Search

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.

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.

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.

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.

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.

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 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.

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.