Showing papers on "Overpotential published in 2005"

Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution

Abstract: The electrochemical hydrogen evolution reaction is catalyzed most effectively by the Pt group metals. As H2 is considered as a future energy carrier, the need for these catalysts will increase and alternatives to the scarce and expensive Pt group catalysts will be needed. We analyze the ability of different metal surfaces and of the enzymes nitrogenase and hydrogenase to catalyze the hydrogen evolution reaction and find a necessary criterion for high catalytic activity. The necessary criterion is that the binding free energy of atomic hydrogen to the catalyst is close to zero. The criterion enables us to search for new catalysts, and inspired by the nitrogenase active site, we find that MoS2 nanoparticles supported on graphite are a promising catalyst. They catalyze electrochemical hydrogen evolution at a moderate overpotential of 0.1−0.2 V.

Thermodynamic Guidelines for the Design of Bimetallic Catalysts for Oxygen Electroreduction and Rapid Screening by Scanning Electrochemical Microscopy. M-Co (M: Pd, Ag, Au)

Abstract: We propose guidelines for the design of improved bimetallic (and related) electrocatalysts for the oxygen reduction reaction (ORR) in acidic media. This guide is based on simple thermodynamic principles assuming a simple mechanism where one metal breaks the oxygen-oxygen bond of molecular O(2) and the other metal acts to reduce the resulting adsorbed atomic oxygen. Analysis of the Gibbs free energies of these two reactions guides the selection of combinations of metals that can produce alloy surfaces with enhanced activity for the ORR when compared to the constituent metals. Selected systems have been tested by fabricating arrays of metallic catalysts consisting of various binary and ternary combinations of Pd, Au, Ag, and Co deposited on glassy carbon (GC) substrates. The electrocatalytic activity of these materials for the ORR in acidic medium was examined using scanning electrochemical microscopy (SECM) in a new rapid-imaging mode. This was used to rapidly screen arrays covering a wide range of catalyst compositions for their activity for the ORR in 0.5 M H(2)SO(4). Using the SECM technique, we have identified combinations of metals with enhanced electrocatalytic activities when compared with the constituent, pure metals. Addition of Co to Pd, Au, and Ag clearly decreases the ORR overpotential, in agreement with the proposed model. Catalyst spots that exhibited enhanced electrocatalytic activity in the SECM screening technique were then examined using classical rotating disk electrode (RDE) experiments. The activity of carbon black supported catalyst mixtures on a GC RDE and the electrocatalytic activity determined using the SECM screening technique showed excellent agreement. C/Pd-Co electrodes (10-30% Co) exhibited remarkable activity for ORR catalysis, close to that of carbon-supported Pt.

Influence of particle agglomeration on the catalytic activity of carbon-supported Pt nanoparticles in CO monolayer oxidation

Abstract: Fuel cell electrocatalysts usually feature high noble metal contents, and these favour particle agglomeration. In this paper a variety of synthetic approaches (wet chemical deposition, electrodeposition and electrodeposition on chemically preformed Pt nuclei) is employed to shed light on the influence of nanoparticle agglomeration on their electrocatalytic properties. Pt loading on model glassy carbon (GC) support is increased systematically from 1.8 to 10.6 μg Pt cm−2 and changes in the catalyst structure are followed by transmission electron microscopy. At low metal loadings (≤5.4 μg Pt cm−2) isolated single crystalline Pt nanoparticles are formed on the support surface by wet chemical deposition from H2PtCl4 precursor. An increase in the metal loading results, first, in a systematic increase of the average diameter of isolated Pt nanoparticles and, second, in coalescence of nanoparticles and formation of particle agglomerates. This behaviour is in line with the previous observations on carbon-supported noble metal fuel cell electrocatalysts. The catalytic activity of Pt/GC electrodes is tested in CO monolayer oxidation. In agreement with the previous studies (F. Maillard, M. Eikerling, O. V. Cherstiouk, S. Schreier, E. Savinova and U. Stimming, Faraday Discuss., 2004, 125, 357), we find that the reaction is strongly size sensitive, exhibiting an increase of the reaction overpotential as the particle size decreases below ca. 3 nm. At larger particle sizes the dependence levels off, the catalytic activity of particles with diameters above 3 nm approaching that of polycrystalline Pt. Meanwhile, Pt agglomerates show remarkably enhanced catalytic activity in comparison to either isolated Pt nanopraticles or polycrystalline Pt foil, catalysing CO monolayer oxidation at ca. 90 mV lower overpotential. Enhanced catalytic activity of Pt agglomerates is ascribed to high concentration of surface defects. CO stripping voltammograms from Pt/GC electrodes, comprising Pt agglomerates along with isolated single crystalline Pt nanoparticles from 2 to 6 nm size, feature double voltammetric peaks, the more negative corresponding to CO oxidation on Pt agglomerates, while the more positive to CO oxidation on isolated Pt nanoparticles. It is shown that CO stripping voltammetry provides a fingerprint of the particle size distribution and the extent of particle agglomeration in carbon-supported Pt catalysts.

Platinum/Carbon nanotube nanocomposite synthesized in supercritical fluid as electrocatalysts for low-temperature fuel cells.

Abstract: Carbon nanotube (CNT)-supported Pt nanoparticle catalysts have been synthesized in supercritical carbon dioxide (scCO2) using platinum(II) acetylacetonate as metal precursor. The structure of the catalysts has been characterized with transmission electron micrograph (TEM) and X-ray photoelectron spectroscopy (XPS). TEM images show that the platinum particles' size is in the range of 5−10 nm. XPS analysis indicates the presence of zero-valence platinum. The Pt−CNT exhibited high catalytic activity both for methanol oxidation and oxygen reduction reaction. The higher catalytic activity has been attributed to the large surface area of carbon nanotubes and the decrease in the overpotential for methanol oxidation and oxygen reduction reaction. Cyclic voltammetric measurements at different scan rates showed that the oxygen reduction reaction at the Pt−CNT electrode is a diffusion-controlled process. Analysis of the electrode kinetics using Tafel plot suggests that Pt−CNT from scCO2 provides a strong electrocata...

About the Choice of the Protogenic Group in PEM Separator Materials for Intermediate Temperature, Low Humidity Operation: A Critical Comparison of Sulfonic Acid, Phosphonic Acid and Imidazole Functionalized Model Compounds

Abstract: Traditionally, sulfonated polymers are used as separator materials in PEM fuel cells. Based on recent experimental results on model compounds this paper critically discusses the potentials and limits of sulfonic acid and alternatively phosphonic acid and heterocycles (imidazole) as protogenic groups for PEM fuel cell electrolytes operating at intermediate temperatures (T > 100 °C) and low humidification. Apart from transport properties, the stability and reactivity of mono-functionalized model compounds (1-heptylsulfonic acid (S-C7), 1-heptylphosphonic acid (P-C7) and 2-heptyl-imidazole (I-C7)) and a few diphosphonic acids are examined under wet and dry conditions. These are characterized with respect to their proton conductivity (ac impedance spectroscopy), proton diffusion coefficient (pulsed-field gradient NMR), thermo-oxidative stability (TGA under air), electrochemical stability (cyclic voltammetry) and their hydration behavior (TGA under water vapor). The sulfonic acid functionalized compound shows reasonable properties only when a minimum hydration level is guaranteed, while phosphonic acid functionalized compounds combine satisfactory proton conductivity even in the water-free state at intermediate temperatures (T < 200 °C), comparatively high thermo-oxidative and electrochemical stability and electrochemical reactivity (hydrogen oxidation and oxygen reduction at platinum surfaces). The presence of water leads to moderate water uptake allowing for reasonable conductivities even at room temperature and prevents condensation reactions at higher temperature. The imidazole based system shows the largest electrochemical stability window, but its moderate proton conductivity and thermo-oxidative stability and the very high overpotential for oxygen reduction on platinum turn out to be severe disadvantages for the envisaged application.

Electrochemical sensing based on redox mediation at carbon nanotubes.

Abstract: An electrochemical sensing platform was developed based on the integration of redox mediators and carbon nanotubes (CNT) in a polymeric matrix. To demonstrate the concept, a redox mediator Azure dye (AZU) was covalently attached to polysaccharide chains of chitosan (CHIT) and interspersed with CNT to form composite films for the amperometric determination of β-nicotinamide adenine dinucleotide (NADH). The incorporation of CNT into CHIT-AZU matrix facilitated the AZU-mediated electrooxidation of NADH. In particular, CNT decreased the overpotential for the mediated process by an extra 0.30 V and amplified the NADH current by ∼35 times (at −0.10 V) while reducing the response time from ∼70 s for CHIT-AZU to ∼5 s for CHIT-AZU/CNT films. These effects were discussed in terms of the AZU/CNT synergy, which improved charge propagation through the CHIT-AZU/CNT matrix. The concept of CNT-facilitated redox mediation in polymeric matrixes has a potential to be of general interest for expediting redox processes in ele...

Molybdenum-sulfur dimers as electrocatalysts for the production of hydrogen at low overpotentials.

Abstract: (CpMoμ-S)2S2CH2, 2, and related derivatives serve as electrocatalysts for the reduction of protons with current efficiencies near 100%. The kinetics of the electrochemical reduction process has been studied, and the effects of varying the proton source, the solvent, the cyclopentadienyl substituents, and the sulfur substituents on the catalyst have been examined. The reduction of excess p-cyanoanilinium tetrafluoroborate under a hydrogen atmosphere in 0.3 M Et4NBF4/acetonitrile buffered at pH 7.6 is catalyzed by 2 at −0.64 V versus ferrocene, with an overpotential of 120 mV. Protonation of the sulfido ligand in 2 is an initial step in the catalytic process, and the rate-determining step at high acid concentrations appears to be the elimination of dihydrogen. The elimination may occur either from adjacent hydrosulfido sites or from a hydrosulfido−molybdenum hydride intermediate.

Superior perovskite oxide-ion conductor; Strontium- and magnesium-doped LaGaO3 : III. Performance tests of single ceramic fuel cells

Abstract: The performance of La0.8Sr0.2Ga0.83Mg0.17O2.815 (LSGM) as an optimized electrolyte of a solid oxide fuel cell was tested on single cells having a 500-µm-thick electrolyte membrane. The reactivity of NiO and LSGM suggested use of an interlayer to prevent formation of LaNiO3. The interlayer Sm-CeO2 was selected and sandwiched between the electrolyte and anode. Comparison of Sm-CeO2/Sm-CeO2+ Ni and Sm-CeO2+ Ni as anodes showed that Sm-CeO2/Sm-CeO2+ Ni gave an exchange current density 4 times higher than that of Sm-CeO2+ Ni. The peak power density of the interlayered cell is 100 mW higher than that of the standard cell without the interlayer. This improvement is due to a significant reduction of the anode overpotential; the overpotential of the cathode La0.6Sr0.4CoO3-delta (LSCo) remained unchanged. Comparison of the peak power density in this study and with that of a previous study, also with a 500-µm-thick electrolyte, indicates a factor of 2 improvement, i.e., from 270 mW/cm2 to 550 mW/cm2 at 800°C. The excellent cell performance showed that an LSGM-based thick membrane SOFC operating at temperatures 600° < Top < 800°C is a realistic goal.

Transistor-like behavior of transition metal complexes.

Abstract: Electron transport through semiconductor and metallic nanoscale structures,1 molecular monolayers,2-6 and single molecules7-15 connected to external electrodes display rectification, switch, and staircase functionality of potential importance in future miniaturization of electronic devices Common to most reported systems is, however, ultrahigh vacuum and/or cryogenic working conditions Here we introduce a single-molecule device concept based on a class of robust redox active transition metal (Os(II)/(III)) complexes inserted between the working electrode and tip in an electrochemical scanning tunneling microscope (in situ STM) This configuration resembles a single-molecule transistor, where the reference electrode corresponds to the gate electrode It operates at room temperature in a condensed matter (here aqueous) environment Amplification on−off ratios up to 50 are found when the redox level is brought into the energy window between the Fermi levels of the electrodes by the overpotential (“gate vol

Tungsten carbide nanocrystal promoted Pt/C electrocatalysts for oxygen reduction.

Abstract: Tungsten carbide nanocrystals on carbon (W2C/C) and tungsten carbide nanocrystals and Pt on carbon (Pt-W2C/C) composite electrocatalysts were prepared by the intermittent microwave heating (IMH) method and tested for the electroreduction of oxygen in the acidic media for the first time. The results revealed that the tungsten carbide nanocrystal promoted Pt/C electrocatalyst was very active for ORR with the onset potential of 1.0 V vs SHE at ambient temperature that is over 100 mV more positive compared with that of traditional Pt/C electrocatalyst. The kinetic parameters were determined. The exchange current densities at both high and low overpotential regions are two orders higher for ORR on Pt-W2C/C than that on Pt/C, showing a synergetic effect to improve the activity for ORR. The novel electrocatalysts show a poisoning resistant property toward methanol.

Analysis of Polarization Curves to Evaluate Polarization Sources in Hydrogen/Air PEM Fuel Cells

Abstract: A step-by-step technique to evaluate six sources of polarization, mainly associated with the cathode, in hydrogen/air proton exchange membrane fuel cells is demonstrated. The six sources of polarization were nonelectrode ohmic overpotential, electrode ohmic overpotential, nonelectrode concentration overpotential, electrode concentration overpotential, activation overpotential from the Tafel slope, and activation overpotential from catalyst activity. The technique is demonstrated as applied in the analysis of hydrogen/air polarization curves of an in-house membrane electrode assembly (MEA) using hydrogen/oxygen polarization curves as a diagnostic tool. The analysis results are discussed at three cell temperature/relative humidity (RH)/oxygen partial pressure (po 2 , atm) conditions at atmospheric pressure: 80°C/100% RH a n o d e /75% RH c a t h o d e /po 2 = 0.136, 100°C/70% RH/po 2 = 0.064, and 120°C/35% RH/po 2 = 0.064, which represent a near fully-humidified, a moderately humidified, and a low humidified condition, respectively. At the higher temperature operating conditions the RH and po 2 decrease resulting in higher electrode ohmic resistance (0.020, 0.020, and 0.035 Ω cm 2 , respectively), lower limiting current (2019, 1314, and 819 mA/cm 2 , respectively), and lower onset current density for significant electrode concentration overpotential (80, 60, and 40 mA/cm 2 , respectively). The technique is useful for diagnosing the main sources of loss in MEA development work, especially for high temperature/low relative humidity operation where several sources of loss are present simultaneously.

On the current-voltage characteristics of charge transfer reactions at mixed conducting electrodes on solid electrolytes.

Abstract: Even though the electrochemical oxygen reduction reaction at mixed conducting oxide electrodes is highly important for several applications of solid electrolytes a thorough discussion of the kinetics of electron and ion transfer steps at the corresponding electrode surface is not available yet. A straightforward application of current-voltage (I-V) relations derived for charge transfer reactions at electrode/electrolyte interfaces turns out to be inappropriate. In this contribution, a model is presented that relates concentrations of electrochemically active species and electrostatic potential steps at the electrode/gas interface of mixed conducting electrodes to the applied overpotential, and thus I-V formulas for the corresponding electron and ion transfer reactions are obtained. Depending on the specific parameters surprising effects are found such as an additional factor of two in the exponents of the I-V relation, irrelevance of the symmetry factor or limiting currents even if charge transfer is rate determining.

Effect of Elevated Temperature and Reduced Relative Humidity on ORR Kinetics for PEM Fuel Cells

Abstract: As a measure of catalytic activity, i η = 0 . 3 v ,the current density at 0.3 V overpotential, was chosen to evaluate the oxygen reduction reaction (ORR) at elevated temperatures (> 100°C) and various relative humidities(RH) for polymer exchange membrane (PEM) fuel cells. The purely kinetic reaction order of the ORR with respect to oxygen partial pressure is less than 1.0 and changes with the RH. The activation energy is 49 kJ/mol at 100% RH and 55 kJ/mol at 50% RH. The active electrochemical surface area of platinum changes little with RH. RH has a strong effect on the catalytic activity under dry conditions (0-60% RH), but under wet conditions (>60% RH) its influence is unclear. The Tafel slope obtained in the 1-100 mA/cm 2 current density range changes significantly with RH: wet conditions produce low Tafel slopes ( 100 mV/dec). Dependence of the RH on the oxygen reduction reaction (ORR) may be explained by the changes of the rate-determining reaction, proton activity, and adsorbed -OH on the platinum surface. The ORR kinetic parameters obtained here are instructive for high-temperature fuel cell data analysis and performance improvement.

Voltammetric analysis using a self-renewable non-mercury electrode

Abstract: Galinstan is a new kind of electrode material and the galinstan electrode is a promising alternative to the commonly used mercury electrodes. The eutectic mixture of gallium, indium and tin is liquid at room temperature (m.p. −19°C) and its voltammetric behaviour is similar to that of mercury. The potential windows of use were determined for different pH values and are similar to those obtained with conventional mercury electrodes. Furthermore, the high hydrogen overpotential, which is characteristic for mercury, can be observed when galinstan is used as electrode material. Galinstan can be employed as a liquid electrode in the voltammetric analysis of different metal ions, such as lead and cadmium, in different supporting electrolytes. Our results indicate that the non-toxic liquid alloy galinstan could therefore become immensely important in electrochemical research as a potential surrogate material for mercury.

In Situ Stress Measurements during Copper Electrodeposition on (111)-Textured Au

Abstract: In situ stress measurements were made during copper electrodeposition onto (111)-textured Au from acidic sulfate electrolyte using the wafer curvature method. In the Cu underpotential deposition region, the intermediate Cu-sulfate honeycomb structure creates a surface stress that is tensile when compared to that of the sulfate-adsorbed electrode at positive potentials or the complete Cu monolayer at more negative potentials. This behavior is consistent with surface-induced charge redistribution models that appear in the literature. During the bulk deposition of Cu, there is a rapid increase in tensile stress during the first 20 nm of growth that we attribute to nuclei coalescence and grain boundary formation. The magnitude of the tensile stress as well as the film thickness at which the maximum stress occurs are both dependent upon the electrode potential due to its influence on the nucleation density. When the films are continuous, the total stress is the superposition of the coalescence-induced tensile stress and a compressive stress which we attribute to the incorporation of mobile adatoms on the surface into the grain boundaries. The tensile stress component dominates thin films deposited at high overpotential, whereas thick films deposited at low overpotential have a net compressive stress. When deposition is interrupted both tensile and compressive components of the stress relax somewhat but are quickly reestablished when deposition is resumed. The development of the growth stress that we describe here is very similar to that which has been reported for Cu deposition from the vapor phase. © 2005 The Electrochemical Society. All rights reserved.

Electrochemical reduction of CO2 mediated by poly-M-aminophthalocyanines (M = Co, Ni, Fe): poly-Co-tetraaminophthalocyanine, a selective catalyst

Abstract: The electrochemical reduction of carbon dioxide was studied on a glassy carbon electrode modified with either polymeric M-tetrakis aminophthalocyanines (M = Co, Ni, Fe) or with the polymeric free ligand, in aqueous electrolyte. The reaction products are dependent on the central ion: for Co-polymer the only reaction product found was formic acid; for Ni polymer, formic acid and formaldehyde were found, whereas formaldehyde and H2 were detected for Fe polymer. For the free ligand polymer only H2 was detected. Spectroelectrochemical experiments show that in the case of Co-polymer, Co(I) is the active site of the electrocatalysis but the reduced cobalt center and the reduced ligand are not enough to promote the reduction of the carbon dioxide and an extra overpotential is necessary. In the case of the Ni polymer, the reaction takes place at the same potential where the complex is double reduced and it is not necessary to apply more potential. On the other hand, there are important differences between the morphologies of both polymers as demonstrated by electrochemical impedance spectroscopy. The experiments show that the metallic center affects the kinetics of polymerization and the polymer morphology. On the other hand, the chemical nature of the metal center of the catalyst is the most important factor in the electrochemical reduction of CO2 and the products involved.