Anodic Reaction Kinetics Research Articles

Effect of rolling technologies on the properties of Pb-0.06wt%Ca-1.2wt%Sn alloy anodes during copper electrowinning

The objective of this work was to study the effect of different rolling technologies on the properties of Pb-0.06wt%Ca-1.2wt%Sn anodes during copper electrowinning and to determine the relationship between the properties of the anodes and rolling techniques during copper electrowinning. The anode process was investigated via anodic polarization curves, cyclic voltammetry curves, electrochemical impedance spectra, and corrosion tests. The microscopic morphology and phase composition of the anodic oxide layers were observed by scanning electron microscopy and X-ray diffraction, respectively. Observable variations in the electrocatalytic activity and reaction kinetics of anodes during electrowinning indicated that the electrochemical behavior of the anodes was strongly affected by the rolling technology. An increase in the rolling number tended to decrease the oxygen evolution overpotential and the corrosion rate of the anodes. These trends are contrary to that of the apparent exchange current density. Furthermore, the intensities of diffraction peaks associated with PbO, PbOx, and α-PbO2 tended to increase with increasing rolling number. In addition, the rolled anodes exhibited a more uniform microstructure. Compared with one-way rolled anodes, the eight-time cross rolled anodes exhibited better electrocatalytic activity and improved corrosion resistance.

In Operando Li+- Activity Measurements in Lithium Ion Batteries - a Method to Develop and Optimize Safe Operating Strategies Even at Unfavourable Conditions

The deposition of lithium on the surface of graphitic anodes in Lithium-ion batteries is a major concern about safety and lifetime of lithium-ion batteries. The deposition can be caused by high anodic overpotentials at low temperatures or exceeding a critical lithium ion concentration at the graphite particle surface. The prediction of the onset of lithium deposition by modelling is rather difficult because there are no reported values for the Li+ activity in the electrolyte or their changes during lithium ion insertion into graphite. Furthermore the modelling of the Li+ activity in the electrolyte is quite challenging because the needed parameters like the diffusion coefficient of Li+inside the particles depend strongly on the state of charge and the absolute value on the measuring method respectively the texture of the electrode coating [1]. The critical Li+ activity in the electrolyte in matters of a lithium deposition follows from the concentration dependency of the Li/Li+potential, which is described by the Nernst equation. In the present study, we take advantage of the concentration dependency and develop a 4-electrode cell design (Figure 1a), which allows to measure potential changes of the metallic lithium reference electrode of the standard three electrode setup. The potential shifts of the metallic lithium reference correlate directly with changes in the Li+ activity in the electrolyte. We studied the interaction of lithium transport and reaction kinetics in dependency of current density, temperature and active material of the anode and the cathode. The measurements indicate a direct correlation between changes in anodic and cathodic reaction kinetics and the Li+concentration in the electrolyte (Figure 1b). Based on the knowledge of the Li+activity and its variation, we were able to improve an operation strategy for Lithium-Ion batteries and match the cell components with each other in terms of avoiding lithium deposition.

The Influence of Sulfur Poisoning and YSZ Substrate Orientation on Electrochemical Properties of Nickel Pattern Anodes

Micro-patterned Ni thin films with well-defined 2D-structures are useful to examine electrochemical reaction kinetics of SOFC anodes. In this work, an electrode preparation method is developed yielding stable Ni pattern electrodes on YSZ, which facilitate long term measurements. Moreover, these stable electrodes allowed investigating the effect of the YSZ substrate orientation on the polarization resistance and interface capacitance. Statistical analysis of data obtained from 3 different substrates revealed no significant difference in polarization resistances and no significant difference in interface capacitance between single crystalline substrates (α=0.05). Also, the effect of hydrogen sulfide on the polarization resistance was examined and the results are qualitatively equivalent to findings on cermet electrodes.

Pt-based bimetallic catalysts for SO2-depolarized electrolysis reaction in the hybrid sulfur process

SO2-depolarized electrolysis (SDE) is pivotal in the hybrid sulfur process, which is a promising approach for mass hydrogen production without CO2 emission. The anode overpotential of SDE is the key component of electrolysis potential. This factor can be reduced by improving anode reaction kinetics. Such improvement is commonly achieved by employing Pt/AC as anode electrode and catalyst, thus also improving economic and electrocatalytic performances. In this work, anode catalysts for SO2 oxidation reaction are experimentally studied. Platinum-based bimetallic catalysts, including Pt–Pd/C, Pt–Rh/C, Pt–Ru/C, Pt–Ir/C, and Pt–Cr/C, are prepared and characterized. Their electrochemical characteristics for SDE in a once-through mode are investigated in SO2-saturated 30 wt% sulfuric acid at room temperature by various approaches such as cyclic voltammetry, linear sweep voltammetry, and polarization curves. Results show that 60 wt% Pt–Cr/C exhibits the highest electrocatalytic activity for SDE. Further studies on the metal proportion in Pt–Cr/C show that at a Pt:Cr atomic ratio of 1:2, this bimetallic catalyst demonstrates equal or even better electrolysis performance than 60 wt% Pt/C at a significantly lower economic cost.

Anodic behavior and microstructure of Al/Pb–Ag anode during zinc electrowinning

Anodic behaviors and oxygen evolution kinetics of Pb–0.8%Ag and Al/Pb–0.8%Ag anodes during the initial 24 h zinc electrowinning were investigated with cyclic voltammetry (CV) curves and electrochemical impedance spectroscopy (EIS). The results reveal that the anodic behaviors and reaction kinetics of the two anodes vary a lot during the anodic polarization which indicate the formation and stabilization of anodic layer. Compared with conventional Pb–0.8%Ag anode, Al/Pb–0.8%Ag anode has longer time of anodic polarization. At the very beginning of anodic polarization, the two anodes all exhibit higher potential of oxygen evolution reaction (OER) since the reaction is controlled by the transformation step of intermediates. Then, its OER potential is largely diminished and OER rate is deduced from the formation and adsorption of the first intermediate (S–OHads). In the prolonged anodic polarization, the anodic potential of Al/Pb–0.8%Ag gradually decreases and the final value is more stable than that of conventional Pb–0.8%Ag anode. On the anodic layer after 24 h of anodic polarization, the OER potential is controlled by the formation and adsorption of intermediate. The microstructures of Al/Pb–0.8%Ag and Pb–0.8%Ag anodes after 24 h of anodic polarization were analyzed by scanning electron microscope (SEM).

Electrochemical Behaviors of Pb–0.3%Ag-0.06%Ca Rolled Alloy Anode during and after Zinc Electrowinning - Tafel Investigations

In this paper, the oxygen evolution potential and kinetics of Pb–0.3%Ag-0.06%Ca rolled alloy anode during the 15 days electrolysis in acid zinc sulphate electrolyte solution were investigated with galvanostatic polarization curve and Quasi-stationary polarization (Tafel) techniques. The results revealed that the anodic oxygen evolution potential and reaction kinetics varied a lot during the electrolysis for it is a process indicating the formation and stabilization of anodic oxide layer. With the increasing electrolysis time, the potentials (MSE) and overpotentials of oxygen evolution mainly present a declining trend while the electrode surface exchange current density a rising trend.

Modeling Heat Transfer Effects In a Solid Oxide Carbon Fuel Cell

A model of a planar geometry solid oxide-based carbon fuel cell is developed. The carbon fuel cell involved a carbon bed in direct contact with the anode, and the oxygen needed for conversion is supplied through the yttria stabilized zirconia (YSZ) solid electrolyte. Using experimentally derived parameters, the model includes mass transport and chemical reaction kinetics inside the carbon bed, anode and cathode reaction kinetics, and ion transport across the YSZ electrolyte. The model also accounts for the effect of heat transfer on temperature distribution inside the anode and cathode compartments, and on the overall cell performance. As expected, inclusion of heat transfer effects provides a realistic predictive tool to anticipate cell performance and efficiency—under exothermic, autothermal, and endothermic operating conditions for the carbon fuel cell.

Performance of Solid Oxide Fuel Cells on H2, NH3 and Hydrocarbon Fuels

The different gases such as H2, CH4, C3H8, NH3 or simulated underground coal gasification (UCG) gas can be used as fuel in SOFC. Basis on the thermochemical calculations, the performances of ceria-based electrolyte anode-supported cells operated on the five fuels were measured. As a result, for C3H8 fuel, the measured OCV decreased rapidly at lower temperatures, indicating sluggish reaction kinetics at the anode. However, the synergy between slow anode reaction kinetics and fast migration of oxide ions to the anode suppresses carbon deposition at low operating temperatures, which leads to better cell durability. Overall, the UCG gas fed cell exhibited much increased stability than any other hydrocarbons, whcih demonstrates that UCG gas is a promising fuel for utilization in SOFCs.

Influence of Mn and Zr on the Corrosion of Al-Free Mg Alloys: Part 2—Impact of Mn and Zr on Mg Alloy Electrochemistry and Corrosion

A total of 47 alloys were produced to explore the changes in electrochemical and corrosion behavior of binary and ternary Mn- and Zr-containing Mg alloys—in the absence of Al. Up to ~2 wt% Mn was found to slightly reduce cathodic kinetics, while Zr in solid solution increased anodic reaction kinetics. We demonstrate that Zr is an efficient “activator” of Mg, which has not been explicitly shown previously. When Mn and Zr are both added to Mg, there can be an optimized combination/interaction where Mn with Zr works in conjunction to moderate the corrosion rate compared to an Mg-alloy with either Zr or Mn as singular additions. This has ramifications in emerging Al-free Mg alloys.

Effect of pH on the Grain Size Dependence of Magnesium Corrosion

Prior works show that grain size can play a role in the corrosion of a metal; however, such works are nominally executed in a single electrolyte/environment at a single pH. In this work, the anodic and cathodic reaction kinetics of pure Mg specimens with grain sizes ranging from approximately 8 μm to 590 μm were compared as a function of pH in 0.1 mol dm−3 sodium chloride (NaCl) electrolytes using anodic polarization experiments and an in situ scanning vibrating electrode technique (SVET). Anodic polarization experiments showed that grain size is important in determining overall electrochemical response, but the environment dictates the form of the grain size vs. corrosion rate relationship (i.e., pH is the overall controlling factor). Consequently, the role of grain size upon corrosion cannot be fully assessed unless a variation in environment is simultaneously studied. For example, the anodic reaction, which dictates active corrosion, also dictates passivation, so the corrosion rate vs. grain size relationship has been shown to “flip” depending on pH. Further, SVET analysis of unpolarized Mg immersed in 0.1 mol dm−3 NaCl electrolyte at neutral pH showed that breakdown of passivity of cast Mg occurred after ~1 h immersion, giving filiform-like corrosion tracks. The front edges of these corrosion features were revealed as intense local anodes, while the remainder of the dark-corroded Mg surface, left behind as the anodes traversed the surface, became cathodically activated. In contrast, grain-refined Mg samples were significantly less susceptible to localized corrosion, and breakdown was not observed for immersion periods of up to 24 h.

High activity and durability of Pt catalyst toward methanol electrooxidation in intermediate temperature alkaline media

To explore strategies for addressing the CO poisoning of Pt-based catalysts and sluggish anode reaction kinetics which have largely hindered the development of low temperature direct methanol fuel cells, electrocatalytic activity and durability of high-surface Pt toward methanol electrooxidation in alkaline media have been investigated in an intermediate temperature of 80–150°C by means of voltammetry, chronoamperometry and single cell measurements. In this intermediate temperature range, Pt electrocatalyst is highly active and durable toward the methanol oxidation in alkaline media. Measured exchange current densities are of the order of magnitude of 10−7–10−6Acm−2 and sustainable chronoamperometric currents are obtained at potentials as low as 0.16V vs RHE. An alkaline methanol fuel cell operated at 120°C using carbon-supported Pt as both anode and cathode electrocatalysts exhibits a peak power density of around 90mWcm−2 and provides stable power output under constant current discharge. Accelerated electrooxidation of CO intermediate whose onset is close to that of the methanol electrooxidation and possible reaction of surface CO and hydroxide ions in the intermediate temperature alkaline media are proposed to account for the high activity and durability of Pt. Therefore, the CO-poisoning of Pt-based catalysts and sluggish methanol electrooxidation could be potentially addressed by the use of intermediate temperature alkaline media.

Electrochemical behaviors of co-deposited Pb/Pb–MnO2 composite anode in sulfuric acid solution – Tafel and EIS investigations

The oxygen evolution kinetics and anodic layer properties of Pb/Pb–MnO2 composite anode during the 72h galvanostatic electrolysis in H2SO4 solution were investigated with Quasi-stationary polarization (Tafel) and Electrochemical Impedance Spectroscopy (EIS) techniques. The results revealed that the anodic activity and reaction kinetics varied a lot during the electrolysis for it is a process indicating the formation and stabilization of anodic layer. At the very beginning of electrolysis, the composite anode exhibited very high oxygen evolution activity since the reaction was controlled by the transformation step of intermediates. Then, its oxygen evolution activity was largely diminished and the rate determination step (rds) became the formation and adsorption of first intermediate, S–OHads. In the prolonged electrolysis, the anodic potential gradually decreased and the final stable value was comparable to industrial Pb–Ag (1.0%) anode. On the stable anodic layer after 72h, the oxygen evolution reaction (OER) was still controlled by the formation and adsorption of intermediate, and the adsorption resistance took a dominant part in the whole impedance. Besides, compared with Pb–Ag and Pb anode, although OER at the stable state were all controlled by the intermediate adsorption process, the adsorption resistance of Pb/Pb–MnO2 anode was much smaller than the other two due to the existence of MnO2 particles and large amount of β-PbO2 in the stable anodic layer.

Advanced electrolyte-free fuel cells based on functional nanocomposites of a single porous component: analysis, modeling and validation

Recently, a fuel cell device constructed with only one layer composited of ceria-based nanocomposites (typically, lithium nickel oxide and gadolinium doped ceria (LiNiO2–GDC) composite materials), called an electrolyte-free fuel cell (EFFC), was realized for energy conversion by Zhu et al. The maxium power density of this single-component fuel cell is 450 mW cm−2 at 550 °C when using hydrogen fuel. In this study, a model was developed to evaluate the performance of an EFFC. The kinetics of anodic and cathodic reactions were modeled based on electrochemical impedance spectroscopy (EIS) measurements. The results show that both of the anodic and cathodic reactions are kinetically fast processes at 500 °C. Safety issues of an EFFC using oxidant and fuels at the same time without a gas-tight separator were analyzed under open circuit and normal operation states, respectively. The reaction depth of anodic and cathodic processes dominated the competition between surface electrochemical and gas-phase reactions which were effected by the catalytic activity and porosity of the materials. The voltage and power output of an EFFC were calculated based on the model and compared with the experimental results.

Enhanced corrosion resistance of Mg alloy ZK60 after processing by integrated extrusion and equal channel angular pressing

It was demonstrated in this work that the corrosion rate of Mg alloy ZK60 can be reduced through the use of an integrated process of extrusion and equal channel angular pressing concomitant with increased strength. The improvement in corrosion resistance, as measured by electrochemical experiments and immersion tests in NaCl electrolytes, was correlated with both grain refinement and the redistribution of Zn and Zr solutes within the microstructure. These microstructural changes impacted the relative anodic and cathodic reaction kinetics sustained by the alloy in a unique manner, whereby structural and chemical variations dictated the anodic reaction rates in the former and the cathodic rates in the latter. The observed decrease in the corrosion rate, combined with improved mechanical properties for ZK60, represents a scenario where two properties that are nominally inversely correlated can be improved simultaneously, offering an insight into how Mg can be unique in terms of its processing-property relationships.

Chloride induced reinforcement corrosion: Rate limiting step of early pitting corrosion

Transition from passive state to stable localised corrosion of reinforcement steel in concrete owing to chloride ingress takes place over a period of time rather than being a one-step-occurrence. The depassivation process was characterised by frequent measurements of corrosion potential, polarisation resistance, and macro-cell currents when short-circuiting the working electrode with additional cathode surface. In addition, the concrete resistivity was continuously monitored and cathodic and anodic polarisation curves were measured. The results are consistent and imply that the localised corrosion process is in the initial phase of pit growth under mixed anodic/ohmic control. With time, the anodic reaction kinetics become more limited and the corrosion rate gets almost entirely determined by anodic control. The observed relationship between achieved maximum corrosion current and concrete resistivity indicates that the extent to which the anodic reaction kinetics are restricted is determined by the concrete microstructure and its ability to retain ionic movement.

Probing anodic reaction kinetics and interfacial mass transport of a direct formic acid fuel cell using a nanostructured palladium–gold alloy microelectrode

The anodic reaction kinetics and interfacial mass transport of a direct polymer electrolyte membrane formic acid fuel cell have been investigated in an all solid-state electrochemical cell using a highly active nanostructured palladium–gold alloy microelectrode as an in situ probe. Well-defined “S-shaped” steady-state cyclic voltammograms exhibiting current-rising region at lower overpotentials and limiting current region at higher overpotentials have been first obtained for the electrochemical oxidation of formic acid at varying temperature. The “S-shaped” steady state polarization curves and chronoamperometric curves enable convenient measurements of the anodic reaction kinetics and interfacial mass transport of formic acid under real polymer electrolyte membrane conditions. It is encouragingly found that formic acid can be directly oxidized to CO 2 with the first electron transfer being the likely rate-determining step and the formation of surface poison can be neglected. The exchange current density for the electrooxidation of formic acid is on the order of magnitude of 10 −7 A cm −2 in the temperature range of 20–60 °C. The permeability and diffusion coefficient of formic acid through a Nafion ® 117 membrane are of the order of magnitude of 10 −9 mol cm −1 s −1 and 10 −6 cm 2 s −1, respectively. The combination of a nanostructured microelectrode and an all solid-state electrochemical cell offers a versatile approach to evaluate potential electrocatalysts for fuel cells and electrochemical sensors employing polymer electrolyte membranes.

Electrochemical corrosion of X65 pipe steel in oil/water emulsion

The electrochemical corrosion behavior of X65 pipeline steel in the simulated oil/water emulsion was investigated under controlled hydrodynamic and electrochemical conditions by rotating disk electrode technique. Results demonstrated that mass-transfer of oxygen plays a significant role in the cathodic process of steel in both oil-free and oil-containing solutions. Electrode rotation accelerates the oxygen diffusion and thus the cathodic reduction. The higher limiting diffusive current density measured in oil-containing solution is due to the elevated solubility of oxygen in oil/water emulsion. The anodic current density decreases with the increase of electrode rotating speed, which is attributed to the accelerated oxygen diffusion and reduction, enhancing the steel oxidation. Addition of oil decreases the anodic dissolution of steel due to the formation of a layer of oily phase on steel surface, increasing the reaction activation energy. The steel electrode becomes more active at the elevated temperature, indicating that the enhanced formation of oxide scale is not sufficiently enough to offset the effect resulting from the enhanced anodic dissolution reaction kinetics. The corrosion reaction mechanism is changed upon oil addition, and the interfacial reaction is activation-controlled, rather than mass-transfer controlled. When sand particles are added in oil/water emulsion, there is a significant increase of corrosion of the steel. The presence of sands in the flowing slurry would impact and damage the oxide film and oily film formed on the steel surface, exposing the bare steel to the corrosive solution.

Ruthenium based DSA® in chlorate electrolysis—critical anode potential and reaction kinetics

Ruthenium based DSA®s have been investigated in chlorate electrolyte using rotating discs made from commercial electrodes. Measurements of the voltammetric charge, q*, and of iR-corrected polarisation curves up to current densities of 40 kA/m 2 were recorded on new anodes and on aged anodes from 3 years of production in a chlorate plant. Anodic polarisation curves in chloride containing electrolytes bend towards a higher slope at approximately 1.2 V versus Ag/AgCl, likely due to oxidation of ruthenium. The potential and current density at which the curves bend have been defined as the critical potential, E cr, and the critical current density, i cr. New anodes that operate at a relatively high potential, > E cr, obtain an increase in real surface area and thereby a decrease in anode potential and in the selectivity for oxygen formation during the first months of operation. Experiments at constant ionic strength under chlorate process conditions showed that E cr decreased with increasing chloride concentration with a factor of −0.09 V/log Cl −, whereas i cr increased with increasing chloride concentration. The chlorine evolution reaction was of the first order with respect to chloride concentration. A possible reaction mechanism for chlorine formation is suggested.

The Effect of Products of Nitrate Ion Reduction on the Kinetics of Electrode Reactions on Stainless Steel

The effects of nitrate ions and possible products of their reduction (ammonium, hydrazine, and hydroxylammonium ions) on electrochemical characteristics of stainless steel are studied in phosphoric acid. Specific properties of nitrates as corrosion inhibitors of a passivating type are explained by a complex effect of their reduction products on the kinetics of both cathodic and anodic reactions. It is found that the products of nitrate reduction, firstly, accelerate cathodic reactions, secondly, accelerate anodic reactions due to the formation of complexes with the ions of corroding steel, and, thirdly, reducte passivation potential due to alkaline properties of certain products. Everything aforementioned substantially affects the possibility of using nitrates as the corrosion inhibitors of stainless steel in acidic media.

The Electrochemistry of θ (Al<sub>2</sub>Cu), S (Al<sub>2</sub>CuMg) and T<sub>1</sub> (Al<sub>2</sub>CuLi) and Localized Corrosion and Environment Assisted Cracking in High Strength Al Alloys

The θ (Al 2 Cu), S (Al 2 CuMg), and T 1 (Al 2 CuLi) intermetallic constituents and precipitates found in age-hardenable aluminum alloys play important roles in the localized corrosion behavior of Al-Cu-(Mg,Li) alloys. Most of the common understanding of the behavior of these phases is based on knowledge or assumptions of their open circuit potentials (OCP). Electrochemical measurements made on these compounds synthesized in bulk form, and detailed investigations of their dissolution from alloys reveal interesting complexities not suggested by OCP measurements. Studies show that both anodic and cathodic reaction kinetics are very fast on these intermetallics. These compounds invariably corrode by dealloying in which the more active Al, Mg, or Li components dissolve to leave behind a Cu-enriched particle remnant. The dealloyed particle remnants are themselves physically unstable and have been observed to coarsen and emit 10 to 100 nm metallic Cu clusters into corrosion product layers and surface films. Overall, a more complete and quantitative understanding of the dissolution characteristics of these intermetallic compounds has helped in understanding localized corrosion and environment assisted cracking behavior of Al alloys.