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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.

Water behavior studies were performed in porous ceramic media based on proton conducting yttrium-doped barium cerate BaCe0.85Y0.15O3-α (BCY15), which is a component of a new high-temperature dual membrane fuel cell (dmFC) design. Complex permittivity measurements were carried out on porous samples at room temperature. A new phenomenon was observed during wetting—a gigantic enhancement of the real component of the capacitance at lower frequencies. A possible explanation is the formation of semi-liquid layer with dipole structure on the pores walls, which is supposed to be organized also at operating temperatures. Since the electrochemically active volumetric layer facilitates the water formation, it should improve the operation of the dmFC by decreasing its resistance, which is experimentally confirmed. This phenomenon can be of importance also for classical proton conducting solid oxide fuel cells, as well as for operation in electrolyzer mode.

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 paper reports impedance studies of the cathode/electrolyte behaviour in solid oxide fuel cells (SOFC), based on comparative investigation of half-cells with yttria stabilized zirconia (YSZ) electrolyte and different cathode materials: lanthanum strontium manganite (LSM), and composite LSM/YSZ with low ionic conductivity as well as the electron conducting Ag, Pt and Au. For improved impedance data analysis the technique of the differential impedance analysis is applied. It ensures structural and parametric identification without preliminary assumptions about the working model. It is found that despite the low ionic conductivity of LSM, the cathode reaction of the oxide cathode materials is a two-step process including: (i) charge transfer with activation energy of the resistivity E a increasing with the temperature and (ii) transport of oxygen ions through the bulk of the electrode (rate-limiting stage) with E a independent on the temperature. For the metal (electron conducting) electrodes, the reaction behaviour is described with one step process with higher E a at higher temperatures. The activation energy of the electrolyte conductivity decreases with the increase of the temperature. The observed changes in E a for the electrolyte and the cathode reaction (the charge transfer step for the LSM-based electrodes) appear in the same temperature interval. This interesting coincidence suggests for correlation between the bulk (electrolyte) and surface conduction properties. Approaches for improvement of both the ionic conductivity and the supply with electrons in LSM should be also searched.

This work shows the enhanced capability of differential impedance analysis (DIA) for recognition of CPE behavior. It is based on the procedure of a secondary DIA, which analyses the frequency dependence of the local operating model parameters' estimates in the zones where there is no adequacy between the object and the local estimator. The new algorithm is successfully examined on synthetic models of CPE, Warburg, Randles, Randles with CPE modification, simple Faradaic reaction with CPE capacitance, as well as on real data obtained on yttrium iron garnet single crystals and yttria stabilized zirconia single crystals.

The effect of tap water contaminants on the kinetics of the hydrogen evolution reaction on a nickel electrode in 1 mol dm(-3) KOH was investigated by galvanostatic polarization and electrochemical impedance spectroscopy techniques. It was found that the tap water contaminants lead to an increase in the overpotential of the hydrogen evolution reaction, especially at low temperatures. The combination of electrochemical techniques, as well as physicochemicals such as SEM and EDAX ones, confirmed that the contaminants are specifically adsorbed and blocked the available electrode surface for the reaction. It was concluded that they do not participate in an electrochemical reaction in the potential region where HER occurs. Besides the short term negative impact on the rate of hydrogen evolution, a 55 h test revealed that the overpotential shows a steady increase over time in presence of tap water contaminants, while in absence of these contaminants the overpotential is constant. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Most electrochemical objects are intrinsically nonstationary or contain valuable evolving phenomena. This paper presents the essentials of the theory of Nonstationary Impedance Spectroscopy (NIS) including the ideas of nonstationary and instantaneous impedance, the new mathematical background and the main approach to the development of nonstationary impedance models. The practical issues for the application of the new techniques are discussed.

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.

Identification of the appropriate model is the aim of experimental data analysis. Some principal problems of the classical approach for parametrical identification are discussed. A new method for structural identification where the working model is derived from the experimental results is developed. Some recent results which expand the applicability of the method are also given.

The problems of non-stationary electrochemical impedance spectroscopy are discussed by an analysis of the errors of the classical Fourier transform applied in non-stationary conditions. The essentials of the theory of generalized Fourier transform, called rotating transform, capable of handling non-stationary signals, are described. The application of the rotating transform to impedance analysis of non-stationary electrochemical systems is emphasized.

The problem of non-stationarity plays an important role in the development of impedance spectroscopy. The main reason is due to the fact that most electrochemical systems possess an intrinsic non-stationary nature. Instantaneous impedance is the theoretical basis for solving this problem. The present paper is devoted to an experimental technique for estimation of the instantaneous impedance. On the basis of the four-dimensional approach the experimental design for measuring the instantaneous impedance is described in detail. A special technique is proposed for validation of the modelling, based on the a posteriori evaluation of the model flexibility.

Structural simulation is a new approach to modeling of more complex electrochemical systems, the impedance of which is hard to present in an explicit analytical form. This method is based on the parameters and structure of the impedance model only and eliminates the solution of the impedance equations. It renders possible the simulation of complex models with variation of both their parameters and structures and provides a complete computerization of the simulation process.