D. I. Bronin

Structure and phase transitions of (La, Sr)(Ga, Mg)O3−α solid electrolyte

Abstract Electrical conductivity, Raman scattering (RS) spectra, and thermal expansion of the solid solution based on lanthanum gallate having the formula La0.88Sr0.12Ga0.82Mg0.18O2.85 (LSGM12–18) were studied as a function of temperature. X-ray diffraction analysis (XDA) of the samples at room temperature was performed. A dilatometric examination demonstrated that phase transitions of the second order took place at 775±10 and 880±20 K. The transition at 880 K was confirmed both by Raman scattering spectra and by a change in the activation energy of the electrical conductivity. This phase transition is probably due to a change of the oxygen sublattice symmetry.

Single solid-oxide fuel cells with supporting ni-cermet anode

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.

Cathodes based on (La, Sr)MnO3 modified with PrO2 − x

The electrochemical characteristics of composite cathodes made of (La, Sr) MnO3-(Zr, Sc)O2 (LSM-SSZ), modified with PrO2 − x additive, and designed for application in solid oxide fuel cells at moderately high temperatures were studied. The relationship between activity of catalytically modified composite LSM-SSZ cathodes and dispersity of electrocatalyst was revealed. The boundaries of the temperature range with the maximum dispersity of electrocatalyst and electrochemical activity of cathodes were found. The composite LSM-SSZ cathodes modified with PrO2 − x were shown inert with respect to oxidation reactions of hydrocarbon fuel (methane) and highly active electrochemically with respect to oxygen reaction in non-equilibrium gas mixture of CH4 and O2. In cells with (Ce, Sm)O2 (SDC) and (Zr, Y)O2 (YSZ) electrolytes, their overvoltage is below 80 mV at the current density about 0.5 A/cm2 and temperature of 600°C. These electrodes can be used as cathodes in single-chamber fuel cells. Long-term experiments were carried out to study time stability of characteristics of the said composite cathodes. The studied electrodes show parabolic or damped exponential time curves of polarization resistance if contacting with YSZ or SDC electrolyte, respectively. According to the forecast based on the experimental regularities, the polarization resistance of LSM-SSZ cathodes in 10,000 h will not exceed 0.4 or 0.13 Ohm cm2, respectively, if YSZ or SDC electrolyte is used.

The use of Tikhonov regularization method for calculating the distribution function of relaxation times in impedance spectroscopy

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 effect of copper on the properties of La1.7Ca0.3NiO4 + δ-based cathodes for solid oxide fuel cells

The aim of the work was to develop the electrochemically active cathodes with a low in-plane resistance for application in the intermediate-temperature solid oxide fuel cells in contact with CeO2-based solid electrolytes. Mixed La1.7Ca0.3NiO4 + δ conductor was chosen as the material for functional cathodic layer. The conductor has the layered perovskite-like structure and is compatible with these electrolytes in the thermal linear expansion coefficient (TLEC). Lanthanum nickelate ferrite LaNi0.6Fe0.4O3, which exhibits a high electronic conductivity and is close to the functional layer material in the TLEC, was used as the collector layer. The peculiarities of sintering and the electrical and electrochemical characteristics of the cathodes were studied as a function of the amount of CuO, which was introduced into the functional layer.

Single fuel cell with supported LSM cathode

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.

Peculiarities of electrical transfer and isotopic effects H/D in the proton-conducting oxide BaZr0.9Y0.1O3 − δ

Electrical conduction of the oxide BaZr0.9Y0.1O3 − δ is studied by electrochemical impedance spectroscopy as a function of temperature (300–600°C) and the oxygen partial pressure in gas phase saturated with H2O or D2O vapor. The full electrical conduction is separated into components, in particular, the bulk and grain boundary conduction. It is shown that at PO2> 1 Pa the BaZr0.9Y0.1O3 − δ conduction is contributed significantly by electron holes whose transference number in air atmosphere may be as high as 0.5–0.6. In reductive conditions, the electrical transfer involves proton (deuteron) charge carriers. Isotopic effect H/D in the conduction in the bulk and along the grain boundaries is determined. Isotopic effect H/D in the hole conduction is also revealed. It is shown that this effect comes out of different solubility of deuterons and protons in the oxide (the thermodynamic isotopic effect).

Electrical double-layer capacitance of the M, O2/O2− interfaces (M=Pt, Au, Pd, In2O3; O2−=zirconia-based electrolyte)

Abstract The behavior of the electrode systems M, O 2 /O 2− , where M=porous Pd, Pt, Au, and dense In 2 O 3 ; O 2− =ZrO 2 -based single-crystal solid electrolyte, was studied by means of impedance measurements. The examination of the Pt, O 2 /O 2− electrode system showed that the constant phase element (CPE) can be attributed to a nonuniform distribution of current on the electrode surface. It was inferred that the CPE parameters n and B in the expression Y CPE = B ( jω ) n , may be related one to the other by B=(C dl ) n (R Ω ) n−1 ,where C dl is the double layer capacitance and R Ω the resistance of the electrolyte in the cell. Then C dl of the electrode–electrolyte interface can be determined. The specific C dl of the oxidized noble metals and india electrodes is nearly one order of magnitude lower than C dl of the electrodes in the metallic state. The C dl value of all the electrodes studied depends little or is independent of temperature and oxygen pressure. It was concluded that the Helmholtz model of a double-layer structure does not contradict the C dl behavior.

Options for adjustment of microstructure and conductivity of cathodic substrates of La(Sr)MnO3

The electrodes of solid-oxide fuel cells (SOFCs) must be characterized by high conductivity to decrease ohmic losses and sufficient porosity to provide high gas diffusion rate. In the cases, when the SOFC electrodes are substrates, they must be synthesized at the temperature above the temperature of formation of their solid-electrolyte coating. Herewith, manufacturing of supporting electrodes with the required micro-structure is rather complicated. The present paper studies the effect of the method of manufacturing of the initial La0.6Sr0.4MnO3 (LSM) powders, their degree of dispersion, introduction of sintering additives and pore agents on their microstructure, conductivity, and possibility of adjusting the temperature of SOFC cathodic substrate formation at which the required characteristics are reached. It is shown that sintering of cathodic substrates to the relative density of 65–70% can be carried out at the temperatures from 1050 to 1350–1400°C, which would allow obtaining electrolyte films of powders with different sintering ability on such substrates. The average pore size in cathodic substrates can be varied in the range of 0.4 to 2.5 μm by using the initial LSM powder with different dispersion degree and by employing graphite as a pore agent. At 900°C, conductivity of cathodic substrates of LSM grows at an increase in their relative density from 50% to 70% approximately from 50 to 100 S/cm and weakly depends on the dispersion degree of the initial powders.

Polarization resistance of platinum electrodes in contact with proton-conducting La0.9Sr0.1613-1

The Pt-La0.9Sr0.1ScO3 − δ interface was studied by impedance spectroscopy at different temperatures (600–900°C) and partial pressures of oxygen (50 < pO2 < 2.1 × 104 Pa) in the gas phase saturated with H2O vapors at 25°C (pH2O = 3.17 kPa). The polarization resistance of electrodes was calculated taking into account the shunt effect of the hole conductivity of the electrolyte. In the range of temperatures and partial oxygen pressures in the gas phase under study, the mechanism of the electrode reaction at the Pt-La0.9Sr0.1ScO3 − δ interface was shown to be the same.