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Project titleHexagonal perovskites - new perspectives in the development of medium-temperature protonics for environmentally friendly electrochemical devices

AffiliationFederal State Autonomous Educational Institution of Higher Education "Ural Federal University named after the First President of Russia B.N. Yeltsin",

Research area 03 - CHEMISTRY AND MATERIAL SCIENCES, 03-601 - Chemistry of new inorganic functional and nano-dimensional materials

Keywordshexagonal perovskites, complex oxides, crystal structure, local structure, hydration, proton conductivity, ionic transport numbers, chemical stability

Annotation
The economy and the basis of the Russian budget are traditionally built on the export of oil and gas. A real opportunity to gradually reduce this dependence is the creation of an industry associated with the production, storage, transportation of hydrogen energy carriers and the production of various types of fuel cells. Recently (October 2020), the Prime Minister of the Russian Federation approved an action plan for the development of hydrogen energy until 2024. All this demonstrates the importance of the task of strengthening the country's position in the field of hydrogen energy. Currently, the most promising and intensively developing are electrochemical methods of direct conversion of fuel energy into electrical energy. The electrochemical method is carried out in fuel cells (FC), in which the chemical energy of the fuel and oxidizer is converted directly into electrical energy with a very high efficiency - up to 70%. Solid oxide fuel cells (SOFCs) are the most promising type of energy sources of this type. The advantages of SOFCs, for example, compared to fuel cells operating at room temperatures, are that they do not need an expensive catalyst (platinum). In addition, one of the advantages of SOFCs is their environmental friendliness, since the product of hydrogen combustion, when used as a fuel, is water. Recently, there has been a global trend towards lowering the operating temperature of SOFCs down to 500-700 ° C, which significantly reduces corrosion of metal and degradation of ceramic components, significantly increases the service life of the fuel cell, and, ultimately, makes it possible to achieve a significant reduction in the cost of electricity produced. Protonic electrolytes based on complex oxides are promising conductors for this temperature range. The main problem that hinders the widespread commercialization of SOFCs is the development of a solid electrolyte with high ionic conductivity, stable in both oxidizing and reducing atmospheres, reliably coupled with electrodes in a wide range of physicochemical, mechanical and functional characteristics. Therefore, the development of new materials with a high level of electrolytic conductivity at relatively low temperatures, making it possible to minimize degradation processes, is one of the main tasks of modern power engineering. The main goal of the Project is the development of new proton conductors based on complex oxides with the structure of anion-deficient hexagonal perovskites, capable of dominant proton transfer in the medium temperature range (T <700oC), possessing chemical stability under oxidizing and reducing conditions, as well as high рСО2 and pH2O. The crystal structure of the compounds studied in this Project is characterized by its own peculiarities - it can be represented as an intergrowth structure with various structural blocks, in which coordination-unsaturated polyhedra are present, capable of changing the coordination number upon intercalation of water. As a result, proton defects are formed, and the material acquires proton conductivity. The structural flexibility of this class of materials allows, within the framework of one compound, to combine a set of properties and, at the same time, to "adjust" the required characteristics. The scientific novelty is determined by the fact that, for the first time, on the basis of an integrated physicochemical approach, new compounds will be studied that were not previously described in the literature as proton-conducting electrolytes. The accumulation of material on the formation of proton conductivity in new structural types will make it possible to understand the role of the crystal chemical factor in the intercalation of water from the gas phase, which is important in understanding the mechanisms of proton transport in complex oxides. Based on the studies of the crystal and local structure, transport, thermal properties, the main regularities of the formation of proton conductivity and the factors providing a significant level of electrolytic properties will be established.

Expected results
During the implementation of the project, there will be: • regimes for the synthesis of new phases based on hexagonal perovskite Ba5In2Al2ZrO13 with a block structure, capable of intercalation of water vapor and dominant proton transfer at medium temperatures, have been developed; • studied the structure, including local, of anhydrous and hydrated phases; • the qualitative and quantitative composition of oxygen-hydrogen groups will be established, the main factors providing the formation of proton defects in the structure will be identified; • on the basis of a complex of transport properties (total and partial conductivity, transfer numbers), the effects of interaction of the proton and anionic subsystems will be revealed when varying T, pO2, pH2O; • hydration processes have been studied (thermogravimetry and IR spectroscopy), which will make it possible to formulate the basic principles of the existence, thermal and chemical stability of hydrated forms of the phases under study, to identify the T-pH2O regions of their stability; the relationship between the concentration of protons and the crystal-chemical characteristics of the studied phases will be established. • chemical resistance to СО2 and Н2О has been studied. On the whole, new results will be obtained, which will make it possible to establish the relationship between the value of proton conductivity, thermal and chemical stability of hydrated phases, and crystal-chemical characteristics of the studied classes of materials. The method of IR spectroscopy in combination with TG studies (partial hydration method) will establish the qualitative and quantitative composition of oxygen-hydrogen groups (sedentary forms of protons) formed in the process of dissociative absorption of water vapor. Separation of overlapping IR peaks will make it possible to isolate individual bands corresponding to the vibrations of various OH groups and to determine their quantitative contribution depending on the degree of hydration. The approach used will make it possible to determine the regularities between the structure (studies of the crystal and local structure), the value of proton conductivity / mobility (electrical studies), and the composition of the obtained hydrated crystalline compounds (IR spectroscopy, TG). All this gives an idea of the fundamental relationship between the composition, structure and properties of substances, which is the basis for a targeted search for new objects with specified useful characteristics. At the technological level, this will provide a unique opportunity to create new materials for use as electrolytic membranes in medium-temperature SOFCs, which are promising and environmentally friendly high-efficiency energy sources.

Annotation of the results obtained in 2023
One of the promising areas for solving the problem of providing electrical energy to consumers in various, including difficult conditions, is developments in the field of fuel cell (FC) technologies. Fuel cells are electrochemical devices for converting fuel energy into electrical energy. In such devices, direct conversion of chemical energy into electrical energy occurs due to the occurrence of redox reactions at the interfaces of electronic conductors with ionic conductors (electrolytes). Since the intermediate stage of burning fuel to produce heat is bypassed and, accordingly, there is no conversion of heat into work in these devices, their efficiency, both theoretically and practically, is significantly higher than that of thermal power plants. Recently, there has been a global trend towards the development of fuel cells based on oxide systems operating in the range of average temperatures of 500-700°C (and even lower). And this presupposes a scientifically based selection of an electrolyte with a sufficiently high ionic conductivity in a given temperature range. Such electrolytes can be materials based on complex oxides with proton conductivity. In addition to high values of proton conductivity, such materials must be stable in both oxidizing and reducing atmospheres, have chemical resistance to components of the gas environment (CO2, H2O), and have good ceramic characteristics (mechanical strength, sinterability). Researchers around the world are currently focusing their efforts on solving these problems. Within the framework of this Project, a specific task was solved - the development of new materials based on hexagonal perovskites, optimization of their functional properties, in order to form a theoretical and experimental basis for the development of highly efficient, environmentally friendly and economically attractive electrochemical devices for various purposes for environmentally friendly energy, in particular hydrogen energy. This Project is developing new proton-conducting electrolytes based on hexagonal perovskite Ba5In2Al2ZrO13 with a block structure, which have not previously been described in the literature as protonics, but which have a chance of creating highly efficient proton-conducting systems based on them. Main results: 1) Hexagonal perovskites (space group P63/mmc) were obtained by solid-phase synthesis: Ba5In2Al2ZrO13 (basic composition), Zn2+-doped phases on the In3+ and Al3+ sublattices Ba5In1.9Zn0.1Al2ZrO12.95, Ba5In2Al1.9Zn0.1ZrO12. 95 and solid solutions Ba5In2-xYxAl2ZrO13 and Ba5In2+xAl2Zr1−xO13−x/2. Doping of the Ba5In2Al2ZrO13 compound with yttrium and indium was accompanied by an increase in the unit cell parameters, which correlates with the ratio of the radii of the substituting atoms. Zinc doping was accompanied by a decrease in the parameters and volume of the unit cell, while the free volume of the unit cell increased. The introduction of zinc did not affect the degree of inversion of indium and aluminum atoms. 2) According to energy-dispersive X-ray spectroscopy data, it was proven that under the conditions of the synthesis, the cationic composition of the samples correlated well with the theoretical one. The results of studying the surface morphology of the samples according to SEM data showed that all ceramic samples had fused round grains with a size of ~5-10 μm. 3) Using thermogravimetry (TG) combined with mass spectrometry (MS), it was established that all phases are capable of intercalating water from the gas phase. The compositions of hydrated samples were determined from thermogravimetric measurements. During hydration, the symmetry of the unit cell does not change; there is a slight increase in the lattice parameters. It was found that the degree of hydration increased with increasing unit cell volume; the presence of oxygen vacancies during acceptor doping did not significantly affect the degree of hydration. The possibility of water intrusion is due to the presence of oxygen-deficient blocks in the structure, in which layers of BaO3 and BaO□2 alternate. The localization of oxygen vacancies in the layers of BaO□2 leads to the fact that some of the barium atoms realize a polyhedron consisting of half a cuboctahedron, that is, they have a coordination number of 9. During hydration, such a polyhedron can easily be expanded to a ten-vertex due to the participation of OH groups in coordination. 4) The presence in the structure of energetically nonequivalent OH groups involved in various hydrogen bonds is confirmed by infrared (IR) spectroscopy studies. Accordingly, this suggests their different thermal stability, which is confirmed by TG and MS data. The most thermally stable were isolated OH- groups involved in weak hydrogen bonds, and the least thermally stable were OH- groups participating in strong hydrogen bonds. Based on the data obtained, we can conclude that methods of iso- and heterovalent doping of intergrowth structures, which make it possible to increase the volume of the unit cell, are a promising strategy for increasing 5) For all synthesized samples, electrical conductivity studies were carried out with varying temperature, partial pressure of oxygen and water vapor. All phases were characterized by an insignificant contribution of grain-boundary resistance, not exceeding 10% of the total resistance, which is a positive point for the characteristics of ionics. - The studied phases at temperatures below 500 °C are characterized predominantly by oxygen-ion transfer in a dry atmosphere (pH2O = 3.5·10−5 atm). In a humid atmosphere (pH2O = 1.92·10−2 atm) at temperatures below 600 °C, all phases exhibit dominant proton transfer. A feature of the proton transport of the studied phases is the low activation energies of proton conductivity ~0.2−0.3 eV compared to classical protonics - doped perovskites based on barium cerates and zirconates. This is a significant advantage of the systems under study. - Using impedance spectroscopy, it was found that doping with indium leads to a slight increase in oxygen-ion conductivity and a more significant increase in proton conductivity in humid air (pH2O = 1.92·10−2 atm) (by 0.5 orders of magnitude compared to the matrix phase). High concentrations of the acceptor dopant lead to a decrease in proton mobility as a result of their capture due to the Coulomb interaction of defects with opposite charges. - With isovalent doping (yttrium in the indium sublattice), the value of oxygen-ion conductivity does not change significantly, but the value of electronic conductivity decreases, which leads to an increase in ionic transfer numbers. It was found that as the yttrium concentration increased, the band gap increased. The magnitude and fraction of proton conductivity increases with increasing yttrium concentration, which is the result of an increase in proton mobility as a result of an increase in unit cell volume. - Acceptor doping (substitution of indium or aluminum for zinc) led to an increase in oxygen-ion and proton conductivity, which is a consequence of an increase in the free volume of migration. 6) It has been established that the phases exhibit chemical resistance in an atmosphere of carbon dioxide and hydrogen during high-temperature annealing. 7) Doping the complex oxide Ba5In2Al2ZrO13 with zinc made it possible to obtain high-density ceramics (~98-99%) at lower temperatures (by 100 °C) and shorter heat treatment times. Based on the results of investigations in 2023, 4 articles were published: https://doi.org/10.31857/S0424857023030039; https://doi.org/10.3390/app13063978 https://doi.org/10.1134/S1023193523030035 Q3; https://doi.org/10.1007/s11581-023-05187-5; and 5 reports were presented at conferences: https://elar.urfu.ru/handle/10995/123331 https://fizteh.urfu.ru/fileadmin/user_upload/site_19855/Conference/2023/Konferencija_FTI-2023_Informacionnoe_soobshchenie.pdf http://www.conference.ihte2023.tilda.ws/ http://hdl.handle.net/10995/117133

Publications

1. Andreev R., Animitsa I. Transport Properties of Intergrowth Structures Ba5In2Al2ZrO13 and Ba7In6Al2O19 Applied Sciences, 2023, 13(6), 3978 (year - 2023) https://doi.org/10.3390/app13063978

2. Andreev R.D., Animitsa I.E. Protonic transport in the novel complex oxide Ba5Y0.5In1.5Al2ZrO13 with intergrowth structure Ionics, V. 29, i. 11. P. 4647 - 4658 (year - 2023) https://doi.org/10.1007/s11581-023-05187-5

3. Andreev R.D., Anokhina I.A., Korona D.V., Gilev A.R., Animitsa I.E. Транспортные свойства In3+- И Y3+-допированного гексагонального перовскита Ba5In2Al2ZrO13 Электрохимия, Т. 59. № 3. C. 143-157 (year - 2023) https://doi.org/10.31857/S0424857023030039

4. R. D. Andreev, I. A. Anokhina, D. V. Korona, A. R. Gilev, and I. E. Animitsa Transport Properties of In3+- and Y3+-Doped Hexagonal Perovskite Ba5In2Al2ZrO13 Russian Journal of Electrochemistry, V. 59, N. 3, pp. 190–203 (year - 2023) https://doi.org/10.1134/S1023193523030035

5. Andreev R.D. ELECTRIC CONDUCTIVITY OF ANION-DEFICIENT HEXAGONAL PEROVSKITES BA5IN2AL2ZRO13 AND BA7IN6AL2O19 Десятая международная молодежная научная конференция. Физика. Технологии. Инновации. ФТИ-2023. 15-19 мая 2023. г. Екатеринбург, Десятая международная молодежная научная конференция. Физика. Технологии. Инновации. ФТИ-2023. 15-19 мая 2023. г. Екатеринбург (year - 2023)

6. Andreev R.D., Animitsa I.E. КИСЛОРОД-ИОННЫЙ И ПРОТОННЫЙ ТРАНСПОРТ ГЕКСАГОНАЛЬНОГО ПЕРОВСКИТА Ba5In1.5Y0.5Al2ZrO13 ПРОБЛЕМЫ ТЕОРЕТИЧЕСКОЙ И ЭКСПЕРИМЕНТАЛЬНОЙ ХИМИИ тезисы докладов XXXIII Российской молодежной научной конференции с международным участием, - (year - 2023)

7. Andreev R.D., Korona D.V., Animitsa I.E. ВЛИЯНИЕ ПРИРОДЫ ДОПАНТА НА КИСЛОРОДНО-ИОННЫЙ И ПРОТОННЫЙ ТРАНСПОРТ ГЕКСАГОНАЛЬНОГО ПЕРОВСКИТА BA5IN2AL2ZRO13 ФИЗИЧЕСКАЯ ХИМИЯ И ЭЛЕКТРОХИМИЯ РАСПЛАВЛЕННЫХ И ТВЕРДЫХ ЭЛЕКТРОЛИТОВ Сборник материалов ХIX Российской конференции, посвященной 65-летию Института высокотемпературной электрохимии Уральского отделения РАН., В сборнике: Физическая химия и электрохимия расплавленных и твердых электролитов. Сборник материалов ХIX Российской конференции, посвященной 65-летию Института высокотемпературной электрохимии Уральского отделения РАН.С. 164-165. (year - 2023)

8. Animitsa I.E. ПРОЦЕССЫ ИНКОРПОРИРОВАНИЯ ПРОТОНОВ В СТРУКТУРУ ПЕРОВСКИТОПОДОБНЫХ ПРОТОННЫХ ПРОВОДНИКОВ ФИЗИЧЕСКАЯ ХИМИЯ И ЭЛЕКТРОХИМИЯ РАСПЛАВЛЕННЫХ И ТВЕРДЫХ ЭЛЕКТРОЛИТОВ Сборник материалов ХIX Российской конференции, посвященной 65-летию Института высокотемпературной электрохимии Уральского отделения РАН. 2023, В сборнике: Физическая химия и электрохимия расплавленных и твердых электролитов. Сборник материалов ХIX Российской конференции, посвященной 65-летию Института высокотемпературной электрохимии Уральского отделения РАН. 2023. С. 166-168. (year - 2023)

Annotation of the results obtained in 2022
One of the promising directions for solving the problem of providing electric energy to consumers in various, including difficult, conditions, are developments in the field of fuel cell (FC) technologies. Fuel cells are electrochemical devices for converting fuel energy into electrical energy. In such devices, there is a direct conversion of chemical energy into electrical energy due to the occurrence of redox reactions at the interphase boundaries of electronic conductors with ionic conductors (electrolytes). Since the intermediate stage of fuel combustion with heat generation is bypassed and, accordingly, there is no conversion of heat into work in these devices, their efficiency is both theoretically and practically much higher than that of thermal power plants. Recently, there has been a global trend towards the development of fuel cells based on oxide systems operating in the range of average temperatures of 500-700°C (and even lower). And this implies a scientifically based selection of an electrolyte with a sufficiently high ionic conductivity in a given temperature range. Such electrolytes can be materials based on complex oxides with proton conductivity. In addition to high values of proton conductivity, such materials should be stable both in oxidizing and reducing atmospheres, have chemical resistance to the components of the gaseous medium (CO2, H2O), and have good ceramic characteristics (mechanical strength, sintering ability). The efforts of researchers around the world are currently focused on solving these problems. Within the framework of this Project, a specific task was solved - the development of new materials based on hexagonal perovskites, optimization of their functional properties, in order to form theoretical and experimental foundations for the development of highly efficient, environmentally friendly and economically attractive electrochemical devices for various purposes for clean energy, in particular, hydrogen energy. In this Project, new proton-conducting electrolytes based on hexagonal perovskite Ba5In2Al2ZrO13 with a block structure are being developed, which have not been previously described in the literature as protons, but which have a chance to create highly efficient proton-conducting systems based on them. The main results: • Hexagonal perovskites (space group P63/mmc) were obtained by solid-phase synthesis: Ba5In2Al2ZrO13 (basic composition) and doped phases Ba5In1.9Y0.1Al2ZrO13 (isovalent doping), Ba5In2.1Al2Zr0.9O12.95 (acceptor doping), Ba5In2Al2Zr0.9Nb0.1O13 .05 (donor doping). Doping of the Ba5In2Al2ZrO13 compound with yttrium and indium was accompanied by an increase in the unit cell parameters, and by niobium, by a decrease, which correlates with the ratio of the radii of the substituting atoms. According to the data of energy dispersive X-ray spectroscopy, it was proved that under the conditions of the synthesis, the cationic composition of the samples correlated well with the theoretical one. The results of studying the surface morphology of the samples according to SEM data showed that all ceramic samples had intergrown rounded grains 3–5 µm in size. • Using thermogravimetry (TG) combined with mass spectrometry (MS), it was found that all phases are capable of water intercalation from the gas phase. The possibility of water intrusion is due to the presence of oxygen-deficient blocks in the structure, in which layers of BaO3 and BaO□2 alternate. The localization of oxygen vacancies in the BaO□2 layers leads to the fact that some of the barium atoms form a polyhedron consisting of half a cuboctahedron, that is, they have a coordination number of 9. Upon hydration, such a polyhedron can easily be supplemented to a ten-vertex due to the participation of OH groups in the coordination . The compositions of hydrated samples were determined from thermogravimetric measurements, which corresponded to Ba5In2Al2ZrO12.7(OH)0.6, Ba5In2.1Al2Zr0.9O12.54(OH)0.82, Ba5In1.9Y0.1Al2ZrO12.61(OH)0.78, and Ba5In2Al2Zr0.9Nb0.1O12.81( OH)0.48. During hydration, the symmetry of the unit cell does not change, there is a slight increase in the lattice parameters. It was found that the degree of hydration increased with an increase in the unit cell volume. The presence in the structure of energetically non-equivalent OH groups involved in various hydrogen bonds is confirmed by infrared (IR) spectroscopy studies. Accordingly, this suggests their different thermal stability, which is confirmed by the TG and MS data. The most thermally stable were isolated OH-groups involved in weak hydrogen bonds and less thermally stable - OH-groups involved in strong hydrogen bonds. Based on the data obtained, it can be concluded that the methods of iso- and heterovalent doping of intergrowth structures, which make it possible to increase the unit cell volume, are a promising strategy for increasing the degree of hydration, respectively, the concentration of protons. • For all synthesized samples, the electrical conductivity was studied by varying the temperature, partial pressure of oxygen, and water vapor. All phases were characterized by an insignificant contribution of grain boundary resistance, not exceeding 10% of the total resistance, which is a positive moment for the characteristics of ionics. The studied phases at temperatures below 500°C are predominantly characterized by oxygen-ion transfer in a dry atmosphere (pH2О = 3.5 10−5 atm). In a humid atmosphere (pH2О = 1.92 10−2 atm) at temperatures below 600°C, all phases show the dominant proton transfer. Using impedance spectroscopy, it was found that doping with indium leads to an increase in conductivity in moist air (pH2О = 1.92 10–2 atm) by 0.5 orders of magnitude compared to the matrix phase. Measurement of the conductivity as a function of the partial pressure of oxygen showed that this is due to the large contribution of ionic conductivity to the value of the total electrical conductivity. Donor doping (Nb5+) makes it possible to reduce the fraction of hole conductivity as a result of the formation of interstitial oxygen and the shift of equilibrium to the left. Acceptor doping makes it possible to increase the proportion of proton transfer as a result of increasing the concentration of protons. A specific feature of the proton transport of the studied phases is the low activation energies of proton conductivity ~0.2–0.3 eV compared to classical protons, doped perovskites based on barium cerates and zirconates. This is a significant advantage of the studied systems. • It has been shown for the first time that modified phases based on Ba5In2Al2ZrO13 can be promising materials for proton-conducting membranes in electrochemical devices such as solid oxide fuel cells (SOFCs). Proton conductivity reaches 10-4 S cm-1 at 400-500°C. Further modification of the composition and a directed search for the optimal concentrations of the doping component can significantly improve the proton conductivity. https://doi.org/10.15826/chimtech.2022.9.4.14 https://doi.org/10.3390/ma15113944

Publications

1. Andreev R., Korona D., Anokhina I., Animitsa I. Proton and Oxygen-Ion Conductivities of Hexagonal Perovskite Ba5In2Al2ZrO13// Materials 2022, 15, 3944. https://doi.org/10.3390/ma15113944 MDPI AG Switzerland, Materials 2022, 15, 3944 (year - 2022) https://doi.org/10.3390/ma15113944

2. Andreev R.D., Korona D.V., Anokhina I.A., Animitsa I.E. Andreev R.D., Korona D.V., Anokhina I.A., Animitsa I.E. Novel Nb5+-doped hexagonal perovskite Ba5In2Al2ZrO13 (structure, hydration, electrical conductivity)// Chimica Techno Acta. 2022, vol. 9(4), No. 20229414. Ural University Press publisher, Chimica Techno Acta. 2022, vol. 9(4), No. 20229414 (year - 2022) https://doi.org/10.15826/chimtech.2022.9.4.14