Showing papers on "Cyclic voltammetry published in 2004"

Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor

Abstract: The charge storage mechanism in MnO2 electrode, used in aqueous electrolyte, was investigated by cyclic voltammetry and X-ray photoelectron spectroscopy. Thin MnO2 films deposited on a platinum substrate and thick MnO2 composite electrodes were used. First, the cyclic voltammetry data established that only a thin layer of MnO2 is involved in the redox process and electrochemically active. Second, the X-ray photoelectron spectroscopy data revealed that the manganese oxidation state was varying from III to IV for the reduced and oxidized forms of thin film electrodes, respectively, during the charge/discharge process. The X-ray photoelectron spectroscopy data also show that Na+ cations from the electrolyte were involved in the charge storage process of MnO2 thin film electrodes. However, the Na/Mn ratio for the reduced electrode was much lower than what was anticipated for charge compensation dominated by Na+, thus suggesting the involvement of protons in the pseudofaradaic mechanism. An important finding o...

Electrochemical biosensing platforms using platinum nanoparticles and carbon nanotubes.

Abstract: Platinum nanoparticles with a diameter of 2-3 nm were prepared and used in combination with single-wall carbon nanotubes (SWCNTs) for fabricating electrochemical sensors with remarkably improved sensitivity toward hydrogen peroxide. Nafion, a perfluorosulfonated polymer, was used to solubilize SWCNTs and also displayed strong interactions with Pt nanoparticles to form a network that connected Pt nanoparticles to the electrode surface. TEM and AFM micrographs illustrated the deposition of Pt nanoparticles on carbon nanotubes whereas cyclic voltammetry confirmed an electrical contact through SWCNTs between Pt nanoparticles and the glassy carbon (GC) or carbon fiber backing. With glucose oxidase (GOx) as an enzyme model, we constructed a GC or carbon fiber microelectrode-based biosensor that responds even more sensitively to glucose than the GC/GOx electrode modified by Pt nanoparticles or CNTs alone. The response time and detection limit (S/N = 3) of this biosensor was determined to be 3 s and 0.5 microM, respectively.

Electrocatalytic corrosion of carbon support in PEMFC cathodes

Abstract: The influence of platinum on the corrosion of carbon catalyst supports has been characterized by on-line mass spectroscopy during cyclic voltammetry, with varying Pt mass fraction, catalyst type, and temperature. The generation rates increased with higher Pt mass fraction (0, 10, and 39% balanced by Vulcan XC-72). A peak observed at approximately for Pt/C was lowered to for PtRu/C. An Arrenhius plot indicated higher apparent activation energy for production at the positive potential limit of the cyclic voltammogram on 0% Pt (carbon-only) electrode than on 39% Pt/C electrode. It was concluded that platinum accelerated the corrosion rate of the carbon catalyst support. © 2003 The Electrochemical Society. All rights reserved.

Synthesis and Electrochemical Characterization of Uniformly-Dispersed High Loading Pt Nanoparticles on Sonochemically-Treated Carbon Nanotubes

Abstract: A sonochemical process was developed to treat carbon nanotubes in nitric and sulfuric acids to create surface functional groups for metal nanoparticle deposition. Carbon nanotubes treated in the sonochemical process are shown to lead to the deposition of uniformly dispersed high loading Pt nanoparticles, which have not been achieved with carbon nanotubes treated in reflux processes. Pt nanoparticles of a size less than 5 nm and loading up to 30 wt % with little aggregation were synthesized on the sonochemically treated carbon nanotubes. Cyclic voltammetry measurements in 1.0 M H2SO4 showed that the Pt nanoparticles on carbon nanotubes are more than 100% active in the electrochemical adsorption and desorption of hydrogen than the Pt nanoparticles supported on carbon black. This enhancement of electrochemical activity is attributed to the unique structures of carbon nanotubes and the interactions between the Pt nanoparticles and the carbon nanotube support. The ability to synthesize high loading Pt on carbo...

Characterization of Vulcan Electrochemically Oxidized under Simulated PEM Fuel Cell Conditions

Abstract: Electrochemical surface oxidation of Vulcan XC-72, a carbon black commonly used in proton exchange membrane (PEM) fuel cells, was studied following potentiostatic treatments up to 120 h at potentials from 0.6 to 1.2 V at room temperature and 65°C. Surface oxidation was followed using cyclic voltammetry (CV), thermal gravimetric analysis coupled to on-line mass spectrum-etry (TGA-MS), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. The analytical techniques all indicate significant surface oxidation occurred during the first 16 h of 1.2 V potential holds at room temperature and a slow increase in surface oxide formation thereafter. An identification of ether, carbonyl, and carboxyl surface oxide species was made by deconvolution of XPS spectra and assigning these functional groups to the observed TGA-MS CO 2 evolution peaks (150-750°C). An increase in CO evolution (>800°C) determined by TGA-MS was consistent with electrochemical CV data, which detected electroactive hydroquinone/quinone species; these electrochemically detected species were a minor fraction of the electrochemically generated surface oxides. Potential holds at 1.0 V at room temperature only resulted in slight oxidation of Vulcan XC-72. However, experiments at 65°C showed clear signs of surface oxidation after only 16 h at potentials ≥0.8 V, verifying that surface oxides can be generated under simulated PEM fuel cell conditions. Overall, these results suggest that changes in component hydrophobicity, driven by carbon surface oxidation, are an important factor in determining long-term PEM performance instability and decay.

Electronic band structure of titania semiconductor nanosheets revealed by Electrochemical and photoelectrochemical studies

Abstract: Electrochemical and photoelectrochemical studies were conducted on self-assembled multilayer films of titania nanosheets on a conductive ITO substrate. Cyclic voltammogram (CV) curves indicated that the titania nanosheet electrode underwent insertion/extraction of Li+ ions into/from the nanosheet galleries, associated with reduction/oxidation of Ti4+/Ti3+. These processes accompanied reversible changes in UV−vis absorption of the titania nanosheet electrodes. Applying a negative bias of −1.3 V (vs Ag/Ag+) and lower brought about absorption reduction where the wavelength is shorter than 323 nm, and vice versa, indicating a flat-band potential of (approximately) −1.3 V and a band gap energy of 3.84 eV. Photocurrents were generated from the titania nanosheet electrodes under a positive bias. The onset potential for photocurrent generation from the titania nanosheet electrodes was around −1.27 V, and the band gap energy estimated from the photocurrent action spectra was 3.82 eV, in excellent agreement with th...

Basal plane pyrolytic graphite modified electrodes: comparison of carbon nanotubes and graphite powder as electrocatalysts.

Abstract: The oxidations of NADH, epinephrine, and norepinephrine are studied using carbon nanotube and graphite powder-modified basal plane pyrolytic graphite electrodes. Immobilization is achieved in two ways: first, via abrasive attachment of multiwall carbon nanotubes or graphite powder by gently rubbing the electrode surface on a fine quality paper supporting the desired material; second, via “film” modification from dispersing either graphite powder or nanotubes in acetonitrile and pipeting a small volume onto the electrode surface and allowing the solvent to volatilize. While electrocatalytic behavior of both types of nanotube-modified electrodes is shown, with enhanced currents and reduced peak-to-peak separations in the voltammetry in comparison with naked basal plane pyrolytic graphite, similar catalytic behavior is also seen at the graphite powder-modified electrodes. Caution is, therefore, suggested in assigning unique catalytic properties to carbon nanotubes.

Anodic stripping voltammetry of arsenic(III) using gold nanoparticle-modified electrodes.

Abstract: A novel method for the detection of arsenic(III) in 1 M HCl at a gold nanoparticle-modified glassy carbon electrode has been developed. The gold nanoparticles were electrodeposited onto the glassy carbon electrode via a potential step from +1.055 to −0.045 V vs SCE for 15 s from 0.5 M H2SO4 containing 0.1 mM HAuCl4. The resulting electrode surfaces were characterized with both AFM and cyclic voltammetry. Anodic stripping voltammetry of arsenic(III) on the modified electrode was performed. After optimization, a LOD of 0.0096 ppb was obtained with LSV.

Electrochemical oxidation of aliphatic amines and their attachment to carbon and metal surfaces

Abstract: The electrochemical oxidation of aliphatic amines (primary, secondary, and tertiary) has been investigated by cyclic voltammetry and preparative electrolysis. The oxidation mechanisms have been established, and the lifetimes of the radical cations have been measured for secondary and tertiary amines. These results have been put in parallel with the attachment of amines to glassy carbon, Au, and Pt electrodes by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and infrared reflection-absorption spectroscopy (IRRAS). It is then possible to show that it is not the radical cation but the radical obtained after the deprotonation which reacts with the electrode surface. XPS results also point to the existence of a covalent bond between Au or Pt and the organic moiety.

Physical and Electrochemical Characterizations of Microwave-Assisted Polyol Preparation of Carbon-Supported PtRu Nanoparticles

Abstract: PtRu nanoparticles supported on Vulcan XC-72 carbon and carbon nanotubes were prepared by a microwave-assisted polyol process. The catalysts were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). The PtRu nanoparticles, which were uniformly dispersed on carbon, were 2−6 nm in diameter. All PtRu/C catalysts prepared as such displayed the characteristic diffraction peaks of a Pt face-centered cubic structure, excepting that the 2θ values were shifted to slightly higher values. XPS analysis revealed that the catalysts contained mostly Pt(0) and Ru(0), with traces of Pt(II), Pt(IV), and Ru(IV). The electro-oxidation of methanol was studied by cyclic voltammetry, linear sweep voltammetry, and chronoamperometry. It was found that both PtRu/C catalysts had high and more durable electrocatalytic activities for methanol oxidation than a comparative Pt/C catalyst. Preliminary data from a direct methanol fuel cell single stack test cell using the Vulcan...

Resolving neurotransmitters detected by fast-scan cyclic voltammetry.

Abstract: Carbon-fiber microelectrodes are frequently used as chemical sensors in biological preparations. In this work, we evaluated the ability of cyclic voltammograms recorded at fast-scan rates to resolve neurochemicals when analyzed by principal component regression. A calibration set of 30 cyclic voltammograms was constructed from 9 different substances at a variety of concentrations. The set was reduced by principal component analysis, and it was found that 99.5% of the variance in the data could be captured with five principal components. This set was used to evaluate cyclic voltammograms obtained with one or two compounds present in solution. In most cases, satisfactory predictions of the identity and concentration of analytes were obtained. Chemical dynamics were also resolved from a set of fast-scan cyclic voltammograms obtained with the electrode implanted in a region of a brain slice that contains dopaminergic terminals. Following stimulation, principal component regression of the data resolved the cha...

Development of anode catalysts for a direct ethanol fuel cell

Abstract: Ethanol electrooxidation was investigated at platinum based electrodes: Pt, Pt—Sn, Pt—Re dispersed on a high surface area carbon powder. The atomic composition of the bimetallic catalyst was varied and the best results were obtained with an atomic ratio Pt:X close to 100:20. The electrocatalytic activity of Pt, PtSn and PtRe was compared using cyclic voltammetry and long-term electrolyses at constant potential. Under voltammetric conditions and in a single direct ethanol fuel cell, PtSn was the most active catalyst. During electrolysis ethanol was oxidized to acetaldehyde (AAL), acetic acid (AA) and carbon dioxide. On PtSn/C and PtRe/C, the ratio AA/AAL was found to be always lower than unity. Otherwise, PtSn electrocatalysts were the most selective towards the production of CO2 compared to Pt and PtRe electrodes.

Carbon Nanostructures in Portable Fuel Cells: Single-Walled Carbon Nanotube Electrodes for Methanol Oxidation and Oxygen Reduction

Abstract: We show here, for the first time, a reproducible way to obtain films of varying amounts of single-walled carbon nanotubes (SWCNTs) on electrode surfaces using electrophoretic deposition. We deposit these nanotubes in a facile manner on an optically transparent electrode (OTE) and investigate its performance as an electrode material in the presence of platinum for methanol oxidation and oxygen reduction. Our focus here is on the deposition of the SWCNT on the electrodes, the characterization of the nanotubes on the electrode surface, and the cyclic voltammetry of methanol oxidation and oxygen reduction using these nanostructured carbon electrodes with platinum electrodeposited on them. The nanotubes retain their structure on the electrode surface, and we can obtain electrodes with relatively thick films of the CNTs. The high surface area and porosity of these films enable us to use relatively small amounts of platinum and yet obtain excellent currents. We see a remarkable enhancement in methanol oxidation ...

Ethanol Electrooxidation on a Carbon-Supported Pt Catalyst: Reaction Kinetics and Product Yields

Abstract: The ethanol oxidation reaction (EOR) on a carbon-supported Pt nanoparticle catalyst was studied by cyclic voltammetry and potential-step measurements as a function of ethanol concentration and reaction temperature (23−60 °C), combining on-line mass spectrometric analysis of the reaction products and electrochemical current measurements. The effect of catalyst loading/electrode roughness was elucidated by comparison with a polycrystalline Pt electrode. Individual, absolute rates for CO2 and acetaldehyde formation were determined via the doubly ionized carbon dioxide molecular ion at m/z = 22 and the CHO+ fragment at m/z = 29, whereas acetic acid yields were calculated as the difference between the Faradaic current (charge) and the sum of the partial currents for oxidation to CO2 and acetaldehyde, calculated from the calibrated mass spectrometric currents. Incomplete ethanol oxidation to acetaldehyde and acetic acid prevails over complete oxidation to CO2 under all conditions, the dominant products being ac...

Electrogenerated Chemiluminescence from a CdSe Nanocrystal Film and Its Sensing Application in Aqueous Solution

Abstract: Electrogenerated chemiluminescence (ECL) of semiconductor quantum dots in aqueous solutions and its first sensing application were studied by depositing CdSe nanocrystals (NCs) on a paraffin-impregnated graphite electrode (PIGE). The CdSe nanocrystal thin film exhibited two ECL peaks at −1.20 (ECL-1) and −1.50 V (ECL-2) in pH 9.3, 0.1 M PBS during the cyclic sweep between 0 and −1.8 V at 20 mV s-1. The electron-transfer reaction between individual electrochemically reduced nanocrystal species and oxidant coreactants such as H2O2 and reduced dissolved oxygen led to ECL-1. When mass NCs packed densely in the film were reduced electrochemically, assembly of reduced nanocrystal species could react with coreactants to produce another ECL signal, ECL-2. ECL-1 showed higher sensitivity to the concentration of oxidant coreactants than ECL-2 and thus was used for ECL detection of coreactant, H2O2. A linear response of ECL-1 to H2O2 was observed in the concentration range of 2.5 × 10-7−6 × 10-5 M with a detection l...

Direct preparation of carbon nanofiber electrodes via pyrolysis of iron(II) phthalocyanine: Electrocatalytic aspects for oxygen reduction

Abstract: A facile method for the direct preparation of carbon nanofiber (CNF) electrodes by pyrolysis of iron(II) phthalocyanine on nickel substrates is reported. Uniform, large area coverage is observed with aligned bundles of CNFs exhibiting bamboo-like, hollow fibril morphology possessing diameters of 40−60 nm and lengths of ∼10 μm. The electrochemical behavior and stability of CNF electrodes as oxygen reduction catalysts were investigated by electrochemical methods. Without necessitation for extensive electrode pretreatment or surface activation, these electrodes demonstrate significant electrocatalytic activity in aqueous KNO3 solutions at neutral to basic pH for the reduction of dioxygen to hydrogen peroxide, O2 + H2O + 2e- ⇌ H + OH-. As determined from chronocoulometry, slopes of Anson plots indicate that the overall electrochemical reaction proceeds by the peroxide pathway via two successive two-electron reductions. pH-dependent cyclic voltammetry studies indicate that the CNF electrodes are very active to...

Electrochemical and bioelectrochemistry properties of room-temperature ionic liquids and carbon composite materials.

Abstract: Room-temperature ionic liquids (RTILs) are liquids at room temperature and represent a new class of nonaqueous but polar solvents with high ionic conductivity. The conductivity property of carbon nanotubes/RTILs and carbon microbeads/RTILs composite materials has been studied using ac impedance technology. Enzyme coated by RTILs-modified gold and glassy carbon electrodes allow efficient electron transfer between the electrode and the protein and also catalyze the reduction of O2 and H2O2.

Electrostatic layer-by-layer assembled carbon nanotube multilayer film and its electrocatalytic activity for O2 reduction.

Abstract: Multilayer films of shortened multiwalled carbon nanotubes (MWNTs) are homogeneously and stably assembled on glassy carbon electrodes with the layer-by-layer (LBL) method, based on electrostatic interaction of positively charged poly(diallyldimethylammonium chloride) and negatively charged and shortened MWNTs. The film assembly and electrochemical property as well as the electrocatalytic activity toward O2 reduction of the MWNT multilayer film are studied. Scanning electron microscopy, the quartz crystal microbalance technique, ultraviolet-visible-near-infrared spectroscopy, and cyclic voltammetry are used for characterization of film assembly. Experimental results revealed that film growth is uniform, almost with the same coverage of the MWNTs in each layer, and that the assembled MWNTs are mainly in the form of small bundles or single tubes on the electrodes. Electrochemical studies indicate that the LBL assembled MWNT films possess a remarkable electrocatalytic activity toward O2 reduction in alkaline media. This property, combined with the well-dispersed, porous and conductive features of the MWNT film illustrated with the LBL method, suggests the potential application of the MWNT film for constructing an efficient alkaline air electrode for energy conversions.