Showing papers on "Cyclic voltammetry published in 2003"

Carbon nanotube purification: preparation and characterization of carbon nanotube paste electrodes.

Abstract: Paste electrodes have been constructed using single-wall carbon nanotubes mixed with mineral oil. The electrochemical behavior of such electrodes prepared with different percentages of carbon nanotubes has been compared with that of graphite paste electrodes and evaluated with respect to the electrochemistry of ferricyanide with cyclic voltammetry. Carbon nanotubes were purified by a treatment with concentrated nitric acid, then oxidized in air. In addition, electrochemical pretreatments were carried out to increase the selectivity of carbon nanotube electrodes. Performances of carbon nanotube paste and carbon paste electrodes were evaluated by studying such parameters as current peak, ΔEp, anodic and cathodic current ratio, and charge density toward several different electroactive molecules. Data interpretation based on the carbon nanotubes and carbon surface area is presented. Carbon nanotube paste and carbon paste electrodes were tested as H2O2 and NADH probes, and several analytical parameters were ev...

Formate, an active intermediate for direct oxidation of methanol on pt electrode.

Abstract: The electro-oxidation of methanol on a Pt thin film electrode in acidic solution has been investigated by in situ surface-enhanced IR absorption spectroscopy. A new IR peak is observed at around 1320 cm-1 when the electrode potential is more positive than 0.5 V, where the bulk oxidation of MeOH occurs. This peak has been assigned to the symmetric stretching of formate species adsorbed on the Pt electrode surface. It is the first observation of formate adsorption during the electro-oxidation of methanol on a Pt surface. A near proportional relationship between the intensity of the IR band of the formate species and MeOH electro-oxidation current is observed. A new reaction scheme via non-CO pathway with formate as the active intermediate is proposed for the methanol electro-oxidation process.

Structured Silicon Anodes for Lithium Battery Applications

Abstract: Pillar arrays fabricated on silicon substrates have been tested as potential anodes for lithium batteries. Electrodes of array characteristics, diameter 580 ′ 150 nm: fractional surface coverage 0.34: height 810 nm are reported here. Cyclic voltammetry (CV) and cyclic galvanostatic tests of alloying/dealloying of electrochemically produced lithium with silicon were carried out, and results correlated with SEM studies. Aerial current densities in the low and fractional mA cm - 2 , and voltage 25 mV to 2 V (vs. L/Li + ) were used. CV features correspond to various Zintl phase compounds (ZPCs). Structured electrodes of Si pillars maintained their structural integrity throughout cycling; planar Si electrodes showed cracks (2 μm features) after 50 cycles. A model is advanced in which lithium diffuses through a layer of ZPC to react with Si: growing ZPCs plastically deforms where necessary. Upon lithium dealloying vacancies coalesce to form voids at the ZPC/Si interface, Si rejoins the substrate. or precipitates out as a nanocrystalline material, and the voids appear as a fine pattern of cracks, looking like dried mud. The extra surface area that a pillar structure can confer on Si electrodes is essential and makes it practical to consider the possible eventual use of such anodes in integrated battery structure;.

Cyclic Voltammetry Studies of Nanoporous Semiconductors. Capacitive and Reactive Properties of Nanocrystalline TiO2 Electrodes in Aqueous Electrolyte

Abstract: Extended networks of nanosized semiconductor particles permeated with an electrolyte display unique electrochemical behaviors. We report on a general investigation of the electrochemical properties of nanoporous electrodes by means of cyclic voltammetry. Models have been developed accounting for the fundamental characteristics of these electrodes: charge accumulation, charge transport, and interfacial charge transfer. These characteristics can be translated into simple electrical equivalents, which allow us to identify and classify the major features of voltammetry response according to the competition of the different processes during a voltammetric scan. A key point for describing the experimental observations is the potential dependence of the intrinsic film capacitance. The physical meaning of this capacitance is discussed in terms of the distribution of electronic states. We describe in detail the numerical simulation methods, and despite the simplicity of our approach, we show that these methods al...

Electrochemical bandgaps of substituted polythiophenes

Abstract: The electrochemical bandgaps for different soluble substituted polythiophenes have been measured by cyclic voltammetry. The effect of substituents on the oxidation/reduction potentials is discussed. Bandgaps obtained by cyclic voltammetry have been found to be in general higher than optical bandgaps. Among regioregular polymers substituted with a phenyl group at position 3 of the thiophene ring, examples are found that give very symmetric voltammograms. Rationalization for this behaviour is discussed from a conformational point of view.

Heterogeneous Electron-Transfer Kinetics for Ruthenium and Ferrocene Redox Moieties through Alkanethiol Monolayers on Gold

Abstract: The standard heterogeneous electron-transfer rate constants between substrate gold electrodes and either ferrocene or pentaaminepyridine ruthenium redox couples attached to the electrode surface by various lengths of an alkanethiol bridge as a constituent of a mixed self-assembled monolayer were measured as a function of temperature. The ferrocene was either directly attached to the alkanethiol bridge or attached through an ester (CO2) linkage. For long bridge lengths (containing more than 11 methylene groups) the rate constants were measured using either chronoamperometry or cyclic voltammetry; for the shorter bridges, the indirect laser induced temperature jump technique was employed to measure the rate constants. Analysis of the distance (bridge length) dependence of the preexponential factors obtained from an Arrhenius analysis of the rate constant versus temperature data demonstrates a clear limiting behavior at a surprisingly small value of this preexponential factor (much lower than would be expect...

Organic Layers Bonded to Industrial, Coinage, and Noble Metals through Electrochemical Reduction of Aryldiazonium Salts

Abstract: The reduction of diazonium salts in an aprotic medium permits the attachment of substituted aryl groups to a variety of metals: Co, Ni, Cu, Zn, Pt, and Au. These aryl groups are strongly bonded to the metal as they resist sustained rinsing under sonication in organic solvents. The organic layers have been characterized by cyclic voltammetry, infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, Rutherford backscattering, electrochemical impedance spectroscopy, and atomic force microscopy. From these data it is possible to propose a structure for these grafted layers.

Determination of quantum confinement in CdSe nanocrystals by cyclic voltammetry

Abstract: Ionization potentials Ip, electron affinities E.A., and the quantum confinement in CdSe nanocrystals were determined by means of cyclic voltammetry. The results were compared to values obtained from spectroscopic measurements, especially UV/vis absorption and photoluminescence emission spectra. Absolute band gap positions were obtained from the electrochemical measurements and discussed with regard to vacuum level values. The results are in good agreement with theoretical expectations and spectroscopic data.

Ionic Conductivity of PEMFC Electrodes Effect of Nafion Loading

Abstract: The effect of Nafion loading in the cathode catalyst layer of proton exchange membrane fuel cell (PEMFC) electrodes was studied by impedance spectroscopy, cyclic voltammetry, and polarization experiments. Catalyst utilization. determined by cyclic voltammetry, peaked at 76% for a Nafion loading of ca. 30 mass %, and this coincides with the optimum performance obtained in H 2 /O 2 fuel cells. However, the small range of utilizations observed (55-76%) cannot explain the wide range of performances. The impedance results show that the ionic conductivity of the cathode increased greatly with increasing Nafion content, and this is the main factor responsible for the increase in performance up to 30% Nafion. The loss of performance at higher Nafion loadings must have been due to an increasing oxygen transport resistance, because the electronic resistance did not increase significantly. In fact, the highest electronic resistances were observed at low Nafion loadings, indicating that Nafion played a significant role as a binder.

Material characterization and electrochemical performance of hydrous manganese oxide electrodes for use in electrochemical pseudocapacitors

Abstract: Hydrous manganese oxide with promising pseudocapacitive behavior was deposited on a carbon substrate at anodic potentials of 0.5-0.95 V vs. saturated calomel electrode ~SCE! in 0.25 M Mn(CH3COO)2 solution at 25°C. The effects of the deposition potential on the material characteristics and electrochemical performances of the hydrous manganese oxide prepared were investigated. Porous manganese oxide with higher crystallinity was formed at a lower deposition potential. When the deposition potential was 0.8 VSCE or higher, the deposited oxide consisted of an inner layer with a laminated structure and a rough outer layer with nodules on the surface. X-ray photoelectron spectroscopy was also carried out to examine the chemical state of the deposited oxide. Analytical results indicated that the oxide was composed of both trivalent and tetravalent manganese oxides at a deposition potential of 0.5 VSCE. However, the tetravalent manganese oxide became the dominant species in the film deposited at above 0.65 VSCE. The manganese oxide formed at 0.5 VSCE exhibited a specific capacitance as high as 240 F/g, as evaluated by cyclic voltammetry ~CV! with a potential scan rate of 5 mV/s in 2 M KCl at 25°C. Increasing the CV scan rate reduced the specific capacitance. Only about 70% of the capacitance at 5 mV/s could be maintained when the CV scan rate was increased to 100 mV/s, for all the manganese oxide electrodes prepared. Moreover, a high deposition potential gave rise to a low specific capacitance of the manganese oxide formed.

Preparation of PtNi nanoparticles for the electrocatalytic oxidation of methanol

Abstract: Carbon supported PtNi nanoparticles were prepared by hydrazine reduction of Pt and Ni precursor salts under different conditions, namely by conventional heating (PtNi-1), by prolonged reaction at room temperature (PtNi-2) and by microwave assisted reduction (PtNi-3). The nanocomposites were characterized by XRD, EDX, XPS and TEM and used as electrocatalysts in direct methanol fuel cell (DMFC) reactions. Investigations into the mechanism of PtNi nanoparticle formation revealed that platinum nanoparticle seeding was essential for the formation of the bimetallic nanoparticles. The average particle size of PtNi prepared by microwave irradiation was the lowest, in the range of 2.9–5.8 nm. The relative rates of electrooxidation of methanol at room temperature as measured by cyclic voltammetry showed an inverse relationship between catalytic activity and particle size in the following order PtNi-1 < PtNi-2 < PtNi-3.

Voltammetry of oxygen in the room-temperature ionic liquids 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide and hexyltriethylammonium bis((trifluoromethyl)sulfonyl)imide: One-electron reduction to form superoxide. Steady-state and transient behavior in the same cyclic voltammogram resulting from widely different diffusion coefficients of oxygen and superoxide

Abstract: The electrochemical reduction of oxygen in two different room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([EMIM][N(Tf)2]) and hexyltriethylammonium bis-((trifluoromethyl)sulfonyl)imide ([N6222][N(Tf)2]) was investigated by cyclic voltammetry at a gold microdisk electrode. Chronoamperometric measurements were made to determine the diffusion coefficient, D, and concentration, c, of the electroactive oxygen dissolved in the ionic liquid by fitting experimental transients to the Aoki model. [Aoki, K.; et al. J. Electroanal. Chem. 1981, 122, 19]. A theory and simulation designed for cyclic voltammetry at microdisk electrodes was then employed to determine the diffusion coefficient of the electrogenerated superoxide species, O2·-, as well as compute theoretical voltammograms to confirm the values of D and c for neutral oxygen obtained from the transients. As expected, the diffusion coefficient of the superoxide species was found to be smaller than that of the oxygen in both ionic liquids. The diffusion coefficients of O2 and O2·- in [N6222][N(Tf)2], however, differ by more than a factor of 30 (DO2 = 1.48 × 10-10 m2 s-1, DO2·- = 4.66 × 10-12 m2 s-1), whereas they fall within the same order of magnitude in [EMIM]-[N(Tf)2] (DO2 = 7.3 × 10-10 m2 s-1, DO2·- = 2.7 × 10-10 m2 s-1). This difference in [N6222][N(Tf)2] causes pronounced asymmetry in the concentration distributions of oxygen and superoxide, resulting in significant differences in the heights of the forward and back peaks in the cyclic voltammograms for the reduction of oxygen. This observation is most likely a result of the higher viscosity of [N6222][N(Tf)2] in comparison to [EMIM][N(Tf)2], due to the structural differences in cationic component.

Photovoltaic Properties of Self-Assembled Monolayers of Porphyrins and Porphyrin−Fullerene Dyads on ITO and Gold Surfaces

Abstract: A systematic series of ITO electrodes modified chemically with self-assembled monolayers (SAMs) of porphyrins and porphyrin-fullerene dyads have been designed to provide valuable insight into the development of artificial photosynthetic devices. First the ITO and gold electrodes modified chemically with SAMs of porphyrins with a spacer of the same number of atoms were prepared to compare the effects of energy transfer (EN) quenching of the porphyrin excited singlet states by the two electrodes. Less EN quenching was observed on the ITO electrode as compared to the EN quenching on the corresponding gold electrode, leading to remarkable enhancement of the photocurrent generation (ca. 280 times) in the porphyrin SAMs on the ITO electrode in the presence of the triethanolamine (TEA) used as a sacrificial electron donor. The porphyrin (H(2)P) was then linked with C(60) which can act as an electron acceptor to construct H(2)P-C(60) SAMs on the ITO surface in the presence of hexyl viologen (HV(2+)) used as an electron carrier in a three electrode system, denoted as ITO/H(2)P-C(60)/HV(2+)/Pt. The quantum yield of the photocurrent generation of the ITO/H(2)P-C(60)/HV(2+)/Pt system (6.4%) is 30 times larger than that of the corresponding system without C(60): ITO/H(2)P-ref/HV(2+)/Pt (0.21%). Such enhancement of photocurrent generation in the porphyrin-fullerene dyad system is ascribed to an efficient photoinduced ET from the porphyrin singlet excited state to the C(60) moiety as indicated by the fluorescence lifetime measurements and also by time-resolved transient absorption studies on the ITO systems. The surface structures of H(2)P and H(2)P-C(60) SAMs on ITO (H(2)P/ITO and H(2)P-C(60)/ITO) have been observed successfully in molecular resolution with atomic force microscopy for the first time.

Nanostructures and capacitive characteristics of hydrous manganese oxide prepared by electrochemical deposition

Abstract: Amorphous manganese oxide deposits with nanostructures (denoted as a-MnO x .nH 2 O) were electrochemically deposited onto graphite substrates from 0.16 M MnSO 4 .5H 2 O with pH 5.6 by means of the potentiostatic, galvanostatic, and potentiodynamic techniques. The maximum specific capacitance of a-MnO x .nH 2 O deposits plated in different modes, measured from cyclic voltammetry at 25 mV s -1 , is about 230 F g -1 in a potential window of 1.0 V. The high electrochemical reversibility, high-power characteristics, good stability, and improved frequency responses in 0.1 M Na 2 SO 4 for these nanostructured a-MnO x .nH 2 O deposits prepared by electrochemical methods demonstrate their promising potential in the application to electrochemical supercapacitors. The nanostructure of a-MnO x .nH 2 O, clearly observed by means of a scanning electron microscope, was found to depend strongly on the deposition mode. The similar capacitive performance of all deposits prepared in different modes was attributable to their nonstoichiometric nature with a very similar oxidation state, demonstrated by XPS spectra.

Analysis of Pharmaceuticals and Biological Fluids Using Modern Electroanalytical Techniques

Abstract: A review of the principles and application of modern electroanalytical techniques, namely, cyclic voltammetry, linear sweep voltammetry, differential pulse voltammetry, differential pulse polarography, square wave voltammetry, square wave polarography, stripping voltammetric, and stripping polarographic techniques, is presented. The use and advantages of these techniques at different electrodes are discussed. The analytical applications of these techniques to pharmaceutical compounds in dosage forms and biological media are also discussed. Various selected studies on these subjects since 1995 are reviewed.

The standard redox potential of the phenyl radical/anion couple.

Abstract: The voltammogram of aryldiazonium tetrafluoroborates in acetonitrile (ACN), at low concentration, shows a first one-electron wave followed at a more negative potential by a small second wave; this last one corresponds to the reduction of the radical formed at the level of the first wave. Simulation of the voltammogram permits one to determine the standard redox potential of the radical/anion couple E°(Ph•/Ph-) = 0.05 V/SCE and the reduction mechanism of the diazonium cation. An electron transfer concerted with the cleavage of the C−N bond furnishes the aryl radical; this radical undergoes two competitive reactions: reduction at the electrode and H-atom transfer.