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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.

In this study the technique of the Differential Impedance Analysis (DIA) is applied for distinction of two models which give similar impedance diagrams presented with slightly depressed semi-circles—two time-constants Voigt model with high degree of mixing and one time-constant model in which the capacitance is replaced with constant phase element (CPE). Those two models are often used for impedance investigations of solid state. The applicability of the recognition procedure is confirmed on synthetic models as well as on real samples of yttria-stabilized zirconia (YSZ) and Er-doped BaTiO 3 .

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.

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.

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.

During the last years the method of the Electrochemical Impedance Spectroscopy has been intensively developed. It is the most powerful electrochemical method, because of it unique ability to investigate the structure and the properties of the objects in a wide frequency range. The Capacitive Impedance Spectroscopy (CIS) is of special interest. The further development of this principle is given in the current work. It gives an opportunity to investigate the electric properties of different kinds of materials. A special measuring system is developed for this purpose. A modern computer techniques with a special data analyze software is also described.

Electrochemical impedance spectroscopy is one of the most informative tools for investigating of the complicated processes in electrochemical power sources. The interest arises from the unique advantage of the method's capability to separate and characterize, in a single measurement, the different steps involved in a complex process. Impedance spectroscopy finds a variety of applications for studies of different energy sources – batteries, fuel cells, supercapacitors, and other systems – and has different targets – studies of basic kinetic processes, materials research, quality control, matching, and diagnostics. The principle of the method and a set of useful techniques providing for reliable experiments and data interpretation are described. The selection of the measurement setup configuration and details of the experimental program are discussed. Techniques for correction of the errors due to cable inductance and to possible time evolution of the object are described. The unique method for soft computing – differential impedance analysis, which corresponds to the nature of power sources – is also explained. Examples of experimental data – raw and processed – illustrate the recommended techniques and their application in different studies.

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 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 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.

The impedance study of non-stationary systems is of a great interest to electrochemistry concerning the investigation of real objects in low and infra low frequency ranges. The method described in this paper requires the measurement of a consecutive series of impedance diagrams during the time of the system evolution. The analysis of this four-dimensional data set is based on the hypothesis of a continuum of the state and parameter spaces. This approach allows the estimation of the non-stationary transfer function of the system presented by its instantaneous impedance projections. The method described is applied to simulated data and the validity of the non-stationary estimation is verified.

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.

The main problem in parameter estimation of electrochemical systems is related to their significant response in a very large frequency range. Using the classical least-squares method leads to the solution of “ill-conditioned” matrices. To avoid this problem, a model reduction method has been developed for parametrical identification of electrochemical systems in the frequency domain. According to this approach, the system is represented by reduced lumped parameter submodels. Parameter estimation of these submodels is consecutively performed using simple least-squares estimators on the basis of the impedance data reduced for the participation of the previous submodels. For more efficient computer handling of the models, a matrix description of the electrochemical impedances is introduced. A number of simulated models and measured data are identified showing the applicability of this method.