Showing papers on "Overpotential published in 1997"
TL;DR: In this article, a charge recombination between dye-sensitized nanocrystalline TiO2 electrodes and the I3/I-couple in nonaqueous solution is described.
Abstract: Charge recombination between dye-sensitized nanocrystalline TiO2 electrodes and the I3-/I- couple in nonaqueous solution is described. The sensitizer was [RuL2(NCS)2] (L = 2,2‘-bipyridyl-4,4‘-dicarboxylic acid). An apparent inequality between the dark current and the recombination current is ascribed to a voltage shift caused by a potential drop at the SnO2/TiO2 interface, ohmic losses in the SnO2 and TiO2, and an overpotential for the redox reaction at the Pt counter electrode. Treating the dye-coated TiO2 electrodes with pyridine derivatives (4-tert-butylpyridine, 2-vinylpyridine, or poly(2-vinylpyridine)) improves significantly both the open-circuit photovoltage Voc (from 0.57 to 0.73 V) and the cell conversion efficiency (from 5.8 to 7.5%) at a radiant power of 100 mW/cm2 (AM 1.5) with respect to the untreated electrode. An analytical expression relating Voc to the interfacial recombination kinetics is derived, and its limitations are discussed. Analysis of Voc vs radiant power data with this expressi...
906 citations
TL;DR: In this paper, the effect of porous composite electrodes on the overall charge transfer process in solid-state devices, such as solid oxide fuel cells, is theoretically examined by taking into account various parameters such as electrolyte thickness, intrinsic charge-transfer resistance, electrode thickness, and porosity.
Abstract: The effect of porous composite electrodes on the overall charge-transfer process in solid-state devices, such as solid oxide fuel cells, is theoretically examined by taking into account various parameters such as electrolyte thickness, intrinsic charge-transfer resistance, electrode thickness, and porosity. A model is presented that accounts for ionic transport within the electrolyte, electronic conduction through electrocatalyst, and charge-transfer at the electrolyte-electrocatalyst interface. Diffusion of gaseous species in porous electrodes is assumed to be rapid so as not to be rate limiting. The conduction of electrons in the electrocatalyst is assumed to introduce negligible resistance. The activation overpotential as a function of current density is assumed to be ohmic, and an effective charge-transfer resistance is defined. The transport equations are solved numerically in two dimensions using a finite difference technique and analytically in one dimension. The analysis predicts that the use of composite electrodes in devices employing solid electrolytes can significantly increase performance under conditions where the intrinsic charge-transfer resistance is high in comparison to the area-specific resistance of the electrolyte. The results indicate a low effective charge-transfer resistance is obtained for relatively thick electrodes with a fine microstructure as long as the porosity is sufficient to ensure negligible concentration polarization.
371 citations
TL;DR: In this paper, the effect of cathode composition, processing and structure on the performance of medium-temperature (600-800 °C) solid oxide fuel cells (SOFCs) is described.
250 citations
TL;DR: In this article, a series of six cathodes Sr0.15La0.85MnO3 (SLM) on yttria-stabilized zirconia with different morphology of the electrode/electrolyte interface were characterized by ac impedance and dc polarization measurements.
Abstract: A series of six cathodes Sr0.15La0.85MnO3 (SLM) on yttria-stabilized zirconia with different morphology of the electrode/electrolyte interface were characterized by ac impedance and dc polarization measurements. It is found that the electrode kinetics at elevated temperature (945°C) are governed by two serial processes. An activation process can be identified to occur at high cathodic overpotential, whereas a transport process competes with charge-transfer at comparatively low overpotential. Attention is drawn to the profound change in the electrocatalytic properties of Sr0.15La0.85MnO3 upon current passage and its influence in elucidation of the interfacial kinetics.
225 citations
TL;DR: In this paper, the incorporation of iron reduced the overpotential required for oxygen evolution by as much as 300 mV at a current density of 100 mA/cm 2 compared to undoped nickel oxide under similar conditions.
Abstract: Iron-doped nickel oxide films were deposited by reactive sputtering from elemental and alloy targets in a 20% oxygen/argon atmosphere and were characterized for use as oxygen evolution catalysts. The incorporation of iron reduced the overpotential required for oxygen evolution by as much as 300 mV at a current density of 100 mA/cm 2 compared to undoped nickel oxide deposited under similar conditions. Tafel slopes were reduced from 95 mV/dec in undoped NiO x films to less than 40 mV/dec for films containing 1.6 to 5.6 mole percent (m/o) iron, indicating a change in the rate-limiting step from the primary discharge of OH ions to the recombination of oxygen radicals. Resistivity, structural, and compositional measurements indicate that high oxygen content is necessary to gain the full benefit of the iron dopant. Initial tests in KOH indicate excellent long-term stability. A film deposited from the FeNi alloy target, which exhibited low oxygen overpotentials and a Tafel slope of 35 mV/dec, had not degraded appreciably following more than 7000 h of operation at an anodic current density of 20 mA/cm 2 . Taken together, the low oxygen evolution reaction overpotentials, the excellent stability in KOH, and the relative insensitivity to iron content indicate that reactively sputtered iron-doped nickel oxide is promising as an oxygen catalyst.
199 citations
TL;DR: In this paper, the authors investigated the interface microstructure of the state-of-the-art cathode material for solid oxide fuel cells, SrxLa1-xMnO3 (SLM), with respect to its electrochemical performance.
Abstract: The interface microstructure of the state-of-the-art cathode material for solid oxide fuel cells, SrxLa1–xMnO3 (SLM), was investigated with respect to its electrochemical performance. The interface microstructure was characterized by grain size and coverage of SLM on the electrolyte surface. Variation of the grain size was obtained by using three different sintering temperatures, whereas variation of the coverage was obtained by using two powders with a different morphology. This resulted in a set of six cathode/electrolyte samples with different combinations of grain size and SLM coverage at the interface. The cathode overpotential, as a measure for the electrochemical performance, could not be related to the length of the three-phase boundary. Based on the constriction resistance occurring in the electrolyte a model was developed which provides an estimate for the width of the active three-phase boundary zone.This zone is most likely to extend outside the cathode particle across the zirconia surface. The width calculated in this way was found to vary in the range of 0.03 to 0.07 µm for the different electrode microstructures. It is argued that the actual values may be smaller by one or two orders of magnitude.
180 citations
TL;DR: In this paper, the electrochemical oxidation of hydrogen in the presence of carbon monoxide (0.05−2%) on a well-characterized Pt75Mo25 alloy surface was examined using the rotating disk electrode technique in 0.5 M H2SO4 at 333 K.
Abstract: The electrochemical oxidation of hydrogen in the presence of carbon monoxide (0.05−2%) on a well-characterized Pt75Mo25 alloy surface was examined using the rotating disk electrode technique in 0.5 M H2SO4 at 333 K. The surface composition of this alloy determined by low-energy ion scattering after sputter cleaning and annealing in UHV was essentially the same as the bulk. The shapes of the polarization curves are qualitatively similar to those for the Pt50Ru50 alloy examined previously: a high Tafel-slope (ca. 0.5 V/dec) region at low overpotential followed by a transition to a highly actively state where the current approaches the diffusion-limiting current; the potential where the transition to the active state occurs decreases with decreasing CO concentration, and the current in the low-overpotential region is roughly inverse half-order in the CO partial pressure. The magnitude of the current in the low-overpotential region on the Pt75Mo25 alloy is nearly the same as on the Pt50Ru50 alloy, but the po...
132 citations
TL;DR: In this paper, the authors investigated the kinetics of CO 2 reduction under long-term potentiostatic conditions using 0.5 M NaHCO 3 solutions and using standard electrochemical equipment.
117 citations
TL;DR: In this article, the electrochemical reduction of nitrite and nitrate in alkaline solution has been studied on glassy carbon electrodes modified with phthalocyanine complexes of Mn, Fe, Co, Ni, Cu and Zn.
Abstract: The electrochemical reduction of nitrite and nitrate in alkaline solution has been studied on glassy carbon electrodes modified with phthalocyanine complexes of Mn, Fe, Co, Ni, Cu and Zn (MnPc, FePc, CoPc, NiPc, CuPc and ZnPc, respectively, and Pc(-2) is phthalocyanine dianion). Voltammetry shows that the reduction of nitrite occurs at potentials slightly less negative than for nitrate reduction. The MPc complexes lower the overpotential for the reduction of nitrite and nitrate as follows: CuPc > FePc > NiPc > CoPc > MnPc > ZnPc > glassy carbon electrode. The main product for the reduction of nitrite and nitrate is ammonia, with high yields of ammonia being obtained, depending on the choices of the electrolysis potential and MPc complex.
117 citations
TL;DR: In this article, the yield of complete oxidation product (CO2) from electrocatalysis of methanol on a Pt(100) electrode in perchloric acid electrolyte at room temperature was determined by combined measurements of chronoamperometry and linear sweep voltammetry.
Abstract: The yield of complete oxidation product (CO2) from electrocatalysis of methanol on a Pt(100) electrode in perchloric acid electrolyte at room temperature was determined by combined measurements of chronoamperometry and linear sweep voltammetry. The fractional yield of CO2 was zero over a potential range of 0.35−0.45 VRHE and increased monotonically to unity over the potential range of 0.5−0.65 V. The results may be explained by a simple parallel mechanism, in which methanol oxidizes directly to CO2, or by a complex serial mechanism, in which the overpotential for CO oxidation is reduced by solution phase methanol.
106 citations
TL;DR: In this article, the degradation of the cell performance due to thermal cycles and due to aging is examined by the AC impedance method, where a large positive overpotential is applied at the cathode in order to accelerate the performance degradation.
TL;DR: In this article, anomalous effects associated with the electrochemical cycling of electrogenerated polypyrrole films after polarization at high cathodic potentials were simulated from a derivation of the electrochemically stimulated conformational relaxation model.
Abstract: Anomalous effects associated with the electrochemical cycling of electrogenerated polypyrrole films after polarization at high cathodic potentials were simulated from a derivation of the electrochemically stimulated conformational relaxation model. The cathodic treatment promotes the compaction of the polymer by conformational movements of polymeric segments. The compacted structure requires, at constant temperature, an anodic overpotential (that is, an extra energy) to be opened during oxidation, allowing the penetration of counterions from the solution in order to keep the electroneutrality in the film. The definition of a conformational relaxation time for the swelling of the polymeric structure, including both thermal and electrochemical energetic contributions, allows the integration of structural parameters on electrochemical equations. Theoretical voltammograms reproduce the effect of the different variables (cathodic potential, sweep rate, and temperature) and fit experimental results for polypyrr...
TL;DR: In this paper, Boron-doped diamond thin films have been examined before and after high-current-density electrolysis to investigate the morphological and microstructural stability of this new electrode material.
Abstract: Boron-doped diamond thin films have been examined before and after high-current-density electrolysis to investigate the morphological and microstructural stability of this new electrode material. The diamond thin films were used to generate chlorine from a solution of 1.0 M HNO{sub 3} + 2.0 M NaCl at current densities of 0.05 and 0.5 A/cm{sup 2} for times up to 20 h. Comparative studies were made using common graphitic electrodes including highly oriented pyrolytic graphite, glassy carbon, and Grafoil{reg_sign}. The electrodes were characterized using four-point probe resistivity measurements, atomic force microscopy, scanning electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and cyclic voltammetry. In all cases, no severe morphological or microstructural damage (i.e., corrosion) was observed on films exposed to the highest current density. There were surface compositional changes in the forms of oxygenation and non-diamond carbon impurity etching that produced an increase in the reaction overpotential. Specifically, the overpotential was supposed to result from a combination of decreased surface conductivity due to the formation of carbon-oxygen functional groups and loss of kinetically active redox sites due to the oxidative etching of nondiamond carbon impurities.
TL;DR: In this article, the influence of a uniform external magnetic field on faradaic reactions has been investigated, and it has been shown that magnetic field-induced reaction overpotential is due to a steady state displacement of charge balancing counterions at the electrode-solution interface, analogous to that recently observed for a rotating microdisk electrode.
TL;DR: In this paper, the applicability of classical liquid electrochemical concepts to porous cathode structures used in high temperature fuel cells is discussed and the importance of the characteristic length term, Lc (D∗k), which indicates the changeover from surface to bulk control is emphasised.
TL;DR: In this paper, a stable electroactive thin film of poly(azure I) (PAI) was deposited on the surface of a glassy carbon electrode by cyclic voltammetry from azure I (AI) in an aqueous solution.
TL;DR: In this paper, functionalized transition metal macrocyclic monomers were synthesized in order to prepare electron-conducting polymer-based electrodes, and the electrocatalytic behaviour of such electrodes, particularly the influence of the macrocycle skeleton towards dioxygen reduction, was examined.
TL;DR: In this paper, a quantitative theory is developed for better understanding of the potentiostatic growth of passivating films on metals following a previously proposed general treatment for voltammetric and galvanostatic transients.
Abstract: A quantitative theory is developed for better understanding of the potentiostatic growth of passivating films on metals following a previously proposed general treatment for voltammetric and galvanostatic transients. The theory is based, in this case, on an ohmic model for the current/overpotential relation inside the film and a Tafel relation for current/over-potential at the metal/film interface. The equations are applied to the potentiostatic growth of the passivating film on Zn in 0.3 M H 3 BO 3 plus 0.15 M Na 2 B 4 O 7 solution. It is shown that the initial charge, after electrode polishing and before potentiostatic growth, can be disregarded. It is then observed that, during potentiostatic growth, the film ionic specific resistivity decays, passes through a minimum value, and then increases, giving rise to an aged film. The time for the occurrence of this last increase is proposed to be called aging time. As a result, there is the appearance in the potentiostatic experiments of a maximum charge for film formation, which depends on the growth potential. This maximum charge presents, with the increase of the growth potential, a maximum followed by subsequent increase. As a consequence, it is proposed that the results can be interpreted through the existence of two kinds of films present, each appearing in two different potential regions. Finally, a general explanation is proposed for the evolution of the ionic specific resistivity in terms of the existence of defect injection at the metal/film (interstitial cations) and film/solution (cation vacancies) interfaces, the migration of the defects and their recombination inside the film
TL;DR: In this article, the reduction of dichromate species in acid solution at noble metal electrodes is an intriguing example of electrocatalytic behaviour as, contrary to conventional expectation in this area, reaction occurs rapidly at a much lower overpotential on gold as compared with platinum.
TL;DR: In this article, the authors studied the phenomenon of the large overpotential at the first reduction of polypyrrole perchlorate films in propylene carbonate (PC) solutions.
TL;DR: The early stages of the electrolytic deposition of gold on carbon electrodes from concentrated LiCl electrolytes have been investigated by voltammetry, chronoamperometry, and microscopy as discussed by the authors.
Abstract: The early stages of the electrolytic deposition of gold on carbon electrodes from concentrated LiCl electrolytes have been investigated by voltammetry, chronoamperometry, and microscopy The analysis of current-time transients according to existing theories indicates that this process occurs by multiple three-dimensional nucleation, followed by diffusion-controlled growth of nuclei Diffusion coefficients calculated on the basis of nucleation theory were found to be higher than those Irom rotating disk experiments (RDE), suggesting that gold species are adsorbed on the electrode surface Gold nucleation from 6 M LiCl cannot be classified as either instantaneous or progressive nucleation The kinetic parameters for nucleation, aN o (nucleation rate) and N o (number density of active sites on the substrate surface), were estimated by using a general Poisson nucleation law Both quantities were found to vary with potential and with concentration of gold The potential dependence of the nucleation rate, aN o , was interpreted according to classical and atomistic theory For low gold concentrations, where adsorption of AuCl or AuCl 3 might occur prior to nucleation, the number of atoms in the critical nucleus (N c ) was less than unity over the entire potential range analyzed For high gold concentrations the number of atoms forming the critical nucleus depends on overpotential In all solutions studied nucleation takes place on a limited number of active sites
TL;DR: In this paper, a stable electroactive thin film of poly(nile blue A) (PNB) has been deposited on the surface of a glassy carbon electrode by cyclic voltammetry in an aqueous solution containing nile blue (NB).
TL;DR: In this paper, an analytical solution for the one-dimensional steady-state transport of ions in an electrolyte towards a planar electrode is obtained, where the mass and charge transport equations give rise to an implicit form of a set of nonlinear algebraic equations which must be solved numerically.
TL;DR: In this article, a gold electrode modified by dopamine covalently bound to self-assembled cysteamine monolayer (dopamine-SAM) is described, and the modified electrode exhibits a quasi-reversible surface wave in phosphate buffer (pH 7.0) containing 0.1 M KCl.
TL;DR: In this article, the effect of square-wave pulsating overpotential on the morphology of lead dendrites is discussed and it is shown that lead morphology strongly depends on the parameters determining the shape of the pulsating OPN wave.
TL;DR: In this paper, the volume fraction of Co3O4 particles in the deposit α reaches a limiting value of ca. 0.008 when α exceeds 0.1, and α decreases with increasing α.
TL;DR: In this paper, the authors investigated the dependence of the stress on the overpotential and made use of the knowledge established in surface science to identify the structure of the resulting films.
TL;DR: In this article, the underpotential and overpotential deposition of copper onto a polycrystalline platinum surface was studied in the presence of organic co-adsorbates.
TL;DR: In this article, the reduction of NAD + to enzymatically active NADH at a substantially lower overpotential (i.e. −0.72 V vs. SCE) was investigated by in situ spectroelectrochemistry.
TL;DR: In this paper, the performance of indium as cathodic material for the electroreduction of small organic molecules is considered, and the results show that the protonation of the adsorbed radical is the rate determining step in the proposed CECE mechanism.
Abstract: The performance of indium as cathodic material for the electroreduction of small organic molecules is considered. The cathodic reduction of formaldehyde (FA) is an ideal model reaction for this purpose since indium has a very large overpotential for the hydrogen evolution reaction with and without FA. Kinetic sets of the reaction pathways, with respect to the Tafel slope and reaction order, are considered on the basis of quasi-potentiostatic measurements and cyclic voltammetry. The value of the Tafel slope bc≈60mVdec-1 indicates that the protonation of the adsorbed radical is the rate determining step in the proposed CECE mechanism. The reaction order with respect to FA is close to one in the limiting current regions but smaller in the Tafel region. The existence and kinetics of the radicals adsorbed during FA reduction are evidenced by very fast potentiodynamic experiments, with scan rates between 40 and 80Vs-1. Electrochemical measurements are carried out on freshly in situ prepared In-electrodes. During cathodic polarization, the surface oxide film is reduced to In-metal via a solid-state mechanism. The crystallization kinetics of indium in the oxide matrix is also discussed.