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

Dual cells with mixed protonic-anionic conductivity for reversible SOFC/SOEC operation

International audienceThe dual cell concept is a novel design for solid oxide fuel cells operating at intermediate temperature. The cell comprises a series of five layers with different compositions, alternating two dense electrolytes and three porous layers, i.e. the outer electrodes and a central membrane. The dual cell concept makes it possible to separate the compartment for water formation from both fuel and oxidant chambers. Such a three-chamber configuration gives many advantages related to fuel dilution, materials corrosion, and reversibility between fuel cell and electrolyser operational modes (SOFC/SOEC) at high temperature. Dual conductivity (protonic/anionic) can be achieved by joining two dense BaCe0.85Y0.15O3-δ (BCY) and Ce0.85Y0.15O2-δ (YDC) electrolytes through a porous ceramic central membrane made up of both materials. Complete anode-supported dual cells have been fabricated by a combination of pressing, casting, printing, wet spraying, and plasma spraying techniques. Electrochemical tests carried out by impedance spectroscopy showed the feasibility of the concept and successful reversible operation of the dual cell. The fabrication route, the microstructural and electrochemical testing results are reported in this work, and partially compared to simulated results from an electrochemical model developed describing the dual cell concept

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

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

Redox Cycling for SOFC Accelerated Degradation

This work aims at development of Accelerated Stress Tests for SOFC via artificial aging of the fuel electrode applying chemical and electrochemical (hydrogen starvation) redox cycling. In principle the degradation processes follows that of calendar aging (Ni coarsening and migration), but in addition it can bring to irreversible damages caused by the development of cracks at the interface anode/electrolyte due to the expansion/shrinkage of the Ni network. The challenge is to introduce conditions which will prevent the formation of cracks which can be done by partial oxidation. The advantage of the proposed methodology is that a mild level of oxidation can be regulated by direct impedance monitoring of the Ni network resistance changes during oxidation/reduction. Once the redox cycling conditions are fixed on bare anode and checked on anode/electrolyte sample for eventual cracks, the procedure can be introduced for AST in full cell configuration. The developed methodology is evaluated by comparative impedance analysis of artificially aged and calendar aged button cells. The results for 20 redox cycles which can be performed for 24 hours are comparable with those obtained for about 1600 hours operation in standard conditions which ensures more than 50 times acceleration

Pile à combustible avec cathode avec canaux

L'invention concerne une pile à combustible comprenant - une anode (10) apte à oxyder un premier composé Ml en premiers ions M(m+), - un premier électrolyte (20) en contact avec cette anode, - une cathode (50) apte à réduire un second composé N2 en seconds ions N(n_), - une membrane centrale (30) poreuse dont une des faces (32) est en contact avec ce premier électrolyte (20), et dont la face opposée (35) est en contact avec cette cathode, le premier électrolyte (20) étant constitué d'un matériau apte à conduire les ions M(m+), la membrane centrale (30) étant constituée d'un matériau apte à conduire à la fois les ions M(m+) et les ions N(n_). La cathode (50) est traversée par un réseau de canaux (52) qui débouchent chacun sur la membrane centrale (30) et sur une surface libre de la cathode (50), la dimension minimale d'une section transversale d'un quelconque de ces canaux (52) étant supérieure à 20 μm de telle sorte que l'évacuation du produit P résultant de la réaction des ions M(m+) et des ions N(n_) depuis la membrane centrale (30) vers l'extérieur de la pile à combustible (1) au travers de ces canaux (52) est possible