Browsing by Author "Ignatova K."
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Item A study on the kinetics of the electrodeposition of Ni, Co and Ni-Co alloy in citrate electrolyte part 1. The kinetic study of the independent electrodeposition of Ni and Co(2015-01-01) Ignatova K.; Lilova D.The elecrodeposition of Ni and Co was studied by electrochemical techniques in varying compositions of slightly acidic citrate electrolyte. Cyclic voltammetry and current transient measurements were used to characterize the initial stages of nucleation, growth mechanism and electrodeposition of Ni and Co. The voltammetry analysis showed that the electrodeposition of both Ni and Co occurs under diffusion-kinetic control associated with a typical nucleation process. The deposition of Ni is in one act of charge transfer with exchange of two electrons, the deposition of Co is a two-stage process of reduction of free cobalt ions to cobalt, and reduction of complex ions type CoCit- to cobalt. The application of the method of chronoammetry proved that for cobalt the mechanism of nucleation is spontaneous, with three-dimensional nucleation and nuclei growth, while for nickel the slower progressive mechanism of nucleation is typical. The differences in the deposition mechanisms of the two metals is a possible explanation for their anomalous co-deposition in the electrolyte examined.Item AN EFFECT OF NH4SCN ON DEPOSITION OF Sn-Co AND Sn-Ni POWDERS IN A FLUORIDE-CHLORIDE ELECTROLYTE(2020-01-01) Ignatova K.; Alakushev M.The cyclic voltammetry method is applied to investigate the kinetics of tin, cobalt and nickel deposition as well as that of Sn-Co and Sn-Ni alloy powders in a fluoride-chloride electrolyte. The content of the components used and that of an ammonium rhodanide (NH4SCN) additive are varied. The conditions of Sn-Co and Sn-Ni alloy powders deposition are determined in electrolytes of a different composition. It is shown that the addition of NH4SCN of about 5 g L−1−10 g L−1 favors the alloying. The further increase of the additive concentration facilitates the formation of smooth coatings greatly suppressing that of powders.Item APPLICATION OF ELECTROCHEMICALLY PREPARED Sn-Co ALLOY POWDERS AS ANODE MATERIAL FOR LITHIUM-ION BATTERIES(2024-01-01) Ignatova K.; Stankulov T.The electrochemical behavior of active electrode (anode) materials (AEMs) for lithium-ion battery (LIB), prepared with Sn-Co powders electrodeposited in three different current modes, was investigated. It is shown that AEM with Sn-Co powder, obtained in constant-potential mode, has the highest tin content (82.1 wt. %) and the best performance and stability in operation. The anode material, made with this Sn-Co powder, shows a first-cycle charge capacity of 450 mAh g-1, which decreases to 295 mAh g-1 for 10 battery cycles at a current load of 0.8 mA (0.1C). The data also showed that the AEM with Sn-Co powder, deposited in pulse-potential mode, which has the lowest tin content (24.5 wt. % Sn) but the highest dispersity compared to the other powders tested, shows better performance than that of a powder with a tin content of 68.6 wt. %, obtained in constant-current mode. It can be concluded that for the high electrochemical activity of the anode materials, prepared with Sn-Co powders, both the high tin content and the high dispersity and morphological uniformity of the powders are of great importance.Item Comparative study on electrodeposition of Co-Cu alloys from ammonia-sulphate and citrate electrolyte(2016-01-01) Ignatova K.; Lilova D.The kinetics of the electrodeposition of Cu and Co in Co-Cu alloy, chemical composition, cathodic current efficiency and morphology of Co-Cu alloys electrodeposited from an ammonia-sulphate electrolyte (AMSE) and from a weakly acidic citrate electrolyte (CE) without and with addition of saccharin (CE+SHR) at constant potential mode are compared. The results indicate that the deposition of Co-Cu alloys from AMSE occurs without initial polarization, while the deposition from CE is characterized by long initial polarization and the overall polarization is higher compared to that in AMSE as a result of formation of more stable citrate complexes of the metals. The individual deposition of Co as well as Cu from CE is a two-stage process of reduction of their complex ions. It is found that the Co-Cu alloys with Co up to 50 - 60 mass % deposited from AMSE have coarse-grained crystalline structure with comparable amount of elongated and spheroid crystals with average size in the range from 2 to 10 μm, as compared to those deposited from CE with average size of about and less than1μm. The increase of the Co content in the alloys up to 80 mass % results in decrease of the average size of the crystallites in AMSE to less than 5 μm, and in CE- of about 500 nm. The addition of saccharin in the citrate electrolyte yields glossy and smooth coatings with close to nano-sized structure with average size of the crystallites in the range from 100 to 300 nm.Item Effect of Electrolyte PH And Pulse Potential Frequency on The Properties of Ni-co Powders(2021-01-01) Ignatova K.; Avdeev G.In this work, we have synthesized finely dispersed Ni-Co powders through two current modes - constant and pulse potential modes in a chloride electrolyte with different pH values and at different pulse frequencies. The powder formation has been found to be difficult when increasing the pH of the solution from 1.4 to 6.8 and by raising the pulse frequency above 500 Hz. With increasing the pulse frequency “anomalous” cobalt deposition and decrease in particle size is observed. The Ni-Co powders, containing more than 45 - 50 wt. % Co, represent three-phase systems based on cubic, f.c.c. β-Ni and β-Co phases and hexagonal, h.c.p. α-Co-phase. Nickel and cobalt are evenly distributed in the structure of the powder particles. The results obtained prove that Ni-Co powders are to a very large extent alloy systems, not a mechanical mixture of individual elements.Item Effect of H3Po3 and Nah2Po2 in the electrolyte on the composition and microstructure of Ni-Co-P alloys(2019-01-01) Ignatova K.; Alakushev M.; Kozhukharov S.; Marcheva Y.; Vladimirova S.; Avdeev G.The effect of two phosphorus-containing additives (phosphorous acid, H3PO3 and sodium hypophosphite, NaH-2PO2) in modified Watt's electrolyte (pH = 2; 80°S) on the deposition kinetics, morphology, component and phase composition of Ni-Co-P coatings was studied. When only one of the studied additives is present in the solution, the P content in the coatings reaches maximum of 15 % and their structure is amorphous. The P and Ni incorporation in the coatings is facilitated when the electrolytes with prevailing concentrations of H3PO3 are used. The maximal P content (over 30 %) in the coatings was reached when both additives were present in the electrolyte. These coatings possess mixed structure with nano-dimensional amorphous phase and crystal hexagonal phase of solid solutions Ni2P and Ni2P-Co2P. The increasing of the current density causes P content decrement and the resulting crystal phase is tetragonal, composed by solid solutions of Ni3P and Ni3P-Co3P.Item Effect of organic additives in citrate electrolyte on the properties of Ni-Co alloy(2015-01-01) Ignatova K.; Lilova D.; Piroeva I.The kinetics of independent and joined deposition of Ni and Co in citrate electrolyte in the presence of three types of additives: saccharine (SCH), urotropine (URT) and gelatine (GLT), is examined. Comparison is made between the chemical composition, the current cathodic efficiency (CCE) and the morphology of Ni-Co alloys depending on the type of the additive, both in constant potential mode (CPM) and in pulse potental mode (PPM). The study specifies the effect of GLT and SCH for formation of smooth, shiny, nanostructured Ni-Co coatings with average crystallite size less than 100 nm with a high CCE (about 80 % for SCH, and over 90 % for GLT), in both potential modes. The presence of SCH in the electrolyte with simultaneous application of PPM increases the content of Ni in the alloy to 25-30 mass % Ni. The optimum additive of SCH containing 0.5 - 1.0 g dm-3 is specified, wherein the electrolyte remains stable during continuous operation.Item Effect of the electrode position conditions on the morphology and corrosion behavior of Ni-Co alloys part 2: Phase composition and corrosion behavior of Ni-Co alloys, electrodeposited from citrate electrolyte(2016-01-01) Ignatova K.; Avdeev G.The changes in the phase composition (through X-Ray analysis) and the corrosion behaviors (through potentiodynamic polarization studies in 4 % NaCl) of Ni-Co alloys, electrodeposited from a citrate electrolyte by changing the Ni/Coratio, content of organic additive (saccharine) and the content of sodium citrate in the solution, are studied. It is found that the increase of Ni/Co ratio from 1 to 5 increases the content of Ni in the Ni-Co alloy to 48 mass %. The proportion between the phases of Co and Ni with cubic face-centered crystal lattice is in approximate compliance with the proportion of the two metals in the alloy. The peaks in the difractograms are weak and stretched, which corresponds to the nano-sized structure of the alloy. The Ni-Co alloys with highest content of Ni have a higher corrosion resistance compared to those containing a higher percentage of Co. It is also shown that the Ni-Co coatings obtained in the presence of addition of saccharine show a much higher corrosion resistance than the coatings obtained in absence of additive. Ni-Co alloys, deposited from electrolyte with more sodium citrate show only a slight growth of Ni content and the effect of sodium citrate on corrosion behaviors of the alloy is insignificant.Item Effect of the electrodeposition conditions on the morphology and corrosion behavior of Ni-Co alloys part 1: Chemical composition, cathodic current efficiency, and morphology of Ni-Co alloys electrodeposited from citrate electrolyte(2016-01-01) Ignatova K.; Piroeva I.; Vladimitova-Atanasova S.Ni-Co alloys with varying cobalt content are electrodeposited in stationary potentiostatic mode employing citrate electrolyte, either with or without addition of saccharin (SHR) at different Ni/Co ratios and variable content of sodium citrate (Na3citrate) in the electrolyte. The changes in the microstructure, chemical composition and cathodic current efficiency of deposition (CCE) of Ni-Co alloys are studied. Scanning electron microscope, energy dispersive X-Ray and atomic absorbtion analysis are used to characterize the alloy coating.The alloy deposition is proved to be of an anomalous type. It is found that with the increase of the value of ratio Ni/Co in the solution from 1 to 5, the percentage of Ni in the Ni-Co alloy grows to 48 mass %, and the CCE decreases to about 70 %, whereas more fine-crystalline Ni-Co coatings (size of crystallites less than 100 nm) are deposited. The addition of SHR results in a strong decrease of CCE (up to 50 %), but also in formation of smooth and shimmering coatings with average size of crystallites much less than 50 nm. The coatings obtained from electrolyte with higher content of sodium citrate contain more Ni but are more coarse-crystalline, as the effect of the complexing agent is generally less pronounced in comparison with the case with SHR addition.Item ELECTRODEPOSITION OF Ni-Co ALLOY POWDERS FROM DIFFERENT ELECTROLYTE COMPOSITIONS(2020-01-01) Ignatova K.The effect of the content of the main components (Co and Ni) and pH on the formation rate, the chemical com-position and the morphology of Ni-Co powders obtained in amino-sulfate, amino-chloride and chloride electrolytes is determined. The powder formation in an ammonium-sulfate electrolyte is performed with a low speed and hence a low current efficiency. It is strongly suppressed in a chloride solution by the increase of pH, Co and especially Ni content of the electrolyte. The decrease of pH from 9.1 to 1.4 increases the Ni content from 45.3 wt.% to 61.4 wt.% forming powders of a smaller average particle size (up to 10 µm) with a trend for agglomeration. The Ni-Co powders obtained have polycrystalline structures with presence of f.c.c. phases of ß-Ni and ß-Co and additional h.c.p phase of a-Co, when the cobalt content is greater than 50 wt.%.Item ELECTRODEPOSITION OF Sn-Co POWDERS BY STATIONARY AND PULSE MODE(2021-01-01) Ignatova K.; Lilova D.; Vladimirova L.; Alakushev M.Sn-Co powders with homogeneous distribution of components in powder particles were obtained by constant and pulse potential modes from a fluoride-chloride electrolyte. Morphology, elemental and phase compositions by SEM, EDX, SEM-Mapping andXDR analysis were investigated. Under stationary conditions, the potential increment leads to an increase in cobalt content from 3.6 wt.% to 56.4 wt.%, and to a structural alteration of the powders from network-shaped to a dendritic form with an average particle size between 15 and 30 µm, depending on the total metal concentration. The powders, obtained by pulse mode at a lower pulse frequency (100 Hz), contain particles with an irregular dendritic structure, characterized by the greatest thickness of the main axes of the dendrites. Increasing the pulse frequency to 1000 Hz results in a finer dispersed structure (r < 10 µm) and the cobalt content achieves 70.5 wt.% under these conditions. The obtained Sn-Co powders are mixed structures of amorphous phase and inclusions of microcrystalline phases of tetragonal β-Sn, hexagonal α-Co, CoO and SnO2 and only in stationary mode an additional phase of SnF2 exists.Item INFLUENCE OF ELECTROLYTE COMPOSITION AND ITS TEMPERATURE ON THE PROPERTIES OF Sn-Co POWDER(2021-01-01) Alakushev M.; Ignatova K.The conditions for electrodeposition of Sn-Co powders in a fluoride-chloride electrolyte with different Co2+/Sn2+ ratios have been determined. Both, the effect of electrolyte temperature and the value of the initial current density on the morphology, component and phase composition of Sn-Co powders, were investigated. A wide range of powder compositions, depending on the increase of Co2+/Sn2+ ratio, was obtained - from 7 wt. % Co to over 85 wt. % Co. The deposited Sn-Co powders are characterized by a very diverse morphology, which varies from needle-like and cross-linked in the Sn-enriched powders to typically dendritic in Co-enriched ones. By increasing the electrolyte temperature from 20oC to 60oC, the share ofparticles with a size of about 5 - 10 µm reaches above 50 %. The main elements (tin, cobalt and oxygen) are evenly distributed in the structure of the individual particles.Item Kinetic study on electrodeposited Ni-Co alloy in citrate electrolyte part 2. the kinetics of joint electrodeposition of Ni and Co in Ni-Co alloy system(2015-01-01) Ignatova K.; Lilova D.In the paper the results from the study of the kinetic of joint electrodeposition of Ni and Co in Ni-Co alloy system are reported. It is found that the co-deposition of Ni and Co in Ni-Co alloy system in slightly acidic citrate electrolyte occurs under combined diffusion-kinetic control with a typical mechanism of nucleation similar to those in the case of Co. The application of the method of chronoammetry proved that the mechanism of nucleation for Ni-Co alloy is spontaneous with three-dimensional nucleation and nuclei growth. The difference in the independent deposition mechanisms of the two metals is a possible explanation for their anomalous co-deposition in the electrolyte examined. The anomalous codeposition is more clearly noticeable in electrolytes with higher content of cobalt than nickel. The SEM-analysis proves that the alloys enriched in cobalt have crystallites with more even and rounded shape, which is expected in the case of spontaneous nucleation.Item Kinetics of electrodeposition of NiP, CoP and NiCoP coatings depending on sodium hypophosphite concentration(2018-01-01) Ignatova K.; Marcheva Y.The effect of NaH2PO2 concentration in the electrolyte (pH = 2) on the kinetics of NiP, CoP and NiCoP alloys electrodeposition at room temperature was studied. It was established that at concentration of NaH2PO2 about 0,2 M the deposition rate of NiP alloys decreases; the deposition rate of CoP alloys increases but at the same time the quality of the coating becomes bad. In both cases it was observed a depolarizing effect on the deposition. With increasing of the content of NaH2PO2, the phosphorus content in NiCoP alloys increases to approximately 7 % and the current efficiency of electrodeposition (CEE) goes over 100 %. The comparison of the kinetic data with those of CEE of NiCoP coatings showed that the mechanism of deposition is mixed chemical-electrochemical.Item Membrane electrochemical preparation of three-polyphosphate non-cyanide electrolyte for coppering(2015-01-01) Gadzhov I.; Lilova D.; Ignatova K.The membrane electrochemical technology for obtainment of three-polyphosphate non-cyanide electrolyte for coppering is developed and optimized. For the purpose of characterizing the selectivity of the ionic membranes used, the processes of mass transfer are examined through determining the transmission numbers of the ions in the three-chamber electrolyzer. Detailed balance of the voltage between the electrodes and quantitative evaluation of the anodic output on current during the membrane preparation of the electrolyte are made. It is found that Cu is actively solved without passivation up to current density ia = 8 A dm-2. The anodic polarization during the operation of the electrolyzer is insignificant and the current efficiency is close to 100 %. It is found that the optimal environment for the membrane preparation of the electrolyte is anolyte of 140 g dm-3 Na5P3O10 and citric acid C6H8O7 up to pH = 5.3, and catholyte solution of 380 g dm-3 Na2CO3.Item STRUCTURE AND CATALYTIC ABILITY OF Sn-Co POWDERS TO OXYGEN REDUCTION REACTION IN A MODEL METAL-AIR BATTERY(2022-01-01) Ignatova K.; Popov I.; Shipochka M.Composition, microstructure, current efficiency and catalytic ability of Sn-Co powders, deposited by constant and pulse potential modes are investigated. SEM, EDS, XPS, XRD and BET methods were applied. It is shown that at higher potentials in stationary mode and at higher pulse frequencies in pulse mode (500-1000 Hz), Sn-Co powders with maximum specific surface area (21 - 23 m2 g-1) and cobalt content (56 wt. % in stationary and 75.5 wt. % in pulse mode) are obtained. XRD and XPS analysis showed that the powders obtained in both modes represent a mixture of amorphous and microcrystalline phases of tetragonal β-Sn, α-Co with hexagonal, hcp lattice, as well as phases of CoO, SnO2 and impurity phase of SnF2 only in stationary mode. It was found that the catalytic ability of Sn-Co powders to the oxygen reduction reaction, ORR in a model Metal-Air battery is better than that of the reference Ag catalyst at low current densities. It increases with an increase in the cobalt content and with an increase in the dispersity of the powders