High-Temperature Behavior, Oxygen Transport Properties, and Electrochemical Performance of Cu-Substituted Nd1.6Ca0.4NiO4+δ Electrode Materials

In this study, Nd1.6Ca0.4Ni1−yCuyO4+δ-based electrode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) are investigated. Materials of the series (y = 0–0.4) are obtained by pyrolysis of glycerol-nitrate compositions. The study of crystal structure and high-temperature stability in air and under low oxygen partial pressure atmospheres are performed using high-resolution neutron and in situ X-ray powder diffraction. All the samples under the study assume a structure with Bmab sp.gr. below 350◦C and with I4/mmm sp.gr. above 500◦C. A transition in the volume thermal expansion coefficient values from 7.8–9.3 to 9.1–12.0 × 10−6, K−1 is observed at approximately 400◦C in air and 500◦C in helium.The oxygen self-diffusion coefficient values, obtained using isotope exchange, monotonically decrease with the Cu content increasing, while concentration dependence of the charge carriers goes through the maximum at x = 0.2. The Nd1.6Ca0.4Ni0.8Cu0.2O4+δ electrode materialdemonstrates chemical compatibility and superior electrochemical performance in the symmetrical cells with Ce0.8Sm0.2O1.9, BaCe0.8Sm0.2O3−δ, BaCe0.8Gd0.19Cu0.1O3−δ and BaCe0.5Zr0.3Y0.1Yb0.1O3−δ solid electrolytes, potentially for application in IT-SOFCs. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.122013100200-2; Ministry of Education and Science of the Russian Federation, Minobrnauka: AAAA-A21-121011390009-1; Ural Branch, Russian Academy of Sciences, UB RAS: 122020100324-3Material synthesis, sample preparation, and electrochemical studies were performed in the framework of budget tasks for the Institute of High Temperature Electrochemistry, UB RAS, project № 122020100324-3. The standard characterization of powder and ceramic materials was carried out at the Shared Access Centre “Composition of Compounds” of the Institute of High Temperature Electrochemistry, UB RAS. The synchrotron XRD experiments were performed at the shared research center SSTRC on the basis of the Novosibirsk VEPP-3 complex at BINP SB RAS. The in situ XRD study was carried out using the facilities of the shared research center “National Center of Investigation of Catalysts” at the Boreskov Institute of Catalysis (BIC). The part of the reported study concerning the crystal structure of the samples was funded within the framework of budget project for Synchrotron radiation facility SKIF, Boreskov Institute of Catalysis.BIC support of the isotope exchange study by the Ministry of Science and Higher Education of the Russian Federation projects AAAA-A21-121011390009-1 and AAAA-A21-121011390007-7 is greatly acknowledged. XPS study of the electrode materials was partly performed in the framework of the budget task for the Institute of Metallurgy, UB RAS, project № 122013100200-2 using the equipment of the Shared Access Centre “Ural-M” of the Institute of Metallurgy, UB RAS.Acknowledgments: Material synthesis, sample preparation, and electrochemical studies were performed in the framework of budget tasks for the Institute of High Temperature Electrochemistry, UB RAS, project №122020100324-3. The standard characterization of powder and ceramic materials was carried out at the Shared Access Centre “Composition of Compounds” of the Institute of High Temperature Electrochemistry, UB RAS. The synchrotron XRD experiments were performed at the shared research center SSTRC on the basis of the Novosibirsk VEPP-3 complex at BINP SB RAS. The in situ XRD study was carried out using the facilities of the shared research center “National Center of Investigation of Catalysts” at the Boreskov Institute of Catalysis (BIC). The part of the reported study concerning the crystal structure of the samples was funded within the framework of budget project for Synchrotron radiation facility SKIF, Boreskov Institute of Catalysis.BIC support of the isotope exchange study by the Ministry of Science and Higher Education of the Russian Federation projects AAAA-A21-121011390009-1 and AAAA -A21-121011390007-7 is greatly acknowledged. XPS study of the electrode materials was partly performed in the framework of the budget task for the Institute of Metallurgy, UB RAS, project № 122013100200-2 using the equipment of the Shared Access Centre “Ural-M” of the Institute of Metallurgy, UB RAS

СИНТЕЗ И СТРУКТУРА ГАЗОЧУВСТВИТЕЛЬНЫХ КОМПОЗИТНЫХ МАТЕРИАЛОВ TIO2–MoO3

Introduction of molybdenum trioxide to gas sensing materials based on titanium dioxide results in a considerable increase of the output signal in the hydrogen-air environment. It is established that the value of the output signal reaches the maximum at the 1 mol. % MoO3 content in the composite material. Improved gas sensing characteristics of TiO2:MoO3 composite correlate with the structural and phase peculiarities of this system – the inhibition of TiO2 crystallization in the TiO2:MoO3 system, the shift of anatase-rutile phase transition to the higher temperature area in comparison with the temperature of this transition in unloaded TiO2, and with the presence of different types of tetrahonal distortions in MoO6 octahedrons, which ensures the MoO3 lattice high activity in the processes of hydrogen catalytic oxidation, and also with the crystallization of highly dispersed molybdenum oxide with a particle size of 10 nm.Введение легирующих добавок оксида молибдена в состав газочувствительных материалов на основе диоксида титана приводит к значительному повышению выходного сигнала в водородо-воздушной среде. Установлено, что величина выходного сигнала максимальна при содержании 1 мол. % МоО3 в композитном материале. Улучшенные газочувствительные характеристики композита TiO2–MoO3 с 1 мол. % МоО3 коррелируют со структурно-фазовыми особенностями данной системы – подавлением кристаллизации диоксида титана в системе TiO2–MoO3, смещением фазового перехода анатаз–рутил в область больших температур по сравнению с температурой данного перехода для нелегированного диоксида титана, наличием различных типов тетрагональных искажений в октаэдрах МоО6 по различным направлениям, обеспечивающих высокую активность кислорода решетки МоО3 в процессах каталитического окисления адсорбированных молекул водорода, а также кристаллизацией высокодисперсного оксида молибдена с размером частиц порядка 10 нм

МАЛОУГЛОВОЕ РАССЕЯНИЕ НЕЙТРОНОВ НА ОБРАЗЦАХ Sr2FeMoO6–δ С РАЗНОЙ СТЕПЕНЬЮ CВЕРХСТРУКТУРНОГО УПОРЯДОЧЕНИЯ КАТИОНОВ Fe/Mo

Single-phase Sr2FeMoO6-δ samples with different-degreesuperstructiral ordering Fe/Mo cations superstructural ordering (P, 76, 86 and 93 %) were obtained by the solid-phase technique. Based on the results of measuring the magnetic characteristics in the samples, we found that an increase in magnetization (26.41, 32.36 and 42.66 A·m2·kg–1), magnetic moment (1.33, 3.07 and 3.58 μB/f.u.) and Curie temperatures (422, 428 and 437 K) withparameter P (76, 86 and 93 %) can be explained by the presence of antistructural defects, as well as antiferromagnetic inclusions. This determines the redistribution of electron density, which is accompanied by the change in electronic configuration of a part of Fe/Mo cations. Based on the temperature dependences of the magnetic moment of the samples measured in ZFC and FC modes, and on small-angle polarized neutron scattering (SANS), we found that the samples are in a magnetically inhomogeneous state. An important result to mention is that we detected the difference between the slope of the SANS curves of samples with different oxygen content, which demonstrates a different microstructure of inhomogeneities. The main inhomogeneities are magnetic inclusions with the dimensions depending on the superstructural ordering of Fe/Mo cations. According to the Porod law, it was shown that the Sr2FeMoO6-δ samples with wave vector values 0.1 > q > 0.002 Å–1 contain polydisperse grains with a smooth surface. For q > 0.1 Å–1 a deviation from the Porod law is observed, confirming the presence of magnetic inhomogeneities with a diameter < 6 nm in the grains.Однофазные образцы Sr2FeMoO6-δ с различной степенью сверхструктурного упорядочения катионов Fe/Mo (P, 76, 86 и 93 %) были получены твердофазным методом. На основании результатов измерения магнитных характеристик в образцах установлено, что увеличение величин намагниченности (26,41, 32,36 и 42,66 А·м2·кг –1), магнитного момента (1,33, 3,07 и 3,58 μБ/ф.е.) и температур Кюри (422, 428 и 437 К) с ростом параметра P (76, 86 и 93 %) можно объяснить присутствием антиструктурных дефектов, а также антиферромагнитных включений. Наличие таковых обуславливает перераспределение электронной плотности, что сопровождается изменением электронной конфигурации части катионов Fe/Mo. На основании данных температурных зависимостей магнитного момента образцов, измеренных в ZFC и FC режимах, и малоуглового рассеяния поляризованных нейтронов (МУРН) установлено, что образцы находятся в магнитно-неоднородном состоянии. Важным результатом является обнаруженное различие между наклоном кривых МУРН образцов с различным содержанием кислорода, что свидетельствует о разной микроструктуре неоднородностей. Основными неоднородностями, на которых происходит рассеяние, являются магнитные включения с характерными размерами, зависящими от сверхструктурного упорядочения катионов Fe/Mo. Показано, что согласно закону Порода при значениях волнового вектора 0,1 > q > 0,002 Å–1 образцы Sr2FeMoO6-δ содержат полидисперсные зерна с гладкой и шероховатой поверхностью, а при q > 0,1 Å–1 наблюдается отклонение от закона Порода, что указывает на наличие в зернах магнитных неоднородностей диаметром менее 6 нм

ФАЗОВЫЕ ПРЕВРАЩЕНИЯ ПРИ КРИСТАЛЛИЗАЦИИ ТВЕРДОГО РАСТВОРА СТРОНЦИЙ-ЗАМЕЩЕННОГО ДВОЙНОГО ПЕРОВСКИТА

The kinetics of phase contents modification in the process of SrBaFeMoO6–δ crystallization from a stoichiometric mixture of SrCO3 + BaCO3 + 0,5Fe2O3 + MoO3 simple oxides using the solid phase method has been investigated. In the temperature region of 300–1200°С, a number of endotermic effects have been detected. Herewith, the first one (with maximum around 552°С) and the third one (with maximum around 743°С) are accompanying by the significant decrease of the mass of specimen. In the temperature range of 946–1200°С, the mass change of specimen is practically not observable, while the thermal effect is still present, and the specimen remains not single-phase one. This indicates the difficulty of the flow of solid phase reactions with the formation of solid solution of barium-strontium ferromolybdate. During analysis of the change of the phase composition consisting of a mixture of initial reagents of stoichiometric relation SrCO3 + BaCO3 + 0,5Fe2O3 + MoO3, it has been observed that with increasing temperature, complex compounds BaMoO4, SrFeO3 appear almost simultaneously, then SrBaFeMoO6–δ appears consequently. Thus, the compounds BaMoO4 и SrFeO3, are structure forming for the solid solution of barium-strontium ferromolybdate. With further temperature increase up to 770°С the formation of new compound ВаFeO3 with disappearing SrFeO3 was detected. In this case, the amount of double perovskite increases faster than that of barium molybdate. The main accompanying compounds at the crystallization of the SrBaFeMoO6–δ double perovskite solid solution are BaMoO4 and SrFeO3. It was established that at the initial stage of the interaction, the resulting solid solution of barium-strontium ferromolybdate is enriched with iron and its composition changes during the reaction in the direction of an increase of the molybdenum content, as in the case of other precursor combinations.Изучена кинетика модификации фазового состава твердого раствора в процессе кристаллизации SrBaFeMoO6–δ твердофазным методом из стехиометрической смеси простых оксидов SrCO3 + BaCO3 + 0,5Fe2O3 + MoO3. В температурном диапазоне 300–1200°С выявлен ряд эндотермических эффектов, при этом первый (с максимумом в районе 552°С) и третий (с максимумом в районе 743°С) из них сопровождаются существенным уменьшением массы образцов. В интервале температур 946–1200°С изменение массы образца не наблюдается, в то время как тепловой эффект не исчезает и образец остается неоднофазным. Это указывает на затрудненное протекание реакций с образованием соединения ферромолибдата бария–стронция в твердой фазе. Анализ фазового состава шихты, состоящей из смеси исходных реагентов стехиометрического состава SrCO3 + BaCO3 + 0,5Fe2 O3 + MoO3, показал, что при возрастании температуры вначале практически одновременно появляется ряд соединений BaMoO4, SrFeO3, а затем и SrBaFeMoO6–δ. Таким образом, соединения BaMoO4 и SrFeO3 можно считать структурообразующими для твердого раствора ферромолибдата бария-стронция. С последующим увеличением температуры до 770°С возникает новое соединение ВаFeO3 и исчезает SrFeO3 . При этом скорость возрастания количества двойного перовскита выше, чем молибдата бария. Основные сопутствующие соединения при кристаллизации твердого раствора двойного перовскита SrBaFeMoO6–δ – это BaMoO4 и BaFeO3. Установлено, что в процессе синтеза твердого раствора ферромолибдата бария-стронция его состав, как и в случае использования других исходных соединений, изменяется от избытка железа в сторону преобладания содержания молибдена

Фазовые превращения при кристаллизации Sr2CrMoO6–δ

The sequence of phase transformations in the process of crystallization of the Sr2CrMoO6 by the solid-phase technique from a stoichiometric mixture of simple oxides SrCO3 + 0.5Cr2O3 + MoO, has been investigated. It was determined that the synthesis of the strontium chrome-molybdate proceeds through a series of sequential-parallel stages. By means of the differential thermal analysis and thermogravimetric analysis data, it has been established that five clearly expressed endothermal effects were observed in the temperature range 300—1300 K. It was found that during the studies of the phase transformations sequence in the process of the double perovskite synthesis, SrCrO3, SrMoO4 and Sr2CrO4 are the main concomitant compounds. Herewith, it has been observed that with the annealing temperature increase from 300 to 1270 K, the complex compounds SrCrO4, SrCrO3 (350—550 K) and SrMoO4, Sr2CrO4 (600—750 K) are emerging initially and practically simultaneously. It has been revealed with a subsequent temperature increase that in the temperature range 940—1100 K, the SrMoO4, Sr2CrO4 and SrCrO3 phase concentration dramatically drops with the emerging and growth of the Sr2CrMoO6-δ double perovskite. With that in the range up to 1120—1190 K, the main XRD reflexes intensity for the SrCrO3 and SrMoO4 lowers substantially, and their content in the samples at 1170 К is no more than 7,9 %. During a consideration of the derivative of the SrCrO3, SrMoO4 and Sr2CrO4 phase transformation degree (|(dα/dt)|mах), at which their crystallization rates are maximal, it has been determined that |(dα/dt)|mах for the Sr2CrO4 corresponds to the maximal temperature 1045 K, which indicates the presence of considerable kinetic difficulties at the formation of the Sr2CrO4 phase. Thereafter this phase does not disappear and at its appearance the slowing down of the double perovskite growth takes place. On the base of investigations of the phase transformations dynamics for the obtaining of the single-phase Sr2CrMoO6-δ compound with the superstructural ordering of the Cr/Mo cations and improved magnetic characteristics, the SrCrO3 and SrMoO4 precursors were used with combined heating modes.Исследована последовательность фазовых превращений в процессе кристаллизации Sr2CrMoO6-δ из стехиометрической смеси простых оксидов SrCO3 + 0,5Cr2O3 + MoO. Установлено, что фазообразование хромомолибдата стронция протекает через ряд последовательно-параллельных стадий. Согласно данным дифференциально-термического и термогравиметрического анализов, обнаружено, что в температурном диапазоне 300—1300 К наблюдается пять ярко выраженных эндотермических эффектов. При изучении последовательности фазовых превращений в процессе синтеза двойного перовскита обнаружено, что основными сопутствующими соединениями являются SrCrO3, SrMoO4 и Sr2CrO4. При этом замечено, что с ростом температуры отжига от 300 до 1270 К в исходной смеси первоначально и практически одновременно появляются сложные соединения SrCrO4, SrCrO3 (350—550 К), а затем и SrMoO4, Sr2CrO4 (600—750 К). Показано, что с последующим увеличением температуры в интервале температур 940—1100 К концентрация фаз SrMoO4, Sr2CrO4 и SrCrO3 резко падает с появлением и ростом двойного перовскита Sr2CrMoO6-δ. При этом в интервале температур до 1120—1190 К основные рентгеновские рефлексы фазы Sr2CrO4 уменьшаются незначительно, тогда как интенсивность рентгеновских рефлексов фаз SrCrO3 и SrMoO4 снижается существенно больше и их содержание в образце при температуре 1170 К составляет не более 7,9 %. Анализ амплитудных значений производной степени превращения фаз SrCrO3, SrMoO4 и Sr2CrO4, при которых скорости их кристаллизации максимальны, показал, что для Sr2CrO4 величина |(dα/dt)|mах соответствует наибольшей температуре T = 1045 К. Это указывает на наличие кинетических трудностей при образовании фазы Sr2CrO4, которая в дальнейшем не исчезает, а при ее появлении наблюдается замедление роста двойного перовскита. На основании результатов, полученных при изучении динамики фазовых превращений для формирования однофазного Sr2CrMoO6-δ со сверхструктурным упорядочением Cr/Mo и улучшенными магнитными характеристиками, были применены прекурсоры SrCrO3 и SrMoO4 с использованием комбинированных режимов нагрева

PHASE TRANSFORMATIONS DURING CRYSTALLIZATION OF A SOLID SOLUTION OF STRONTIUM-SUBSTITUTED DOUBLE PEROVSKITE

The kinetics of phase contents modification in the process of SrBaFeMoO6–δ crystallization from a stoichiometric mixture of SrCO3 + BaCO3 + 0,5Fe2O3 + MoO3 simple oxides using the solid phase method has been investigated. In the temperature region of 300–1200°С, a number of endotermic effects have been detected. Herewith, the first one (with maximum around 552°С) and the third one (with maximum around 743°С) are accompanying by the significant decrease of the mass of specimen. In the temperature range of 946–1200°С, the mass change of specimen is practically not observable, while the thermal effect is still present, and the specimen remains not single-phase one. This indicates the difficulty of the flow of solid phase reactions with the formation of solid solution of barium-strontium ferromolybdate. During analysis of the change of the phase composition consisting of a mixture of initial reagents of stoichiometric relation SrCO3 + BaCO3 + 0,5Fe2O3 + MoO3, it has been observed that with increasing temperature, complex compounds BaMoO4, SrFeO3 appear almost simultaneously, then SrBaFeMoO6–δ appears consequently. Thus, the compounds BaMoO4 и SrFeO3, are structure forming for the solid solution of barium-strontium ferromolybdate. With further temperature increase up to 770°С the formation of new compound ВаFeO3 with disappearing SrFeO3 was detected. In this case, the amount of double perovskite increases faster than that of barium molybdate. The main accompanying compounds at the crystallization of the SrBaFeMoO6–δ double perovskite solid solution are BaMoO4 and SrFeO3. It was established that at the initial stage of the interaction, the resulting solid solution of barium-strontium ferromolybdate is enriched with iron and its composition changes during the reaction in the direction of an increase of the molybdenum content, as in the case of other precursor combinations

The influence of cation ordering and oxygen nonstoichiometry on magnetic properties of Sr2FeMoO6–x around Curie temperature

Sr2FeMoO6–x polycrystalline samples with different oxygen content (6−x) and various degrees of superstructural ordering of Fe/Mo cations (P) were obtained by the solid-phase method from the SrFeO2.52 and SrMoO4 precursors. Fromthe investigation on theinfluence ofoxygennon-stoichiometry and theP parameteron the magnetic properties of Sr2FeMoO6–x, it was found that with an increase in P and a decrease in the (6−x) value from 5.99 to 5.94, an increase in the magnetization values is observed in the temperature range 77–600 K. For all the Sr2FeMoO6–x samples there is a tendency that P value rises with increasing x, where, accordingly, the volumefraction ofregionsinwhichtherearenoanti-structural defectsincreasesaswell.Thisisalsoindicatedby Mössbauer spectroscopy data, confirming an increase in the area of the S1 sextet corresponding to Fe ions in highly ordered regions and a decrease in the area of the S2 sextet associated with disordered regions. Using the temperature scanning method, the temperatures of the onset and completion of the transition from the paramagnetic to the ferrimagnetic state and, correspondingly, the blurring width of the transition have been estimated. It turned out that with decreasing P, the blurring of the transition increases, which is associated with an increase in the concentration of anti-structural defects

Tuning the high-temperature properties of Pr2NiO4+δ by simultaneous Pr- and Ni-cation replacement

Novel Pr2−xSrxNi1−xCoxO4±δ (x = 0.25; 0.5; 0.75) oxides with the tetragonal K2NiF4-type structure have been prepared. Room-temperature neutron powder diffraction (NPD) study of x = 0.25 and 0.75 phases together with iodometric titration results have shown the formation of hyperstoichiometric oxide for x = 0.25 (δ = 0.09(2)) and a stoichiometric one for x = 0.75. High-temperature X-ray powder diffraction (HT XRPD) showed substantial anisotropy of the thermal expansion coefficient (TEC) along the a- and c-axis of the crystal structure, which increases with increasing the Co content from TEC(c)/TEC(a) = 2.4 (x = 0.25) to 4.3 (x = 0.75). High-temperature NPD (HT NPD) study of the x = 0.75 sample reveals that a very high expansion of the axial (Ni/Co)–O bonds (75.7 ppm K−1 in comparison with 9.1 ppm K−1 for equatorial ones) is responsible for such behaviour, and is caused by a temperature-induced transition between low- and high-spin states of Co3+. This scenario has been confirmed by high-temperature magnetization measurements on a series of Pr2−xSrxNi1−xCoxO4±δ samples. For compositions with high Ni content (x = 0.25 and 0.5) we synthesised K2NiF4-type oxides Pr2−x−ySrx+y(Ni1−xCox)O4±δ, y = 0.0–0.75 (x = 0.25); y = 0.0–0.5 (x = 0.5). The studies of the TEC, high-temperature electrical conductivity in air, chemical stability of the prepared compounds in oxygen and toward interaction with Ce2−xGdxO2−x/2 (GDC) at high temperatures reveal optimal behaviour of Pr1.35Sr0.65Ni0.75Co0.25O4+δ. This compound shows stability in oxygen at 900 °C and does not react with GDC at least up to 1200 °C. It features low TEC of 13 ppm K−1 and high-temperature electrical conductivity in air of 280 S cm−1 at 900 °C, thus representing a promising composition for use as a cathode material in intermediate temperature solid oxide fuel cells (IT-SOFC)