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Ce0.8R0.2O2–a (R = Y, Sm, Tb) (further Се20R) submicron barrier layers, which were obtained by the dip-coating method on a YSZ (ZrO2 + 9.5 mol.% Y2O3) solid electrolyte to prevent the chemical interaction between YSZ and the La2NiO4+δ-based cathode material, were studied. The final thermal treatment of the barrier layers was performed at 1200 °С. A porous composite of the La2NiO4+δ-Ce0.8Sm0.2O2–а composition with the LaNi0.6Fe0.4O3 collector layer served as the cathode. The polarization characteristics of the cathodes were studied by impedance spectroscopy for both the uncoated electrolyte and the electrolyte with Се20R (R = Sm, Tb, Y) thin-layer coatings of various thicknesses. The impedance spectra were analyzed using the DRT method. Even single-layer coatings (≈200 nm) were found to prevent the chemical interaction between the electrode and the electrolyte and to greatly increase (by 5.5–6 orders of magnitude) the electrode polarization conductivity.

The method of electrochemical impedance is used to study the polarization conductivity of porous platinum electrodes in the Pt|YSZ|Pt cells over the temperature range from 500 to 700°C in air. Electrodes are impregnated with microquantities of praseodymium oxide PrOx (from 10–4 to 0.1 mg/cm2) by the impregnating them with praseodymium nitrate solution, with further thermal treatment: slow heating (50°С/h) up to 850°C. This processing resulted in the formation of the praseodymium oxide nanofilm on the electrolyte surface. The introducing of PrOx, even in an amount of 10–4 mg/cm2, is found to significantly increase the electrode polarization conductivity. Analysis of the electrochemical impedance spectra was performed by DRT (distribution of relaxation times) method, which revealed a change of the rate-determining stage of the electrode process in the concentration range of praseodymium oxide from 1 × 10–3 to 3.3 × 10–3 mg/cm2. The studied electrodes are discussed in terms of the island model of dense-oxide-PrOx-electrodes in which platinum plays the role of the current collector.

Composite powders (66 wt % NiO + 34 wt % Zr0.84Y0.16O1.92 were obtained according to the solution combustion synthesis technique by crystallization of nickel oxide particles on the surface of Zr0.84Y0.16O1.92particles. The samples pressed from powders after sintering and reduction in moist hydrogen had high electrical conductivity, more than 16 × 103 S cm–1 at room temperature. Impedance spectroscopic studies have shown that in a humid atmosphere of Ni – Zr0.84Y0.16O1.92, electrodes formed on the surface of a dense Zr0.84Y0.16O1.92 electrolyte have low electrochemical activity. The impregnation of the electrodes with a solution of cerium nitrate followed by thermolysis made it possible to increase the electrochemical activity of the electrode by two to three orders of magnitude. An analysis of the electrochemical impedance spectra through the calculation of the distribution function of the relaxation time showed that the rate of hydrogen oxidation at the electrodes is limited by surface–adsorption processes. After impregnation, the gas diffusion resistance significantly limits the electrode reaction rate.

The anodic and cathodic polarization dependences for the oxygen electrode based on lanthanum-strontium manganite and the fuel Ni-cermet electrode are studied in the temperature range of 700–900°С in gas media that correspond to working conditions of a reversible fuel cell. The temporal behavior of these electrodes is studied in the course of periodic polarity changes of current with the density of 0.5 A/cm2. The electrode overvoltage is shown to be about 0.1 V in modes of power generation and water electrolysis at 900°С and the current density of 0.5 A/cm2. A single electrolyte supported tubular solid-oxide fuel cell was fabricated and tested in the fuel-cell and hydrogen-generation modes. It is found that at 900°С and overvoltage of 0.7 V, the cell generates the specific electric power of 0.4 W/cm2 when the 50% H2 + 50% H2O gas mixture is used as the fuel and air is used as the oxidizer. At the water electrolysis with the current density of 0.5 A/cm2, which under normal conditions corresponds to generation of about 0.2 and 0.1 L/h of hydrogen and oxygen, respectively, the consumed power is about 0.55 W/cm2. The efficiency of the conversion cycle electric power–hydrogen–electric power is 70–75%.

Characteristics of fuel cells with supporting Ni-YSZ anode, bilayer YSZ/GDC electrolyte with the thickness of 10 μm, and La2NiO4 + δ cathode are studied. It is shown that when humid (3% water) hydrogen is supplied to the anode and air is supplied to the cathode, the maximum values of cell’s power density are 1.05 and 0.75 W/cm2 at 900 and 800°С, respectively. After the introduction of praseodymium oxide and ceria into the cathode and the anode, respectively, the power density is ca. 1 W/cm2 at 700°С. It is found that the power density of a cell with impregnated electrodes weakly increases with the increase in temperature to ca. 1.4 W/cm2 at 900°С. The analysis of impedance spectra by the distribution of relaxation times shows that such behavior is associated with the gas-diffusion resistance of the SOFC anode. The latter is explained by the low porosity of the anode and the high rate of fuel consumption.

The state-of-the-art in realization of the method of distribution of relaxation times (DRT) as applied to the analysis of data of electrochemical impedance spectroscopy is briefly surveyed. The theoretical fundamentals of the DRT method are described, the methods of solving the Fredholm equation of the 1st order with respect to the unknown DRT function are considered as an ill-defined problem. The Tikhonov regularization method presently considered as the most suitable for solving this equation is discussed. For several numerical experiments, the high resolution of the DRT method and its stability with respect to noise in impedance spectra are demonstrated. Among the problems and limitations of the DRT methods, the choice of the optimal regularization coefficient is considered as the most significant. Particularly, it is shown that in those cases where several relaxation processes with the constant phase angle appear in the response of objects under study to ac disturbances, different regularization coefficients should be selected for each of these elements in order to obtain adequate results.

The work contains the results of studies of a promising composite material of Sr2Fe1.5Mo0.5O6 + Ce0.8Sm0.2O1.9 for electrodes of symmetrical solid oxide fuel cells. It is shown that conductivity of the composite at 800°C is about 10 and 15 S/cm, for air and humid hydrogen, respectively, and polarization resistance of the electrodes in contact with the electrolyte based on doped lanthanum gallate under the same conditions is about 0.26 and 0.12 Ohm cm2. Tests of a symmetrical fuel cell with a planar design and the supporting gallate electrolyte with the thickness of 300 μm show that the cell can develop the power of about 0.5 W/cm2 at 800°C when air is supplied to the cathode and humid hydrogen is supplied to the anode. Analysis of polarization losses shows that the polarization of the oxygen electrode considerably exceeds the polarization of the anode.

Study of the physical and electrical properties of the Sr2Fe1.5Mo0.5 + Ce0.8Sm0.2O1.9 composite material and its electrochemical properties as an electrode in contact with lanthanum gallate based electrolyte based on in reducing media revealed an increase in the dilatometric curve slope at around 400°C. This corresponds to an increase in the thermal expansion coefficient from 12.8 × 10–6 to 19.3 × 10–6 °C–1. An analysis of electrochemical impedance spectra by the relaxation-time-distribution method demonstrated the electrode reaction is localized in the frequency range 500–0.01 Hz. The electrical conductivity of the material under study was found to be about 17 S cm–1 at 800°C in the atmosphere of humid hydrogen. The polarization resistance under the same conditions was about 0.15 Ω cm2. The dependence of the polarization resistance on the partial pressure of hydrogen is linear with a reaction order of about–0.4, whereas that on the partial pressure of water has the opposite slope with a reaction order of about 0.2.

The behavior of polarization resistance of 50% Ni + 50% Zr0.9Sc0.1O1.95 electrodes in contact with Zr0.84Y0.16O1.92 electrolyte both in their initial state and after the introduction of cerium oxide into their porous structure is studied by the method of electrochemical impedance. The 1500 h experiments carried out at 800°С in the 60% H2O + 40% H2 gas medium demonstrate that the degradation rate of the electrodes in their initial state is somewhat lower than after the introduction of CeO2, while the initial polarization resistance of thus modified electrodes is 5-times lower. It is found that the introduction of cerium oxide can lower down the polarization resistance of electrodes subjected to short-term 500 h exposures.

The 61%NiO + 39%Zr 0.84 Y 0.16 O 1.92 (NiO-YSZ) and 56%NiO + 44%Zr 0.83 Sc 0.16 Ce 0.01 O 1.92 (NiO-CeSSZ) composite powders have been prepared using two-steps and one-step combustion synthesis, respectively. The Ni-YSZ anode substrate with a low level of electrical resistance (less than 1 mOhm cm) and porosity of about 53% in the reduced state was fabricated. The functional layer of the anode with the high level of electrochemical activity was made of NiO-CeSSZ. The single anode-supported solid oxide fuel cell with the bi-layer Ni-cermet anode, Zr 0.84 Sc 0.16 O 1.92 film electrolyte and the Pt + 3% Zr 0.84 Y 0.16 O 1.92 cathode was fabricated. The power density and the U–I curves of the fuel cell at initial state and after impregnation of the cathode and anode by praseodymium and cerium oxides, respectively, have been measured at different temperatures. The maximum of power density of the initial fuel cell was 0.35 W cm −2 at conditions of wet hydrogen (air) supply to the anode (cathode) at 900 °C. After the electrodes were impregnated, the value of power density increased by seven times and was approximately 2.4 W cm −2 at 0.6 V. It was suggested that after the electrodes impregnation the polarization resistance of the fuel cell was determined by the gas diffusion in the supported anode.

Time dependences of the total and partial polarization resistances of Ni-cermet electrode with addition of Cu and impregnated with CeO 2-δ in contact with the YSZ electrolyte in the reducing gas mixtures of different composition were studied by means of impedance spectroscopy at the temperature of 900 °C. In H 2 + H 2 O binary gas mixtures the degradation rate (decrease of electrochemical activity over time) of the Ni-cermet electrode increases sharply at the partial pressure of water over 45 vol.%. The degradation rate of the Ni-cermet electrode in the 10%CO + 90%CO 2 gas mixture is significantly slower than in the 10%H 2 + 90%H 2 O gas mixture. The analysis of the impedance spectra showed that at the equilibrium potential the electrode reaction is limited by three stages. Major changes in the characteristics of the electrode during long exposures under working condition are associated with the changes ranger of the partial polarization resistances high- and middle-frequency.

The supported Ni-YSZ (50 wt.% Ni + 50 wt.% Zr0.84Y0.16O1.92) anodes were produced of powders, obtained by the ceramic method, combustion synthesis, deposition of nickel oxide onto the YSZ ceramics, and deposition of 28 wt.% of nickel oxide onto the 39 wt.% NiO + 61 wt.% YSZ powders. The influence of the NiO-YSZ powder production technique, the amount of pore former and sintering temperature on the porosity, gas permeability, thermal expansion, and anode conductivity were studied. The porosity of anodes made of powders obtained by the ceramic method is always lower than the porosity of the anodes made of powders produced by combustion synthesis under otherwise equal conditions. The anode electrical conductivity greatly depends on the powder production techniques, while the anode thermal expansion is only slightly influenced by them.

Single fuel cells with bilayer supported cathodes are manufactured and tested. The cathodes consist of a high-porous La0.6Sr0.4MnO3 support with the thickness of approximately 1 mm and a functional composite layer with the thickness of 13–15 μm made of La0.75Sr0.2MnO3 and 8YSZ. Voltammetric and power characteristics of single fuel cells with a supported cathode, thin-film YSZ electrolyte, and platinum cathode are determined. The conclusion as to the significant contribution into the polarization overpotential losses on the cathode is made on the basis of the measurements of electric fuel cell characteristics. It decreases significantly as a result of the supported cathode modification by praseodymium oxide. At 850°C and voltage of 0.81 V, electric power density of a fuel cell was 1.65 W/cm2.

Single solid-oxide fuel cells (SOFCs) with a porous (36-41%) supporting Ni-cermet anode are manufactured and tested. The effect of the thickness of the supporting Ni-cermet anode on the electrochemical characteristics of single SOFCs is studied. It is shown that polarization losses on electrodes at the current density of 1.2 A/cm2 increase by about 2 times from 0.13 to 0.25 V at an increase in the thickness of the supporting Ni-cermet anode from 0.40 to 1.27 mm. The impedance spectroscopy method is used to identify relaxation processes responsible for the behavior of the fuel cell anode and cathode. It is found that a significant percentage of polarization losses on the anode is due to transport limitations in fuel supply to the three-phase nickel/electrolyte/gas phase interface and removal of the reaction products away from it.

The method of isotopic exchange was used to study the oxygen exchange kinetics in the oxides of La0.88Sr0.12Ga0.82Mg0.18O3 − δ and La0.80Sr0.20Ga0.85−x Mg0.15Co x O3 − δ (x = 0.05, 0.15, 0.20, 0.25). The rates of oxygen exchange and its diffusion coefficients were determined in the temperature range of 600–900°C at the oxygen pressure of 5 torr. The fractions of the three exchange types for the oxides studied were determined at the temperature of 817°C and oxygen pressure of 5 torr. The total conductivity of the oxides of La0.80Sr0.20Ga0.85−x Mg0.15Co x O3 − δ (x = 0.05, 0.15, 0.20, 0.25) was measured in the temperature range of 550–850°C in air and at the temperature of 800°C in the range of oxygen pressures of 1–760 torr. It was shown that an increase in the electronic conductivity component due to an increase in the cobalt fraction in the gallium sublattice results in growing interfacial exchange rate, total conductivity of the studied systems, and a decrease in the effective conductivity activation energy.

Characteristics of nickel-cermet anodes obtained from weakly aggregated NiO nanopowders made by wire electric blasting (NiO/WEB) were studied. Electrodes made of NiO/WEB nanopowders have low sheet resistance (

The time curves of full polarization resistance of Ni cermet electrode modified with CeO2 − δ additive were studied by means of impedance spectroscopy in binary gas mixtures x% H2 + (100 − x)% H2O, 10% CO + 90% CO2 and multicomponent gas mixtures H2 + CO2 + H2O + CO + Ar of various composition at the temperature of 900°C. The Ni cermet electrode degradation rate in binary gas mixtures H2 + H2O was shown to increase sharply at the partial water pressure over 45%. The Ni cermet electrode degradation rate in the mixture of 10% CO + 90% CO2 was significantly lower than that in 10% H2 + 90% H2O. The major changes in the electrode characteristics upon long exposure in working conditions were accounted for by changes in the high-frequency partial polarization resistance. In the course of long testing, the electrode microstructure was not significantly changed. In the presence of hydrogen-containing components (H2 and H2O), the carbon-containing components (CO and CO2) were shown to make an insignificant contribution to the current generation processes in Ni cermet electrode. It was suggested that strong degradation of Ni cermet electrode was caused by poisoning its reaction sites with strongly linked adsorption forms of water (hydroxyls) at the positive charge of electrode.

The electrochemical behavior of Ni cermet electrode with CeO2 − x additive in contact with YSZ electrode was studied by means of impedance spectroscopy in H2, H2O, CO2, CO, He, and Ar gas media of various composition within the temperature range of 700 to 950°C. Near the equilibrium potential, the electrochemical impedance spectra of the studied electrodes indicate to three stages of electrode reaction. The polarization conductivity of the low-frequency stage of electrode reaction (σlf) is characterized with the following regularities: (a) temperature dependence of σlf has a positive slope in Arrhenius coordinates; (b) σlf increases upon replacement of gas mixture with lower mutual diffusion coefficient by mixture with higher mutual diffusion coefficient, while polarization conductivity values of other stages remain practically invariable; (c) concentration relationships of 1/σlfrecorded for constant activity of oxygen in the gas phase are linear in the 1/σlf vs. 1/P CO 2 (P CO) coordinates; (d) no low-frequency stage of the electrode reaction is observed upon electrochemical inflow (outflow) of the gas reagents (reaction products) to (from) the test electrodes (current passing through closely pressed specimens and central specimen impedance measurement); and (e) no change in the gas flow rate affects σlf value. The observed regularities were explained by assuming the gas diffusion nature of the low-frequency stage of the electrode reaction. The gas diffusion layer thickness was estimated.