Showing papers on "Overpotential published in 2007"

Study of the Exchange Current Density for the Hydrogen Oxidation and Evolution Reactions

Abstract: The exchange current density for the hydrogen oxidation/evolution reactions was determined in a proton exchange membrane fuel cell. Ultralow Pt-loaded electrodes (0.003 mg pt /cm 2 ) were used to obtain measurable kinetic overpotential signals (50 mV at 2 A/cm 2 ). Using a simple Butler-Volmer equation, the exchange current density and transfer coefficient were determined to lie within the range of 235-600 mA/cm 2 Pt and 0.5-1, respectively. Due to the fast kinetics, no measurable voltage losses are predicted for pure-H 2 /air proton exchange membrane fuel cell applications when lowering the anode Pt loadings from its current value of 0.4 mg pt /cm 2 to the automotive target of 0.05 mg pt /cm 2 .

Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes

Abstract: The compound CsH2PO4 has emerged as a viable electrolyte for intermediate temperature (200–300 °C) fuel cells. In order to settle the question of the high temperature behavior of this material, conductivity measurements were performed by two-point AC impedance spectroscopy under humidified conditions (p[H2O] = 0.4 atm). A transition to a stable, high conductivity phase was observed at 230 °C, with the conductivity rising to a value of 2.2 × 10−2 S cm−1 at 240 °C and the activation energy of proton transport dropping to 0.42 eV. In the absence of active humidification, dehydration of CsH2PO4 does indeed occur, but, in contradiction to some suggestions in the literature, the dehydration process is not responsible for the high conductivity at this temperature. Electrochemical characterization by galvanostatic current interrupt (GCI) methods and three-point AC impedance spectroscopy (under uniform, humidified gases) of CsH2PO4 based fuel cells, in which a composite mixture of the electrolyte, Pt supported on carbon, Pt black and carbon black served as the electrodes, showed that the overpotential for hydrogen electrooxidation was virtually immeasurable. The overpotential for oxygen electroreduction, however, was found to be on the order of 100 mV at 100 mA cm−2. Thus, for fuel cells in which the supported electrolyte membrane was only 25 μm in thickness and in which a peak power density of 415 mW cm−2 was achieved, the majority of the overpotential was found to be due to the slow rate of oxygen electrocatalysis. While the much faster kinetics at the anode over those at the cathode are not surprising, the result indicates that enhancing power output beyond the present levels will require improving cathode properties rather than further lowering the electrolyte thickness. In addition to the characterization of the transport and electrochemical properties of CsH2PO4, a discussion of the entropy of the superprotonic transition and the implications for proton transport is presented.

Kinetics of consumption of fermentation products by anode-respiring bacteria

Abstract: We determined the kinetic response of a community of anode-respiring bacteria oxidizing a mixture of the most common fermentation products: acetate, butyrate, propionate, ethanol, and hydrogen. We acclimated the community by performing three consecutive batch experiments in a microbial electrolytic cell (MEC) containing a mixture of the fermentation products. During the consecutive-batch experiments, the coulombic efficiency and start-up period improved with each step. We used the acclimated biofilm to start continuous experiments in an MEC, in which we controlled the anode potential using a potentiostat. During the continuous experiments, we tested each individual substrate at a range of anode potentials and substrate concentrations. Our results show low current densities for butyrate and hydrogen, but high current densities for propionate, acetate, and ethanol (maximum values are 1.6, 9.0, and 8.2 A/m2, respectively). Acetate showed a high coulombic efficiency (86%) compared to ethanol and propionate (49 and 41%, respectively). High methane concentrations inside the MEC during ethanol experiments suggest that methanogenesis is one reason why the coulombic efficiency was lower than that of acetate. Our results provide kinetic parameters, such as the anode overpotential, the maximum current density, and the Monod half-saturation constant, that are needed for model development when using a mixture of fermentation products. When we provided no electron donor, we measured current due to endogenous decay of biomass (~0.07 A/m2) and an open-cell potential (−0.54 V vs Ag/AgCl) associated with biomass components active in endogenous respiration.

Detection of NADH and Ethanol Based on Catalytic Activity of Soluble Carbon Nanofiber with Low Overpotential

Abstract: The excellent catalytic activity of a novel carbon nanomaterial, soluble carbon nanofiber (CNF) with good dispersion and wettability, to the oxidation of dihydronicotinamide adenine dinucleotide (NADH) was described for biosensing application. The edge plane sites and oxygen-rich groups presented on the CNF surface could be partially responsible for its electrocatalytic behavior, which induced a substantial decrease by 573 mV in the overpotential of NADH oxidation reaction (compared to a bare electrode). The CNF-coated electrode thus allowed highly sensitive amperometric detection of NADH with a low limit of detection (0.11 μM), low applied potential (+0.06 V), and minimization of surface fouling. Such ability of CNF to promote the electron transfer between NADH and the electrode suggested a new, promising biocompatible platform for development of dehydrogenase-based amperometric biosensors. With alcohol dehydrogenase (ADH) as a model, the ADH/CNF-modified electrode could be constructed by a simple castin...

Diffusion-Controlled Oxygen Reduction on Multi-Copper Oxidase-Adsorbed Carbon Aerogel Electrodes without Mediator

Abstract: Bioelectrocatalytic reduction of O2 into water was archived at diffusion-controlled rate by using enzymes (laccase from Trametes sp. and bilirubin oxidase from Myrothecium verrucaria, which belong to the family of multi-copper oxidase) adsorbed on mesoporous carbon aerogel particle without a mediator. The current density was predominantly controlled by the diffusion of dissolved O2 in rotating-disk electrode experiments, and reached a value as large as 10 mA cm–2 at 1 atm O2, 25 °C, and 8,000 rpm on the laccase-adsorbed electrode. The overpotential of the bioelectrocatalytic reduction of O2 was 0.4–0.55 V smaller than that observed on a Pt disk electrode. Without any optimization, the laccase-adsorbed biocathode showed stable current intensity of the O2 reduction in an air-saturated buffer at least for 10 days under continuous flow system.

Water Electrolysis under a Magnetic Field

Abstract: The energy efficiency of water electrolysis was considerably improved under a high magnetic field. This was proved by measuring the cell voltage, the IR-drop, and the electrode potentials for the electrolysis which was galvanostatically operated in alkaline (4.46 and 0.36 M KOH) and acidic (0.05 M H 2 SO 4 ) solutions. A large reduction in the cell voltage was achieved in a magnetic field, especially at a high current density. The decrease of the IR-drop, which was measured by the current interrupter method, depended on the concentration of electrolyte solutions. In a magnetic field, the oxygen overpotential was reduced more than the hydrogen overpotential.

Ultrasensitive nanostructured platform for the electrochemical sensing of hydrazine

Abstract: Ultrasensitive electrochemical detection of hydrazine using nanosized Au particles self-assembled on a sol−gel-derived 3D silicate network is described. The citrate-stabilized gold nanoseeds (GNSs) were self-assembled on the thiol groups of the silicate network, which was preassembled on a polycrystalline Au electrode. The size of the GNSs on the network was enlarged by a seed-mediated growth approach, and the GNSs were characterized by UV−visible spectroscopy, X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and electrochemical measurements. The enlarged nanoparticles (GNEs) on the silicate network have a size distribution between 70 and 100 nm and behave as a nanoelectrode ensemble. This nanostructured platform is highly sensitive toward the electrochemical oxidation of hydrazine. A very large decrease in the overpotential (∼800 mV) and significant enhancement in the peak currents with respect to the bulk Au electrode were observed without using any redox mediator. The nanos...

On the influence of the metal loading on the structure of carbon-supported PtRu catalysts and their electrocatalytic activities in CO and methanol electrooxidation

Abstract: PtRu (1 : 1) catalysts supported on low surface area carbon of the Sibunit family (SBET = 72 m2 g−1) with a metal percentage ranging from 5 to 60% are prepared and tested in a CO monolayer and for methanol oxidation in H2SO4 electrolyte. At low metal percentage small (<2 nm) alloy nanoparticles, uniformly distributed on the carbon surface, are formed. As the amount of metal per unit surface area of carbon increases, particles start coalescing and form first quasi two-dimensional, and then three-dimensional metal nanostructures. This results in a strong enhancement of specific catalytic activity in methanol oxidation and a decrease of the overpotential for CO monolayer oxidation. It is suggested that intergrain boundaries connecting crystalline domains in nanostructured PtRu catalysts produced at high metal-on-carbon loadings provide active sites for electrocatalytic processes.

Experimental and Theoretical Study of Cobalt Selenide as a Catalyst for O2 Electroreduction

Abstract: Cobalt sulfides have been known for more than 30 years to be active toward oxygen reduction, and cobalt selenides have shown less activity. In this paper, a theoretical analysis is made of the four-electron reduction reaction of oxygen to water over the mixed anion and cation (202) surface of the pentlandite structure Co9Se8, one of several selenide phases. Reversible potentials for forming adsorbed reaction intermediates in acid are predicted using adsorption energies calculated with the Vienna ab initio simulation program (VASP) and the known bulk solution values together in a linear Gibbs energy relationship. Comparison with an earlier theoretical analysis of pentlandite structure Co9S8 shows that the overpotential is predicted to be larger for the selenide by around 0.22 V. Cobalt selenide electrodes of unspecified stoichiometry were prepared chemically on glassy carbon discs, and polarization curves were measured using rotating discs. When heat-treated at 900 °C, the onset potential for O2 reduction ...

Electrochemical Determination of Ascorbic Acid in Room Temperature Ionic Liquid BPPF6 Modified Carbon Paste Electrode

Abstract: A room temperature ionic liquid N-butylpyridinium hexafluorophosphate (BPPF6) was used as a binder to make an ionic liquid modified carbon paste electrode (IL-CPE), which showed good characteristics such as simple preparation procedure, fast electrochemical response and good conductivity. The electrochemical oxidation of ascorbic acid (AA) on the new IL-CPE was carefully studied. The oxidation peak potential of AA on the IL-CPE appeared at 109 mV (vs. SCE), which was about 338 mV decrease of the overpotential compared to that obtained on the traditional carbon paste electrode (CPE) and the oxidation peak current was increased for about four times. The electrochemical parameters of AA on the IL-CPE were calculated with the charge transfer coefficient (α) and the electrode reaction rate constant (ks) as 0.87 and 0.800 s−1, respectively. Based on the relationship of the oxidation peak current and the concentration of AA a sensitive analytical method was established with cyclic voltammetry. The linear range for AA determination was in the range from 1.0×10−5 to 3.0×10−3 mol/L with the linear regression equation as Ip (μA)=−2.52–0.064C (μmol/L) (n=13, γ=0.9942) and the detection limit was calculated as 8.0×10−6 mol/L (3σ). The proposed method was free of the interferences of coexisting substances such as dopamine (DA) and amino acids etc., and successfully applied to the vitamin C tablets determination.