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In this study the methods to increase electrochemical activity and stability of air electrodes based on LaNi0·6Fe0·4O3-δ (LNF) and Сe0.8Sm0.2O1.9 (SDC) with different collector layers has been developed. The influence of the electrode layers’ composition and sintering conditions on the electrode performance in contact with the SDC electrolyte is considered. The polarization resistance of the electrodes with the LNF collector containing a combined Bi–Cu additive is in a range of 0.60–0.67 Ωcm2 at 600 °C. Appearance of low-melting liquid phases during the collector formation gives rise to both additional sintering and electrochemical activation of the functional layers. The optimized electrodes exhibit a low degradation (13–15%) preserving a polarization resistance level of 0.15–0.21 Ω cm2 at 700 °C after long-term testing for 600 h. To elucidate reasons of the electrode ageing, changes in the electrode performance and microstructure are analyzed using DRT technique and autocorrelation function analysis of SEM images.

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.

In this study the bilayer composite electrodes based on LaNi0.6Fe0.4O3-δ (LNF) electronic conductor and Bi2O3-based electrolytes doped with Er (Bi1.6Er0.4O3, EDB) and Y (Bi1.5Y0.5O3, YDB) have been developed and their performance has been investigated in the dependence on the electrolyte content and sintering conditions. The polarization resistance of the optimized electrodes with electrolyte content of 50 wt % in the functional layer and with the LNF-EDB-CuO collector is in a range of 0.65–1.09 Ω cm2 at 600 °C and 0.10–0.12 Ω cm2 at 700 °C. The polarization characteristics of the Bi-based electrodes are compared with those for the composite electrodes based on LNF and Ce0.8Sm0.2O1.9 (SDC). The developed electrodes have been tested in a SOFC mode in the anode-supported cells with a thin film electrolyte of YSZ/YDC (Y-doped zirconia/ceria). The single cells with such cathodes are shown to have performance characteristics that are several times higher than that for the cell with a standard platinum cathode. This is due to the optimized content and dispersity of the components; high conductivity of ionic and electronic constituents of the composite electrodes; greatly extended triple phase boundary (TPB) of the electrochemical reaction and advanced electrode design with collector providing uniform current distribution.

A-site deficient Ruddlesden–Popper phases now attract a lot of attention as promising materials for intermediate temperature solid oxide fuel cells' cathodes and oxygen separation membranes. However, controversial information on the A-site deficiency effect on structural, transport and electrochemical properties of these materials apparently requires further studies of such systems. In this work, structural and transport features of (Nd2-xCax)0.975NiO4+δ system were studied. (Nd2-xCax)0.975NiO4+δ (x = 0–0.4) samples were synthesized via a solution-assisted solid state reaction method and characterized by X-ray diffraction. Oxygen transport was studied by the temperature programmed isotope exchange of oxygen with C18O2 in the flow reactor. A high oxygen mobility of materials (D* ~ 10−7 cm2/s at 700 °C) is provided by the cooperative mechanism of oxygen migration involving both regular and highly-mobile interstitial oxygen. As typical for Ruddlesden–Popper phases, doping with Ca leads to increase of total conductivity due to holes' formation and decrease of oxygen mobility due to hampering cooperative mechanism, however, this effect is less pronounced compared to A-site stoichiometric Nd nickelates which can be attributed to oxygen vacancies formation and their participation in cooperative migration as well as to other features of A-site deficiency. High mixed ionic-electronic conductivity of materials makes them promising in using as solid oxide fuel cells’ cathodes and oxygen separation membranes.

This work presents investigation of novel composite electrodes comprising misfit layered oxide Ca3Co4O9+δ and protonic conductor BaCe0.5Zr0.3Y0.1Yb0.1O3-δ. Materials for the study were synthesized by the pyrolysis of organic salt compositions, and their structure, conductivity, chemical, and thermal compatibility were comprehensively studied. Composite two-layered electrodes with functional layers of (1-x)Ca3Co4O9+δ-xBaCe0.5Zr0.3Y0.1Yb0.1O3-δ (x = 0.0; 0.3; 0.4; 0.5), and oxide collectors were successfully fabricated by layer-by-layer painting on BaCe0.5Zr0.3Y0.1Yb0.1O3-δ substrates. Polarization characteristics of the electrodes were studied by the method of impedance spectroscopy. It was shown that the developed two-layered electrode with a functional layer of 0.7Ca3Co4O9+δ-0.3BaCe0.5Zr0.3Y0.1Yb0.1O3-δ and an oxide collector layer of LaNi0.6Fe0.4O3-δ + 3 wt.% CuO had superior performance in comparison with Ca3Co4O9+δ electrodes with collectors based on noble metals and can be recommended as a prospective low-cost cathode composition for protonic ceramic electrochemical cells.

In order to develop economically competitive solid oxide fuel cell (SOFC) systems it is necessary to design new functional materials with the purpose of enhancing their performance, extending operational lifetime and reducing the cost. The present work focuses on the study of Pr2-xCaxNiO4+δ cathode materials prepared by a simple and cheap conventional solid state reaction method. The structure, oxygen nonstoichiometry, electrical properties and chemical compatibility of the materials with a number of well-known oxygen ion and proton conducting electrolytes were systematically investigated. Chemical composition (Ca content) and technological factors (powders pre-history, electrodes' sintering temperature), as well as external parameters (temperature, air humidity) were correlated with the electrochemical performance of the electrodes to determine the optimal compositions and conditions. Based on this study and testing results of the single anode-supported cell with BaCe0.89Gd0.1Cu0.01O3-δ electrolyte, the Pr1.7Ca0.3NiO4+δ-based electrode compositions are proposed for preferred usage in SOFCs in place of Pr2NiO4+δ electrode.

Mixed ionic-electronic conductors are of great interest for development of oxygen separation membranes and air electrodes for intermediate temperature solid oxide fuel and electrolysis cells. This work presents the results of studying the structure, transport properties and oxygen mobility in Nd2−xCaxNiO4 oxides (x = 0–0.5), synthesized via a solution-assisted solid state reaction method, in correlation with the over-stoichiometric oxygen content. High oxygen mobility was demonstrated in the low-doped materials (DO ~ 10−10–10−8 cm2/s at 700 °C) which corresponded to the ionic (oxygen) conductivity values ~ 10−4–10−2 S/cm, comparable to those for widely used solid electrolytes and state-of-the-art cathode materials. The polarization resistance of the Nd2−xCaxNiO4+δ electrodes in contact with Ce0.8Sm0.2O1.9 electrolyte was shown to have non-linear dependency on Ca content, which may be caused by the influence of two opposite tendencies for decreasing over-stoichiometric oxygen and oxygen diffusivity and increasing the total conductivity with Ca doping. High electrical and electrochemical characteristics along with oxygen mobility mean the materials developed display high potential for electrochemical and catalytic applications.

Possibility of forming a gas-tight thin-film solid electrolyte by the electrophoretic deposition method on a modified multilayer cathode was examined. The main goal of the study was to find such technological parameters at which the resulting structure of the cathode substrate would make it possible to preserve its porous structure and functional properties after all the stages of deposition and sintering of a defect-free thin-film electrolyte. The electrode materials LaNi0.6Fe0.4O3−δ (LNFO) and La2NiO4+δ (LNO), used to form the electrode-substrate, were produced by the modified Pechini method and the ceramic method. The influence exerted by the specific surface area of the starting LNFO powders, introduction of a pore-forming agent (graphite), and sintering temperature of the cathode substrate on its porosity, gas tightness, and electrical conductivity was examined. The method of cyclic electrophoretic deposition with intermittent calcination stages on a multilayer porous cathode substrate constituted by a 1-mm-thick collector LNFO layer with a deposited functional LNO layer (3–5 µm) was used to form a defect-free thin-film solid electrolyte Ce0.8(Sm0.8Sr0.2)0.2O2−δ (CSSO) (thickness 5 µm, gas-tightness coefficient 0 µm2). A test sample of the cathode substrate preserved its porous structure and gas tightness after calcination cycles. The results obtained can be used to develop a technology for formation of a thin-film electrolyte on porous multilayer cathode substrate, with their porous structure preserved.

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.

Progress in hydrogen energy and promising directions for its modern development are closely related to design of fuel cells, including solid oxide fuel cells, and solid state membranes for hydrogen, oxygen and synthesis gas production. A necessary condition for fabrication of economically competitive devices in this area is the use of cheap electrode materials combining high electrochemical activity and long-term stability. Ln2NiO4+δ oxides with the Ruddlesden–Popper layered structure possessing a high mixed ion-electron conductivity and moderate values of the coefficients of thermal expansion are promising materials for development of oxygen-conducting membranes and cathodes of intermediate-temperature solid oxide fuel cells. In this paper structural characteristics, electrical conductivity, oxygen mobility and electrochemical properties of Ln2-xCaxNiO4+δ (Ln = La, Pr, Nd; x = 0; 0.3) samples were studied to determine factors, which have the most significant effect on the electrochemical activity of electrodes and their stability. It was found that doping with calcium lead to stabilization of the structure and increased the electrical conductivity of materials. However, addition of calcium decreased the electrochemical activity of electrodes in varying degrees depending on the nature of lanthanide. There is no direct interrelation of such a decrease of activity with either the electrical properties or the interstitial oxygen content. We have revealed correlation of the polarization resistance of electrodes with characteristics of oxygen transfer in the electrode material (self-diffusion coefficient, surface exchange constant). Using the C18O2 SSITKA method, the total oxygen mobility in the doped materials was shown to fall with doping due to a decrease in the content of highly mobile interstitial oxygen and hampering of the cooperative oxygen transport mechanism. In the case of La1.7Ca0.3NiO4+δ, this leads to the appearance of a slow diffusion channel and a substantial decrease in the total diffusion coefficient value which results in a sharp increase in the polarization resistance of the electrodes. This phenomenon is not observed in materials with praseodymium and neodymium. The electrodes based on Pr1.7Ca0.3NiO4+δ and Nd1.7Ca0.3NiO4+δ, developed in this work, have an acceptable level of the electrochemical activity along with a high electrical conductivity and increased stability in comparison with undoped compositions and can be recommended for use as cathodes for intermediate temperature fuel cells.

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%.

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.

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.

A row of Co-free and Co-containing electrode materials were synthesized and characterized and the electrode polarization resistance in contact with a proton-conducting BaCe 0.89 Gd 0.1 Cu 0.01 O 3 − δ electrolyte was studied by means of electrochemical impedance spectroscopy in a temperature range of 600–800 °C, an oxygen partial pressure range of 10 − 3 –0.21 atm and a water partial pressure range of 4 × 10 − 4 –0.1 atm. The electrodes with Pr 1.9 Ca 0.1 NiO 4 + δ –BaCe 0.89 Gd 0.1 Cu 0.01 O 3 − δ and Y 0.8 Ca 0.2 BaCo 4 O 7 + δ functional layers was found to exhibit the lowest polarization resistance equal to 0.49 and 0.46 Ω cm 2 at 700 °C, respectively, with less pronounced deterioration of their characteristics in wet conditions than for all the other investigated electrodes. The shunt effect of the electronic conductivity in the electrolyte on the electrode response was shown to be an important factor for consideration when comparing behavior of the electrodes in contact with different proton-conducting electrolytes.

A new method of obtaining gastight ceramic based on CaZr0.95Sc0.05O3 – δ is presented. The microstructure and electric properties of the obtained samples, same as the behavior of composite electrodes in contact with this electrolyte, are studied for application of the obtained results in the technology of formation of electrochemical devices. The design of bilayer electrodes is suggested, in which the materials tested as the functional layer were layered lanthanum nickelate La2NiO4 + δ and substituted lanthanum nickelate La1.7Ca(Sr,Ba)0.3NiO4 + δ in combination with the electrolyte components of Ce0.8Sm0.2O2 – δ and BaCe0.89Gd0.1Cu0.01O3 – δ. The collector layer used was lanthanum nickelate–ferrite LaNi0.6Fe0.4O3 – δ and manganite La0.6Sr0.4MnO3 – δ that are characterized by high electron conductivity, low layer resistance and are close by their values of coefficient of linear thermal expansion to the materials of functional layers. Electrochemical activity of the obtained electrodes are compared with the characteristics of composite electrodes based on lanthanum ferrite–cobaltite La0.6Sr0.4Fe0.8Co0.2O3 – δ and deficient lanthanum manganite La0.75Sr0.2MnO3 – δ.

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 present work focuses on the structural, thermo-mechanical and electrical properties of La 2–x Ca x NiO 4 + δ with different calcium content (x = 0–0.4) and the electrochemical performance of the electrodes on their base in contact with a Ce 0.8 Sm 0.2 O 1.9 electrolyte. It was shown that the introduction of even a small amount of calcium (x = 0.1) stabilized the crystal structure in the tetragonal syngony across the whole temperature range 25–800 °C. Dilatometric study of the compact samples in the entire concentration range of Ca showed no phase transitions up to 800 °C. The total electrical conductivity, measured by a dc four-probe technique, increased with an increase in calcium content. However, electrochemical activity of the La 2–x Ca x NiO 4 + δ – based electrodes, contrarily, drastically decreased. The polarization resistance R η , derived from the impedance spectroscopy data, increased from 0.18 Ω·cm 2 (x = 0) to 1.24 Ω·cm 2 (x = 0.3) at 800 °C. The coefficients of oxygen surface exchange ( k ⁎ ) and oxygen tracer diffusion ( D ⁎ ) calculated from the oxygen isotope exchange data for La 2 NiO 4 + δ were considerably higher than those for La 1.7 Ca 0.3 NiO 4 + δ (4.54 ∙ 10 − 7 cm ∙ s − 1 and 3.02·10 − 8 cm 2 ∙ s − 1 (x = 0); 2.91 ∙ 10 − 8 cm ∙ s − 1 and 5.82·10 − 10 cm 2 ∙ s − 1 (x = 0.3) at 800 °C, respectively). It was found that for La 1.7 Ca 0.3 NiO 4 + δ the rate-determining stage of oxygen surface exchange is incorporation of oxygen, while for La 2 NiO 4 + δ it is oxygen dissociative adsorption.

La2NiO4 + δ (LNO) and LаNi0.6Fe0.4O3 (LNF) powders were synthesized using the two-step ceramic, glycine–nitrate combustion and modified Pechini method. The microstructure of the bilayer electrodes on their base was inspected using the original algorithm for scanning electron microscope (SEM) image processing. The electrochemical performance of the electrodes in contact with the Ce0.8Sm0.2O1.9 (SDC) electrolyte was studied with respect to the synthesis method, powder's dispersion and the sintering temperature of the electrode layers and their microstructural features. Irrespective of the powder's processing the increased sintering temperature of the functional layers (1300 °C) was found to give better results in terms of adhesion and reduction in polarization resistance. Applying the LNF collector onto the LNO functional layer resulted in a sharp reduction in serial resistances due to a more uniform current distribution along the surface of the electrode. The lowest polarization resistance (ASR = 0.18 Ω cm2 at 700 °C), which is among the best of those presented in the literature, was obtained for the bilayer electrode made of LNO and LNF powders produced by the glycine–nitrate combustion method.

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.

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.

The manuscript focuses on the electrochemical performance of the La1.7Ca0.3NiO4+δ (LCNO) and La1.7Sr0.3NiO4+δ (LSNO) based composite cathodes with Ce0.8Sm0.2O1.9 (SDC) or BaCe0.89Gd0.1Cu0.01O3 (BCGCu) ceramic components in contact with BCGCu or BaCe0.5Zr0.3Y0.2O3+1wt.%CuO (BCZYCu) proton conducting electrolytes in comparison with La0.6Sr0.4Fe0.8Co0.2O3 (LSFC) and La0.75Sr0.2MnO3 (LSM) composite cathodes.

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.

Powders of composite materials based on lanthanum nickelate-ferrite with different contents of lanthanides were synthesized using Ln-Ni-Fe-O (LnNF) mischmetal. The phase composition of the powders was determined by XRD. The crystal structure of the main phase in LnNF obtained using a mischmetal with a high lanthanum content is perovskite-like, while the structure with a high cerium content is fluorite-like. The temperature coefficients of linear expansion of the synthesized materials were determined by dilatometry. The temperature dependences of electric conductivity of the compact samples and porous electrodes from these materials were studied by the DC four-probe method. The electric conductivity of compact samples from materials prepared using the mischmetal with the perovskite-like structure of the main phase exceeds the electric conductivity of the materials based on the mischmetal with a high cerium content by approximately three orders of magnitude. The temperature dependences of polarization conductivity of electrodes in cells with the Ce0.8Sm0.2O1.9 (SDC) electrolyte at 600–900°C in air were studied by impedance spectroscopy. The LnNF electrodes with a perovskite-like crystal structure of the main phase showed high electrochemical activity.

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).

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 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.

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 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 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.

The sintering features, electroconductivity, and electrochemical characteristics of bilayer electrodes with functional composite layers based on La(Sr)MnO3 (LSM) and La(Sr)Fe(Co)O3 with LSM collector layer and Bi(Y)O1.5 (YDB) electrolyte additive in contact with Ce (Sm)O2(SDC), La(Sr)Ga(Mg)O3, and Zr(Sc)O2 electrolytes were studied. YDB additive to the electrode collector layer was shown to produce a positive effect to the properties of the studied electrode systems. The maximum electrochemical activity and electroconductivity was observed for the electrodes with 5 wt % of YDB electrolyte additive in the collector layer. Thus, electroconductivity of electrodes is almost doubled and 100 mV cathode overvoltage current density is increased by 30% at the temperatures of 800 to 900°C and up to 10-fold at 650 to 700°C. The collector layer sintering temperature of bilayer electrodes can be reduced from 1150 to 1000°C without loss of electrochemical activity. The service life tests (about 1200 h) of composite electrodes with LSM2-SDC functional layer and 90% LSM2 + 10% YDB collector layer in contact with SDC electrolyte showed the time dependences of polarization resistance tending to saturation and described with damped exponent.

A novel copper-based anode for low-temperature solid oxide fuel cells was prepared through the conventional ceramic technology and using CuO and SDC (Ce 0.8 Sm 0.2 O 1.9 ) powders with controlled particle size. The new Cu–SDC anode also contained highly dispersed CeO 2 and Ni particles to increase its surface area and fuel cell performance. The specific surface area of the Cu–SDC bare anode, CeO 2 and Ni-dispersed phases were estimated to be 1.53, 39.4 and 86.4 m 2 g −1 , respectively. Solid oxide fuel cells having the new anode were tested for both humid hydrogen and methane. Power densities of ca. 250 mW cm −2 were achieved in H 2 at 600 °C and in CH 4 700 °C, even if the SDC–electrolyte supporting membrane was 250-μm thick. Short term stability tests (maximum 64 h) showed an initial impairment, but not dramatic, of the new anode performance and the formation of carbon deposits. The addition of MoO x to the new anode did not prevent the formation of carbon deposits.

The cathodic overvoltage of composite cathodes 50 wt % La0.8Sr0.2MnO3 (LSM) + 50 wt % La10Ge6O27 (LGO) (further on, LSM-LGO), LSM-SSZ (Zr0.835Sc0.165O2−δ), Ag-Pd-LGO, and Ag-Pd-SSZ in contact with the LGO electrolyte is measured. The temperature dependences of the polarization conductivity and the working-current densities of the same composite cathodes are investigated. The study is performed at 700–900°C. A comparison with the SSZ electrolyte is conducted. The chemical interaction in the LSM-LGO composition is studied. It is demonstrated that the interaction of lanthanum-strontium manganite with lanthanum germanate occurs with the dissolution of the initial phases in one another and with the formation of fresh phases at elevated temperatures. Coefficients of linear thermal expansion of the LGO and SSZ electrolytes and the LSM, LSM-LGO, and LSM-SSZ electrode materials are compared at 40–900°C. Most of the studied electrodes in contact with the LGO electrolyte demonstrate thermomechanical stability and high electrochemical activity.

The electric and electrochemical characteristics of cathodes made of La0.6Sr0.4Fe0.8Co0.2O3−δ (LSFC) and intended for fuel cells with electrolytes based on ceric oxide are studied. Adding cupric oxide into the LSFC cathode is shown to exert a favorable effect of the properties of the LSFC-CuO/SDC electrode system, where SDC stands for the CeO2-Sm2O3 electrolyte. The effect produced by cupric oxide when added in the form of nanopowder is perceptibly greater than in the case of micropowdered CuO. Adding a mere 0.5 wt % of nanopowdered CuO reduces the LSFC cathode resistance nearly tenfold. The cathode’s adhesion to the electrolyte substantially improves as well, which makes it possible to lower the cathode’s firing temperature by 100°C. The maximum of electrochemical activity is intrinsic to cathodes containing 2 wt % CuO, with the caking temperature of 1000°C. According to a 2011-h life test of the LSFC-SDC composite cathodes containing nanopowdered CuO, temporal stability of their electrochemical characteristics improves with the SDC content. The time dependences of the polarization resistance of cathodes containing 40–50 wt % SDC look like decaying exponential curves. The steady-state polarization resistance, calculated on the basis of this, is equal to 0.1–0.2 ohm cm2. At an overvoltage of less than 100 mV, the cathodes may provide for a current density of 0.5–1.0 A cm−2.

Oxygen-ion conduction in apatite-like compounds based on silicates and germanates of lanthanum La x A6O12 + 1.5x (A = Si, Ge; x = 9.11–10.22) is studied. The compounds are shown to be purely ionic conductors at 600–900°C and partial oxygen pressures 10−16 to 105 Pa. The electroconductivity of the best conducting specimens of La x A6O12 + 1.5x (A = Si, Ge; x = 9.77–10) exceeds that of electrolyte YSZ at moderate temperatures. The electroconductivity of lanthanum germanate is substantially greater than that of lanthanum silicate, specifically, 7.85 × 10−2 and 2.35 × 10−2 S cm−1, respectively, at 800°C. An inflection is discovered at ∼750°C in the temperature dependences of electroconductivity of La x Ge6O12 + 1.5x (x = 9.77–10.22). A dilatometric examination points to a second-kind phase transition that may be due to the oxygen sublattice disordering. The behavior of apatite-like electrolytes La x A6O12 + 1.5x (A = Si, Ge) during long exploitation periods in the interval of working temperatures of electrochemical devices is studied for the first time ever. The electrolytes’ aging at 800°C in air for 1000 h was investigated by the electroconductivity method. The electroconductivity of lanthanum germanates decayed with time by 5% and that of lanthanum silicates, by 9.5%. The steady-state values of electroconductivity of all compounds studied is reached after 600–700 h. The compounds studied form a class of materials that hold some promise as solid electrolytes for medium-temperature fuel cells and other electrochemical devices.

A complex investigation of solid electrolytes Ce1−x Sm x O2−δ (x = 0.10−0.30) is performed in a broad temperature interval by methods of x-ray diffraction analysis, electroconduction, dilatometry, differential scanning calorimetry, and thermogravimetry. Ceramic technology is used for the synthesis. The density of the obtained specimens is no lower than 95%. The dilatometry measurements reveal the presence of singularities in the curves obtained at temperatures in the region of 180–230°C, which may be ascribed to a phase transition of the type order-disorder of hypothetical phase Ce7SmO15.5. The electroconduction and dilatometry methods also point to the presence of singularities in the temperature dependences in the temperature interval extending from 800 to 870°C.

The ion-oxygen conductivity of apatite-like compounds based on lanthanum silicates and germanates La10A6O27 (A = Si, Ge), La10−x CaxSi6O27−δ (x = 0.25, 0.5, 1.0), La9.75Ca0.25Ge6O27−δ and La9.33+δSi6−x AlxO26(x=0.4, 0.8, 1.5) is studied in the interval of partial oxygen pressures pO2 extending from 10−16 to 105 Pa, at temperatures of 500–1000°C. The electroconductivity of undoped compounds La10A6O27 (A = Si, Ge) exceeds that of yttria-stabilized zirconia. The electroconductivity of lanthanum germanate (1.7 × 10−2 and 8.5 × 10−2S cm−1 at 700 and 900°C, respectively) is substantially higher than that of lanthanum silicate (9.8 × 10−3 and 3.5 × 10−2 S cm−1 at 700 and 900°C). Doping lanthanum germanate with calcium raises its electroconductivity (2.7 × 10−2 and 1.3 × 10−1 S cm−1 for La9.75Ca0.25Ge6O27−δ at 700 and 900°C). Conversely, doping lanthanum silicate with ions of calcium or aluminum reduces the conductivity. In the pO2 interval studied, the above compounds are ionic conductors and represent a class of solid electrolytes of promise for various electrochemical devices.

The electrochemical cells with electrodes based on La0.8Sr0.2MnO3 (LSM) and supporting solid electrolytes La0.88Sr0.12Ga0.82Mg0.18O2.85 (LSGM) and Ce0.80Sm0.20O1.90 (SDC) were studied comparatively. Characteristics of LSM electrodes and composite electrodes comprising a mixture of LSM and electrolytes of different origins [LSGM, SDC, and Zr0.82Sc0.18O1.91 (SSZ) in the mass ratio of 1:1] were analyzed. It was shown that: 1) the electrode polarization conductivity and the ohmic resistance of the cells with the LSM–LSGM composite electrodes on the LSGM and SDC electrolytes had very similar values, while they were largely different from all the other electrodes, 2) the electrochemical activity of the electrodes on the SDC electrolyte was much higher than on the LSGM electrolyte, and 3) the ohmic resistance of the cells with the SDC electrolyte corresponded to the electrolyte resistance, whereas, the ohmic resistance of the cells with the LSGM electrolyte was much larger than the electrolyte resistance. The obtained results are due to the interaction between the LSM and LSM-containing electrodes with the LSGM electrolyte during sintering, leading to the formation of a product with a very low conductivity.

For two types of electrochemical cells with oxygen-conducting solid electrolytes based on lanthanum gallate (LSGM) and cerium oxide (SDC) studied are the temperature dependences of the polarization conductivity of air electrodes prepared from lanthanum-strontium manganite (LSM) and composites LSM-LSGM, LSM-SDC, and LSM-SSZ (SSZ is zirconium dioxide-based electrolyte). Effect of praseodymium oxide, added into these electrodes as a modifier, on their electrochemical properties is examined. Electrochemical systems with an LSM/LSGM interface exhibit low electrochemical activity toward the oxygen reaction, because during the formation of electrodes, LSM interacts with LSGM to form a poorly conducting product.

The phase composition and electroconduction in air of solid electrolytes (Ce0.8Sm0.2)1 − xCuxO2 − δ (CSCu), where x = 0, 2, 5, 10, and 20 mol % and which are synthesized using the ceramic technology, are studied. Adding an additive of CuO lowers the CSCu sintering temperature by 100– 200°C and leads to the formation of single-phase solid solutions of a fluorite type up to x = 10 mol %. The electroconductivity of the CSCu electrolytes remains practically invariant upon adding up to 5 mol % Cu and equals 0.089–0.095 and 0.017–0.021 S cm−1 at 800 and 600°C. The sintering, adhesion, and electroconductance of composite cathodes based on La0.8Sr0.2MnO3 with 40% CSCu and their electrochemical behavior in air in the temperature interval 900–1000°C on carrying electrolyte Zr0.9Y0.1O1.95 with a CSCu sublayer containing 2 mol % Cu are studied.

X-ray photoelectron spectroscopy was used to identify the oxidation states of cations in La0.7Ca0.3MnO3 and La0.7Ca0.3Mn0.97Cu0.03O3 perovskite oxides, to determine the surface composition of the samples, and to assess the effect of the Cu dopant on the electronic spectrum of the material. The binding energies of the La 4d, Ca 2p, Mn 2p, and O 1s core levels were determined, and the effect of the Cu dopant on the shape of the La 4d and O 1s peaks was analyzed.

La0.8 – xCexSr0.2MnO3 (x= 0–0.3) mixed-conducting manganites are studied by x-ray photoelectron spectroscopy. The effect of A-site Ce substitution on the surface composition of the samples, charge states of the constituent cations, and bonding configuration is analyzed. The binding energies of the constituent elements are determined. The spectral shapes of the O 1s, La 4d, La 3d, Sr 3d, Mn 2p, Ce 4d, and Ce 3d peaks are examined as a function of Ce content.

X-ray photoelectron spectroscopy was used to identify the oxidation states of ions in La1 – xCaxCr1 – yMyO3 (M = Mn, Fe, Ni, Cu) perovskite oxides, to determine the surface composition of the samples, and to assess the effect of B-site substitutions on the electronic spectrum of the chromites. The binding energies of the La 4d, Ca 2p, Mn 2p, Ni 2p, Cr 2p, and O 1s core levels were determined, and the effect of the dopant on the shape of the La 4d and O 1s peaks was analyzed.

Measurements of steady-state potentials of various electrodes made of oxide compositions are performed in cells containing electrolyte ZrO2 + 10 mol % Y2O3 at a temperature of 773 K in nonequilibrium gas mixtures N2 + O2 + 20 vol % CO2 + CO, with variable CO (0.1–5 vol %) and O2 (0.5–10 vol %) contents. After adding CO into a gas phase, potentials of all electrodes shift, to one degree or another. The shift is substantially affected by the O2 content. The electrochemical systems under study are of some interest for analyzing partial pressures of CO in gas mixtures. The discovered effect must be taken into account when employing such electrodes for the determination of partial concentrations of other components of a gas mixture, for example, oxygen.