Search

International audience; This study describes the development of a shaping process involving GDC10 (Ce0.9Gd0.1O1.95) as an electrolyte for Solid Oxide Cells operating at low temperature (500−600 °C). The goal is to build a single cell of porous/dense/porous GDC10 layers while adapting the porous layer thicknesses, which are infiltrated by catalysts to prepare efficient GDC-base composites. Two shaping processes were used to make the cell. The first process is die pressing of the electrolyte powder followed by the infiltration of a screen-printed porous backbone. The second shaping process consists of preparing a porous/dense/porous (30 μm/≈ 400 μm/30 μm) cell in a single step by combining tape-casting and screen-printing processes followed by the infiltration of the previous catalyst.

International audience

International audience; Layered Ruddlesden–Popper (RP) lanthanide nickelates, Lnn+1NinO3n+1 (Ln = La, Pr and Nd; n = 1, 2 and 3), are considered potential cathode materials in solid oxide fuel cells. In this study, the thermal evolution of the structure, oxygen nonstoichiometry, electrical conductivity and oxygen transport properties of La2NiO4+δ, Nd2NiO4+δ, La3Ni2O7−δ, La4Ni3O10−δ, Pr4Ni3O10−δ and Nd4Ni3O10−δ are investigated. Phase transitions involving a disruption of the cooperative tilting of the perovskite layers in the low-temperature structure thereby transforming it to a more symmetric structure are observed in several of the materials upon heating in air. Pr4Ni3O10−δ and Nd4Ni3O10−δ show no phase transition from room temperature up to 1000 °C. High density ceramics (>96%) are obtained after sintering at 1300 °C and (for n = 2 and n = 3 members) post-sintering annealing at reduced temperatures. Data for the electrical conductivity measurements on these specimens indicate itinerant behaviour of the charge carriers in the RP nickelates. The increase in p-type conductivity with the order n of the RP phase is interpreted as arising from the concomitant increase in the formal valence of Ni. The observations can be interpreted in terms of a simple energy band scheme, showing that electron holes are formed in the σx2−y2↑ band upon increasing the oxidation state of Ni. Electrical conductivity relaxation measurements reveal remarkable similarities between the surface exchange coefficients (kchem) of the different RP phases despite the differences in the order parameter n and the nature of the lanthanide ion. Calculation of the oxygen self-diffusion coefficients (Ds) from the experimental values of the chemical diffusion coefficients (Dchem), using the corresponding data of oxygen non-stoichiometry from thermogravimetry measurements, shows that these are strongly determined by the order parameter n. The value of Ds decreases almost one order of magnitude on going from the n = 1 members La2NiO4+δ and Nd2NiO4+δ to the n = 2 member La3Ni2O7−δ, and again one order of magnitude on going to the n = 3 members La4Ni3O10−δ, Pr4Ni3O10−δ and Nd4Ni3O10−δ. The results confirm that oxygen-ion transport in the investigated RP nickelates predominantly occurs via an interstitialcy mechanism within the rock-salt layer of the structures.

Compositions in the La2−xPrxNiO4+δ series offer an attractive balance of chemical stability and electrochemical performance for use as cathode materials in solid oxide fuel cells (SOFCs). A detailed crystallographic study of this system has been performed, combining both high resolution synchrotron and neutron powder diffraction data, in order to investigate structural details of the series as a function of composition, temperature and oxygen over-stoichiometry. The monoclinic structure (space group F2/m) of ambient temperature Pr-rich compositions for 1.0 < x ≤ 2.0 is discussed in terms of octahedra tilt arrangements and possible long-range structural modulations. In situ synchrotron diffraction experiments and TEM are employed to examine the role of temperature and interstitial oxygen on these structural distortions. With increasing La substitution, a region of mixed monoclinic and tetragonal phases is described for 0.5 ≤ x ≤ 1.0. La-Rich compositions are found to be single phase tetragonal (F4/mmm for 0 < x < 0.5) or orthorhombic (Fmmm for x = 0). Possible origins and electrochemical property consequences of the refined structural trends are considered.

International audience; Electrical conductivity relaxation (ECR) and oxygen permeation measurements were conducted, at 750 °C, to assess the long-term oxygen transport characteristics of the mixed ionic–electronic conducting Pr2NiO4+δ with a K2NiF4 structure. The results show that the apparent values for the oxygen diffusion and surface exchange coefficients extracted from the data and the associated oxygen flux increase over 120 h by 1–2 orders of magnitude. The results of post-mortem X-ray diffraction analysis of the samples show partial to virtually complete decomposition of Pr2NiO4+δ under the conditions of the experiments to Pr4Ni3O10+δ, PrNiO3−δ, Pr6O11, and traces of NiO. Pulse 18O–16O isotopic exchange (PIE) measurements confirmed fast surface exchange kinetics of the higher-order Ruddlesden–Popper phase Pr4Ni3O10+δ and Pr6O11 formed upon decomposition. Additional factors related to the microstructure, however, need to be considered to explain the observations.

International audience; This paper reports the study of the densification of 20% Gd doped ceria (Ce0.8Gd0.2O1.9 (GDC)) interlayers in SOFC cathodes through two different routes: the well-known addition of sintering elements, and an innovative densification process by infiltration. First, Li, Cu, and Zn nitrates were added to GDC powders. The effect of these additives on the densification was studied by dilatometry on pellets, and show a large decrease of the sintering temperature from 1330 °C (pure GDC), down to 1080 °C, 950 °C, and 930 °C for Zn, Cu, and Li addition, respectively. However, this promising result does not apply to screen-printed layers, which are more porous than pellets and in which the shrinkage is constrained by the substrate. The second approach consists in preparing a pre-sintered GDC layer, which is subsequently infiltrated with Ce and Gd nitrates and sintered at 1250 °C to increase its density. Such an approach results in highly dense GDC interlayers. Using La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) as electrode, the influence of the interlayers on the cathode performance was studied. The addition of sintering aids dramatically increases the cell resistances, most likely because the additives increase the reactivity between GDC and either Yttria Stabilized Zirconia (YSZ) or LSCF, thus losing the expected benefit related to the decrease of sintering temperatures. The interlayers prepared by infiltration do not induce additional resistances in the cell, which results in power densities of single cells 40–50% higher than those of cells prepared with commercial GDC interlayers, making this approach a valuable alternative to sintering aids.

International audience; 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 Co 4+ and Fe 3+ 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 cm 2 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.

Solid oxide fuel cells (SOFCs) operating at intermediate temperature (500 – 700 °C) have attracted much attention due to high energy conversion efficiency, fuel adaptability and low emissions [1]. Both composition and microstructural /architectural design of the cathode film play an important role in obtaining the optimal electrochemical performances in this temperature range [2-9]. Recently, Ln2NiO4+δ (Ln = La, Pr, Nd) rare-earth nickelates have displayed promising oxygen electrode performance because of their interesting structure, transport and catalytic performance. Among nickelates, Pr2NiO4+δ presents excellent electrochemical properties at intermediate temperatures while it is characterized by a rather poor chemical stability in comparison to La2NiO4+δ. The preparation of efficient oxygen electrode materials which present very low degradation rates is still a challenge. The present study is focused on designing Pr doped lanthanum nickelates, La2-xPrxNiO4+δ with 0 ≤ x ≤ 2 with the aim of finding the best compromise between chemical stability and optimized electrochemical performance. First, single and double layered La2NiO4+δ electrodes with controlled microstructure were deposited on Ce0.9Gd0.1O2-δ (CGO) electrolyte using electrostatic spray deposition (ESD) or/and screen-printing (SP). The microstructure of La2NiO4+δ films has been optimized by varying some key process parameters. A coral-like microstructure of La2NiO4+δ films was successfully obtained resulting from nozzle to substrate distance, flow rate, substrate temperature deposition time equal to 50 mm, 1.5 mL/h, 350 °C and 3 h, respectively [6]. Electrochemical properties of La2NiO4+δ deposited symmetrically on CGO were measured by impedance spectroscopy. They are found to be strongly dependent on the cathode microstructure and architecture. For example, a double layer La2NiO4+δ cathode, consisted of 3-D coral nanocrystalline film (average particle size ~ 150 nm) with a continuous nanometric dense sub-layer (100 nm) at the electrode/electrolyte interface, prepared by ESD and topped by a La2NiO4+δ current collector prepared by SP, shows the best performance leading to the polarization resistance (Rpol) down to 0.42 Ω cm² at 600 °C (Fig 1) [6]. In a second part, this double layer design was extended toLa2-xPrxNiO4+δ (with x = 0, 0.5, 1 and 2) cathodes with a view to taking advantage of the complimentary properties of the two extreme compositions Pr2NiO4+δ and La2NiO4+δ, i.e. higher electronic conductivity of Pr2NiO4+δ superior stability of La2NiO4+δ. The chemical stability and polarization resistance (Rpol) were found to decrease when the Pr content was increased. Polarization resistance (Rpol) equal to 0.42, 0.12 and 0.08 Ω cm2 at 600 °C has been obtained for La2NiO4+δ, LaPrNiO4+δ and Pr2NiO4+δ, respectively. Among the complete La2-xPrxNiO4+δ solid solution, LaPrNiO4+δ shows the best compromise between electrochemical properties (the lowest Rpol value available in the literature for this composition, 0.12 W cm2 at 600 °C) and chemical stability in air (up to 30 days at 700 °C) [9]. Finally, the role of the electrode/electrolyte interface has been investigated on the polarization resistance of La2NiO4+δ and Pr2NiO4+δ cathodes. For this purpose, the thin dense nickelate sub-layer was replaced by a thin porous (~3-4 mm) composite layer based on a mixture of CGO (deposited by SP) and La2NiO4+δ or Pr2NiO4+δ (deposited by ESD). As a consequence, further reduction in Rpol down to 0.16 (Fig 1) and 0.04 Ω cm2 was successfully obtained for La2NiO4+δ and Pr2NiO4+δ, respectively [8]. This composite LnNO-CGO sub-layer is playing an important role in enhancing the electrochemical properties of these cathodes leading to the lowest Rpol values available in the literature for these compositions, to the best of our knowledge. This talk will end with our latest results incorporating a LnNO-CGO composite sub-layer to the double layer LaPrNiO4+δ electrode, the most promising composition in terms of electrochemical properties and chemical stability. References: [1] J.R. Wilson, A.T. Duong, M. Gameiro, H.Y. Chen, K. Thornton, D.R. Mumm, S.A. Barnett, Electrochemistry Communications. 11 (2009) 1052-1056. [2] D. Marinha, L. Dessemond, J.S. Cronin, J.R. Wilson, S.A. Barnett, E. Djurado, Chem. Mater. 23 (2011) 5340-5348. [3] D. Marinha, J. Hayd, L. Dessemond, E. Ivers-Tiffée, E. Djurado, J. Power Sources. 196 (2011) 5084–5090 [4] D. Marinha, L. Dessemond, E. Djurado, J. Power Sources. 197 (2012) 80-87. [5] R.K. Sharma, M. Burriel, E. Djurado, J. Mater. Chem. A. 3 (2015) 23833-23843. [6] R.K. Sharma, M. Burriel, L. Dessemond, V. Martin, J.M. Bassat, E. Djurado, J. Power Sources. 316 (2016) 17-28. [7] R.K. Sharma, M. Burriel, L. Dessemond, J.M. Bassat, E. Djurado, J. Power Sources. 325 (2016) 337-345. [8] R.K. Sharma, M. Burriel, L. Dessemond, J.M. Bassat, E. Djurado, J. Mater. Chem. A. 4 (2016) 12451-12462. [9] R.K. Sharma, S.K. Cheah, M. Burriel, L. Dessemond, J.M. Bassat, E. Djurado, J. Mater. Chem. A. (2017) DOI: 10.1039/C6TA08011A Figure 1

International audience; The mean oxidation state of cationic elements in Pr2NiO4+δ was studied using XANES measurements versus temperature in air. Unexpectedly, praseodymium is solely found as Pr3+ while Pr4+ was also foreseeable. Pr3+ cation is well accommodated in the large rare-earth site with the nine-fold coordination in this K2NiF4 – T-type structure; conversely, the small size of the Pr4+ cation seems not compatible with this site. Moreover, XANES measurements show a progressive decrease of the mean oxidation state of nickel ions when the temperature increases. It is directly correlated to the decrease of the oxygen over-stoichiometry of the material evidenced by TGA measurements.

International audience; The preparation and characterization of pure fluorite-type phases allowed exploring the CeO2–ZrO2–PrOx (CZP) phase diagram. On the basis of magnetic susceptibility measurements, the Pr4+/Pr3+ molar ratio of several oxides annealed at T = 700 °C under air was determined; the higher the Zr content, the lower the Pr4+ concentration. Thermogravimetric analysis and temperature-programmed reduction measurements showed various Pr and Ce reduction steps. The Pr4+ reduction starts at T = 250 °C and is maximum around T = 400 °C. For the most reducible compositions, which exhibit the lowest Zr content and the highest Pr rate, the reduction phenomena strongly depend on the Pr/Ce molar ratio. As a remarkable result, Pr and Ce reductions can simultaneously take place at lower temperature (T > 430 °C) than for oxides of the CeO2–ZrO2 solid solution evidencing that the increase of Pr content also allows enhancing the reducibility of Ce4+ at low temperature. On the basis of a discussion taking into account the probability of oxygen surroundings in disordered fluorite networks and the rate of oxygen released in materials after the first reduction step performed at T < 500 °C, a mapping of the most probable labile oxygen sites in the CZP phase diagram is proposed. In particular, it is shown that for the oxides containing 10 atom % Zr, the most labile oxygen site should be systematically coordinated with one Zr atom, one Ce, and two Pr atoms. In the same series (10 atom % Zr), the electronic transport properties allowed showing semiconducting behavior with a strong increase of the total conductivity as the Pr content increases. On the basis of the thermal variation of the Seebeck coefficient, these phenomena are associated with hopping of electrons and holes, involving intra-atomic charge transfers, which depend on the reduction temperature of Pr4+ ions under air. Finally, the oxygen mobility strongly increases with the Pr content in this series. The oxygen tracer self-diffusion coefficient D* has been estimated by two independent measurements, and the best value is around 10–8 cm2/s at T = 400 °C for the Ce0.45Zr0.1Pr0.45O2–x composition, which is quite high in this temperature range. These fundamental properties of CZP phases design very promising new materials like automotive exhaust catalysts, gas sensors, electrolytes, or oxygen electrodes for solid oxide fuel cells.

International audience; Architecturally designed La2−xPrxNiO4+δ (with x = 0, 0.5, 1 and 2) cathodes on the Ce0.9Gd0.1O2−δ (CGO) electrolyte have been prepared with a view to take advantage of the complimentary properties of the two extreme compositions La2NiO4+δ and Pr2NiO4+δ, i.e. the superior stability of La2NiO4+δ and the higher electronic conductivity of Pr2NiO4+δ. The design consists of stacking of two layers starting with a 3D tree-like microstructure (∼20 μm thick) over a thin dense base layer (∼100 nm) fabricated in one step by electrostatic spray deposition (ESD) and then topped by using a screen-printed (SP) current collecting layer of the same composition. X-ray diffraction confirms the formation of a complete solid solution crystallizing in a single phase orthorhombic structure with the Fmmm space group. The thermodynamic stability and polarisation resistance (Rpol) decrease by increasing the Pr content. Among the complete La2−xPrxNiO4+δ solid solutions, LaPrNiO4+δ shows the best compromise between electrochemical properties (the lowest Rpol value available in the literature for this composition, 0.12 Ω cm2 at 600 °C) and thermodynamic stability in air. Moreover, an anode supported single cell (Ni-3YSZ/Ni-8YSZ/8YSZ/CGO) including the LaPrNiO4+δ double layer electrode shows a maximum power density of 438 mW cm−2 at 700 °C.

International audience; La2NiO4+δ is a mixed ionic–electronic conducting material with the 2D K2NiF4-type structure garnering much interest as a potential intermediate-temperature solid-oxide fuel cell (IT-SOFC) cathode. The oxygen diffusion along the ab-plane exhibiting the highest oxygen ionic conductivity governs the behavior of the bulk materials in this family of oxides. The oxygen surface exchange processes, however, are not well-understood and large differences in the surface exchange coefficient (k*) values can be found in the literature for this and other cathode materials. The isotopic exchange depth profiling (IEDP) technique was used in combination with low energy ion scattering (LEIS) measurements on two sets of La2NiO4+δ single crystals with precisely cut crystal faces but different thermal histories. For each set of single crystals, the oxygen diffusion and surface exchange coefficient were evaluated for two different orientations in the temperature range of 450–600 °C. The differences in k* have been correlated with differences in surface chemistry: surface termination, near-surface rearrangement, and the presence of extrinsic impurities. Finally, the predominant La termination at the immediate outer surface is evidenced, confirming recent results for other Ruddlesden–Popper phases with mixed ionic–electronic conducting properties.

International audience; The present study is focused on the development of alternative oxygen electrodes for Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolyzers Cells (SOECs). Rare earth nickelates with general formula Ln2NiO4+δ (Ln = La, Pr or Nd) have shown good performance as oxygen electrodes with various electrolytes. To further enhance the physico-chemical properties, electrochemical performance as oxygen electrode and durability of Solid Oxide Cells (SOCs), herein, we have performed the substitution of nickel with cobalt in these nickelates. Two compositions (x=0.1 and 0.2) were mainly considered and completely characterized using several techniques. The single cells were then prepared and electrochemically characterized using DC- and AC-techniques in the temperature range 700-900 °C. The durability test up to 250h were also investigated at 1 A.cm-2 current density at 800 °C under both SOFC (dry conditions) and SOEC conditions (with 50% H2 and 50% H2O feed gas mixture) indicating different degradation behavior.

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.

Compositions in the La2-xPrxNiO4+δ series offer an attractive balance of chemical stability and electrochemical performance for use as cathode materials in solid oxide fuel cells (SOFCs). A detailed crystallographic study of this system has been performed, combining both high resolution synchrotron and neutron powder diffraction data, in order to investigate structural details of the series as a function of composition, temperature and oxygen over-stoichiometry. The monoclinic structure (space group F2/m) of ambient temperature Pr-rich compositions for 1.0x ≤ 2.0 is discussed in terms of octahedra tilt arrangements and possible long-range structural modulations. In situ synchrotron diffraction experiments and TEM are employed to examine the role of temperature and interstitial oxygen on these structural distortions. With increasing La substitution, a region of mixed monoclinic and tetragonal phases is described for 0.5 ≤ x ≤ 1.0. La-Rich compositions are found to be single phase tetragonal (F4/mmm for 0x0.5) or orthorhombic (Fmmm for x = 0). Possible origins and electrochemical property consequences of the refined structural trends are considered.

International audience; The ionic conductivity of CaTi0.9Fe0.1O3−δ was determined as functions of oxygen partial pressure and temperature, using two experimental methods: total electrical conductivity measurements by impedance spectroscopy and oxygen isotopic exchange. The data were compared with previously published results. It was demonstrated that, in contradiction with the hypothesis often assumed, the ionic conductivity is not constant in the whole oxygen pressure range. The variation of the ionic conductivity as a function of the oxygen pressure, in the domain where an additional p-type electronic conductivity is observed, was calculated for CaTi0.9Fe0.1O3−δ using our results, and for CaTi0.8Fe0.2O3−δ using data from the literature. At 900 °C, under air, a decrease of the ionic conductivity of 30% compared to the value under 10−7 bar was calculated for the CaTi0.9Fe0.1O3−δ and an increase of 100% was determined for the CaTi0.8Fe0.2O3−δ. This feature was interpreted in terms of the complex defect chemistry of iron-doped calcium titanates.

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; The double perovskite oxides La2-xNiMnO6-δ (x = 0.0, 0.1, 0.2, 0.3) are evaluated as potential cathodes for intermediate temperature solid oxide fuel cells. The effects of La-deficiency on the crystal structure, high temperature thermal and electrical properties, and catalytic activity for oxygen reduction reaction are comprehensively investigated. La2-xNiMnO6-δ crystallizes in monoclinic structure with the space group P21/n. The cell volume expands with the increase of La-deficiency. The highest electrical conductivity of 25 S cm−1 is found for La1.8NiMnO5.71 at 800 °C in air. Introducing La3+ deficiency greatly improves the amount of chemical adsorbed oxygen species on material surface, which promotes the oxygen reduction reaction of La2-xNiMnO6-δ cathode. The optimum composition, La1.8NiMnO5.71 exhibits the lowest area specific resistance of 0.21 Ω cm2 at 700 °C in air. Meanwhile the peak power density of the anode-supported single cell with La1.8NiMnO5.71 cathode reaches 0.6 W cm−2 at 700 °C. The electrochemical impedance spectra measurements combining with distribution of relaxation times analysis prove that the charge transfer process is the rate-limiting step of the ORR reaction.

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; Electrochemical impedance spectroscopy measurements carried out on La2NiO4+δ(LNO)//Ce0.8Gd0.2O2−δ(GDC)//3mol.%Y2O3-ZrO2(TZ3Y) half-cells from 500°C to 800°C revealed that the impedance of the LNO electrode is Gerischer-like. Based on experimental data of the porosity, tortuosity, surface area, molar volume and thermodynamic factor of the LNO electrode sintered at 1200°C, the ionic conductivity σi(LNO) and the surface exchange coefficient ℜ0 of the LNO electrode are calculated using the Adler-Lane-Steele (ALS) model. Surprisingly, the polarization resistance of the half-cell is shown to depend on the sintering temperature of the GDC interlayer, which is varied from 1200°C up to 1450°C. Whatever the sintering conditions of GDC, the calculated ℜ0 values agree with those obtained by recently published pulse isotopic exchange (PIE) experiments. Indeed, the oxygen reduction reaction mostly occurs at the LNO grain surface. However, the calculated ionic conductivity of the electrode vary depending on the GDC sintering temperature. X-ray diffraction studies of the LNO//GDC and GDC//TZ3Y interfaces reveal that cations interdiffusion occurs in both cases. The interfacial GDC layer may act as a bottleneck for the oxide ion transfer from the LNO electrode toward the TZ3Y electrolyte, and vice-versa, resulting in an increase of the series and polarization resistances.

International audience; This paper is devoted to the study of composite cathodes of La2NiO4+δ infiltrated into a Gd-doped ceria backbone. Porous Gd-doped ceria backbones are screen printed onto yttria-stabilized zirconia or Gd-doped ceria dense electrolytes, and infiltrated with a La and Ni nitrate solution (2:1 stoichiometry ratio). The influence of the preparation parameters on the polarization resistance, such as the concentration of the infiltration solution, the amount of infiltrated phase, the annealing temperature, the thickness of the electrode, and the nature of the electrolyte, is characterized by impedance spectroscopy performed on symmetrical cells. The optimization of these parameters results in a decrease of the polarization resistance down to 0.15 Ω cm2 at 600 °C. Using the Adler-Lane-Steele model, the modelling of the impedance diagrams leads to the determination of the ionic conductivity as well as the surface exchange rate of the infiltrated electrode.

The present study is focused on alternative oxygen electrodes for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs) using Metal Supported Cells (MSCs) conditions. To prevent detrimental oxidation of the metal support, sintering of the cell components at high temperature under low pO2 is required. Ln2NiO4+δ (Ln = La, Pr) compounds with the K2NiF4-type structure act as cathode materials for IT-SOFC due to their mixed ionic and electronic conductivity (i.e. MIEC properties). Pr2NiO4+δ shows excellent electrochemical properties at intermediate temperature (i.e. low polarization resistance Rp value, Rp = 0.03 Ω.cm² at 700 °C), while La2NiO4+δ exhibits higher chemical stability. Thus, the properties of La2 -x Pr x NiO4+δ mixed nickelates were investigated with the aim to find best compromise between chemical stability and electrochemical performances. Herein, the chemical stability of the nickelates under air at operating temperatures as well as the evolution of the polarization resistances during ageing (recorded under air at idc = 0 and idc ≠ 0 conditions) were studied for duration up to one month. La2NiO4+δ is chemically stable whereas Pr2NiO4+δ dissociates after 1 month at 600, 700 and 800 °C. At idc = 0 condition, the ageing of the half cells during 1 month under air shows no change in Rp , despite their various degrees of chemical stability. A different behavior is observed under idc ≠ 0 conditions with a large increase in Rp for Pr-rich phases in SOFC mode while interestingly, Rp remain stable in SOEC mode. In this presentation, the electrochemical properties of La2 -x Pr x NiO4+δ under ageing and postmortem analysis will be discussed in details.

The layered perovskite PrBaxCo2O5+δ (PBxCO, x = 0.90–1.0) oxides have been synthesized by a solid-state reaction technique, and evaluated as the potential cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Room temperature X-ray diffraction patterns show the orthorhombic structures which double the lattice parameters from the perovskite cell parameter as a ≈ ap, b ≈ ap and c ≈ 2ap (ap is the cell parameter of the primitive perovskite) in the Pmmm space group. There is a good chemical compatibility between the PBxCO cathode and the Ce0.9Gd0.1O1.95 (CGO) electrolyte at 1000 °C. The electrical conductivity and thermal expansion coefficient of PBxCO are improved due to the increased amount of electronic holes originated from the Ba-deficiency. The results demonstrate the high electrochemical performance of PBxCO cathodes, as evidenced by the super low polarization resistances (Rp) over the intermediate temperature range. The lowest Rp value, 0.042 Ω cm2, and the cathodic overpotential, −15 mV at a current density of −25 mA cm−2, are obtained in the PrBa0.94Co2O5+δ cathode at 600 °C in air, which thus allow to be used as a highly promising cathode for IT-SOFCs. A CGO electrolyte fuel cell with the PrBa0.94Co2O5+δ cathode presents the attractive peak power density of ∼1.0 W cm−2 at 700 °C. Furthermore, the oxygen reduction kinetics of the PrBa0.94Co2O5+δ cathode is also studied, and the rate-limiting steps for oxygen reduction reaction are determined at different temperatures.

In order to clearly identify the rate determining step (rds) in oxygen semi-permeation of dense Ln2NiO4+δ membranes (with Ln = La, Pr and Nd), a specific setup has been designed, which allowed measuring the oxygen semi-permeation flux as well as the oxygen activity gradient at both membrane surfaces. From the results of the measurements, the coefficients of oxygen diffusion and surface exchange at both membrane surfaces have been determined. Their values have been compared with the values reported in the literature for similar membrane materials. The main conclusion is that the oxygen flux through these membranes is limited by the oxygen surface exchange at the oxygen-lean face, and not, as previously claimed, by oxygen volume diffusion.

The third generation of IT-SOFCs concerns the metal supported cells. With respect to the sintering conditions of the complete cell, a current challenge is the chemical stability of the components, more especially the cathode, under low pO2 (e.g. under N2, pO2 ≈ 10-4 atm). In this scope, the properties of the mixed rare earth nickelates formulated Pr2-xLaxNiO4+δ have been studied; indeed Pr2NiO4+δ is a very promising material in terms of electrochemical properties while La2NiO4+δ exhibits a higher chemical stability. The aim of this study is to find the best compromise between both characteristics. The properties of mixed phases were found intermediate between those of Pr2NiO4+δ and La2NiO4+δ.

This work aims to associate two innovative promising oxide materials, the perovskite-type BaIn0.3Ti0.7O3-δ (BIT07) electrolyte and the lanthanide nickelate Pr2-xNiO4+δ cathode, in a planar anode-supported SOFC for an efficient operation at 700°C. The cell elaboration protocol will be briefly described, and the results of electrochemical characterizations of complete cells will be discussed. Adding a GDC barrier layer in between BIT07 electrolyte and the cathode decreased the polarization resistance by a factor 4, e.g. for Pr1.97NiO4+δ a competitive value of 0.29 Ω cm² is reached. It also led to increased OCV values, as a small p-type conductivity of BIT07 in oxidizing atmosphere associated with a few microscopic cracks observed in the electrolyte layer could be compensated by an electron-blocking effect of GDC and a filling of the porosities of the electrolyte. This cell was operated beyond 500 hours of under current, with a fairly high extrapolated degradation rate of 27% / kh.

The cell manufacturing of Metal Supported Cells (MSC) for IT-SOFC application (typically 600 °C) requires to operate under non oxidizing conditions at high temperature (typically > 950 °C). The most sensitive layer appears to be the cathode one, usually a mixed ionic and electronic oxide. The behavior (TGA, XRD analyses, TECs) of the lanthanide nickelates Ln2NiO4+δ (Ln = La, Pr or Nd) is reported when sintered at high temperature, under low pO2, down to 10-4 atm. Electrochemical Impedance Spectroscopy measurements on Ln2NiO4+δ//GDC//YSZ half cells sintered either in air or under argon at various temperatures show polarization resistance values that meet the target requirements (< 0.20 W.cm² at 600 °C), especially for La2NiO4+ δ and Pr2NiO4+ δ. These results are discussed in terms of structural and microstructural features.

International audience; The state-of-the-art materials for SOFCs are yttria-stabilized zirconia as electrolyte and lanthanum strontium cobalt ferrite as cathode. However, the formation of insulating phases between them requires the use of diffusion barriers, typically made of gadolinia doped ceria. The study of the stability of this layer during the fabrication and in operando is currently one of the major goals of the SOFC industry. In this work, the cation inter-diffusion at the cathode/barrier layer/electrolyte region is analysed for an anode-supported cell industrially fabricated by conventional techniques, assembled in a short-stack and tested under real operation conditions for 3000 h. A comprehensive study of this cell, and an equivalent non-operated one, is performed in order to understand the inter-diffusion mechanisms with possible effects on the final performance. The analyses evidence that the cation diffusion is occurring during the fabrication process. Despite the significant diffusion of Ce,Gd, Zr, Y and Sr cations, the formation of typically reported CGO-YSZ solid solution is not observed while the presence of isolated grains of SrZrO3 is proved. All in all, this study presents new insights into the stability of the typically employed diffusion barriers for solid oxide cells that will guide future strategies to improve their performance and durability.

International audience; The electrochemical performance of Pr2NiMnO6 (PNMO)-xCe0.9Gd0.1O1.95 (CGO) (x = 0–40 wt%) composite oxides as intermediate-temperature solid oxide fuel cell (IT-SOFC) cathode materials are evaluated. The electrochemical impedance spectroscopy (EIS) analysis results identify two consecutive electrode processes on the composite cathode. Among the various composites, PNMO-30CGO cathode exhibits the best electrochemical performance with the minimum polarization resistance of 0.23 Ω cm2 and the maximum exchange current density of 75 mA cm−2 at 700 °C in air. These values are almost constant even after 30-h operation. The oxygen reduction reaction (ORR) mechanism studies prove that the major rate-determining step is the charge-transfer process. Introducing CGO significantly improves the charge-transfer process, by increasing the triple phase boundary (TPB) length and oxygen vacancy concentration in the composite cathode.

International audience; In the solid oxide fuel cell field, the gadolinium doped ceria (GDC) is widely used as interlayer between oxygen electrode and electrolyte materials, as for instance La2NiO4+δ (LNO) and 3 mol.% Y2O3-ZrO2 (TZ3Y), respectively. However, cation interdiffusion occurs between GDC and LNO/TZ3Y during their sintering steps at high temperature, which finally lowers the electrochemical performance of the cathode for the oxygen reduction reaction (ORR). In this paper, an alternative interlayer is proposed, the praseodymium doped ceria (PrDC with 30 at.% Pr). It is shown that reactivity occurs between PrDC and TZ3Y, but the compounds formed seem to favour the ORR. Besides, little reactivity is detected between LNO and PrDC. As a result, the total resistances, i.e. series and polarization resistance obtained with a LNO//PrDC//TZ3Y half-cell from 500 °C to 800 °C are much lower than those obtained with a LNO//GDC//TZ3Y half-cell, sintered in the same conditions. In addition, differences in the shape of the impedance diagrams are observed depending on the interlayer: this feature is discussed throughout the paper.

Composite electrodes of Pr2NiO4+δ and Gd-doped ceria (Ce0.8Gd0.2O1.90 so-called GDC) have been prepared starting from a Pr-Ni nitrate solution (2:1 stoichiometry ratio) that was infiltrated into a porous GDC backbone made by screen printing onto 3 % yttria-stabilized zirconia membrane (90 μm thick). The crystallization of the Pr2NiO4+δ powder was first studied by X-ray diffraction: it highly depends on the annealing atmosphere (air or N2) and the formation of this phase as infiltrate is more difficult than for the powder. The electrochemical study performed on symmetrical cells shows that the polarization resistance decreases when the annealing temperature of the infiltrate electrode decreases. The lowest R p value is obtained for an annealing at 900 °C, for 2 h in N2, the value being 0.075 Ω cm2 at 600 °C in air.

International audience; Nd1−xBaCo2O6−δ (N1−xBCO) is evaluated as cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The effects of Nd-deficiency on the crystal structure, thermal expansion behavior, electrical conductivity and electrochemical performance are studied. N1−xBCO oxides crystallize in the orthorhombic symmetry with Pmmm space group. A good chemical compatibility between N1−xBCO and CGO electrolyte is found at 1100 °C in air. Introducing Nd-deficiency promotes the formation of oxygen vacancy, and significantly improves the electrochemical performance of N1−xBCO cathodes. The lowest area specific resistance (ASR) value of 0.043 Ω cm2 is obtained on the N0.96BCO cathode at 700 °C in air. The rate limiting step for electrochemical oxygen reduction reaction (ORR) is charge transfer process at the interface. The power output of the electrolyte supported cell Ni-CGO/CGO/N0.96BCO reaches 0.6 W cm−2 at 700 °C.

In CeO2-based solid electrolytes, it has been shown that point defects are directly responsible for oxygen ionic conduction. The ionic conductivity is strongly affected by the anion vacancy concentration which is enhanced by doping with aliovalent cations. When rare earth sesquioxides such as La2O3, Gd2O3, Sm2O3, Y2O3 are added to CeO2, the dopant cation substitutes for the cerium ion, and oxygen vacancies are created for charge compensation. Incorporation of trivalent dopants into CeO2 at the Ce4+ sites can be depicted by the following defect reaction (expressed in Kröger-Vink notation)

The praseodymium nickelate oxide Pr2NiO4+δ, a mixed conducting oxide, was evaluated as cathode for Low Temperature Solid Oxide Fuel Cell at T = 873 K, using zirconia-type electrolytes. The electrochemical performance of the porous cathode has been improved by optimisation of its microstructure combined with the use of a doped ceria barrier layer in between the cathode and zirconia electrolyte. Both low polarization and ohmic resistances were obtained using Pr2NiO4+δ powders with a median particle size of 0.4 µm. These manufacturing conditions resulted in a cathode microstructure with well established connections between the cathode particles and good adhesion of the cathode on the electrolyte. Full-sized anode supported cells have been manufactured using the same process conditions and tested. The best results were obtained when using a dense Ce0.8Gd0.2O1.9 (20CGO) barrier layer. Moreover, we developed a new route for depositing ceria thin films: the supercritical fluid deposition (SFD) technique. Interestingly, we could evidence that the deposit of a dense interlayer of a few tens of nanometer contributes to the lowering of the polarization resistance, but also that this result can be achieved with undoped ceria, although this material is never used for SOFC because of its low ionic conductivity level.

International audience; Neodymium nickelate, with composition Nd2NiO4+δ is integrated as oxygen electrode in a solid oxide electrolyte supported cell made of a TZ3Y electrolyte and a Ni-CGO hydrogen electrode. This cell is tested in both fuel cell (SOFC) and electrolysis (SOEC) mode and the reversible operation is proven, ASR values being slightly lower in electrolysis mode. Performances in SOEC mode are compared with a commercial cell based on the same electrolyte and cathode, but with lanthanum strontium manganite (LSM) as anode. For a voltage of 1.3 V, current densities of 0.40, 0.64 and 0.87 A cm−2 are measured at 750, 800 and 850 °C, respectively; they are much higher than the ones measured in the same conditions for the LSM-containing cell. Indeed, for a voltage of 1.3 V, current densities are respectively 1.7, 3 and 4.2 times higher for the Nd2NiO4+δ cell than for the LSM one at 850, 800 and 750 °C, respectively. Consequently, Nd2NiO4+δ can be considered as a good candidate for operating below 800 °C as oxygen electrode for high temperature steam electrolysis.

International audience; The effect of an 80 nm YSZ interfacial layer, deposited onto a YSZ pellet by different techniques (dip-coating, sputtering and atomic layer deposition) at the SOFC cathode/electrolyte interface to allow a maximum adhesion of the thick cathode layer, was thoroughly analysed. The LSM cathode was deposited on the solid electrolyte by painting and sintered in air at 1200°C for 2 h. The morphological and structural analyses of the samples were performed by scanning electron microscopy and X-ray diffraction; their electrical properties were examined between 390 and 700°C by electrochemical impedance spectroscopy. The impedance responses showed three different contributions. The highfrequency arc is attributed to the YSZ electrolyte. The electrode processes associated with the medium- and low frequency arcs were discussed. The electrochemical performance was influenced by the microstructure at the electrode/electrolyte interface.

Two rare-earth nickelates, with compositions Nd2NiO4+δ and La2NiO4+δ have been investigated as oxygen electrodes. Their polarization resistance was determined on symmetrical half-cells and was found much lower than for classical Sr-doped LaMnO3 (LSM) perovskite. Then Nd2NiO4+δ was integrated in an electrolyte supported cell and tested in both fuel cell (SOFC) and electrolysis (SOEC) modes. The performances of this cell were higher than the one of a LSM-containing one. ASR values measured for this cell at 0.7 V were equal to 0.60 and 1.09 Ω.cm², at 850 and 750{degree sign}C, respectively. At 850{degree sign}C, this value is 1.7 times lower compared to the LSM-containing cell. At 750{degree sign}C it is 5.6 times lower. These results seem to show that nickelates are particularly suitable for a use below 800{degree sign}C. Similar conclusions were obtained in electrolysis mode.

Chemical surface exchange coefficients k_chem and chemical diffusion coefficients D_chem of Nd2NiO4+δ have been determined by the conductivity relaxation technique at 575{less than or equal to}T/{degree sign}C{less than or equal to}725 between oxygen partial pressures of 0.01 and 0.02 bar. At 700 {degree sign}C values of 1E-4 cm/s and 6E-7 cm2/s were measured for k_chem and D_chem, respectively. A strong decrease upon cooling was observed for both kinetic parameters due to high activation energies. Electronic conductivities range between 100 and 120 S/cm and are found to decrease for increasing temperature in the investigated temperature region of 575{less than or equal to}T/{degree sign}C{less than or equal to}725. Thermogravimetry/mass spectroscopy was performed at 20

Until now, most cathode materials used in high temperature ceramic solid electrolyte oxide cells (SEOC) are perovskite-type MEIC compounds, AMO3-δ, showing oxygen sub-stoichiometry. Recently, a new family of overstoichiometric oxides, formulated A2MO4+δ with A = La, Nd, Pr, Sr and M = Ni, Cu, has been investigated. Due to their basic properties (large D* and k coefficients, high electrical conductivity), the nickelate compounds have been especially used in various applications. Nd2NiO4 and Pr2NiO4 exhibit very promising electrochemical properties down to temperature as low as 600{degree sign}C, as ITSOFC cathodes as well as PCFC cathodes (they are stable under moist air (3 -10 % H2O). In addition, encouraging cell tests (SOFC and PCFC) have been performed with both nickelates. Furthermore, these materials have been also used as HTSE anodes and first results show excellent performances. Thus, it appears that these materials show an extremely high flexibility for being used as air electrode in very different conditions in terms of oxygen partial pressure or/and water content. This is explained on the basis of structural features of these materials and the great ability of these compounds for accommodating the oxygen non-stoichiometry.

International audience; The effects of yttrium Y and zinc Zn substitution on calcium Ca and cupper Cu sites in Bi2Sr2CaCu2O8+δ compounds have been investigated by resistivity measurements. At high temperature, after a superconducting transition, the electrical behaviour evolves from a metallic state in compounds with a weak Y concentration into a semi-conducting state (hopping conduction) in compounds with a large Y concentration. Similarly, the Zn substitution modifies the electrical behaviour from metallic to insulating with a much lower concentration comparatively to Y. In the two cases, the semi-conducting behaviour is described by hopping between localised states in the conduction band. This localization is explained by the random distribution of the doping elements which originates from the Y/Zn and Cu wave-functions overlap. The obtained results show the localization is improved in the case of Zn. Consequently, it is suggested that the direct substitution of Zn into the planes leads to a greater overlap. Moreover, the superconducting transition (critical temperature: TC) shows an irregular depression when Zn is substituted. According to an X-Ray diffraction study, this result can be interpreted by internal micro-strains in the structure.

International audience; We used electron spin resonance (ESR) and magnetic susceptibility to study the magnetic correlations in polycrystalline La2NiO4.12 prepared by modified sol–gel method. We observed an evolution of the ESR signal with temperature, from 4 K to 300 K, with marked changes at 220 K and 150 K. Such changes are related to the particular order of holes and spins previously observed in this system by neutron diffraction by other authors. Magnetic susceptibility measurements confirm the charge and spin order temperatures in this composition.

International audience; The promising SOFC cathode material Pr2NiO4.22 has been studied in situ under a pure oxygen atmosphere from 25 to 950 °C by high resolution synchrotron X-ray powder diffraction. At room temperature (RT) δ = 0.22(1), the average crystal structure turns out to be monoclinic. The subtle monoclinic distortion (γ = 90.066(1)° at RT), retained up to 460 °C, is interpreted in terms of specific tilt schemes of the NiO6 octahedra. It is also shown that Pr2NiO4.22 is incommensurately structurally modulated already at room temperature, in the same manner as the homologous cobaltate La2CoO4.14. The phase transition to the High Temperature Tetragonal (HTT) phase was completed at 480 °C without any evidence for the Low Temperature Orthorhombic (LTO) phase allowing clarifying the phase diagram of this K2NiF4-type ternary oxide. Moreover, it turns out that above 800 °C, the HTT phase transforms reversibly into two coexisting isomorphous tetragonal phases. The incommensurate modulation subsists up to 950 °C, although modified concomitantly with the two abovementioned phase transformations. In addition, the role of kinetics on the decomposition process is highlighted through thermo-gravimetric analyses