Showing papers on "Electrode potential published in 2014"

Self-Supported PdxBi Catalysts for the Electrooxidation of Glycerol in Alkaline Media

Abstract: Highly active self-supported PdxBi catalysts are synthesized by the sacrificial support method. Self-supported PdxBi catalysts have a porous nanostructured morphology with high surface areas (in the range from 75 to 100 m2 g–1), making PdxBi a state-of-the-art catalyst. Pd4Bi displays the highest activity toward glycerol oxidation. In situ Fourier transform infrared spectroscopy highlights the unique catalytic behavior of self-supported PdxBi materials due to their particular structure and morphology. The confinement of reactants and intermediates in pores acting as nanoreactors is responsible for the high selectivity as a function of the electrode potential: aldehyde and ketone at low potentials, hydroxypyruvate at moderate potentials, and CO2 at high potentials. Moreover, the selectivity depends on the electrode history: it is different for the positive potential scan direction than for the reverse direction, where the catalyst becomes selective toward the production of carboxylates.

Long-Range Electron Transfer over Graphene-Based Catalyst for High-Performing Oxygen Reduction Reactions: Importance of Size, N-doping, and Metallic Impurities

Abstract: N-doped carbon materials are considered as next-generation oxygen reduction reaction (ORR) catalysts for fuel cells due to their prolonged stability and low cost. However, the underlying mechanism of these catalysts has been only insufficiently identified, preventing the rational design of high-performing catalysts. Here, we show that the first electron is transferred into O2 molecules at the outer Helmholtz plane (ET-OHP) over a long range. This is in sharp contrast to the conventional belief that O2 adsorption must precede the ET step and thus that the active site must possess as good an O2 binding character as that which occurs on metallic catalysts. Based on the ET-OHP mechanism, the location of the electrode potential dominantly characterizes the ORR activity. Accordingly, we demonstrate that the electrode potential can be elevated by reducing the graphene size and/or including metal impurities, thereby enhancing the ORR activity, which can be transferred into single-cell operations with superior sta...

A unified model for surface electrocatalysis based on observations with enzymes

Abstract: Despite being so large, many enzymes are not only excellent electrocatalysts – making possible chemical transformations under almost reversible conditions – but they also facilitate our understanding of electrocatalysis by allowing complex processes to be dissected systematically. The electrocatalytic voltammograms obtained for enzymes attached to an electrode expose fundamental aspects of electrocatalysis that can be addressed in ways that are not available to conventional molecular or surface electrocatalysts. The roles of individual components, each characterisable by diffraction or spectroscopy, can be tested and optimised by genetic engineering. Importantly, unlike small-molecule electrocatalysts (RMM < 1000) that are structurally well-defined but invariably altered by being attached to a surface, the enzyme is a giant, multi-component assembly in which the active site is buried and relatively insensitive to the presence of the electrode and solvent interface. A central assertion is that for a given driving force (electrode potential) a true catalyst has no influence on the direction of the reaction; consequently, ‘catalytic bias’, i.e. the common observation that an enzyme or indeed any electrocatalyst operates preferentially in one direction, must arise from secondary effects beyond the elementary catalytic cycle. This Perspective highlights and extends a general model for electrocatalysis by surface-confined enzymes, and explains how two secondary effects control the bias: (i) the electrode potential at which electrons enter or leave the catalytic cycle; (ii) potential-dependent interconversions between states of the catalyst differing in catalytic activity due to changes in the composition and arrangements of atoms. The model, which is easily applied to enzymes that have been studied recently, highlights important considerations for understanding and developing surface-confined electrocatalysts.

Instrument-free control of the standard potential of potentiometric solid-contact ion-selective electrodes by short-circuiting with a conventional reference electrode.

Abstract: A simple, instrument-free method to control the standard potential (E°) of potentiometric solid-contact ion-selective electrodes (SC-ISE) is described. In this method, the electrode potential of a SC-ISE is reset by short-circuiting the electrode with a metallic wire to a conventional Ag/AgCl/3 M KCl reference electrode (RE) in a solution containing primary ions. The method is studied experimentally for SC-ISEs where the conducting polymer poly(3,4-ethylenedioxythiophene) doped with the bulky anion poly(sodium 4-styrenesulfonate), PEDOT(PSS), is used as the solid contact. Three different types of ion-selective membranes (ISMs) are studied: two potassium-selective membranes, with and without the lipohilic additive tetradodecylammonium tetrakis(4-chlorophenyl)borate (ETH-500) and a cation-sensitive membrane without an ionophore. When the SC-ISE is short-circuited with the RE, the PEDOT(PSS) layer is oxidized or reduced, thereby shifting the potential of the SC-ISE to the proximity of the potential of the RE...

Direct electrochemical regeneration of the cofactor NADH on bare Ti, Ni, Co and Cd electrodes: The influence of electrode potential and electrode material

Abstract: The regeneration of enzymatically-active reduced form of enzymatic cofactor nicotinamide adenine dinucleotide (1,4-NADH) from the oxidized form (NAD+) in a batch electrochemical reactor employing bare (non-modified) metal electrodes was investigated as a function of electrode potential and electrode material (Ti, Ni, Co and Cd). It was found that the regeneration of 1,4-NADH employing the electrodes is feasible; all the electrodes were capable of producing more than an 80% enzymatically-active product (1,4-NADH), reaching a 96% product purity on Ti. The product purity was found to be highly potential-, and material dependant. The origin of the material/potential dependency was related to the strength of the metal–hydrogen (M Hads) bond, and thus to the potential dependence of the Hads electrode surface coverage. In contradiction to literature, bare (non-modified) metal electrodes were found to be good candidates for electrochemical regeneration of enzymatically-active 1,4-NADH, when the regeneration is performed at a specific overpotential.

Molecular dynamics simulations of the structure of the graphene-ionic liquid/alkali salt mixtures interface.

Abstract: We performed molecular dynamics simulations of mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate with lithium tetrafluoroborate and potassium tetrafluoroborate between two charged and uncharged graphene walls, in order to analyze the structure of the well-known formation of layers that takes place on liquids under confinement. For this purpose, we studied the molecular density profiles, free energy profiles for bringing lithium and potassium cations from the bulk mixture to the graphene wall and the orientational distributions of imidazolium rings within the first adsorbed layer as a function of salt concentration and electrode potential. The charge densities in the electrodes were chosen to be zero and ±1 e nm−2, and the salt molar percentages were %salt = 0, 10 and 25. We found that the layered structure extends up to 1–2 nm, where the bulk behaviour is recovered. In addition, whereas for the neutral surface the layers are composed of both ionic species, increasing the electrode potential, the structure changes to alternating cationic and anionic layers leading to an overcompensation of the charge of the previous layer. We also calculated the distribution of angles of imidazolium rings near neutral and charged graphene walls, finding a limited influence of the added salt. In addition, the average tilt of the imidazolium ring within the first layer goes from 36° with respect to a normal vector to the uncharged graphene wall to 62° in the presence of charged walls. The free energy profiles revealed that lithium and potassium ions are adsorbed on the negative surface only for the highest amount of salt, since the free energy barriers for approaching this electrode are considerably higher than kBT.

Elucidating Redox-Level Dispersion and Local Dielectric Effects within Electroactive Molecular Films

Abstract: The electron exchange between a redox-active molecular film and its underlying electrode can be cleanly tracked, in a frequency-resolved manner, through associated capacitive charging. If acquired data is treated with a classical (non quantum) model, mathematically equivalent to a Nernst distribution for one redox energy level, redox site coverage is both underestimated and environmentally variable. This physically unrealistic model fails to account for the energetic dispersion intrinsically related to the quantized characteristics of coupled redox and electrode states. If one maps this redox capacitive charging as a function of electrode potential one not only reproduces observations made by standard electroanalytical methods but additionally and directly resolves the spread of redox state energies the electrode is communicating with. In treating a population of surface-confined redox states as constituting a density of states, these analyses further resolve the effects of electrolyte dielectric on energ...

Electrochemical synthesis of copper nanoparticles using cuprous oxide as a precursor in choline chloride–urea deep eutectic solvent: nucleation and growth mechanism

Abstract: The electrochemical nucleation and growth kinetics of copper nanoparticles on a Ni electrode have been studied with cyclic voltammetry and chronoamperometry in the choline chloride (ChCl)–urea based deep eutectic solvent (DES). The copper source was introduced into the solvent by the dissolution of Cu(I) oxide (Cu2O). Cyclic voltammetry indicates that the electroreduction of Cu(I) species in the DES is a diffusion-controlled quasi-reversible process. The analysis of the chronoamperometric transient behavior during electrodeposition suggests that the deposition of copper on the Ni electrode at low temperatures follows a progressive nucleation and three-dimensional growth controlled by diffusion. The effect of temperature on the diffusion coefficient of Cu(I) species that is present in the solvent and electron transfer rate constant obeys the Arrhenius law, according to which the activation energies are estimated to be 49.20 and 21.72 kJ mol−1, respectively. The initial stage of morphological study demonstrates that both electrode potential and temperature play important roles in controlling the nucleation and growth kinetics of the nanocrystals during the electrodeposition process. Electrode potential is observed to affect mainly the nucleation process, whereas temperature makes a major contribution to the growth process.

Oxidizing Electrode Potentials Decrease Current Production and Coulombic Efficiency through Cytochrome c Inactivation in Shewanella oneidensis MR-1

Abstract: Previous transcriptomic profiling of Shewanella oneidensis MR-1 had suggested that electron transfer to an anode in a bioelectrochemical system may induce a general stress response (similar to a heat-shock response) and/or an increase in protein turnover rates. Analysis of this microbe grown with a wide variety of electron acceptors also indicated that protein turnover may be related to the redox potential of the terminal electron acceptor. To investigate whether electrodes can induce stress and increase protein turnover, S. oneidensis was grown at potentiostatically poised electrodes at five redox potentials versus the standard hydrogen electrode (SHE) between −3 and +797 mVSHE. Subsequently, current production, coulombic efficiency, and transcription levels of marker genes for general stress and protein turnover were measured. Maximal current production was found at +397 mVSHE, and maximal coulombic efficiency was observed at +197 mVSHE. Both values decreased at more positive (oxidizing) potentials, that is, extracellular electron transfer of S. oneidensis is optimal at moderate electrode potentials. In contrast to previous findings, transcript measurements of a stress-marker gene indicate that extracellular electron transfer does not increase general stress in comparison with aerobic respiration. Although overall protein turnover is not related to electrode potential, increased expression of a protease suggests that protein degradation increases at oxidizing electrode potentials. Cyclic voltammetry revealed decreased activity of c-type cytochromes at the higher potentials, which indicates that oxidizing electrodes directly damage electron-transfer proteins at the electrode surface.

Influence of the CO Adsorption Environment on Its Reactivity with (111) Terrace Sites in Stepped Pt Electrodes under Alkaline Media

Abstract: The effect of the electrode potential in the reactivity of platinum stepped single crystal electrodes with (111) terraces toward CO oxidation has been studied. It is found that the CO adlayer is significantly affected by the potential at which the adlayer is formed. The electrochemical and FTIR experiments show that the adsorbed CO layer formed in acidic solution at ∼0.03 V vs SHE is different from that formed at −0.67 V vs SHE in alkaline solutions. The major effect of the electrode potential is a change in the long-range structure of CO adlayer. The adlayer formed in alkaline media presents a higher number of defects. These differences affect the onset and peak potential for CO stripping experiments. The stripping voltammogram for the adlayer formed at −0.67 V vs SHE always shows a prewave and the peak potential is more negative than that observed for the adlayer formed at 0.03 V vs SHE. This means that the apparent higher activity for CO oxidation observed in alkaline media is a consequence of the diff...

Sodium molybdate – an additive of choice for enhancing the performance of AC/AC electrochemical capacitors in a salt aqueous electrolyte

Abstract: Sodium molybdate (Na2MoO4) has been used as an additive to 1 mol L−1 lithium sulfate electrolyte for electrochemical capacitors based on activated carbon (AC) electrodes, in order to reduce the corrosion of stainless steel current collectors. We demonstrate that the MoO42− anions improve the overall capacitance owing to pseudofaradaic processes. In a two-electrode cell, capacitance values of 121 F g−1 have been achieved up to 1.6 V using 1 mol L−1 Li2SO4 + 0.1 mol L−1 Na2MoO4, as compared to 103 F g−1 when 1 mol L−1 Li2SO4 is used. Further, by using a two-electrode setup equipped with a reference electrode, we could demonstrate that, at 1.6 V, the positive electrode potential reaches a value of 0.96 V vs. NHE in 1 mol L−1 Li2SO4, crossing the thermodynamic potential limit of oxygen evolution (Eox = 0.846 V vs. NHE), and the pitting potential, Epit = 0.95 V vs. NHE. By contrast, in 1 mol L−1 Li2SO4 + 0.1 mol L−1 Na2MoO4, the pseudofaradaic contribution occurring at −0.05 V vs. NHE due to MoO42− anions drives the positive electrode to reach only 0.798 V vs. NHE. Hence, the oxidation of the AC and corrosion of the stainless steel current collector at the positive electrode are unlikely in Li2SO4 + Na2MoO4 when the capacitor operates at 1.6 V. During potentiostatic floating of the capacitor at 1.6 V for 120 hours in Li2SO4 + Na2MoO4, the capacitance and resistance remain constant at 125 F g−1 and ∼1.0 Ω, respectively, while the resistance increases from 1.4 Ω to 3.1 Ω in Li2SO4. Overall, the addition of MoO42− anions to Li2SO4 aqueous electrolyte allows the capacitance to be enhanced, corrosion of the positive stainless steel current collector to be inhibited and the AC/AC electrochemical capacitor to demonstrate stable performance up to 1.6 V.

In operando observation system for electrochemical reaction by soft X-ray absorption spectroscopy with potential modulation method.

Abstract: In order to investigate local structures of electrolytes in electrochemical reactions under the same scan rate as a typical value 100 mV/s in cyclic voltammetry (CV), we have developed an in operando observation system for electrochemical reactions by soft X-ray absorption spectroscopy (XAS) with a potential modulation method. XAS spectra of electrolytes are measured by using a transmission-type liquid flow cell with built-in electrodes. The electrode potential is swept with a scan rate of 100 mV/s at a fixed photon energy, and soft X-ray absorption coefficients at different potentials are measured at the same time. By repeating the potential modulation at each fixed photon energy, it is possible to measure XAS of electrochemical reaction at the same scan rate as in CV. We have demonstrated successful measurement of the Fe L-edge XAS spectra of aqueous iron sulfate solutions and of the change in valence of Fe ions at different potentials in the Fe redox reaction. The mechanism of these Fe redox processes is discussed by correlating the XAS results with those at different scan rates.

Electrochemical oxidation of dichlorvos on SnO2Sb2O5 electrodes

Abstract: The effects of electrode potential and the initial concentration of 2,2-dichlorovinyl dimethyl phosphate (dichlorvos, DDVP) on its oxidation/mineralization reaction kinetics, using an electrochemical oxidation system based on SnO 2 Sb 2 O 5 anodes, have been studied. Electrochemical degradation followed the Langmuir–Hinshelwood mechanism, with adsorption equilibrium constant K = 0.082 L mg −1 of dichlorvos on the electrode material surface, and reaction rate constant k = 0.021 mg L −1 s −1 of the organic compound-electrocatalyst adduct. Chemical oxygen demand and CO 2 measurements in neutral media suggest that the rate limiting step for mineralization is the same as for the electrochemical oxidation. The results show that the electrochemical mineralization of dichlorvos was readily possible at potentials more positive than 2.5 V vs. SCE, with lower reaction half-lives than obtained with other advanced oxidation process. The fastest overall degradation rate constants were obtained at limiting low concentrations of dichlorvos in aqueous solution, k obs = kK = 0.0017 s −1 .

Poly(luminol) based sensor array for determination of dissolved chlorine in water

Abstract: An optical sensor for determination of free chlorine content of drinking water was prepared and tested. The function of the sensor is based on detecting chemiluminescence signal provided by thin immobilized poly(luminol) reagent layer. The poly(luminol) reagent film was prepared by electropolymerization of luminol onto planar indium-tin-oxide (ITO) electrode. Different methods, like electrode potential cyclization (cyclic voltammetry, CV), pulsed potential electrolysis (pulsed amperometry, PA) and potentiostatic electrolysis (constant potential electrolysis, PSE) were employed for preparation of the poly(luminol) layer. The chemoluminescence (chemiluminescence) of the differently prepared films was investigated in the poly(luminol) – hypochlorite – hydrogen peroxide reaction. Highest luminescence signal was obtained by the films prepared with CV. Poly(luminol) layers deposited with pulsed potential showed 80% less luminescence while almost no signal was obtained in case of films made with constant potential technique. The effects of the buffer composition and pH on the analytical properties of the electro polymerized sensing layer were investigated. The lower concentration limit of free chlorine detection was 5 × 10 −7 M in phosphate buffer at pH = 8.0. It was found that the chemiluminescence signal decreased significantly when hypochlorite concentrations over 1 mM were applied. An array of 24 micro wells was fabricated on ITO glass slab of about microscope slide size. The individual micro wells had identical volume and the poly(luminol) layer immobilized on their bottom had identical activity. The wells could be used for “single shot” determination of free chlorine content of drinking water. The long storage stability, the simple measurement procedure and low feasible concentration range makes the array an attractive analytical tool. Its applicability was proved measuring dissolved chlorine concentration of tap water samples.

A precise pH microsensor using RF-sputtering IrO2 and Ta2O5 films on Pt-electrode

Abstract: In this study, an easily implemented surface modification scheme is reported employing Ta2O5 membrane which covers IrO2 electrode in response to H+ and eliminating redox species interference. Evidence shows that H+ can pass through Ta2O5 films and react with IrO2/Pt electrodes due to proton–electron double injection. A Ta2O5 membrane, an ionic conductor with an insulating property, blocks the transport of electrons generated from oxygen perturbation in the solution. The conduction of both electrons and protons preserve the current continuity across the interface. Owing to proton–electron double injection, IrO2 will be reduced to Ir(OH)3 during pH detection. The [IrO2]/[Ir(OH)3] will remain constant and therefore the Nernstian electrode potential performs stably as a function of pH (−59.447 to −59.504 mV/pH, 2