.     

Phase composition and conductivity of La1 ? x Sr x ScO3 ? α (x = 0.01?0.20) under oxidative conditions

The phase composition was studied and overall conductivity of oxides La_{1 ? x} Sr _x ScO_{3 ? ??} (x = 0.01?0.20) was measured as dependent on air humidity (pH₂O = 0.04?2.35 kPa) in the temperature range from 100 to 900°C. The samples were synthesized in air at 1600°C. They are single-phase, with a perovskitetype structure with orthorhombic distortions and the density of 94?C99%. The conductivity measurements were carried out using the impedance technique and four-probe dc technique. The contributions of bulk and grain boundary resistances were determined, effective conductivity activation energies were calculated.     

Exchange kinetics and diffusion of oxygen in systems based on lanthanum gallate

The oxygen exchange and diffusion kinetics are studied by means of isotopic exchange in complex oxides La_0.80Sr_0.20Ga_{0.85 ? x} Mg_0.15Co _x O_{3 ? δ} (x = 0.00, 0.05, 0.15, 0.20, 0.25) and in electrolyte La_0.88Sr_0.12Ga_0.82Mg_0.18O_{3 ? δ} with gold-coated surface. The oxygen exchange rates and diffusion coefficients are determined in the bulk material and near the surface within the range of temperatures from 600 to 900°C and oxygen pressures from 0.3 to 1.0 kPa. The activation energies of the oxygen exchange and diffusion processes are calculated. By cobalt doping, gallium sublattice is shown to increase the interphase exchange rate; however, the oxygen transport properties in the bulk phase are not significantly affected by cobalt, while cobalt additive decreases the activation barrier of oxygen diffusion in the near-surface area. Gold activates the electrolyte surface, affecting the exchange rate and increasing the first-type exchange share. Cobalt substitution increases the third-type exchange share as compared to that of La_0.88Sr_0.12Ga_0.82Mg_0.18O_{3 ? δ} (x = 0.00, 0.05, 0.15, 0.20, 0.25) and in elec. The oxygen exchange with gas phase is assumed to be limited by molecular oxygen adsorption-desorption process on the electrolyte surface.     

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).     

Die Bestimmung der Bildungsenthalpie von RhI3,s mit Hilfe einer weiterentwickelten Wägemethode zur Messung temperaturabhängiger Partialdrücke in geschlossenen Systemen

Determination of Temperature Dependent Partial Pressures in Closed Systems. A New Improved Method. The Heat of Formation of RhI_3,s An improved method to determine temperature dependent partial pressures of gaseous species in equilibria with condensed phases in closed systems (silica ampoules) at temperatures up to 1000 °C and pressures p_i 0.1 < p_i < 15 bar is presented. The method is based on the determination of the change of mass in the gasphase caused by solid gas transition at higher temperatures concerning substances which are deposited at one end of the ampoule. The results of the measurements give informations about reaction mechanisms, and enthalpies. The reliability of the method is demonstrated at the example of the heat of formation of RhI_3,s (δ_BH°(RhI_3,s, 298) = –142.2 kJ · mol^–1).     

Cathodic materials for intermediate-temperature solid oxide fuel cells based on praseodymium nickelates-cobaltites

A unique combination of methods (TPD of O₂, thermogravimetry, isotopic heteroexchange of oxygen in different modes) was used to carry out detailed studies of oxygen mobility and reactivity in mixed praseodymium nickelates-cobaltites (PrNi_{1 ? x} Co _x O_{3 + δ}) and their composites with doped cerium dioxide (Ce_0.9Y_0.1O_{2 ? δ}) as promising cathodic materials stable towards the effect of CO₂ in the intermediate-temperature region. It is shown that in the case of composites of PrNi_{1 ? x} Co _x O_3+δ-Ce_0.9Y_0.1O_{2 ? δ} synthesized using the Pechini method and ultrasonic treatment, stabilization of the disordered cubic perovskite phase due to redistribution of cations between the phases provides high oxygen mobility. Preliminary results on tests of cathodic materials of this type supported on planar NiO/YSZ anodes (H.C. Starck) with a thin layer of YSZ electrolyte and a buffer Ce_0.9Y_0.1O_{2 ? δ} layer showed that power density of up to 0.4 W/cm² was reached in the region of medium (600–700°C) temperatures, which was close to typical values for fuel cells of this type with cathodes based on strontium-doped perovskites and their composites with electrolytes.     

Electrocatalytic properties of an Sr0.25Bi0.5FeO3– δ/LSGM interface

Sr_0.25Bi_0.5FeO_{3– δ} (SBF) has been studied as a cathode material for low- and intermediate-temperature (600–850 °C) solid oxide fuel cells (SOFCs) based on the La_0.9Sr_0.1Ga_0.9Mg_0.1O₃ (LSGM) electrolyte. The observed cathodic current density passing through an SBF/LSGM interface at 840 °C in pure oxygen is about 1 A·cm^–2 at an overpotential of 40 mV, much higher than that for an La_xSr_1–xMnO₃ electrode under similar conditions reported in the literature. Analysis indicates that the electrode kinetics is controlled primarily by mass transfer at high temperatures and by charge transfer at low temperatures. The inductive loops of the impedance spectra further suggest that the adsorption of intermediate species is involved in the interfacial reaction. Electronic Publication     

Electrotransfer in nonstoichiometric lanthanum scandate LaSc1 ? x O3 ? �� (x = 0?0.1)

The phase composition and the total and partial conductivities of ceramic samples LaSc_{1 ? x} O_{3 ? ??} (x = 0?0.1) are studied as a function of the air humidity (pH₂O = 0.04?2.35 kPa) and the oxygen partial pressure (?15 ?? logpO₂ ?? ?0.68) in a wide temperature range from 100 to 900??C as well as their thermal expansion.     

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     

Nonstoichiometry,point defects and magnetic properties in Sr2FeMoO6−δ double perovskites

The phase stability, nonstoichiometry and point defect chemistry of polycrystalline Sr₂FeMoO_6?δ (SFMO) was studied by thermogravimety at 1000, 1100, and 1200 °C. Single-phase SFMO exists between ?10.2≤log pO₂≤?13.7 at 1200 °C. At lower oxygen partial pressure a mass loss signals reductive decomposition. At higher pO₂ a mass gain indicates oxidative decomposition into SrMoO₄ and SrFeO_3?x. The nonstoichiometry δ at 1000, 1100, and 1200 °C was determined as function of pO₂. SFMO is almost stoichiometric at the upper phase boundary (e.g. δ=0.006 at 1200 °C and log pO₂=?10.2) and becomes more defective with decreasing oxygen partial pressure (e.g. δ=0.085 at 1200 °C and log pO₂=?13.5). Oxygen vacancies are shown to represent majority defects. From the temperature dependence of the oxygen vacancy concentration the defect formation enthalpy was estimated (ΔH_OV=253±8 kJ/mol). Samples of different nonstoichiometry δ were prepared by quenching from 1200 °C at various pO₂. An increase of the unit cell volume with increasing defect concentration δ was found. The saturation magnetization is reduced with increasing nonstoichiometry δ. This demonstrates that in addition to Fe/Mo site disorder, oxygen nonstoichiometry is another source of reduced magnetization values.     

Electric properties of oxyfluorides Ba2In2O5−0.5x F x with brownmillerite structure

Synthesis of fluoro-substituted substances based on brownmillerite Ba₂In₂O₅ is carried out. The width of the homogeneity region of the Ba₂In₂O_5?0.5x F _x (0 < x ≤ 0.25) solid solution was established using X-ray analysis. Measurement of temperature dependences of conductivity in atmospheres with different partial pressure of water vapor (pH₂O = 3.3 and 2 × 10³ Pa) showed an increase in conductivity at T ≤ 550°C in a humid atmosphere, which is due to appearance of proton transport. The dependence of conductivity on partial oxygen pressure (pO₂ = 0.21 × 10⁵ to 10^?15 Pa) is studied in the temperature range of 500–1000°C; ion transport numbers are calculated. The method of polarization measurements was used to determine transport numbers of fluoride. Total conductivity is divided into ion (proton, oxygen, and fluoride ion) and electron components. Analysis of concentration dependences of conductivities showed that low concentrations of fluoride allow increasing both the total and partial conductivities (oxygen-ion and proton) and, besides, allow shifting the “order-disorder” phase transition by 100°C to the low temperature range.     

Synthesis and characterization of Sr<Subscript>0.75</Subscript>Y<Subscript>0.25</Subscript>Co<Subscript>0.9</Subscript>Ru<Subscript>0.1</Subscript>O<Subscript>3−<Emphasis Type="Italic">δ</Emphasis></Subscript> perovskite as a solid oxide fuel cell cathode material

The oxygen-deficient Sr_0.75Y_0.25Co_0.9Ru_0.1O_3?δ (SYCR) cathode is systematically evaluated for the application of solid oxide fuel cells. X-ray diffraction analysis indicates that SYCR presents a tetragonal structure with space group of I4/mmm (139). In the measured high oxygen partial pressure (pO₂) region (0.01–0.21 atm), the conductivity increases with increasing pO₂ because of the oxygen vacancy annihilation and hole creation, relating to a general p-type semiconducting mechanism. To get an insight into the rate-limiting step of SYCR cathode, behaviors of individual polarization resistance (R ₁ and R ₂) are investigated in different pO₂. The obtained fitting results reveal that R ₁ is nearly independent on the pO₂, while R ₂ presents a (pO₂)^?0.5 dependence. At 800 °C, SYCR cathode exhibits an R _p value of 0.14 Ω cm², moreover, when using the wet hydrogen (～ 3% H₂O) as fuel and ambient air as oxidant, the maximum power density of single cell Ni-YSZ (yttria-stabilized zirconia)|YSZ|SYCR reaches 452.9 W cm^?2.     

Nanocomposite proton conductors containing mesoporous oxides as the promising fuel cell membranes

Composite proton-conducting electrolytes are synthesized based on H₄SiW₁₂O₄₀ · xH₂O, CsHSO₄, (CsH₂PO₄)_0.9(CsHSO₄)_0.1 and mesoporous matrices SBA-15 and MCM-41 and their transport and structural characteristics are studied. Composites based on silicotungstic acid demonstrate the conductivity from ～10^?3 to 10^?4 S/cm in the temperature range of 25–140°C at the increased partial pressure of water vapor. The conductivity of systems CsHSO₄-SBA-15 (at T = 140–200°C) and (CsH₂PO₄)_0.9(CsHSO₄)_0.1-SBA-15 (at T = 200–230°C) reaches 10^?2 S/cm and is independent of humidity. The electrolytes studied are promising as proton-exchange membranes of fuel cells operating at low and medium temperatures.     

Electroconductivity and nature of ion transfer in La1 − x Sr x Sc1 − y Mg y O3 − α system (0.01 ≤ x = y ≤ 0.20) in dry and humid air

The conductivity and ion and proton transfer numbers were measured in La_{1 ? x} Sr _x Sc_{1 ? y} Mg _y O_{3 ? α} system (x = y = 0.10–0.20). The partial conductivities (total ion, proton, oxygen, hole) and their effective activation energies were calculated. The measurements were carried out in air with respect to humidity (pH₂O = 0.04?2.65 kPa) within the temperature range from 630 to 920°C.     

Nickel-cermet electrodes for high-temperature electrochemical devices made using nanomaterials

Characteristics of nickel-cermet anodes obtained from weakly aggregated NiO nanopowders made by wire electric blasting (NiO/WEB) were studied. Electrodes made of NiO/WEB nanopowders have low sheet resistance (<0.1 Ohm) and high electrochemical activity (R _η = 0.06–0.09 Ohm cm² at 850–900°C). Prolonged studies of symmetric cells of the (0.9H₂ + 0.1H₂O) Ni-SSZ + CeO₂/YSZ/CeO₂ + Ni-SSZ (0.9H₂ + 0.1H₂O) type at the temperature of 850°C showed that the electrodes preserve sufficiently high activity (R _η < 0.1 Ohm cm²) for 1000 h. Using a NiO-WEB powder allows not performing presynthesis of nickel-cermet and decreasing the anode baking temperature to 1200°C.     

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.     

Ion–electron transport in strontium ferrites: relationships with structural features and stability

The total electrical conductivity of strontium ferrites, including intergrowth Sr₄Fe₆O_13+δ, Sr₃Fe₂O_6+δ with a Ruddlesden–Popper structure, and SrFeO_2.5+δ where the cubic perovskite lattice transforms into vacancy-ordered brownmillerite at p(O₂)<10 Pa and T<850 °C, was measured at 650–1000 °C in the oxygen partial pressure range 10⁻¹⁵ Pa to 50 kPa. The data were used in order to determine partial ion, p- and n-type electron contributions in the vicinity of electron–hole equilibrium point. The ferrites with brownmillerite and Ruddlesden–Popper structures exhibit substantial ion transport due to thermally-activated disordering of oxygen vacancies and oxygen ions in the perovskite structural slabs, whereas the ion conductivity of Sr₄Fe₆O_13+δ remains below 0.01 S cm⁻¹ in the studied conditions. The bonding energy of oxygen ions, evaluated from the formation enthalpy of n-type charge carriers, increases in the sequence Sr₄Fe₆O_13+δ<SrFeO_3+δ<Sr₃Fe₂O_6+δ. These values correlate with thermodynamic stability of strontium ferrites at low p(O₂). The transition of SrFeO_2.5+δ brownmillerite into disordered cubic phase above 850 °C leads to higher stability in reducing atmospheres. The level of p-type conductivity is mainly governed by the concentration of electron holes, which was calculated from the oxygen content determined by coulometric titration technique. The hole mobility, which is quite similar for all strontium ferrites and has a temperature-activated character, varies in the range 0.005–0.05 cm² V⁻¹ s⁻¹ indicative of small-polaron conduction mechanism.     

Phase diagram and thermodynamic properties of compounds in the SrO-SrF2-Mn2O3-O2 system

Temperature dependences of the Gibbs energy of the formation of strontium manganite (SrMn₃O_{6 ? δ}, SrMnO₃, Sr₂MnO₄) and fluorine manganite (Sr_2.5Mn₆O_{12.5 ? δ}F₂) from simple oxides, oxygen, and SrF₂, respectively, are determined by EMF with fluorine- and oxygen-ion electrolytes in the 1000–1300 K range of temperatures. Phase relations in the SrO-SrF₂-Mn₂O₃-O₂ system at p(O₂) = 1 atm are calculated on the basis of the obtained data.     

Solubility of Li2WO4 under hydrothermal conditions

Experimental studies of water solubilities of Li₂WO₄ at temperatures up to 450°C and pressures up to 147 MPa show that this salts with water forms a type 2 binary system, where the solubility of the salt under the vapor pressure decreases as temperature rises to end at the critical end-point p (g=l-s) near the H₂O critical temperature. High-temperature Li₂WO₄ solutions are shown to not react with silica, but they cause corrosion of metallic walls of an autoclave accompanied with gas evolution and formation of dark-colored solid corrosion products.