Bulgarian Chemical Communications, Volume 50, Special Issue-D, 2018

Pages 5-6
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Professor Zdravko Stoynov- The Scientist Who Created New Horizons (16.05.1936 – 09.09.2017)
Original Research Article
Pages 7 – 20
D. Vladikova
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Differential Resistance Analysis – Current Achievements and Applications Original Research Article
Pages 212 – 30
Z. Stoynov, D. Vladikova, B. Burdin
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Some of the most severe hurdles towards deployment of fuel cells are lifetime and durability. For Solid Oxide Fuel Cells (SOFCs) the degradation rates (DR) currently reported are below 1% kh-1, with a tendency for further decrease to 0.1% kh-1. The SoA degradation already requires long term testing, which takes several years. In addition to the accelerated stress tests, the testing time can be shortened by increasing the sensitivity and accuracy of the monitoring and diagnostic tools. This work presents a new method for degradation evaluation with higher sensitivity due to its operation with derivatives, named Differential Resistance Analysis (DRA). It applies the Differential Resistance Rd extracted from the current-voltage (i-V) characteristics and its time-evolution. The introduction of a spectral transformation procedure additionally increases the noise immunity and sensitivity of the method. The DRA works with two new performance indicators: Rd,min and ΔU* which are more sensitive respectively to transport and to activation losses. A more detailed information illustrated with examples on SOFC about the methodology, the advantages in respect to sensibility and noise immunity, the possibilities to evaluate the state of health and to register early warning signals, is presented. The results show about 10 times higher sensitivity towards evaluation of the DR in comparison with the classical approach.


A Perspective on the Role of the Three-Phase Boundary in Solid Oxide Fuel Cell Electrodes
Original Research Article
Pages 31 – 38
A. Bertei, E. Ruiz-Trejo, D. Clematis, M. P. Carpanese, A. Barbucci, C. Nicolella, N. Brandon
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Within composite electrodes for solid oxide fuel cells (SOFCs), electrochemical reactions take place in the proximity of the so-called three-phase boundary (TPB), the contact perimeter where the electron-conducting, the ionconducting and the porous phases meet. Strictly speaking, the TPB is a line and efforts have been made to increase its length per unit of electrode volume in order to reduce the activation losses. In this communication, by integrating physically-based modelling, 3D tomography and electrochemical impedance spectroscopy (EIS), a renovated perspective on electrocatalysis in SOFCs is offered, showing that the electrochemical reactions take place within an extended region around the geometrical TPB line. Such an extended region is in the order of 4 nm in Ni/Sc0.2Zr0.9O2.1 (Ni/ScSZ) anodes while approaches hundreds of nanometres in La0.8Sr0.2MnO3-x/Y0.16Zr0.92O2.08 (LSM/YSZ) cathodes. These findings have significant implications for preventing the degradation of nanostructured anodes, which is due to the coarsening of the fractal roughnes of Ni nanoparticles, as well as for the optimisation of composite cathodes, indicating that the adsorption and surface diffusion of oxygen limit the rate of the oxygen reduction reaction (ORR). In both anodes and cathodes, the results point out that the surface properties of the materials are key in determining the performance and lifetime of SOFC electrodes.


A Comprehensive Approach to Improve Performance and Stability of State-of-the-Art Air Electrodes for Intermediate Temperature Reversible Cells: An Impedance Spectroscopy Analysis
Original Research Article
Pages 39 – 47
M. P. Carpanese, D. Clematis, M. Viviani, S. Presto, G. Cerisola, M. Panizza, M. Delucchi, A. Barbucci
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Solid oxide fuel cells (SOFC) are devices for the transformation of chemical energy in electrical energy. SOFC appear very promising for their very high efficiency, in addition to the capability to work in reverse mode, which makes them suitable for integration in production units powered with renewables. Research efforts are currently addressed to find chemically and structurally stable materials, in order to improve performance stability during long-term operation. In this work, we examine different approaches for improving stability of two state-of-the-art perovskite materials, La0.6Sr0.4Co0.2Fe0.8O3- (LSCF) and Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF), very promising as air electrodes. Two different systems are considered: (i) LSCF and BSCF porous electrodes impregnated by a nano-sized La0.8Sr0.2MnO3- layer and (ii) LSCF-BSCF composites with the two phases in different volume proportions. The study considers the results obtained by electrochemical impedance spectroscopy investigation, observing the polarisation resistance (Rp) of each system to evaluate performance in typical SOFC operating conditions. Furthermore, the behaviour of polarisation resistance under the effect of a net current load (cathodic) circulating for hundreds of hours is examined, as parameter to evaluate long-term performance stability.


IT-SOFC Based on a Disaggregated Electrospun LSCF Nanofiber Electrode Deposited onto a GDC Electrolyte Disc: Preparation Technique and Morphological Characterization
Original Research Article
Pages 48 – 54
C. Sanna, A. Lagazzo, E. M. Sala, R. Botter, P. Costamagna
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Laboratory-size intermediate temperature solid oxide fuel cells (IT-SOFCs) are manufactured, based on La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) nanofiber electrodes applied onto Ce0.9Gd0.1O1.95 (GDC) electrolyte discs. The LSCF nanofiber electrodes are produced through electrospinning. The electrospun tissue is gently disaggregated in α-terpinol before been applied onto the electrolyte, in order to break the fibers into segments, while preserving their morphology. GDC electrolytes are obtained by uniaxial pressing of the GDC powders, followed by sintering. The dispersed nanofibers are deposited onto the electrolyte to form symmetrical IT-SOFCs, which are then heat treated. SEM characterisation of the heat treated IT-SOFCs proves that the nanofibers morphology is preserved, forming a 3-D structure with many contact points among the fibers themselves, which is expected to feature simultaneously enhanced charge conduction and electrochemical reaction. The cells are ready for electrochemical impedance spectroscopy (EIS), which is the ideal tool to characterize the electrochemical performance of the disaggregated electrospun LSCF nanofiber electrodes.


Ionic Conductivity of Na-doped SrSiO3
Original Research Article
Pages 55 – 61
M. Viviani, A. Barbucci, M. P. Carpanese, R. Botter, D. Clematis, S. Presto
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The conductivity of a newly proposed ionic conductor Na-doped SrSiO3 was studied. Powders were prepared by mixing of SrCO3, Na2CO3 and SiO2 in water or ethanol in order to explore the effect of solvents on the formation of secondary phases. X-ray diffraction was employed to study the phase composition of mixtures treated in air at different temperatures in the range 950-1050 C for 20 hours. Various heating schemes were applied to help the incorporation of Na in the monoclinic SrSiO3 structure. Pressed pellets were sintered at 1000 °C for 20 hours and electroded with Ag paste for electrochemical characterization by impedance spectroscopy. For most compositions and thermal treatments, the formation of the insulating Na2Si2O5 phase was observed as a matrix around grains of the monoclinic SrSiO3 phase. Double calcination limited conductivity but increased its thermal stability. When ethanol was used for powder mixing, the material exhibited higher conductivity after long term ageing at 650 °C, also thanks to its low activation energy, without appreciable crystallization of other silicates.


Chronoamperometrically poised electrodes mimic the performance of yeast-based bioanode in MFC
Original Research Article
Pages 62 – 67
Y. Hubenova, E. Hubenova , M. Mitov
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Poised electrodes in a three-electrode mode is recently used as an alternative tool for establishment of favorable redox conditions for extracellular electron transfer (EET) from biocatalysts to the anode in bioelectrochemical systems. It has been demonstrated that the optimal imposed potentials differ for a particular microorganism species, depending on its specific metabolic pathways. In this study, carbon felt electrodes were potentiostatically poised at +0.005, +0.405, +0.505 and + 0.605 V (vs. SHE) in the presence of exoelectrogenic yeast strain Candida melibiosica 2491. The results from chronoamperometric experiments show that the yeast is capable of performing EET at potentials higher than +0.5 V. The current generated at + 0.605 V (vs. SHE) follows the yeast growth phases, reaching stable maximal outputs of ca. 40 mA/m2. The cyclic voltammetry analysis carried out reveals that the observed electrochemical activity is due to production and secretion of endogenous mediator of EET, mimicking the already established yeast performance in real biofuel cells. The differences in the electrochemical impedance spectra obtained with exploited yeast suspension and cellular-free fraction describe the contribution of the cellular processes to the anodic current generation.


Redox interactions between dye 4-(E)-1-ethyl-4-(2-(4-hydroxynaphthalen-1-yl)vinyl)quinolinium bromide and NAD+/NADH
Original Research Article
Pages 68 –74
Y. Hubenova, R. Bakalska, E. Hubenova, M. Mitov
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An intrinsic property of the merocyanine dyes is the intramolecular charge transfer, which determines a variety of their applications in different fields like optical communication technology, molecular electronics, as optical chemosensors for analytical purposes and voltage-sensitive dyes for mapping membrane potential changes in excitable cells, etc. In this study, the electrochemical behavior of recently synthesized styrylquinolinium dye 4-(E)-1-ethyl-4-(2-(4-hydroxynaphthalen-1-yl)vinyl)quinolinium bromide and its possible interaction with NAD+/NADH redox couple have been investigated. It has been established that in neutral and alkaline buffer solutions the dye co-exists in benzenoid (reduced) and quinoid (oxidized) forms and the equilibrium between both forms can be shifted by change of pH. A quasi-reversible electrochemical behavior, assigned to consecutive electroreduction and electrooxidation of dye, has been observed by means of cyclic voltammetry. The redox coupled interaction between the dye and NAD in solution is evaluated by juxtaposing the results obtained by UV-Vis spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy.


Hydrogen Evolution Reaction at Zirconium and Si-Modified Zirconium Electrodes. Electrochemistry at Fractal Interfaces
Original Research Article
Pages 75 – 81
M. Hromadová, J. Kocábová, L. Pospíšil, S. Cichoň, V. Cháb, M. Novák, J. Macák
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Hydrogen evolution reaction was studied by electrochemical impedance spectroscopy at a high purity grade zirconium and Si-modified zirconium electrode in borate buffer. Roughness factor and fractal dimension of the electrode surface were determined for both electrodes from topographic images obtained by atomic force microscopy. Electrochemical impedance spectra were fitted by a simple equivalent circuit containing constant phase element, which provided the exponent n that can be directly related to fractal dimension Df of the electrode surface. For Zr electrodes a reasonable agreement between Df values calculated from n exponent of the constant phase element and the experimentally-obtained fractal dimension was found, whereas this approach failed for Si-modified Zr electrodes. Further implication of this result for the determination of hydrogen evolution rate parameters is discussed.


EIS as a Tool to Characterize Nanostructured Iron Fluoride Conversion Layers for Li-Ion Batteries
Original Research Article
Pages 82 – 89
B. Guitián, X.R. Nóvoa, A. Pintos
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The present study aims at developing FeF3 conversion layer on cheap mild steel substrates for use as cathode materials in Li-ion batteries. The coatings, grown in an ethylene glycol based electrolyte are characterised by SEM, FIB and EDX techniques that show the formation of similar porous structures but thicker for the mild steel substrate than for the iron substrate. The porous structure is characterised by EIS using a transmission line model that allows obtaining relevant parameters of the layer as the conductivity or the reactivity at pore walls. The conductivity of the FeF3 layer is higher for the mild steel substrate than for the iron substrate. The FeF3 layers react to some extent in LiBOB electrolyte which is interpreted as iron corrosion and Fe3+ reduction with Li+ insertion in the FeF3 structure.


Slow Charging/Discharging Processes of the Electrochemical Double Layer
Original Research Article
Pages 90 – 98
T. Pajkossy
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It is demonstrated for two cases of apparently ideal, ideally polarizable electrochemical interfaces that chargingdischarging following potential changes is not instantaneous hence the double layer is not (solely) of electrostatic nature– as assumed in „classical” double layer theories. That is, physico-chemical interactions must also be considered to account for the slow double layer rearrangement processes.


Molten Carbonate Fuel Cells in Integrated Systems for the Exploitation of Poor Fuels and the Segregation of CO2
Original Research Article
Pages 99 – 107
E. Audasso, P. Campbell, M. Della Pietra, M.C. Ferrari, B. Bosio, E. Arato
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A multi-scale approach for the analysis of reactant systems is a very important mean for the improvement of the understanding of the phenomena occurring at different scales of space and time, and for the support it brings to the experimental and design activities in particular for scale-up purposes. In previous works, the authors developed kinetics, cell and stack models for molten carbonate fuel cells, integrating all these scales of simulation in a code called SIMFC, which has been successfully validated on experimental data. The aim of the present work is to show the benefits that the coupling of plant simulation and small scale models can bring to the study of innovative applications and the design of particular configurations. In the frame of this paper, the authors analysed the integration of the calcium-looping technology coupled with molten carbonate fuel cells for the capture and sequestration of carbon dioxide using a series of different sources of fuel gas to evaluate the best options. In particular, this work analyses the exploitation of poor fuels in fuel cell that would not be possible without enrichment. The availability of a small scale model allowed to identify critical operating conditions (of both local temperature and composition) that decrease the performance and that cannot be easily identified with the use of only large scale model commonly employed for plant design and simulations.


Strategies to Optimise Organic Coating Systems
Original Research Article
Pages 108 – 113
M. Delucchi, A. Barbucci, G. Cerisola
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Organic coatings are generally used for metal protection. The coating process involves several steps, such as chemical pretreatment, priming and application of top coats. Electrochemical Impedance Spectroscopy (EIS) is a valuable tool to tailor each step and/or optimise the protective performances of the complete coating system. EIS on deformed, T-bent and damaged samples of complete systems can be used as a tool to obtain a selective evaluation of the best chemical pretreatment/primer combinations. Impedance data analysis and equivalent circuit modeling on undamaged and artificially damaged coatings exposed to aggressive environments can be used to evaluate the influence of formulation parameters, as Pigment Volume Concentration (PVC), on the protective performance and the retention of the coating adhesion, respectively. EIS data obtained for intact coatings can be analysed to evaluate water permeability coefficients, often influenced by the molecular weight and the chemical nature of the resins constituting the coatings. Several case histories will be presented to gain further insight on the power of EIS for the optimisation of organic coating systems.


Methanol Synthesis from Renewable Electrical Energy: A Feasibility Study
Original Research Article
Pages 114 – 122
M. Rivarolo, D. Bellotti, L. Magistri
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This paper aims to present a feasibility study of an innovative plant for methanol synthesis from carbon dioxide and hydrogen, produced by water electrolyser fed by renewable electrical energy. The analysis aims to examine a methanol production plant, based on 1MW of installed electrolyser, from both the management and economic standpoints: the 1MW plant size has been chosen to represent a modular plant for the power to fuel distributed generation, which may be powered by renewable energy. The thermo-economic investigation is performed using two different approaches: a detailed design point analysis, carried out in order to identify the optimal component sizes and operating parameters followed by a time-dependent plant management optimization. Both the studies are carried out with two simulation tools, named WTEMP (Web-based Thermo-Economic Modular Program) and W-ECoMP (Web-based Economic Poly-generative Modular Program), both developed by the Thermochemical Power Group at University of Genoa.


Rotating Fourier Transform - Engine for Non-Stationary Impedance Spectroscopy
Original Research Article
Pages 123 – 130
Z. Stoynov †
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Explanatory Notes: In the last year of his unwearied active scientific life Zdravko Stoynov was focused on intensive theoretical and experimental activities for effective illustration of his powerful advanced mathematical tool Rotating Fourier Transform (RFT). He would describe it as a “powerful engine for non-stationary impedance spectroscopy which opens up the exploration of the low and infra-low frequencies where many important and interesting phenomena, still hidden, can be measured precisely.” He was expecting the development of a new “4th Generation” marketable impedance analyzers, applying RFT and MRFT (Multiple RFT) in the near future. In order to accelerate the coming of this "near future" and be able to see it, he was working both on the mathematical tool and on the experimental verification. We are presenting his last manuscript, as written by him, expecting, that there will be an interest in his work on RFTs and support of his idea for the 4th Generation of impedance analyzers to fruition. We are open to collaboration for the continuation of Zdravko Stoynov’s work.
Daria Vladikova,
Electrochemical Methods Department, Acad. E. Budevski Institute of Electrochemistry and Energy Systems – BAS The conventional Impedance Spectroscopy is based on the original Fourier Transform (FT), which is the best estimator of periodic signals in stationary conditions. Many practical applications however require impedance measurements of typically non-stationary objects. This paper presents an advancement of the classical Fourier Transform which provides for precise measurements of sinusoidal signals in presence of non-stationary noise. The new mathematical tool was called Rotating Fourier Transform (RFT). Its architecture contains multiple integrals converting the time-domain phenomenon into its frequency domain complex image. The classical Fourier Transform is used as a kernel of the multiple integrals. The new transform filtrates orthogonally the derivatives of the drifting potential which is in this case an additive aperiodic noise. The paper reports the first practical application of the new mathematical instrument in a real laboratory experiment.
More complex is the case of measurements of impedance, which is changing with the time. In this case the changes can be defined as a multiplicative aperiodic noise. The derived analytical expressions are showing that every simple element which is changing with the time, produces methodical errors increasing with the frequency decrease. Those errors are changing the original structure of the model producing an artificial substructure, which is a product of the applied mathematical tool - the FT. When applied transform is the RFT, the artificial structure is eliminated. It was proven that the RFT filtrates orthogonally the first derivative of this noise and provides for the estimation of the proper Instantaneous Impedance. The paper presents also the first practical application of the new mathematical tool for measurement of battery impedance.


Data Exchange Formats
Original Research Article
Pages 131 – 134
Z. Stoynov †
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Explanatory Notes: With the increasing number of Electrochemical Impedance Applications, the problem of unified data exchange formats becomes important. In addition, the existing computerized experimental facilities produce automatically large sets of data files, which have to be stored, sorted, archived and possibly exchanged via Internet. The efficiency of the data banking and the speed of the virtual research depend notably on the selected Data Exchange Formats (DEF). The material below describes the DEF for impedance measurements proposed by Zdravko Stoynov [1]. It will be nice if the equipment producers discuss this topic with the end users of their equipment and introduce common data banking as a convenient and appropriate tool, especially for friendly scientific communication in the big EU Framework programs projects. We offer Zdravko Stoynov’s approach since we use it for more than 15 years and find it extremely convenient. Our group is ready to develop and upload free of charge Data Convertor. We shall be thankful for every remark, correction, recommendation or suggestion. The DEF description which we have included in this special issue of Bulgarian Chemical Communications can be also downloaded from the web site of the European Internet Centre for Impedance Spectroscopy www.accessimpedance.eu (in the Section Information Kit/News). The web site of EICIS will be upgraded in September/October 2018 with its new name “Zdravko Stoynov Internet Centre for Impedance Spectroscopy New ideas and approaches are welcome.
Daria Vladikova,
Electrochemical Methods Department, Acad. E. Budevski Institute of Electrochemistry and Energy Systems – BAS