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Structure, electrical conductivity and electrochemical performance for a new oxygen deficient perovskite material with composition SrCo0.85Fe0.15O2.62 is explored here. It crystallizes in a '314-type' oxygen-vacancy ordered structure (Space Group: I4 /mmm) with 2ap × 2ap × 4ap unit cell, and contains alternate oxygen replete [Co2/Fe2-O] and oxygen deficient [Co1/Fe1-O] planes along c-direction. Room temperature Mössbauer spectroscopy and room temperature neutron powder diffraction confirm the presence of mixed valence states (3+ and 4+) for both Fe and Co. Polaronic behavior in the electrical conductivity is observed below 400 °C, and attributed to the presence of Co4+ and Fe3+ cations in the structure. The electrical conductivity at 600 °C is high enough (∼200 S cm−1) to act as a cathode material. At the same time, the polarization resistance measurements show an excellent value of 0.1 Ω cm2 at 600 °C. These results suggest that at 600 °C, this material is a mixed oxide-ion and electronic conductor exhibiting excellent activity for the oxygen reduction reaction, making it a promising cathode material for an intermediate temperature solid oxide fuel cell. [ABSTRACT FROM AUTHOR]

The present study is focused on alternative structured oxygen electrodes for solid oxide electrolysis cells (SOEC). The Ln2NiO4+δ (Ln = La or Pr) nickelate oxides were selected as innovative electrode materials with respect to their mixed electronic and ionic conductivity. A thin interfacial ceria-based layer was added in between the electrode and the zirconia-based electrolyte to improve mechanical and electrochemical properties and to limit the chemical reactivity. These structured cells were characterized by electrochemical impedance spectroscopy on symmetrical cells, under zero dc conditions and anodic polarization. Low polarization resistance RP and improved anodic overpotential ηA versus current density curves were obtained for the Pr2NiO4+δ electrode with Ce0.8Y0.2O2-δ interlayer: RP is decreased down to 0.06 Ω cm2 at 800 °C, under air and zero dc conditions. Then, complete hydrogen electrode-supported cells including Pr2NiO4+δ as oxygen electrode were electrochemically characterized. At 800 °C, when the inlet gas composition is 90 vol.% H2O-10 vol.% H2 at the hydrogen electrode, air being swept at the oxygen electrode, the current density determined at 1.28 V reaches -0.9 A cm-2, the corresponding steam to hydrogen conversion ratio being 58%. These results are compared to those obtained with a reference cell including the oxygen deficient perovskite La0.6Sr0.4Fe0.8Co0.2O3-δ as oxygen electrode. [ABSTRACT FROM AUTHOR]

Four mixed ionic-electronic conducting (MIEC) perovskite-related oxides were studied as potential H+-SOFC cathode materials: La0.6Sr0.4Fe0.8Co0.2O3-δ, Ba0.5Sr0.5Co0.8Fe0.2O3- δ, PrBaCo2O5+δ and Pr2NiO4+δ. Their hydration properties were measured by TGA: Ba0.5Sr0.5Co0.8Fe0.2O3-δ shows the largest water uptake. Their electrochemical performances were characterized using BaCe0.9Y0.1O3-δ as electrolyte; polarization resistances as low as 0.5 δcm2 were found at 600°C, for PrBaCo2O5+δ and Pr2NiO4+δ. The rate determining steps of the oxygen reduction reaction were determined on the basis of electrochemical studies performed as a function of pH2O, in air. Proton transfer and water release appear to be the rate determining steps for Ba0.5Sr0.5Co0.8Fe0.2O3-δ, PrBaCo2O5+δ and Pr2NiO4+δ. No rate determining step involving proton was found for La0.6Sr0.4Fe0.8Co0.2O3-δ. On the basis of this study, one can suggest that Ba0.5Sr0.5Co0.8Fe0.2O3-δ, PrBaCo2O5+δ and Pr2NiO4+δ show some protonic conduction as well as oxide diffusivity and can be labeled Triple Conducting (e-/O2-/H+) Oxides, so-called TCO. [ABSTRACT FROM AUTHOR]

La-Si metallic coatings with La/Si ratios between 1.55 and 1.66 were synthesized by Direct Current Magnetron Sputtering of lanthanum and silicon targets in pure argon atmosphere. After the deposition stage, the ceramic apatite-structure coatings were obtained by thermal oxidation in air for 2 hours at 1173 K. The structural and chemical features of these films have been assessed by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive Spectroscopy (EDS). The electrical properties were determined by Complex Impedance Spectroscopy in planar configuration. The films with near apatite structure La/Si atomic ratio in range of La10Si6027 and La9.33Si6026 deposited on different substrates were initially amorphous. After thermal oxidation at 1173 K in air, the coating crystallised under the expected apatite structure. SEM observation revealed that the film compactness and thickness increased after thermal oxidation. The film adhesion on SOFCs material is guaranteed if the anode support was not reduced before the deposition stage. A specific method was performed to measure the electrical conductivity, i.e. to determine the ionic conductivity of the film: it uses the four-point probe technique combined with ac impedance spectroscopy. The good agreement between measurements performed in this work (Ea = 0.75 eV, 973 K, σ= 3-4 10-3 S·cm-1) and literature data allows concluding that deposited layers are of good quality and should be used as electrolyte in an Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC). [ABSTRACT FROM AUTHOR]

Abstract: Solid state ionic devices based on high-temperature proton conductors can be used for various applications, especially in a new class of fuel cells, the Protonic Ceramic Fuel Cell (SOFC-H+). These systems are currently operating at intermediate temperatures (500–600°C) and one of the major problems is the overpotential at the cathode side. In this context, various perovskite oxides AMO3− δ (A=La, Ba, Sr; M=Mn; Fe, Co, Ni) and A2MO4-type compounds (A=La, Nd, Pr or Sr; M=Ni) have been investigated. Their properties under moist cathodic atmosphere have been studied. Actually, they are stable and exhibit high electrical conductivity (σ >100Scm−1) as well as good electrocatalytic properties towards oxygen reduction. The electrochemical properties of these oxides deposited on the protonic electrolyte BaCe0.9Y0.1O3− δ have been studied and the Area Specific Resistances have been measured under air/H2O (3%) atmosphere. The obtained values at 600°C, especially for Ba0.5Sr0.5Fe0.8Co0.2O3− δ and Pr2NiO4+ δ show to be promising cathode materials for Protonic Ceramic Fuel Cell applications. [Copyright &y& Elsevier]

An in-depth analysis of the very low frequency impedance arcs observed when measuring efficient solid oxide fuel cell electrodes by electrochemical impedance spectroscopy (EIS) is reported in this paper. The study was carried out on Pr 2 NiO 4+ δ //Ce 0.8 Gd 0.2 O 2- δ //3 mol. % Y 2 O 3 -ZrO 2 symmetrical half-cell. In the temperature range 500–900 °C, three impedance arcs related to O 2 molecular diffusion were distinguished from the EIS measurements. Based on theoretical calculations using the Adler-Lane-Steele (ALS) and Dusty gas models, the arc at highest frequencies was ascribed to the diffusion of O 2 in the porous structures of the electrode and collecting gold grid. It obeys the ALS model, i.e. a parallel R//C impedance with a capacitance coming mainly from the solid phase. The second arc at medium frequencies was ascribed to the diffusion of O 2 in the porous structure of the ceramic part used to maintain the gold grid. It follows the Dusty gas model, i.e. a Warburg impedance with relaxation time depending on the gas phase properties. Finally, the third one at lowest frequencies was ascribed to the “gas conversion” phenomenon, coming from a difference in the local p O 2 above the active sites of the working and counter electrodes. This gas conversion impedance largely increases when clogging the channels of the gas distribution system. [ABSTRACT FROM AUTHOR]

Abstract: The neodymium-deficient nickelate Nd1.95NiO4+δ , mixed conducting K2NiF4-type oxide, was evaluated as cathode for solid oxide fuel cells. The electrochemical properties were investigated on planar Ni–YSZ anode-supported SOFC based on co-tape casted and co-fired HTceramix® cells. Using a layer of strontium doped lanthanum cobaltite as current collector, a current density of 1.31Acm−2 (at 0.70V) was obtained at 800°C using hydrogen fuel with small single cells, after optimizing the cathode sintering temperature. Impedance spectroscopy measurements were performed; the different resistive contributions and the values of the corresponding equivalent capacities are discussed. [Copyright &y& Elsevier]

Abstract: La2− x NiO4+ δ (x =0 ; 0.05) has been prepared by modified Pechini method and characterized by XRD, TGA, SIMS and Electrochemical Impedance Spectroscopy. La2− x NiO4+ δ phases show no reaction with YSZ at 900 °C for 4 h. The materials have promising high oxygen diffusion coefficient and surface exchange coefficient. The electrode reaction process is identified to be the reaction rate limiting step at low temperature (T <700 °C) and small applied d.c. potential, whereas at high temperature (T >700 °C) and large d.c. potential, it is the interface diffusion process which becomes limiting. [Copyright &y& Elsevier]