Reversible Electrode Processes Research Articles

Genuine anodic and cathodic current components in cyclic voltammetry

Implicit anodic and cathodic current components associated with the real net current at a given potential of a simple quasireversible electrode reaction can be accurately estimated using basic mathematical modeling within the framework of Butler-Volmer electrode kinetics. This methodology requires only prior knowledge of the formal potential of the dissolved redox couple, offering direct insight into the electrode kinetics. The proposed approach facilitates a unique transformation of a conventional cyclic voltammogram, allowing the replacement of the common, net current with authentic anodic and cathodic current components. This simple methodology introduces a novel perspective in analyzing voltammetric data, particularly enabling the kinetic characterization of fast, seemingly electrochemically reversible electrode processes on macroscopic electrodes at slow scan rates. Theoretical predictions are experimentally demonstrated using the electrode reaction for the reduction of the hexaammineruthenium(III) complex, serving as an example of one of the fastest electrode processes involving a dissolved redox species.

Genuine differential voltammetry

A novel form of differential voltammetry is proposed, developed through the implicit anodic and cathodic current components of the experimentally accessible conventional net current measured in a voltammetric experiment. By employing basic mathematical modelling of an electrode reaction of a dissolved redox couple at a conventional, macroscopic electrode within the framework of the Butler-Volmer electrode kinetic model, the implicit anodic and cathodic current components of the net conventional current are clearly defined and can be estimated. Consequently, a novel form of differential current, calculated as the difference between anodic and cathodic implicit current components associated with a single potential of the voltammetric experiment, can be established. This differential current demonstrates remarkable characteristics in terms of electrode kinetics and analytical performance, particularly in cases involving fast, seemingly electrochemically reversible electrode processes. It holds promise to be analytically superior to the best-known differential voltammetric techniques so far (e.g., square-wave voltammetry), as well as provides a means for estimating the rate constants of very fast, apparently reversible electrode processes at macroscopic electrodes under mild experimental conditions (i.e., studied at slow potential scan rates). The practical implication of the novel methodology is significant: a simple linear sweep voltammogram of a quasi-reversible electrode reaction with unknown electrode kinetic parameters can be readily transformed into the novel type of differential voltammogram through a convolution procedure of the conventional net current, paving a new way for studying electrode processes by voltammetry.

Electrochemical properties of Gd(III) ions in LiCl-KCl-GdCl3 at 723–1023 K

Electrochemical behavior of Gd(III) ions in molten LiCl-KCl-GdCl3 was investigated at 723–1023 K via cyclic voltammetry using tungsten as a working electrode, Gd-Bi (mole fraction, xGd = 0.16) as a reference electrode, and Gd-Bi (xGd = 0.02) as a counter electrode. A single reduction–oxidation wave was observed, confirming a single-step, 3-electron transfer Gd(III)/Gd transition. The cathodic peak potential exhibited minimal change (<13 mV) over a wide range of scan rates (0.05–0.30 V s−1), indicating facile charge transfer kinetics (i.e., a reversible electrode process). A nucleation overpotential associated with solid Gd deposition was observable at low temperatures (T < 823 K). The mass transport properties of Gd(III) ions were estimated using the Berzins and Delahay relation based on diffusion-limiting peak current. The diffusivity values were determined to be DGd(III) = 0.5–2.7 × 10−5 cm2 s−1 at 723–1023 K with an associated activation energy of Ea = 33.9 (±1.0) kJ mol−1. The two-phase [liquid + GdBi] Gd-Bi alloy reference electrode experienced less than 0.5 mV of drift over 5 days of repeated electrochemical measurements, indicating high stability.

Electrochemical sensor modified with gold and arenediazonium salts for the determination of copper and mercury in human’s hair

Current article presents the results of using a solid carbon-containing electrochemical sensor modified with arenediazonium salts (ME-ADT-COOH) for the joint determination of copper and mercury in human hair. The sensitivity of the determination of copper and mercury ions using a graphite electrode modified with gold and arenediazonium salts with a substituent carboxy group is higher compared to the other carbon-containing modified electrodes. The influence of the concentration of arenediazonium tosylates with various substituents on the obtained analytical signal was studied. The conditions for the manufacturing the new carbon-containing modified electrochemical sensor have been developed. The effective surface area of the modified electrode was calculated, which was estimated using a cyclic voltammogram against the background of 0.1 M KCl with the addition of potassium hexacyanoferrate (С=5*10-4mol/l) salts and calculated using the Randles-Ševčík equation for the reversible electrode process. A linear range of determined concentrations in the voltametric determination of microelements on the arendiazonium-modified gold-graphite electrode was established - from 0.1 to 12 µg/g. The error of the determination did not exceed 25 %. The correctness of the results of the determination of copper and mercury in real objects by the “introduced-found” method, which correlates well with the known values, has been verified.

How to appropriately assess the oxygen reduction reaction activity of platinum group metal catalysts with rotating disk electrode

SummaryThe sluggish oxygen reduction reaction (ORR) has becoming the bottleneck of largescale implementation of proton exchange membrane fuel cells. However, when it comes to the ORR activity assessing of platinum group metals (PGMs) with rotating disk electrode, the corresponding potential conversion vs. reversible hydrogen electrode, test protocols, and activity calculation processes are still in chaos in many published literatures. In this work, two standard calculation processes for PGM ORR activities are demonstrated, followed by a specification for the usage of reference electrodes. Then a 4-fold discrepancy in ORR activities obtained via different test protocols is found for the same Pt/C, and an average adsorption model and the “coverage effects” are proposed to illustrate the hysteresis loop between negative and positive-going ORR polarization plots. Finally, four motions over appropriate assessment of PGM ORR activity are emphasized, hoping to bring a fair communication platform for researchers from different groups.

How to Appropriately Assess the Oxygen Reduction Reaction Activity of Platinum Group Metal Catalysts with Rotating Disk Electrode

The sluggish oxygen reduction reaction (ORR) has becoming the bottleneck of largescale implementation of proton exchange membrane fuel cells (PEMFCs). However, when it comes to the ORR activity assessing of platinum group metals (PGMs) with rotating disk electrode, the corresponding potential conversion vs. reversible hydrogen electrode (RHE), test protocols and activity calculation processes are still in chaos in many published literatures. In this work, two standard calculation processes for PGM ORR activities are demonstrated, followed by a specification for the usage of reference electrodes (REs). Then a 4-fold discrepancy in ORR activities obtained via different test protocols is found for the same Pt/C, and an average adsorption model and the “coverage effects” are proposed to illustrate the hysteresis loop between negative and positive-going ORR polarization plots. Finally, four motions over appropriate assessment of PGM ORR activity are emphasized, hoping to bring a fair communication platform for researchers from different groups.

Phenolcarboxylic Acids in the Reaction with Electrogenerated Hydroperoxyl Radical

Using the chronovoltammetric method, it has been shown that the reduction of oxygen on a mercury electrode in the presence of phenolcarboxylic acids in an acidic medium occurs as reversible electrode process followed by rate-limiting chemical reaction involving the hydroperoxyl radical. The rate constants of the phenolcarboxylic acids reaction with the radical have been calculated using the theory of electrode process followed by chemical reaction. The suggested reaction mechanism (electron transfer from the acid molecular form to the radical) has been confirmed by the correlation between the experimental rate constants and the ionization potentials of phenolcarboxylic acids simulated using the DFT method.

Non-unity stoichiometric reversible electrode reactions. The effect of coupled kinetics and the oxidation of bromide

We explore via simulation, the cyclic voltammetry of the following mechanism involving a reversible electrode process coupled with a follow up reaction in which the product (B) of the electrode reaction reacts chemically with the reactant, A, to form a species C:2A−2e−⇄BB+A⇌kbkfC Such a mechanism is shown, under certain critical combinations of rate constant and reactant concentration to give rise to two voltammetric peaks despite the assumption of a single redox couple (A/B). This results from the formation of C during the first peak whilst the second peak, seen at more oxidizing potentials, reflects the dissociation of C releasing A. A mechanism of the type investigated may hold during the oxidation of bromide(Br−, A) in solution to form bromine(Br2, B) and tribromide (Br3−,C). Simulations are used to explore the conditions for which two peaks are expected in the bromide oxidation system in aqueous solution and for which the two following limiting cases operate3Br−−2e−⇄Br3− and2Br−−2e−⇄Br2 We report experimental data for the bromide oxidation in aqueous solution. These are interpreted via simulations using independent literature values for the rate and equilibrium constants for tribromide formation.

CHRONOVOLTAMMETRIC STUDY OF ANTIRADICAL PROPERTIES OF FLAVONOIDS IN REACTION WITH REACTIVE OXYGEN SPECIES

Using the chronovoltammetric method it was shown that on a mercury film electrode in an acidic medium the reduction of oxygen to hydrogen peroxide in the presence of flavonoids is realized as a reversible electrode process followed by a first-order limiting chemical reaction with the participation of the primary product of the electrode reaction – the hydroperoxyl radical. For this type of reaction, in the case of reduction processes, the peak of potential shifts relative to the reversible half-wave potential in the direction of positive potentials. This shift is the larger then the higher the rate constant of the subsequent chemical reaction and the lower the sweep speed of the polarization voltage. To calculate the constants, the half-wave potential of oxygen electroreduction is determined without the addition of a flavonoid. Its value equals to –0.27 V and can be used both in acidic and alkaline media, since it does not change on mercury electrodes in the pH range of 1–9. The reaction rate constants of flavonoids with hydroperoxyl are a characteristic of their antiradical activity. The flavonone groups are the most reactive towards the radical compounds – morin and myricetin. For all studied compounds, the number of electrons participating in the limiting stage varies from 0.96 to 1. This indicates the occurrence of an elementary chemical act of a flavonoid with a radical and the absence of an effect on the kinetics of the investigated reaction of the products of the conversion of flavonoids. The applicability of the presented method for calculating the rate constants to reactions involving active oxygen forms is confirmed by the presence of a high correlation between the data obtained and the reaction rate constants of flavonoids with 2,2′-diphenyl-1-picrylhydrazyl radical.

General Explicit Mathematical Solution for the Voltammetry of Nonunity Stoichiometry Electrode Reactions: Diagnosis Criteria in Cyclic Voltammetry.

Electrochemical reactions can effectively follow nonunity stoichiometries as can be found in the electrochemistry of halides, hydrogen, and metal complexes. The voltammetric response of these systems shows peculiar deviations with respect to the well-described features of the 1:1 stoichiometry. With the aim of specifying such differences, a rigorous and manageable analytical theory is deduced for the complete characterization of reversible electrode processes with complex stoichiometry in cyclic voltammetry (CV) at macroelectrodes. Particularly, the main features of the CV of 2:1, 1:2, 3:1, and 1:3 processes (that is, the peak currents and potentials and the influence of the scan rate and of the species concentration and diffusion coefficients) are given and compared with the 1:1 case in order to propose unambiguous diagnostic criteria of the stoichiometry of the electrode reaction. Also, expressions for the concentration profiles and surface concentrations of the redox species are given.

High-Energy and High-Power Nonaqueous Lithium-Ion Capacitors Based on Polypyrrole/Carbon Nanotube Composites as Pseudocapacitive Cathodes.

The energy density of present lithium-ion capacitors (LICs) is greatly hindered by the limited specific capacities of porous carbon electrodes. Herein, we report the development of a nonaqueous LIC system by integrating two reversible electrode processes, that is, anion doping/undoping in a core-shell structured polypyrrole/carbon nanotube (Ppy@CNT) composite cathode and Li+ intercalation/deintercalation in a Fe3O4@carbon (C) anode. The hybrid Ppy@CNT is utilized as a promising pseudocapacitive cathode for nonaqueous LIC applications. The Ppy provides high pseudocapacitance via the doping/undoping reaction with PF6- anions. Meanwhile, the CNT backbone significantly enhances the electrical conductivity. The as-developed composite delivers noteworthy capacities with exceptional stability (98.7 mA h g-1 at 0.1 A g-1 and retains 89.7% after cycling at 3 A g-1 for 1000 times in Li-half cell), which outperforms state-of-art porous carbon cathodes in present LICs. Furthermore, when paired with Fe3O4@C anodes, the as-developed LICs demonstrate a superior energy density of 101.0 W h kg-1 at 2709 W kg-1 and still maintain 70 W h kg-1 at an increased power density of 17 186 W kg-1. The findings of this work provides new knowledge on the cathode materials for LICs.

Electrochemical Kinetics of Diffusion Problems with a Moving Phase Boundary

The growth and dissolution of nuclei and a deposit film on a solid electrode are considered. The diffusion problems of the growth and dissolution of hemispherical nuclei and a thin deposit film are solved. Equations are derived to describe the I–V characteristics and chronoamperograms of reversible, irreversible, and quasi-reversible electrode processes, and the dependences of the peak current, the amount of deposit on an electrode, the potential of the peak current, and the full width at half-maximum of an I–V characteristic on the preelectrolysis parameters are determined. The formation of a new phase at the solid–solution interface and the mass-exchange processes at the weakly soluble film–solution interface are discussed. Anodic films are experimentally studied. In the case of nuclei or a thin anodic film, the amount of compound formed on the electrode surface linearly depends on the ionic concentration and the preelectrolysis time.

Electrochemical properties and sensing applications of nanocarbons: A comparative study

Nanocarbons like graphene, carbon nanotubes, and expanded graphite feature different morphology and surface functional groups. They are thus expected to have different properties and applications. Herein, electrochemical properties of several nanocarbons including graphene nano platelets (GNPs), multi-walled carbon nanotubes (MWCNTs), graphene oxide (GO), aminated graphene (NH2-G), expanded graphite (EG) have been studied together with their composites of EG. Reversible and diffusion-controlled electrode processes for faradaic reactions of Fe(CN)63−/4− are seen on all these electrodes. The capacitive current of MWCNTs is the biggest, while that of GO is the smallest. The composites of EG with nanocarbons feature bigger active areas than nanocarbons but smaller capacitive currents. Voltammetric response of positively and negatively charged inorganic ions (Pb2+, NO2−) as well as neutral organic colorants (sunset yellow and tartrazine) has been studied on these nanocarbons and their composites. The sensitive monitoring of three species has been realized on different nanocarbons or composites. Such a phenomenon is intercepted by different interactions of charged or neutral target species with nanocarbons or the composites. This study provides a systematic investigation of electrochemistry of nanocarbons and their composites. It is thus of significance to explore different electrochemical applications of these nanocarbons and their composites.

NiCo2O4 surface coating Li[Ni0.03Mn1.97]O4 micro-/nano- spheres as cathode material for high-performance lithium ion battery

Abstract Here we report a novel transitional metal oxide (NiCo 2 O 4 ) coated Li[Ni 0.03 Mn 1.97 ]O 4 micro-/nano- spheres as high-performance Li-ion battery cathode material. A thin layer of ∼10 nm NiCo 2 O 4 was formed by simple wet-chemistry approach adjacent to the surface of Li[Ni 0.03 Mn 1.97 ]O 4 micro-/nano- spheres, leading to significantly enhanced battery electrochemical performance. The optimized sample(1 wt%) not only delivers excellent discharge capacity and cycling stability improvement at both room temperature and elevated temperatures, but also effectively prevents Mn dissolution while retaining its coating structure intact according to XRF and TEM results. The CV and EIS break-down analysis indicated a much faster electrochemical reaction kinetics, more reversible electrode process and greatly reduced charge transfer and Warburg resistance, clearly illustrating the dual role of NiCo 2 O 4 coating to boost electron transport and Li + diffusion, and alleviation of manganese dissolving. This approach may render as an efficient technique to realize high-performance lithium ion battery cathode material.

High Volumetric Energy Density Capacitors Based on New Electrode Material Lanthanum Nitride

LaN is synthesized via calcining La2O3 in NH3 and studied as capacitive material for energy storage. A volumetric capacitance of 951.3 F cm–3 was found in 1 mol dm–3 Na2SO4 using a current density of 1 A g–1, with less than 1% loss of capacitance being experienced after 5000 cycles. In addition, 87.3% of the initial capacitance remained at a current density of 10 A g–1. LaN exhibits high capacitance that is attributed to subsurface space charge accumulation with a possible electric double-layer capacitor component. A reversible electrode process ensures long cycle life and favorable electrical charge transfer. The assembled LaN symmetrical capacitor showed high volumetric energy densities, facilitating high-duty applications.

Higher order derivative voltammetry for reversible and irreversible electrode processes under spherical diffusion

Theoretical expressions for the first, second and third derivatives of voltammetric curves are analytically derived for reversible, quasi-reversible, and irreversible processes under spherical diffusion. The shapes of the curves are analyzed n terms of peak-potentials, peak-currents, and peak-widths, and the differences and ratios among them. The results obtained with spherical electrodes are compared with those with the planar electrodes, which exhibits striking differences between two electrodes. Derived parameters – such as ratios of peak-currents (ipa/ipc), and ratios of half-peak-widths (W½a/W½c), and ratios of the differences in peak potentials (ΔEpa/ΔEpc), for various derivatives are analyzed. As electrode sphericity increases, these ratios (i. e., measures of symmetry in the curves) for a quasi-reversible and irreversible electron transfer process approaches to one, which is the same as that for a simple reversible electrode process on a planar electrode. Namely, the asymmetry which was exhibited on planar electrodes for quasi and irreversible processes disappears on a spherical electrode. This suggests that the planar electrode is better suited for kinetic study of slower electron transfer than spherical electrodes for this derivative approach.

Capacitive Intermetallic Manganese Nitride with High Volumetric Energy Densities

A capacitive intermetallic MnN was synthesized via nitridation of MnO2 at 800°C under NH3 atmosphere. A volumetric capacitance of 1044.3 F cm−3 was found in 1 mol dm−3 Na2SO4 (aq) using a current density of 1 A g−1, and less than 1% of capacitance was lost after 5000 ladder cycles. 89.3% of the initial volumetric capacitance remained at a current density of 10 A g−1. A reversible electrode process controlled by diffusion and a high electrical conductivity during charging-discharging ensured a long cycle life. The assembled symmetrical capacitor of MnN/ Na2SO4 (aq)/ MnN shows a high volumetric energy density of 112.2 W h L−1, facilitating heavy-duty applications.

Aqueous Rechargeable Alkaline CoxNi2-xS2/TiO2 Battery.

An electrochemical energy storage system with high energy density, stringent safety, and reliability is highly desirable for next-generation energy storage devices. Here an aqueous rechargeable alkaline CoxNi2-xS2 // TiO2 battery system is designed by integrating two reversible electrode processes associated with OH(-) insertion/extraction in the cathode part and Li ion insertion/extraction in the anode part, respectively. The prototype CoxNi2-xS2 // TiO2 battery is able to deliver high energy/power densities of 83.7 Wh/kg at 609 W/kg (based on the total mass of active materials) and good cycling stabilities (capacity retention 75.2% after 1000 charge/discharge cycles). A maximum volumetric energy density of 21 Wh/l (based on the whole packaged cell) has been achieved, which is comparable to that of a thin-film battery and better than that of typical commercial supercapacitors, benefiting from the unique battery and hierarchical electrode design. This hybrid system would enrich the existing aqueous rechargeable LIB chemistry and be a promising battery technology for large-scale energy storage.

Surface modification imparts selectivity, facilitating redox catalytic studies: quinone mediated oxygen reduction

Modifying a gold electrode surface with hydroxy-alkyl-thiols significantly reduces the observed rate of electron transfer. This designed and controlled decrease in electron transfer rate readily allows reversible and irreversible electrode processes to be more clearly delineated. Separation of such voltammetric responses can facilitate the direct study of redox catalytic processes, which would, under other experimental conditions be obscured. The reduction of oxygen by anthraquinone hydroxy derivatives are used as a paradigmatic example, demonstrating the importance of the hydroxyl groups in the reduction process. Specifically, the reduced form of 1,8-dihydroxy-anthraquinone is shown to be significantly more reactive towards oxygen reduction than the 1,4-dihydroxyl analogue.

On the meaning of the diffusion layer thickness for slow electrode reactions

A key concept underpinning electrochemical science is that of the diffusion layer - the zone of depletion around an electrode accompanying electrolysis. The size of this zone can be found either from the simulated or measured concentration profiles (yielding the 'true' diffusion layer thickness) or, in the case of the Nernst ('linear') diffusion layer by extrapolating the concentration gradient at the electrode surface to the distance at which the concentration takes its bulk value. The latter concept is very well developed in the case of fast (so-called reversible) electrode processes, however the study of the linear diffusion layer has received scant attention in the case of slow charge transfer processes, despite its study being of great interest in the analysis of the influence of different experimental variables which determine the electrochemical response. Analytical explicit solutions for the concentration profiles, surface concentrations and real and linear diffusion layers corresponding to the application of a potential step to a slow charge transfer process are presented. From these expressions the dependence of the diffusion layer thickness on the potential, pulse time, heterogeneous rate constant and ratio of bulk concentrations of electroactive species and of diffusion coefficients is quantified. A profound influence of the reversibility degree of the charge transfer on the diffusion layer thickness is clear, showing that for non-reversible processes the real and linear diffusion layers reveal a minimum thickness which coincides with the equilibrium potential of the redox couple in the former case and with the reversible half-wave potential in the latter one.