Tafel slope

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Avoid the quasi equilibrium assumption when evaluating the electrocatalytic oxygen evolution reaction mechanism by Tafel slope analysis

Avoid the quasi equilibrium assumption when evaluating the electrocatalytic oxygen evolution reaction mechanism by Tafel slope analysis

would be observed. However in our simulations, when the parameters are defined such that the system falls under these conditions, a Tafel slope of 15 mV is not observed. This is because the potential range where the required conditions are meet is relatively small and thus does not result is clear Tafel behaviour (we normally consider Tafel behaviour is observed when the slope is the same for at least 1 decade in current density). Interestingly, we find no literature reporting experimental Tafel slopes of 15 mV, which may confirm our conclusion that the conditions required to achieve this Tafel slope only occur over a small potential range.

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Ionic Conduction at High Field in Anodic Oxide Films on Tantalum Metal in Aqueous Electrolyte at Various Temperatures Jitender 1, Naveen Verma*2 , Krishan Chander Singh 2

Ionic Conduction at High Field in Anodic Oxide Films on Tantalum Metal in Aqueous Electrolyte at Various Temperatures Jitender 1, Naveen Verma*2 , Krishan Chander Singh 2

anodically. Depending upon the idea of thermally activated hopping of ionic point defects in accordance with Boltzmann statistics, Diggle and Vijh derived a fundamental transport equation for the space charge gradient model and concluded that under steady current conditions, the total amount of space charge inside of a film increases with thickness, with a tendency to level off towards some constant value with increasing thickness at lower current densities. At a given current density, the region of large space charge becomes relatively more confined to the interfaces as the film thickness increases. The whole amount of space charge held by the anodic oxide film increases with the increased current level in accordance to the order of current density profiles for a given thickness. Young‟s conclusion on Nb and Ta metals showed a temperature autonomous Tafel slope and consequently single barrier hypothesis of Cabrera and Mott totally disagreed. Diggle and Vijh [12] concluded that although the Dewald hypothesis seemed to imply that Tafel slope changed with thickness because of space charge in an extremely distinct manner. According to them, space charge caused the rate limiting step to alter from an interface barrier to conduction within the anodic film. Hence, Tafel slope go through a corresponding transition with diverse hopping separations accepted for each of these breaking points. These examinations show to observe the investigation of space charge effect

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Oxygen evolution at oxidized iron electrodes: a tale of two slopes

Oxygen Evolution at Oxidised Iron Electrodes: A Tale of Two Slopes

surfaces. They too found, for a surface saturated with adsorbed OH, that the highest energy intermediate was a surface oxygen species, in this case an oxo species. Considering these studies, the present mechanistic interpretation brings together a number of strands in the current understanding of the OER at metal oxides, and is clearly more satisfying than our previous presentation.A full kinetic analysis of the mechanism outlined above using the quasi-steady state approximation is presented a recent publication [21]. However, it is gratifying to note here that the current mechanism is in excellent agreement with the kinetic data. At low overpotentials the chemical formation of SO − (eqn. 3) is rate limiting with a predicted Tafel slope of ca. 60 mV dec -1 and a reaction order of unity. For high overpotentials it is reasonable to expect high surface coverage of SOH. Hence, the electrochemical formation of SO (eqn. 4) becomes rate limiting with a Tafel slope of ca. 120 mV dec −1 and a unit reaction order. The decrease in the low overpotential Tafel slope to 40 mV dec −1 for thick hydrous layers can be accounted for if the formation of SOOH (eqn.5) is assumed to be rate limiting. This step predicts a Tafel slope of ca. 40 mV dec −1 and a reaction order of unity. Finally if the decomposition of the peroxide SOOH species is rate determining, steady state analysis predicts a Tafel slope of 40 mV dec -1 and a reaction order with respect to hydroxide ion activity of 2. The observed diagnostic parameters of 40 mV dec -1 and reaction order of 1.5 with respect to hydroxide ion activity observed for type B hydrous oxides may well be attributed to the steps outlined in eqn.5 and eqn.6 being rate determining in different regions of the oxide film. The physical reason for the change in Tafel slope from 60 to 40 mV dec -1 is currently uncertain though a possible explanation might lie in increased stabilization of the previous intermediates in thicker layers. This would likely occur through intermolecular hydrogen bonding interactions with neighboring surfaquo groups.

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Stress Corrosion Behavior of X65 Steel Welded Joint in Marine Environment

Stress Corrosion Behavior of X65 Steel Welded Joint in Marine Environment

Cracks were generated near the fusion line of the U-shaped bend in deep sea environment but no crack was produced in shallow sea environment. SCC sensitivity in deep sea environment was higher than that in shallow sea environment. Tafel slope and corrosion potential of welded joint is low in deep sea environment indicting there existed hydrogen evolution reactions and cathodic polarization of X65 steel welded joint was controlled by a mixture of oxygen reduction and hydrogen evolution reactions in deep sea environment but only by oxygen diffusion reactions in shallow sea environment, which showed high SCC sensitivity in deep sea environment.

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The Catalytic Activity of Ruthenates ARuO3 (A= Ca, Sr or Ba) for the Hydrogen Evolution Reaction in Acidic Medium

The Catalytic Activity of Ruthenates ARuO3 (A= Ca, Sr or Ba) for the Hydrogen Evolution Reaction in Acidic Medium

HER starts with the proton discharge (Volme reaction, Eq. (1)), and follows either electrodesorption step (Heyrovsky reaction, Eq. (2)), or the proton recombination step, physical desorption, (Tafel reaction, Eq. (3)). The distinction between steps (1), (2) and (3), as the rate controlling, is usually accomplished in terms of Tafel slopes or by calculating the rate constants of the forward and backward reactions through simultaneous fitting of polarization and impedance data. According to the general model for the HER mechanism, if the Volmer reaction is the rate determining step, the resulting Tafel curve should yield a slope of 118 mV.dec -1 at 20°C. If Heyrovsky step is rate determining, the measured Tafel slope would yield a value of about 40 mV.dec -1 , or 30 mV.dec -1 for the Tafel desorption step [22].

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Oxygen evolution at oxidized iron electrodes: a tale of two slopes

Oxygen evolution at oxidized iron electrodes: a tale of two slopes

surfaces. They too found, for a surface saturated with adsorbed OH, that the highest energy intermediate was a surface oxygen species, in this case an oxo species. Considering these studies, the present mechanistic interpretation brings together a number of strands in the current understanding of the OER at metal oxides, and is clearly more satisfying than our previous presentation.A full kinetic analysis of the mechanism outlined above using the quasi-steady state approximation is presented a recent publication [21]. However, it is gratifying to note here that the current mechanism is in excellent agreement with the kinetic data. At low overpotentials the chemical formation of SO − (eqn. 3) is rate limiting with a predicted Tafel slope of ca. 60 mV dec -1 and a reaction order of unity. For high overpotentials it is reasonable to expect high surface coverage of SOH. Hence, the electrochemical formation of SO (eqn. 4) becomes rate limiting with a Tafel slope of ca. 120 mV dec −1 and a unit reaction order. The decrease in the low overpotential Tafel slope to 40 mV dec −1 for thick hydrous layers can be accounted for if the formation of SOOH (eqn.5) is assumed to be rate limiting. This step predicts a Tafel slope of ca. 40 mV dec −1 and a reaction order of unity. Finally if the decomposition of the peroxide SOOH species is rate determining, steady state analysis predicts a Tafel slope of 40 mV dec -1 and a reaction order with respect to hydroxide ion activity of 2. The observed diagnostic parameters of 40 mV dec -1 and reaction order of 1.5 with respect to hydroxide ion activity observed for type B hydrous oxides may well be attributed to the steps outlined in eqn.5 and eqn.6 being rate determining in different regions of the oxide film. The physical reason for the change in Tafel slope from 60 to 40 mV dec -1 is currently uncertain though a possible explanation might lie in increased stabilization of the previous intermediates in thicker layers. This would likely occur through intermolecular hydrogen bonding interactions with neighboring surfaquo groups.

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Inhibitive Assesment of Stearamide as a Corrosion Inhibitor for Mild Steel in HCl Solution

Inhibitive Assesment of Stearamide as a Corrosion Inhibitor for Mild Steel in HCl Solution

Although in both solvents, Stearamide is adsorbed on metal surface, however, no important changes in Tafel slope could be observed. Therefore, the inhibition role of this compounds is not through the interference on the reactions of metal dissolution and reduction of protons [20]; and Stearamide acts as adsorptive inhibitor, i.e., it reduces anodic dissolution and also retards the hydrogen evolution reaction by blocking the active reaction sites on the metal surface or adsorbed molecules, mechanically screens the coated part of the electrode and therefore, protects it from the action of the corrosion medium [2]. Figure 3 shows this phenomenon schematically. As it was discussed earlier, Stearamide is

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Mechanism of Oxygen Reactions at Porous oxide Electrodes  Part 2  Oxygen evolution at ruthenium dioxide, iridium dioxide and mixed ruthenium iridium oxide electrodes in aqueous acid and alkaline solution

Mechanism of Oxygen Reactions at Porous oxide Electrodes Part 2 Oxygen evolution at ruthenium dioxide, iridium dioxide and mixed ruthenium iridium oxide electrodes in aqueous acid and alkaline solution

The classical approach to the oxygen evolution reaction (OER) has usually been in terms of the formation of discrete adsorbed intermediates such as OH radicals, which subsequently react with other intermediates or with the electrolyte to liberate oxygen gas. According to this adsorption type approach, if the stability of the adsorbed intermediate formed in the primary discharge step can be increased, then the rate control can shift to a later step. Thus the kinetics of oxygen generation on ruthenium 66 , which possesses substantial electrocatalytic activity in acid solution (b = 2.303RT/2F) has been interpreted in terms of direct interaction between strongly adsorbed hydroxyl groups, while the behaviour of platinum 67 which is an intrinsicallyinferior electrocatalyst in acid solution [b = 2(2.303RT/F)], is usually rationalized in terms of rate determining radical generation in the primary step. More recently the concept of the cyclic formation/decomposition of higher valent state surface bonded oxycation moeities in the oxygen evolution pathway has received attention since the early work of Burke, Lyons and McCarthy 68 . This type of approach has received attention especially in connection with electrocatalysis exhibiting relatively low Tafel slope values, where, according to a more conventional interpretation, only strong surface intermediate adsorption interaction is considered to be involved. It is useful at this stage to summarize the diagnostic mechanistic parameters obtained in the present work. Pure RuO 2 electrodes (corresponding to x = 0) exhibit low overpotential

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Corrosion of Nickel-Aluminum-Bronze Alloy in Aerated 0.1 M Sodium Chloride Solutions under Hydrodynamic Condition

Corrosion of Nickel-Aluminum-Bronze Alloy in Aerated 0.1 M Sodium Chloride Solutions under Hydrodynamic Condition

surface across a diffusion layer whose concentration gradient is determined by the electrode potential. Mathematical analysis of such behavior showed that the apparent Tafel slope should be 59 mV/decade at room temperature, consistent with experimental data. Dhar [35] found the average anodic Tafel slopes of 70Cu–30Ni to be 60 mV/decade in 0.5 M NaCl, in good agreement with that of Cu. As E was further increased, the anodic Cu curve produced a small peak consistent with formation of a CuCl film [32,33] at a potential of __0.092VSCE. Beyond the peak, limiting-current behavior was obtained where the rate of anodic reaction was controlled by the formation and dissolution of a CuCl film [36].

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Investigation of Corrosion Inhibition of Vitamins B1 and C on Mild Steel in 0.5 M HCl Solution: Experimental and Computational Approach

Investigation of Corrosion Inhibition of Vitamins B1 and C on Mild Steel in 0.5 M HCl Solution: Experimental and Computational Approach

method for the cathodic and anodic branches of the polarization curves. The E corr as function of time data for VitB1 revealed that E corr was shifted toward more positive value, while i corr value was much lower than VitC and corroded steel. These results clearly support the previously obtained EIS results. Moreover, according to the electrochemical parameters (i corr , E corr , b a and b c ) given in Table 3, it can be observed that the anodic Tafel slope (ba) increases with the addition of the inhibitors while the cathodic Tafel slope (bc) remains almost constant, this illustrates the high anodic corrosion inhibition and the high stability of the oxide film promoted by the inhibitors in acid solution, by blocking the reaction sites of the metal surface and decreasing the surface area responsible for corrosion. The positive shift of corrosion potential is evidently caused by the restriction of anodic reaction process. Furthermore, from the Tafel slopes (ba and -bc), corrosion rates were calculated to be 2.3 and 1.6 mm/year for VitC and VitB1, respectively, reflecting the corrosion resistance enhancement by replacing addition of the vitamins.

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Numerical simulation of the current, potential and concentration distributions along the cathode of a rotating cylinder Hull cell

Numerical simulation of the current, potential and concentration distributions along the cathode of a rotating cylinder Hull cell

was used as the domain for the numerical simulations. In primary current distribution, the non-uniform current distribution profile was influenced by the geometry of the RCH cell. In secondary and tertiary current distributions cases, the Tafel approximation was used for the electrochemical kinetics. For the tertiary current distribution, a Nernst diffusion layer model was used with the boundary layer thickness estimated from a literature mass trans- port correlation [13,14]. Under the turbulent flow conditions considered, the Nernst diffusion layer thickness can be assumed to be uniform [17,18] but the local rates of reaction (overpo- tential, current density and concentration) were assumed to be distributed non-uniformly along the length of the working elec- trode. Two electrolyte subdomains were defined: diffusion layer region and free electrolyte region. It was assumed that the poten- tial drop of the metal resistivity along the cathode was negligible. The current density, j at any point inside the RCH cell was determined from the gradient of local potential, φ according to Ohm’s Law [2]:

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A comparative study of the oxygen evolution reaction on oxidized nickel, cobalt and iron electrodes in base

A comparative study of the oxygen evolution reaction on oxidized nickel, cobalt and iron electrodes in base

as ‘‘16th” was typical of those recorded prior to the measurement of Tafel lines with 45 6 b 6 48 mV dec 1 . It is noteworthy that the redox charge capacity of the A III/C II peak pair, which is associated with the outer hydrous region of the oxide (recall Section 3.3), is greatly enhanced in the ‘‘5th” CV relative to the ‘‘16th”. By contrast the A IV peak is more significant in the ‘‘16th” CV, implying that, with repeated utilisation, the inner compact region develops a greater relative influence on the overall properties of the passive film. This observation suggests an identification of the film charge transport barrier with the anhydrous inner oxide or with the inter- face between this region and more dispersed hydrous outer oxide. The duplex layer model for anodic oxides developed by Burke and co-workers [69,80,81], envisages that the ions of the inner region are held in place by a rigid network of polar covalent bonds, through which ionic transport is difficult, thus limiting growth to perhaps no more than five monolayers. It is significant that this description fits the profile of a ‘‘barrier oxide” as outlined by Meyer [77] and MacDonald [76]. On the other hand, charge percolation proceeds comparatively easily and quickly through the outer, hy- drous, polymeric oxide region – e.g. we have recently calculated [38] that the average rate of charge diffusion for a hydrous oxide covered Fe electrode in base is comparable to that of electrodes modified by redox polymers such as poly(pyrrole). Interestingly, a duplex model was also specifically proposed [82] for the anodic oxide formed on Au electrodes – the very system for which the twin barrier model was originally applied to the OER [76].

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Effect of Titanium Carbide Content on the Corrosion Behavior of Al-TiC Composites Processed by High Energy Ball Mill

Effect of Titanium Carbide Content on the Corrosion Behavior of Al-TiC Composites Processed by High Energy Ball Mill

CPP technique is one of the strongest methods that have been successfully employed to report the mechanism of corrosion and corrosion mitigation for metals and alloys in corrosive media. We have been used the CPP technique in understanding the uniform and pitting corrosion as well as corrosion inhibition phenomena for different metallic materials in different aggressive solutions [12,13,19]. Figure 6 shows CPP curves obtained for (1) Al-15%TiC, (2) Al-25%TiC and (3) Al- 35%TiC composites after (a) 1 h and (b) 24 h immersions in 3.5% NaCl solutions at room temperature. The values of corrosion parameters such as cathodic Tafel slope (βc), anodic Tafel slope (βa), corrosion current density (j Corr ), corrosion potential (E Corr ), polarization resistance (Rp), and corrosion

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Comparing Electrochemical Behaviour of Amorphous Ni–P Powders and Film

Comparing Electrochemical Behaviour of Amorphous Ni–P Powders and Film

sample (Fig. 4b). According to our earlier argument [26], the peak at 0.5 V is attributed to the destruction of passive film. The damage of passive film is companied with the come off of fragments into solution, and then the combination of OH - and Ni into new passive film resulting the decrease of current density. There are visible Tafel regions in the cathodic polarization sides in Fig. 4. Hence, we can use the curves of cathodic polarization sides to obtain the kinetic parameters of corrosion such as corrosion current density (i corr ), cathodic Tafel slope ( c ) and polarization resistance (R p ). The

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Synthesis, Physicochemical and Electrochemical Properties of Nickel Ferrite Spinels Obtained by Hydrothermal Method for the Oxygen Evolution Reaction (OER)

Synthesis, Physicochemical and Electrochemical Properties of Nickel Ferrite Spinels Obtained by Hydrothermal Method for the Oxygen Evolution Reaction (OER)

Electrochemical studies, namely, cyclic voltammograms (CV), electrochemical impedance spectra (EIS) and Tafel polarization were performed in a conventional three-electrode single compartment Pyrex glass cell using a computerized potentiostat/galvanostat (Autolab, PGSTAT30). The commercial reference and the auxiliary electrodes were Hg/HgO/1M KOH (Koslow, scientific testing instruments; Model: 5088) and pure Pt-foil, respectively. All potentials mentioned in the text are based on the Hg/HgO (E 0 = 0.098 V vs. NHE at 25 0 C) electrode only. The software used for impedance, cyclic voltammetry, and Tafel polarization measurements was NOVA 1.5. The EIS of the oxide catalysts/1M KOH interface was measured over the frequency range 0.1-10 5 Hz at a potential 0.55 V, which is the region of the commencement of the OER. The double layer capacitance (C dl ) was

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Electrocatalytic Properties of Bimetallic Surfaces for the Oxygen Reduction Reaction

Electrocatalytic Properties of Bimetallic Surfaces for the Oxygen Reduction Reaction

The Tafel plots at all temperatures show linear regions at low current density from which kinetic parameters were deduced. The electrode potential shift to more positive values and an increase of the catalytic current density is observed with raised temperature. This behavior is similar for RuSe and RuMo to 40 h of milling and it is attributed to an enhancement of the electrocatalytic kinetic reduction of the adsorbed oxygen with temperature. Figures 8 and 9 (a), show the temperature dependence of the mass transfer corrected Tafel coefficient for the oxygen reduction to RuSe and RuMo respectively, the Tafel slopes are temperature invariant with a constant slope of - 0.100 and - 0.135 Vdec -1 , indicating that in the range of 293-333 K, the rate determining step in ORR is the first electron charge transfer, i.e.,   O 2 ads  H   e    HO 2  ads . The charge transfer coefficient, , calculated from the Tafel slopes are plotted as a function of temperature in Figure 8, for RuSe slope, increases lineally with the absolute temperature (d/dT = 5 x 10 -4

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Corrosion inhibition of carbon steel in 1M HCl solution by Ruta graveolens extract

Corrosion inhibition of carbon steel in 1M HCl solution by Ruta graveolens extract

Potentiodynamic polarization measurements were carried out using computerized electrochemical analyzer model 35415. The carbon steel specimen was used as the working electrode, a platinum wire as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The polarization measurements were made to evaluate the corrosion current, corrosion potential and Tafel slopes. A luggin capillary arrangement was used to keep the reference electrode close to the working electrode. 400 ml of the electrolyte was added to the test cell and the luggin capillary was filled with the same test solution. Ten minutes was given for each experiment to attain the steady state open circuit potential [3]. Then the carbon steel specimen was polarized to about ±200 mV anodically and cathodically from the open circuit potential to obtain the corrosion potential, corrosion current and Tafel slopes [4].

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Hydrogen evolution reaction measurements of dealloyed porous NiCu

Hydrogen evolution reaction measurements of dealloyed porous NiCu

For the as-deposited samples, the Tafel slopes tend to be around 100 to 125 mV/dec. In contrast, the Tafel slopes for the dealloyed samples are generally higher, most above 175 mV/dec. One possible reason for these larger Tafel slopes is a decrease in effective area available for reac- tion at higher overpotentials due to larger gas evolution rates. This effect may be increased by the more porous nature of the dealloyed samples, allowing gas bubbles to be trapped more easily. To confirm this hypothesis, additional measurements of the effective surface areas at different applied potentials during HER conditions are needed.

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High Performance Sn–In Cathode for the Electrochemical Reduction of Carbon Dioxide to Formic Acid

High Performance Sn–In Cathode for the Electrochemical Reduction of Carbon Dioxide to Formic Acid

is only 159 mV decade −1 , much smaller than that of the other electrodes. A smaller Tafel slope is advantageous for practical applications because it results in a lower rate-limiting activation energy barrier and hence remarkably higher catalytic activity,[51] also indicating surface-area-independent enhancement of the intrinsic catalytic activity.[52] It can be seen that the FEs of the Sn–In electrodes increase with more negative potential (Figure 7C), however, the Sn 42 In 58 electrode exhibits an

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Experimental Study of Corrosion on A36 Mild Steel Towards Aqueous 2-Amino-2-Ethyl-1, 3-Propanediol and Diethanolamine

Experimental Study of Corrosion on A36 Mild Steel Towards Aqueous 2-Amino-2-Ethyl-1, 3-Propanediol and Diethanolamine

samples have been mounted in epoxy and hardener with 5:1 ratio to approximately justified space by using aluminum foil and samples were dried for 24 hours. One side of the mild steel samples was ground to expose and polished for corrosion test. Measure the area of exposed mild steel for further measurements. The electrochemical tests were implemented out in a three-electrode corrosion cell for the quantitative examination of mild steel corrosion. The working electrode is the steel sample which is intended to observe and measure the corrosion rate. Current measurements could be changed into current densities for further analysis and calculation. The counter/auxiliary electrode is the name specified for the second electrode which is extant precisely to transmit the current in the circuit formed by the observations. The third electrode is called “reference electrode” and is existent to deliver exact steady datum point across the potential of the working electrode that can be deliberate. Other essential components needed for the experiment is a source of the potential, current and potential device. A solution of amine poured into the corrosion cell container, and the all the electrodes are immersed in it. The wires are connected to the corrosion cell, and the equipment is connected to the WEIS software which is installed on the connected computer. Tafel Plot results obtained from WEIS software. The trend of the plot is observed for every trial, and other parameters such as voltage, current, are also monitored. The result from WEIS is exported into IVMAN software which is used for Tafel analysis for corrosion rate and corrosion coefficients.

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