Zdravko Stoynov – the scientist who created new scientific horizons

This paper is in the memory of the big Bulgarian scientist Professor Zdravko Stoynov. An original and non-standard personality with the spirit of a gentleman, he turned all that he touched into a masterpiece. His innovative thinking which made him one of the world’s leaders in Electrochemical Impedance Spectroscopy, chiseled away at the scientific taboos and created new scientific horizons. Zdravko Stoynov left us an abundant heritage – his spirit, his interminable capacity to wonder, his developments and inventions – all that he has taught us and brought up with. It has no dimensions but rather carries a unique value

Editorial

This Special Issue of the Journal of Electrochemical Science and Engineering presents papers form the Fifth Regional Symposium on Electrochemistry – South East Europe (RSE - SEE5) held in Pravetz, Bulgaria, June 7 - 11, 2015

Permeability of gases In the anode of An anode-supported SOFC

International audienceIn the high current density regime, the performance of SOFCs is limited by concentration overpotentials when the demand for reactants exceeds the capacity of the porous cermet anode to supply them by gas diffusion mechanisms, and when the rate at which water is produced exceeds the ability of the anode to evacuate water. A compromise should be found between maintaining a high level of activation in the anode, which means a high density of Triple Phase Boundaries (TPB) usually associated with small grains size and small pores, and high gas permeability generally associated with large interconnected pores. The present work aims at determining experimentally the gas permeability of the anode as a function of the percolation, tortuosity and volume fraction of the pores. Anodes with varying porosity ensured by different quantities of pore former were obtained by cold pressing and sintering. SEM image analysis, mercury intrusion porosimetry and permeability measurements for different gases were performed. However, non-linearity as a function of the gases molecular weight is observed

Zeolite based carbon-free gas diffusion electrodes for secondary metal-air batteries

In recent years, secondary metal air batteries have received considerable attention as promising technology for energy storage in combination with renewable energy sources. The oxidation of carbon in conventional gas-diffusion electrodes reduces the life of the secondary metal-air batteries. Replacement of the carbon-based material with zeolite is a possible solution for overcoming this problem which is the aim of this work. Zeolite is a natural or synthetic porous material which provides the necessary gas permeability. The required hydrophobicity of the electrodes is ensured by mixing the zeolite with an appropriate amount of polytetrafluoroethylene following a specially developed procedure. The experiments are performed in a home designed test cell which ensures measurements in both half-cell and full cell configuration. In this study the testing is carried out in 3-electrode homemade half-cell configuration with hydrogen reference electrode. The cell was subjected to cycling at charge/discharge current ±2 mA cm-2 respectively. The obtained results show that the replacement of carbon with zeolite in the gas diffusion layer is a promising direction for optimization of the gas diffusion electrode

Zeolite Based Air Electrodes for Secondary Batteries

In recent years, secondary batteries received considerable attention as promising technology for energy storage in combination with renewable energy sources. The oxidation of carbon in conventional air electrodes reduces the life of secondary batteries. One possible solution for overcoming this problem is the replacement of carbon material with zeolite.Zeolite is a natural or synthetic porous material with crystalline structure which provides the necessary gas permeability. The required hydrophobicity of the electrode is ensured by mixing zeolite with an appropriate amount of polytetrafluoroethylene following a specially developed procedure. The main purpose of the present research is to discover the optimum level of hydrophobicity (impregnation) of zeolite. Moreover, appropriate amount of PTFE will ensure better mechanical stability and long charge/discharge cycle life.The results from this study show that the replacement of carbon with zeolite in the gas diffusion layer is a promising direction for optimization of the bi-functional air electrode. The relationship between the particle size and the hydrophobicity of the electrode was found. It was found that the mechanical stability and hydrophobicity of the electrode improved with the replacement of the emulsion powder. The gas permeability is maintained in the norms, which guarantees the good performance of the electrode. More than 200 charge/discharge cycles were reached

Accelerated Stress Tests for Solid Oxide Cells via Artificial Aging of the Fuel Electrode

Solid Oxide Cells (SOCs) are under intensive development due to their great potential to meet the 2030 targets for decarbonization. One of their advantages is that they can work in reversible mode. However, in respect to durability, there are still some technical challenges. Although the quick development of experimental and modeling approaches gives insight into degradation mechanisms, an obligatory step that cannot be avoided is the performance of long‐term tests. Taking into account the target for a commercial lifetime is 80,000 h, experiments lasting years are not acceptable for market needs. This work aims to develop accelerated stress tests (ASTs) for SOCs by the artificial aging of the fuel electrode via redox cycling, which follows the degradation processes of calendar aging (Ni coarsening and migration). However, it can cause irreversible damage by the formation of cracks at the interface anode/electrolyte. The advantages of the developed procedure are that it offers a mild level of oxidation, which can be governed and regulated by the direct impedance monitoring of the Ni network resistance changes during oxidation/reduction on a bare anode sample. Once the redox cycling conditions are fixed and the anode/electrolyte sample is checked for cracks, the procedure is introduced for the AST in full‐cell configuration. The developed methodology is evaluated by a comparative analysis of current voltage and impedance measurements of pristine, artificially aged, and calendar‐aged button cells, combined with microstructural characterization of their anodes. It can be applied in both fuel cell and electrolyzer mode. The results obtained in this study from the electrochemical tests show that the artificially aged experimental cell corresponds to at least 3500 h of nominal operation. The number of hours is much bigger in respect to the microstructural aging of the anode. Taking into consideration that the duration of the performed 20 redox cycles is about 50 to 60 working hours, the acceleration factor in respect to experimental timing is estimated to be higher than 60, without any damaging of the sample. This result shows that the selected approach is very promising for a large decrease in testing times for SOCs

Electrochemical testing of an innovative dual membrane fuel cell design in reversible mode

Solid oxide fuel Cells (SOFC) are intrinsically reversible which makes them attractive for the development of reversible devices (rSOC). The main hurdles that have to be overcome are the higher degradation in electrolyzer (EL) mode and the slow and difficult switching form mode to mode. This work aims at the development and experimental validation of a concept for rSOC based on a new dual membrane fuel cell (dmFC) design which can overcome the existing problems of the classical SOFC. The kernel of the system is additional chamber - central membrane (CM) for water formation/evacuation in FC mode and injection in El mode. Its optimization in respect of microstructure and geometry in laboratory conditions is carried out on button cells. The electrochemical performance is evaluated based on volt-ampere characteristics (VACs) combined with impedance measurements in different working points. The influence of a catalyst in the water chamber is also examined. The VACs which give integral picture of the cell performance are in excellent agreement with the impedance studies which ensure deeper and quantitative information about the processes, including information about the rate limiting step. The results from the optimization of the water chamber show that the combination of design and material brings to important principle advantages in respect to the classical rSOC \u2013 better performance in electrolyzer mode combined with instantaneous switching

Electrochemical performances and post-operational characterization of a segmented sofc operated under load for 15k hours

In the frame of the ENDURANCE FCH-JU-FP7 project (2014-2017) a segmented cell (20 segments regularly distributed from fuel inlet to fuel outlet) was operated for 15k hours in co-flow at 750\ub0C (average temperature) in hydrogen under load. Each segment was carefully monitored during operation by periodically acquiring the impedance spectra and constantly checking the voltage under current load. After 15k hours of operation the test was stopped and the cell used for further investigations in order to compare the cell evolution with the segment degradation. The overall observation in cross section of the cell has shown a good stability, however some differences were observed in the electrodes that might be related to the local operating conditions: temperature, H2 /H2O ratio in the fuel stream. The gathered results will contribute to increase the understanding the evolution of a SOFC in real operating conditions. Evidences of the effect of temperature, time and fuel pollutants were found

Impedance investigation of BaCe0.85Y0.15O3-delta properties for hydrogen conductor in fuel cells

International audienceThe 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