Tafel Plot Analysis Research Articles

Redox Switching and Oxygen Evolution at Hydrous Nickel Oxide Films in Aqueous Alkaline Solution

The redox switching reaction and the oxygen evolution reaction at multi-cycled nickel oxy-hydroxide films in aqueous alkaline solution are discussed. The hydrous oxide is considered as a porous assembly of interlinked octahedrally coordinated anionic metal oxyhydroxide surfaquo complexes which form an open network structure. The latter contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution. The dynamics of redox switching has been quantified in terms of a diffusive frequency using the Laviron-Aoki diffusion model. Steady state Tafel plot analysis has been used to elucidate the kinetics and mechanism of oxygen evolution with slopes of ca. 60 mVdec-1 and ca. 120 mVdec-1 at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity remains invariant at ca. 1.0 as the potential is increased. These observations are rationalized in terms of a kinetic scheme involving surfaquo groups.

The Oxygen Evolution Reaction at Hydrous Iron Oxide Films in Base: Kinetics and Mechanism

The oxygen evolution reaction at multi-cycled iron oxy-hydroxide films in aqueous alkaline solution is discussed. Steady-state Tafel plot analysis and electrochemical impedance spectroscopy have been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slopes of ca. 60 mV dec−1 and ca. 120 mV dec−1 are found at low and high overpotentials, respectively, with the former reducing to 40 mV dec−1 for very thick films. A reaction order of ca. 1.0 is observed irrespective of overpotential and film thickness. A mechanistic scheme involving the active participation of octahedrally coordinated anionic iron oxyhydroxide surfaquo complexes, which form the porous hydrous layer, is proposed. The latter structure contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution.

Electrochemical Properties of Oxo–Manganese Complex Biomimicking Enzyme Active Sites and Its Electrocatalytic Application for Dopamine Determination

This work describes the characterization of the [Mn2IV,IVO2(terpy)2(H2O)2]4+ complex in aqueous solution by UV–vis spectrophotometry, cyclic voltammetry, and linear sweep voltammetry with a rotating disk electrode. The pH effect, potential scan rate, effect of perfluorosulfonate polymer, and anion of supporting electrode on the electrochemical behavior of the modified electrode for better performance were investigated. The potential peak of the modified electrode was linearly dependent upon the ratio [ionic charge]/[ionic radius]. The modified electrode exerted an electrocatalytic effect on dopamine oxidation in aqueous solution with a decrease in the overpotential compared with the unmodified glassy carbon electrode. This way, the modified electrode showed an enzymatic biomimicking behavior. Tafel plot analyses were used to elucidate the kinetics and mechanism of dopamine oxidation.

Kinetics and Mechanistic Aspects of the Oxygen Evolution Reaction at Hydrous Iron Oxide Films in Base

The oxygen evolution reaction at multi-cycled iron oxy-hydroxide films in aqueous alkaline solution is discussed. The factors affecting the growth and electrocatalytic properties of the latter are described. In particular, steady-state Tafel plot analysis and electrochemical impedance spectroscopy have been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slopes of ca. 60 mV dec−1 and 40 mV dec−1 are found at low overpotentials depending on the oxide growth conditions, with an apparent Tafel slope of ca. 120 mV dec−1 at high overpotentials. Reaction orders of ca. 0.5, 1.0 and 1.5 are observed at low and high overpotentials, again depending on the oxide growth conditions. A mechanistic scheme involving the active participation of octahedrally coordinated anionic iron oxyhydroxide surfaquo complexes, which form the porous hydrous layer, is proposed. The latter structure contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution. This work brings together current research in both heterogeneous electrocatalysis and homogeneous molecular catalysis for water oxidation.

An electrochemical impedance study of the oxygen evolution reaction at hydrous iron oxide in base

The oxygen evolution reaction at multi-cycled iron oxy-hydroxide films in aqueous alkaline solution is discussed. Steady-state Tafel plot analysis and electrochemical impedance spectroscopy have been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slopes of ca. 60 mV dec(-1) and 40 mV dec(-1) are found at low overpotentials depending on the oxide growth conditions, with an apparent Tafel slope of ca. 120 mV dec(-1) at high overpotentials. Reaction orders of ca. 0.5 and 1.0 are observed at low and high overpotentials, again depending on the oxide growth conditions. A mechanistic scheme involving the active participation of octahedrally coordinated anionic iron oxyhydroxide surfaquo complexes, which form the porous hydrous layer, is proposed. The latter structure contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution. This work brings together current research in heterogeneous electrocatalysis and homogeneous molecular catalysis for water oxidation.

Improved active anticorrosion coatings using layer-by-layer assembled ZnO nanocontainers with benzotriazole

In the present work, a novel approach for the synthesis of nanocontainers by encapsulation has been presented, which are capable of responsive release of corrosion inhibitors (benzotriazole). Nanocontainers have been prepared using layer-by-layer (LbL) assembly of oppositely charged species of polyelectrolytes and inhibitor on the surface of ZnO. Synthesis of nanostructured zinc oxide was carried out initially using the sonochemical precipitation approach. The thickness of the layer, surface charge and functional groups present on each layer were identified using particle size distribution (PSD), zeta potential and FTIR analysis, respectively. The average initial size of the core ZnO was in nanosize range as observed in the TEM image. XRD analysis shows that ZnO contains wurtzite as a major phase. The release properties of polyelectrolyte modified ZnO nanocontainer have been investigated and the observed release rate characteristics show that these nanocontainers would be useful in the multifunctional coating formulations. Responsive release of inhibitor was quantitatively evaluated in water at different pH. Performance of nanocontainers was studied in corrosive environment by incorporating the nanocontainers in alkyd resin. Results of UV-spectroscopy at different pH, corrosion rate analysis and TAFEL plots of nanocontainers coatings on mild steel (MS) panel have been presented. It has been found that electrochemical current density decreased from 0.00355 to 0.0008A/cm2, when neat alkyd resin combined with 5% nanocontainers has been used in the coating. Corrosion results from Tafel plot and corrosion rate analysis shows that the 5wt% loading of nanocontainers is useful and optimum for the sustained release of inhibitor for the applications in the marine coatings irrespective of the operating pH.

Development of an electrochemical sensor for determination of dissolved oxygen by nickel–salen polymeric film modified electrode

Abstract An amperometric oxygen sensor based on a polymeric nickel– salen ( salen = N , N ′-ethylene bis (salicylideneiminato)) film coated platinum electrode was developed. The sensor was constructed by electropolymerization of nickel– salen complex at platinum electrode in acetonitrile/tetrabutylammonium perchlorate by cyclic voltammetry. The voltammetric behavior of the sensor was investigated in 0.5 mol L −1 KCl solution in the absence and presence of molecular oxygen. Thus, with the addition of oxygen to the solution, the increase of cathodic peak current (at −0.25 V vs. saturated calomel electrode (SCE)) of the modified electrode was observed. This result shows that the nickel– salen film on electrode surface promotes the reduction of oxygen. The reaction can be brought about electrochemically, where the nickel(II) complex is first reduced to a nickel(I) complex at the electrode surface. The nickel(I) complex then undergoes a catalytic oxidation by the molecular oxygen in solution back to the nickel(II) complex, which can then be electrochemically re-reduced to produce an enhancement of the cathodic current. The Tafel plot analyses have been used to elucidate the kinetics and mechanism of the oxygen reduction. A plot of the cathodic current vs. the dissolved oxygen concentration for chronoamperometry (fixed potential = −0.25 V vs. SCE) at the sensor was linear in the 3.95–9.20 mg L −1 concentration range and the concentration limit was 0.17 mg L −1 O 2 . The proposed electrode is useful for the quality control and routine analysis of dissolved oxygen in commercial samples and environmental water. The results obtained for the levels of dissolved oxygen are in agreement with the results obtained with a commercial O 2 sensor.

Effect of Charge Current Density on Electrochemical Performance of Fe/C Electrodes in Alkaline Solutions

A composite Fe2O3/C powder was applied as a negative material in iron/air batteries. The effect of charge current density on the electrochemical behavior of an electrode in KOH electrolyte without or with K2S additives was investigated. In KOH electrolyte, the discharge capacity increased in proportion to the logarithm of charge current density in the range of 10–40 mA cm−2. According to an analysis of Tafel plots of the electrodes, the hydrogen evolution was found to reach its diffusion limit earlier than the reduction reaction of Fe(OH)2 to Fe as charge current density increased. This was deduced to be the reason for the increase of discharge capacity with current density. But as the density increased further, the discharge capacity decreased because both above reactions reached their diffusion limits. When K2S additives were used, a similar phenomenon was observed, with a higher discharge capacity than that without additives. It indicated that the practical capacities of iron/air batteries could be increased through the depression of hydrogen evolution by optimizing the charge current density for the batteries. At a charge current density of 50 mA cm−2, the Fe2O3/C electrode showed the highest discharge capacity of 810 mAh g−1-Fe2O3 in K2S-added electrolyte.

Hydrous Nickel Oxide: Redox Switching and the Oxygen Evolution Reaction in Aqueous Alkaline Solution

The redox switching reaction and the oxygen evolution reaction at multi-cycled nickel oxy-hydroxide films in aqueous alkaline solution are discussed. The hydrous oxide is considered as a porous assembly of interlinked octahedrally coordinated anionic metal oxyhydroxide surfaquo complexes which form an open network structure. The latter contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution. The dynamics of redox switching has been quantified in terms of a diffusive frequency using the Laviron-Aoki diffusion model. Steady state Tafel plot analysis has been used to elucidate the kinetics and mechanism of oxygen evolution with slopes of ca. 60 mVdec−1 and ca. 120 mVdec−1 at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity remains invariant at ca. 1.0 as the potential is increased. These observations are rationalized in terms of a kinetic scheme involving surfaquo groups.

Electrochemical investigation of the dimeric oxo-bridged ruthenium complex in aqueous solution and its incorporation within a cation-exchange polymeric film on the electrode surface for electrocatalytic activity of hydrogen peroxide oxidation

Electrochemical behavior of oxo-bridged dinuclear ruthenium(III) complex ([(bpy)2(H 2O)Ru III-O-Ru III(H 2O)(bpy)2] 4+) has been studied in aqueous solution (KCl 0.5 mol L −1) by both cyclic and rotating disk electrode (RDE) voltammetry in order to identify and elucidate the reaction mechanism. Modified electrode containing the oxo-bridged ruthenium complex incorporated into a cation-exchange polymeric film deposited onto platinum electrode surface was studied. Cyclic voltammetry at the modified electrode in KCl solution showed a single-electron reduction/oxidation of the couple Ru III-O-Ru III/Ru III-O-Ru IV. The modified electrode exhibited electrocatalytic property toward hydrogen peroxide oxidation in KCl solution with a decrease of the overpotential of 340 mV compared with the platinum electrode. The Tafel plot analyses have been used to elucidate the kinetics and mechanism of the hydrogen peroxide oxidation. The first at low overpotential region there is no significant change in the Tafel slope (∼0.130 V dec −1) with varying peroxide concentration. The second region at higher overpotential the slope values (0.91–0.47 V dec −1) were depended on the peroxide concentration. The apparent reaction order for H 2O 2 varies from 0.16 to 0.50 in function of the applied potential. The apparent reaction order (at constant potential) with respect to H + concentration of 10 −5 to 10 −1 mol L −1 was 0.25. A plot of the anodic current vs. the H 2O 2 concentration for chronoamperometry (potential fixed = +0.61 V) at the modified electrode was linear in the 1.0 × 10 −5 to 2.5 × 10 −4 mol L −1 concentration range.

Characterization of Cr-Ni-ZrO<sub>2</sub> Composite Coatings Electrodeposited from Cr(III) Plating Bath

New Cr-Ni-ZrO2 composite coatings are prepared by composite electrodeposition. The morphology, composition and corrosion resistance of as-deposited Cr-Ni-ZrO2 coatings are systematically characterized by means of SEM, EDS. The hardness of as deposited and annealed Cr-Ni-ZrO2 coatings are also discussed. The electrochemical corrosion tests are conducted by means of Tafel plots. Results of SEM and EDS show that Cr-Ni-ZrO2 composite coatings with homogeneous and crack-free morphology have been prepared successfully; the mass proportion of Zr in the as-deposited coatings is up to 7.85%. The hardness values of Cr-Ni-ZrO2 coatings are higher than that of Cr-Ni alloy coatings. Furthermore, the hardness values increase and then decrease with the temperature increasing when annealed. Analysis of Tafel plots demonstrates that Cr-Ni-ZrO2 coatings have much better corrosion resistance than that of Cr-Ni coatings.

Redox switching and oxygen evolution at oxidized metal and metal oxide electrodes: iron in base

Outstanding issues regarding the film formation, redox switching characteristics and the oxygen evolution reaction (OER) electrocatalytic behaviour of multicycled iron oxyhydroxide films in aqueous alkaline solution have been revisited. The oxide is grown using a repetitive potential multicycling technique, and the mechanism of the latter hydrous oxide formation process has been discussed. A duplex layer model of the oxide/solution interphase region is proposed. The acid/base behaviour of the hydrous oxide and the microdispersed nature of the latter material has been emphasised. The hydrous oxide is considered as a porous assembly of interlinked octahedrally coordinated anionic metal oxyhydroxide surfaquo complexes which form an open network structure. The latter contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution, and also charge compensating cations. The dynamics of redox switching has been quantified via analysis of the cyclic voltammetry response as a function of potential sweep rate using the Laviron-Aoki electron hopping diffusion model by analogy with redox polymer modified electrodes. Steady state Tafel plot analysis has been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slope values of ca. 60 mV dec(-1) and ca. 120 mV dec(-1) are found at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity changes from ca. 3/2 to ca. 1 as the potential is increased. These observations are rationalised in terms of a kinetic scheme involving Temkin adsorption and the rate determining formation of a physisorbed hydrogen peroxide intermediate on the oxide surface. The dual Tafel slope behaviour is ascribed to the potential dependence of the surface coverage of adsorbed intermediates.

Electrochemical Proton-Coupled Electron Transfer of an Osmium Aquo Complex: Theoretical Analysis of Asymmetric Tafel Plots and Transfer Coefficients

Electrochemical proton-coupled electron transfer of an osmium aquo complex attached to a self-assembled monolayer on a gold electrode is studied with a recently developed theoretical formulation. The calculated hydrogen/deuterium kinetic isotope effect for the standard rate constant, the cathodic transfer coefficient at zero overpotential, and the Tafel plot are in excellent agreement with experimental data. The input quantities to the heterogeneous rate constant expressions were calculated with density functional theory in conjunction with dielectric continuum models, and no parameters were fit to experimental data. The theoretical calculations indicate that the asymmetry of the Tafel plot and the deviation of the transfer coefficient at zero overpotential from the standard value of one-half arise from the change in the equilibrium proton donor-acceptor distance upon electron transfer. The direction of the asymmetry and deviation from one-half is determined by the sign of this distance change, and the magnitude of these effects is determined by the magnitude of this distance change, as well as the reorganization energy and the distance dependence of the overlap between the initial and final proton vibrational wave functions. This theory provides experimentally testable predictions for the impact of specific system properties on the qualitative behavior of the Tafel plots.

Investigation of a DNA‐Based Biosensor with Chitosan‐Carbon Nanotubes Interface by Cyclic and Elimination Voltammetry

AbstractThe paper presents a disposable DNA biosensor based on a composite of multiwalled carbon nanotubes (MWCNT) in chitosan (CHIT) used as an interface at the screen printed carbon electrode (SPCE). Double stranded herring sperm DNA was immobilized over the MWCNT‐CHIT layer. The response of the modified SPCE with and without DNA was characterized using the redox probe [Fe(CN)6]3−/4− in the mode of cyclic voltammetry and elimination voltammetry with linear scan (EVLS), which enables the elimination of selected partial voltammetric currents contributing to the total current. The elimination functions providing a significant increase in the voltammetric current sensitivity and improving the peak resolution enable to indicate an adsorbed state of depolarizers, to discover hidden electrode processes, and to determine their nature. The SPCE surface modification was evaluated using (a) the charge transfer coefficient α of the redox probe calculated from the Tafel plot analysis (TPA) and EVLS and (b) the polarization resistance RP for both cathodic and anodic processes.

Redox switching and oxygen evolution electrocatalysis in polymeric iron oxyhydroxide films

Outstanding issues regarding the redox switching characteristics and the oxygen evolution reaction (OER) electrocatalytic behaviour of multicycled iron oxyhydroxide films in aqueous alkaline solution have been examined. Charge percolation through the hydrous layer has been quantified, using cyclic voltammetry, in terms of a charge transport diffusion coefficient D(CT) which admits a value of ca. 3 x 10(-10) cm2 s(-1). Steady-state Tafel plot analysis and electrochemical impedance spectroscopy have been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slope values of ca. 60 mV dec(-1) and ca. 120 mV dec(-1) are found at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity changes from ca. 3/2 to ca. 1 as the potential is increased. These observations are rationalised in terms of a kinetic scheme involving Temkin adsorption and the rate determining formation of a physisorbed hydrogen peroxide intermediate on the oxide surface. The dual Tafel slope behaviour is ascribed to the potential dependence of the surface coverage of adsorbed intermediates.

Influences of Metal-Precoated Layers on Pulsed Current Electrodeposition of ZnO Nanorods on Indium Tin Oxide Substrates

The influences of the Au- and Pt-precoated layers on ZnO nanorod growth have been investigated using the pulsed current electrodeposition method. Enhancement of the densities of the ZnO nanorod arrays is achieved using both metal-coated indium tin oxide (ITO) substrates whereas more significant improvement is obtained using the Au-precoated substrates. Tafel-plot analyses for the reaction of formation by and , which is a crucial step for the electrodeposition of ZnO using electrolyte reveal that formation through this reaction occurs in the limiting-current region at the Au/ITO substrate in the ZnO nucleation stage while the side reaction of formation also takes place at the Pt/ITO and the bare ITO substrates under the same pulse current density in this study. We suggest that the low-density ZnO nanorods formed on the Pt-precoated and the bare ITO substrates are ascribed to the formation of which would influence on the surface. Because of an appropriate potential build up at the Au/ITO substrate, a high exists on the surface of the Au/ITO substrate without being disturbed by significantly, resulting in high-density ZnO nanorods deposited on the Au/ITO substrates.

Oxidation and corrosion resistance of TiAl3 coatings

Titanium aluminides based on TiAl, TiAl3 and Ti3Al are potential materials for high temperature aerospace applications. Their low density, high temperature creep resistance, high temperature strength and high oxidation resistance make them excellent coating materials. However these coatings are likely to be subjected to high temperature and corrosive environments during service. Hence it is aimed to study the oxidation and corrosion resistance of TiAl3 coatings on various types of substrates. In the present work, TiAl3 is coated on high speed steel, stainless steel 304, stainless steel 316, copper and aluminum substrates by physical vapor deposition technique. X-ray diffraction analysis confirms the presence of TiAl3 phase. The hardness studies reveal that better hardness can be achieved with thick coatings. The oxidation behavior of the coatings is studied by carrying out step stress experiments at elevated temperatures. Coated samples are heated up from 400°C in the steps of 100°C for 1h in each step to 1000°C. The mass gain caused by oxidation was determined. The oxidation curve drawn as a function of mass gain versus temperature reveals that TiAl3 film started to oxidize above 800°C, where as oxidation of the uncoated substrates began at a much lower temperature of 550°C. The excellent oxidation resistance of the coatings can be attributed to the formation of an amorphous Al2O3 film. Scanning electron microscope (SEM) and EDAX analysis confirm the presence of an amorphous Al2O3 film. The corrosion behavior of TiAl3 coatings are investigated by the polarization resistance experiments in NaCl aqueous solution at ambient temperature. According to the Tafel plot analysis, the coatings show lower corrosion rate than the untreated substrates. The major corrosion in the coatings arose from electrolyte penetration into the pores of the coatings. In fact, a dense coating showed a high corrosion resistance in an aqueous medium.

Alcohol electrooxidation at Pt and Pt–Ru sputtered electrodes under elevated temperature and pressurized conditions

The electrooxidation properties of methanol and 2-propanol, which are both promising candidates for direct alcohol fuel cells (DAFCs), have been studied under elevated temperature and pressurized conditions. Sputter-deposited Pt and Pt–Ru electrodes were well-characterized and utilized for the electrochemical measurement of the alcohol oxidation at 25–100°C. The Pt electrode prepared at 600°C had a flat surface, and the Pt–Ru formed an alloy. The electrochemical measurements were carried out in a gas-tight cell under elevated temperature, which accompanies the pressurized condition. This is a representative example of the DAFC rising temperature operation. As a result, at 25°C, the onset potential of the 2-propanol oxidation is about 400mV more negative than that of the methanol oxidation, and current density of the 2-propanol oxidation exceeds that of the methanol oxidation. Conversely, at 100°C, the methanol oxidation current density overcomes that of 2-propanol, and the onset potentials of the two are almost the same. The highest current density for the methanol oxidation is obtained at the Pt:Ru=50:50 electrode, whereas at the Pt:Ru=35:65 for the 2-propanol oxidation. A Tafel plot analysis was employed to investigate the reaction mechanism. For the methanol oxidation, the number of electrons transferred during the rate-determining process is estimated to be 1 at 25°C and 2 at 100°C. This suggests that the methanol reaction mechanism differs at 25 and 100°C. In contrast, the rate-determining process of the 2-propanol oxidation at 25 and 100°C was expected to be 1-electron transfer which accompanies the proton-elimination reaction to produce acetone. Consequently, it is deduced that methanol and 2-propanol have an advantage under the rising temperature and room temperature operation, respectively.

Corrosion study of PVD coatings and conductive polymer deposited on mild steel: Part I: Polypyrrole

Electrochemical deposition of polypyrrole on mild steel substrate was carried out by cyclic voltammetry and galvanostatic technique from neutral aqueous sodium tartrate medium. Homogenous polypyrrole layer for further studies was achieved by galvanostatic technique at constant current density of 5 mA/cm 2. TiN, CrN and diamond-like carbon (DLC) coatings deposited by physical vapor deposition (PVD) technique on mild steel were used for comparison of corrosion properties with polypyrrole film. Electrochemical synthesis of polypyrrole was also applied as a solution for the PVD coatings porosity problem. Polypyrrole film was successfully deposited on all studied PVD coatings by the galvanostatic technique under special conditions. The protection against corrosion was studied by several methods — measurement of potentiodynamic polarization curves, Tafel plot analysis and electrochemical impedance spectroscopy in 3% NaCl solution.

Study of Corrosion Properties of PVD Coatings and Electrochemically Deposited Polypyrrole Film on Mild Steel

Electrochemical synthesis of pyrrole on mild steel substrate was carried out by potentiodynamic and galvanostatic techniques from neutral aqueous sodium tartrate medium. Homogenous and adherent PPy layer was achieved by galvanostatic technique at current density 5 mA cm-2. Basic physical vapor deposition (PVD) coatings (TiN, CrN and DLC) deposited on mild steel were used for comparison of corrosion properties with PPy film. The main problem of PVD coatings in the corrosive media is their porosity. Electrochemical deposition of PPy was applied as a solution for the porosity. PPy film was successfully deposited on PVD coatings by the galvanostatic technique. The protection against corrosion was studied in 3% NaCl solution by several electrochemical methods - measurement of potentiodynamic polarization curves, Tafel plot analysis and EIS method.