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International audience; The performances of mixed ionic and electronic conductors is used in many applications, such as oxygen transport membranes or electrodes in solid oxide fuel cells. The performances of these systems depend mainly on two fundamental parameters including oxygen diffusion (DO) and the oxygen exchange coefficient (k). This work focuses on the impact of the oxygen partial pressure on oxygen diffusion and surface exchange coefficients of mixed conducting materials, as reported studies are scarce in the literature. In this way, two different mixed conducting materials are studied, namely, La0.6Sr0.4Fe0.6Ga0.4O3-δ, a perovskite-type material, and the nickelate La2NiO4+δ. The DO and k coefficients are determined by a specific oxygen permeation measurement and by the isotopic exchange depth profile method.

International audience; The praseodymium nickelate PrNiO3-δ was recently identified as one of the decomposition products of Pr2NiO4+δ oxygen electrode, raising the question of the role of this perovskite compound on the Pr2NiO4+δ performance and ageing behaviour. Herein, the electrochemical properties of PrNiO3-δ as cathode for Solid Oxide Fuel Cells (SOFCs) is reported. PrNiO3-δ, synthesized from the citrate-nitrate route, cristallizes in an orthorhombic structure (S.G.: Pnma). Under air, a transition to high temperature rhombohedral structure (S.G.: R-3c) occurs at around 600 °C, while its oxygen vacancy amount increases from δ 0.05 at room temperature up to 0.10 at high temperature. Thermal gravimetry analysis (TGA) measurements coupled with X-ray diffraction (XRD) characterizations show that above 1000 °C, in air, PrNiO3-δ starts to decompose into Pr2NiO4+δ and NiO through an intermediate transformation into Pr4Ni3O10+δ. The value of the polarization resistance (Rp) of co-sintered GDC-PrNiO3-δ electrode, at 950 °C in air for 2 h, is 0.91 Ω cm2 at 600 °C under air. It remains much higher than that of co-sintered GDC-Pr2NiO4+δ electrode (Rp = 0.15 Ω cm2), which means that the perovskite does not play any significant role in the electrochemical performance during long term operation of the Pr2NiO4+δ electrode

International audience; In this work, La2NiO4+δ is used as cathode material for Intermediate temperature solid oxide fuel cells (IT-SOFC). Scanning electron microscopy, X-ray diffraction, and electrochemical impedance spectroscopy are used to investigate the effect of the presence of an ultrathin La2NiO4+δ layer (80 nm) deposited by dip-coating or sputtering at the interface YSZ/La2NiO4+δ. The thick porous cathode layer of La2NiO4+δ is deposited by screen-printing, and sintered at 1000 °C for 2 h or 1200 °C for 20 min in order to study the effect of sintering temperature on the electrochemical properties. The results show that the electrochemical performances of the cathode are influenced by the deposition technique. The best electrochemical properties are obtained with the use of the nano film interface layer deposited by sputtering. The introduction of ultrathin interface with nano grained between the cathode and electrolyte is a promising technique to reduce polarization resistance associated with oxygen reduction reaction (ORR) on the cathode.

International audience; In this work we present a modified citrate-nitrate route using citric acid as a chelating agent as an effective and facile strategy to obtain nanocrystalline La3Ni2O7+δ (L3N2) and La4Ni3O10−δ (L4N3) powders for the preparation of solid oxide fuel cell cathodes. Both samples crystallize in a Fmmm orthorhombic layered Lan+1NinO3n+1 Ruddlesden-Popper structure, with n = 2 and 3, respectively. The oxygen non-stoichiometry, determined by TGA is equal to 0.05 and 0.06 for L3N2 and L4N3, respectively. The thermal expansion coefficient values of L3N2 and L4N3 are 11.0 × 10−6 K−1 and 11.5 × 10−6 K−1, respectively. This study focused on L3N2, L4N3 and on novel composite electrodes with CGO (Ce0.9Gd0.1O2−δ): L3N2-CGO and L4N3-CGO with a view to taking advantage of their complimentary properties, i.e. high ionic conductivity of CGO and high electronic conductivity of Lan+1NinO3n+1 (n = 2 and 3). A significant improvement of the polarization resistance, from 1.0 to 0.03 Ω cm2 and from 1.5 to 0.52 Ω cm2 at 700 °C, is obtained when 50 wt% CGO is added to L3N2 and L4N3, respectively. In addition, the L3N2-CGO composite shows good long-term stability at 900 °C for 2 weeks in air, confirming its suitability as a SOFC cathode.

In this study, high-quality mesoporous Fe 3 O 4 nanocages (MFONs) have been synthesized by a facile complex-coprecipitation method at 100 °C with addition of triethanolamine and ethylene glycol. The as-prepared Fe 3 O 4 nanocages possess a mesoporous structure and highly uniform dispersion. When used as an anode material for rechargeable lithium-ion batteries, MFONs anode shows high specific capacities and excellent cycling performance at high and low current rates. At a current density of 200 mA g −1 , the discharge specific capacities are 876 mAh g −1 at the 2nd cycle and 830 mAh g −1 at the 100th cycle. Even at the high current density of 1000 mA g −1 , MFONs anode still retains a stable capacity of 573 mAh g −1 after 300 cycles. This superior electrochemical performance is attributed to the unique mesoporous cage-like structure and high specific surface area (133 m 2 g −1 ) of MFONs, which may offer large electrode/electrolyte contact area for the electron conduction and Li + storage. Furthermore, the good mechanical flexibility of the mesoporous nanocages can readily buffer the massive volume expansion/shrinkage associated with the reversible electrode reaction. These results indicate that MFONs can be used as a promising high-performance anode material for lithium-ion batteries.

International audience; The chemical stability of lanthanide nickelates Ln2NiO4+δ (Ln=La, Pr or Nd) has been studied in the temperature range 25–1300 °C, either in air or at low pO2 (down to 10−4 atm). Thermal gravimetry analysis (TGA) measurements coupled with X-ray diffraction (XRD) characterization have shown that all compounds retain their K2NiF4-type structure in these conditions, while remaining over-stoichiometric in oxygen up to 1000 °C. Only Nd2NiO4+δ starts to decompose into Nd2O3 and NiO above 1000 °C, at pO2=10−4 atm. In addition, a careful analysis of the lanthanide nickelates structural features has been performed by in situ XRD, as a function of temperature and pO2. For all compounds, a structural transition has been always observed in the temperature range 200–400 °C, in air or at pO2=10−4 atm. In addition, their cell volume did not vary upon the variation of the oxygen partial pressure. Therefore, these materials do not exhibit a chemical expansion in these conditions, which is beneficial for a fuel cell application as cathode layers. Additional dilatometry measurements have revealed that a temperature as high as 950 °C for Pr2NiO4+δ or 1100 °C for La2NiO4+δ and Nd2NiO4+δ has to be reached in order to begin the sintering of the material particles, which is of primary importance to obtain an efficient electronic/ionic conduction in the corresponding designed cathode layers. Besides, excellent matching was found between the thermal expansion coefficients of lanthanide nickelates and SOFC electrolytes such as 8wt% yttria stabilized zirconia (8YSZ) or Ce0.8Gd0.2O2−δ (GDC), at least from 400 °C up to 1400 °C in air or up to 1200 °C at pO2=10−4 atm.

International audience; This study is focused on the structural characteristics, oxygen nonstoichiometry, electrical conductivity, electrochemical performance and oxygen reduction mechanism of YBa1−xSrxCo2O5+δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5). The high oxygen nonstoichiometry, δ = 0.18-0.43 at 700 °C, indicates the large oxygen vacancy concentrations in oxides. The electrical conductivity is improved due to the greater amount of electronic holes originated from the increased interstitial oxygen, and the conductivities of all samples are above 100 S cm−1 at 400-700 °C in air. The results demonstrate the promising performance of YBa1−xSrxCo2O5+δ cathodes at intermediate temperatures, as evidenced by low area-specific resistances (ASRs) e.g. 0.21-0.59 Ω cm2 at 700 °C. The lowest ASR, 0.44 Ω cm2, and the cathodic overpotential, −40 mV at a current density of −136 mA cm−2, are obtained in YBaCo2O5+δ cathode at 650 °C. The dependence of polarization resistance on oxygen partial pressure suggests that the charge transfer process is the rate-limiting step for oxygen reduction reaction in YBaCo2O5+δ cathode.

Brownmillerite oxide Ca 2 Fe 1.3 Mn 0.7 O 5+ δ is synthesized and evaluated as a new cathode material for solid oxide fuel cells (SOFCs). The average thermal expansion coefficient (TEC) of Ca 2 Fe 1.3 Mn 0.7 O 5+ δ is 13.3 ± 0.2 × 10 −6 K −1 , close to that of the typical electrolyte materials. The lowest polarization resistance ( R p ) (0.22 ± 0.02 Ω cm 2 at 750 °C in air) and cathodic overpotential values are achieved when the material is sintered at 1000 °C on Ce 0.9 Gd 0.1 O 1.95 (CGO) electrolyte. Two main oxygen reduction reaction (ORR) processes are identified. The rate limiting step for ORR is found to depend on the oxygen partial pressure. The excellent electrochemical performance indicates that Ca 2 Fe 1.3 Mn 0.7 O 5+ δ can be a promising cathode material for SOFCs.

This paper is dedicated to the study of the relationship between the Nd 2 NiO 4+ δ powder micro-structural properties (especially particle size and crystallinity) and electrochemical properties when the oxide is used as SOFC cathode deposited on 8YSZ electrolyte coated with thin doped ceria. Nano-structured particles of Nd 2 NiO 4+ δ with controlled crystallinity, size and morphology have been synthesized using ultrasonic spray pyrolysis (USP). The series and polarization resistances measured on symmetrical half cells Nd 2 NiO 4+ δ /YDC/8YSZ/YDC/Nd 2 NiO 4+ δ are both found to be dependent on the cathode microstructure and present a similar evolution with temperature. The best results are obtained for highly crystalline cathode powders combined with a small particle size.

In this chapter, after a brief overview of the oxygen reduction reaction at the cathode side of a solid oxide fuel cell (SOFC), the main requirements for cathode materials are discussed as well as the conventional compounds that have been intensively studied during the last two decades. In the second part of the chapter, innovative materials (mixed ionic and electronic conducting oxides) are considered from the viewpoint of structural aspects and transport properties. Special attention is devoted to the layered compounds, Ln2NiO4 + δ, which exhibit promising properties (oxygen diffusivity, electrocatalytic properties). Their performances as SOFC cathode materials are compared to those of other materials. Finally, the flexibility of these materials as oxygen electrodes in protonic ceramic fuel cells and electrolyzers is emphasized. This is discussed in terms of the materials’ structural features.

{sup 57}Fe doped La{sub 4}Ni{sub 2.97}Fe{sub 0.03}O{sub 9.95} was synthesized by a citrate method and, afterwards, successfully oxidized and reduced by electrochemical methods. The compounds obtained were investigated by X-ray diffraction, electrical measurements and Moessbauer spectroscopy. The study allowed to follow the variation of the two nickel sites environment with the oxygen stoichiometry and a deeper understanding of the electrical behavior versus oxygen non-stoichiometry was achieved. The Moessbauer study revealed that after both oxidation and reduction treatments, the major modifications were observed on the octahedra adjacent to the La{sub 2}O{sub 2} layers, while the middle octahedra of the triple perovskite block remained almost unchanged. The oxygen intercalation (oxidized treatment) takes place essentially in the La{sub 2}O{sub 2} layers and the oxygen desintercalation (reduction treatment) occurs in the octahedral sites adjacent to those layers. - Grapical abstract: Moessbauer spectra of oxidized and reduced Ruddlesden-Popper compounds La{sub 4}Ni{sub 2.97}Fe{sub 0.03}O{sub 10{+-}}{sub y}.

International audience; La2 − xNiO4 + δ (x = 0 ; 0.05) has been prepared by modified Pechini method and characterized by XRD, TGA, SIMS and Electrochemical Impedance Spectroscopy. La2 − xNiO4 + 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.

International audience; 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 1.31 A cm−2 (at 0.70 V) 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.

The clathrate form of silicon, Si 136 (otherwise known as Si 34 ), having a residual sodium content as low as 35 ppm (i.e., x ∼0.0058 in Na x Si 136 ), has been prepared by thermal decomposition of NaSi under high vacuum, followed by several other treatments under vacuum, and completed by repeated reactions with iodine. The residual amount of sodium has been determined by a combination of analytic and spectroscopic methods involving XRD, electron probe microanalysis, atomic absorption, NMR and EPR. This latter technique proved to be very appropriate to the characterisation of very diluted sodium atoms in such clathrate structure and to the quantitative determination of its residual concentration.

The 18 O/ 16 O Isotope Exchange Depth Profile technique (IEDP) was applied to BIMEVOX.10 (ME=Cu, Co) materials to determine their oxygen exchange coefficients and bulk oxygen tracer diffusion coefficients. These results are compared to the exchange current densities obtained from current–voltage curves and bulk conductivity determined by impedance spectroscopy. These results confirm that, in spite of a noticeably higher ionic conductivity than that of the classical oxide electrolytes such as yttria stabilized zirconia (YSZ) or gadolinia-doped ceria (GDC), the oxygen surface exchange coefficients are low.