Synthesis, crystal structure and properties of complex oxides with the perovskite structure based on neodymium, alkaline earth and 3d-transition metals : dissertation for the degree of candidate of chemical sciences : 02.00.04

The increasing demand on global electrical power consumption, environmental problem and depletion of natural sources stimulate finding a modern and alternative way of renewable energy. Solid oxide fuel cells (SOFC) is one of the reliable alternative sources of renewable energy [1]. The reduction of working temperature down to the intermediate temperature range (600–800 ° C) is one of the main challenges in creating reliable long-term operating devices with improved performance. Various perovskite-type oxides have been studied in order to progress the cathode performance at intermediate temperature [1]. Among these perovskites cobalt based materials attracted much attention due to their high conductivity and good electrochemical properties, however thermal expansion coefficient (TEC) value is too high comparing to the possible electrolytes, like La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) or Ce0.9Gd0.1O2-δ (CGO). Thus, thermomechanical incompatibility, and as a result, short-term stability of the cells with the cobalt-based cathode materials is the main drawback.
The efficiency of standard SOFCs cathode material based on LaMnO3 can be noticeably enhanced when lanthanum is substituted by neodymium. The moderate TEC value was also detected for the Co-doped materials, as well as Ni-substituted oxides reported as potential cathode materials for the intermediate-temperature solid oxide fuel cells (IT-SOFCs) [2]. Although, there are significant amount of reports available on lanthanum manganites, but limited data presents on A-site substituted neodymium manganite and their iron-, cobalt-, and nickel-doped derivatives.
The aforementioned information confirms the relevance of the present work, which have been performed in the Department of Physical and Inorganic Chemistry, Institute of Natural Science and Mathematics, Ural Federal University named after the first President of Russia B.N. Yeltsin. This work was
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supported by the Act 211 of the Government of the Russian Federation, agreement 02.A03.21.0006.
Goals and Objectives of the Work
The purpose of this work was the systematic study of crystal structure, oxygen nonstoichiometry and transport properties of complex oxides Nd1- xAxMn0.5B0.5O3-δ (A = Ba, Sr, Ca; B = Mn, Fe, Co, Ni; x = 0, 0.25) in order to establish the relationship between the chemical composition, structure and functional properties as well as verification of possibility for their use as cathode materials in SOFCs.
Following tasks were set to achieve the aforementioned goal:
1. Synthesis of Nd1-xAxMn0.5B0.5O3-δ (A = Ba, Sr, Ca; B = Mn, Fe, Co, Ni; x = 0, 0.25) complex oxides and their crystal structure refinement.
2. Determination of oxygen nonstoichiometry in Nd1-xAxMn0.5B0.5O3-δ (A = Ba, Sr, Ca; B = Mn, Fe, Co, Ni; x = 0, 0.25) versus temperature in air following with a comparative study of doping effect.
3. Determination of thermal expansion for the studied oxides using HT-XRPD and dilatometry measurements.
4. Determination of total conductivity and Seebeck coefficient for the studied oxides versus temperature.
5. Examination of chemical compatibility between the studied oxides with moderate TEC and high total conductivity (Nd0.5Ba0.5Mn0.5Fe0.5O3-δ) and Ce0.8Sm0.2O2-δ electrolyte.
6. Impedance spectroscopic measurements for the Nd0.5Ba0.5Mn0.5Fe0.5O3-δ cathodes for evaluation of possible application in SOFCs.
Theoretical and practical significance
The experimental results obtained in the work can be treated as basic knowledge which can be used in theoretical calculations and technical design
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for the best performance as cathode materials in SOFCs. The experimental results on crystal structure, temperature dependent oxygen nonstoichiometry, TEC values, total conductivity and Seebeck coefficient of the studied materials will serve as a basis to establish theoretical links between composition, structure and properties. The calculated values of activation energy in Nd1- xAxMn0.5Fe0.5O3−δ provide additional information for understanding of the charge transference mechanism.
Thus, the experimental measurements and theoretical study of Nd1- xAxMn0.5B0.5O3-δ (A = Ba, Sr, Ca; B = Mn, Fe, Co, Ni; x = 0, 0.25) could support a choice of most optimal material for SOFCs application.
Methodology and research methods
1. The synthesis of the studied complex oxides was carried out by citrate- nitrate method.
2. The crystal structure was investigated by X-ray diffraction using a Maxima XRD-7000 and an Equinox 3000 diffractometers. The unit cell parameters were refined using the Le Bail method and structural parameters were refined by the Rietveld method using FullProf software.
3. Thermal expansion coefficient was determined using dilatometry and high-temperature X-ray diffraction analysis. Netzsch DIL 402 dilatometer and high-temperature cameras: HTK 1200N (Anton Paar) installed on Maxima XRD-7000 diffractometer were used as instruments.
4. Oxygen nonstoichiometry was investigated by thermogravimetric analysis using a Netzsch STA 409 PC instrument. The absolute value of oxygen content at room temperature was calculated from the results of Red-Ox titration with Mohr salt and iodometric titration methods using an automatic titrator Aquilon ATP-02.
5. Measurement of total electrical conductivity and thermo-emf was carried out simultaneously using the dc 4-probe method. The oxygen partial
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pressure was adjusted and monitored inside the cell of the original design
using the Zirconia 318 device.
6. Chemical compatibility of complex oxides with respect to electrolyte was
studied by contact annealing at a temperature of 1458 K in air.
7. Impedance measurements were performed using an Elins Z-2000
instrument.
Defence items
1. The information on synthesis and crystal structure of Nd1-xAxMn0.5B0.5O3-δ (A = Ba, Sr, Ca; B = Mn, Fe, Co, Ni; x = 0, 0.25) at room temperature.
2. The high temperature structural parameters for Nd1-xBaxMn0.5Fe0.5O3-δ (x = 0.25 and 0.5), NdNi0.5Mn0.5O3-δ and calculated values of thermal expansion.
3. Temperature dependencies of oxygen nonstoichiometry for Nd1- xAxMn0.5B0.5O3-δ (A = Ba, Sr, Ca; B = Mn, Fe, Co, Ni; x = 0, 0.25) complex oxides in air.
4. The values of thermal expansion for the studied complex oxide in air, obtained by high-temperature dilatometry.
5. The temperature dependent total conductivity and Seebeck coefficient for Nd1-xAxMn0.5B0.5O3-δ (A = Ba, Sr, Ca; B = Mn, Fe, Co, Ni; x = 0, 0.25) in air.
6. The results of chemical compatibility test for Nd0.5Ba0.5Mn0.5Fe0.5O3-δ with
Ce0.8Sm0.2O2-δ electrolyte and Impedance spectroscopic results.
Reliability of results and approbation of work
The reliability of the results is achieved by an integrated approach using a variety of methods, which are independent and complement each other. The approbation of the work has been made in a form of presentations at the
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international and Russian conferences and in the journal publications. The main results of the work were presented and discussed at: XXVII Conference “Problems of Theoretical and Experimental Chemistry” (Yekaterinburg, 2017); Sino-Russian ASRTU Conference Alternative Energy: Materials, Technologies, and Devices (Ekaterinburg, 2018); National Seminar on Design Synthesis, Characterization, Reactivity, Theoretical Study and Application of Different Advanced Functional Materials (Barddhaman, India, 2017); II International conference on Modern Synthetic Methodologies for Creating Drugs and Functional Materials (MOSM2018) (Ekaterinburg, 2018).
Publications
Main issues of the thesis are published in 3 articles and 5 abstracts of presentations at All-Russian and international conferences.
Structure and scope of work:
The thesis work consists of introduction, 5 chapters, conclusions and bibliography. The material is presented on 123 pages, the work contains 23 tables, 42 figures, and list of references contains 158 items.