Showing papers on "Electrode potential published in 2008"
TL;DR: The results present a view of the ionic liquid-metal electrode interface having a very thin "double layer" structure where the ions form a single layer at the surface to screen the electrode charge, and raise many other fundamental questions as to the detailed nature of the interfacial structure and interpretations of both electrochemical and spectroscopic data.
Abstract: Room-temperature ionic liquids are a new class of liquids with many important uses in electrical and electrochemical devices. The liquids are composed purely of ions in the liquid state with no solvent. They generally have good electrical and ionic conductivity and are electrochemically stable. Since their applications often depend critically on the interface structure of the liquid adjacent to the electrode, a molecular level description is necessary to understanding and improving their performance. There are currently no adequate models or descriptions on the organization of the ions, in these pure ionic compounds, adjacent to the electrode surface. In normal electrolytic solutions, the organization of solvent and ions is adequately described by the Gouy-Chapman-Sterns model. However, this model is based on the same concepts as those in Debye-Huckel theory, that is a dilute electrolyte, where ions are well-separated and noninteracting. This is definitely not the situation for ionic liquids. Thus our goal was to investigate the ionic liquid-metal interface using surface-specific vibrational spectroscopy sum frequency generation, SFG. This technique can probe the metal-liquid interface without interference from the bulk electrolyte. Thus the interface is probed in situ while the electrode potential is changed. To compliment the vibrational spectroscopy, electrochemical impedance spectroscopy (EIS) is used to measure the capacitance and estimate the "double layer" thickness and the potential of zero charge (PZC). In addition, the vibrational Stark shift of CO adsorbed on the Pt electrode was measured to provide an independent measure of the "double layer" thickness. All techniques were measured as a function of applied potential to provide full description of the interface for a variety of imidazolium-based (cation) ionic liquids. The vibrational Stark shift and EIS results suggest that ions organize in a Helmholtz-like layer at the interface, where the potential drop occurs over the a range of 3-5 A from the metal surface into the liquid. Further, the SFG results imply that the "double layer" structure is potential-dependent; At potentials positive of the PZC, anions adsorbed to the surface and the imidazolium ring are repelled to orient more along the surface normal, compared with the potentials negative of the PZC, at which the cation is oriented more parallel to the surface plane and the anions are repelled from the surface. The results present a view of the ionic liquid-metal electrode interface having a very thin "double layer" structure where the ions form a single layer at the surface to screen the electrode charge. However, the results also raise many other fundamental questions as to the detailed nature of the interfacial structure and interpretations of both electrochemical and spectroscopic data.
446 citations
TL;DR: In this article, the authors analyzed the double layer formation in room temperature ionic liquids and compared the results with the mean-field theory and the charge conservation law at large electrode polarizations.
387 citations
TL;DR: Solid oxide fuel cells (SOFCs) as mentioned in this paper operate at higher temperatures than solution-based electrolysis cells, which results in a lower Nernst potential, the thermodynamic potential required for water splitting, and in lower electrode overpotentials.
Abstract: When the economy is based on renewable energy resources, such as wind and solar, the major source of H2 for chemical production and energy storage will be from the electrolysis of water. The ability to reduce CO2 efficiently by a similar process could also play a role in reducing greenhouse gas emissions and moving us toward a more sustainable economy. 1 CO produced in this manner could be used in chemical production or reacted with H2 to produce liquid fuels via the Fischer‐Tropsch reaction. 2 Solid oxide electrolyzers SOEs, which are essentially solid oxide fuel cells SOFCs operated in reverse, are capable of higher water electrolysis efficiencies compared to solution-based electrolysis cells because they operate at higher temperatures 925 K. The higher operating temperatures result in a lower Nernst potential, the thermodynamic potential required for water splitting, and in lower electrode overpotentials. 3 The electrode overpotential is the difference between the actual electrode potential and the Nernst potential, and is a measure of the lost efficiency in the cell. SOEs also differ from low-temperature, solution-based electrolyzers in that the electrolyte membrane conducts oxygen anions, rather than protons. The material most often used for the electrolyte is yttria-stabilized zirconia YSZ, a material that is a good oxygen-anion conductor and an electronic insulator. In an SOE, the cathode the fuel-side electrode reaction for water electrolysis is the electrochemical dissociation of steam to produce H2 and O 2 anions, Reaction 1, while recombination of the oxygen ions to O2, Reaction 2, occurs at the anode the air-side electrode H2 O+2 e → O 2 +H 2
205 citations
TL;DR: Density functional theory simulations were used to calculate activation barriers for different possible reaction mechanisms, including oxidation by hydroxyl radicals at different sites on the PFOS molecule, and direct electron transfer, and indicated that the most likely rate-limiting step for PFOS oxidation was direct electronTransfer.
Abstract: This research investigated the oxidative destruction of perfluorooctane sulfonate at boron-doped diamond film electrodes. Experiments measuring oxidation rates of PFOS were performed over a range in current densities and temperatures using a rotating disk electrode (RDE) reactor and a parallel plate flow-through reactor. The oxidation of PFOS yielded sulfate, fluoride, carbon dioxide, and trace levels of trifluoroacetic acid. Reaction rates in the RDE reactor were zeroth order in PFOS concentration. Reaction rates in the flow-through reactor were mass-transfer-limited and were pseudo-first-order in PFOS concentration, with a half-life of 5.3 min at a current density of 20 mA/cm2. Eyring analysis of the zeroth order rate constants at a fixed electrode potential yielded an apparent activation energy of 4.2 kJ/mol for PFOS oxidation. Density functional theory (DFT) simulations were used to calculate activation barriers for different possible reaction mechanisms, including oxidation by hydroxyl radicals at different sites on the PFOS molecule, and direct electron transfer. A comparison of the experimentally measured apparent activation energy with those calculated using DFT indicated that the most likely rate-limiting step for PFOS oxidation was direct electron transfer.
204 citations
TL;DR: The substrate-generation/tip-collection mode of scanning electrochemical microscopy was used to detect hydrogen peroxide formed as an intermediate during oxygen reduction at various electrodes to limit the extension of the diffusion layer above the sample.
Abstract: The substrate-generation/tip-collection mode of scanning electrochemical microscopy was used to detect hydrogen peroxide formed as an intermediate during oxygen reduction at various electrodes. The experiment is conceptually similar to rotating ring−disk experiments but does not require the production of a ring−disk assembly for the specific electrode material in question. In order to limit the extension of the diffusion layer above the sample, the sample electrode potential is pulsed while the Pt ultramicroelectrode probe (UME) is held at a constant potential for oxidative amperometric detection of hydrogen peroxide. The signal at UME is influenced by the sample region within the diffusion length of hydrogen peroxide during the pulse of 2.5 s. The method is tested with three model electrodes showing different behavior with respect to the oxygen reduction reaction (ORR) in acidic solution. Simple analytical models were used to extract effective rate constants for the most important reaction paths of ORR a...
116 citations
TL;DR: It was found that performance degradation of the ITO electrodes toward the reduction of Cr(VI) can be suppressed by modifying the electrode surface with gold nanoparticles (AuNPs), which were formed on ITO electrode by potential-sweeping or potential-step electrodeposition in a 0.01 M NaCl aqueous solutions.
96 citations
TL;DR: In this paper, a simple generic model to predict the influence of electrode porosity on the cyclic voltammetric response of an electrode is presented, and conditions under which deviation from the behavior of a perfectly flat, planar electrode can be expected are predicted.
Abstract: A simple generic model to predict the influence of electrode porosity on the cyclic voltammetric response of an electrode is presented. The conditions under which deviation from the behavior of a perfectly flat, planar electrode can be expected are predicted. The scope for misinterpretation when conventional flat electrode theory is applied to porous electrodes is highlighted, especially in respect to the extraction of electrode kinetic parameters and the influence of ‘electrocatalysis’.
96 citations
TL;DR: In this article, the formation of the SEI phase is general behavior at the electrode/electrolyte interface of solid-state batteries, and the fast electrochemical reaction is attained as a result of optimization of the electrode and electrolyte combination.
95 citations
TL;DR: The proposed membrane electrode revealed good selectivity for Al(III) ions over a wide variety of other cations, and was used in non-aqueous solvents and also as indicator electrode in potentiometric determination of Al( III) ions in some real samples.
86 citations
TL;DR: The purpose of the study was to examine how the Marcus curves depend on the applied potential and on the distance of the redox species from an electrode on a atomistic simulation designed to model the electrochemical interface with an ionic liquid.
Abstract: The calculation of the Marcus free energy curves for electron transfer events between a redox species and a metallic electrode in an atomistic simulation designed to model the electrochemical interface with an ionic liquid is described. The calculation is performed on a system comprising a molten salt mixture confined between model metallic electrodes [Reed et al., J. Chem. Phys. 126, 084704 (2007)] which are maintained at a constant electrical potential. The calculation therefore includes a self-consistent description of the screening of the electrode potential by the liquid and the polarization of the electrode by the ions (image charge effects). The purpose of the study was to examine how the Marcus curves depend on the applied potential and on the distance of the redox species from an electrode. The pronounced oscillations in the mean electrical potential seen in molten salt systems in the "double-layer" region are not reflected in the reaction free energy for the electron transfer event. The reorganization energy depends markedly on the distance of the redox ion from the electrode surface because of image charge effects.
86 citations
TL;DR: In this paper, the authors measured the length change in response to changes in the surface stress during scans of the electrode potential for nanoporous platinum samples immersed in aqueous NaF, an electrolyte with weak ion adsorption.
TL;DR: In this paper, an electrochemical method for evaluating the rate constants of recombination and transfer to solution for electrons generated upon illumination of TiO2 photocatalysts is presented, based on the combination of voltammetric measurements in the dark and open circuit photopotential relaxation measurements done with nanoporous thin film electrodes.
Abstract: An electrochemical method for evaluating the rate constants of recombination and transfer to solution for electrons generated upon illumination of TiO2 photocatalysts is presented. It is based on the combination of voltammetric measurements in the dark and open circuit photopotential relaxation measurements done with nanoporous thin film electrodes. Not only the average first-order rate constants for electron consumption are obtained in such a way but also the values of such constants as a function of the electrode potential (microcanonical rate constants). This method is applied to different titanium dioxide samples as to elucidate the effect of fluorination on the rate of both electron recombination (with surface trapped holes) and electron transfer to dissolved oxygen. In both cases, but especially for recombination, there is, upon fluorination, a significant retardation of the electron consumption process in agreement with several photocatalytic studies found in the literature. Finally, we take advant...
TL;DR: In this paper, the influence of the electrode potential on impedance pattern in high concentration of glucose was studied and a mathematical model was put forward to quantitative account for the impedance behavior of glucose oxidation.
TL;DR: In this article, a stable suspension of ordered mesoporous carbon (OMC) was obtained by dispersing OMC in a solution of Nafion and then coating the suspension onto glassy carbon (GC) electrode.
TL;DR: Atomic force microscopy has been used to characterize the formation of a phospholipid bilayer composed of 1,2-dimyristyl-sn-glycero-3-phosphocholine at a Au(111) electrode surface andForce-distance curve measurements reveal that the thickness of the corrugated phase is consistent with the Thickness of a bilayer lipid membrane.
Abstract: Atomic force microscopy (AFM) has been used to characterize the formation of a phospholipid bilayer composed of 1,2-dimyristyl-sn-glycero-3-phosphocholine (DMPC) at a Au(111) electrode surface. The bilayer was formed by one of two methods: fusion of lamellar vesicles or by the combination of Langmuir−Blodgett (LB) and Langmuir−Schaefer (LS) deposition. Results indicate that phospholipid vesicles rapidly adsorb and fuse to form a film at the electrode surface. The resulting film undergoes a very slow structural transformation until a characteristic corrugated phase is formed. Force−distance curve measurements reveal that the thickness of the corrugated phase is consistent with the thickness of a bilayer lipid membrane. The formation of the corrugated phase may be explained by considering the elastic properties of the film and taking into account spontaneous curvature induced by the asymmetric environment of the bilayer, in which one side faces the gold substrate and the other side faces the solution. The e...
TL;DR: The potential-dependent behavior contrasts that determined at a constant surface charge or from an applied electric field, illustrating the importance of considering the electrochemical potential at the fully-solvated interface in determining reaction energetics, even for non-redox reactions.
Abstract: Influences of coadsorbed sodium and water, aqueous solvent, and electrode potential on the kinetics of O2 dissociation over Pt(111) are systematically investigated using density functional theory models of vacuum and electrochemical interfaces. Na coadsorption alters the electronic states of Pt to stabilize the reactant (O2*), transition, and product (2O*) states by facilitating electron donation to oxygen, causing a more exothermic reaction energy (−0.84 eV for Na and O2, −0.81 eV for isolated O2) and a decrease in dissociation barrier (0.39 eV for Na and O2, 0.57 eV for isolated O2). Solvation decreases the reaction energy (−0.67 eV) due to enhanced hydrogen bond stabilization of O2* compared to 2O*. The influence of Na is less pronounced at the solvated interface (barrier decreases by only 0.11 eV) because H2O screens Na charge-donation. In the electrochemical model system, the dissociation energy becomes more exothermic and the barrier decreases toward more positive potentials. Potential-dependent behavior results from changes in interfacial dipole moment and polarizability between O2*, the dissociation transition state, and 2O*; each are influenced by changes in adsorption and hydrogen bonding. Coadsorption of Na in the solvated system dampens the dipole moment change between O2* and 2O* and significantly increases the polarizability at the dissociation transition state and for 2O*; the combination causes little change in the reaction energy but reduces the activation barrier by 0.08 eV at 0 V versusNHE. The potential-dependent behavior contrasts that determined at a constant surface charge or from an applied electric field, illustrating the importance of considering the electrochemical potential at the fully-solvated interface in determining reaction energetics, even for non-redox reactions.
TL;DR: In this paper, a double layer structure of water at a Cu(111) electrode surface was investigated by surface X-ray diffraction, scanning tunneling microscopy and infrared reflection absorption spectroscopy methods.
TL;DR: In this paper, the kinetics of electrocatalytic reduction of nitrate in perchloric acid were studied with cyclic voltammetry at a very low sweep rate of 1mV s−1, where pseudo-steady state condition was assumed to be achieved at each electrode potential.
TL;DR: In this article, a molecular theory is presented to describe chemically modified electrodes by redox polymers, which is based on writing the free energy functional of the system which includes the size, shape, charge distribution, and conformations of all of the molecular species as well as all of inter and intramolecular interactions, the acid-base equilibrium for the ionizable groups of the weak polyelectrolyte, and the redox equilibrium of the electrochemical active sites with the metal.
Abstract: A molecular theory is presented to describe chemically modified electrodes by redox polymers. The theory is based on writing the free energy functional of the system which includes the size, shape, charge distribution, and conformations of all of the molecular species as well as all of the inter and intramolecular interactions, the acid-base equilibrium for the ionizable groups of the weak polyelectrolyte, and the redox equilibrium of the electrochemical active sites with the metal. The minimization of the free energy leads to the molecular organization of the film as a function of bulk pH, salt concentration, and applied electrode potential. The approach is applied to the experimental system composed by osmium pyridine-bipyridine complex covalently bound to poly(allylamine) backbone, which is adsorbed onto a mercapto-propane sulfonate thiolated gold electrode. The redox and nonredox capacity of the electrode and its dependence on the electrode potential calculated with the molecular theory shows very good agreement with linear scan voltammetric experiments under reversible conditions (equilibrium scans) without the use of any free adjustable parameter. The predicted film thickness is in line with ellipsometric measurements. Further, the theory predicts the swelling of the film as a function of the electrode potential. The molecular theory provides the link between the molecular organization within the film and the electrochemical behavior. It is shown that the electrostatic, excluded volume, and van der Waals interaction fields are strongly coupled in a nontrivial way. Furthermore, the degree of charge regulation and distribution of oxidized states couples to the molecular distributions and the interaction fields. The application of the theory to different model systems demonstrates the importance of incorporating molecular information into the theoretical approach and the very strong coupling that exists between molecular structure, film organization, interactions fields, and electrochemical behavior.
TL;DR: Capillary zone electrophoresis coupled with electrochemical detection using a platinum working electrode as detector was convenient, highly sensitive, highly repeatable and could be used in the rapid determination of practical samples.
Abstract: The simultaneous determination of three isomers of phenylenediamines (o, m, and p-phenylenediamine) and two isomers of dihydroxybenzenes (catechol and resorcinol) in hair dyes was performed by capillary zone electrophoresis coupled with amperometric detection (CZE–AD) The effects of working electrode potential, pH and concentration of running buffer, separation voltage, and injection time on CZE–AD were investigated Under the optimum conditions the five analytes could be perfectly separated in 030 mol L−1 borate–040 mol L−1 phosphate buffer (pH 58) within 15 min A 300 μm diameter platinum electrode had good responses at +085 V (versus SCE) for the five analytes Their linear ranges were from 10 × 10−6 to 10 × 10−4 mol L−1 and the detection limits were as low as 10−7 mol L−1 (S/N = 3) This working electrode was successfully used to analyze eight kinds of hair dye sample with recoveries in the range 910–1080% and RSDs less than 50% These results demonstrated that capillary zone electrophoresis coupled with electrochemical detection using a platinum working electrode as detector was convenient, highly sensitive, highly repeatable and could be used in the rapid determination of practical samples
TL;DR: In this article, the authors investigated the effects of the volume change associated with the formation of Li-Sn compounds on the electrode potential from the viewpoint of the Gibbs free energy and associated elastic-strain energy.
18 Dec 2008
TL;DR: In this article, a solution to the transport equations for oxide ions, electrons and holes, potential profiles across the cell are calculated for fuel cell and electrolyzer applications, and the main result of the analysis is the verification that in steady state operation, the oxygen pressure inside a cavity can never exceed the value corresponding to the electrode potential.
Abstract: Solid oxide fuel cells and electrolyzers are usually based on yttrium stabilized zirconia (YSZ) as electrolyte. Although the transport number of oxide ions in YSZ at normal conditions is not below 0.999 the electron and hole conductivity may be of practical importance. In long term operation it may cause oxygen pressure to build up in closed cavities inside the electrolyte. Fundamental concepts as electric and electrochemical potential are discussed, and revive the concept electromotive potential originally introduced by Bronsted. This potential is equivalent to the Fermi level and is useful in the discussion of electronic conduction in mixed conductors. On the bases of a solution to the transport equations for oxide ions, electrons and holes, potential profiles across the cell are calculated for fuel cell and electrolyzer applications. One main result of the analysis is the verification that in steady state operation, the oxygen pressure inside a cavity can never exceed the value corresponding to the electrode potential.
TL;DR: A simple and highly sensitive electroanalytical method based on suppression of 1,4-benzoquinone peak current on addition of caffeine that exhibited a well-defined peak with reproducible peak current values for repetitive measurements and showed a decrease in peak current value with an increase in caffeine content is presented.
TL;DR: A photoelectrochemical signal chain sensitive to the presence of superoxide radicals was developed on the basis of CdSe/ZnS quantum dots which were immobilized on gold electrodes using a dithiol compound and the electrode was found to be sensitive to higher nanomolar concentrations of the radical.
TL;DR: The results indicate that electrochemical oxidation using a boron-doped diamond electrode has the potential to become a useful and easily accessible tool for conducting oxidative surface mapping experiments.
Abstract: We report here the novel use of electrochemistry to generate covalent oxidative labels on intact proteins in both non-native and physiologically relevant solutions as a surface mapping probe of higher order protein structure. Two different working electrode types were tested across a range of experimental parameters including voltage, flow rate, and solution electrolyte composition to affect the extent of oxidation on intact proteins, as measured both on-line and off-line with mass spectrometry. Oxidized proteins were collected off-line for proteolytic digestion followed by LC-MS/MS analysis. Peptide MS/MS data were searched with the InsPecT scoring algorithm for 46 oxidative mass shifts previously reported in the literature. Preliminary data showed reasonable agreement between amino acid solvent accessibility and the resulting oxidation status of these residues in aqueous buffer, while more buried residues were found to be oxidized in non-native solution. Our results indicate that electrochemical oxidation using a boron-doped diamond electrode has the potential to become a useful and easily accessible tool for conducting oxidative surface mapping experiments.
TL;DR: In this paper, different MEAs using different catalyst powders, carbon supported and unsupported catalysts with different proton conducting electrolyte powder (Nafion) content were produced by using a dry powder spraying technique developed at German Aerospace Research Center (DLR, Deutsches Zentrum fuer Luft- und Raumfahrt).
TL;DR: Wang et al. as mentioned in this paper extended the analysis to dual-layer catalyst layers, where the electrode is characterized by a two-layer configuration with specific properties assigned to each layer, and explicit solutions were obtained for the profiles of the reaction rate and electrolyte phase potential across the dual-Layer electrode.
Abstract: This paper presents a one-dimensional (1D) theoretical study on the electrochemical phenomena in the cathode dual-layer electrode of polymer electrolyte fuel cells (PEFCs). Following our previous work on transport and electrochemical processes in single-layer cathodes [Y. Wang and X. H. Feng, J. Electrochem. Soc., 155, B1289 (2008)], we extend the analysis to dual-layer catalyst layers, where the electrode is characterized by a two-layer configuration with specific properties assigned to each layer. A ID model is developed, and explicit solutions are obtained for the profiles of the reaction rate and electrolyte phase potential across the dual-layer electrode. Effort is also made to analyze the solutions to explore the impacts of each layer's properties on their performance (i.e., the average reaction current in each layer) with particular focus on the ratios of the ionic conductivity (related to the ionomer content), specific area, and exchange current density (related to Pt loading and reaction interface roughness). The results can be applied to optimize electrode performance through dual-layer configuration for high-performance cost-effective electrodes for PEFCs.
TL;DR: In this paper, the effect of the material property and the electrode microstructure on the performance of mixed conducting oxide anodes has been investigated, and the results showed that high ionic conductivity and catalytic activity with a certain level of electronic conductivity is required for a high-performance oxide anode.
Abstract: To clarify guidelines for a high-performance mixed conducting oxide anode, electrochemical behaviors of mixed conducting oxide anodes were studied on the oxides, La 0.9 Ca 0.1 Cro 0.8 Al 0.2 O 3 , La 0.9 Ca 0.1 Cr 0.2 Al 0.3 O 3 , Sr 0.9 La 0.1 TiO 3 , Sr 0.8 La 0.2 TiO 3 , SrTi 0.97 Nb 0.03 O 3 , Ce 0.9 Gd 0.1 O 1.95 , and Ce 0.992 Nb 0.008 O 2 . The hydrogen oxidation on the oxide anodes is studied by ac impedance and steady-state polarization measurements. In the ac impedance measurements, a slow relaxation process of the order of 10 -2 Hz was observed with the CeO 2 -based and La 0.9 Ca 0.1 Cr 0.2 Al 0.8 O 3 anodes. The corresponding pseudocapacitances are 10 4 -10 6 μF cm -2 . These pseudocapacitances are identified as the chemical capacitance due to the variation of the nonstoichiometric oxygen content of the electrode material. At the same electrode potential, the CeO 2 -based anodes showed a far higher steady-state current than the LaCrO 3 - and the SrTiO 3 -based anodes. The extension of the reaction zone beyond the three phase boundary is estimated from our experimental results. The reaction zone of the Ce 0.9 Gd 0.1 O 1.95 anode extends from the three-phase boundary to the electrode/gas interface. To estimate what determines the electrode performance of the oxide anodes, the effect of the material property and the electrode microstructure was studied. The effect of the material property is much larger than that of the electrode microstructure. High ionic conductivity and catalytic activity with a certain level of electronic conductivity is required for a high-performance oxide anode.
TL;DR: The previously proposed electrooxidation mechanism for glucose on GC/NiCu electrode was found to reproduce the experimental impedance plots and a quantitative explanation for the impedance behavior of glucose oxidation is put forward by a proposed mathematical model.
Abstract: Nickel-copper alloy modified glassy carbon electrodes (GC/NiCu) prepared by galvanostatic deposition were used for the electrocatalytic oxidation of glucose in alkaline solutions. The electro-oxidation of glucose in a 1 M NaOH solution at different concentration of glucose was studied by the method of ac-impedance spectroscopy. The impedance behavior show different patterns, capacitive, and inductive loops and negative resistances, at different applied anodic potential. The influence of the electrode potential on the impedance pattern is studied and a quantitative explanation for the impedance behavior of glucose oxidation is put forward by a proposed mathematical model. At potentials higher than 0.5 V/Ag-AgCl, a pseudoinductive behavior is observed while at higher than 0.53 V/Ag-AgCl, impedance pattern is reversed to the second, third, and forth quadrants. The conditions required for the reversing of impedance pattern are delineated with the use of the impedance model. The previously proposed electrooxidation mechanism for glucose on GC/NiCu electrode was found to reproduce the experimental impedance plots.
TL;DR: In this article, the dependence of contact resistance on the electrode potential versus reference electrode was measured using two-and four-point measurements of conductive polymer by two and four point techniques.