Electrochemical phase formation (ECPF) and macrogrowth Part I. Hemispherical models

The phenomenological theory of hemispherical growth is generalised to time-dependent nucleation and growth-rates. Special cases, which include models with diffusion-controlled rates, are analysed. Expressions are obtained for small and large time behaviour and peak characteristics of potentiostatic transients, and their use in model parameter estimation is discussed. Two earlier equations are corrected. Numerically calculated transients which are presented exhibit some interesting features such as a maximum preceding the steady state, oscillations and shoulder.

Electrochemical phase formation (ECPF) and macrogrowth Part II. Two-rate models

A general theory is evolved for a class of macrogrowth models which possess two independent growth-rates. Relations connecting growth-rates to growth geometry are established and some new growth forms are shown to result for models with passivation or diffusion-controlled rates. The corresponding potentiostatic responses, their small and large time behaviours and peak characteristics are obtained. Numerical transients are also presented. An empirical equation is derived as a special case and an earlier equation is corrected. An interesting stochastic result pertaining to nucleation events in the successive layers is proved.

A Comparative Study of Electrochemical Kinetic Parameters by the Potential Step Method. Electrode Reactions of CyDTA and EDTA Complexes of Cu(II) at DME

By using the same current-time (I-t) curves, electrochemical kinetic parameters are determined by two methods, (a) using the ratio of current at a given potential to the diffusion-controlled limiting current and (b) curve fitting method, for the reduction of Cu(II)–CyDTA complex. The analysis by the method (a) shows that the rate determining step involves only one electron although the overall reduction of the complex involves two electrons suggesting thereby the stepwise reduction of the complex. The nature of I-t curves suggests the adsorption of intermediate species at the electrode surface. Under these circumstances more reliable kinetic parameters can be obtained by the method (a) compared to that of (b). Similar observations are found in the case of reduction of Cu(II)–EDTA complex.

Physicochemical, Electrochemical, and Spectroscopic Characterization of Zinc-Based Room-Temperature Molten Electrolytes and Their Application in Rechargeable Batteries

Room-temperature zinc ion-conducting molten electrolytes based on acetamide, urea, and zinc perchlorate or zinc triflate have been prepared and characterized by various physicochemical, spectroscopic, and electrochemical techniques. The ternary molten electrolytes are easy to prepare and can be handled under ambient conditions. They show excellent stability, high ionic conductivity, relatively low viscosity, and other favorable physicochemical and electrochemical properties that make them good electrolytes for rechargeable zinc batteries. Specific conductivities of 3.4 and 0.5 mS cm(-1) at 25 degrees C are obtained for zinc-perchlorate-and zinc-triflate-containing melts, respectively. Vibrational spectroscopic data reveal that the free ion concentration is high in the optimized composition. Rechargeable Zn batteries have been assembled using the molten electrolytes, with gamma-MnO2 as the positive electrode and Zn as the negative electrode. They show excellent electrochemical characteristics with high discharge capacities. This study opens up the possibility of using acetamide-based molten electrolytes as alternate electrolytes in rechargeable zinc batteries. (C) 2009 The Electrochemical Society.

Synthesis, spectral and electrochemical properties of donor/acceptor substituted fluoroarylporphyrins

Spectroscopic and electrochemical redox properties of a series of fluorinated porphyrins bearing donor-acceptor groups and their Zn(II) and Cu(II) derivatives are presented. The magnitude of the ring reduction potentials and charge transfer properties derived from spectral data depend on the nature and position of the substituent(s), (nitro/dimethylamino) and the central metal ions.

Electrochemical reduction of hydrogen peroxide on stainless steel

Electrochemical reduction of hydrogen peroxide is studied on a sand-blasted stainless steel (SSS)electrode in an aqueous solution of NaClO4.The cyclic voltammetric reduction of H2O2 at low concentrations is characterized by a cathodic peak at -0 center dot 40 V versus standard calomel electrode(SCE).Cyclic voltammetry is studied by varying the concentration of H2O2 in the range from 0 center dot 2 mM to 20 mM and the sweep rate in the range from 2 to 100 mV s(-1)Voltammograms at concentrations of H2O2 higher than 2 mM or at high sweep rates consist of an additional current peak, which may be due to the reduction of adsorbed species formed during the reduction of H2O2. Amperometric determination of H2O2 at -0 center dot 50 V vs SCEprovides the detection limit of 5 A mu M H2O2. A plot of current density versus concentration has two segments suggesting a change in the mechanism of H2O2 reduction at concentrations of H2O2 a parts per thousand yen 2 mM. From the rotating disc electrode study, diffusion co-efficient of H2O2 and rate constant for reduction of H2O2 are evaluated.

Hydrogel membrane electrolyte for electrochemical capacitors

Polymer electrolytes are known to possess excellent physicochemical properties that are very useful for electrochemical energy systems. The mobility in polymer electrolytes is understood to be mainly due to the segmental motion of polymer chains and the ion transport is generally restricted to the amorphous phase of the polymer. Gel polymer electrolytes (GPE) that are formed using plastizicers and polymers along with ionic salts are known to exhibit liquid-like ionic conductivity while maintaining the dimensional stability of a solid matrix. In the present study, the preparation and characterization of poly(vinyl alcohol)-based hydrogel membranes (PHMEs) as electrolyte for electrochemical capacitors have been reported. VaryingHClO4 dopant concentration leads to different characteristics of the capacitors. The EC comprising PHME doped with 2 M HClO4 and black pearl carbon (BPC) electrodes has been found to exhibit a maximum specific capacitance value of 97 F g(-1), a phase angle value of 78A degrees, and a maximum charge-discharge coulombic efficiency of 88%.

Crystallite size effects in stacking faulted nickel hydroxide and its electrochemical behaviour

P-Nickel hydroxide comprises a long range periodic arrangement of atoms with a stacking sequence of AC AC AC-having an ideal composition Ni(OH)(2). Variation in the preparative conditions can lead to the changes in the stacking sequence (AC AC BA CB AC AC or AC AC AB AC AC) This type of variation in stacking sequence can result in the formation of stacking fault in nickel hydroxide. The stability of the stacking fault depends on the free energy content of the sample. Stacking faults in nickel hydroxide is essential for better electrochemical activity. Also there are reports correlating particle size to the better electrochemical activity. Here we present the effect of crystallite size on the stacking faulted nickel hydroxide samples. The electrochemical performance of stacking faulted nickel hydroxide with small crystallite size exchanges 0.8e/Ni, while the samples with larger crystallite size exchange 0.4e/Ni. Hence a right combination of crystallite size and stacking fault content has to be controlled for good electrochemical activity of nickel hydroxide. (C) 2008 Elsevier B.V. All rights reserved.

Effect of Crystallographic Structure of MnO2 on Its Electrochemical Capacitance Properties

MnO2 is currently under extensive investigations for its capacitance properties. MnO2 crystallizes into several crystallographic structures, namely, α, β, γ, δ, and λ structures. Because these structures differ in the way MnO6 octahedra are interlinked, they possess tunnels or interlayers with gaps of different magnitudes. Because capacitance properties are due to tercalation/deintercalation of protons or cations in MnO2, only some crystallographic structures, which possess sufficient gaps to accommodate these ions, are expected to be useful for capacitance studies. In order to examine the dependence of capacitance on crystal structure, the present study involves preparation of these various crystal phases of MnO2 in nanodimensions and to evaluate their capacitance properties. Results of α-MnO2 prepared by a microemulsion route (α-MnO2(m)) are also used for comparison. Spherical particles of about 50 nm, nanorods of 30−50 nm in diameter, or interlocked fibers of 10−20 nm in diameters are formed, which depend on the crystal structure and the method of preparation. The specific capacitance (SC) measured for MnO2 is found to depend strongly on the crystallographic structure, and it decreases in the following order: α(m) > α δ > γ > λ > β. A SC value of 297 F g-1 is obtained for α-MnO2(m), whereas it is 9 F g-1 for β-MnO2. A wide (4.6 Å) tunnel size and large surface area of α-MnO2(m) are ascribed as favorable factors for its high SC. A large interlayer separation (7 Å) also facilitates insertion of cations in δ-MnO2 resulting in a SC close to 236 F g-1. A narrow tunnel size (1.89 Å) does not allow intercalation of cations into β-MnO2. As a result, it provides a very small SC.

Electrochemical Functionalization of a Gold Electrode with Redox-Active Self-Assembled Monolayer for Electroanalytical Application

The electrochemical functionalization of a Au electrode with a redox-active monolayer and the electroanalytical applications of the functionalized electrode are described. Reaction of the electrochemically derived o-quinone on the self-assembled monolayer (SAM) of 6-mercaptopurine (MPU) on a Au electrode gives a redox-active 4-(6-mercapto-purin-9-yl)benzene-1,2-diol (MPBD) self-assembly under optimized conditions. Electrochemical quartz crystal microbalance technique has been employed to follow the functionalization of the electrode in real time. Electrochemically derived o-quinone reacts at the N(9) position of the self-assembled MPU in neutral pH. Raman spectral measurement confirms the reaction of o-quinone on MPU self-assembly. MPBD shows a well-defined reversible redox response, characteristic of a surface-confined redox mediator at 0.21 V in neutral pH. The anodic peak potential (Epa) of MPBD shifts by −60 mV while changing the solution pH by 1 unit, indicating that the redox reaction involves two electrons and two protons. The surface coverage (Γ) of MPBD was 7.2 ± 0.3 × 10-12 mol/cm2. The apparent heterogeneous rate constant (ksapp) for MPBD was 268 ± 6 s-1. MPBD efficiently mediates the oxidation of nicotinamide adenine dinucleotide (NADH) and ascorbate (AA). A large decrease in the overpotential and significant increase in the peak current with respect to the unmodified electrode has been observed. Surface-confined MPBD has been successfully used for the amperometric sensing of NADH and AA in neutral pH at the nanomolar level.

Effect of laser surface treatment on microstructure and properties of MRI 230D Mg alloy

A creep resistant Mg alloy MRI 230D was subjected to laser surface treatment using Nd:YAG laser equipped with a fiber optics beam delivery system in argon atmosphere. The laser surface treatment produced a fine dendritic microstructure and this treatment was beneficial for the corrosion and wear resistance of the alloy. Long-term linear polarisation resistance and Electrochemical Impedance Spectroscopy measurements confirmed that the polarisation resistance values of laser treated material were twice as high as that for the untreated material. This improved behaviour was due to the finer and more homogenous microstructure of the laser treated surface. The laser treatment also increased surface hardness two times and reduced the wear rate by 25% due to grain refinement and solid solution strengthening.

Exfoliated graphite–ruthenium oxide composite electrodes for electrochemical supercapacitors

The performance of exfoliated graphite (EG)–ruthenium oxide (RuOx) composites as binderless electrodes is evaluated for electrochemical capacitors (ECs). A composite of EG–RuOx is prepared by a modified sol–gel process. The material is characterized using X-ray diffraction and microscopy. Electrochemical capacitors with the composite electrodes in the presence of aqueous sulfuric acid (H2SO4) electrolyte are evaluated using voltammetry, impedance and charge–discharge studies. Cyclic voltammetry reveals very stable current–voltage behaviour up to several thousands of cycles, as well as high specific capacitances, e.g., a few hundreds of farads per gram for the composite that contains 16.5 wt.% RuOx.

Gelatin hydrogel electrolytes and their application to electrochemical supercapacitors

Gelatin hydrogel electrolytes (GHEs) with varying NaCl concentrations have been prepared by cross-linking an aqueous solution of gelatin with aqueous glutaraldehyde and characterized by scanning electron microscopy, differential scanning calorimetry, cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic chronopotentiometry. Glass transition temperatures for GHEs range between 339.6 and 376.9 K depending on the dopant concentration. Ionic conductivity behavior of GHEs was studied with varying concentrations of gelatin, glutaraldehyde, and NaCl, and found to vary between 10(-3) and 10(-1) S cm(-1). GHEs have a potential window of about 1 V. Undoped and 0.25 N NaCl-doped GHEs follow Arrhenius equations with activation energy values of 1.94 and 1.88 x 10(-4) eV, respectively. Electrochemical supercapacitors (ESs) employing these GHEs in conjunction with Black Pearl Carbon electrodes are assembled and studied. Optimal values for capacitance, phase angle, and relaxation time constant of 81 F g(-1), 75 degrees, and 0.03 s are obtained for 3 N NaCl-doped GHE, respectively. ES with pristine GHE exhibits a cycle life of 4.3 h vs 4.7 h for the ES with 3 N NaCl-doped GHE. (c) 2007 The Electrochemical Society.

Synthesis and characterization of nano-MnO2 for electrochemical supercapacitor studies

Nanostructured MnO2 was synthesized at ambient condition by reduction of potassium permanganate with aniline. Powder X-ray diffraction, thermal analysis (thermogravimetric and differential thermal analysis), Brunauer-Emmett-Teller surface area, and infrared spectroscopy studies were carried out for physical and chemical characterization. The as-prepared MnO2 was amorphous and contained particles of 5-10 nm diameter. Upon annealing at temperatures >400°C, the amorphous MnO2 attained crystalline α-phase with a concomitant change in morphology. A gradual conversion of nanoparticles to nanorods is evident from scanning electron microscopy and transmission electron microscopy (TEM) studies. High-resolution TEM images suggested that nanoparticles and nanorods grow in different crystallographic planes. Capacitance behavior was studied by cyclic voltammetry and galvanostatic charge-discharge cycling in a potential range from -0.2 to 1.0 V vs SCE in 0.1 M sodium sulfate solution. Specific capacitance of about 250 F g-1 was obtained at a current density of 0.5 mA cm-2(0.8 A g-1).

Physicochemical, spectroscopic and electrochemical characterization of magnesium ion-conducting, room temperature, ternary molten electrolytes

Room temperature, magnesium ion-conducting molten electrolytes are prepared using a combination of acetamide, urea and magnesium triflate or magnesium perchlorate. The molten liquids show high ionic conductivity, of the order of mS cm(-1) at 298 K. Vibrational spectroscopic studies based on triflate/perchlorate bands reveal that the free ion concentration is higher than that of ion-pairs and aggregates in the melt. Electrochemical reversibility of magnesium deposition and dissolution is demonstrated using cyclic voltammetry and impedance studies. The transport number of Mg2+ ion determined by means of a combination of d.c. and ac. techniques is similar to 0.40. Preliminary studies on the battery characteristics reveal good capacity for the magnesium rechargeable cell and open up the possibility of using this unique class of acetamide-based room temperature molten electrolytes in secondary magnesium batteries. (C) 2010 Elsevier B.V. All rights reserved.