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The electrochemical behaviour of a Sr2Fe1.5Mo0.5O6–δ based double-layer electrode decorated with a “symmetric” catalyst by the wet impregnation technique for the simultaneous acceleration of cathodic and anodic reactions was investigated for the first time. As a “symmetric” catalyst, a solution of precursor for the synthesis of praseodymium nickelate (Pr2NiO4+δ) was considered. Since the catalyst consists of NiO and Pr6O11 in an oxidizing atmosphere and of Ni and Pr2O3 in a reducing atmosphere, it effectively accelerates the rate of oxygen reduction at the cathode and hydrogen oxidation at the anode of solid oxide fuel cells with symmetric electrodes. It was shown that the rate of oxygen reduction after introduction of the catalyst into the electrode increased due to an increase in the rate of oxygen interfacial exchange between the electrode and the gas phase. The rate of hydrogen oxidation increased due to an increase in the rate of dissociation of adsorbed hydrogen. During tests of the fuel cell with a 300 μm LaGaO3-based supporting electrolyte and decorated electrodes, a maximum power density of about 0.83 W cm−2 at 800 °C under wet hydrogen/air condition was obtained.

The electrochemical behaviour of Ni-based anodes for H+ solid oxide fuel cells (SOFCs) with a composition consisting of Ni–BaCe0.89Gd0.1Cu0.01O3–δ and Ni–BaCe0.8Y0.2O3–δ was studied. Ni–BaCe0.8Y0.2O3–δ exhibited poorer performance in comparison with Ni–BaCe0.89Gd0.1Cu0.01O3–δ, which was explained by the formation of the BaY2NiO5 phase at the electrode–electrolyte interface during electrode sintering. It was found that the hydrogen oxidation rate on the Ni–BaCe0.89Gd0.1Cu0.01O3–δ anode in the temperature range of 700–900 °C was determined by the rate of the three stages. It was shown that the electrode reaction mechanism did not change in this temperature range. At the temperature of 600 °C, the reaction mechanism became more complicated, most likely due to the influence of increased protonic conductivity in the electrolyte. The nature of the probable rate-determining stages of hydrogen oxidation is suggested.

Results of the redox cycling of the Sr2Fe1.5Mo0.5O6-δ electrode in contact with the LaGaO3-based electrolyte at 800°С in air - CO/CO2 - Ar/H2 are presented for the first time. The electrochemical impedance method was used to demonstrate that the electrode polarization resistance at fast gas cycling remained almost unchanged. In the long-term cycling from oxidizing to reducing atmospheres (and vice versa) the polarization resistance equilibrated rapidly (slowly). The kinetics of the CO oxidation in CO/CO2 gas mixtures at 800°С (pO2 ≈ 10−16 – 10−19 atm) near the equilibrium electrode potential was studied. By means of a complex approach to the impedance spectra analysis using the distribution of relaxation times and non-linear least squares methods, it was determined that the CO oxidation is limited by three steps: CO adsorption (CO2 desorption) at the electrode surface, oxygen hetero-exchange and the oxygen ion transport in the electrode bulk. We assume that the charge transfer and the gas diffusion do not limit the CO oxidation reaction on the Sr2Fe1.5Mo0.5O6-δ electrode.

The co-doped double perovskite-type cobaltite Pr0.9Y0.1BaCo1.8Ni0.2O6–δ (PYBCN) is synthesized via organo-metallic precursors. The concentrations of electronic and ionic defects in PYBCN at variations of gaseous environment are obtained from the analysis of the experimental isothermal plots for equilibrium oxygen content (6 – δ) vs. oxygen partial pressure p O 2 in the ambient gaseous phase. It is argued that nickel doping is beneficial for enhanced formation of vacancies in O2– positions controlling oxygen ion transport in the cobaltite. The cobaltite based dual-phase composite with 30 wt% addition of samarium doped ceria Ce0.8Sm0.2O1.9 (SDC) is found to have considerably reduced thermal expansion compared to the parent cobaltite. The impedance measurements show that the area specific resistance (ASR) of the composite decreases to 0.11 Ω cm2 at 973 K with the activation energy 1.2 eV. Further increase of the temperature is accompanied by the decline of the activation energy to 0.9 eV, and ASR attains 0.045 Ω cm2 at 1073 K. The observed small ASRs are attributed to a large concentration of O2–vacancies in the cobaltite component of the composite. The cathodic overvoltage for oxygen reduction that does not exceed 40 mV at 1073 K and current density 1000 mA cm−2, moderate thermal expansion, good conductivity and high electro-chemical activity distinguish the developed composite as one of the promising cathode materials for intermediate temperature solid oxide fuel cell applications.

The row of oxide cathode materials with Ruddlesden-Popper structure and Sr2Fe1.5Mo0.5O6−δ anode material were investigated as promising electrodes for La10(SiO4)6O3 electrolyte with apatite-type structure. XRD analysis confirmed their chemical compatibility with the electrolyte. It was observed that the cathode electrochemical activity increased in the row Pr2NiO4+δ-Nd1.9NiO4+δ-La2NiO4+δ. The lowest value of polarization resistance was obtained for the double-layer cathode La2NiO4+δ-LaFe0.6Ni0.4O3−δ and was about 0.3 Ω·cm2 at 800 °C. It was established that three relaxation processes determined the overall cathode polarization resistance. The oxygen diffusion in the gas phase as the low-frequency process was proposed. The satisfactory activity of Sr2Fe1.5Mo0.5O6−δ anode in H2 + H2O and H2O + CH4 (1:1 vol%) gas mixtures at 800 °C was detected about 0.4 Ω·cm2. The oxygen transport in Sr2Fe1.5Mo0.5O6−δ anode and surface dissociation of hydrogen were suggested as the rate-determining stages of hydrogen oxidation on the Sr2Fe1.5Mo0.5O6−δ anode.

This paper presents, for the first time, the results of studies of the electrochemical reaction of oxygen reduction on Sr2Fe1.5Mo0.5O6-δ and Sr-deficient Sr1.95Fe1.5Mo0.5O6-δ, as promising electrodes for solid state electrochemical devices, by the electrochemical impedance method with the subsequent interpretation of the data using the concepts outlined in the Adler et al. model. It was established that the oxygen reduction reaction for both electrodes is determined by two relaxation processes associated with oxygen diffusion, oxygen surface exchange, and Knudsen diffusion in the pores of the electrode. Strontium deficiency was found to have a positive effect on the electrochemical activity of the electrodes, enhancing the stage related to oxygen diffusion and surface exchange. Also, for the first time, for Sr2Fe1.5Mo0.5O6-δ, a quantitative correlation for the surface oxygen exchange and diffusion coefficients obtained from the impedance data and by the isotopic exchange method is demonstrated. © 2021 Elsevier B.V. The authors are grateful to A.S. Farlenkov for the SEM images and A.V. Khrustov for the calculation of the microstructure parameters. The facilities of shared access center "Composition of Compounds" of IHTE UB RAS were used.

Performance of Ca-doped strontium ferrite-molybdate Sr2-xCaxFe1.5Mo0.5O6-δ (x = 0, 0.05, 0.15, 0.3, 0.5) has been studied under reducing conditions. Thermogravimetry (TG) analysis in 5% H2/Ar and DSC measurements in high-purity argon demonstrated that the compositions are highly stable and there are no phase transitions. The best electrochemical and electrical performances of about 0.12 Ω cm2 and 33 S/cm at 800 °C in wet hydrogen, respectively, were obtained for the Sr1.85Ca0.15Fe1.5Mo0.5O6-δ anode. The EIS spectra were analyzed by means of distribution relaxation times and non-linear least squares methods, which made it possible to determine the behavior of the rate-limiting steps of hydrogen oxidation. Solid oxide fuel cells (SOFC) of planar design with the 1.5 mm LaGaO3-based supporting electrolyte, with the Sr1.85Ca0.15Fe1.5Mo0.5O6-δ symmetrical electrodes and with the Sr2-xCaxFe1.5Mo0.5O6-δ (x = 0.15 and 0.3) anode and the 70 wt% Pr0.9Y0.1BaCo2O6–δ + 30 wt% SDC cathode have been studied. The obtained results demonstrated a low anode overvoltage, which was about 0.12 V at 1 A/cm2 and 800 °C in wet hydrogen. The maximum power density of SOFC with the Sr1.85Ca0.15Fe1.5Mo0.5O6-δ anode and cobaltite cathode was about 0.18 W/cm2 at 800 °С under air/wet hydrogen conditions. The obtained results elucidate that Sr1.85Ca0.15Fe1.5Mo0.5O6-δ is a promising anode for solid oxide fuel cells.

The electrochemical behavior of Ni – Zr0.9Sc0.1O1.95 anode in contact with the YSZ electrolyte was studied in H2 + H2O + Ar mixtures at 900°С by means of impedance spectroscopy. The analysis of the impedance spectra performed by the distribution of relaxation times and non-linear least square methods demonstrated that the electrode process is limited by several rate-determining steps. High-frequency resistance at equilibrium potential does not depend on the gas mixture composition, whereas low-frequency resistance has the 0.5 reaction order and depends on the water partial pressure. The distribution of relaxation times dependencies under polarization demonstrated that the frequency range of the high-frequency step does not depend on the electrode potential and the contribution of the low-frequency step decreases as the cathode polarization increases. An anomalous behavior of the hydrogen oxidation and water reduction rates in the point of zero charge was observed. The abnormal behavior of polarization curves is for the first time explained by the influence of the difference of potential in the dense part of the double electric layer.

Oxygen electroreduction kinetics and electrochemical performance of the Pr6O11-impregnated Sr2Fe1.5Mo0.5O6-δ – Ce0.8Sm0.2O1.9 (SFM–SDC) cathode have been first studied. By means of distribution of relaxation times and non-linear least squares analysis of impedance spectra were shown that the addition of Pr6O11 into the cathode leads to the increase in the rate of the low-frequency step. It is suggested that the observed phenomenon is associated with the increase in the rate of surface oxygen exchange. It is shown that the introduction of praseodymium oxide into the cathode results in a decrease in the area specific polarization resistances of the cathode at equilibrium potentials from 0.23 to 0.06 Ω cm2 at 800 °C in air. The maximum power density of symmetrical solid oxide fuel cell (SOFC) with impregnated SFM–SDC electrodes and supporting 760 μm La0.85Sr0.15Ga0.85Mg0.15O3-δ electrolyte without buffer/barrier and collector layers under air/wet hydrogen (dry CH4) condition was about 0.5 (0.26) W cm−2 at 800 °С. The overvoltage of the cathode was higher than that of the anode under air/wet hydrogen and vice versa when methane was supplied to the anode. The obtained results elucidate that the impregnated SFM–SDC is a promising cathode for SOFC application.

The dual-phase composites are obtained by mixing perovskite-like cobaltite Pr0.9Y0.1BaCo2O6–δ and samarium-doped ceria Ce0.8Sm0.2O1.9 in weight proportions 90/10, 70/30, and 50/50. The absence of chemical interactions between the components at heating up to 1100 °С is confirmed by X-ray powder diffraction measurements. The partial dilution of the cobaltite is accompanied with decreasing thermal expansion of the composite. The impedance measurements reveal linear changes of the area-specific resistance with inverse temperature that correspond to the activation energy values of about 0.9 and 0.8 eV for 70/30 and 50/50 composites, respectively. The overvoltage for oxygen electro-reduction with current density 1 A/cm2 does not exceed 80 mV at 700 °С. The high electrical conductivity, moderate thermal expansion, and appreciable electrochemical activity single out the mixture of Pr0.9Y0.1BaCo2O6–δ (70 wt%) and Ce0.8Sm0.2O1.9 (30 wt%) as a promising composite cathode material for intermediate temperature solid oxide fuel cell applications (IT–SOFC).

The results of detailed studies of the kinetics of hydrogen oxidation on a Sr2Fe1.5Mo0.5O6-δ (SFM) electrode in contact with La0.85Sr0.15Ga0.85Mg0.15O3 electrolyte are presented for the first time. For a detailed understanding of the mechanism of the electrode reaction, studies were carried out in H2+H2O+Ar gas mixtures at a constant partial pressure of hydrogen, water and at a constant hydrogen/water ratio. Also, to understand the pathways of the electrode reaction, the results of the comparison of the mechanism of hydrogen oxidation on the SFM electrode with different organisation of current probes to the studied electrode are presented, as well as a comparison of the kinetics of electrode reaction on the SFM electrode and the SFM electrode decorated with nickel particles. It is shown that the rate of hydrogen oxidation on the SFM electrode is determined by the low rate of oxygen heterogeneous exchange, as well as by the slow dissociation of hydrogen on the SFM surface. At 800 °C, the rates of these processes are comparable; with temperature decrease the rate of hydrogen oxidation is mainly limited by hydrogen dissociation. Decorating the SFM electrode with nickel particles significantly increases the rate of hydrogen dissociation and increases the activity of the electrode more than two-fold, up to a polarisation resistance of about 0.04 Ω cm2 at 800 °C.

The double perovskites Sr2-xCaxMgMoO6–δ with x = 0, 0.25, and 0.5 were obtained by combustion of organometallic precursors and were shown to have a tetragonal structure with the crystalline lattice parameters a = b = 5.569, c = 7.923, 7.899, 7.885 A, respectively. Calcium doping was found to improve the conductivity and decrease the activation energy values. At x = 0.5, the temperature dependence of the total conductivity has a metallic-like behavior under reducing conditions. The electrical conductivity for Sr1.5Ca0.5MgMoO6−δ was about 25 and 30 S/cm at 800 and 500 °C, respectively. Besides, the enhancement of the calcium content lead to the reduction in the area specific resistance of the Sr2−x Ca x MgMoO6−δ anode. In conjunction with the electron paramagnetic resonance study, the experimental data on the electrical conductivity demonstrated that calcium doping is accompanied by an enhanced electron component under reducing conditions. The magnetic susceptibility revealed the coexistence of itinerant electrons and paramagnetic centers in calcium-rich molybdate Sr1.5Ca0.5MgMoO6−δ.

A promising Sr 2 Fe 1.5 Mo 0.5 O 6–δ + Ce 0.8 Sm 0.2 O 1.9–δ IT-SOFC anode material in contact with the lanthanum gallate based electrolyte in H 2 +H 2 O + Ar gas mixtures was studied. The anode has the polarization resistance nearly 0.3 Ω cm 2 at 700 °C that is considered as the acceptable value for the practical application in IT-SOFC. The distribution of relaxation time curves showed that the electrode reaction was limited by two rate-determining steps within the frequencies range 10–0.01 Hz. The opposite behavior of the polarization resistance vs. water and hydrogen partial pressure was founded. This behavior was explained by the low electrochemical activity of the anode to the water dehydrogenation. The additive of nickel into the anode resulted in the reduction of the polarization resistance and increase the rate of the water dehydrogenation. A long-term test carried out during 500 h at 700°С in a humid hydrogen demonstrated the exponential behavior of the polarization resistance. The weight decrease of about 4.5% for Sr 2 Fe 1.5 Mo 0.5 O 6–δ powder in Ar + 5%H 2 at 700 °C within the 300 h the exposure was observed.

Determination of the polarization resistance of the anode and cathode in a solid oxide fuel cell (SOFC) with a thin-film electrolyte is a complicated task due to the difficulty of the manufacturing the reference electrode. In this case, only the total polarization resistance of the electrodes is available for analysis. When the relaxation times of the electrode processes are close, the separation of the electrode reactions, in most cases, is impossible by the conventional nonlinear least-squares method due to а several reaction processes in anode and cathode. In this paper, using a fuel cell with a supporting cathode as an example it is shown that the distribution of relaxation times (DRT) technique for the analysis of the impedance spectra of the fuel cell can be used to determine the polarization resistance of the cathode and anode. This becomes possible at comparing and analyzing the DRT functions obtained by varying the composition of gas mixtures at the cathode and anode of the fuel cell, using an electrocatalyst to accelerate the cathode reaction as well as comparing the DRT functions of the fuel cell with the supporting cathode to the DRT functions of cells with supporting electrolyte and symmetric electrodes.

PrBaCo2O6–δ – PrBaCoTaO6 (PBC/PBCT) composites were studied with respect to their chemical and thermomechanical compatibility with ceria electrolyte (SDC), electrical conductivity, electrochemical performance and reversibility at variations of CO2 content in the oxidizing atmosphere. The electrochemical impedance spectra of the composite electrodes were analyzed by the distribution of relaxation times technique. The best properties combination was observed for the 75/25 composite containing 25 wt% of PBCT where polarization resistance did not exceed 0.025 Ohm⋅cm2 in the air at 1073 K. Though subjected to some degradation in CO2 rich atmospheres, the 75/25 composite was shown to completely restore its electrochemical activity after annealing in the air at 973 K.

Polarization resistance of the Ni-Zr0.84Y0.16O1.92 anode impregnated with cerium oxide in H2 + H2O + CO + CO2 + Ar gas mixtures of different composition was studied at temperatures of 700–950 °C by means of EIS and subsequent DRT and NLLS analysis of EIS spectra. The electrochemical activity of the anode in the H2 + H2O + CO + CO2 + Ar was found to be defined basically by the hydrogen-containing components (H2 and H2O). The EIS spectra analysis revealed that the anode polarization resistance is determined by three processes, registered at high-, middle- and low-frequencies, respectively. The high-frequency resistance had an activation energy of about −0.93 eV and it did not depend on the gas mixture composition. Both the middle-frequency resistance (activation energy of about −1.04 eV) and the low-frequency resistance (0.31 eV) depended on the gas atmosphere composition. Suggestions about the nature of the rate-determined steps were proposed.

The development of solid oxide electrochemical devices with symmetrical electrodes, is a promising direction in the field of resource saving and green energy. A feature of symmetrical electrodes is their stability to reduction–oxidation (redox) cycling, which can be useful in the case of carbon or sulphur deposition on a fuel electrode. The results of studies of a double-layer symmetrical electrode, where the functional layer consist of 50 wt.% Sr2Fe1.5Mo0.5O6-δ + 50 wt.% Ce0.8Sm0.2O1.9-δ and the collector is Sr2Fe1.5Mo0.5O6-δ are presented for the first time. It was shown that the introduction of Ce0.8Sm0.2O1.9-δ into the electrode functional layer, as well as the use of the current layer, does not lead to a change in the mechanism of oxygen reduction and hydrogen oxidation reactions. An analysis of the fuel cell impedance spectra by the distribution of relaxation times (DRT) method and comparison of the DRT functions with the results of the analysis of symmetric cells made it possible to conclude that during redox cycling the resistance of the oxygen exchange stage increases. It has been suggested that the cause of this is the formation of iron under reducing conditions, followed by its oxidation to oxide in an oxidizing atmosphere.

In a recent paper of Zhang et al. (2016) a variation of the Tikhonov regularization method for calculating the distribution of relaxation times (DRT) in relation to analysis of the electrochemical impedance spectroscopy data has been proposed. The novelty of the proposed method is that it is free from the regularization parameter, the adjusting of which usually causes the main issue when applying the Tikhonov regularization method. In the present paper, we investigate by means of numerical experiments the frequency resolution of the method by Zhang et al. and its robustness to resist noise embedded in the impedance spectra. In doing so, we also consider how changing certain matrices used in the method affects the results. In addition, the DRT function calculation for a real electrochemical object by means of this new approach is shown.

The paper illustrates the electrode kinetics studies results on Ni - Ce0.8Sm0.2O2-δ (Ni-SDC) anodes in a wet hydrogen atmosphere in the temperature range of 600–850°С by the electrochemical impedance spectroscopy before and after ceria addition to the anode. The addition of ceria to the anode by the solution impregnation technique is shown to result in the anode electrochemical activity growth by 10–24 times. An approach to the analysis of the impedance spectra based on the combination of the distribution of relaxation times method (DRT) and non-linear least squares method was verified. Based on data obtained from DRT functions, we found that the Ni-SDC anode reaction rate was limited by three processes, whereas the ceria-impregnated Ni-SDC anode electrode reaction rate was limited by two processes. The correlation of the DRT functions of both anodes demonstrated unambiguously that ceria addition to the Ni-SDC anode caused a significant decrease in the middle-frequency step of the electrode reaction, while the resistances of the other electrode reaction steps decreased less. Highly disperse ceria addition to the Ni-SDC anode was assumed to increase significantly the rate of reactions on the anode surface between the adsorbed species.

The current work focuses on the investigation of the structural and electrical properties of La2NiO4±δ, La1.7Sr0.3NiO4±δ and La1.7Ca0.3NiO4±δ layered perovskites and electrochemical performance of the cathodes on their base in contact with the Ce0.8Sm0.2O1.9 electrolyte with special attention given to the influence of the introduction of the sintering additives (CuO, Bi0.75Y0.25O1.5) into cathode layers on their in-plane and polarization resistances. Studies by the dc four-probe technique and impedance spectroscopy were performed on the samples with/without the collector layer on a base of LaNi0.6Fe0.4O3 with different sintering temperatures of the electrode layers. © 2014 The Authors. Published by Elsevier Ltd. This work has been partly done using facilities of the shared access center "Composition of compounds" IHTE, UB RAS, and supported by the Russian Foundation for Basic Research, the Government of Sverdlovsk region and Presidium of Ural Branch of RAS (projects №№13-03-96098, 14-03-00414, 14-08-31030, 12-S-3-1016).