Electrode Reaction Mechanism Research Articles

Two-Electron, Two-Proton Oxidation of Catechol: Kinetics and Apparent Catalysis

The study of proton-coupled electron transfer reactions is of great current interest. In this work, the catechol redox process was studied voltammetrically in the pH range from 1.0 to 14.0 using a glassy carbon electrode. Analysis of the peak potentials and currents together with Tafel analysis allowed the inference of the likely transition states and electrode reaction mechanism. Modification of the glassy carbon electrode surface with sparse coverages of alumina particles was shown to lead to strong apparent catalysis of the catechol redox process at low pH. A possible mechanism for this is proposed.

PH Dependent Electrochemistry of Anthracenediones at a Glassy Carbon Electrode

The voltammetric response of 1-hydroxy-2(hydroxymethyl)anthracene-9,10-dione (HAC) and 1,8-dihydroxy-4,5-dinitro anthracenedione (DHDN) was investigated at a glassy carbon electrode in acidic, neutral and basic media by cyclic, square wave and differential pulse voltammetry. The oxidation of DHDN was evidenced by the appearance of one irreversible and two reversible peaks in the square wave voltammograms. The shift of peak potentials with change in pH demonstrated proton coupled electron transfer reactions. DHDN exhibited two steps pH dependent reduction while HAC reduced in a single step involving two electrons and two protons. The square wave voltammetric response showed the quasi-reversible nature of the reduction process of HAC. However, its oxidation followed irreversible behavior. The results of sensitive voltammetric techniques helped in proposing the electrode reaction mechanism of HAC and DHDN. Moreover, physical parameters such as diffusion coefficient, pKa and electron transfer rate constant were determined from scan rate, pH and concentration effects.

The FFC Cambridge process and its relevance to valorisation of ilmenite and titanium-rich slag

The Fray–Farthing–Chen (FFC) Cambridge process was patented in 1998 for low cost and clean electrochemical extraction of metals and synthesis of alloys directly from the mineral precursors, particularly oxides (including slags) and sulfides, with the aid of molten salts. In this paper, the technical success of this unique method is explained from the basic electrode reaction thermodynamics and mechanisms at the compound/metal/electrolyte three-phase interlines (boundaries). Selected recent innovations are then introduced towards more efficient cathode design and practices. Finally, the prospects of the FFC Cambridge process are discussed for processing the ilmenite ore and the so-called titanium-rich slag to produce Fe–Ti alloys and its commercial potential in the future titanium industry.

Sulfur-based composite cathode materials for high-energy rechargeable lithium batteries.

There is currently an urgent demand for highly efficient energy storage and conversion systems. Due to its high theoretical energy density, low cost, and environmental compatibility, the lithium sulfur (Li-S) battery has become a typical representative of the next generation of electrochemical power sources. Various approaches have been explored to design and prepare sulfur cathode materials to enhance their electrochemical performance. This Research News article summarizes and compares different sulfur materials for Li-S batteries and particularly focuses on the fine structures, electrochemical performance, and electrode reaction mechanisms of pyrolyzed polyacrylo-nitrile sulfur (pPAN@S) and microporous-carbon/small-sulfur composite materials.

Diagnostic criteria for the characterization of quasireversible electron transfer reactions by cyclic square wave voltammetry.

Theory for cyclic square wave voltammetry of quasireversible electron transfer reactions is presented and experimentally verified. The impact of empirical parameters on the shape of the current-voltage curve is examined. From the trends, diagnostic criteria enabling the use of this waveform as a tool for mechanistic analysis of electrode reaction processes are presented. These criteria were experimentally confirmed using Eu(3+)/Eu(2+), a well-established quasireversible analyte. Using cyclic square wave voltammetry, both the electron transfer coefficient and rate were calculated for this analyte and found to be in excellent agreement with literature. When properly applied, these criteria will enable nonexperts in voltammetry to assign the electrode reaction mechanism and accurately measure electrode reaction kinetics.

Electrochemical behaviour and determination of rimsulfuron herbicide by square wave voltammetry

The voltammeric behaviour of rimsulfuron herbicide has been studied by square wave stripping voltammetry on static hanging mercury drop electrode. It exhibited a well-defined peak within the pH range of 1.0–6.0, having a maximum peak response at −600 mV (vs.Ag/AgCl) at pH 3.0. The factors such as accumulation potential (Eacc), accumulation time (tacc), frequency (f), pulse amplitude (ΔE) and step potential (ΔEs) have been optimised. The calibration plot was a straight line in the range of 4.4–134.4 μg L−1 with a detection limit of 1.3 μg L−1. The validity of the method was assessed from the recoveries of spiked lake water, tomato juice and agrochemical formulation of Doncep®. The results of the experiments conducted for five recoveries were 48.8 ± 1.7 and 49.7 ± 1.0 μg L−1, which are very close to the rimsulfuron spiked to lake water and tomato juice (50 μg L−1), with a relative error of –2.4% and −0.6%, respectively. The electrode reaction mechanism was also postulated.

Designing the next generation high capacity battery electrodes

Much of current research in electrochemical energy storage is devoted to new electrode chemistries and reaction mechanisms that promise substantial increases in energy density. Unfortunately, most high capacity electrodes exhibit an unacceptably large hysteresis in their voltage profile. Using a first-principles multi-scale approach to examine particle level dynamics, we identify intrinsic thermodynamic and kinetic properties that are responsible for the large hysteresis exhibited by many high capacity electrodes. Our analysis shows that the hysteresis in the voltage profile of high capacity electrodes that rely on displacement reactions arises from a difference in reaction paths between charge and discharge. We demonstrate that different reaction paths are followed (i) when there is a large mismatch in ionic mobilities between the electrochemically active species (e.g. Li) and displaced ionic species and (ii) when there is a lack of a thermodynamic driving force to redistribute displaced ions upon charging of the electrode. These insights motivate the formulation of design metrics for displacement reactions in terms of fundamental properties determined by the chemistry and crystallography of the electrode material, properties that are now readily accessible with first-principles computation.

Role of Ceria-Zirconia Solid Solution with High Oxygen Storage Capacity in Cermet Anodes of Solid Oxide Fuel Cells

A solid solution of ceria and zirconia is investigated as the ceramic component of the cermet anode for solid oxide fuel cells (SOFCs). Homogeneously dispersed NiO-Ce0.75Zr0.25O2-δ (CZO) composite powder is synthesized via a single-step combustion process based on the glycine-nitrate method. The electrolyte-supported cell composed of a Ni - CZO anode, yttria-stabilized zirconia (YSZ) electrolyte and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) - gadolinia-doped ceria (GDC) cathode is fabricated, and its electrochemical performance is compared with that of a reference cell employing the conventional Ni-YSZ anode. The Ni-CZO anode exhibits substantially higher performance than the Ni-YSZ anode, indicating that CZO actively participates in the anode's catalytic process, in contrast to YSZ. Impedance spectra of full-cells and half-cells are correlated to understand the electrode reaction mechanisms, and heavily overlapping impedance arcs are successfully deconvoluted using a differential relaxation time (DRT) technique. It is found that CZO promotes fuel oxidation through an oxygen spillover mechanism and enhances gas-solid interaction via H2 adsorption on the ceramic component of the cermet anode, resulting in a reduction of the electrode polarization resistance. The potential of the Ni-CZO anode for practical applications and a strategy for further development are discussed in detail.

Analysis of the hydrogen electrode reaction mechanism in thin-layer cells. 2. Study of hydrogen evolution on microelectrodes by scanning electrochemical microscopy

This work presents an updated equation for analyzing experimental dependences of the tip current (iT) on the tip potential (ET) and the tip-substrate distance (d) by scanning electrochemical microscopy (SECM). The model includes the contribution of a general thin layer cell (TLC) with no aprioristic restrictions about the reaction mechanism. The iT(ET, d) dependence for a particular reaction is governed both by the microelectrode response (at large d values) and by the TLC response (more important at small d values) that result from solving the particular reaction mechanism. This equation was applied for studying the mechanism and for quantifying the elementary kinetic parameters of the hydrogen evolution reaction (her) on platinum and gold microelectrode tips. A dependence of the TLC current on ET previously developed for the her operating through the Volmer–Heyrovsky–Tafel (VHT) mechanism was included in the model to correlate experimental iT(ET, d) curves measured on these two metals. The transition from the VT to the VH route was clearly detected on both metals under conditions of high mass transport rates.

Probing of the pH-Dependent Redox Mechanism of a Biologically Active Compound, 5,8-Dihydroxynaphthalene-1,4-dione

The redox behaviour of a potential anticancer organic compound, 5,8-dihydroxynaphthalene-1,4-dione (DND), was investigated in 1 : 1 buffered aqueous ethanol using cyclic, differential pulse, and square wave voltammetry. The redox processes were found to occur in a pH-dependent diffusion-controlled manner. Presence of an α-hydroxyl group stabilised semiquinone radical of DND, formed by the gain of 1 e– and 1 H+, prevented the second step reduction, which is in contrast to the general mechanism previously reported for quinines in protic and aprotic media. In addition, our results supported an independent oxidation and reduction process. Square wave voltammetry provided evidence about the reversible and quasi-reversible nature of oxidation and reduction peaks. Based on the voltammetric results, the electrode reaction mechanism of DND was proposed. Parameters including pKa, transfer coefficient, diffusion coefficient, and electron transfer rate constant were evaluated. The values of pKa obtained from cyclic voltammetry and ultraviolet-visible spectroscopy not only agreed with each other, but also with reported values of structurally related compounds evaluated by other techniques.

Nano Dimensionality: A Way towards Better Li-Ion Storage

Nanodimensional materials such as transition metal oxides, polyanionic based materials and metal fluorides can be used as cathode while metal nanoparticles, alloys and metal oxides are preferred as anode for next-generation lithiumion batteries (LIBs) in order to obtain high reversible capacity, rate capability, safety, and longer cycle life. These nanomaterials can offer relatively short ionic and electronic pathways which leads to better transportation of both lithium ions and electrons to the particles core. This article emphasize on the effect of nanodimension on the electrochemical performance of cathode and anode materials. Their synthesis processes, electrochemical properties and electrode reaction mechanisms are briefly discussed and summarized. Furthermore, the article highlights recent past scientific works and new progresses in the field of LIBs. It also highlights the direction to overcome the existing issues of current lithium storage technology. In future, we may overcome all the existing issues of LIBs and can deliver excellent cathode and anode combinations to fulfill maximum practical efficiency with low cost and ultimate safety for high end applications. Keywords: Anode, cathode, lithium-ion battery, nanodimension, nanomaterials.

Redox behavior of juglone in buffered aq.: Ethanol media

Abstract The present work reports the redox mechanism of 5-hydroxynaphthalene-1,4-dione (HND), commonly known as juglone, in buffered aqueous media having 50% of ethanol. HND followed different mechanistic routes depending upon the pH of the media and more than one pKa were evaluated from the changes in the slope of the Ep vs. pH plot. The change of pH from acidic to neutral conditions was found to switch the mechanism from CEC to EE mechanism. Pulse techniques were utilized to determine the number of electrons involved in the oxidation and/or the reduction step and to ensure the nature of the redox process. Based upon the obtained results, an electrode reaction mechanism was proposed. Computational studies of HND supported the experimental results. UV-Visible spectroscopy was also employed for the detailed characterization of the compound in a wide range of pH and for the determination of its pKa.

Study of spinel Li4Ti5O12 electrode reaction mechanism by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) for the initial Li-ion insertion and extraction in Li4Ti5O12 (LTO) anode at different potentials were studied. At the intermediate stage of Li-ion insertion process, as the frequency decreased, the EIS spectra exhibited three semicircles and a slightly inclined line, which correspond to the Schottky contact reflecting the electronic properties of the material, the charge transfer step and solid state diffusion process respectively. At the intermediate stage of Li-ion extraction process, it was observed that the low-frequency semicircle turned into two joint semicircles. This interesting and significant phenomenon was attributed to gas generation in LTO cells. Kinetic parameters obtained from fitting the experimental impedance spectra in the first charge–discharge cycle on LTO side have been analyzed in detail. A new model was proposed to interpret the mechanism of Li-ion insertion/extraction reaction along with side reactions (gas generation) in the LTO electrode.

Mechanisms and kinetics of electrode processes at bismuth and antimony film and bare glassy carbon surfaces under square-wave anodic stripping voltammetry conditions

Mechanism and kinetics of anodic stripping electrode processes of Zn(II), Cd(II) and Pb(II) at bismuth (BiFE) and antimony (SbFE) film as well as bare glassy carbon (GCE) electrodes have been studied with respect to our recent published theoretical considerations developed for square-wave voltammetry (SWV). Mechanistic aspects of electrode reactions have been elucidated by a qualitative comparison of simulated and experimental data for three different electrode reaction mechanisms, i.e. simple anodic stripping mechanism, anodic stripping mechanism coupled with adsorption of metal analyte ions, and mechanism affected by interactions within the particles of the metal deposit on the electrode surface. The electrode kinetics has been estimated by the method of quasireversible maximum and by analyzing the peak potential separation between forward and backward SW components, under variation of the SW amplitude. Electrode mechanisms and kinetics are different at bismuth compared to antimony film electrodes. The electrode reactions of Pb(II) and Cd(II) at BiFE involve adsorption phenomena, while electrode processes of all three analytes are free of adsorption at SbFE. At BiFE, the electron transfer standard rate constants of all three analytes are quite comparable, ranging within the interval from 1 to 3cms−1. At SbFE the electrode kinetics of Cd(II) and Pb(II) are almost the same, while the electrode reaction of Zn(II) is significantly slower. Our results showed that the kinetics of all examined electrode reactions are slower at SbFE as compared to those observed at BiFE.

Kinetics and Mechanism of [PdClx(H2O)4 − x]2 − x(x = 3,4) Complexes Electro-Reduction

This work presents an analysis of the kinetics and mechanism of electrode reactions taking place during polarization of an Au polycrystalline electrode in the solution containing aqua- and aquachloro- complexes of palladium(II). Stability range of [PdClx(H2O)4 − x]2 − x(x = 3,4) was defined from the thermodynamic analysis of the electrolyte. Spectrophotometric measurements were applied for stability tests of complexes present in the electrolyte. Besides classical electrochemical tests, electrogravimetric tests were also performed. Determined values of M/z demonstrated that a metallic Pd deposit is formed as a result of a 2-electron reduction reaction of Pd(II) complexes. A deposition started from potential of ∼0.5 V. Initially M/z values suggest a coarse palladium(II) complexes adsorption. All the examined electrode reactions of [PdClx(H2O)4 − x]2 − x(x = 3,4) took place within the overpotential range and were diffusion controlled. Diffusion coefficient of [PdClx(H2O)4 − x]2 − x(x = 3,4) was determined with the use of Berzins – Delahay equation. Cyclic voltammograms (CV) indicated also reactions of hydrogen adsorption and absorption by Pd deposit formed during cathodic sweep.

Electrochemical investigations of unexplored anthraquinones and their DNA binding

The redox behaviour of two potential anticancer anthraquinones, 9,10-anthraquinone and 2-chloromethyl-9,10-anthraquinone was investigated in a wide pH range. Cyclic voltammetry based assay was developed for the assessment of the effect of medium, substituents, potential scan rate and number of scans on the voltammetric response of anthraquinones. The electrode reaction mechanism was suggested on the basis of cyclic and differential pulse voltammetric results. The effect of DNA on anthraquinones was also probed at physiological pH which could lead to further investigation of possible citotoxic activity in vitro. The results revealed that anthraquinones interact with DNA more strongly than the clinically used anticancer drug, epirubicin.

Microstructural Insights into Dual-Phase Infiltrated Solid Oxide Fuel Cell Electrodes

3D microstructure of solid oxide fuel cell (SOFC) electrodes by a dual-phase infiltration procedure is constructed numerically from a phenomenological standpoint. The present work studies such a dual-phase infiltration procedure, from generation of backbone matrix to infiltration of backbone phase nanoparticles, followed by infiltration of electrocatalytic nanoparticles. Important geometric properties are calculated under various electrocatalytic nanoparticle loadings, including total and percolated surface areas and percolation probability of electrocatalytic nanoparticles, and total and percolated three-phase boundary (TPB) lengths. The effects of backbone nanoparticles, including infiltration loading, particle size and aggregation risk are studied systematically. One important finding is that the infiltration of backbone nanoparticles increases TPB length but shows negligible influence on the surface area of electrocatalytic nanoparticles. It demonstrates that the dual-phase infiltration has little advantage to reduce electrode resistance compared to the catalyst-phase infiltration when electrode reaction is limited to catalytic surface. However, the different influences of dual-phase infiltration on TPB length and surface area of electrocatalytic nanoparticles offers a potential strategy to identify electrode reaction mechanisms.

Geometric Properties of Nanostructured Solid Oxide Fuel Cell Electrodes

3D microstructures for nanostructured solid oxide fuel cell (SOFC) electrodes fabricated by infiltration/impregnation method are constructed numerically, by using a phenomenological procedure. Key geometric properties of the constructed electrodes are calculated at various infiltration loadings, including the percolation probabilities of pores and infiltrated nanoparticles, the total and active three-phase boundary (TPB) length, backbone and nanoparticles surface areas, and backbone-nanoparticles boundary area. The effects of backbone particle size, backbone porosity, nanoparticle size, and its aggregation risk are studied systematically. Analytical models are developed to predict these geometric properties, and agree well with the numerical infiltration results, as well as the literature data. It is found that the peak TPB length can be achieved at 63% coverage of the backbone surface by infiltrated nanoparticles. More interestingly, the backbone structure has little effect on nanoparticles surface area, but significantly affects TPB length, suggesting an strategy to identify electrode reaction mechanisms. Decreasing infiltrated particle size increases its surface area, enhances the peak TPB length, and decreases the optimal infiltration loading, indicating small infiltrated particles essentially benefits electrode performance. The results provide valuable information for understanding the geometric properties of the infiltrated SOFC electrodes and contribute to the design of high performance SOFC electrodes.

Electrochemical behavior of Zn/Zn(II) couples in aprotic ionic liquids based on pyrrolidinium and imidazolium cations and bis(trifluoromethanesulfonyl)imide and dicyanamide anions

Motivated by the potential of using room temperature ionic liquids (RTILs) as electrolytes to replace traditional aqueous electrolytes for Zn-anode secondary batteries, Zn/Zn(II) redox reactions have been studied in four aprotic RTILs based on pyrrolidinium ([Pyrr]+) and imidazolium ([Im]+) cations, and bis(trifluoromethanesulfonyl)imide ([TFSI]−) and dicyanamide ([DCA]−) anions. Cyclic voltammetry results suggest a smaller overpotential for Zn redox in [Im]+ cation based and [DCA]− anion based RTILs than in [Pyrr]+ and [TFSI]− based RTILs. Potentiodynamic polarization experiments indicate a strong dependence of the electrode reaction mechanism for the Zn species on the RTIL anions. In [TFSI]− based RTILs, Zn2+ ions are the electroactive species, with the electrode reaction being a single-step, two-electron transfer process. In [DCA]− based RTILs, two-step, single-electron reactions account for the electrode mechanism. The exchange current densities derived from Tafel analysis for the Zn species in the four RTILs are greater than 10−3mA/cm2, with the [Im]+ cation based RTIL possessing the highest value of 9.9×10−3mA/cm2. The results obtained will assist in obtaining a better understanding of the electrochemical behavior of Zn in RTILs, shedding light on the development of RTILs for Zn-anode secondary batteries.

A graph theoretical approach to the elucidation of reaction mechanisms: Analysis of the chlorine electrode reaction

A methodology and algorithm are proposed in this work to conceive a limited set of dependent elementary reaction steps for a reaction mechanism by exploiting the reaction route (RR) graph of the mechanism. These steps are constrained by the conservation principles and must be subjected to thermodynamic feasibility studies. This method is expected to aid in the conception of new steps, which might otherwise be ignored, especially in the case of large mechanisms. In order to illustrate the method, it is applied to the chlorine electrode reaction mechanism. The result indicated that a reaction pathway consisting of one of the newly proposed steps is the dominant one. This example illustrated the potential of the proposed method to identify new steps to a mechanism and thereby help in mechanism elucidation. However, these new steps must be subjected to further critical study before they can be accepted.