Editorial– SED’2017
Pages 5
Download PDF (360 KB)
Abstract
National conference “Sofia Electrochemical Days’2017” (SED’2017)
10-13 May, 2017, Sofia, Bulgaria.

Durability of anodic aluminum oxide (AAO) films formed on technically pure AA1050 alloy against corrosion
Original Research Article
Pages 6 – 12
Ch. A. Girginov, S. V. Kozhukharov, M. J. Milanes
Download PDF (1225 KB)
Abstract
The present research provides data about the remarkable durability of Anodized Aluminum Oxide (AAO) film grown on technically pure AA1050 alloy during and after extended exposure to 3.5% NaCl model corrosive medium. The samples were cut from thick aluminum foil with approximate dimensions 30 x 30 mm. The anodization was performed for 48 min in 15%wt. H2SO4 in galvanostatic (15 mA cm-2) and isothermal (20 °C) regime. This process was carried out in a two electrode cell with continuous stirring. The obtained AAO films were submitted to regular weekly electrochemical measurements via Electrochemical Impedance Spectroscopy (EIS) and Linear Sweep Voltammetry (LSV). The EIS spectra have shown almost pure capacitive behavior of the investigated samples. This fact is an indication for the well-expressed insulating capability of the obtained AAO films. Furthermore, the specimens kept their capacitance during the entire exposure period, showing remarkable AAO durability. Only gradual EIS spectra shape evolution was registered, without any abrupt changes. The obtained spectra were further submitted to quantitative data fitting analysis to suitable equivalent circuits. The EIS results were further confirmed by the LSV measurements. The registered currents were in the range of the equipment minimum detection threshold, confirming the assumption for the purely capacitive AAO behavior. The obtained data reveal that the formed AAO films efficiently protect the metallic substrates even after 5208 hours of exposure without any indication for corrosion damage..

Comparative electrochemical and topographical elucidation of Anodic Aluminum Oxide (AAO) layers formed on technically pure aluminum (TPA) and AA2024-T3 aircraft alloy
Original Research Article
Pages 13 – 21
S. V. Kozhukharov, Ch. A. Girginov
Download PDF (1215 KB)
Abstract
The remarkable difference between the mechanical properties of pure aluminum and its industrial alloys predetermines significant variations in their chemical comportment. This fact imposes more detailed investigations on the correlation between the alloy’s chemical composition and its behavior during the preliminary chemical treatments and further exposure to corrosive media. In this sense, the present research is devoted to the comparison of Anodic Aluminum Oxide (AAO) films, formed at the same conditions on technically pure aluminum (TPA) and the highly doped (AA2024-T3) aircraft alloy. The anodization process was performed galvanostatically with simultaneous in-situ chrono-potentiometric curve recording. The electrochemical behavior of the investigated anodized specimens in 3.5 %NaCl medium was elucidated by Electrochemical Impedance Spectroscopy (EIS) and Linear Voltammetry (LVA). The topology of the obtained AAO films was observed by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Remarkable differences between the AAO films formed on TPA and AA2024-T3 were registered by all analytical techniques used in the present research. The electrochemical methods used have shown completely different curve shapes and the further numerical analysis confirmed the observed significant dissimilarities. The SEM and AFM surface observations also revealed entirely different surface morphologies, for both investigated aluminum compositions. The remarkably distinguishable morphologies were observed in both cases: prior to and after anodization. Summarizing the results of both types of electrochemical measurements and the topological observations, it can be inferred that the TPA forms uniform barrier AAO film, whereas the oxide layer on AA2024-T3 is rather cracked and possesses lower durability in the model corrosive medium.

Application of electro-Fenton process for the treatment of Methylene Blue
Original Research Article
Pages 22 – 26
D. Clematis, N. Klidi, A. Barbucci, M.P. Carpanese, M. Delucchi, G. Cerisola, M. Panizza
Download PDF (334 KB)
Abstract
The electrochemical removal of an aqueous solution containing 0.25 mM of methylene blue (MB), one of the most important thiazine dye, has been investigated by electro-Fenton process using a graphite-felt cathode to electrogenerate in situ hydrogen peroxide and regenerate ferrous ions as catalyst. The effect of operating conditions such as applied current, catalyst concentration, and initial dye content on MB degradation has been studied. MB removal and mineralization were monitored during the electrolysis by UV–Vis analysis and TOC measurements. The experimental results showed that MB was completely removed by the reaction with •OH radicals generated from electrochemically assisted Fenton’s reaction, and in any conditions the decay kinetic always follows a pseudo-first-order reaction. The faster MB oxidation rate was obtained applying a current of 300 mA, with 0.3 mM Fe2+ at T=35 °C. In these conditions, 0.25 mM MB was completely removed in 45 min and the initial TOC was removed in 90 min of electrolysis, meaning the almost complete mineralization of the organic content of the treated solution.

Electrochemical studies of two pyrrolo[1,2-c]pyrimidines
Original Research Article
Pages 27 – 36
M.-L. Tatu, F. Harja, E.-M. Ungureanu, E. Georgescu, L. Birzan, M.-M. Popa
Download PDF (1552 KB)
Abstract
The electrochemical characterization of two pyrrolo[1,2-c]pyrimidines has been performed by cyclic voltammetry (CV), differential pulse voltammetry, and rotating disk electrode voltammetry. Their diffusion coefficients were determined from the scan rate influence on CV anodic currents. Modified electrodes were prepared in pyrrolo[1,2-c]pyrimidines solutions in acetonitrile containing tetrabutylammonium perchlorate by cycling the potential or by controlled potential electrolysis at different anodic potentials and charges.

Electrodeposition and structure of Ni-Co-P alloy coatings in stationary and pulse potentiostatic mode
Original Research Article
Pages 37 – 43
K. N. Ignatova, Y. S. Marcheva, S. A. Vladimirova, G. V. Avdeev, D. S. Lilova
Download PDF (705 KB)
Abstract
The rate of growth, composition, morphology and microstructure of Ni-Co-P coatings, deposited at Constant Potential Mode (CPM) and at Pulse Potential Mode (PPM) were investigated depending on the content of NaH2PO2 in the electrolyte, temperature, cathodic polarization and the pulse frequencies.
It was established, that with the rising of NaH2PO2 content (from 0.189 М up to 0.5 М), the rate of growth and the phosphorous content in the Ni-Co-P coatings in both modes of deposition increase. Maximum phosphorous content in the coatings are received in pulse mode at pulse frequencies 100 Hz at a temperature of the electrolyte 20ºC (9.7 mass.% P) and at 80ºC (14.8 mass. % P) at cathodic polarization ΔЕ = -1.15 V. With the raising of pulse frequencies the rate of growth of the coating and the phosphorous content decrease. The pulse potential mode (PPM) of deposition results in weaker manifestation of the phenomenon “anomalous” deposition of cobalt in the alloy and this being explained by stabilizing the pH of the solution in the pulse mode. The different conditions of electrocrystallization in the both mode of deposition reflect in formation of different type structures. Ni-Co-P coatings, deposited at stationary mode are monophasic and polycrystalline. The identified phase of the type NiCoP has a hexagonal symmetry. The coatings, deposited at pulse mode (PPM) are typical amorphous.

Anomalous electrodeposition of gold-indium alloys
Original Research Article
Pages 44 – 49
B.-K. Choi, F. Sauer, G. Beck, R. Stauber, Ts. Dobrovolska
Download PDF (516 KB)
Abstract
The process of anomalous deposition of gold–indium alloys from different electrolytes on the basis of ammonium sulphate (with addition of oxalic, malonic and lactic acid, and glycine) was investigated. The oscillations, observed at some current densities from the first three mentioned electrolytes were investigated in galvanostatic regime. By linear sweep voltammetry technique, besides the methods of cyclic voltammetry and conventional X-ray analysis the phase composition of the deposited alloys from glycine electrolytes were determined.

Network modification of phosphate materials by transition metals doping
Original Research Article
Pages 50 – 54
O.Kostadinova, D. Kochnitcharova, E. Lefterova, M. Shipochka, P.Angelov, T.Petkova
Download PDF (968 KB)
Abstract
This work is focused on structural modification of phosphate materials by doping with transition metal oxides. The structure of (TiO2)x(V2O5)y(P2O5)100-x-y composites is discussed in terms of composition, structural units and valence variations. XRD diffraction, IR and XPS spectroscopic techniques are used to analyze the materials. The addition of V2O5 and TiO2 destroys Р-О-Р bridge structure, generates mixed Р-О-V bonds and non-bridged oxygen atoms leading to the appearance of isolated PO43- units.

Thin phosphate films on aluminum surfaces
Original Research Article
Pages 55 – 60
G. P. Ilieva, D. I. Ivanova, L. B. Fachikov
Download PDF (843 KB)
Abstract
The effects of different factors such as concentration (2.0÷11 vol. %) and temperature (20÷70oC) on formations, compositions and morphologies of coatings obtained by treatment of aluminum (Al – 99.5 %) surfaces in solutions containing ammonium and sodium phosphates, MoO42- promoting ions, buffers, softeners, inorganic activators, as well as surfactants, have been investigated. Gravimetric and electrochemical methods, optical microscopy, scanning electron microscopy with EDX-analysis and X-ray photoelectron spectroscopy have been used. The optimal operating conditions (concentration 4 – 7 vol. % and temperature range 50 – 70оС), under which dense and homogeneous coatings with thickness of about 1 μm can be produced have been estimated. The coating contents include phosphorus, oxygen, molybdenum and nickel, and consequently a conjecture about probable chemical compounds

Structure and corrosion resistance of Ni-P, Со-Р and Ni-Co-P alloy coatings
Original Research Article
Pages 61 –69
K. N. Ignatova, St. V. Kozhukharov, G. V. Avdeev, I. A. Piroeva
Download PDF (917 KB)
Abstract
The present research work reports the results acquired from the systematical characterization regarding the morphology, the phase composition, the barrier properties and the corrosion resistance, possessed by Ni-P, Co-P and Ni-Co-P coatings on copper substrates. The investigated coatings were galvanostatically deposited at similar conditions from sulfate-chloride electrolytes, with pH = 2, at 80°C. The XRD analysis results have revealed that the Ni-P alloy coating possesses typically amorphous structure, whereas the Со-Р coatings are with homogeneous polycrystalline structures, composed by orthorhombic Co2P phase. The Ni-Co-P coating hasnano-sized structure and possesses variable composition consisted by series of solid solution Ni2P -Co2P depending on the content of the main elements. The results obtained from Electrochemical Impedance Spectroscopy (EIS) and Linear Sweep Voltammetry (LSV) for the barrier ability and the corrosion durability reveal clear corelation between themselves. The quantitative data aqcuired by both EIS and LVA methods applied during exposure from 24 to 672 hours to 3.5% NaCl have shown that the superior corrosion protective characteristsics belong to the Ni-Co-P three-component layer. Besides, both Ni-Co-P and Ni-P alloy coatings preserve their barrier properties during the entire 672 h exposure cycle. For comparison, the Со-Р coating was completely broken after 168 hours of exposure to the model corrosive medium.

Corrosion behavior in model solutions of steels suggested as construction materials for mining industries
Original Research Article
Pages 70 – 76
D.I. Ivanova, G. P. Ilieva, L. B. Fachikov
Download PDF (1133 KB)
Abstract
A comparative study concerning corrosion behaviours of two steels suggested as construction materials in mining industries have been carried out. Samples of widely used carbon steel st.25 and the new conceived, low-alloyed, steel КР-355 (0.96 % Cr, 0.36 % Cu) have been tested. The tests have been held in model media containing the main aggressive components appearing in real mining waste waters. The main parameters characterizing the mechanical and corrosion-electrochemical behaviour of the tested materials have been determined by physical and electrochemical methods. It has been estimated that the steel КР-355 exhibited better mechanical strength behaviour and higher corrosion resistance in all media used in the tests.

Sulfite driven fuel cell for environmental purposes: optimization of the oxidation conditions
Original Research Article
Pages 77 – 81
S.Stefanov, M. Martinov, E. Razkazova-Velkova
Download PDF (633 KB)
Abstract
The growing world population puts ever-increasing environmental requirements on all industries, be it construction, pharmacology or energy generation. One of the more potent and hard-to-neutralize industrial waste products are sulfites (typically sodium or calcium) generated by flue-gas desulfurization processes. Our project aims at neutralizing these dangerous compounds by oxidizing them in a fuel cell (FC) of our own design while simultaneously gaining electrical power. The present study’s goal is to find suitable catalysts for the oxidation process. The experiments show that a nickel coated graphite fiber is an appropriate candidate to be used as electrode for the anode compartment of the fuel cell. A comparison of the electrochemical characteristics of the chosen fuel cell with different oxidizing agents (aerated seawater, hydrogen peroxide and ammonium chloride) is presented as well.

Electrocatalysts with reduced noble metals aimed for hydrogen/oxygen evolution supported on Magneli phases. Part I: Physical characterization
Original Research Article
Pages 82 – 88
P.Paunović, O. Popovski, G. Načevski, E. Lefterova,A. Grozdanov and A. T. Dimitrov
Download PDF (1600 KB)
Abstract
The subject of this study is preparation and physical characterization of nano-scaled electrocatalysts for water electrolysis, consisted of Magneli phases as a support and different metallic phases (Co, Pt, Ru, CoPt (Co:Pt = 1:1 wt.), CoRu (Co:Ru = 1:1 wt.) and CoPtRu (Co:Pt:Ru = 1:0,5:0,5 wt.)). Magneli phases were mechanically treated (top-down approach) to reduce their particles from micro to nano scale. Electrocatalytic materials were prepared by sol-gel procedure using organometallic precursors (Me-acetylacetonate) deposited on dispersed Magneli phases in wt. ratio 10% Me and 90% Magneli phases. Pure Magneli phases and the studied electrocatalysts were characterized by means of TEM, XRD and BET analysis. The obtained results have shown that the size of the micro-scaled Magneli phases after the applied mechanical treatment, were reduced to 180÷200 nm. Specific surface area of 4.2 m2g–1 was determined by BET analysis. After grafting of the metallic phase over the Magneli phase, good dispersion of the catalytic centers over the support surface was achieved, that is appropriate for catalytic purpose. Metallic particles are nano-scaled in the range of 2 to 15 nm, thus the BET surface area of the electrocatalysts is higher (4.3 to 11 m2g–1) related to the BET surface area of pure Magneli phases.

Electrocatalysts with reduced noble metals aimed for hydrogen/oxygen evolution supported on Magneli phases. Part II: Electrochemical characterization
Original Research Article
Pages 89 – 94
P.Paunović, O. Popovski, G. Načevski, A. Grozdanov and A. T. Dimitrov
Download PDF (397 KB)
Abstract
This study is concerned with electrochemical testing of nano-scaled electrocatalysts consisted of Magneli phases as a support and different metallic phases (Co, Pt, Ru, CoPt (Co:Pt = 1:1 wt.), CoRu (Co:Ru = 1:1 wt.) and CoPtRu (Co:Pt:Ru = 1:0,5:0,5 wt.)). Electrocatalytic materials were prepared by sol-gel procedure using organometallic precursors (Me-acetylacetonate) deposited on dispersed Magneli phases in wt. ratio 10% Me and 90% Magneli phases. Electrochemical testing of the studied electrocatalysts was performed in aqueous alkaline electrolyte. The order of activity for hydrogen evolution was the following: CoPt > CoPtRu > CoRu, while for oxygen evolution: CoRu > CoPtRu > CoPt. Co has been shown as promoter for reducing Pt particles (the most active monometallic phase), so reduction of Pt quantity in the metallic phase was compensated with smaller particles. Also, mutual interaction between metallic phases (shifting the centre of d-band to values close to Fermi level) increases the intrinsic catalytic activity for both hydrogen and oxygen evolution. Due to large particle of Magneli phases, the catalytic activity is lower compared with corresponding electrocatalysts deposited on carbon nanomaterials. But, they are very good catalysts for oxygen evolution, because Magneli phases in this case, behave not only as support material, but also, as an active oxide electrode.

Characterisation of La0.6Sr0.4Co0.2Fe0.8O3-δ- Ba0.5Sr0.5Co0.8Fe0.2O3-δ composite as cathode for solid oxide fuel cells
Original Research Article
Pages 95 – 101
Maria Paola Carpanese, Davide Clematis, Massimo Viviani, Sabrina Presto, Marco Panizza, Giacomo Cerisola, Antonio Barbucci
Download PDF (1218 KB)
Abstract
Mixture of La0.6Sr0.4Co0.2Fe0.8O3-δ and Ba0.5Sr0.5Co0.8Fe0.2O3-δ, was investigated as promising cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The two perovskites possess high catalytic activity for the oxygen reduction (ORR), although some problems related to their chemical and structural stability have still to be overcome in view of improving durability of the cell performance. The achievement of a stable and high-performing composite material is the aim of this study. In principle, chemical equilibrium at the LSCF-BSCF interface may be reached through ions interdiffusion during the sintering process, resulting in the chemical stabilization of the material. The composite-cathode deposited on Ce0.8Sm0.2O2-δ electrolyte was then investigated by Electrochemical Impedance Spectroscopy (EIS) as a function of temperature, overpotential and time. The results exhibited an interesting electrochemical behavior of the electrode toward oxygen reduction reaction. XRD analysis was performed to detect structural modification during thermal or operation stages and it was found that after the sintering the two starting perovskites were no longer present; a new phase with a rhombohedral La0,4Sr0,6FeO3-type structure (LSF) is formed. An improvement in composite cathode durability has been detected under the considered operating conditions (200 mAcm-2, 700 °C) in comparison with the pure BSCF electrode. The results confirmed this new electrode as promising system for further investigation as IT-SOFC cathode.

On the contamination of membrane-electrode assemblies of water electrolysers based on proton exchange membrane in the course of operation
Original Research Article
Pages 102 – 107
S. A. Grigoriev, D. G. Bessarabov, A. S. Grigoriev, N. V. Kuleshov, V. N. Fateev
Download PDF (630 KB)
Abstract
The issues of contamination of membrane-electrode assemblies (MEAs) of water electrolysers based on proton exchange membrane (PEM) currently pose a subject for investigation. The analysis of structure and chemical composition of the MEA after its long-term testing shows, that the ions and/or the clusters of several contaminating elements (Ti, Pt, Ir, Fe, Ni, Cr, Si), originated from the constructional materials of MEA, electrolysis stack and external piping, are accumulated in the membrane and electrocatalytic layers. Quantitatively, the most predominant contaminating element is Ti (up to 12 wt. %). The cathode area (cathode active layer and membrane band adjacent to them) is extremely exposed to the contaminant’s deposit.

Application of inkjet printing technology for SOFCs anode fabrication and modification
Original Research Article
Pages 108 – 113
R.I. Tomov, A. Fakeeh, T.B. Mitchell-Williams, M. Krauz, R.V. Kumar, B.A.Glowacki
Download PDF (797 KB)
Abstract
Feasibility of inkjet printing technology for fabrication and modification of SOFC electrodes and electrolytes was studied. Drop on demand inkjet printing offers fast, scalable and cost efficient processing path by reproducibly dispensing droplets in the range of nL to pL volumes at high rates (kHz) and high velocity (1-10 m/s). Electromagnetic print heads were utilized to dispense droplets of various inks (doped ceria, Ni oxide) on demand. Printing parameters including pressure, nozzle opening time were studied in order to optimize the inks jetting and delivery. Reduction of SOFC anode polarization losses was pursued via infiltration nano-engineering of the electrode’s scaffolds. Two - step fabrication using inkjet printing was implemented. In the first step porous electrode scaffolds (Gd:CeO2-NiO) were created by printing suspension inks. During the second step inkjet printing infiltration was utilized for controllable loading of Gd:CeO2 nano-decorations on the scaffolds. Anode symmetrical cells were characterized by Electrochemical Impedance Spectroscopy in order to reveal the relation between the surface nano-structure and the electrochemical performance. Electrochemical impedance spectroscopy measurements confirmed a significant reduction and convergence of the area specific resistance (ASR) values for infiltrated anodes with different NiO/Gd:CeO2 volume ratios. This work demonstrated the feasibility of achieving significant improvements in SOFC electrodes performances via simple industrially scalable procedure.

Nanosized Ag particles as catalyst in gas-diffusion electrodes for ORR
Original Research Article
Pages 114 – 118
B. B. Mladenova, Y. D. Milusheva, M. I. Karsheva, I.D. Hinkov, T. E. Stankulov, G. R. Borisov, R. I. Boukoureshtlieva
Download PDF (785 KB)
Abstract
Silver nanosized particles were prepared via original process. The obtained silver particles size varied between 10 to 50 nm with a shape that was mainly spherical. The UV-visible spectroscopy investigation showed Plasmon resonance peak at between 440 and 470 nm for silver nanoparticles. Transmission electron microscopy study showed hexagonal and cubic crystal structures of silver particles.In order to verify the catalytic activity of the obtained silver nanoparticles they were incorporated in the active layer of the air gas-diffusion electrodes. The active layer of the gas diffusion electrode was prepared via four methods. The catalyst activity of these electrodes was studied in three-electrode half-cell with Ag/AgCl reference electrode using 4M NaCl solution as electrolyte. The results obtained showed stable operation of all electrodes up to 50 mA/cm2, and for some – even up to 100 mA/cm2.

Ni incorporation in pSOFC anode ceramic matrix: Part II. Wet chemical reduction in an anhydrous medium
Original Research Article
Pages 119– 126
M.V. Gabrovska, D. A. Nikolova, E. A. Mladenova, D. E. Vladikova, S. K. Rakovsky, Z. B. Stoynov
Download PDF (684 KB)
Abstract
The well-known proton conductive electrolyte yttrium-doped barium cerate BaCe0.85Y0.15O2.925 (BCY15) was used as an anode ceramic matrix for synthesis of Ni-based cermet anode with application in proton conducting solid oxide fuel cell (pSOFC). A cost-effective and energy-efficient wet-chemical reduction approach was presented by using of nickel chloride hexahydrate as precursor, ethylene glycol as anhydrous medium, hydrazine hydrate as reducing agent and alkaline solution as pH regulator. The characterization of the Ni-cermet was performed by Powder X-ray diffraction, N2-physisorption and SEM techniques. The electrochemical properties of anode cermet were determined by impedance spectroscopy after high-temperature sintering followed by reduction in hydrogen atmosphere.It was found that the preparation of BCY15/Ni cermet in ethylene glycol medium leads to (i) Structure preservation of the proton conducing ceramic matrix in the anode composite;(ii) Increase the specific surface area as result of metal Ni phase formation, a precondition for existence of numerous active sites for fuel electrochemical oxidation; (iii) Obtaining of homogeneous, nano-scaled, uniform distributed and non-agglomerated metal nickel particles.The cermet elaborated by ethylene glycol assisted route possesses a capacity to be promising anode in BCY-based pSOFC devices because of the anode ceramic matrixstructure preservation and demonstrated electrochemical performance.

One step solid-state synthesis of lanthanum cobalt oxide perovskites as catalysts for oxygen evolution in alkaline media
Original Research Article
Pages127 – 132
S.Enache, M. Dragan, A. Soare, D. Ion-Ebrasu, A. Zaulet, M. Varlam, K. Petrov
Download PDF (788 KB)
Abstract
Lanthanum cobalt oxide perovskites exhibit good catalytic activity and stability towards oxygen evolution reaction in alkaline media. This makes them especially attractive as electrodes for water electrolysis since they may substitute the more expensive ones used so far, such as iridium and ruthenium oxides. Although various preparation methods are well documented in literature, the fabrication of lanthanum cobalt oxides is often laborious, involving intermediate grinding and annealing steps. Different from that, we show that lanthanum cobalt oxide perovskites can be readily and thoroughly obtained by solid-state synthesis from lanthanum and cobalt oxide precursors, without making use of intermediate processing steps. The resulted powders are essentially within a polycrystalline single phase with well-defined structural and morphological properties. In order to assess the activity towards the oxygen evolution reaction, electrodes obtained from the resulted powders are investigated in a three electrode electrochemical cell in 1N KOH.

Electrochemical characterization of metal oxides as catalysts for oxygen evolution in alkaline media
Original Research Article
Pages 133 – 138
Daniela Ion-Ebrasu, Adnana Zaulet, Stanica Enache, Mirela Dragan, Dorin Schitea, Elena Carcadea, Mihai Varlam and Konstantin Petrov
Download PDF (1197 KB)
Abstract
An investigation of LaCoO3 catalysts for Oxygen Evolution Reaction (OER) in alkaline media is presented. The material has been obtained by an original one step method. The effect of various parameters such as the catalyst and PTFE loading, catalyst layer thickness and fabrication technique on the electrode performance are described. The catalytic activity of the OER in 0.1 M KOH was determined from Tafel plots, obtained from galvanostatic current-voltage measurements as well as by the Rotating Disk Electrode (RDE) method. Long term tests have been conducted showing a good stability of the catalysts for the OER in 0.1 M KOH

Electrochemical testing of an innovative dual membrane fuel cell design in reversible mode
Original Research Article
Pages 139 – 145
Z.Stoynov, D. Vladikova, B. Burdin, A. Thorel, A. Chesnaud, P. Piccardo, M. Slavova1,
R.Spotorno
Download PDF( 958 KB)
Abstract
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 – better performance in electrolyzer mode combined with instantaneous switching.

Effect of the concentration of MnO2 in the composite electrode and the electrolyte on the electrochemical properties of a hybrid supercapacitor
Original Research Article
Pages 146 – 152
G.D. Ivanova, A. E. Stoyanova, M. A. Mladenov, R. G. Raicheff, D. G. Kovacheva
Download PDF (1178 KB)
Abstract
A novel hybrid supercapacitor based on a composite of acetylene black and MnO2 as positive electrode and activated carbon as negative electrode has been developed. The effect of the electrolyte (K2SO4, KOH and KOH+LiOH) and the concentration of MnO2 in the composite electrode is investigated. A supercapаcitor using 50 wt.% MnO2/XC composite electrode and mixed KOH electrolyte demonstrates highest and most stable discharge capacitance (about 1400 cycles), as well as a highest effectiveness of the charge-discharge process. This result can be related one hand with the better electroconductivity of KOH solution in comparison to K2SO4electrolyte and on the other - with the presence of Li+ ions, which favors the faradaic charge-transfer reaction.

Electrochemical impedance study of HTSC ceramics YBCO and BSCCO in presence of electrolyte
Original Research Article
Pages 153 – 157
P.А. Lilov, A.Y. Vasev, A.E. Stoyanova, Y.G. Marinov, A.K. Stoyanova-Ivanova
Download PDF (315 KB)
Abstract
Electrochemical studies of high temperature superconductuctors (HTSC) began immediately after their discovery and numerous studies of ionic transport in HTSC solid electrolytes were conducted. The search for approaches to the problems in HTSCs was largely based on analogies with electrochemical studies of semiconductors, since the majority of HTSC materials exhibit semiconducting properties at ambient temperatures. The analogy thus stimulated the electrochemical studies on the degradation of HTSC in water and acid solutions and their relative stability in strong alkaline solutions. HTSC cuprate ceramics find applications also as an additive in the zinc electrode in nickel-zinc batteries. Such cells are a potentially cheaper and easier to recycle, replacement for toxic nickel-cadmium batteries. Electrochemical tests showed that the Ni-Zn battery cells with YBCO and BSCCO superconducting ceramic additives in the zinc electrode exhibited good cyclic operation ability and capacity stability, as well as a higher specific capacity than the cells with a zinc electrode with a “classic” carbon conducting additive. The study presented here is focused on the electrochemical characterization of nanocomposite conductive cuprate ceramics - Bi2Sr2CaCu2Ox (BSCCO -2212), Bi2Sr2CuOy (BSCO - 2201) and YBa2Cu3Ox (YBCO - 123) in the following electrolytes: 7M KOH, the electrolyte used in Ni-Zn cells, an alkaline phosphate (AF) - electrolyte containing KOH and Na3PO4.12H2O, and a proprietary PSPAA electrolyte. The electrical conductivity of ceramic/electrolyte system was investigated by Electrochemical Impedance Spectroscopy measurements performed by a Bio-logic SP-200 potentiostat. The impedance responses for all ceramics are compared and discussed in terms of equivalent circuits.

Key parameters determining the performance of lithium sulfur batteries
Original Research Article
Pages 158 – 162
P. Półrolniczak, M. Przybylczak, K. Wasiński, M. Walkowiak
Download PDF (648 KB)
Abstract
Lithium sulfur battery is a promising electrochemical system offers high energy density of 2500 Wh kg-1, which is almost five folds higher than those of state-of-art Li-ion batteries. However, commercialization of Li/S batteries still cannot be realized due to many problematic issues, such as poor electronic conductivity of sulfur, solubility of lithium polysulfides in organic electrolytes and the use of highly reactive metal lithium as anode. In this paper we discuss key parameters, in particular carbon/sulfur ratio, carbon porosity, sulfur loading and electrolyte volume, which impact the battery performance. We found that increasing the carbon content in the sulfur cathode from 20 to 40% resulted in 30% capacity improving of the Li/S battery. The optimized sulfur loading was found to be 2.2 mg S per cm-2 and electrolyte volume should not be lower than 75 μl per mg of sulfur.

Lithium ion batteries: active electrode materials based on manganese dioxide
Original Research Article
Pages 163 – 170
K.Banov, D. Ivanova, L. Fachikov, V. Kotev, T. Stankulov, B. Banov
Download PDF (1090 KB)
Abstract
Lithium cobaltate used in commercial rechargeable lithium ion batteries possesses good cyclability, high discharge potential and acceptable energy density. Its synthesis is easy and can be realized on a large industrial scale. However, the price of cobalt is very high and determines the end user price of the lithium battery. Cobalt and its oxides are very toxic. Lithium manganese dioxide spinel combines high specific energy density – volumetric and gravimetric, high coulomb efficiency - close to the theoretical one, long cycle life. It is non-toxic, environmentally friendly and with low cost but possesses some disadvantages as low starting capacity and low stability at elevated temperatures. Optimising the synthesis methods and elucidating the factors influencing the electrochemical stability, we have obtained manganese dioxide spinel, which is very attractive and prospective cathode material for large scale application. Cathode materials based on manganese dioxide spinel with high coulomb efficiency close to 90%, discharge rates of 4C and cycle life up to 1000 cycles are presented.

High voltage cathode materials based on lithium cobaltate with nickel and manganese doping
Original Research Article
Pages 171 – 176
K. Banov, T. Petkov, R. Boukoureshtlieva, D. Ivanova, L. Fachikov, V. Kotev, B. Bano
Download PDF (845 KB)
Abstract
Doped lithium cobaltates LiCo1-yMyO2 (M = Mn, Ni, y = 0.1, 0.3, 0.5) have been prepared by low temperature solid-state reaction. The structure of the samples was characterized by X-ray diffraction (XRD). The specific surface area (SSA) of the materials was examined by B.E.T. method. The synthesized powders were found to have rhombohedral structure, except for LiCo0.5Mn0.5O2, which crystallises with cubic spinel-like structure (space group Fd3m). The electrochemical performances of the compounds were studied by galvanostatic cell cycling in the high-voltage range between 3.0 and 4.8 V vs. Li/Li+ electrode. It was shown that the type and the amount of the doping element greatly affect the structure, electrochemistry and cycle life characteristics of the investigated materials. LiCo0.9Mn0.1O2 has shown better cycling results compared to all other compounds.

Electrochemical behaviour of LiMn2O4 and LiCoO2 in water electrolyte
Original Research Article
Pages 177 – 182
T.Petkov, T. Stankulov, K. Banov, A. Momchilov
Download PDF (1127 KB)
Abstract
Li-Ion Batteries can afford quite large energy densities based on organic electrolytes with highly toxic and flammable solvents and its preparation is complicated and expensive, partly because the electrolyte is moisture and air sensitive. Utilization of aqueous electrolytes in rechargeable lithium batteries will increase their safety, economical, and environmental issues. The electrochemical properties of LiMn2O4 and LiCoO2as positive electrode materials for Li-ion battery with water electrolytes against LiV3O8 as negative electrode were investigated. The used salts are 6M LiNO3 and 2M Li2SO4 dissolved in distillated water. LiMn2O4 was prepared by conventional solid state reaction heating stoichiometric mixtures of LiNO3 and MnO2 at temperature under 650oC for 24 h. LiCoO2 was obtained via sol-gel method with final thermal treatment at800oCfor 24 h. Lithium vanadate was prepared via solid state reaction. All materials are stable in the aqueous solution and intercalation/deintercalation of lithium ions occurs within the window of electrochemical stability of the water, which was determined using slow cycling voltammetry. Manganese spinel exhibit specific capacity of 99mAh.g-1 in 6M LiNO3and 94mAh.g-1 in 2M Li2SO4water electrolyte, while its capacity in organic electrolyte is 135mAh.g-1. The obtained capacities from lithium cobaltate in water electrolytes were 99mAh.g-1 and 79mAh.g-1respectively while that in organic electrolyte is 145mAh.g-1. The cycling behaviorand specific capacityof these active compounds in electrochemical cell with vanadate as a negative active mass is not as stable as in organic electrolytes.

The electrochemical behavior of LiV3O8 obtained via different syntheses as negative active material in aqueous Li-ion battery
Original Research Article
Pages 183 – 188
T.Petkov, T. Stankulov, K. Banov, A. Momchilov
Download PDF (1080 KB)
Abstract
The electrochemical properties of the lithium trivanadate (LiV3O8, LVO) as a negative electrode material for aqueous Li-ion battery was studied. Two methods of synthesis were applied to prepare LiV3O8: sol-gel method followed by solid-state reaction (SSR) at 500 and 550 °C and melting process with subsequent hydrothermal treatment and drying.As a counter electrode was used LiMn2O4 (LMO) prepared by conventional SSR synthesis. The intercalation/de-intercalation of lithium ions occurs within the window of electrochemical stability of the water. LiV3O8 prepared by melting process, hydrothermally treated and dried (MP65DR), shows a poorer performance in galvanostatic mode compared to these obtained via sol-gel. The sample obtained via sol-gel followed by SSR at 550 °C (SG55) shows an initial specific capacities of ~75 (intercalation) and ~52 (de-intercalation) mAh.g-1 which is about 4-5 times less in comparison to the capacities delivered in non-aqueous electrolytes. Although the lower capacity (30-33 mAh.g-1), the other sol-gel sample annealed at 500 °C (SG50) displays better capacity retention and coulombic efficiency throughout the cycles.

Screening impedance analysis of Zn-air cells
Original Research Article
Pages 189 – 194
Z. B. Stoynov, D. E. Vladikova, B. I. Abrashev, M. P. Slavova, B. G. Burdin, E. S. Mihaylova-Dimitrova, L. C. Colmenares, A. R. Mainar, J. A. Blázquez
Download PDF (1061 KB)
Abstract
As a promising technology, zinc-air secondary batteries have gained considerable attention over the past few years. The presented work aims at application of Electrochemical Impedance Spectroscopy (EIS) as a tool to develop new nanostructured and environmentally friendly materials for rechargeable zinc-air cells. The results are obtained on both half- and full-cell configuration during cycling. They ensure electrochemical characterization of the cell and its components (electrodes and electrolyte) in similar state of charge. This offers experimental evidence to determine the rate limiting stages. The accumulated information will be used for further studies and optimization of the zinc-air cell.