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Solid Oxide Fuel Cells (SOFCs) are regarded as a promising technology for economic power generation due to their high efficiency and large fuel flexibility. Durability is a severe hurdle towards their deployment. The near future targets in respect to Degradation Rate (DR) are about 0.1% kh−1, which needs improved monitoring and diagnostics. This work aims at introducing a new approach based on Differential Analysis of the i-V curves, named DiVA. It operates with the Differential Resistance Rd and its evolution during long term testing. Two new performance indicators are introduced. Since derivatives are more sensitive to small deviations, the Differential Resistance Analysis (DRA) ensures increased sensitivity and information capability in respect to degradation monitoring and diagnostics, which is demonstrated on a small stack during thermal cycling conditions – before and after the first thermal cycle, on button cells tested up to 9000 h, as well as on button cells operating in fuel cell and in electrolysis mode. The results show that DRA is several times more sensitive in comparison with the classical DR evaluation based on registration of the voltage decrease at constant current.

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 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.

This work presents results of a measurement setup which was constructed for understanding the performance contributions of the individual compartments and reactions taking place in the patented concept of IDEAL-Cell (described in detail in[1]). By inserting a third measurement point at the central membrane of an ideal cell it was possible to measure the cell in three different modes corresponding to hydrogen, oxygen compartment and the full cell. Recorded maximum power densities of (~4, ~7, 11, 16) mW/cm² at (600, 650, 700, 750) °C in H2-O2 atmosphere respectively were observed which were similar to the values of proof of concept ideal cell samples tested so far. Moreover in our setup it was possible to record higher maximum power densities for the hydrogen (proton conducting) compartment which was (~14, 22, 33*, ~38*) mW/cm² at (600, 650, 700, 750) °C.

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.

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.

Permittivity Spectroscopy is a branch of the Impedance Spectroscopy specially tuned for measurements and analyses of dielectrics permittivity properties. The present paper presents experimental results on permittivity properties of composite objects in which a polarizable dielectric is distributed in a fine non-polarizable matrix (solid or liquid) measured in frequency range 1 MHz down to 0.01 Hz. Two types of objects are studied - water in porous functional ceramics and lubricating oils. In both systems gigantic enhancement of the effective capacitance is observed. The first series of experiments was performed on porous membranes of yttrium doped barium cerate, which is a proton conducting ceramics with hydrophilic properties. At a given level of watering the measured capacitance is sharply increasing (3 to 5 orders of magnitude) in the lower frequency range. The second example covers permittivity study of lubricating oils, where the increase is 2-3 orders of magnitude. The phenomenon of gigantic enhancement of the effective capacitance could be related to a formation of dipole volume structures induced by the external alternating electrical field.

The purpose of this study is to compare the investigations of amorphous magnetic ribbons combining the usual structural methods with physical impedance spectroscopy (PIS) and X-ray photoelectron spectroscopy (XPS). The samples are commercially available rapidly quenched ribbons for torroidal transformers. Two types of ribbons were thermally treated below the temperature of amorphous–crystalline phase transition. Two binding energies of B 1 s electrons are determined by XPS in one of the as-quenched ribbons, which show impedance time constant spectra without dispersion. The other ribbon, possessing only one value of binding energy of the B 1 s electron (B I state) in a rapidly as-quenched state, undergoes transition to a still amorphous state but acquires high electromagnetic losses and show another value of the binding energy of the B 1 s electron (B II state) after the heat-treatment procedure. This latter type of ribbons show structural impedance spectra containing various time constant values.

The nanosized Co, B, H, particles synthesized by the ‘tea’ and ‘antigravity’ methods using a borohydride reduction process have been subjected to structure and composition studies by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and selected area diffraction (SAD). The amounts of the elements Co, B, O2, H2 and C as mean volume values, and surface values for the as-prepared particles, as well as after Ar+ etching to a depth of about 15 nm and 30 nm from the initial particle surface, are determined. About 1.5 atoms of cobalt per atom of boron correspond to samples obtained by the ‘antigravity’ method. The binding energy (BE) of 1s electrons of boron atoms has only one value. These particles are angular and are in the typical nanocrystalline state. In the case of samples prepared by the ‘tea’ method, two atoms of cobalt per atom of boron are found. The presence of two kinds of BE (B1 and B11) of 1s electrons of boron atoms in the particles obtained by the ‘tea’ method is observed and almost equal amounts of these two states are established in the spectrum. The particles' shape and structure are typical of the amorphous state. The fact that there is one peak when the ‘antigravity’ method is applied, in contrast to the two peaks with the ‘tea’ method indicates the presence of a metal amorphous state in the latter case.

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.

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.

The optimization of Ni-YSZ anodes is an important step in obtaining electrically efficient anode supported SOFCs. Despite the continuous studies concerning optimization of the anode reduction conditions, degradation phenomena, oxidation-reduction cycling behaviour and microstructural changes, the anode reduction continues to be an active field for deeper insight [1,2], since as a final stage of the cell preparation, it offers an opportunity for optimization in respect to mechanical stability, electronic conductivity, catalytic activity and gas permeation. This is additionally complicated due to the counter-flow of the fuel and exhaust water vapour. This study combines two approaches – impedance spectroscopy and gas permeability studies that ensure experimental information about the main processes occurring during anode reduction: (i) formation of a new microstructure influencing the diffusive mass transfer and (ii) development of an electrically conductive network. The gas permeability studies [3] are carried out on anode pellets at room temperature before and after reduction performed at different conditions (temperature, reduction atmosphere). The observed changes are discussed and compared with SEM microstructural analysis. The impedance measurements are carried out directly during the reduction of button cells, segmented cells and NiO-YSZ anode pellets and thus the reduction process is studied during its evolution. The influence of the reduction conditions, including the duration of the process, is discussed in respect to the formation of an optimal Ni network with good electrical connectivity and catalytic activity. The presented figure shows two impedance diagrams measured on a segment from segmented fuel cell in different stages of the reduction process . Acknowledgement:The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU-2013-1) under grant agreement No 621207. References S.-W. Baek, J. Bae, Internat. J. Hydrogen energy 36 (2011) 689 B. Liu, Y. Zhang, B. Tu, Y. Dong, M. Cheng, J. Power Sources 165 (2007) 14 E. Mladenova, D. Vladikova, Z. Stoynov, A. Chesnaud, A. Thorel, M. Krapchanska, Bulg. Chem. Communic. 3 (2013) 366 Figure 1

The successful application of the Impedance Spectroscopy for measurement and quality control of batteries should obey a number of specific requirements, correspondent to the nature of the batteries as objects under investigation. Batteries are nonlinear and non-stationary large statistical systems with distributed parameters in macro- and micro-scale. Processes of charge and mass-transfer take place in their electrodes, changing the chemistry and the habitus of the active electrode materials. Starting with the very first successful measurement of a small Lead-acid battery in 1976 [1], we developed a set of complementary techniques providing for reliable measurements of battery impedances, data handling and analysis. The techniques for inductive errors evaluation and correction, for the 4-Dimentional data treatment [2] of the evolving battery State of Charge will be discussed in details. Special attention will be given to the interpretation of the battery impedance as a property of a fuzzy system with un-known parameters distribution. The Differential Impedance Analysis which solves this problem and the related Spectral Transforms are powerful tools for batteries evaluation and quality control [3]. These techniques will be illustrated with practical examples of impedance measurements of large industrial Lead-acid and Li-ion batteries and stacks fabricated by different technologies and for different purposes. Some related side phenomena as: (i) presence and destruction of the passive layer; (ii) increased inductance of stacks as well as impedance observation of local in-homogeneities in Li-ion batteries will be also discussed. References 1. M. Keddam, Z. Stoynov, H. Takenouti, J. Appl. Electrochem. 7 (1977) 539. 2. Z. Stoynov in: C. Julien, Z. Stoynov (Eds.), Materials for Lithium- ion Batteries, Kluver Academic Publishers, 3/85 (2000) 359. 3. Z. Stoynov and D. Vladikova, in Encyclopedia of Electrochemical Power Sources, edited by U. Garche ( Elsevier, 2009) pp. 632-642.

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