Annotation of the results obtained in 2022
Microextrusion printing of composite anodic NiO/(CeO2)0.80(Sm2O3)0.20 (NiO/SDC) and cathodic La0.6Sr0.4Co0.2Fe0.8O3-δ/(CeO2)0.80(Sm2O3)0.20 (LSCF/SDC) nanostructures of complex geometry on the surface of previously printed hierarchically organized planar (CeO2)0.80(Sm2O3)0.20 solid electrolytes was investigated. Taking into account the optimized parameters of microextrusion printing (pressure over ink, pulse duration, pulse interval, speed of dispenser movement along the three coordinate axes), composite electrode structures of complex geometry were applied according to the previously prepared digital trajectory and miniature membrane-electrode blocks of micro-SOFCs (NiO/SDC)/SDC/(LSCF/SDC) were successfully formed, differing in SDC microstructure characteristics. The obtained planar materials were studied by X-ray diffraction analysis, diffuse reflectance infrared spectroscopy and Raman spectroscopy, the results of which confirmed the spectral characteristics preservation of the oxide nanomaterials used in the printing process, as well as the absence of any impurity signals. The results of scanning electron microscopy (SEM) show that the fabricated micro-SOFC samples can be divided into two groups according to the microstructure features of the solid electrolyte, the first of which (samples #2-5) is characterized by the highly dispersed state of SDC, and in the case of the second group (samples #6-8) the solid electrolyte has a polymodal particle size distribution, including rather large one-dimensional structures (up to 5-7 μm long). It was demonstrated that the microstructure of SDC electrolytes fully correlates with the microstructure of the corresponding powders used. The study of the electrodes microstructure in the phase contrast mode made it possible to further confirm the composite character of these materials; it was found that SDC particles were evenly distributed between the films of complex geometry characteristic of LSCF powder (in the case of cathode), as well as between NiO nanosheets (in the case of anode). Examination of "electrolyte/electrode" interfaces showed that between SDC solid electrolyte and electrodes applied on its surface there are sufficiently clear and even borders, and variations of functional layer thicknesses at the investigated sections are minimal. Analysis of "electrolyte/anode" and "electrolyte/cathode" cross sections by example of sample No. 5 included into the first group showed that the solid electrolyte layer is dense enough, and the composite electrode layers are characterized by higher porosity. In the case of micro-SOFC #6, which is included in the second group of samples differing in the solid electrolyte microstructure, it was found that the SDC electrolyte layer is significantly more porous than that for sample #5. The anode and cathode surface, as in the case of sample No. 5, is characterized by a composite structure. Plotting the chemical element distribution maps on the surface of the studied cross sections by Energy Dispersive X-Ray (EDX) analysis allowed to specify that nickel is concentrated in the anodic layer, cerium and samarium are mainly present in the lower layer of the solid electrolyte SDC. Ce and Sm also appear in the anodic NiO/SDC and cathodic LSCF/SDC layers (but in smaller amounts), confirming their composite nature. The obtained element distribution maps also prove the concentration of the elements included in the composite cathode structure. The elemental analysis performed during EDX mapping confirmed the given composition of the materials. The results of the topography investigation of the obtained micro-SOFCs samples by atomic force microscopy (AFM) are in good agreement with the SEM data and confirm that the microstructure of the printed electrode coatings is inherited from the corresponding oxide powders of composition (CeO2)0.80(Sm2O3)0.20 (samples 2-7), NiO #6 and LSCF #3 used for the ink preparation. The Kelvin Probe Force Microscopy (KPFM) data and volt-ampere characteristics obtained within AFM show high electrical conductivity of composite anode and cathode structures formed by microextrusion printing, while for solid electrolyte layers SDC electrical conductivity has a much lower value. According to the results of scanning capacitance microscopy data, there is also a slight shift of charge carriers to the grain boundaries for planar SDC structures, which may indicate a predominantly intergranular character of the materials’ conductivity. Using KPFM, it was found that the electron work function value of the SDC coatings’ surface is in the range of 4.32-4.71 eV. For the composite NiO/SDC anode structures, the value of the work function varies from 4.64 (sample No. 6) to 4.80 eV (sample No. 8). In the case of LSCF/SDC cathode materials, this parameter ranges from 4.56 (sample #5) to 4.77 eV (sample #3). The electrophysical characteristics of the formed single-chamber micro-SOFCs were studied by impedance spectroscopy. It was found that the total conductivity value in the 450-700°C temperature range increases with a change in the gas medium composition in the following row: dry nitrogen < dry air < wet air, which agrees well with the tendency to a decrease in the activation energy in the same row. The summary analysis of the total conductivity values in a dry air atmosphere for the whole range of micro-SOFCs samples under study allowed to establish that the highest values in the whole temperature range under study (450-700°С) are demonstrated by sample No. 5, characterized by the most homogeneous electrolyte microstructure, consisting of nanospheres with an average size of about 120 nm. Sample #3, which has a similar electrolyte microstructure, is characterized by generally comparable conductivity values, whereas samples #2 and #4, containing porous hemispherical SDC agglomerates, probably tend to form more porous coatings, which leads to a 6.4-11.7-fold decrease in conductivity. The lowest conductivity values are demonstrated by samples No. 6-8, which is due to the presence in their electrolytes not only porous nanospheres but also rather large one-dimensional structures (about 5-7 µm long), which under the selected conditions of additional heat treatment (600°C, 2h) do not provide the necessary contact between SDC particles, therefore, an additional decrease in the value of total conductivity is observed. Thus, a systematic investigation of hydrothermal and glycol-citrate synthesis of nanosized (CeO2)0.80(Sm2O3)0.20, La0.6Sr0.4Co0.2Fe0.8O3-δ and NiO oxides, characterized by different types of hierarchical organization was performed within the project. Procedures for obtaining stable disperse systems (including binary ones) based on the resulting anisotropic nanoparticles of the above compositions, characterized by the necessary rheological properties for their application as functional inks, have been developed. Procedures for obtaining stable disperse systems (including binary ones) based on the resulting anisotropic nanoparticles of the above compositions, characterized by the necessary rheological properties for their application as functional inks, have been developed. Using a set of complementary physico-chemical analysis methods, it is shown that the proposed microextrusion printing technology with the use of the prepared functional inks is promising and effective for automated layer-by-layer formation ("composite anode - solid electrolyte - composite cathode") of miniature functional components for micro-SOFCs. As a result of the performed electrophysical measurements the relationship between the solid electrolyte microstructural properties and the functional characteristics of the formed planar structures based on it was established, which is important in the context of improving the efficiency of such poorly studied alternative energy devices as miniature single-chamber solid oxide fuel cells of planar type.

 

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Annotation of the results obtained in 2021
The process of hydrothermal synthesis of nanomaterials with anisotropic structure of (CeO2)0.80(Sm2O3)0.20 and NiO was investigated in the carried out work. The influence of synthesis conditions (temperature, time of heat treatment, concentration of reagents, steam pressure in the reactor) on the shape, degree of hierarchical organization and dispersity of the formed nanostructures was shown. It has been determined that in the oxide of (CeO2)0.80(Sm2O3)0.20 composition the concentration of reagents is the factor which has the most essential influence on the process of hydrothermal synthesis in comparison with its temperature and duration. It was shown that in the case of low (0.005 mol/L) concentration of reagents there is a tendency to the formation of products containing fluorite-type crystalline phase (sp.gr Fm3m), while a 2-fold increase in the concentration of metal salts leads to the formation of crystalline substances with an orthorhombic structure (sp.gr Pnma) characteristic of cerium and samarium hydroxocarbonates. Additional heat treatment at 600ºC (1 h) leads to complete transformation of the intermediate products crystal structure and formation in all cases of highly dispersed (average size of CSR 5-14 nm) CeO2-based solid solutions without crystalline impurities. The microstructure of obtained nanopowders of (CeO2)0.80(Sm2O3)0.20 composition is characterized by hierarchical organization of its particles. The samples obtained at 120ºС consist of nanospheres whose average size increases from 120 to 150 nm with increasing concentration of reagents in the reaction system. As the synthesis temperature increases up to 180ºC, hemispherical agglomerates (sized about 60-75 nm) formed from nanoparticles whose size increased from 6 to 9 nm with increasing synthesis time. As the concentration of the starting reagents increases, in addition to nanospheres and hemispheres, nanosheets (about 20 nm thick), which form one-dimensional structures (about 5-7 micrometers long) as the synthesis temperature rises to 180ºC, are also observed in the microstructure of the samples. Not only the concentration of nickel chloride but also the temperature and duration of hydrothermal treatment significantly affect the NiO synthesis process. The phase composition of powders after drying changes from α-Ni(OH)2 to γ-NiOOH and β-Ni(OH)2 when passing to harsher synthesis conditions. Additional heat treatment of the obtained semiproducts at 350°C (1h), leads to their complete decomposition and formation of nanocrystalline NiO. The average size of the oxide formed depends on the phase composition of the semiproduct. The obtained NiO powders consist of nanosheets, which organize into spherical agglomerates at lower temperature of hydrothermal treatment. As the synthesis temperature increases and the concentration of reagents decreases, there is a tendency for the formation of individual nanosheets. The synthesis process of La0.6Sr0.4Co0.2Fe0.8O3-δ oxide was studied. The influence of oxidant (ammonium nitrate) and reducing agents (ethylene glycol and citric acid) concentration on the synthesis process and the phase composition and microstructure of the obtained oxide were determined. The process of initiation of the redox reaction by heating the reaction systems consisting of lanthanum, strontium and cobalt nitrates and iron chloride with the addition of oxidizing and reducing agents in an air current was studied using a simultaneous thermal analysis. It was found that when reaching 180 °С in all cases the initiation of the redox reaction and active outgassing occurred, mass loss was observed, with the main chemical transformations in the reaction systems occurring when heated to about 250 °С, and the oxidation of the resulting carbon was completed by 600 °С. According to the data of the X-ray diffraction analysis it was established that the proposed conditions of synthesis allow to receive the target products, the crystal structure of which corresponded to the target cubic structure of perovskite type (pr.gr.Pm3 ̅m) and contained no crystalline impurities already after initiation of the redox reaction and heat treatment at 300°С. The obtained powders of composition La0.6Sr0.4Co0.2Fe0.8O3-δ have a foamy morphology, traditional for products of glycol-citrate synthesis, and consist of agglomerates in the form of films of complex shapes with thickness from tens of nanometers to several micrometers. The inner volume of the oxide films is a highly porous material, consisting of particles 20-30 nm in size, while their outer surface is characterized by high density. In the case of the sample with a minimum concentration of ammonium nitrate and ethylene glycol, oxide films with the smallest thickness are formed. In the case of simultaneous increase in the oxidizer (NH4NO3) and one of the reducing agents (C2H6O2) the product of glycol-citrate synthesis consists of oxide films of the greatest thickness. The results of the X-ray spectral elemental microanalysis confirmed the compliance of the chemical composition of the powders under study with the specified one. It was demonstrated that the character of the oxide powder microstructure essentially depends on the composition of the reaction system, which allows by varying the oxidizer and reducing agent concentrations to control and predict the morphology features, particle size, specific surface area and, accordingly, the functional properties of the obtained material. Methods have been developed for obtaining functional inks based on anisotropic nanoparticles of composition (CeO2)0.80(Sm2O3)0.20, NiO and La0.6Sr0.4Co0.2Fe0.8O3-δ suitable in their rheological characteristics for forming hierarchically organized solid electrolytes as well as composite anode and cathode nanostructures by microextrusion printing. Optimal rheological characteristics ensuring the required sedimentation stability, wettability of the used substrates and preservation of the geometry of the formed oxide coatings were achieved by using alpha-terpineol as a solvent and ethylcellulose (23-25 wt%) as a binder. A technique was developed for microextrusion printing of hierarchically organized planar-type solid electrolytes of (CeO2)0.80(Sm2O3)0.20 composition (using functional inks based on appropriate anisotropic nanoparticles of different geometry and dispersion) on the surface of polycrystalline Al2O3 substrates equipped with Pt-microheaters. The printing parameters such as the speed of the dispenser moving over the surface of the substrates c, the pulse duration and the pulse interval during ink dosing were optimized. After coating were subjected to drying at about 40ºC for 3 hours followed by heat treatment at 600ºC (1h) to remove the solvent and organic components. The formed oxide coatings of composition (CeO2)0.80(Sm2O3)0.20 preserved the cubic crystal structure of fluorite type, there are no crystalline impurities. The average CSR size for the obtained coatings varies from 6.6 to 12.8 nm, which agrees well with the microscopy results. The character of the morphology of the studied materials agrees well with the microstructural characteristics of the powders used in the preparation of functional inks. The results of Kelvin probe force microscopy (KPFM), demonstrate that the surface potential has a greater value in the recesses of the material, indicating a shift in the density of charge carriers in the corresponding areas. Capacitance gradient maps (scanning capacitance microscopy results) demonstrate that there is a shift of charge carriers to inter-particle boundaries, confirming the KPFM results. The values of the electron yield work from the surface of the obtained oxide coatings were calculated using the KPFM results, which allow us to estimate the concentration of defects (in particular, oxygen vacancies).

 

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