Isothermal crystallization of poly(vinylidene fluoride) in the presence of high static electric fields. II. Effect of crystallization temperature and electric field strength on the crystal phase content and morphology

The melt crystallization of poly(vinylidene fluoride) in a static electric field was studied for different fields strengths and undercooling conditions. The γ-phase nucleation process was examined directly by polarized optical microscopy and indirectly by small-angle light scattering. The crystal phase content was assessed by differential scanning calorimetry. It is shown that the γ-phase nucleation density and γ-phase content increase with field strength and that the higher the crystallization temperature, the larger the effect of the field. These experimental results confirm the predictions of the model of nucleation in an electric field that we published previously. It is also noted that the degree of crystallinity and the perfection of crystal orientation along the γ-phase spherulite radical direction decrease with field strength. The homogeneity of morphology resulting from the crystallization in the field is also examined by polarized optical microscopy on specimens microtomed across their thickness. When the crystallization is carried out under high field (E ≈ 0.1 MV/cm) and high temperature (T > 166°C) a nonuniform morphology results, characterized by a higher nucleation density at the positive electrode than at the negative electrode. Near the negative electrode very large disklike spherulites are seen to grow parallel to the substrate.     

Transferable ordered ni hollow sphere arrays induced by electrodeposition on colloidal monolayer

We report an electrochemical synthesis of two-dimensionally ordered porous Ni arrays based on polystyrene sphere (PS) colloidal monolayer. The morphology can be controlled from bowl-like to hollow sphere-like structure by changing deposition time under a constant current. Importantly, such ordered Ni arrays on a conducting substrate can be transferred integrally to any other desired substrates, especially onto an insulting substrate or curved surface. The magnetic measurements of the two-dimensional hollow sphere array show the coercivity values of 104 Oe for the applied field parallel to the film, and 87 Oe for the applied field perpendicular to the film, which is larger than those of bulk Ni and hollow Ni submicrometer-sized spheres. The formation of hollow sphere arrays is attributed to preferential nucleation on the interstitial sites between PS in the colloidal monolayer and substrate, and growth along PSs' surface. The transferability of the arrays originates from partial contact between the Ni hollow spheres and substrate. Such novel Ni ordered nanostructured arrays with transferability and high magnetic properties should be useful in applications such as data storage, catalysis, and magnetics.     

Copper deposition and dissolution in seemingly parallel electric and magnetic fields: Lorentz force distributions and flow configurations

In two different cylindrical electrochemical cells, copper deposition and dissolution in the presence of a magnetic field mostly parallel to the electric field was investigated. Particle image velocimetry measurements show that even under such a field configuration where Lorentz forces are often a priori neglected may in fact dominate the flow. Further, depending on the electrode radius, a reversal of the secondary flow was found. This feature can be explained by the different Lorentz force configurations calculated from the magnetic field and the primary current distributions. The components of the magnetic field were determined by means of a traversed Hall probe and were calculated by a commercial finite element package. Experimentally and numerically determined field distributions match very well. A good agreement between the measured and calculated velocity distributions was also found, suggesting that in the present case the effect of the Lorentz force alone is sufficient to explain the magnetic field influence on the flow. Contribution to the special issue “Magnetic field effects in Electrochemistry”     

Effects of Anions on the Zinc Electrodeposition onto Glassy-Carbon Electrode

The zinc electrodeposition onto glassy-carbon electrode from the sulfate, chloride and acetate solutions is examined using cyclic voltammetry and chronoamperometry. The surface morphology of zinc deposited films is observed by scanning electron microscopy. The results show that all mechanisms of the zinc electrocrystallization on glassy-carbon electrode in the three solutions follow the same three-dimensional instantaneous nucleation and growth. The anions mainly affect the nucleation densities during zinc deposition, which results in different surface morphology. In the presence of acetate and chloride ions, the deposited zinc film tends to grow in a multi-layered pattern, while in sulfate solution the zinc deposition forms irregular grains.     

Investigation of nucleation processes under the influence of magnetic fields

The present work demonstrates the possibilities and the limits of the in situ electrochemical scanning tunneling microscopy for investigation of nucleation processes in magnetic fields on the examples of Cu and Co electrodeposition onto Au(111) electrodes from sulfate electrolytes with pH 3. Cyclic voltammograms of Cu in the underpotential range (UPD) exhibit no significant change in the cathodic and anodic peaks recorded in magnetic fields parallel to the surface. In magnetic fields of a permanent magnet, the reconstruction of Au has been annihilated during UPD of Cu. In the overpotential range, the dissolution of Cu is inhibited. This triggers the formation of a Cu–Au surface alloy. The UPD deposition of Co onto Au(111) could be proven without magnetic field, which leads to the formation of two monolayers. The nucleation in an applied field could not be observed due to higher induced fluctuations and microconvective effects. Contribution to special issue “Magnetic field effects in Electrochemistry”.     

Magnetic field effect on electrochemical oscillations during iron dissolution

A fairly low magnetic field of ca. 30 mT was found to affect the period and the amplitude of the self-sustained current oscillation of an iron electrode. This was observed much distinctively when the direction of the applied field was normal to the electrode surface, i.e., non-magnetohydrodynamic (MHD) configuration. The Flade potential of the iron electrode was not affected by the magnetic field intensity of up to 4 T. The observed magnetic field effect was attributed to the depression of the natural convection in the vicinity of the electrode surface which was caused by the two local paramagnetic body forces, the magnetic field gradient force and/or the concentration gradient force.     

Transport of redox probes through single pores measured by scanning electrochemical-scanning ion conductance microscopy (SECM-SICM)

We report scanning electrochemical microscopy-scanning ion conductance microscopy (SECM-SICM) experiments that describe transport of redox active molecules which emanate from single pores of a track-etch membrane. Experiments are performed with electrodes which consist of a thin gold layer deposited on one side of a nanopipet. Subsequent insulation of the electrode with parylene results in a hybrid electrode for SECM-SICM measurements. Electrode fabrication is straightforward and highly parallel. For image collection, ionic current measured at the nanopipet both controls the position of the electrode with respect to the membrane surface and reports the local conductance in the vicinity of the nanopipet, while faradaic current measured at the Au electrode reports the presence of redox-active molecules. Application of a transmembrane potential difference affords additional control over migration of charged species across the membrane.     

磁场作用下铁电极的不均匀阳极溶解

采用电化学方法以及扫描电镜形貌观察研究了磁场对铁在硫酸溶液中的阳极溶解的影响 .恒电位极化测试结果表明 ,随外加阳极电位的增加 ,磁场的存在将加速阳极溶解 ,使振荡态或钝态变为活性溶解态 ,维持钝态 .于特定电位下与重力方向平行的电极表面两侧将因局部溶解加速而出现凹陷 .由于电极周边浓度梯度场的特殊性以及磁场的作用方向导致了铁的不均匀阳极溶解     

Mg-Li电极在NaCl溶液中的电化学行为

应用熔炼法制备含Li量为8.5%和14%的两种Mg-Li合金,分别由电势线性扫描、计时电流、交流阻抗和失重法等检测Mg-Li电极在NaCl溶液中的电化学特性,SEM观察其放电表面形貌.结果表明:Mg-14Li电极比Mg-8.5Li电极有较负的开路电位、更大的放电电流和较高的放电效率,但附着电极表面的疏松产物易于脱落.Mg-8.5Li电极的放电效率高于Mg-14Li电极的放电效率.两种电极在低恒电位放电电流效率均高于较高恒电位的放电电流效率.     

Hydrophobic silica sol-gel films for biphasic electrodes and porotrodes

Hydrophobic sol-gel films from methyltrimethoxysilane (MTMOS) are deposited onto glass and tin-doped indium oxide (ITO) coated glass substrates. Uniform and microporous films of ca. 200 nm thickness are obtained and investigated by scanning electron microscopy and by electrochemical techniques. From cyclic voltammograms for the oxidation of ferrocenedimethanol in aqueous 0.1 M KNO3 apparent diffusion coefficients and free volume data for processes within the film are derived and it is demonstrated that the film morphology can be controlled by the deposition timing. Two novel types of biphasic electrodes for observing liquid/liquid ion transfer reactions are introduced: (i) an ITO electrode coated with a hydrophobic sol-gel film and (ii) a hydrophobic sol-gel film on glass sputter-coated with 20 nm porous gold (porotrode). For the t-butylferrocene redox system deposited in the form of an organic liquid, very low and morphology dependent current responses are observed on modified ITO electrodes. However, the porotrode system allows biphasic electrode reactions to be driven with high efficiency and with no significant morphology effect of the hydrophobic sol-gel film. This type of nanofilm-modified electrode system will be of interest for biphasic sensor developments.     

Orientation Ordering of Nanoparticle Ag/Co Cores Controlled by Electric and Magnetic Fields

The effect of electric and magnetic fields on the sandwich structure Pt/hydrogenated amorphous silicon (a‐Si:H)/stearic acid monolayer/Langmuir–Blodgett film of Ag/Co nanoparticles encapsulated in an organic envelope is studied. This structure is used as a working electrode in an electrochemical cell filled with NaCl solution (1 mM ) and equipped with an Ag/AgCl reference electrode. Reversible changes in voltammograms are observed due to treatments (negative or positive bias voltage and simultaneous laser irradiation) applied to the designed structure before measurements. As an explanation of the observed phenomena we suggest that both the Co‐up and Ag‐up (on the a‐Si:H surface) orientation orderings of nanoparticle Ag/Co cores are repeatedly reached. The role of the photovoltaic material (a‐Si:H) in the observed behavior is explained. Voltammetric measurements with an applied magnetic field support our idea about the orientation ordering of nanoparticle cores.     

Magnetic field effects on bioelectrocatalytic reactions of surface-confined enzyme systems: enhanced performance of biofuel cells

The effect of a constant magnetic field on bioelectrocatalytic transformations of three different enzyme assemblies linked to electrodes is examined and correlated with a theoretical magnetohydrodynamic model. The systems consist of surface-reconstituted glucose oxidase (GOx), an integrated lactate dehydrogenase/nicotinamide/pyrroloquinoline quinone assembly (LDH/NAD+ -PQQ), and a cytochrome c/cytochrome oxidase system (Cyt c/COx) linked to the electrodes. Pronounced effects of a constant magnetic field applied parallel to the electrode surface are observed for the bioelectrocatalyzed oxidation of glucose and lactate by the GOx-electrode and LDH/NAD+ -PQQ-electrode, respectively. The enhancement of the bioelectrocatalytic processes correlates nicely with the magnetohydrodynamic model, and the limiting current densities (iL) relate to B1/3 (B = magnetic flux density) and to C4/3 (C* = bulk concentration of the substrate). A small magnetic field effect is observed for the Cyt c/COx-electrode, and its origin is still questionable. The effect of the constant magnetic field on the performance of biofuel cells with different configurations is examined. For the biofuel cell consisting of LDH/NAD+ -PQQ anode and Cyt c/COx cathode, a 3-fold increase in the power output was observed at an applied magnetic field of B = 0.92 T and external load of 1.2 kOhms.     

Construction of an electrochemical sensor based on the electrodeposition of Au-Pt nanoparticles mixtures on multi-walled carbon nanotubes film for voltammetric determination of cefotaxime

Mixtures of gold-platinum nanoparticles (Au-PtNPs) are fabricated consecutively on a multi-walled carbon nanotubes (MWNT) coated glassy carbon electrode (GCE) by the electrodeposition method. The surface morphology and nature of the hybrid film (Au-PtNPs/MWCNT) deposited on glassy carbon electrodes is characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) techniques. The modified electrode is used as a new and sensitive electrochemical sensor for the voltammetric determination of cefotaxime (CFX). The electrochemical behavior of CFX is investigated on the surface of the modified electrode using linear sweep voltammetry (LSV). The results of voltammetric studies exhibited a considerable improvement in the oxidation peak current of CFX compared to glassy carbon electrodes individually coated with MWCNT or Au-PtNPs. Under the optimized conditions, the modified electrode showed a wide linear dynamic range of 0.004-10.0 μM with a detection limit of 1.0 nM for the voltammetric determination of CFX. The modified electrode was successfully applied for the accurate determination of trace amounts of CFX in pharmaceutical and clinical preparations.     

Effect of magnetic field on properties of AuPt particles magneto-electrodeposited on carbon paper

AuPt nano particles are bi-functional catalysts for Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER) that were taken place on air electrodes in lithium air batteries. Magnetic field has been applied during electrodeposition for the preparation of AuPt particles. With the increase of the magnetic flux density under constant current density, the grain size decreases from about 1μm to 200nm and the activity of the AuPt catalyst increases. The magnetic field oriented vertical to the electric field has a promotion effect on increasing the catalytic ability of AuPt/carbon electrode. By pulse plating, the grain size decreases to about 100nm. By adjusting parameters of the electric field and the magnetic field, controllable in-situ preparation of AuPt catalyst with various morphology and catalytic activity could be achieved.     

Electrochemical Characterization of a Pyrolytic Carbon Film Electrode and the Effect of Anodization

A pyrolytic carbon (PC) film electrode was fabricated by the chemical vapor deposition (CVD) method. This report deals with the preparation, characterization and electrochemical behavior of this carbon film. Cyclic voltammetry, linear sweep voltammetry, Raman spectroscopy and scanning electron microscopy were employed to characterize the electrode. Low background current and capacitance were observed and the rate of charge transfer for Fe(CN) redox couple was determined via cyclic voltammetry. Also the effect of the anodic activation on the electrochemical activity was evaluated and characterized with respect to the sequence of voltage applied to the electrode. The excellent electrochemical activity and low background current are the reasons why this electrode is attractive for electroanalysis measurements with lower detection limit.     

Electrochemical characterization of MnO2 as electrocatalytic energy material for fuel cell electrode

 Development of inexpensive non Pt based high electrocatalytic energy materials is the need of the hour for fuel cell electrode to produce clean alternative green energy from synthesized bio alcohol using biomass. MnO₂, electro synthesized at different current density is found to be well performed electrocatalytic material, comparable to Pt, with higher current density, very low overvoltage for the electrochemical oxidation of methanol. From EIS study, the polarization resistance of the coated MnO₂ is found to be much low and electrical double layer capacitance is high, the effect increases with increase in current density of electro deposition. XRD, EDX and AAS analysis confirm the MnO₂ deposition. The morphology of SEM images exhibits an enhanced 3D effective substrate area, for electro oxidation of the fuel. A few nano structured grains of the deposited MnO₂ is also observed at higher current density. The fact supports that a high energetic inexpensive electro catalytic material has been found for fuel cell electrode to synthesis renewable energy from methanol fuel.     

Growth and magnetic properties of oriented alpha-Fe2O3 nanorods

alpha-Fe(2)O(3) nanorods have been deposited on Si substrates using the metal-organic chemical vapor deposition method. Structural analyses indicated that alpha-Fe(2)O(3) nanorods are preferentially oriented in the [104] direction on Si(100) substrates, and the nanorod possesses the single-crystalline structure. MFM image suggests that a spin domain is formed in the alpha-Fe(2)O(3) nanorod. Anisotropic magnetic property of the alpha-Fe(2)O(3) nanorods, i.e., the discrepancy of the saturation magnetization, is observed from SQUID measurements when the magnetic field are applied parallel and perpendicular to the substrate. A lower Morin temperature than that of the macroscopically crystalline hematite is observed when the magnetic field is applied parallel to the substrate.     

The rods-like manganese dioxide films grown on nickel foam for electrochemical capacitor applications

MnO₂ films grown on nickel foam (NF) with a desirable 3D structure are investigated as electrochemical pseudocapacitor materials for potential energy storage applications. The prepared MnO₂ films are characterized by X-ray diffraction, FT-IR and scanning electron microscopy. Results indicate that the products are typical hexagonal ?-MnO₂ with a uniform nanorod structure. The electrochemical measurements showed that the MnO₂ films with rods-like morphology have excellent electrochemical performances and its specific capacitance value as single electrode is up to 664 F g^?1 at a discharge current density of 5.5 A g^?1, which is higher than that of most reported corresponding materials. The specific capacitance retention ratio is 76.7% at the current density range from 5.5 to 30 A g^?1. Furthermore, we found that the deposition conditions such as deposition potential and deposition mass have a pronounced effect on their electrochemical activities.     

A Novel V<Subscript>2</Subscript>AlC Electrode Material for Supercapacitors

A novel V₂AlC electrode material for supercapacitors was investigated in this study. The structure and surface morphology were examined using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The formation of irregularly shaped V₂AlC with different particle size distribution was confirmed by XRD and FESEM. The electrochemical measurements were performed by cyclic voltammetry (CV), galvanostatic charge discharge and electrochemical impedance spectroscopy (EIS). V₂AlC electrode exhibited 27.6 F g^–1 of specific capacitance at the current density of 0.5 A g^–1. The specific capacitance of V₂AlC electrode remained 93.8% of the first cycle after 2000 cycles. V₂AlC has great potential for application in supercapacitors.     

Effects of an electrostatic field on crystallization of poly(vinylidene fluoride)

Melt crystallization of poly(vinylidene fluoride) (PVF2) under a static electric field was studied by optical microscopy. For most crystallization temperatures a mixture of α and γ spherulites was obtained. The growth rate of the spherulites was measured in the plane perpendicular to the direction of the field. The effect of an electric field on the growth rate of the γ spherulites corresponds in a qualitative way with the dependence predicted theoretically for the primary nucleation of the nonpolar form. It is shown, that the growth rate of γ spherulites is always reduced by the electric field. This reduction was larger the lower the undercooling. In the case of γ spherulites the interpretation of the results is more complicated. Both the morphology and the growth rate of γ spherulites under an electric field depend on electrode configuration. An increase of the growth rate of γ spherulites in an electric field was detected only when the sample was in direct contact with both electrodes. Under these circumstances, γ spherulites nucleate and grow at the PVF2-positive electrode interface; and the increase of the growth rate is higher the lower the undercooling, and is greater in comparison with the decrease found for the nonpolar α phase. The reason for the sensitivity of the morphology of the γ form to the electrode configuration is not completely clear. © 1994 John Wiley & Sons, Inc.