Showing papers on "Overpotential published in 2003"

Oxygen Is Electroreduced to Water on a "Wired" Enzyme Electrode at a Lesser Overpotential than on Platinum

Abstract: The first enzyme-based catalyst that is superior to platinum in the four-electron electroreduction of oxygen to water is reported. The smooth Pt cathode reached half and 90% of the mass transport-limited current density at respective overpotentials of −0.4 and −0.58 V in 0.5 M sulfuric acid, and only at even higher overpotentials in pH 7.2 phosphate buffer. In contrast, the smooth “wired” bilirubin oxidase cathode reached half and 90% of the mass transport-limited current density at respective overpotentials as low as −0.2 and −0.25 V. The mass transport-limited current density for the smooth “wired” enzyme cathode in PBS was twice that with smooth Pt in 0.5 M sulfuric acid. Under 1 atm O2 pressure, O2 was electroreduced to water on a polished carbon cathode, coated with the “wired” BOD film, in pH 7.2 saline buffer (PBS) at an overpotential of −0.31 V at a current density of 9.5 mA cm-2. At the same overpotential, the current density of the polished platinum cathode in 0.5 M H2SO4 was 16-fold lower, only...

Experimental Study on Heat Generation Behavior of Small Lithium-Ion Secondary Batteries

Abstract: The overpotential resistance and the entropy change of two small lithium-ion secondary batteries, which are the heat source terms to increase the battery temperature, have been measured by several methods, changing the battery temperature and the state of charge. The temperature increase and the total heat generation rate of the batteries were calculated during the discharge cycle by using the measured resistance and entropy, being compared with the experimental results of the temperature increase and the total heat generation rate. The overpotential resistance was estimated by four measurement methods, i.e., the battery voltage-current characteristics during the constant current discharge, the difference between the open-circuit voltage and the cell voltage, the voltage change during the intermittent discharge for 60 s, and the ac impedance measurement. The overpotential resistance by the voltage-current characteristics is almost the same as by the difference between the open-circuit voltage and the cell voltage. However, in some cases the resistances by the intermittent discharge and the ac impedance are smaller than the former two resistances. The entropy change measured by the temperature change of the open-circuit voltage agrees almost with the measured by the heat production difference between the charge and discharge cycle. The temperature increases and the total heat generation rates estimated from the overpotential resistances by the voltage-current characteristics and by the temperature change of open-circuit voltage agree well with the measured ones for the two batteries during the constant current discharge. © 2003 The Electrochemical Society. All rights reserved.

Development of (La,Sr)MnO3-based cathodes for intermediate temperature solid oxide fuel cells

Abstract: Conventional (La, Sr)MnO 3 (LSM) electrodes modified by ion impregnation methods showed promising potential as cathodes for intermediate temperature solid oxide fuel cells. After impregnation of ionic conducting-type Gd 0 . 2 Ce 0 . 8 (NO 3 ) x nitrite salt solution into the LSM structure, electrochemical activity of LSM electrodes for the O 2 reduction reaction was substantially enhanced. At 700°C, the electrode polarization resistance of impregnated LSM electrodes decreased to 0.72 Ω cm 2 as compared to 26.4 Ω cm 2 for pure LSM electrodes, a reduction in electrode polarization resistance by 36 times. The polarization losses were also reduced substantially. At 300 mA cm - 2 and 700°C, overpotential for the O 2 reduction was reduced from 0.79 V on pure LSM to 0.19 V on impregnated LSM electrodes, a reduction in overpotential by four times. Ion-impregnated LSM electrodes showed better performance than those of LSM/Y 2 O 3 -ZrO 2 and LSM/(Gd,Ce)O 2 composite electrodes as reported in the literature.

Effect of ambient conditions on performance and current distribution of a polymer electrolyte membrane fuel cell

Abstract: The performance and current distribution of a free-breathing polymer electrolyte membrane fuel cell (PEMFC) was studied experimentally in a climate chamber, in which temperature and relative humidity were controlled. The performance was studied by simulating ambient conditions in the temperature range 10 to 40 °C. The current distribution was measured with a segmented current collector. The results indicated that the operating conditions have a significant effect on the performance of the fuel cell. It was observed that a temperature gradient between the fuel cell and air is needed to achieve efficient oxygen transport to the electrode. Furthermore, varying the air humidity resulted in major changes in the mass diffusion overpotential at higher temperatures.

The Effects of Processing Conditions and Chemical Composition on Electronic and Ionic Resistivities of Fuel Cell Electrode Composites

Abstract: A major goal of polymer electrolyte fuel cell (PEFC) efforts is an understanding of how process conditions and material composition affect sources of overpotential in an operating cell. Limited ionic conductivity within the catalyst layer is one such source of overpotential. We report the results of a study of the impact of processing conditions, specifically hot pressing and boiling in acid, on the ionic and electronic resistivities of catalyst layers made from platinized VULCAN XC-72 (XC-72) carbon. Our results show that the greatest gains in ionic conductivity for a PEFC catalyst coated membrane comes from acid exchange of the active layer in tetrabutylammonium form. We also probe the dependence of ionic and electronic resistivities of catalyst layers on their chemical composition. We determine the ionic and electronic conductivities of surface modified unplatinized XC-72 carbon with phenyl sulfonic acid of varying weight percentage. Nafion composites with the modified materials display an increase in ionic conductivity of more than an order of magnitude when compared to a composite layer consisting of plain XC-72 and Nafion.

Competition Between Bulk and Surface Pathways in Mixed Ionic Electronic Conducting Oxygen Electrodes

Abstract: In mixed ionic electronic conductors (MIECs) the electrochemistry of oxygen reduction and evolution is complicated by the competition between surface adsorption/desorption, interaction of these surface species with bulk vacancies and diffusion of these species to the MIEC/electrolyte interface. Charge transfer may occur by diffusion of oxygen adsorbates to the triple phase boundary or by diffusion of vacancies from the electrolyte with subsequent exchange with oxygen adsorbed on the porous MIEC wall. A model describing the competition between these two charge transfer pathways is developed. Key to the model is treatment of the boundary condition at the MIEC/electrolyte interface. The absolute potential across this interface is used to relate the surface overpotential of each pathway in terms of the other and terms arising from the concentration overpotentials. Results show the conditions under which one process may dominate over the other.

Metal/insulator/metal junctions for electrochemical surface science

Abstract: We describe the preparation and characterization of Al-AlOx-Ag tunnel junctions and calculate the energy distribution of the tunneling hot electrons in the range 0–2.5 eV above the Fermi level of silver. Because the mean free path of the hot electrons is of the order of the thickness of the silver film of the junction, which is at the same time the electrode in contact with an electrolyte, new surface effects can be studied. Hot electrons can be injected into the nonhydrated electron band in water. Hot electrons also cause hydrogen evolution at electrode potentials more positive than the ones needed in common electrochemistry. We observed the emission of hot electrons into silver during transients of hydrogen oxidation at silver and during oxidation of overpotential hydrogen on platinum clusters deposited on the silver electrode. The tunnel current at constant tunnel voltage can be changed by faradaic reactions, but surprisingly also by nonfaradaic reactions; this is assigned to a mesoscopic quantum phenomenon.

Production and properties of composite layers based on an Ni–P amorphous matrix

Abstract: Composite layers Ni–P + Co, Ni–P + W and Ni–P + Ti were obtained in galvanostatic conditions, at jdep = 0.200 A cm−2. The x-ray diffraction method was used to determine the phase composition of the layers and atomic absorption spectrometry was applied to specify their chemical composition. A metallographic microscope, stereoscopic microscope and Form Talysurf-type profilograph were used for cross-section and surface morphology characterization of the layers. The behaviour of the obtained layers was investigated in the process of hydrogen evolution reaction from 5 M KOH using classical methods (voltammetry, steady-state polarization) and electrochemical impedance spectroscopy (EIS). Based on recorded steady-state polarization curves, the Tafel equation parameters for this process were determined. EIS was used to study the interfacial properties at electrode overpotential ΔE = −0.200 V. It was found that the investigated Ni–P + Co layer is characterized by increased electrochemical activity for hydrogen evolution compared to Ni–P + W and Ni–P + Ti layers. Greater activity of the Ni–P + Co layer in this process may be attributed to the developed electrode surface. The values of surface roughness factor Rf were also determined.

Online Monitoring of Anode Outlet CO Concentration in PEM Fuel Cells

Abstract: Reformed gas produced from conventional fuels such as gasoline, methanol, or natural gas are likely to be used in electric vehicle or stationary applications of proton exchange membrane ~PEM! fuel cells. Regardless of the reforming processes employed, e.g., steam reforming ~SR! or autothermal reforming ~ATR!, the reformate gas largely consists of H2 ,N 2 ,C O 2 , CO, and H2O. The strong poisoning effect of CO in the reformate gas on Pt-based anode catalyst has long been known 1,2 and extensively addressed. 3-12 However, attention in these studies has focused almost exclusively on the voltagecurrent characteristics of the PEM fuel cell under different operating conditions with Pt or Pt-based alloy catalysts and with a simulated reformate as anode feed. We have reported a significant effect of the anode flow rate on PEM fuel cell performance with H2/108 ppm CO feed and Pt as anode catalyst. 11 The results were explained on the basis of a CO inventory model which simulates the effect of flow rate on the anode overpotential via CO material balance in the anode chamber including terms for flow in and flow out, as well as electrocatalytic oxidation. The model predicts that the anode CO concentration is a function of anode flow rate, which had not been previously reported. The present work was motivated by our desire to confirm the earlier model predictions 11 by direct on-line measurement of the CO concentration in PEM fuel cell anode outlet gas. In this paper, we provide such results based on using an infrared ~IR! analyzer to monitor the anode outlet CO concentration of PEM fuel cell. Results are provided using both Pt and PtRu as anode catalyst at conventional PEM fuel cell operating conditions. The experimental data on the anode CO concentration as a function of anode flow rate for Pt catalyst agrees well with our previous predictions. Based on the CO material balance in the anode, the CO electro-oxidation rate is calculated for both Pt and PtRu catalyst. These results provide the first direct experimental evidence from in situ fuel cell experiments that both the ‘‘bifunctional electro-oxidation mechanism’’ and the ‘‘ligand effect’’ resulting in reduced CO affinity for the surface proposed in the literature coexist for PtRu catalyst.

Fabrication of Poly(allylamine)ferrocene Monolayer Based on Electrostatic Interaction and Its Electrocatalytic Oxidation of Ascorbic Acid

Abstract: A poly(allylamine)ferrocene monolayer was built on the surface of gold electrode modified with negatively charged alkanethiol based on electrostatic interaction. The electrochemical behavior of the modified electrode was characterized by cyclic voltammetry in detail. The modified electrode was shown to exhibit excellent electrocatalytic response to the oxidation of ascorbic acid. The anodic overpotential was reduced by about 170 mV compared with that obtained at a bare gold electrode. The modified electrode possesses several attractive features, such as simple preparation, fast response and good chemical and mechanical stability.