Rate‐determining steps of oxygen surface exchange kinetics on Sr2Fe1.5Mo0.5O6−δ

The oxygen surface kinetics of Sr2Fe1.5Mo0.5O6−δ was determined using the 16O2/18O2 isotope exchange method with gas phase analysis at 600-800 °C. The heterogeneous exchange rates (rH) and the oxygen diffusion coefficients (D) were calculated by processing the concentration dependences of the 18O fraction using Ezin's model. The rates of oxygen dissociative adsorption (ra) and incorporation (ri) were calculated based on a model using the three exchange type rates. It has been established that the rates ra and ri were comparable in this temperature range. Assumptions were made about the effect of the chemical composition of the surface on the rate of oxygen adsorption. It was found that the oxygen exchange coefficient (k) of Sr2Fe1.5Mo0.5O6−δ is comparable to that of La0.6Sr0.4MnO3±δ oxide. High values of the oxygen diffusion coefficient were found for Sr2Fe1.5Mo0.5O6−δ. The values were comparable to those of the double cobaltite praseodymium-barium and exceed by more than an order those of lanthanum‐strontium manganite. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).Russian Foundation for Basic Research, RFBR: 00161Government Council on Grants, Russian FederationThe study was partly financially supported by the Russian Foundation for Basic Research (17-08-00161) and the Russian Federation Government, agreement 02.A03.21.0006 (no. 211)

Formation of conductive oxide scale on 33NK and 47Nd interconnector alloys for solid oxide fuel cells

Two grades of chromium-free alloys were studied in order to apply them as interconnectors for solid oxide fuel cells. The surface modification methods were proposed for each alloy with the purpose of forming of oxide scales considering the required physicochemical properties. Investigations of the structure and properties of the obtained oxide scales were performed and the efficiency of the chosen surface modification methods was approved. The samples with the surface modification exhibited higher conductivity values in comparison with the nonmodified samples. A compatibility study of samples with surface modification and glass sealant of chosen composition was accomplished. The modified samples demonstrated good adhesion during testing and electrical resistance less than 40 mOhm/cm2 at 850 ◦C in air, which allowed us to recommend these alloys with respective modified oxide scales as interconnectors for SOFC. © 2019 by the authors.Russian Foundation for Basic Research, RFBR: 17-58-10006This research was funded by the Russian Foundation of Basic Research grant number 17-58-10006. The facilities of the shared access center "Composition of Compounds" of IHTE UB RAS were used in this work

Understanding the Oxygen Reduction Kinetics on Sr2-xFe1.5Mo0.5O6-δ: Influence of Strontium Deficiency and Correlation with the Oxygen Isotopic Exchange Data

This paper presents, for the first time, the results of studies of the electrochemical reaction of oxygen reduction on Sr2Fe1.5Mo0.5O6-δ and Sr-deficient Sr1.95Fe1.5Mo0.5O6-δ, as promising electrodes for solid state electrochemical devices, by the electrochemical impedance method with the subsequent interpretation of the data using the concepts outlined in the Adler et al. model. It was established that the oxygen reduction reaction for both electrodes is determined by two relaxation processes associated with oxygen diffusion, oxygen surface exchange, and Knudsen diffusion in the pores of the electrode. Strontium deficiency was found to have a positive effect on the electrochemical activity of the electrodes, enhancing the stage related to oxygen diffusion and surface exchange. Also, for the first time, for Sr2Fe1.5Mo0.5O6-δ, a quantitative correlation for the surface oxygen exchange and diffusion coefficients obtained from the impedance data and by the isotopic exchange method is demonstrated. © 2021 Elsevier B.V.The authors are grateful to A.S. Farlenkov for the SEM images and A.V. Khrustov for the calculation of the microstructure parameters. The facilities of shared access center "Composition of Compounds" of IHTE UB RAS were used

Application of promising electrode materials in contact with a thin-layer ZrO2-based supporting electrolyte for solid oxide fuel cells

The paper presents the results of an investigation into thin single- and triple-layer ZrO2-Sc2O3-based electrolytes prepared using the tape-casting technique in combination with promising electrodes based on La2NiO4+δ and Ni-Ce0.8Sm0.2O2-δ materials. It is shown that pressing and joint sintering of single electrolyte layers allows multilayer structures to be obtained that are free of defects at the layer interface. Electrical conductivity measurements of a triple-layer electrolyte carried out in longitudinal and transverse directions with both direct and alternating current showed resistance of the interface between the layers on the total resistance of the electrolyte to be minimal. Long-term tests have shown that the greatest degradation in resistance over time occurs in the case of an electrolyte with a tetragonal structure. Symmetrical electrochemical cells with electrodes fabricated using a screen-printing method were examined by means of electrochemical impedance spectroscopy. The polarization resistance of the electrodes was 0.45 and 0.16 Ohm∙cm2 at 800 °C for the fuel and oxygen electrodes, respectively. The distribution of relaxation times method was applied for impedance data analysis. During tests of a single solid oxide fuel cell comprising a supporting triple-layer electrolyte having a thickness of 300 microns, a power density of about 160 mW/cm2 at 850 °C was obtained using wet hydrogen as fuel and air as an oxidizing gas. © 2020 by the authors.Russian Foundation for Basic Research, RFBR: 17‐08‐ 01227Government Council on Grants, Russian FederationFunding: The study was partly financially supported by the Russian Foundation for Basic Research (17‐08‐ 01227), Russian Federation Government, agreement 02.A03.21.0006 (No. 211) and “InEnergy” LTD agreement of 2018

Oxygen diffusion and surface exchange kinetics for the mixed-conducting oxide La0.6Sr0.4Co0.8Fe0.2O3–δ

Received: 14.11.2018. Accepted: 11.12.2018. Published: 31.12.2018.Studies of oxygen surface exchange kinetics for La0.6Sr0.4Co0.8Fe0.2O3–δ oxide were performed using the technique of isotopic exchange of molecular oxygen with analysis of gas phase isotopic composition in a static circulation system at the temperatures of 600–800 °С in the oxygen pressure range of 0.27–2.13 kPa. The values of interphase exchange rate and oxygen diffusion coefficient were determined. The effective activation energies for oxygen exchange and diffusion processes as well as the exponents in the dependence of these values versus oxygen pressure in the double logarithmic coordinates were calculated. The process of oxygen dissociative adsorption at the surface of La0.6Sr0.4Co0.8Fe0.2O3–δ oxide was found to be the rate-determining stage.This work is partly supported by the Russian Science Foundation (Project No. 17‑73‑10196) using facilities of shared access center “Composition of Compounds” of IHTE UB RAS

Oxygen isotope exchange in oxides La2−xCaxZr2O7−α

The oxygen isotope exchange method with equilibration of the isotope composition of the gas phase was used to obtain temperature dependences of oxygen diffusion coefficients and oxygen heterogeneous exchange rates with oxides La2−xCaxZr2O7−α (x = 0; 0.05; 0.1) in the temperature range of 600–900 °C and oxygen partial pressure equals 1 kPa. The oxygen diffusion coefficient was found to increase with increasing temperature and calcium content in the oxide. It is shown that an increase in the concentration of calcium leads to a decrease in the rate of dissociative adsorption through the segregation of the dopant on the surface and blocking of the active adsorption sites La-O, and also to an increase in the rate of incorporation due to an increase in the concentration of oxygen vacancies.В работе методом изотопного обмена кислорода с уравновешиванием изотопного состава газовой фазы исследована кинетика обмена кислорода газовой фазы с оксидами La2−xCaxZr2O7−α (x = 0; 0.05; 0.1), имеющих структуру пирохлора. Получены температурные зависимости коэффициентов диффузии и скоростей обмена кислорода с исследуемыми оксидами в температурном интервале 600–900 °C и давлении кислорода 1 кПа. Установлено, что с ростом температуры и с повышением содержания кальция в оксиде коэффициент диффузии кислорода увеличивается. Показано, что увеличение концентрации кальция приводит к уменьшению скорости диссоциативной адсорбции из-за сегрегации допанта на поверхности и, как следствие, блокирования активных центров адсорбции La-O, а также увеличению скорости инкорпорирования вследствие увеличения концентрации кислородных вакансий

Electrode kinetics in the system SmBaCo2O6–δ | Ce0.8Sm0.2O1.9 | SmBaCo2O6–δ

The polarization resistance dependences for the SmBaCo2O6-δ electrode in contact with the Ce0.8Sm0.2O1.9 electrolyte in the temperature range 600–700 °C and oxygen pressures of 2.3-520 kPa were obtained. Using the isotopic exchange and impedance spectroscopy data it was established that in the investigated ranges of T and pO2 the electrode process in the system is determined by three stages: oxygen exchange and diffusion in SmBaCo2O6-δ and oxygen diffusion over the gas phase.Получены зависимости поляризационного сопротивления для электрода SmBaCo2O6–δ в контакте с Ce0.8Sm0.2O1.9 электролите в интервале температур 600–700 °С и давлений кислорода 2,3–520 кПа. С помощью данных изотопного обмена и импедансной спектроскопии установлено, что в указанном интервале температур и рО2 электродный процесс в исследуемой системе определяется тремя стадиями: обмен и диффузия кислорода в SmBaCo2O6–δ и диффузия кислорода по газовой фазе

ЗАЩИТНЫЕ ПОКРЫТИЯ La–Mn–Cu–O НА СТАЛИ-ИНТЕРКОННЕКТОРЕ 08Х17Т ДЛЯ ТВЕРДООКСИДНЫХ ТОПЛИВНЫХ ЭЛЕМЕНТОВ, ПОЛУЧЕННЫЕ МЕТОДОМ ЭЛЕКТРОКРИСТАЛЛИЗАЦИИ ИЗ НЕВОДНЫХ РАСТВОРОВ ЭЛЕКТРОЛИТОВ

A novel method was developed to form a protective layer on 08KhG17T stainless steel used to make interconnectors for solid oxide fuel cells. The method was based on the electrocrystallization of metals from non-aqueous electrolyte solutions on the stainless-steel interconnector surface with subsequent thermal treatment. Chemical composition of electrolyte was selected so that the surface is coated with an oxide protective layer of the following composition: LaMn0,9Cu0,1O3. As a result, a uniform oxide layer was formed on the stainless steel interconnector surface to protect stainless steel against high-temperature oxidation resulting in degraded functional properties of the interconnector. The coatings formed were characterized by means of grazing incidence X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy from the surface and in the cross section. Elemental and phase composition analyses have shown that the main components of the protective coatings are compounds with perovskite and spinel structures. The protective coating in contact with cathode material based on lanthanum strontium manganite have shown significantly lowered chromium penetration from steel as a result of diffusion annealing in comparison with the sample without the protective coating. Interconnector bonding to the protective coating has shown no noticeable degradation during at least 500 h at 850 °C in ambient air.Разработан новый метод формирования защитного покрытия на нержавеющей стали марки 08Х17Т, используемой для изготовления интерконнекторов твердооксидных топливных элементов. Он основан на электрокристаллизации металлов из неводных растворов электролитов на поверхности интерконнектора из нержавеющей стали с последующей термической обработкой. Химический состав электролита подбирался из расчета получения на поверхности оксидного слоя состава LaMn0,9Cu0,1O3. В результате на поверхности нержавеющей стали-интерконнера сформировался сплошной оксидный слой, защищающий сталь от высокотемпературного окисления, ведущего к деградации функциональных свойств интерконнектора. Полученные покрытия исследованы методами рентгенофазового анализа в геометрии скользящего пучка, рентгенофотоэлектронной спектроскопии и растровой электронной микроскопии с поверхности и в поперечном сечении. Анализ элементного и фазового составов покрытия показал, что основными его компонентами являются соединения со структурой перовскита и шпинели. В контакте с катодным материалом на основе манганита лантана–стронция защитное покрытие показало заметное ухудшение проникновения хрома из стали в результате диффузионного обжига по сравнению с образцом без покрытия. Соединение интерконнектора с покрытием не показывает заметной деградации в течение не менее 500 ч при температуре 850 °C в воздушной атмосфере