An electrochemical quartz crystal impedance study on the rising of an aqueous solution meniscus for a partially immersed gold electrode during the electrochemical reduction of oxygen.

An electrochemical quartz crystal impedance system (EQCIS) was used to study the resonance behavior of an AT-cut 9-MHz piezoelectric quartz crystal (PQC) with its Au electrode partially immersed in KCl, Na2SO4 and NaClO4 aqueous solutions, respectively. An in situ determination of the immersed area and the height of the electrode was achieved by simultaneous measurements of the PQC electroacoustic admittance and the electrochemical impedance. The rising of the solution meniscus for a gold electrode partially immersed in aqueous solutions was found at oxygen reduction potentials and evaluated versus the electrolyte, electrolyte concentration, solution pH and oxygen concentration. The solution meniscus rising was explained based on a lowering of the contact-angle hysteresis and a continued collection of the water product at the solid-gas-solution interface during oxygen reduction.     

Mercury meniscus on solid silver amalgam electrode as a sensitive electrochemical sensor for tetrachlorvinphos

The in-house prepared mercury meniscus modified solid silver amalgam electrode (m-AgSAE) was successfully applied for the detection of organophosphate pesticide tetrachlorvinphos in pH 7 buffer solution. The electrochemical performance of m-AgSAE for the reduction of tetrachlorvinphos was evaluated using cyclic voltammetry (CV), differential pulse voltammetry (DPV), and square wave voltammetry (SWV), respectively. The surface morphology of solid silver electrode (AgE), as-amalgamated solid silver amalgam electrode (AgSAE), and polished solid silver amalgam electrode (p-AgSAE) was examined by field emission scanning electron microscopy (FESEM). Among the applied techniques, DPV and SWV analysis showed a remarkable increase in the reduction peak current and provided a simple, fast, and sensitive method for the determination of tetrachlorvinphos. The electrochemical impedance spectroscopy (EIS) was used to correlate the electrocatalytic activity of AgSAE, p-AgSAE and m-AgSAE with their interfacial charge transport capabilities. Under the optimized experimental conditions, the DPV and SWV responses were linear over the 1–9 μM and 10–50 μM concentration ranges with a detection limit of 0.06 μM for DPV and 0.04 for SWV. The estimation of tetrachlorvinphos in the ground and waste water samples with the proposed method was in good agreement with that of the added amount. The proposed electrochemical method not only extends the application of non-toxic m-AgSAE, but also offers new possibilities for fast and sensitive analysis of tetrachlorvinphos and its structural analogs in environmental samples.     

Effect of the electrode material on the oxygen reduction at the nonstoichiometric lanthanum fluoride/electrode interface

The voltammetry method with a linear potential scan is used for investigating the effect the electrode material (Ni, Co, electrodes on the basis of cobalt oxides modified with carbon) exerts on the reduction of gaseous oxygen at interfaces solid fluoride-conducting electrode LaF₃:Eu²⁺/electrode, O₂, and conjugated processes. Properties of the modified electrodes are characterized by the impedance spectroscopy, scanning electron microscopy, and x-ray photoelectron spectroscopy methods. The oxygen reaction is irreversible at the LaF₃:Eu²⁺|Ni (or Co) interfaces. At the interface of LaF₃:Eu²⁺ with modified electrodes Co (C n at %), where n = 5 and 9, mobile forms of oxygen are reversible and the reduction of gaseous and chemisorbed oxygen is controlled by diffusion with different effective kinetic parameters.     

Catalysis of the electrochemical oxygen reduction in room-temperature ionic liquids on a pyrolytic graphite electrode by iron-containing superoxide dismutase

Catalysis of the electrochemical oxygen reduction reaction (ORR) on a pyrolytic graphite electrode (PGE) by iron-containing superoxide dismutase (Fe-SOD) is investigated for the first time using cyclic voltammetry and electrochemical impedance spectroscopy. The study is carried out in three room-temperature ionic liquids (RTILs), namely, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄), 1-propyl-3-methylimidazolium tetrafluoroborate (PMIBF₄), and 1-butyl-3-methylimidazolium tetrafluoroborate (EMIBF₄). The results demonstrate that in EMIBF₄, Fe-SOD exhibits the most satisfactory catalysis for ORR, with the standard rate constant of ORR on bare PGE, k _s, increasing from 3.9 to 5.1 times 10⁻³ cm s⁻¹, while in PMIBF₄ and BMIBF₄ containing Fe-SOD k _s increases from 2.6 to 3.6 and from 1.4 to 2.2 times 10⁻³ cm s⁻¹, respectively. In addition to the increased k _s, adding Fe-SOD renders the formal potential of ORR more positive. To accelerate the electron transfer, multi-walled carbon nanotubes (MWCNTs) are employed to modify PGE, consequently, yielding the dramatically increased peak current and k _s. For MWCNTs-modified PGE in EMIBF₄ free of Fe-SOD, k _s increases from 3.9 to ∼7.1 times 10⁻³ cm s⁻¹. The ORR catalysis by Fe-SOD in the presence of Fe-SOD is also evidenced by the formal-potential shift in the positive direction. With MWCNTs accounting for the larger k _s and Fe-SOD being responsible for the formal-potential shift, the catalysis of ORR is satisfactory. Chronocoulmetry experiments proved that some Fe-SOD could be adsorbed on PGE. After analyzing the results, dismutation of superoxide anion O ₂ ⁻ by Fe-SOD is thought to be the main reason for the formal-potential shift. The different polarity of RTILs is probably partly responsible for different k _s obtained in different RTILs. Basing on an earlier proposition, the catalysis of ORR by MWCNTs in RTILs is discussed. Published in Russian in Elektrokhimiya, 2007, Vol. 43, No. 9, pp. 1137–1146. The text was submitted by the authors in English.     

Electrochemical reactions at the electrode/solution interface:Theory and applications to water electrolysis and oxygen reduction

Theoretical simulations on complex electrochemical processes have been developed on the basis of the understanding in electrochemistry,which has benefited from quantum mechanics calculations.This article reviews the recent progress on the theory and applications in electrocatalysis.Two representative reactions,namely water electrolysis and oxygen reduction,are selected to illustrate how the theoretical methods are applied to electrocatalytic reactions.The microscopic nature of these electrochemical reaction...     

Peculiarities of the adsorption behavior of adamantanol-1 on a mechanically renewable silver electrode

Using the method of electrochemical measurements on electrodes with mechanically renewable surface, the adsorption behavior of adamantanol-1 (AdOH) on the Ag electrode interface with solutions of a surface-inactive electrolyte (NaF) is studied. Based on the results of impedance and voltammetric measurements, it is shown that the kinetics of AdOH adsorption in the ideal polarizability potential range (near the zero charge potential) is described within the framework of the mechanism of the quasichemical reaction Ag(H₂O)_ads + AdOH = Ag(AdOH)_ads + H₂O on metal surfaces with energy uniformity. A phenomenological model is proposed that makes it possible to consistently describe the temporal effects on renewable Ag electrodes in the potential range of its initial oxidation in solutions containing AdOH.     

Anion adsorption in the course of dichromate reduction on a smooth gold electrode in aqueous acid solutions

The specific adsorption of anions (HSO₄ ⁻, Cl⁻) present in low concentration (c < 10⁻³ mol dm⁻³) was studied by radiotracer techniques in the course of the reduction of dichromate (chromate) species in 1 mol dm⁻³ HClO₄ supporting electrolyte. In accordance with the results of preliminary studies reported earlier, enhancement of the anion adsorption was found, induced by some adsorbed intermediates of the reduction process. Potential dependence of the induced adsorption and its correlation with the reduction rate was investigated. The role of adsorption competition between various anions is discussed. It is concluded that study of the induced anion adsorption could be a tool for the investigation of the sorption of intermediates formed in the course of the reduction. Received: 3 May 1999 / Accepted: 10 June 1999     

Electrocatalytic activity of dispersed platinum and silver alloys and manganese oxides for the oxygen reduction in alkaline electrolyte

This work reviews the studies conducted in this laboratory of the oxygen reduction reaction (ORR) on electrocatalysts formed by Pt-M/C (M = V, Cr, Co) and Ag-Pt/C alloys and on different Mn oxides (MnO/C, Mn₃O₄/C, MnO₂/C) in KOH electrolyte. The physical and electronic properties of the materials are investigated by in situ XAS (x-ray absorption spectroscopy) in the XANES (x-ray absorption near edge structure) region. The electrocatalytic activity for the ORR on the different catalysts is compared through mass-transport-corrected Tafel plots. The XANES results for the Pt-M/C and Ag-Pt/C composites at high electrode potentials show lower vacancy of the Pt 5d band compared to pure Pt/C, while for the results indicate a chance of the Mn oxidation state as a function of the electrode potential. The electrochemical measurements evidence increased electrocatalytic activity of the Pt alloys compared to pure Pt and this is attributed to a lowering of the adsorption strength of adsorbed oxygen species caused by the reduced Pt reactivity. An activity enhancement of the Ag atoms on the Ag-Pt/C alloys compared to pure Ag is ascribed to an electronic effect induced by the presence of Pt, increasing the Ag-O adsorption strength. In the case of the Mn_yO_x/C materials, the electrochemical results show low activity for MnO/C and higher activity for MnO₂/C and Mn₃O₄/C. This is explained based on the activation for the ORR, which is higher for the material with higher MnO₂ contents and the occurrence of a mediation processes involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 12, pp. 1417–1426. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Specific features of the behavior of an electrochemical system in the case of the Hopf instability for a spherical electrode

Model approximations are developed that allow establishing a quantitative relationship between the geometrical parameters of a spherical electrode, the faradaic impedance, and instabilities of the electrochemical system for an electrode reaction under potentiostatic conditions for the adsorption of species preceding their discharge. It is shown that the control parameter ω_H in the Hopf bifurcation point depends on the electrode size. The effect of the Nernst diffusion layer is observed at low frequencies in the range of negative faradaic impedance values.     

Density functional theory-based electrochemical models for the oxygen reduction reaction: comparison of modeling approaches for electric field and solvent effects

A series of density functional theory (DFT) based electrochemical models are applied to systematically examine the effect of solvent, local electric field, and electrode potential on oxygen reduction reaction (ORR) kinetics. Specifically, the key elementary reaction steps of molecular oxygen dissociation, molecular oxygen protonation, and reduction of a hydroxyl adsorbate to water over the Pt(111) surface were considered. The local electric field has slight influence on reaction energetics at the vacuum interface. Solvent molecules stabilize surface adsorbates, assisting oxygen reduction. A collective solvation-potential coupled effect is identified by including long range solvent-solvent interactions in the DFT model. The dominant path of the ORR reaction varies with electrode potential and among the modeling approaches considered. The potential dependent reaction path determined from the solvated model qualitatively agrees with experiment ORR kinetics.     

Studying the process of oxygen ionization on a porous metal hydride electrode

Problems that are connected with utilization of oxygen evolving during overcharge of the nickel oxide electrode in sealed nickel metal hydride batteries are considered. It is established experimentally that the rate of the process of oxygen reduction in conditions of forced gas supply into pores of a metal hydride electrode increases by two orders of magnitude as compared with the intensity of this process during natural convection. Up to 80% of evolved oxygen undergo ionization on a metal hydride electrode in these conditions even in a regime of forced (hour-long) charge of a model sealed nickel-metal hydride battery. The dependence of the current density of oxygen reduction at a metal hydride electrode on the filling of the electrode’s porous space by oxygen is estimated with the aid of manometric and potentiostatic methods. It is shown that practically all the oxygen ionization current is generated at the walls of gas-filled electrode pores, under thin electrolyte films, with a local current intensity of 1–3 mA cm^?2.     

Simultaneous impedance measurements of two one-face sealed resonating piezoelectric quartz crystals for in situ monitoring of electrochemical processes and solution properties

Through admittance measurements of two piezoelectric quartz crystals in parallel on one impedance analyzer and then non-linear fitting according to an equivalent circuit of two parallel Butterworth-Van Dyke circuits, we have simultaneously obtained accurate and precise impedance responses of two one-face sealed crystals to changes in solution density and viscosity, temperature, conductance, and/or electrode mass. A series of sucrose aqueous solutions, ferri-/ferrocyanide redox switching, hot water cooling, a series of NaClO₄ aqueous solutions, bovine serum albumin adsorption and silver electrodeposition/stripping were selected as model systems for such purposes. Galvanostatic charging/discharging reactions at positive and negative poles in a Ni-Zn battery were synchronously monitored, with some quartz crystal microbalance (QCM) insights into the second reduction process of nickel hydroxide film. In all cases, the crystal immersion angle effect was found to be negligible. The present method as a versatile one is highly recommended for informative two-electrode monitoring of two concurrent chemical or biological events, or for check and/or compensation of effects due to solution density, viscosity, temperature and/or conductance during QCM researches.     

Palladium and the electrochemical quartz crystal microbalance: a new method for the in situ analysis of the precious metal in aqueous solutions

A new method for the quantitative determination of palladium(II) by the electrochemical quartz crystal microbalance (EQCM) technique has been developed. Using a bare carbon-coated quartz crystal, Pd(II) ions are directly deposited from aqueous solution as palladium metal onto the crystal surface, and the Pd(II) concentration is determined with a detection limit of 0.0156 mM, or 1.66 ppm. No complexing agent or preconcentration of palladium is required for the analysis. The palladium is stripped from the crystal through its electrochemical oxidation, regenerating the crystal for subsequent multi-cycle palladium analyses. A conventional gold-coated quartz crystal was incapable of carrying out the same measurements. The EQCM technique presented is simple, sensitive, and reproducible for the detection of this widely used precious metal.     

In situ synthesis of silver nanoparticles on densely amine‐functionalized polystyrene: Highly active nanocomposite catalyst for the reduction of methylene blue

In this contribution, polystyrene (PS) bearing nitrogen‐rich ligands as chelation moieties for both Ag⁺ ions and Ag⁽⁰⁾ nanoparticles was prepared through successive chemical modifications of native PS including nitration (treatment with HNO₃/H₂SO₄), reductive amination (treatment with SnCl₂/HCl), Michael addition of methyl acrylate, and grafting of ethyelenediamine. The as‐synthesized PS derivative was further used to support silver nanoparticles through initial chelation of the silver nanoparticle ions precursor and subsequent chemical in situ reduction with sodium borohydride. Chemical structure of the PS derivatives was confirmed after each synthesis step by using complementary characterization methods including infrared and energy‐dispersive X‐ray spectroscopies, elemental analysis, X‐ray diffraction, thermogravimetric analysis, and scanning electron microscopy. The catalytic activity of the PS‐EAD/AgNP nanocomposite was demonstrated using the reduction of methylene blue to leucomethylene blue, as a model reaction. The reaction was monitored by UV‐vis spectrophotometry and achieved with an excess of sodium borohydride allowing for a pseudo‐first‐order analysis of the kinetic reaction parameters. Quantitative reduction of the methylene blue was obtained upon successive catalytic cycles with a rate constant value of 0.4016 minute^?1.     

Hidden in plain sight: unlocking the full potential of cyclic voltammetry with the thin-film rotating (ring) disk electrode studies for the investigation of oxygen reduction reaction electrocatalysts

Cyclic voltammetry conducted with a thin-film rotating disk electrode, or ring-disk electrode (CV-TF-R(R)DE) is a very popular ‘ex situ’ tool for the rapid screening of electrocatalysts for their activity in oxygen reduction reaction. Despite its popularity and broad use, in most instances only a small part of the information that could be accessed by CV-TF-R(R)DE is actually used by scientists in their research. This work outlines both innovative and more traditional (but half-forgotten) ways of using CV-TF-R(R)DE to its optimal or full potential in the ongoing quest to study the most relevant features of oxygen reduction reaction electrocatalysts and quickly identify the most promising candidates for their applications in fuel cells or other electrochemical devices.     

Modification of glassy carbon electrode with a polymer/mediator composite and its application for the electrochemical detection of iodate

A modified glassy carbon electrode was prepared by depositing a composite of polymer and mediator on a glassy carbon electrode (GCE). The mediator, flavin adenine dinucleotide (FAD) and the polymer, poly(3,4-ethylenedioxythiophene) (PEDOT) were electrochemically deposited as a composite on the GCE by applying cyclic voltammetry (CV). This modified electrode is hereafter designated as GCE/PEDOT/FAD. FAD was found to significantly enhance the growth of PEDOT. Electrochemical quartz crystal microbalance (EQCM) analysis was performed to study the mass changes in the electrode during the electrodeposition of PEDOT, with and without the addition of FAD. The optimal cycle number for preparing the modified electrode was determined to be 9, and the corresponding surface coverage of FAD (Γ_FAD) was ca. 5.11 × 10⁻¹⁰ mol cm⁻². The amperometric detection of iodate was performed in a 100 mM buffer solution (pH 1.5). The GCE/PEDOT/FAD showed a sensitivity of 0.78 μA μM⁻¹ cm⁻², a linear range of 4–140 μM, and a limit of detection of 0.16 μM for iodate. The interference effects of 250-fold Na⁺, Mg²⁺, Ca²⁺, Zn²⁺, Fe²⁺, Cl⁻, NO₃⁻, I⁻, SO₄²⁻ and SO₃²⁻, with reference to the concentration of iodate were negligible. The long-term stability of GCE/PEDOT/FAD was also investigated. The GCE/PEDOT/FAD electrode retained 82% of its initial amperometric response to iodate after 7 days. The GCE/PEDOT/FAD was also applied to determine iodate in a commercial salt.     

Bio‐inspired iron/sulfur/graphene nanocomposite and its use in the catalysis of the oxygen reduction reaction at room temperature in alkaline media on a glassy carbon electrode

This work demonstrates the performance of a bio‐inspired iron/sulfur/graphene nanocomposite as a non‐platinum electrocatalyst for the oxygen reduction reaction (ORR) in an alkaline medium. The catalyst shows the most positive ORR onset potential (1.1 V vs. RHE) according to its unique structure in the alkaline medium (KOH solution, pH = 13) at low temperature (T = 298 K). The catalyst is evaluated by the rotating‐disk electrode (RDE) method under various rotating speeds (0–2,000 rpm) in the potential range ?0.02–1.18 V vs. a rechargeable hydrogen electrode (RHE). The number of transferred electrons, as one of the most important parameters, is almost constant over a wide range of potentials (0.1–0.8 V), which indicates a more efficient four‐electron pathway from O₂ to H₂O on the FePc‐S‐Gr surface. The mean size of catalyst centers are in the nanoscale (<10 nm). The estimated Tafel slope in the appropriate range is about ?110 mV per decade at low current density, and E_1/2 of FePc‐S‐Gr displays a negative shift of only 7.1 mV after 10,000 cycles.     

Effect of sodium cation on the electrochemical reduction of CO<Subscript>2</Subscript> at a copper electrode in methanol

The electrochemical reduction of CO₂ with a Cu electrode in methanol was investigated with sodium hydroxide supporting salt. A divided H-type cell was employed; the supporting electrolytes were 80 mmol dm⁻³ sodium hydroxide in methanol (catholyte) and 300 mmol dm⁻³ potassium hydroxide in methanol (anolyte). The main products from CO₂ were methane, ethylene, carbon monoxide, and formic acid. The maximum current efficiency for hydrocarbons (methane and ethylene) was 80.6%, at −4.0 V vs Ag/AgCl, saturated KCl. The ratio of current efficiency for methane/ethylene, r _f(CH₄)/r _f(C₂H₄), was similar to those obtained in LiOH/methanol-based electrolyte and larger relative to those in methanol using KOH, RbOH, and CsOH supporting salts. In NaOH/methanol-based electrolyte, the efficiency of hydrogen formation, a competing reaction of CO₂ reduction, was suppressed to below 4%. The electrochemical CO₂ reduction to methane may be able to proceed efficiently in a hydrophilic environment near the electrode surface provided by sodium cation.