Electrochemical impedance spectroscopic studies on niobium anodic dissolution in HF

Journal of Solid State Electrochemistry - The dissolution of niobium electrode in anodic regime in hydrofluoric acid was studied using electrochemical techniques. In the active region, complex...     

Electrochemical deposition of Zn on TiN microelectrode arrays for microanodes

Zn was electrochemically deposited onto square TiN electrodes with edge dimensions of 490 μm and 40 μm. These were fabricated by standard microfabrication techniques, which provide an extremely reproducible electrode for experimentation. Reliable constant-potential electrodeposition of Zn on the TiN was performed at −1.2 V, just below the Zn/Zn²⁺ redox potential. At more negative potentials, the hydrogen evolution reaction on TiN interfered with bulk metal electrodeposition, resulting in poor quality Zn films. A two-step plating procedure was shown to be most efficient for electrochemical deposition of Zn, with Zn nucleation on the TiN substrate at high cathodic overpotential during the first step and a second step of bulk metal growth on the nucleated layer at low cathodic overpotential. These results were most consistent with 3D progressive nucleation of Zn on the TiN surface. Using this procedure, deposits of Zn on 490 μm TiN electrodes were uniform. In contrast, Zn deposits on 40 μm electrodes formed high-surface area and volume surface structures resulting from preferential growth at the electrode corners due to enhanced hemispherical diffusion at these sites. This should enable the formation of high surface area, high current density Zn anodes on biocompatible TiN microelectrodes, which could find application as improved microanodes for implantable miniature power supplies, e.g., implantable glucose sensors and internal cardioverter defibrillators.     

Electrochemical deposition of nickel from aqueous electrolytic baths prepared by dissolution of metallic powder

A new method of preparation of aqueous electrolyte baths for electrochemical deposition of nickel targets for medical accelerators is presented. It starts with fast dissolution of metallic Ni powder in a HNO₃-free solvent. Such obtained raw solution does not require additional treatment aimed to removal nitrates, such as the acid evaporation and Ni salt precipitation-dissolution. It is used directly for preparation of the nickel plating baths after dilution with water, setting up pH value and after possible addition of H₃BO₃. The pH of the baths ranges from alkaline to acidic. Deposition of 95% of ca. 50 mg of Ni dissolved in the bath takes ca. 3.5 h for the alkaline electrolyte while for the acidic solution it requires ca. 7 h. The Ni deposits obtained from the acidic bath are physically and chemically more stable and possess smoother and crack-free surfaces as compared to the coatings deposited from the alkaline bath. A method of estimation of concentration of H₂O₂ in the electrolytic bath is also proposed.

     

Electrochemical impedance spectroscopic studies of copper dissolution in arginine–hydrogen peroxide solutions

Anodic dissolution of copper in arginine and hydrogen peroxide-based medium suitable for chemical mechanical planarization slurry formulation was investigated using electrochemical impedance spectroscopy. Potentiodynamic polarization and impedance data were acquired for copper dissolving in hydrogen peroxide and arginine solution. Reaction mechanism analysis (RMA) was employed to determine the mechanistic pathway of copper dissolution. RMA analysis indicates that a stable passivating film does not form on the copper surface, and the direct dissolution of copper was also ruled out. A two-step mechanism involving cupric oxide as an intermediate species is proposed. The data were also fit to an electrical equivalent circuit to obtain insight into the variation of the parameters with the overpotential. The modeled data for the two-step mechanism captures the essential features of the impedance spectra at various overpotentials.     

Platinum dissolution and deposition in the polymer electrolyte membrane of a PEM fuel cell as studied by potential cycling

The behavior of platinum dissolution and deposition in the polymer electrolyte membrane of a membrane-electrode-assembly (MEA) for a proton-exchange membrane fuel cell (PEMFC) was studied using potential cycling experiment and high-resolution transmission electron microscopy (HRTEM). The electrochemically active surface area decreased depending on the cycle number and the upper potential limit. Platinum deposition was observed in the polymer electrolyte membrane near a cathode catalyst layer. Platinum deposition was accelerated by the presence of hydrogen transported through the membrane from an anode compartment. Platinum was transported across the membrane and deposited on the anode layer in the absence of hydrogen in the anode compartment. This deposition was also affected by the presence of oxygen in the cathode compartment.     

Electrochemical quartz crystal microbalance determination of nickel formal partial charge number during nickel-underpotential deposition on platinum in sulphate media

We studied nickel-underpotential deposition (UPD) on polycrystalline platinum in sodium sulphate media using Electrochemical Quartz Crystal Microbalance (EQCM). EQCM enables to separate hydrogen and nickel adsorption/desorption contributions, which was impossible solely with electrochemistry. Thus, nickel coverage can be determined: nearly 0.8 Ni_UPD monolayer over platinum is deposited above nickel Nernst potential for pH 5.5. Coupling the gravimetric signal with nickel stripping peak coulometry yields the nickel formal partial charge number: ι_Ni,EQCM = 1.3 ± 0.13. Electrochemical measurements using Swathirajan and Bruckenstein’s method give: ι_Ni = 1.5 ± 0.17. Within the errors bars, both methods give similar results and ι_Ni lower than ${\mathrm{z}}_{{\text{Ni}}^{2+}}$: nickel adatoms apparently remain partially charged upon UPD on platinum following anion and nickel co-adsorption.     

Instantaneous dissolution of cellulose in organic electrolyte solutions

Herein a novel class of solvent systems for cellulose is introduced. Surprisingly, organic electrolyte solutions, which contain just a small molar fraction of ionic liquid, dissolve instantaneously large amounts of cellulose. The solvation properties of the solvent systems, required for dissolving the polymer, are discussed here.     

Chronoamperometric studies in an ammonia citrate electrolyte for the deposition of a nickel-molybdenum alloy

Dependences of the current and the chemical composition of cathodic deposits on the electrolysis time are studied in an ammonia-citrate electrolyte for the deposition of a nickel-molybdenum alloy in potentiostatic conditions. At potentials less negative than ?0.93 V, no nickel-molybdenum alloy deposition occurs and in the course of electrolysis there is observed a decrease in the current, which is connected with the blocking of the electrode surface by the formed molybdenum oxides. In the region of potentials of deposition of a nickel-molybdenum alloy there is realized the following shape of a time dependence of the current. Within the first few seconds of the electrolysis process, the current somewhat decreases. Then there occurs its increase, which is connected with the alloy deposition on the cathode. In the course of electrolysis there occurs a decrease in the molybdenum content in the cathodic deposit.     

Electrochemical dissolution of gold in acidic medium

Polycrystalline gold dissolves at high potentials in acidic medium even without the presence of complexing agents. In the present work the dissolution of gold is quantitatively studied by an inductively coupled plasma mass spectrometer (ICP-MS) directly connected to an electrochemical scanning flow cell (SFC). It is shown that the onset of gold dissolution coincides with the onset of the formation of 2D surface oxide (ca. + 1.3 V_RHE). An increase in the upper potential limit (UPL) during potential cycling or an increase in the holding potential during potential steps leads to an enhancement in the amount of dissolved gold. Moreover, a change in the dominating mechanism for gold dissolution with increasing potential is found in transient experiments. At lower potentials, gold is dissolved mostly during the gold oxide reduction, while at higher potentials anodic dissolution is the dominating process. Finally, the possible application of gold for stabilization of PtAu-based fuel cell catalysts is discussed.     

Electrochemical dissolution of aluminium in electrocoagulation experiments

Six experiments are presented to highlight important features of aluminium dissolution when used in electrocoagulation procedure employed to remove oily contaminations from water. First, using a common oil-in-water emulsion: diluted milk, we show that the electrochemically generated coagulant ions are active only in the first few seconds following their generation—hence, the electrocoagulation cells’ construction should promote the mixing of the nascent Al colloid with the water phase. For this reason, the use of the narrow-gap cells is suggested. Second, in experiments with Al-Al electrode pairs and dilute, neutral, unbuffered, aqueous solutions we (i) estimate the maximum amount of Al dissolution on the cathode and (ii) show how the rate of Al dissolution changes with frequency if the cell voltage polarity is alternating.     

Electrochemical dissolution of magnetite in acid solutions

The present paper deals with the electrochemical behavior of magnetite microcrystals in an acid medium. A voltammetric method employing a carbon-paste electroactive electrode (CPEE) with an organic binder was used. It was found that the cathodic voltammograms, which were recorded at different scan rates, formed a set bounded in the space of i–E parameters by a generalizing voltammetric curve corresponding to the effective potential scan rate _eff. In other words, all curves are situated under one enveloping curve, just as the smaller dolls sit in the largest doll of a Russian doll. Reverse currents (a cathodic current in the anodic direction of the potential scan) were observed on the cyclic voltammogram. Forward and reverse currents obey the same laws and have one and the same generalizing curve, which could be taken as the magnetite characteristic.     

Electrochemical dissolution of CrIII and CrIV oxides

Oxidative dissolution of Cr oxides can be easily performed using voltammetry of immobilised microparticles. A combined procedure based on the dependence of current on time and potential (chronoamperometry with potential jumps) is suitable for the determination of the sensitivity of the dissolution rate to electrochemical potential. Applying this procedure, it was found that the rate determining step is preceded by two-electron oxidation in the solid phase that probably proceeds as a reversible equilibration between surface sites of Cr^III, Cr^IV and Cr^V. Voltammetry is sensitive to the phase composition, and so the voltammetric peak potentials obtained under the same conditions increase in the order LaCrO₃<CrO₂<Me^IICr₂O₄<α-Cr₂O₃. The influence of Fe- and Ni-for-Fe substitution on dissolution reactivity is also discussed. Received: 20 February 1998 / Accepted: 31 March 1998     

The effect of electrolyte conductivity on electrophoretic deposition

In this work, the influence of electrolyte conductivity on the electrophoretic deposition of alumina particles from ethanol suspensions was studied. Deposition experiments in a Hull cell showed that high-conductivity ethanol-based suspensions yield uniform deposits, while low-conductivity suspensions result in nonuniform deposits. The difference in the deposition, behavior is due to the resistance increase over the deposit during polarization. Impedance measurements during electrophoretic deposition showed that during EPD the relative deposit resistance increases much faster for the high- than for the low-conductivity suspension. The impedance measurements also showed that the resistance increase dropped almost to the suspension resistance after the electric field was turned off. This means that the resistance over the deposit is caused by the interaction of ions with the deposit and by the depletion of ions at the deposition electrode. Negatively charged ions are depleted in the deposit by migration toward the positively charged counterelectrode, while positively charged ions undergo electrochemical reactions at the deposition electrode. This change in ion concentrations near the deposition electrode changes the acid/base properties of the particles in the deposit, as proven by adsorbed pH indicators on the particles. The change in acid/base behavior is quasi-irreversible and results in a memory effect of the deposit resistance when the voltage is reapplied.     

Electrochemical behaviour of tin in alkaline electrolyte

The electrochemical behaviour of two types of tin electrodes, Sn rod and electrodeposited Sn is investigated in 0.1 M KOH_aq in order to evaluate processes related to anodic Sn dissolution. Potential regions of formation of soluble Sn(II) and Sn(IV) are identified by means of a rotating ring disk electrode. An anodic reactivation peak observed for cathodic potential scans for partially passivated electrodes is accompanied by formation of soluble Sn(II) species and a minimum in the imaginary part of the impedance. This confirms that the reactivation peak is due to active dissolution of metallic Sn exposed to the electrolyte during rupture of the oxidised layer.     

DTA studies on the barium sulphate—Calcium sulphate eutectoid mixture

Samples of a nearly eutectoid mixture of 60 mole% BaSO₄–40 mole% CaSO₄, a highly effective system for the selective sorption of strontium, were studied using the DTA method. The effects of the time of thermal treatment and the number of thermal cycles of two different types on the homogenisation degree and thus on the formation of the metastable barium—calcium sulphate were examined. The conditions of the thermal decomposition of the metastable sulphate, the activation energy and the heat released during this process were estimated.

---

Zusammenfassung Eine durch ihre selektive Fähigkeit zur Sorption von Strontium ausgezeichnete, beinahe eutektische Mischung von 60 Mol% BaSO₄ und 40 Mol% CaSO₄ wurde differentialthermoanalytisch geprüft. Der Einfluß der Zeit der thermischen Behandlung und von zwei verschiedenen thermischen Zyklen auf den Homogenisierungsgrad, und so auch auf die Bildung des metastabilen Barium-Kalzium Sulfates wurden untersucht. Die Bedingungen der thermischen Zersetzung des metastabilen Sulfats, die Aktivierungsenergie und die während des Vorgangs freigesetzte Wärme wurden ermittelt.

---

Résumé Etude par ATD d'échantillons provenant d'un mélange BaSO₄ 60 moles% -CaSO₄ 40 moles %, de composition voisine de celle de l'eutectoide et très efficace pour la sorption sélective du strontium. Etude de l'influence de la durée du traitement thermique et du nombre de cycles thermiques (de deux types différents) sur le degré d'homogénéisation, c'est-à-dire sur la formation de sulfate de baryum-calcium métastable. On a estimé les conditions de la décomposition thermique du sulfate métastable, l'énergie d'activation et la quantité de chaleur dégagée par le processus.

---

, 60 % BaSO₄—40 % CaSO₄. . . , .

---

---

Thanks are due to Dr. L. Berák for the provision of results of dilatometric measurements, for useful assistance and continual encouragement.     

Electrochemical and in-situ X-ray diffraction studies of Ti3C2Tx MXene in ionic liquid electrolyte

2D titanium carbide (Ti₃C₂T_x MXene) showed good capacitance in both organic and neat ionic liquid electrolytes, but its charge storage mechanism is still not fully understood. Here, electrochemical characteristics of Ti₃C₂T_x electrode were studied in neat EMI-TFSI electrolyte. A capacitive behavior was observed within a large electrochemical potential range (from − 1.5 to 1.5 V vs. Ag). Intercalation and de-intercalation of EMI⁺ cations and/or TFSI⁻ anions were investigated by in-situ X-ray diffraction. Interlayer spacing of Ti₃C₂T_x flakes decreases during positive polarization, which can be ascribed to either electrostatic attraction effect between intercalated TFSI⁻ anions and positively charged Ti₃C₂T_x nanosheets or steric effect caused by de-intercalation of EMI⁺ cations. The expansion of interlayer spacing when polarized to negative potentials is explained by steric effect of cation intercalation.     

Tracer-diffusion of Zn2+ ions in cobalt sulphate

Tracer-diffusion of Zn²⁺ ions in the presence of CoSO₄ is studied at 25°C in 1% agar gel over a concentration range of 10^–5 to 0.25M using a zone-diffusion technique. The deviations observed between experimental and theoretical values of diffusion coefficients are explained by considering different types of interactions occurring in the ion-gel-water system. Further, study of the obstruction, effect in the diffusion of Zn²⁺ ions at different concentrations of CoSO₄ reveals that the -value decreases with increasing concentration of the electrolyte. This observation is accounted on the basis of competitive hydration between ions and agar molecules.     

Solvent extraction studies on thorium sulphate and halides

The effect of water-miscible alcohols, acetone and haloacids in the aqueous phase of sulphuric acid solutions has been studied on the extraction of thorium sulphate and halides by bis(2-ethylhexyl) phosphoric acid (HDEHP) and tri-n-butyl phosphate (TBP). The partition coefficient (E) is greatly affected by these additives, and this gives useful separations from many other elements. Besides, the log-log relationship between the E values and solvent concentration leads to some suggestions on the extraction mechanisms of this mixed system.