Potentiostatic Regime Research Articles

Relationship between the kinetic parameters and morphology of electrochemically deposited lead

The processes of lead electrodeposition from electrolytes of various concentrations of sodium nitrate as the supporting electrolyte have been examined by chronoamperometry and by the scanning electron microscopic (SEM) analysis of deposits obtained in the potentiostatic regime of electrolysis. The good agreement between the diffusion coefficents determined by Cottrell equation and non-linear fitting method was observed. For the first time, the transition from the mixed ohmic-diffusion to the full diffusion control was defined from the analysis of Cottrell equation. The parameters, such as the number density of active sites and the nucleation rate constant, obtained by non-linear fitting method were discussed in accordance with the fact that lead belongs to the group of metals characterized by the high values of the exchange current density. The data obtained by the chronoamperometric analysis were succesfully correlated with morphologies of electrodeposited lead obtained in the different types of electrodeposition control.

Teorell instability in concentration polarization.

We investigate the development of electro-osmotic (Teorell) oscillations at a weakly charged microporous membrane without a preimposed transmembrane electrolyte concentration drop. This drop, necessary for the occurrence of oscillations, develops spontaneously as a result of concentration polarization in the solution layers adjacent to the membrane. A three-layer model comprising a membrane flanked by two diffusion layers is proposed and analyzed for galvano- and potentiostatic regimes of operation.

Electrochemical and Crystallographic Aspects of Lead Granular Growth

Lead granules synthesized by the potentiostatic regime of electrolysis were characterized by the scanning electron microscopy technique. Effect of the different parameters of electrolysis, such as solution composition, overpotential of electrodeposition, and quantity of the electricity, on lead granular growth has been systematically investigated. Aside from the electrochemical aspects of lead granular growth, crystallographic aspects of the obtained granules were also analyzed. In the dependence of the electrodeposition conditions, granules of various shapes were obtained. The granules, such as octahedrons and hexagons, as well as many various types of twinned particles: single-twinned, multiply-twinned, lamellar-twinned, and many other complicated shapes denoted as polyparticles, were synthesized through regulation of the parameters of electrolysis. Increasing both the concentration of Pb2+ ions and overpotential of the electrodeposition favored the formation of more complicated forms. Formation of granules of specified crystallographic characteristics was also correlated with the basic principle of metal electrocrystallization.

Electrochemical deposition of zinc from deep eutectic solvent on barrier alumina layers

The direct use of high-ordered porous templates of anodic aluminium oxide for formation of nanostructures via electrochemical deposition is difficult because the presence of the of barrier alumina layer in the bottom of pores. In this paper, zinc electrodeposition from a solution of ZnCl2 in a choline chloride/ethylene glycol eutectic mixture on dense alumina layers of different thicknesses was studied. In a potentiostatic regime, the deposition on a barrier layer was found to be hardly possible even when the layer is a native alumina film (about 2nm thick). Choline and ethylene glycol form an adsorbed stable layer at the alumina/electrolyte interface and thereby block the access of zinc chloride anions to the electrode.It has been shown that zinc can be directly deposited on a barrier layer by means of application of a combined potential mode in which an AC component of 50mV superimposed on the DC potential of −1.6V. Increasing the temperature of the eutectic solution allows zinc electrodeposition in a potentiostatic regime already at 50°C. By application of any of these conditions, zinc can be deposited on a barrier alumina layer up to about 60nm thick. The range of frequencies of the AC component and the temperature range where the deposition is optimal were considered. The mechanism of unblocking of the electrode by the alternating potential and by the temperature rise has been suggested.

The potentiostatic current transients and the role of local diffusion fields in formation of the 2D lead dendrites from the concentrated electrolyte

Electrodeposition of lead from the concentrated nitrate electrolyte in the potentiostatic regime of electrolysis has been investigated by the analysis of the potentiostatic current transients and scanning electron microscope (SEM). It was found that the process of Pb nucleation from the concentrated electrolyte follows Scharifker model based on the 3D (three-dimensional) instantaneous nucleation with diffusion-controlled growth. The deviations of the obtained dependencies from the theoretical prediction for this model have been also discussed. Needle-like and fern-like dendrites, as well as crystals of irregular shape (precursors of dendrites) were formed in the diffusion controlled electrodeposition. The SEM analysis of these dendritic forms revealed their 2D (two-dimensional) growth. The size of needle-like dendrites was considerably larger than the size of both the fern-like dendrites and the irregular crystals. Although the electrodeposition process occurred inside the linear diffusion layer of the macroelectrode, the shape and size of dendrites were determined by the effect of local diffusion fields formed around tips (spherical diffusion) and top edges (cylindrical diffusion) of protrusions formed in the initial stage of the electrodeposition. The growth rates under the conditions of spherical and cylindrical diffusion control were mutually compared and a faster growth under the conditions of the spherical, in relation to the cylindrical diffusion, was proved. The effect of the current density distribution on formation of the final forms of Pb dendrites was also discussed.

In Vitro Corrosion Studies of Surface Modified NiTi Alloy for Biomedical Applications

Electropolishing was conducted on NiTi alloy of composition 49.1 Ti-50.9 Ni at.% under potentiostatic regime at ambient temperature using perchloric acid based electrolyte for 30 sec followed by passivation treatment in an inorganic electrolyte. The corrosion resistance and biocompatibility of the electropolished and passivated alloys were evaluated and compared with mechanically polished alloy. Various characterization techniques like scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy were employed to analyze the properties of surface modified and mechanically polished alloys. Water contact angle measurements made on the passivated alloy after electropolishing showed a contact angle of 35.6°, which was about 58% lower compared to mechanically polished sample, implying more hydrophilicity. The electrochemical impedance studies showed that, for the passivated alloy, threefold increase in the barrier layer resistance was obtained when compared to electropolished alloy due to the formation of compact titanium oxide. The oxide layer thickness of the passivated samples was almost 18 times higher than electropolished samples. After 14 days immersion in Hanks’ solution, the amount of nickel released was 315 ppb which was nearly half of that obtained for mechanically polished NiTi alloy, confirming better stability of the passive layer.

Experimental Realization of TiO<sub>2</sub> Nanosponge/Spin-coated P3HT Heterojunction Solar Cells

Production and characterization of multilayered polymer solar cells consisting of P3HT thin films deposited on titanium dioxide nanosponges are reported. The photovoltaic response is remarkable, considering the simplicity of the thin film production method employed here. The nanostructured TiO2 anodic films were grown in buffered aqueous phosphoric acid solutions under potentiostatic regime at room temperature and then characterized using scanning electron microscopy. The P3HT films were prepared by spin coating technique and the samples were annealed at 180°C. Conversion efficiencies around 0.4% were obtained, with open circuit voltages and short circuit current densities as high as 560 mV and 2.7 mA/cm2, respectively.

Niobium Speciation in NaCl-KCl Based Melts: An Electrochemical and Spectroelectrochemical Study

Molten chlorides can be used for niobium electrowinning and electrorefining. Understanding niobium electrochemical properties and speciation in chloride melts is essential for designing or optimizing the industrial electrochemical process. Apart from the technological value, the information on the kinetics of the electrode reactions can also be used for verifying theoretical predictions concerning the nature of the heterophase electrochemical reactions.In the present study a variety of electrochemical techniques (polarization curves measurements (PCM), chronoamperometry (CA), chronopotentiometry (CP), linear (LVA), cyclic (CVA) and square-wave voltammetry (SWVA), impedance spectroscopy) were applied for studying electrode processes involving niobium species in NaCl-KCl-based melts. In addition the most valuable and reliable information about niobium speciation was obtained by combining electrochemical and spectroscopic (measuring electronic absorption spectra, EAS) methods.The results of spectroelectrochemical experiments showed that the major product of niobium anodic dissolution in chloride melts at current densities up to 40 mA/cm2 were niobium(III) species, NbCl6 3-. In the potentiostatic regime of anodic dissolution of niobium metal niobium(III) complex ions were formed at potentials below -1.15 V vs.chlorine reference electrode. The polarization curves characterizing the anodic dissolution of niobium consist of three or more waves. The first wave is associated with reaching limiting current density of niobium ionization according to the three-electron scheme. Further increase of applied current leads to the oxidation of Nb(III) to Nb(IV) ions. These conclusions were confirmed by determined values of potentials of niobium or indifferent electrodes in the equilibrium with the melts containing Nb(III) and Nb(IV) ions.When studying anodic dissolution of niobium we found that at high current densities there is sudden increase of the ohmic part of voltage drop sometime accompanied by increase of the anode potential. As a result stationary and non-stationary polarization dependencies exhibit unstable behavior. This is explained by the salt passivation, SP. The product of the SP of niobium is unstable NbCl4, which decomposes to NbCl3.13 and volatile NbCl5.The results of electrochemical experiments (PCM, CVA, LVA, CA, CP and SWVA) showed that soluble niobium(V) species were formed during Nb(IV) electrooxidation and represented the highest oxidation state of niobium in NaCl-KCl melts. We noticed that peak potential value (both cathodic and anodic) on CV changes with scan rate, whereas the ip/ν1/2 value remains constant. The formation of Nb(V) species was confirmed by the disappearance of absorption bands in the visible part of the spectrum due to Nb(V) species formation during elecrooxidation of niobium-containing melts at potentials up to -0.5 V. The quasireversibility of Nb4+↔Nb5+reaction was explained by the instability of niobium(V) species.Niobium(IV) ions were formed when potentiostatic electrolysis of NbCl3-NaCl-KCl melts (prepared by anodic dissolution of niobium metal) was performed at -(1.0÷0.8) V vs.chlorine reference electrode on a glassy-carbon anode, Fig. 1.The electroreduction of niobium on a tungsten electrode in NbCl3-NaCl-KCl melts at potentials below -1.45 V led to the formation of metallic niobium (Fig. 2), no evidence of any intermediate product formation was obtained.Linear voltammetry measurements showed that intermediate products can be formed during elecroreduction of NaCl-KCl-NbCln (n=3.8-4.1) melts. Most likely the electroreduction of Nb(IV) results in the formation of niobium(III) ions. The shape of cyclic voltammograms recorded in niobium containing melts depended on the scan rate. Similar sets of cyclic voltammograms were obtained at different temperatures and niobium concentrations. SWVA measurements performed on different electrodes also confirmed that there is more than one step of electroreduction. The obtained results were explained by overlapping of the following electrode reactions: Nb(IV) → Nb(III), Nb(IV) → Nb(0) and Nb(III) → Nb(0).

Oscillatory electro-oxidation of thiosulfate on gold

We report experimental results on the oscillatory electro-oxidation of thiosulfate on polycrystalline gold electrode in buffered media of pH 6.0. Importantly, we observed oscillations in the current density (potential) when the system was studied under linear sweep of potential (current density). The system displayed supercritical Hopf, period-doubling, homoclinic and bursting bifurcations at different applied currents and potentials. After presenting the dynamics under both potentiostatic and galvanostatic regimes, we characterized the system in terms of its electrical (ohmic drop compensation and electrochemical impedance spectroscopy) and chemical (Capillary Electrophoresis analysis) aspects. Ohmic resistance compensation and electrochemical impedance spectroscopy confirmed the hidden N-shaped negative differential resistance oscillatory character. Capillary electrophoresis analysis revealed the predominance of different oxidation products as a function of the applied potential. Finally, we suggest a tentative mechanism underlying the kinetic instabilities.

Electrodeposited hard-magnetic Fe50Pd50 nanowires from an ammonium-citrate-based bath

Fe–Pd nanowires were synthesised in anodic alumina templates by applying both potentiostatic and pulsed electrodeposition regimes. When using potentiostatic deposition, only fragmented nanowires were obtained; however, the use of pulse deposition was shown to be effective for producing solid nanowires. In order to achieve this, different on-times for the deposition and off-times between the pulses in an electrolyte with a constant concentration of Fe(III) and Pd(II) ions at pH 9 were employed. Homogeneous nanowires with the composition Fe55±5Pd45±5, lengths of 2.5μm and diameters of 200nm were synthesised under the following pulsed conditions Eon=−1.4V, ton=2s and Eoff=−0.1V, toff=10s for 5000 cycles. The as-deposited nanowires had a fcc crystal structure and were magnetically soft (HC≈5kA/m) with the easy axis of magnetization perpendicular to the long axes of the nanowires, mainly due to the dipolar coupling within the template. In order to promote the ordering into the L10 phase, annealing in the temperature range 400–700°C for 1–9h in Ar+7% H2 was performed. The highest coercivity of 122kA/m was achieved by annealing at 600°C for 5h.

Templating Effect of Polymeric Electrolytes on the Electropolymerization of Aniline

Preparation of polyaniline (PANI) in the presence of polymeric electrolytes is known to produce the polymer with improved characteristics such as mechanical properties, thermal stability, adherence, controlled morphology, wide pH-range of electroactivity, etc. This contribution describes the results of systematic studies on the influence of the structure of water-soluble polymeric sulfonic acids on PANI electrodeposition and properties. The course of polymerization in potentiostatic, galvanostatic and potential sweep regimes was traced by chronoamperometry, chronopotentiometry, cyclic voltammetry and in situ AFM measurements. It was shown that the structure of polyacid, namely, the rigidity of the polymer backbone, the distance between sulfonic groups on the backbone, the length and rigidity of the side chains containing sulfonic groups, strongly affect the onset potential of aniline oxidation, the character and the rate of the initial stages of PANI/polyacid film growth. The in situ AFM studies performed during potentiostatic polymerization showed that the structure of polyacid (primarily the backbone rigidity) has a direct influence on the shape of PANI “nuclei” and the morphology of the film in the early stages of the electropolymerization (Fig. 1). The obtained films were also characterized by UV-Vis-NIR spectroscopy and spectroelectrochemistry and cyclic voltammetry.This work was supported by the Russian Foundation for Basic Research (grant No.12-03-01087).

Electrodeposition of thin metallic layer for solar cell electrodes

Purpose – The paper aims to show application of the electrochemically deposited coatings for thickening of the screen printed electric paths potentially applied in photovoltaic cells. Design/methodology/approach – The electric paths were screen printed with the use of silver-based paste. The paths were thickened by electrodeposition of thin copper layer in potentiostatic regime from surfactant-free plating bath. The morphology and surface quality of the paths were studied by imaging with scanning electron microscopy. Findings – The electric paths can be thickened successfully, but quality for the screen printed substrate determines quality of deposited layer. The EDX analysis confirmed that the deposited copper layer covered uniformly the printed paths. Research limitations/implications – The adhesion of the copper-covered path to the silicon wafer surface depends on adhesion of the original screen printed path. Originality/value – This paper confirms that electrodeposited copper can be applied for screen printed silver paths thickening in a controllable way.

Functional materials based on manganese dioxide deposited on carbon fiber

The composites manganese oxide/carbon fiber are obtained by chemical and electrochemical deposition. The process of electrodeposition was carried out in the presence of tetraalkylammonium cations of neutral SAA Triton X-100 in a potentiostatic regime by oxidating Mn2+ salts at +1.0 V (with respect to Ag/AgCl). Cathodic deposition was carried out in aqueous electrolytes NH4Cl and KCl and in the presence of polyelectrolyte of chitosan with oxidation of the formed precipitates by oxygen in air. The obtained composites are characterized by scanning electron microscopy, X-ray diffraction, and cyclic volt-amperometry techniques. The mechanism of film formation at the surface of the carbon fiber and the electrochemical properties of the composites are discussed.

Growth and Etch Rate Study of Low Temperature Anodic Silicon Dioxide Thin Films

Silicon dioxide (SiO2) thin films are most commonly used insulating films in the fabrication of silicon-based integrated circuits (ICs) and microelectromechanical systems (MEMS). Several techniques with different processing environments have been investigated to deposit silicon dioxide films at temperatures down to room temperature. Anodic oxidation of silicon is one of the low temperature processes to grow oxide films even below room temperature. In the present work, uniform silicon dioxide thin films are grown at room temperature by using anodic oxidation technique. Oxide films are synthesized in potentiostatic and potentiodynamic regimes at large applied voltages in order to investigate the effect of voltage, mechanical stirring of electrolyte, current density and the water percentage on growth rate, and the different properties of as-grown oxide films. Ellipsometry, FTIR, and SEM are employed to investigate various properties of the oxide films. A 5.25 Å/V growth rate is achieved in potentiostatic mode. In the case of potentiodynamic mode, 160 nm thickness is attained at 300 V. The oxide films developed in both modes are slightly silicon rich, uniform, and less porous. The present study is intended to inspect various properties which are considered for applications in MEMS and Microelectronics.

Templating effect of polymeric sulfonic acids on electropolymerization of aniline

The electrodeposition of polyaniline (PANI) was carried out in potentiostatic, galvanostatic and potential sweep regimes in the presence of water-soluble polymeric sulfonic acids with different structure. The course of polymerization was traced by chronoamperometry, chronopotentiometry, cyclic voltammetry and in situ AFM measurements. The rigidity of the polyacid backbone, the distance between sulfonic groups on the backbone, the length and rigidity of the side chains containing sulfonic groups were shown to affect the onset potential of aniline oxidation, the character and the rate of the initial stages of PANI/polyacid film growth. AFM data indicated that the structure of the polyacids has a direct influence on the shape of PANI “nuclei” and the morphology of the film in the early stages of the electropolymerization. The obtained films were characterized by UV-Vis-NIR spectroscopy and cyclic voltammetry.

Characterization and photocatalytic properties of nanoporous titanium dioxide layer fabricated on pure titanium substrates by the anodic oxidation process

Abstract Nanoporous titanium oxide (TiO 2 ) layers were fabricated on commercially pure titanium (Cp-Ti) substrates by the anodic oxidation method under a potentiostatic regime. Anodic oxidation was performed in an aqueous solution containing 1 wt% hydrofluoric acid at room temperature for 30 min. Subsequently one of the anodized samples was annealed at 480 °C for 2 h in air in order to obtain anatase transformation and increased crystallinity. The average pore diameter of both anodized and anodized+annealed samples were found to be nearly 52 nm and 66 nm, whilst the average inter-pore distances were approximately 65 nm and 76 nm, respectively. Characterization of nanoporous TiO 2 layers was carried out through X-ray diffractometer (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV visible spectroscopy. Effects of heat treatment on properties such as crystallinity, morphology and photocatalytic activity were investigated in details. Finally photocatalytic degradation ratios and changes in degradation kinetics under the presence of both anodized and anodized+annealed catalysts were calculated. Depending on the obtained results, the annealed film with anatase structure demonstrated highly efficient photacatalytic performance.

Microstructure, Magnetic and Electronic Transport Properties of Co–TiO2 Nanocomposite Films in Metal Matrix

In this work, Co–TiO2 metallic composite films with a novel nanostructure have been electrodeposited in potentiostatic regime onto copper substrates, from a solution based on cobalt sulfate containing suspended TiO2 nanoparticles, with magnetic stirring of the electrolyte. The effect of deposited film thickness on the morphology, microstructure, and composition of the films was investigated by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive spectroscopy. Functional properties (magnetic and electronic transport) of films with different thicknesses were studied in a view to find out the possibility for some technological applications. Nanocomposite Co–TiO2 films contain three main phases: hcp Co crystalline grains (9–10 nm average size), TiO2 nanoparticles (28 nm average diameter) embedded in Co metallic matrix and Co(OH)2 adsorbed on the crystallite frontiers. The films display hysteresis (coercive field of 7.8÷11.9 kA/m) and significant values of magnetoresistance (with a maximum of −59 % in the case of 0.07 μm film thickness). These properties can be qualitatively explained both by the elastic spin-dependent scattering of the conduction electrons at the interface between the magnetic Co matrix grains and the nonmagnetic regions, and by occurrence of antiferromagnetic coupling between Co crystallites, favored by inclusion in film of TiO2 nanoparticles.

A new insight into the mechanism of lead electrodeposition: Ohmic-diffusion control of the electrodeposition process

A new insight into the mechanism of lead electrodeposition from nitrate solutions is presented by analysis of the polarization characteristics and by scanning electron microscopic (SEM) analysis of the surface morphology of the lead deposits obtained in the potentiostatic regime of electrolysis. In dependence on the concentration of Pb(II) ions, the electrodeposition process is either mixed ohmic-diffusion or completely ohmic controlled. The ratio of the ohmic control to the total control of the electrodeposition process increased with increasing concentration of Pb(II) ions and at the higher concentrations of Pb(II) ions, the electrodeposition process became completely ohmic controlled. The polarization characteristics were well correlated with the morphologies of the obtained lead deposits. The formation of regular crystals was a characteristic of ohmic control of the electrodeposition process, while needle-like and fern-like dendrites were the predominant morphological forms obtained under diffusion control. The critical overpotentials for both dendritic growth initiation and instantaneous dendritic growth were analyzed and a mechanism of lead electrodeposition based on this analysis is presented. It was shown that the values of these critical overpotentials decreased and approached one another with increasing concentration of Pb(II) ions and finally became the same when full ohmic control was attained. The presented results were in good agreement with the predications of the general theory for the formation of disperse metal electrodeposits.

Spatiotemporal Pattern Formation during Electrochemical Oxidation of Hydrogen on Platinum

0\. Title, Abstract, and Contents 1\. Introduction 1 2\. Background 7 3\. Experimental 35 4\. Complex Voltammetric Response 43 5\. Electrochemical Turbulence 71 6\. Ohmic Drop Compensation and Negative Global Coupling 93 7\. Turbulence Suppression and Emerging Coherence 107 8\. Trapping Electrochemical Oscillations between Self-Organized Potential Walls 133 9\. Summary 151 References 155 Curriculum Vitae, Acknowledgements 171

Formic acid electrooxidation over carbon-supported nanoparticles of non-stoichiometric palladium carbide

Catalytic performance of the 5 wt.% Pd/C and 5 wt.% PdC0.14/C catalysts for formic acid electrooxidation was compared under potentiodynamic (0.06–1.0 V vs. RHE, sweep rate 50 mV s−1) and potentiostatic (0.2 and 0.3 V vs. RHE) regimes in 0.5 M HCOOH + 0.1 M H2SO4 solution at 25 °C. The non-stoichiometric palladium carbide catalyst demonstrates superior catalytic performance and stability during HCOOH electrooxidation as compared to the unmodified Pd/C precursor. The observed phenomenon is explained in terms of modification of electronic and adsorption properties of palladium caused by the embedding of carbon atoms in the interstitial cites of the crystalline lattice of Pd nanoparticles.