Zdravko Stoynov

Impedance modelling and data processing: structural and parametrical estimation

Abstract 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.

Nonstationary impedance spectroscopy

Abstract 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.

Four-dimensional estimation of the instantaneous impedance

Abstract 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.

Rotating fourier transform—new mathematical basis for non-stationary impedance analysis

Abstract 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.

IDEAL-Cell, Innovative Dual mEmbrAne fueL-Cell: Fabrication and Electrochemical Testing of First Prototypes

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°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.

Gigantic enhancement of the dielectric permittivity in wet yttrium-doped barium cerate

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.

Towards Understanding the Dual Membrane Fuel Cell (IDEAL-Cell) Using a Metallic Central Membrane

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 this 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.