Search

Solid Oxide Fuel Cells (SOFCs) are a promising technology that can provide efficient and clean energy production. The general barriers hindering their market entry are durability, i.e. resistance to aging, and costs. In parallel to the deeper insight into the different degradation sources and improved understanding of ageing factors and their interactions, work towards higher accuracy for the assessment and monitoring of real-world fuel cell ageing in necessary. The requirements for operational stability formulate the parameter “degradation rate” (DR). Most often long term durability tests are performed at constant current load and the decrease of the voltage is used for its definition. In this work a new approach based on analysis of the volt-ampere characteristics, named Differential Resistance Analysis (DRA), is presented. It operates with the differential resistance, i.e. with the derivative of the voltage in respect to the current (dU/dI = Rd) which is more sensitive to small deviations and thus increases the sensitivity of the analysis. Two performance indicators are derived (Rd, min and ∆U*) with differing selectivity: ∆U* is more sensitive to activation losses and Rd, min - to transport hindrances. The application of the DRA is demonstrated on examples from measurements in fuel cell and in reverse (fuel cell/electrolyzer) mode, as well as on modeling data. The results show that the method is at least 10 times more sensitive to DR evaluation in comparison with the classical approach.

International audience; The present work proposes to explore how the presence of a periodic pattern at electrode/electrolyte interfaces of a SOFC could impact the electrochemical performances, from theoretical and experimental points of view. The model results demonstrate that a patterned interfaces along with an electrolyte having a thickness smaller than the dimensions of the pattern, lead to a strong increase of the exchange surface, hence to the exchange currents (up to 64%) with respect to flat interfaces. With the use of laboratory standard ceramic processes, this architecturation was experimented on YSZ-Ni self supported anodes on top of which a thin YSZ electrolyte was deposited. The first electrical tests for such a cell with a non-optimized thickness show an increase of the current density with respect to a cell with flat interfaces, from 130 to 300 mA cm-2 at 0.7 V, that is even higher than anticipated by the modeling.

International audience; Nowadays, the valorization of biomass, biowastes and by-products is among the key issue to be considered in the development of renewable energies from bioresources. Accurate analysis and characterization of these feedstocks is a crucial aspect in the understanding of their behaviour for further use. This chapter is focused on different characterization techniques which are commonly used up-to-date. They are classified in different categories: Sampling and storage; Proximate analysis; Ultimate analysis; Thermal analysis, Physical characterizations; Physico-chemical characterizations; Structural and textural characterizations; and Mechanical characterizations. For each of them, a general description of the technique is presented, followed by useful information on machines and experimental conditions such as sample preparation, sample pre-treatment, gas atmosphere, temperature program etc. Finally, examples, results treatment and exploitations will be provided to illustrate. This chapter provides an insight on generic and advanced characterization techniques for complex materials, such as biomass, biowastes and related bio-products, that will be again discussed along the handbook in the other chapters.

International audience; Solid co-products from biochemical, chemical and thermochemical processes of biomass and biowaste have gained momentum in utilization as secondary raw materials. These solids are carbon-based or mineral-based materials, and for their suitable use in a number of fields, various properties should be determined. This chapter addresses advanced techniques used to determine physical and chemical properties of these solid residues. For each technique, the basics and protocols are described. Post-treatment procedures and interpretation of the results obtained are also provided for some residues.

International audience; The catalytic activity of pyrolysis chars from bio-waste was investigated for the cracking of model tar compounds (ethylbenzene and benzene). Two pyrolysis chars were produced at 700 °C from (1) used wood pallets (UWP), and (2) a 50/50 dry% mixture of food waste (FW) and coagulation-flocculation sludge (CFS). Steam activation at 850 °C was used to study the influence of the porous structure. While coke deposition is known to be responsible for the deactivation of carbonaceous chars and metal catalysts during tar cracking reactions, the deactivation of complex materials such as bio-waste chars has scarcely been studied. For this reason, special attention was paid on the relationships between the physicochemical properties of the chars, the operating conditions, and the deactivation mechanisms. To this aim, the cracking tests were performed over a wide temperature range: 400–650 °C for the ethylbenzene cracking, and 850–950 °C for benzene cracking. After the ethylbenzene cracking tests at 650 °C, the characterisations performed with SEM, BET, FTIR and Raman revealed that coke deposition was responsible for the char’s deactivation. The high specific surface area of activated chars explained their higher catalytic activity, and mesoporous catalysts were proved to be more resistant to coke deactivation than microporous catalysts. For these reasons, the higher ethylbenzene conversion (85.8%) was reached with the activated char from food waste and sludge (ac.FW/CFS). For benzene cracking at higher temperature (850 and 950 °C), the chars from food waste and sludge (FW/CFS) were the most active catalysts, despite their deactivation by the melting, diffusion and sintering of the inorganic species. This original deactivation mechanism, reported for the first time, led to the formation of an inorganic layer composed of P and Ca species at the char surface, with some areas rich in KCl and NaCl. Non-activated char from food waste and sludge (c.FW/CFS) was surprisingly proved to be more resistant to deactivation by inorganic species than the activated char (ac.FW/CFS) during the benzene cracking tests at 950 °C. This extended catalytic activity was explained by the activation of the non-activated char (c.FW/CFS) with the CO2 contained in the syngas which simultaneously developed the porosity and created new available active sites. This study marks a step forward in the understanding of the relationships between the deactivation mechanisms, the physicochemical properties of the chars, and the cracking temperature. Finally, a proposal for process integration is presented to consider the possibility to valorise the chars as catalysts to decompose the tar generated in the same pyro-gasification process.

A new concept of a high temperature fuel cell based on a dual H+ and O2- conducting membrane was successfully developed recently (“IDEAL-Cell”, FET-Energy/FP7, 2008-2011). It operates in the range of 600-700 °C, and was shown to be superior of standard SOFCs and PCFCs at equivalent overall thickness. It is based on the junction between the anodic part of a PCFC and the cathodic part of a SOFC through a mixed H+ and O2- conducting porous composite ceramic membrane, avoiding all disadvantages associated to the presence of water at the electrodes. In the initial configuration, the porous mixed conductivity central membrane was made of a composite BCY15/YDC15, in which the active sites lay along the triple contact lines (TPB) between the protonic conducting phase (BCY15), the anionic conducting phase (YDC15) and the gas phase in the pores. During the course of the European project cited above, it was discovered and modeled that, in addition to be a protonic conductor, BCY15 was also an excellent oxygen conductor at 600-700 °C provided that it is fed with oxygen. Therefore, a second generation of dual membrane cell was developed in which YDC15 is replaced by BCY15, leading to a strongly simplified cell, much easier to shape and sinter, with potentially higher performances, i.e. the TPB become full active surfaces, magnifying the number of active sites; i.e. the dual membrane being solely made of BCY15, the tortuosity of the conductive phase is strongly diminished and its volume fraction increased, hence the electric resistance becomes much lower. As a whole, this so-called “monolithic concept” (for the cell is almost essentially made of a single BCY15 phase) improves the chemical and mechanical compatibility, increases the global cell conductivity and represents an important step towards simplifying the technology for industrialization. Moreover this new concept shows a good reversibility between the SOFC and SOEC modes; since this cell is based on 3 separate compartments (oxygen at the cathode, hydrogen at the anode, water at the dual central membrane), the dynamic of the device when shifting from one mode to the other is very high (no need to adjust the gas mixture at electrodes). The present work proposes to further increase the electrochemical properties of this monolithic dual membrane high temperature fuel cell by studying the catalytic properties on the mixed H+ and O2- conducting membrane with or without addition of Pt nanoparticles or Ni foam, which have been evaluated by impedance spectroscopy and polarization measurements. These additions led to slight adjustments of the shaping parameters to obtain flat and comparable samples. Finally, the electrochemical performances of the different configurations of the monolithic cell have been evaluated in real operating conditions via a dedicated 3-chamber set-up named Real Life Tester (RLT℗). Results show satisfactorily OCV and good performances for all samples, but a rapid degradation for the configuration with Ni foam probably due to aqueous electrochemical corrosion.

International audience; This study aims at understanding the structural changes occurring in the carbonaceous matrix of wood-based chars during their thermal conversion. Although chars are routinely characterized by porosity measurements or scanning electron microscopy, the composition and structure of the carbonaceous matrix is often not investigated. Here, advanced characterization using X-ray synchrotron microtomography, transmission electron microscopy, Raman spectroscopy and X-ray diffraction provided a precise description of the char properties, allowing for an accurate discussion of their catalytic properties. Two chars were produced by slow pyrolysis of wood waste (400 and 700 °C) and a third one was fabricated by activation under steam at 850 °C of the char obtained at 700 °C. The results show that the pyrolysis temperature and the activation performed did not affect the macrostructure of the chars and that the pores were interconnected at the macroscopic scale. However, at 700 °C, the micro- and nanostructures were modified: short-range organized graphene fringes were observed. The activated char showed a homogeneous microstructure similar to that of its precursor. Besides, the ratio of graphene-like structures, the local organization of graphene sheets, and the imperfections in graphene-like sheets were clearly improved by the post-treatment. To our knowledge, this is the first time that such an approach, combining various tools, is applied for the study of pyrolysis chars.

Steam electrolysis by means of reversible Fuel Cells is a promising concept for compensating the intermittency of renewable energy sources. Several tests on SOFC stacks operated in reverse mode (SOEC) reported faster degradation than in power production mode. Although not completely clarified, the degradation mechanisms appear partly related to the higher steam concentration at the electrode compartments. This work introduces a novel type of cell with dual anionic-protonic conductivity, which allows the separation of the steam compartment from the electrodes compartments. The concept is presented together with a set of experimental results pointing out the potential advantages of this approach. © The Electrochemical Society.

International audience; Syngas from thermochemical conversion of waste or biomass is a renewable energy carrier that may contain pollutants – such as tar – that should be removed before further syngas utilisation. Chars have proved to be promising catalysts for tar cracking, but the influence of the physico-chemical properties on their reactivity is still unclear. This work aimed to better understand the structure and the composition of the mineral species of pyrolysis char, as well as their catalytic role in tar cracking. For this purpose, a characterisation of the minerals has been performed at bulk, surface (studied at micro and nano-scale) and crystallite scale. Pyrolysis chars were produced from wastes generated on cruise ships – namely used wood pallets (UWP), food waste (FW) and coagulation flocculation sludge (CFS) – having different mineral amount and content. Ethylbenzene was used as surrogate of light aromatic hydrocarbons in a tar cracking process. The results showed that ethylbenzene was converted into lighter gases meaning that the chars were efficient for this. Ethylbenzene conversion at 650 °C was found to be significantly higher with the char from a mixture of sludge and food waste (c.FW/CFS) compared to that of wood-based char (c.UWP): 71 wt.% against 45 wt.%, respectively. The combination of multi-scale and complementary techniques has highlighted that the higher catalytic activity of this char was mainly attributed to the mineral content. Well dispersed mineral particles with various morphologies and natures were observed on the surface of c.FW/CFS using Scanning and Transmission Electron Microscopy (SEM and TEM). Especially, Ca, Al and P were the main mineral species identified using XRFS and SEM. These mineral species in form of oxides and hydroxyapatite were considered to be the main active mineral components for tar cracking. Oxides were identified using EDX-analysis. XRD analysis highlighted the presence of crystalised particles of hydroxyapatite (Ca5(PO4)3(OH)), while Raman spectroscopy revealed that these particles were embedded in the carbon matrix.

International audience; Novel ionic conductors were prepared by substituting Ca2+ and Sr2+ for Gd3+ in Gd3GaO6. A microwave-assisted combustion technique was used to synthesize these compounds at 900 °C. SEM observations showed that both substituents promote grain growth during sintering. XRD proved that the Gd3-x(Ca,Sr)xGaO6-x/2 solid solutions are formed up to x = 0.10. Below 600 °C, the level of conductivity under wet Ar is higher than that of measured under dry atmospheres, thereby demonstrating the contribution of proton defects to the overall conductivity. The highest level of proton conduction, i.e. σ600°C = 1 × 10−3 S cm−1, was measured for Gd2.9Sr0.1GaO5.95 at 600 °C in wet Ar. At higher temperatures, only oxygen ions contribute to the conductivity. At 800 °C, a total oxide-ion conductivity of σ800°C = 1 × 10−2 S cm−1 was measured for the highest substitution level, i.e. x = 0.10. In both temperature ranges, activation energy associated with ionic transport decreases with the Me content as a result of an increase in grain size. Stability tests were successfully achieved as the structure of materials remains unchanged after different treatment under severe conditions. Conductivity measurements under varying oxygen partial pressures demonstrated that materials are purely oxide-ion conductors up to pO2 = 1 × 10−5 atm. At higher pO2, a p-type contribution appears.

International audience; This paper reports impedance studies of a dual membrane fuel cell - an innovative design based on the idea for a junction between an oxygen ion conducting cathode/electrolyte part and proton conducting anode/electrolyte part through a mixed oxygen ion and proton conducting porous membrane. Thus oxygen, hydrogen and water are located in three independent chambers. This concept allows avoiding all the severe pitfalls connected to the presence of water at electrodes in both SOFC and PCFC. The performed measurements of the 3 compartments and the data analysis are improved by introducing some specialized approaches and techniques, which are discussed. The impedance contribution in the proof of the new concept is also presented.

The acceptor-substituted BaSn0.875M0.125O3-δ (M = Sc, In, Y, and Gd) proton conductors were prepared by a wet chemical method. The effects of dopant nature on the structure, hydration energy and proton conduction have been investigated. Analysis of XRD data showed that all compounds present a cubic perovskite structure (space group: P m 3 − m ). Thermogravimetry analysis showed that the significant mass of water was gained in wet Ar during measurements, and the most negative hydration energy is observed for Sc-substituted BaSnO3 (−86 kJ/mol). However, Y-substituted BaSnO3 exhibits the highest proton conductivity at low temperature under wet Ar, such as the total conductivity is 4.8 × 10−4 S/cm at 600 °C.

Des materiaux tels que les apatites a base d'oxydes de lanthane et de silicium ou des perovskites conductrices protoniques, potentiellement utilisables au sein de piles a combustible, presentent une grande resistance au frittage. Celle-ci limite d'autant plus leur utilisation au sein de piles a combustible, surtout s'ils doivent etre employes comme electrolytes. Plusieurs strategies peuvent etre envisagees pour remedier a ce probleme parmi lesquelles l'emploi de nouvelles methodes de frittage ou le choix d'une methode de synthese efficace (permettant par exemple de diminuer la taille des grains ou de limiter celle des agregats souvent redhibitoires au moment du frittage). Nous presentons ici les resultats de frittage par Spark Plasma Sintering (appele par la suite SPS) en les comparants a ceux obtenus par frittage conventionnel haute temperature. Les materiaux etudies ont des compositions derivees des phases La 9,33 □ 0,67 Si 6 O 26 et BaZr 0,9 Y 0,1 O 2,95 □0,05 pour lesquelles des problemes de frittage ont ete rencontres. Nous insisterons sur les particularites des materiaux obtenus par SPS en termes de structure et microstructure et des consequences sur les proprietes de transport anionique.

International audience; Images of a hematite-based epoxy coating are obtained by scanning electron microscopy (SEM). At the scale of a few micrometers, they show aggregates of hematite nano-particles organized along thin curved channels. We first segment the images and analyze them using mathematical morphology. The heterogeneous dispersion of particles is quantified using the correlation function and the granulometry of the embedding (epoxy) phase. Second, a two-scales, 3D random microstructure model with exclusion zones is proposed to simulate the spatial distribution of particles. This simple model is parametrized by four geometrical parameters related to the exclusion zones solely. The microstructure is numerically optimized, in the space of morphological parameters, on the granulometry of the embedding epoxy phase and on the microstructure correlation function, by standard gradient-descent methods. Excellent agreement is found between the SEM images and our optimized model. Finally, the size of the representative volume element associated to the optimized microstructure model is compared with that of the SEM images.

In the high current density regime, the performance of SOFCs is limited by concentration overpotentials, which are particularly significant at the anode side where a competition occurs between incoming hydrogen and outgoing water. In standard SOFCs where the anode is made of YSZ and Ni, this becomes crucial when the demand for reactants exceeds the capacity of the porous cermet anode to supply them by gas diffusion mechanisms. A delicate compromise should be found between maintaining a high level of activation in the anode, i.e. high density of triple phase boundaries (TPB), favored by a distribution of small interconnected pores with high tortuosity and small grains size, and high gas permeability, favored by a distribution of large interconnected pores with low tortuosity, while the percolation of both electronic and anionic conductivity phases is maintained. The present work aims at determining experimentally anodes gas permeability as a function of the experimentally determined percolation, tortuosity and volume fraction of the pores, assuming that the diffusion of gases is described by the Darcy Law. Anodes with varying porosity ensured by different quantities of the pore former were obtained by cold pressing and sintering. SEM image analysis, mercury intrusion porosimetry and permeability measurements for different gases were performed. The obtained results show that permeability increases with the volume fraction of the pore former. However, non-linearity as a function of the gases molecular weight is observed and discussed as a function of the mean free path and thus of possible mixed Darcy and Knudsen diffusion.

The route for the increase in SOFC performances is many-fold: i/ in the low current density domain, through the enhancement of the catalytic properties of the electrodes, i.e. via a higher Triple Phase Boundary (TPB) density, hence a higher exchange current and a lower activation overpotential, ii/ in the ohmic loss region, through lower resistance, i.e. via thickness reduction, materials with a higher conductivity, iii/ in the high current density region, via the optimization of the electrodes microstructure, i.e. the control of the porosity, of the tortuosity and percolation of phases. Taking ideas from the batteries community, where the conceptual design of electrodes is much more mature, the present work proposes to explore how the corrugation of electrode/electrolyte interfaces impacts the performances. This approach was applied to the anode/electrolyte interface of a SOFC based on standard compositions, YSZ (or YDC) for the electrolyte, and YSZ (or YDC) + nickel for the anode. The corrugation of surfaces was obtained through the patterning of this interface with different geometries (flat, pyramids, shallow and deep ellipsoids) at the 10-100 µm scale by cold pressing, templating or serigraphy. Thin electrolyte layers have been deposited on top of these architectures by Atomic Layer Deposition (ALD). In parallel, an electrochemical model was carried out and implemented by considering masses and charges conservation, gas transport and electrochemical reaction kinetics throughout the interface in FEM (finite element method) with COMSOL Multiphysics. The results showed a 25 % increase in the total current density for a certain ellipsoid geometry.

The Ln4[Ga 2(1-x) Ge 2x O 7+x □ 1-x ]O 2 (Ln = La, Nd, Gd; x ≤ 0.4) compounds have been prepared first by solid-state reaction and then by a new regenerative sol-gel method which allows their sintering. They exhibit a Cuspidine-like structure. Upon substituting Ge 4+ for Ga 3+ , the Cuspidine structure is maintained up to x = 0.4. For x ≥ 0.15 a 1D modulation of the structure was observed and analyzed as driven by composition. The Ge 4+ substitution for Ga 3+ with concomitant filling of oxygen vacancies is beneficial to the anionic conductivity which increases rather regularly with x to reach 10 -3 S/cm at 800 °C for x = 0.4 and Ln = Nd. Conductivity measurements on Ln4[Ga 2(1-x) Ge 2x O 7+x □ 1-x ]O 2 (Ln = La, Nd, Gd) compounds with a fixed Ge content (x = 0.2) show a clear increase of the activation energy when the ionic radii of the Ln 3+ cation decreases. La and Nd compounds exhibit similar conductivities, significantly larger than that of the Gd counterpart.

International audience; The oxide-ion and proton conduction properties of RE3NbO7 (RE = La, Gd, Y, Yb, Lu) compounds were investigated. For the bigger rare-earth cation, i.e. La3+, the compound crystallises in a weberite-type structure and the oxide-ion conductivity is low owing to the lack of intrinsic oxygen vacancies. Consequently, the resultant proton incorporation and conductivity in La3NbO7 are also low. For small rare-earth cations, i.e. from Gd3+ to Lu3+ and for RE = Y, materials adopt a fluorite-like structure confirmed from X-ray powder diffraction. In this latter case, materials include intrinsic oxygen vacancies leading to a higher oxygen conductivity. For these compounds, a proton incorporation takes place at low temperature under wet conditions giving rise to proton conductivity. Nevertheless, both oxygen and proton conductivities are low in these materials, which can be explained by the ordering of oxygen vacancies observed by Transmission Electron Microscopy.

The Nd4[Ga2(1−x)M2xO7+x□1−x]O2 (M=Ti, Ge) compounds have been prepared by solid state reaction. Nd4(Ga2O7□)O2 and Nd4(Ti2O8)O2 both exhibit a cuspidine-like structure and they form a complete solid solution. Upon substituting Ge for Ga, the cuspidine structure is maintained up to x=0.3. Although both Ti and Ge substitutions for Ga induce a concomitant filling of oxygen vacancies, opposite effects are observed on the oxide ion conductivity: Ti plays a detrimental role whereas the Ge improves the conductivity. A possible interpretation is that additional oxygen atoms are trapped in the coordination polyhedron of Ti4+ whereas they are forced to increase the coordination number of Ga3+ when Ge4+ is introduced in the structure. The highest conductivity at 800 °C, σ≈0.5×10−3 S/cm, is obtained for Nd4[Ga1.8Ge0.2O7.1□0.9]O2.

The development routes to improve the performances of SOFCs are globally well identified and lead to shift the characteristic i/v curve towards higher potentials and current densities, an effect that is consistent with a lowering of the three overpotentials (activation, resistance and concentration). An increase of the Triple Phase Boundary (TPB) density at the electrodes, a reduction of the materials thickness, the development of materials with improved performances, the optimization of electrodes microstructure all serve as guidelines that aim to this goal. Taking as an example the specific design of battery electrodes, we have explored how the corrugation of electrode/electrolyte interfaces at a mesoscopic scale in a SOFC could improve the electrochemical performances, from theoretical and experimental points of view. First, an electrochemical model was established and implemented in the COMSOL Multiphysics software, taking into account masses and charges conservation, gas transport and electrochemical reaction kinetics. The model results demonstrated that the presence of a periodic pattern (parallelepipeds, pyramids, ellipsoids) at the electrolyte/electrode interfaces, along with an electrolyte the thickness of which is significantly smaller than the dimensions of the pattern, could lead to a strong increase of the exchange surface, hence to higher exchange currents and cell performances. To that extent, it is theoretically shown that the patterns must have concave and convex singularities so as to confine the cathode material on the cathode side and the anode material on the anode side, hence the active layers on both sides, so that a much larger number of TPB are solicited and involved in the chemical reactions, reducing then the activation overpotential, as compared with a flat surface. It is also shown that the geometrical feature can be chosen so that it minimizes the concentration overpotential. In addition, the modeling provided us with key information regarding the characteristics of the patterns (shape, width and depth, distance between them, …) that should lead to a large gain in exchange current throughout the interface. For example, an increase of the exchange current of about 60% was calculated for a parallelepipedic pattern with 70 μm width and 100 μm depth, each geometric feature separated from the other by a few hundred microns. With the use of laboratory standard ceramic processes (tape casting, screen printing, bar coating, cold pressing, cold stamping), we have implemented such mesoscopic architectures on green self supported anodes (YSZ + Ni) on top of which a thin layer of electrolyte (YSZ) was deposited. As anticipated the electrical testing and impedance spectroscopy results show that such corrugation at anode/electrolyte interfaces improves significantly the electrochemical performances of the cell even though the sintering tends to alter and blunt the geometrical features of the pattern. Based on the electrochemical performances and modeling, the geometry of the pattern and its evolution during sintering is discussed in terms of activation and concentration overpotentials.

A new concept of a high temperature fuel cell based on a porous dual-conducting (H+ and O2-) central membrane was successfully developed as part of a European project (“IDEAL-Cell”, FET-Energy/FP7, 2008-2011). This additional layer ensures the connectivity between both SOFC cathode/electrolyte and PCFC electrolyte/anode “water-free” compartments so that the system operates with three independent chambers. During the development of this concept, it was discovered that BCY15, used so far as a protonic conductor, exhibited a very significant anion conduction at operating temperature under oxygen atmosphere, opening up the route for a simplified and more efficient configuration; since then, BCY15 is used in all the constituent layers to lead to the so-called monolithic concept. Therefore the porous dual-conducting central membrane is made of a single BCY15 phase instead of a composite mixture of a proton conductor (i.e. BCY15) and an anion conductor (i.e. YDC15). This CM is thus sandwiched between 2 BCY15 electrolytes with a BCY15 + Ni anode on one side and a LSCF48 + BCY15 cathode on the other side. Recent works proved that electrical performances of the monolithic configuration at 600°C are higher than those measured for standard SOFCs and PCFCs at equivalent geometry. The present work aims at presenting the first results of the manufacturing and electrochemical testing of a two-cells short stack specifically designed with 3 independent chambers in which central membrane (CM) supported monolithic cells are integrated. The supported-CMs were fabricated by using BCY15 and a polymer PMMA as a pore former agent. The CMs were sintered at 1400°C for 5 hours under static air leading to a porosity content of around 50 ± 3 vol. %. The electrolytes were then deposited onto the CMs by tape casting from an ethanol-based suspension containing BCY15 and 1 wt. % of ZnO, and were sintered at 1350°C for 3 hours. Anode ink containing 60 vol. % BCY15, 40 vol. % NiO and 10 wt. % of graphite was deposited on one electrolyte and sintered at 1350°C for 3 hours. Cathode ink containing 50 vol. % BCY15, 50 vol. % LSCF48 and 10 wt. % of graphite was deposited on the other electrolyte and sintered at 1100°C for 3 hours to avoid delamination and creeping. This processing sequence leads to flat cells having a total thickness lower than 1 mm. 2 cells were integrated in a specifically designed short stack made of INOX 440C, and sealed with Thermoculite 866 LS. The electrochemical performances of the short-stack have been measured in real operating conditions via a dedicated 3-chamber set-up named Real Life Tester (RLT), and compared to those of single cells operated under the same conditions. Results are discussed in terms of cells geometry and microstructure, and stack design improvements are proposed.

International audience; Recently a new design of solid oxide fuel cell (SOFC) named dual membrane fuel cell (dmFC) has been introduced and proved. It is based on the introduction of three independent compartments for hydrogen, oxygen and water, which eliminate the dilution of the fuel or the oxidizer respectively in the SOFC construction or in its proton conducting (pSOFC) modification. Due to the registered sufficient mixed ionic conductivity of BaCe0.85Y0.15O2.925 (BCY15), a simplified modification in respect to shaping technology and construction is introduced. It's performance is better than the one of pSOFC cell produced with the same material and shaping procedure: 76 mW/cm2 (dmFC thickness 1.1 mm) against 71 mW/cm2 (pSOFC thickness 0.66 mm) at 700 °C. Impedance measurements carried out down to 1 mHz ensure information about the water formation and behaviour in the separate central membrane compartment of the dmFC design, which brings to performance improvements, registered also in electrolyser mode of operation.

International audience; In-substituted BaSnO 3 (BaSn 1Àx In x O 3Àδ , x ¼ 0:125, 0.25 and 0.50) compounds were prepared by a gel polymerization method. Their microstructure, hydration energy and electrical properties were investigated. Single phases were confirmed by X-ray dif-fraction analysis over the whole range of substitution, and were identified as cubic perovskite-like structure. The grain size and conductivities of samples measured under dry and wet atmospheres increase significantly with indium content as such, 1À2 μm for BaSn 0:875 In 0:125 O 3Àδ (x ¼ 0:125) and 20 μm for BaSn 0:50 In 0:50 O 3Àδ (x ¼ 0.50). The highest proton conductivity at 600 ○ C, ¼ 9:5 Â 10 À4 SÁcm À1 , is obtained for BaSn 0:5 In 0:5 O 2:75 (x ¼ 0:50). Barium stannates with chemical formula derived from BaSnO 3 has recently been proposed as protonic conductors with potential applications in fuel cells. 1À6 It was shown that Y-substituted BaSnO 3 materials exhibit high proton conductivity in wet atmospheres. 5,6 Nevertheless, barium stannate based compounds exhibit a strong aversion against densification, i.e. the relative density reaches only about 90% with difficulty although sintered at 1600 ○ C for 12 h. 5,6 Such a high sintering temperature possibly results in barium volatilization leading to the decrease of proton conductivity, and also makes this oxide difficult to integrate electrochemical devices. 7,8 In the literature, the In element has received much less attention than the rare earth elements as a substituant for proton-conducting perovskites. 9À21 Matsumoto et al. 22 reported that electrical performances of BaCe 0:90 In 0:10 O 3Àδ are much more lower than those of the counterparts substituted with other trivalent dopants (Y, Tm, Yb and Sc). Zhang et al. 20 pointed out that the proton concentration and electrical properties of BaCe 0:80Àx Sm 0:20 In x O 3Àδ decrease when the indium content increases. However, Giannici et al. 21 reported that a high content of vacancies can be created by the substitution of Sn 4þ for In 3þ in BaSnO 3. Shober et al. 2 also point out that BaSn 0:5 In 0:5 O 2:75 (x ¼ 0:50) go along with high proton defects, then exhibits a high level of grain conductivity, nevertheless it is relatively unstable in severe reducing atmospheres. The literature showed that in substitution in BaCeO 3 plays a beneficial role on the stability and sinterability of proton-conducting perovskites in CO 2 atmosphere. 9,20 Moreover, indium as a second substituent improve the sinterability of BaZr 0:9 Y 0:1 O 3Àδ while yttrium enhance the proton conductivity. 18 In this paper, the effect of the substitution of Sn 4þ for In 3þ in BaSnO 3 on the micro-structure, water hydration and electrical properties has been investigated. BaSn 1Àx In x O 3Àδ (denoted x ¼ 0:125, 0.25 and 0.50 as BSIn125, BSIn25 and BSIn50 in the following) powders have been synthesized by a gel polymerization route from an aqueous solution containing cation salts. 5,6,23 The resultant precursors were heat-treated at 1200 ○ C for 4 h to obtain §

In this study, the synthesis, structural and electrical properties of BaSn 0.75 M 0.25 O 3− δ (M = Fe, Sc, In, Y, Gd, Sm, Nd and La) compounds have been investigated. Single phase powders of these materials were successfully synthesized by a gel polymerization route and sintered to obtain dense pellets. X-ray diffraction reveals that all compounds crystallise in a cubic perovskite like-structure (space group: Pm 3 ¯ m ) at room temperature. However, cell parameters of the substituted compounds do not evolve in accordance with the substituent radius, i.e. an abnormal drop was observed for elements with a large radius, i.e. Sm, Nd and La. A possible interpretation could be the partial incorporation of substituents into the Ba-site. The site incorporation selectivity of substituents into the Ba- and Sn-sites was investigated based on the study of lattice parameters and empirical defect models. Experiments show that the thermal expansion coefficients and electrical properties have no specific correlation with the nature of the substituent.

International audience; The influence of the sintering conditions on the electrochemical properties of the proton conducting electrolyte BaCe0.85Y0.15O3-delta (BCY15) and Ni - based BCY15 cermet anode for application in high temperature proton conducting fuel cell are investigated by electrochemical impedance spectroscopy. The results show that at lower sintering temperatures due to the formation of parasitic Y2O3 phase an increase of both the electrolyte and electrode resistances is observed. This effect is strongly reduced by enhancement of the sintering temperature. The obtained BCY15 conductivity (sigma = 2.5x10(-2) S/cm at 700 degrees C) is comparable with that of the best proton conducting materials, while the BCY15-Ni cermet (with ASR = 2.5 Omega cm(2) at 700 degrees C) needs further optimization. The results of impedance investigations of BCY15 as proton conducting electrolyte and cermet anode have been applied in development of innovative high temperature dual membrane fuel cell.

International audience; The effects of ZnO additive on the phase formation, microstructure and electrical conduction of Y-doped BaSnO3 have been investigated. The single-phase and dense BaSn0.75Y0.25O3−δ compound with 4 mol% ZnO additive was successfully prepared after sintering at 1300 °C, which significantly reduces the sintering temperature. The conductivities measured under dry and wet air atmospheres reveal that the bulk conductivity of BaSn0.71Y0.25Zn0.04O3−δ is much lower than that of BaSn0.75Y0.25O3−δ. However, ZnO as a sintering aid does not affect the bulk conductivity. The total conductivity of BaSn0.75Y0.25O3−δ with ZnO as the sintering aid is slightly higher than that of unmodified BaSn0.75Y0.25O3−δ, and reaches 2.4 × 10−3 S cm−1 at 621 °C. Therefore, this material can be used as a proton-conducting electrolyte for intermediate temperature solid oxide fuel cells.

This paper presents results for the conductivity of BaCe0.85Y0.15O2-δ (BCY15) measured by electrochemical impedance spectroscopy in wet hydrogen and in Air, as well as test results from the application of the material in a new dual membrane fuel cell configuration (“IDEAL Cell”), in which the water is produced and evacuated through a separate chamber. The conductivity of dense BCY15 in wet hydrogen atmosphere at 700 °C is 2.0 10-2 S/cm. The measured values in air are of the same order. Preliminary tests of the material in the new cell design, where the three types of conductivity (protonic, oxide ion and mixed) are used in different cell compartments, are successfully performed.

International audience; Ba2SnYO5.5 nanopowders were synthesized by a gel polymerization method. In this process, a gel containing Ba, Sn and Y cations has been obtained by the polymerization of acrylic acid using N,N′-methylene bis-acrylamide as a cross-linking reagent and hydrogen peroxide as an igniting reagent. The gel was calcined at 1200 °C, giving rise to the Ba2SnYO5.5 single-phase nanopowders with the grain size ranging from 50 nm to 70 nm. Nanopowders were sintered at 1150 °C by spark plasma sintering (SPS) to obtain dense nanostructure materials (> 95%) containing grains whose size ranges between 70 nm and 100 nm. Nanostructured Ba2SnYO5.5 shows a good chemical stability in wet atmosphere. However, its protonic conductivity decreases compared with that of microcrystallin Ba2SnYO5.5 ceramics.

IDEAL-Cell is a new concept of a high temperature fuel cell operating in the range 600-700 °C. It is based on the junction between the anode part of a PCFC and the cathode part of a SOFC through a mixed H+ and O2- conducting porous ceramic membrane. This concept, extensively described in the present paper, aims at avoiding all the severe pitfalls connected with the presence of water at the electrodes in both SOFC and PCFC concepts. Spark Plasma Sintering samples were designed specifically for proving the IDEAL-Cell concept. The first electrochemical results obtained at 600 °C under hydrogen on millimeter thick samples show that IDEAL-Cell behaves like a high temperature fuel cell. It is estimated that the overall efficiency of this new concept should greatly surpass that of standard SOFCs and PCFCs and that the material constraints, especially in the case of interconnect materials, should significantly decrease.

International audience; Atomistic simulations have been used to investigate energetic aspects of defects in pure and doped BaSnO3, in close comparison with experimental data. Semiempirical potentials were used to determine the solution energies of trivalent cation on either A or B site of the perovskite structure. Density functional theory calculations were also performed to gain further insight into the redox properties of this material. Both approaches give a complete view of the way doping atoms will be inserted inside the BaSnO3 structure. For instance, we calculated with good precision the evolution of cell parameters with dopant nature and concentration. Then, by taking into account dopant vacancy interactions, we also show that the oxygen vacancy preferentially locates close to the dopant in the case of small size dopant while stays at the second neighbor position in the case of bigger dopant. These calculations also show that In and Y are the dopants giving the most homogeneous energetic map for oxygen vacancy movements and might thus be anticipated as the best acceptor-like dopants for enhanced anion conduction properties. Finally, we calculated an oxygen ion migration activation energy of 0.65 eV.

International audience; This work explores experimental procedures for tape-cast proton conducting ceramic fuel cells (PCFC) based on Yttrium-doped Barium Cerate (BCY10). The work is based on several years experience on aqueous tape-casting applied to the shaping of YSZ-based SOFC: however, water-based tape casting of BCY10 appeared to be impracticable for reasons associated with the high basicity of this material that results in rapid hydrolysis when in contact with water. Organic tape casting was therefore developed for BCY10, but only on Electrolyte (BCY10)/Anode (BCY10 + NiO) half cells since up to now no cathode material is available. Planar 20 mm diameter circular half-cells were obtained with the aid of a small load on top of the bi-layer to counterbalance the inevitable warping of the samples. Back-scattered SEM and X-Ray computer-controlled microtomography showed sedimentation of some large grains in the green tapes which are believed to have formed by a mechanism associated with a porosity gradient. The deformation occurring during sintering was modelled taking into account the elastic, thermal, viscoplastic and sintering components of the total deformation. 2D and 3D Finite Element numerical simulations showed that the driving force for deformation is associated with this porosity gradient.

The IDEAL-Cell concept is based on the junction between a PCFC anode/electrolyte section and a SOFC cathode/electrolyte section, through a mixed proton and oxide ion conducting porous ceramic membrane, operating in the temperature range 600-700{degree sign}C. Recombination of ions takes place within the junction, or central membrane (CM), and water vapor is evacuated through open porosity. The first results on the fabrication of multilayered samples reproducing the IDEAL-Cell structure are reported here, together with electrochemical tests carried out on selected samples in order to evaluate the performances of the chosen materials and to demonstrate the feasibility of this innovative concept of fuel cell. Stable OCV and power generation were obtained in the multilayered structures. Anomalies on the I/V curves and impedance measurements under large perturbation could be considered as proofs of water formation inside the central membrane.

Oxy-apatite materials La 9.33 + x Si 6 O 26 + 3 x /2 are thought as zirconia-substitutes in Solid Oxide Fuel Cells due to their fast ionic conduction. However, the well-known difficulties related to their densification prevent them from being used as such. This paper presents strategies to obtain oxy-apatite dense materials. First, freeze-drying has been optimized to obtain ultrafine and very homogeneous La 9.33 + x Si 6 O 26 + 3 x /2 (0 ≤ x ≤ 0.67) nanopowders. From these powders, conventional and Spark Plasma Sintering (SPS) have been used leading to very dense samples obtained at temperatures rather lower than those previously reported. For instance, SPS has allowed to prepare fully dense and transparent ceramics from 1200 °C under 100 MPa. The microstructure and transport properties of such samples have been then evaluated as a function of sintering conditions and lanthanum content. It will be show that for lanthanum content higher than 9.60 per unit formula, the parasitic phase La 2 SiO 5 appears leading to a degradation of conduction properties. We also show that grain boundaries and porosity (for conventionally-sintered materials) seem to have blocking effects on oxygen transport. The highest overall conductivity values at 700 °C, i.e. σ 700 °C = 7.33.10 − 3 S cm − 1 , were measured for La 9.33 Si 6 O 26 material conventionally-sintered at 1500 °C which contains bigger grains' size by comparison with σ 700 °C = 4.77.10 − 3 S cm − 1 for SPS-sintered materials at the same temperature but for few minutes. These values are associated with activation energies close to 0.83–0.91 eV, regardless of sintering condition, which are commonly encountered for anionic conductivity into such materials.

An alternative route to acrylamide gel polymerisation is presented for the preparation of oxide nanopowders. Within this method, cations first dissolved in an aqueous solution are subsequently trapped inside a 3D chelating gel beforefast-drying, which leads to a stable precursor. In this study, we have prepared compositions corresponding to a wide range of state-of-the-art fuel cells materials such as: Zr0.85Y0.15O1.875, Ce0.9Gd0.1O1.95, Nd1.95NiO4+x, GdBaCo2O5+x, Ba0.5Sr0.5Co0.8Fe0.2O3–x and La0.75Sr0.25Cr0.5Mn0.5O3. Thermogravimetry and differential thermal analysis studies have been performed to determine the best calcination conditions. After calcination, the resulting powders have been characterised by X-ray diffraction, FEG-scanning electron microscopy and TEM. The synthesised powders are of nanometric crystallite size and have flake-like aggregates; pure phases are obtained at low temperature. We made this method more attractive by using a nontoxic polymer and a nonexplosive initiator. This route allows the easy preparation of more than tens of grams of nanometric powders in one batch. Finally, since this process is largely independent of the formula of the compound, it can easily be extended to other kinds of oxides such as ferroelectric, CMR (colossal magnetoresistance) or gas sensor compounds.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

International audience; The hydration properties of Lanthanum-doped barium stannate have been studied using density functional calculations. The interaction energy between elementary defects (proton-dopant and oxygen vacancy-dopant) have been calculated in charged states corresponding to p-type conditions. Surprisingly, the most attractive energies are not found for first-neighbor relative positions. The geometric distortions and the electronic densities of state in the different configurations involved are presented. With these data, two simple models are used to estimate the energy of the hydration reaction.

Lanthanum silicate oxyapatites, ion-conducting materials presenting a strong aversion against densification, have been obtained in the form of dense transparent ceramics, by combining the beneficial use of freeze-drying and spark plasma sintering methods.

In the frame of a FP7 project (FET-Energy “IDEAL-Cell”, 2008-2011), we have developed with a European consortium a new type of high temperature fuel cell based on a dual membrane with mixed proton and oxygen ions conductivity. This dual conductivity porous membrane, made of a proton conducting phase (BCY15) and of an oxygen conducting phase (YDC15) was sandwiched between a dense BCY15 electrolyte and a dense YDC15 second electrolyte, forming a tri-layer itself sandwiched by a cathode (LSCF48) and an anode (BCY15 + Ni). In this configuration, water is formed neither at the cathode nor at the anode, minimizing electrodes concentration overpotentials, but rather within the dual membrane which interconnected porosity ensures water evacuation. The IDEAL-Cell project i) proved the concept, ii) showed that this new fuel cell was performing better at 750°C than PCFCs and SOFCs having equivalent thickness, iii) showed that the concept was fully reversible with a high dynamic when shifting from the fuel cell regime to the electrolyzer regime, iv) showed that BCY15 was also an excellent oxygen ions conductor when fed with oxygen, and then demonstrated that the cell could be fabricated by using solely BCY15 (replacing YDC15 in the dual membrane and for the oxygen electrolyte), leading to a drastic simplification of the concept, hence of the shaping process. Today, this simplified cell is fabricated at the laboratory scale by a sequence of successive steps (1/ cold pressing, rolling and sintering of the porous dual membrane at 1350°C, 2/ deposition of both electrolytes by dip coating, and sintering at 1350°C, 3/ deposition of both cathode and anode by bar-coating, and sintering at 1150°C), which is hardly cost effective in view of stacking and further development. The present work proposes to demonstrate that co-sintering of the whole cell in a single step is possible, and to determine under what conditions of geometry, starting materials and sintering cycle a flat and stress-free cell in view of stacking, efficiency and durability can be produced via this simplified process. The research described in this paper was based on the thermomechanical modelling of the multilayer deformation occurring during sintering via a finite element numerical simulation, in which debinding, elastic and irreversible deformations, kinetics of grain growth and pores shrinkage are integrated. The thermomechanical parameters were obtained on a differential CTE measurement set-up, and the microstructural ones by image analysis on SEM images.

The EVOLVE cell, named after a running FP7 project, is an innovative SOFC concept that integrates advanced materials, providing multiple functionalities (electrochemical activity, ion or/and electron conduction, gas diffusion…), and an inventive anode current collector made of a NiCrAl preoxidized foam impregnated with a conductive perovskite. Therefore, this anode combines the beneficial characteristics of MSC and ASC technologies, and strong improvements in terms of reliability and durability (inter-diffusion, Ni coarsening and agglomeration, sulphur tolerance, mechanical robustness and chemical stability under redox cycling in temperature) are expected. In order to enhance the electrode efficiency, the 3D microstructure needs to be precisely described at the microscopic scale by a morphology model. An optimized microstructure (controlled pores size, morphology and connectivity, grains size distribution, 3D tortuosity and 3D percolation of phases, volume fraction of phases, final thickness of components …) is closely related to the starting parameters (nature and quantity of starting powder, binder, shaping aids…) and to the control of the shaping process parameters (sintering treatment and atmosphere). It should meet the best compromise between a good electrocatalytic activity and a low ohmic resistivity while ensuring a stisfactory long term thermomechanical stability on the life time of the device. Based on a mathematical morphology approach applied on symmetrical LST/CGO anode layers, the present work aims at showing how a morphology model can be establised and 3D microstructural data relevant for shaping and performances can be derived from back-scattered electrons (BSE) SEM observations. For each sample, a series of images were used as input information for the microstructural modelling. All the original images were first filtered to remove the noise, thresholded and finally a series of morphological operators (openings, surface openings, closings and reconstruction with markers) were applied to remove artefacts. A 3D pluri-Gaussian model has been generated from 2D images, and then computed to predict volume fraction of the 3 phases, 3D tortuosity, gas permeation and ionic conductivities. These results were used for the optimization of the shaping process that includes the integration of the metal foam.

7 pages; International audience; New oxygen ion conductors have been prepared by substituting Ga3+ for Ge4+ in Nd4GeO8, Zn2+ and Mg2+ for Ga3+ in Nd3GaO6, and Ca2+ and Sr2+ for Nd3+ in Nd3GaO6. A combustion technique using ethylenediamine tetraacetic acid has been developed to synthesize these materials at ~900 C, leading to powders with spherical particles of about 100-200 nm. The green pellets obtained through the combustion-synthesized powders could be sintered to ~98% at 1250 C. It was found that Nd4Ge1-xGaxO8-x/2 are formed up to x = 0.10, Nd3Ga1-xMxO6-x/2 (M = Zn, Mg) up to x = 0.03, and Nd3(1-x)M'3xGaO6-3x/2 up to x = 0.03 and 0.015 for M' = Ca and Sr, respectively. These relatively small substitution rates induce a significant increase in oxygen ion conductivity (800C = 0.2 × 10-2 S cm-1 for Nd4Ge0.9Ga0.1O7.95; 800C = 0.5 × 10-2 and 0.4 × 10-2 S cm-1 for Nd3Ga0.97M0.03O5.985 with M = Zn and Mg, respectively, and 800C = 0.6 × 10-2 and 0.7 × 10-2 S cm-1 for Nd2.91Ca0.09GaO5.955 and Nd2.955Sr0.045GaO5.9775, respectively) with respect to pure Nd4GeO8 (800C = 2.8 × 10-4 S cm-1) and pure Nd3GaO6 (800C = 2.7 × 10-4 S cm-1).

The structural, dynamical and dielectric properties of the cubic phase of perovskite barium stannate BaSnO3, a potential candidate as protonic conductor for solid oxide fuel cells, have been investigated by the means of first-principles density functional calculations, and the structural and electrical properties have been explored at low temperature. From density functional perturbative calculations, the phonon modes, the Born effective charges and the dielectric tensor are derived and analyzed, at zero pressure. The phonon band-structure of the cubic phase does not exhibit unstable modes, in good agreement with x-ray diffraction, which shows that BaSnO3 remains perfectly cubic down to 10 K. The dielectric response in BaSnO3 as measured and calculated is lower than in titanate and zirconate perovskites.

This work is dedicated to the development of a numerical model to optimize the anodeelectrolyte interface geometry for improving the electrochemical performance of a SOFC cell in a 3- Dimensional SOFC simulation process. A SOFC repeated unit composed of anode, electrolyte and cathode is taken as a modeling target. The model is implemented by considering electron and ions conservation, gas transport and electrochemical reaction kinetics in FEM (finite element method) software COMSOL Multiphysics. Four different interface geometries—flat, pyramid, ellipsoid and cube, which are predicted to show better electrochemical performance due to increased TPBs and less activation resistance, are investigated to find the best morphology according to their simulated performance. Results reveal that the cubic interface case achieves the highest current density at fixed 0.7V cell operating voltage, 63.55% higher than the flat reference interface. Distribution of different electrochemical variables is also obtained in each condition, giving valuable information for further optimization.