Galvanostatic Regime Research Articles

Seeking for Electrochemical Instabilities in Lithium-Oxygen Batteries Using Halides As Redox Mediator

One of the greatest challenges of contemporary civilization is the need for batteries that supply the global energy demand because of the increasing use of mobile devices and electric vehicles. Lithium-oxygen batteries are promising alternatives to meet this energy demand, due to its high theoretical energy capacity since its operating mechanism is based on the anodic dissolution of the Li metal combined with the cathodic oxygen reduction reaction (ORR) that culminates in the formation of Li2O2..1 However, there are still some challenges to overcome for the improvement of this technology, such as charging voltage which is considerably larger than the discharging one and the poor lifetime and cyclability, demanding efforts to improve the capacity retention during cycling.Recently, spontaneous potential oscillations have been identified during the process of discharging in lithium-oxygen batteries.2 Nonlinear dynamics in these systems are known to occur under conditions far from thermodynamic equilibrium as a result of the interaction between kinetic instabilities and mass transport properties.3 Understanding, therefore, the reaction mechanism can provide important clues to the birth and evolution of this temporal complexity, thereby enabling precise control and prevention of battery efficiency collapse.In this work, it is proposed the use of lithium halides salts acting as redox mediator in the electrolyte composition. During the charging process, that is, the oxidation of Li, the X- of LiX can be oxidized to X3 - leading to a decrease in the charge overpotential.4 Besides, the halides can act as a diffusional component that could help the emergence of electrochemical instabilities. Thus, it was evaluated the role of LiBr in the electrolyte composition in different gavanostatic regimes, through the assembly of Li-O2 batteries cells with and without LiBr which was submitted to different galvanostatic charge/discharge conditions to comprise the effect of this species in the electrolyte. It has been observed that the presence of bromide reduced the charge overpotential and improved the cyclability (Figures 1a, 1b). Besides, the role of applied current was evaluated and higher it was, less completed cycles were obtained.Figure 1 presents the results obtained in cyclability of Li-O2 batteries tests with and without LiBr as redox mediator and using different galvanostatic conditions. Figure 1a shows cycling tests conducted with (black line) and without (red line) LiBr in the electrolyte composition. With this result, it was possible to conclude the improvement in battery lifetime given by the presence of the redox mediator. Figure 1b highlights the first and fifth cycles, showing the overpotential reduction with LiBr. It was also evaluated the evolution of the maximum capacity of each cycle (Figure 1c) and the results reinforce the LiBr performance. Figure 1d presents the evaluation of different galvanostatic conditions, with lithium bromide, and it was observed that lower the operating current, more cycles have been completed. Finally, Figure 1e highlights from a preliminary study an oscillatory behavior in the battery potential during the first charge of the cell with LiBr in the electrolyte composition, in galvanostatic condition. This is a promising result that indicates the system susceptibility for electrochemical instabilities.After determining an interval of current operation, galvanodynamic experiments will be conducted to obtain the conditions under which spontaneous potential oscillations emerges. Thereby, it will be designed appropriated cells in which in operando experiments will be conducted to monitor spontaneous emergence of patterns with temporal resolution. Then, it is hoped to understand how experimental parameters can impact the nonlinear dynamics observed during the battery operation. Figure 1: (a) Cyclability Li-O2 batteries tests in galvanostatic conditions (±50 µA) with different electrolytes: with and without LiBr in its composition. (b) 1st and 5th cycles for cyclability Li-O2 batteries tests with different electrolytes: with and without LiBr in its composition. (c) Evolution of Li-O2 battery capacity with and without LiBr in the electrolyte composition. (d) Cyclability Li-O2 batteries tests in different galvanostatic conditions with LiBr in the electolyte. (e) Oscillatory behavior during the charge in a galvanostatic regime at (±50 µA) in a battery assembled with LiBr in electrolyte composition.[1] G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson, W. Wilcke, J. Phys. Chem. Lett. 2010, 1, 2193–2203.[2] Y. Hase, Y. Komori, T. Kusumoto, T. Harada, J. Seki, T. Shiga, K. Kamiya, S. Nakanishi, Nat. Commun. 2019, 10, 596.[3] K. Krischer, Nonlinear Dynamics in Electrochemical Systems, 2003.[4] Z. Liang, Y. C. Lu, J. Am. Chem. Soc. 2016, 138, 7574–7583. Figure 1

The Efficiency of the Electro-Oxidation of Small Organic Molecules: Oscillations and Mechanism

Many hydrogen-carriers have been considered for the direct use in fuel cells and also in the electrochemical reform, where they are oxidized instead of water at lower potentials. We have investigated the electro-oxidation of formaldehyde, formic acid, methanol, and ethanol,[1] and of ethylene glycol, glycerol, and glucose[2] on polycrystalline platinum in acidic media, and compare the activity under conventional and oscillatory regimes. The comparison is carried out by different means and generalized by the use of simple identical experimental conditions in all cases. The activity along slow potentiodynamic curves, inferred by the peak current, decreases in the following sequence: formaldehyde ~ formic acid > methanol > ethanol ~ ethylene glycol > glucose > glycerol. The ubiquitous occurrence of potential oscillations under galvanostatic regime is associated with excursions of the electrode potentials to lower values, which noticeably decreases the overpotential of the anodic reaction, when compared to that in the absence of oscillations. Considerable enhancement in the power density was observed in an idealized fuel cell operated with those fuels when operated under oscillatory regime. Furthermore, the spontaneous periodic self-cleaning processes, present important advantages to the use of autonomous oscillations to reach both higher and long-term activities, and stability [3]. The discussion of these half-cell studies is complemented with experiments with proton exchange membrane fuel cells, namely direct liquid fuel cells (DLFCs). We have investigated the effect of temperature on DLFCs under conventional and oscillatory conditions.[4] Methanol, formic acid, ethanol, and dimethyl ether (DME) were used as fuels and identical experimental conditions were used in all cases. Some findings observed in half-cells are also observed in the DLFCs studied, but few peculiarities of the experimental configuration adopted in the later arise. Finally, mechanistic aspects underlying the observed dynamics are discussed.[1] M. V. F. Delmonde, L. F. Sallum, N. Perini, E. R. Gonzalez, R. Schlögl, H. Varela, J. Phys. Chem. C 120 (2016) 22365.[2] G. B. Melle, T. A. Ferreira, R. L. Romano, H. Varela, in preparation (2020).[3] J. A. Nogueira, P. P. Lopes, N. M. Markovic, H. Varela, Electrochem. Commun. 121 (2020) 106853.[4] J.A. Nogueira, H. Varela, Energy Fuels 34 (2020) 12995.

The Role of Self-Organized Reaction Rates in the Micro-Kinetic Description of Electrocatalytic Reactions

Kinetic modeling of electro-catalytic reactions based on micro-kinetic approaches allows at obtaining information on reaction mechanisms and determine how some parameters, such as rate coefficients, are affected by the conditions under which the catalyst operates.1 Unlike a kinetic model, which only considers the overall reaction rate, micro-kinetic modeling provides access to fundamental information of elementary reaction steps. In this sense, we present a micro-kinetic model linked to a proposed reaction mechanism for the formic acid electro-oxidation reaction (FAEOR) on platinum. The model was tested by numerical simulations under voltammetric and oscillatory regimes.2 We formulated the micro-kinetic model using the following workflow stages: i) Gathering information from literature, including spectroscopy and computational chemistry studies; ii) Mechanism proposal and model description that consists of a set of ordinary differential equations involving charge and mass balances on the electrocatalytic surface; iii) Numerical resolution of the model using a set of test parameters; iv) Comparison with different sets of experiments including cyclic voltammetry and potential oscillations under the galvanostatic regime. The initial electrokinetic parameters, associated with the rate coefficients, were adjusted and re-evaluated by an iterative procedure to improve the description of the experimental observations.The type of electrochemical experiments performed in the micro-kinetic model is fundamental to the success of validation. Under oscillatory conditions, galvanostatic experiments are an excellent complement to other electrodynamic techniques because the rate of several processes that are happening in the electrode-solution interface can be associated with oscillatory features such as frequency, amplitude, and oscillation format.3 Thus, the self-organized mode offers a sensitive tool to evaluate the proposed reaction mechanism. In this work, we studied the oscillatory characteristics in the electrochemical response of the FAEOR to determine the rate coefficients as a function of the electrode potential. The consistency of the numerical solution of our model (Figure 1), namely frequency and amplitude of the potential oscillations, with the experimental response makes our proposal a plausible possibility for the FAEOR, providing evidence to the clarification of this controversial process through a micro-kinetic approach. References (1) Campbell, C. T. Micro- and Macro-Kinetics: Their Relationship in Heterogeneous Catalysis. Top. Catal. 1994, 1 (3–4), 353–366. https://doi.org/10.1007/BF01492288.(2) Calderón-Cárdenas, A.; Hartl, F. W.; Gallas, J. A. C.; Varela, H. Modeling the Triple-Path Electro-Oxidation of Formic Acid on Platinum: Cyclic Voltammetry and Oscillations. Catal. Today 2021, 359, 90–98. https://doi.org/10.1016/j.cattod.2019.04.054.(3) Machado, E. G.; Varela, H. Kinetic Instabilities in Electrocatalysis. Encyclopedia of Interfacial Chemistry; Elsevier, 2018; pp 701–718. https://doi.org/10.1016/B978-0-12-409547-2.13369-4. Figure 1

Correlation of Morphology and Crystal Structure of Metal Powders Produced by Electrolysis Processes

In this review paper, morphologies of metal powders produced by the constant (potentiostatic and galvanostatic) regimes of electrolysis from aqueous electrolytes are correlated with their crystal structure at the semiquantitative level. The main parameters affecting the shape of powder particles are the exchange current density (rate of electrochemical process) and overpotential for hydrogen evolution reaction. Depending on them, various shapes of dendrites (the needles, the two-dimensional (2D) fern-like, and the three-dimensional (3D) pine-like dendrites), and the particles formed under vigorous hydrogen evolution (cauliflower-like and spongy-like particles) are produced by these regimes of electrolysis. By decreasing the exchange current density value, the crystal structure of the powder particles is changed from the strong (111) preferred orientation obtained for the needle-like (silver) and the 2D (lead) dendrites to the randomly orientated crystallites in particles with the spherical morphology (the 3D dendrites and the cauliflower-like and the spongy-like particles). The formation of metal powders by molten salt electrolysis and by electrolysis in deep eutectic solvents (DESs) and the crystallographic aspects of dendritic growth are also mentioned in this review.

Spectroelectrochemical investigation of electrodeposited polypyrrole complexes with sulfonated polyelectrolytes

Films of polypyrrole(PPy)-polyelectrolyte(PE) complexes have high potential as active material in bio-sensing and biomedical applications due to high hydrophilicity and biocompatibility, as well as, as an electroactive polymer matrix in electrode materials for rechargeable lithium batteries and supercapacitors. Comparative study is presented on the influence of PE structure on the properties of PPy-PE complexes. PPy films were prepared by pyrrole electrochemical polymerization in cyclic voltammetry, potentio- and galvanostatic regimes in aqueous solutions of acid or salt forms of sulfonated polyelectrolytes (PEs) with different flexibility of the polymer backbone. Electrochemical polymerization of pyrrole in these conditions results in obtaining high-quality polymer films efficiently rechargeable in aqueous and nonaqueous media. The films obtained were characterized by cyclic voltammetry, UV-Vis-NIR spectroelectrochemistry in aqueous and nonaqueous media, as well as, by Raman spectroelectrochemistry in aqueous medium. Gaussian deconvolution of the Raman spectra in the range of 850–1150 cm−1 allowed one to trace peculiarities of polaron-bipolaron transitions depending on the PE structure. It was shown that the films of PPy complexes with the salt forms of rigid-chain PEs had redox numbers comparable to those of the complexes with the flexible-chain PEs, but they were more stable during CV cycling in nonaqueous medium.

Preparation of fine copper powders by galvanostatic regime of electrolysis of copper scrap in a cylindrical electrochemical cell

A cylindrical electrochemical cell is designed. The mean residence time and volume fraction of stagnant zone under perpendicular inflow (PI) and tangential inflow (TI) are studied by hydraulics experiments. Fine copper powders are prepared by galvanostatic regime of electrolysis of copper scrap. The effects of current density on current efficiency, cell voltage, power consumption, and the particle size of copper powder (D50) are investigated both under PI and TI. The results show the residence time of fluid particles is prolonged with decreasing flow rate, which is beneficial to the reaction of substances, and volume fraction of stagnant zone decrease. Maximum current efficiency and minimum power consumption are obtained under PI, which corresponds to the maximum D50 particle size. According to SEM figures, deposits on the surface of cathode are honeycomb-like forms. There are more arborous secondary dendrites under PI, but more compact and stout dendrites under TI in comparison.

Kinetics of galvanostatic anodic polarization of Zn in NaOH solutions and characterization of the resulting layers

The kinetics of film formation on zinc in water solutions of sodium hydroxide was investigated. The concentrations of the forming electrolyte were varied between 0.001 and 0.08 mol dm−3. The anodic polarization was carried out in galvanostatic regime with current densities between 5 and 35 mA cm−2. A point at which the rate of film formation changes was found on the kinetic curves. The dependencies of the slopes of the kinetic curves before and after this point, the position of this point, as well as the breakdown voltage on the concentration of the forming electrolyte, on the applied current density and on the temperature were investigated. By means of X-ray diffractometry and Raman spectrometry was found, that the formed film consists of polycrystalline zinc oxide. Further investigations using ATR-FTIR and XPS confirmed these observations and suggested the possible presence of small quantities of impurities in the layers. SEM and AFM studies show that in the initial stages of anodic polarization a film is formed, which fills the concavities of the metal surface and afterwards the electrode is covered with hexagonal plate-like crystals. Subsequently the surface becomes smoother again. Some thermodynamic considerations, as well as some additional experiments with sandwich structures formed on the electrode surface suggest that the shift in the kinetic curves is caused by a change in the specific resistivity of the layer, occurring at a temperature of about 305 K.

Improvement corrosion resistance of Ni-Ti alloy by TiO2 coating and hydroxyaptite/TiO2 composite coating using micro arc oxidation process

Titanium oxide ceramic coatings were prepared by micro-arc oxidation (MAO) in galvanostatic regime on biomedical NiTi alloy produce via powder metallurgy, also the composite coating of HA/TiO2 was prepared on the surface of nitinol using a one-step micro-arc oxidation (MAO), the process time for each case at 280 V was 30 min. The surface of TiO2 coating exhibited a typical porous and rough structure. In this study, the hydroxyapatite, Ca–P precipitate it assisted to close the porous and the morphology of the surface coating showed a flower-like structure owing to the incidence of self-assembly because of the longtime of precipitate. Surface characterization of the coatings was determined utilizing SEM, XRD, and AFM. The surface morphology, topography and coating thickness were determined as well. In this study, the properties of corrosion were studied utilizing potentiostat test unit in Ringer's solution. Based on the results, the corrosion properties of NiTi alloys considerably improved due to the existence of TiO2 and HA/TiO2 coatings.

Nickel Phosphide Nanomaterials for Hydrogen Evolution Reaction

Among non-noble-metal-based materials considered for hydrogen evolution reaction (HER) electrocatalysts, metal phosphides are quite popular, as they are chemically and mechanically stable, have good conductive properties and a high number of potential active sites on the surface [1,2]. Nickel phosphide, in particular, has been investigated – with promising results – as a HER catalyst in a variety of environments [3].When it comes to the morphology of an electrocatalyst, ordered nanowire arrays seem promising. Not only do they have a large and easily accessible active surface area, but also offer a simplified electron transfer path and can be synthesized directly on a conducting substrate, which greatly reduces or removes the need for additional binders. Moreover, such a nanomaterial should be less prone to damage during usage, especially when gas evolution occurs. Nanowire arrays are often synthesized by electrodeposition into nanoporous anodic aluminum oxide (AAO) membranes. However, aqueous solutions typically used in electrosynthesis of various metal-based materials have several limitations, which may cause brittleness or sponginess of structures [5,6]. These problems can be overcome by using non-aqueous solutions, like those based on deep eutectic solvents (DESs), which offer extended potential windows and low toxicity and are easy to prepare.In this research electrodeposition of nickel in a DES-based environment is used to obtain nickel and nickel phosphide nanowire arrays of good quality, and the prepared nanostructures are then tested as HER electrocatalysts.Ordered arrays of nickel nanowires were prepared by electrodeposition of Ni into AAO templates. A non-aqueous bath based on a mixture of choline chloride and ethylene glycol in molar ratio of 1:2 was used; Ni concentration in the bath was 1.14 M. The electrodeposition was performed at 70 ºC under galvanostatic regime. Nickel phosphide nanowire arrays were obtained via phosphorization of previously prepared nickel nanowires. In order to do so, the AAO membranes with Ni nanowires inside were annealed with red phosphorus in argon atmosphere at 500 ºC for 2 h. The morphology and chemical composition of nickel and nickel phosphide nanowires were examined by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Electrochemical tests were carried out in a three-electrode cell, in 0.5 M H2SO4. For all experiments except XRD AAO templates were removed in 1 M NaOH.AcknowledgementsThis work was supported by the National Science Centre Poland (Grant No. 2017/26/M/ST5/00715). The participation in this conference was made possible by financial support from the Polish Ministry of Science and Higher Education within a conceptual non-competitive project entitled "Best of the best! 4.0." (Measure 3.3). The project is co-financed from the European Social Fund under the Operational Programme Knowledge, Education, Development 2014-2020.

Thorough Analysis of the Effect of Temperature on the Electro-Oxidation of Formic Acid

In this work, the effect of temperature on the electro-oxidation of formic acid on platinum was modeled and numerically investigated. Numerical simulations were carried out using an electrokinetic model recently validated through voltammetric and galvanostatic experiments. We show that the intrinsic electrocatalytic activity of the working electrode for the overall electrochemical reaction can hardly be interpreted from the apparent activation energy due to the complexity of the reaction scheme. A detailed analysis is possible through the estimation of the activation energies determined from the individual rate coefficients. By doing so, we observed that the direct pathway, with an activation energy of 91 kJ mol–1 at 0.40 V and 72 kJ mol–1 at 0.80 V, is the energetically easiest pathway for the formation of CO2 in the proposed reaction scheme. Regarding the self-organized potential oscillations under the galvanostatic regime, our model was able to reproduce experimentally observed results including the phenomena of temperature compensation and overcompensation. Importantly, we have introduced a formalism to classify the elementary steps that contribute to the increase and decrease of the oscillatory frequency in electrochemical systems. Our results shed light on the understanding of the temperature dependence of complex electrocatalytic reactions, and the developed methodology was proven to be robust and of general applicability.

Early-Stage Growth Mechanism and Synthesis Conditions-Dependent Morphology of Nanocrystalline Bi Films Electrodeposited from Perchlorate Electrolyte.

Bi nanocrystalline films were formed from perchlorate electrolyte (PE) on Cu substrate via electrochemical deposition with different duration and current densities. The microstructural, morphological properties, and elemental composition were studied using scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy-dispersive X-ray microanalysis (EDX). The optimal range of current densities for Bi electrodeposition in PE using polarization measurements was demonstrated. For the first time, it was shown and explained why, with a deposition duration of 1 s, co-deposition of Pb and Bi occurs. The correlation between synthesis conditions and chemical composition and microstructure for Bi films was discussed. The analysis of the microstructure evolution revealed the changing mechanism of the films’ growth from pillar-like (for Pb-rich phase) to layered granular form (for Bi) with deposition duration rising. This abnormal behavior is explained by the appearance of a strong Bi growth texture and coalescence effects. The investigations of porosity showed that Bi films have a closely-packed microstructure. The main stages and the growth mechanism of Bi films in the galvanostatic regime in PE with a deposition duration of 1–30 s are proposed.

Aluminium electrodeposition under novel conditions from AlCl3–urea deep eutectic solvent at room temperature

The electrodeposition of aluminium on glassy carbon and aluminium substrates from AlCl3–urea deep eutectic solvent (DES) system at near room temperatures was investigated, without additional purification of the chemicals used to prepare the electrolyte and without rigorous control of moisture and oxygen present in the working space. The effects of changing temperature, working potential, controlled deposition current density and deposition time on the morphology of deposited aluminium without stirring of the electrolyte were recorded. Using potentiostatic and galvanostatic techniques, aluminium was electrodeposited from the deep eutectic solvent (n(AlCl3):n(urea)=1.6:1) onto glassy carbon and aluminium substrates at temperatures ranging from 25 to 50 °C. Using SEM, EDS and XRD techniques, substrates were studied and confirmed the presence of aluminium deposits following both potentiostatic and galvanostatic regimes. The shape and size of Al grains deposited depended on the time of deposition and varied in size from nanometers to micrometers and in shape from regular crystal forms to needle-like and flake-like structures.

Study of the Mn2+ ions influence in the deposition electrolyte on the electrochromic properties of obtained Ni(OH)2 films

An attempt was made to co-deposit nickel and manganese hydroxide films to be used as anodic electrochrome. Cathodic template method with polyvinyl alcohol was used for this. Deposition was conducted in the galvanostatic regime from the solution containing nickel and manganese nitrates in an 8:1 molar ratio.As a result of the work, two films were deposited: one from pure nickel nitrate and one from nickel and manganese nitrate solutions. Analysis of the synthesized films, revealed significant differences in structural, electrochemical and optical properties. The film deposited from the pure nickel nitrate solution was composed of a single α-like form Ni(OH)2. On the other hand, the film deposited from the manganese-containing solution was composed of two phases. Morphology comparison revealed that the surface of the undoped film is rather flat, with small bumps up to 160 nm. The Mn-doped film had many ridges of up to 1200 nm.Electrochemical properties of the film deposited in the presence of Mn were inferior to the film deposited from the pure solution. This is manifested in lower current densities and lower specific capacities of oxidation and reduction processes. Electrochromic properties of the film deposited in the presence of manganese were somewhat worse as well.A mechanism explaining the decrease of specific characteristics of the film in the case of using such deposition method was suggested. The mechanism lies in the formation of the second manganese-containing phase. This phase is rather inert and decreases the content of electrochemically active Ni(OH)2 in the film.The authors also suggested possible uses of the resulting structure

The study of activation impact during formation and testing of Ni(OH)2 electrochromic films in the presence of Al3+ and WO42- ions

The present study is devoted to an attempt to improve the characteristics of nickel hydroxide electrochromic films. The study of optical and electrochemical characteristics of the films, prepared through the codeposition of nickel and aluminum and additional modification during the cycling of the prepared samples was conducted. The modification was realized through the cycling of films in the solution of potassium hydroxide with small amounts of potassium tungstate. Over the course of the study, a series of films were obtained, which demonstrated different electrochemical and optical behavior. Thus, it was found that both the deposition method and the cycling regime have a significant effect on the characteristics of electrochromic films. It is shown that for the potentiodynamic regime, the best characteristics were demonstrated by the film without modifications or additives. On the other hand, all modified films, codeposited with aluminum, demonstrated better characteristics in galvanostatic regimes over the reference film. It would appear that this is due to the physico-chemical features of the films deposited in the presence of additives. In turn, the use of high-rate cycling regime leads to a decrease of electrochromic characteristics, namely higher degree of irreversibility. Despite the difference of currents in different galvanostatic regimes by 4 times, the coloration degree did not differ much. It was also found that the presence of 0.3 mM potassium tungstate in the cycling electrolyte resulted in a significant decrease in the electrochromic properties of the samples

A study of an electrochromic device based on Ni(OH)2/PVA film with the mesh-like silver counter electrode

The study is devoted to the development and testing of the electrochromic device based on Ni(OH)2/PVA (polyvinyl alcohol) composite and mesh counter-electrode. A copper wire with a layer of electroplated silver layer is proposed as a mesh material. Glass coated with fluorine-doped tin oxide after special treatment was used as a substrate for electrochrome deposition. The treatment lies in the shallow dissolution of the surface by means of soft electrochemical etching. The distance between the mesh and electrochromic electrodes was small and equal to 1.5 mm.The proposed design of the electrochromic device can lead to lower cost. However, it limits the range of possible applications for light windows or upper parts of view windows, building partitions.Over the course of the studies, it was demonstrated that the electrochromic device is operational and can be used for further scaling. Parameters of electrochemical cycling – working voltage window and current density, were found. It was found that the use of galvanostatic regime for color switching results in linear characteristics of the device.The use of the chosen voltodynamic regime results in a decrease of specific characteristics of the device – coloration degree and reversibility during bleaching.It was found that due to the small potential difference of nickel oxide and silver electrodes, the polarity of voltage during coloration and switching changes. Additionally, it is noted that no gas evolution was observed over the course of the experiments

Anodic behavior of zinc in aqueous borate electrolytes

The behavior of zinc in aqueous borate electrolytes at anodic polarization in galvanostatic regime was investigated. Induction periods and voltage breakdown phenomena were observed in the respective kinetic curves. The dependence of the duration of these induction periods on the polarization conditions (j = 5 ÷ 60 mA cm−2; H3BO3 content = 0.1 ÷ 3 wt %; pH 5.8 ÷ 8.2; t = 10 ÷ 30 °C) was studied. The registered breakdown voltages obey the law of Burger and Wu, and exceeded 200 V. Clear periodical oscillations of the forming voltage were registered at high current densities. The obtained films were submitted to X-ray diffractometry (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR). The data obtained by these observations have revealed that these layers are composed of amorphous 2ZnO.3BO3.7H2O. Furthermore, on the basis of the scanning electron microscopy (SEM) observations a conceptual model was developed for qualitative explanation of the film growth mechanism. The appearance of high amplitude oscillations was explained by complete disjoining of the layer due to accumulation of mechanical stress. These conclusions were confirmed by obtained atomic force microscopy (AFM) images and (ICP-OES) measurements of the electrolytes after the respective polarizations.

Influences of edge effect on microstructure and corrosion behaviour of PEO coating

ABSTRACTPlasma electrolytic oxidation (PEO) coatings were prepared on AA1060 alloy substrate by a galvanostatic regime in an alkaline sodium hexametaphosphate ((NaPO3)6) electrolyte, and the influences of electric field edge effect on the microstructure, surface roughness, semiconducting behaviour and corrosion behaviour of the coatings were investigated by means of scanning electron microscopy (SEM), laser scanning confocal microscopy (LSCM), Mott–Schottky analysis and electrochemical impedance spectroscopy (EIS) tests. The results showed that an ordered porous film was formed before the presence of micro-discharges during the PEO treatment. The electric field strength at the specimen edge region was about twice as large as that in the specimen centre region, which resulted in the differences in the microstructure, surface roughness and corrosion behaviour of the specimens at the edge and centre regions. All PEO-coated specimens showed n-type semiconducting behaviour, and the coating at the edge region had a lower carrier concentration.

Re–Ni cathodes obtained by electrodeposition as a promising electrode material for hydrogen evolution reaction in alkaline solutions

Re–Ni cathodes catalytic activity towards hydrogen evolution reaction (HER) in the solution of sodium hydroxide is studied. Re–Ni coatings were deposited onto the surface of a copper substrate from ammonium-citrate solutions in the galvanostatic regime. The obtained deposits were examined by the complex of modern physical methods, namely: XPS, XRD, and SEM with X-ray microanalyzer. It was established that the full reduction of nickel species occurs during electrodeposition, while rhenium species electroreduction is incomplete. The obtained electrolytic deposits contained a high amount of hydrogen sorbed by them during electrodeposition. The increase in rhenium content in Re–Ni deposits caused the increase in their catalytic activity towards HER that was accompanied by Tafel slope decreasing. Heat treatment at 450 °C in vacuum resulted in a decrease in catalytic activity towards HER. It was suggested that high catalytic activity towards HER is caused by rhenium oxides included in the Re–Ni coatings. Heat treatment results in rhenium oxides reduction by sorbed hydrogen that causes a decrease in catalytic activity towards HER.

A possible mechanism of formation of flower-like MgO/Mg(OH)2 structures by galvanostatic molten salt electrolysis: The concept of local diffusion fields

The processes of electrodeposition from magnesium nitrate hexahydrate melt by galvanostatic regime of electrolysis have been analyzed. The structure of the formed particles was examined by X-ray diffraction (XRD), which revealed formation of MgO/Mg(OH)2 mixture by molten salt electrolysis. Morphologies of the produced deposits were characterized by scanning electron microscopy (SEM) technique. The flower-like forms constructed from very thin needles with sharp tips were predominately formed by electrodeposition at various current densities and with various amounts of electricity. Aside from that, holes formed from detached hydrogen bubbles of various shapes and sizes, from dish-like holes to those resulting in honeycomb-like structures were also observed. Mechanism of formation of the flower-like forms has been elucidated applying the general theory of disperse deposits formation which is based on the concept of local (spherical and cylindrical) diffusion fields. It is shown that the growth of the needles occurs under the conditions of predominant spherical diffusion control, while the contribution of the cylindrical diffusion to the final shapes was negligible. Comparison with morphologies of deposits obtained by electrodepositions from aqueous magnesium salt electrolytes was also made and discussed.

Overpotential controls a morphology of electrolytically produced copper dendritic forms

The morphologies of copper dendritic forms obtained in both potentiostatic and galvanostatic regimes of electrolysis with various amounts of the electricity were analyzed by the scanning electron microscopy (SEM) technique. Irrespective of amount of passed electricity, 3D (three dimensional) pine-like dendrites with sharp tips were formed in the potentiostatic regime of electrolysis. On the other hand, the amount of passed electricity had a strong effect on the shape of the 3D pine-like dendrites formed in the galvanostatic regime of electrolysis. Dendrites with sharp tips were formed with smaller amount of passed electricity, while dendrites with globular tips were formed with larger amounts. The change in the shape of the galvanostatically synthesized 3D pine-like dendrites was explained by comparison with copper deposits obtained potentiostatically at overpotentials that corresponded to the final overpotentials during galvanostatic regime of electrolysis for the analyzed amounts of electricity. Based on the similarity of the obtained morphologies at the macro level, it was concluded that the overpotential plays a crucial role in the formation of the electrolytically synthesized dendrites and that the controlled conditions of electrolysis could represent a suitable way for a synthesis of spherical Cu particles by electrolysis.