Cyclic reciprocal derivative chronopotentiometric behavior of electrode process in the presence of adsorptive reactants: A theoretical study of the electrolysis sequence of adsorptive and diffusing electroactive reactants

Theoretical modeling and analyzing of electrode process in the presence of adsorptive reactants is presented using cyclic reciprocal derivative chronopotentiometry (CRDCP) with symmetrical programmed current applied. Electrode process proposed based on the assumption of the electrolysis sequence of diffusion and adsorbed electroactive species is described as “on first, off last” and denoted as Onfol model. The numerical simulated (dt/dE)-E waves obtained from the analytical potential-time expressions of the Onfol model present peaks during both “diffusion processes” and “adsorption processes”. Influences of maximum surface excess, adsorbed monolayer thickness, adsorption coefficient and current steps on CRDCP characteristic parameters are discussed. CRDCP behavior allows us to distinguish the form of the electroactive species contributing to the system current and electrolysis sequence can therefore be determined.     

Application of chronopotentiometry and derivative chronopotentiometry with an alternating current to the study of a slow charge transfer in a surface confined redox system

General analytical expressions for the potential-time (E-t) and derivative-potential ((dt/dE)-E) curves of a electroactive monolayer exhibiting a quasi-reversible behaviour, corresponding to the application of an alternating current time function of the form I(t) = I₀ cos(ωt), are presented. The use of this programmed current gives rise to singular and characteristic electrochemical responses. The alternating current also allows to obtain cathodic or both anodic and cathodic responses depending on whether the depletion of the adsorbed species was complete or not, and without using more than one applied current. Moreover, in this last case, it is possible to distinguish a reversible or an irreversible process by means of a simple visualisation of the E-t or (dt/dE)-E curves. Easy methods for estimating thermodynamic and kinetic parameters of the electroactive film are proposed and experimentally tested and compared with those previously obtained by using Cyclic Voltammetry.     

Theoretical investigation on cyclic reciprocal derivative chronopotentiometry: Kinetic study of totally irreversible electrode processes

Theoretical expressions and mathematical analysis in cyclic reciprocal derivative chronopotentiometry (CRDCP) are presented for totally irreversible electrode processes corresponding to the application of symmetrical and unsymmetrical programmed currents. For two successive unsymmetrical programmed currents, the effect of the currents ratio b (b = |I₂(t)/I₁(t)|) on the (dt/dE)–E curves is discussed. The electrochemical behavior of totally irreversible electrode processes has been studied corresponding to the application of the unique unsymmetrical programmed Φ^m(I₀) proposed recently. CRDCP characteristic parameters obtained for totally irreversible electrode processes are different from those of reversible electrode processes. Therefore, a comparison of CRDCP between both mechanisms is presented. Based on the mathematical derivation, alternative methods for kinetic measuring are described. It is prospected that CRDCP is convenient and applicable for studying the reversibility of the electrode processes in form of CRDCP characteristic parameters.     

Anodic depassivation of potassium electrode in propylene carbonate medium

Depassivation of potassium electrode in a highly dried propylene carbonate-KPF₆ electrolyte (< 0.01 ppm H₂O) was studied by using galvanostatic E-t and polarization i-E curves at various scan rates. The results suggest that the passivating layer is strongly absorbed on the electrode surface even during anodic dissolution of potassium, which proceeds only on a certain depassivated fraction of the surface. A mechanism of the anodic depassivation is proposed.     

Theoretical study of a catalytic mechanism using cyclic and derivative chronopotentiometric techniques with spherical electrodes

A general analytical solution for a pseudo-first order catalytic process in chronopotentiometric techniques (reversal chronopotentiometry, cyclic chronopotentiometry and reciprocal derivative chronopotentiometry) when using spherical electrodes of any size is presented. The evolution from transient to stationary potential-time responses, characterized by the disappearance of the periodical signal in the neighbourhood of the steady state, is analyzed in cyclic and reversal chronopotentiometry. From reversal potential-time responses, the reciprocal derivative dt^1/2/dE-E curves - which are more sensitive than traditional dt/dE-E curves - have been obtained. The characteristic peaks presented by the dt^1/2/dE-E curves are quantitatively related to the rate constants of the chemical reaction and show a very different behaviour in catalytic and EC processes, allowing the easy discrimination between both mechanisms. Several types of working curves have been proposed to obtain the rate constants of the chemical reaction in a catalytic process.     

Lithium transport through a sol-gel derived LiMn2O4 film electrode: analyses of potentiostatic current transient and linear sweep voltammogram by Monte Carlo simulation

Lithium transport through a sol-gel derived LiMn₂O₄ film electrode was theoretically investigated by analyses of the potentiostatic current transient and the linear sweep voltammogram in consideration of the interactions between lithium ions by using Monte Carlo simulation. The anodic current transients experimentally measured on the film electrode ran with the slope of logarithmic current with logarithmic time flatter than −0.5 in the early stage, and then did in an upward concave shape in the time interval between t_T1 and t_T2. The linear sweep voltammograms experimentally measured on the film electrode showed two anodic peak currents I_p1 and I_p2 which increased linearly with scan rate v to the power of 0.66 and 0.70, respectively, (i.e. I_p1∝v^0.66 and I_p2∝v^0.70) at the scan rates higher than 0.5 mV s⁻¹. Moreover, the higher v was, the larger appeared the positive deviations of the first and second peak potentials E_p1 and E_p2 from the first and the second transition potentials E°_p1 and E°_p2, respectively, in the inverse derivative of the electrode potential curve. The current transients and the linear sweep voltammograms were analyzed in consideration of the interactions between lithium ions in the electrode by using the Monte Carlo simulation under two different constraints of the diffusion-controlled lithium transport and the cell-impedance-controlled lithium transport. The current transients and the linear sweep voltammograms, theoretically calculated under the cell-impedance-controlled constraint in consideration of the interactions between lithium ions, were in good agreement with the experimental results in shape. The disorder to order phase transition in the LiMn₂O₄ film electrode during the cell-impedance-controlled lithium transport at the potential jump and scan was discussed with the aid of the concentration profiles and the local cross-sectional snapshots of the configuration of lithium ions simulated by the Monte Carlo method.     

Influence of mass transport on the performance of carbon gas-diffusion air electrodes in alkaline solution

The effect of mass transport on the electrochemical behaviour of carbon gas-diffusion air electrodes in alkaline solution was investigated on the basis of ΔE(i) curves. These curves are obtained by subtraction of potential values for an electrode operating with airE _air(i) from potential values for the same electrode operating with pure oxygenE _oxygen(i) at the same current densityi. Three different regions on these curves connected with different modes of mass transport are recognized. A model of the gas-diffusion air electrode which takes into account the diffusion of the gas, diffusion of the dissolved gas, electrochemical reaction and IR drop is used to explain the experimental results.     

Automation of electrode kinetics—V. The dissolution of Al in Cl− and F− containing aqueous solutions

The dissolution of Al in Cl^? and F^? containing aqueous solutions has been investigated over a wide potential range. The steady state current-potential curves and the impedance have been measured using a new electrochemical measurement system. Using an equivalent circuit the C_dl,-E,θ-E, σ_A1-E and R_w-E curves are derived and with the primary i-E and Z(w)-E data are compared for each anion.     

Ferroelectric and electromechanical performance of diverse engineered states of Mn-doped 0.75Pb(Mg1/3Nb2/3)O3-0.25PbTiO3 ceramics

The materials processing history has a great influence on their properties and finally determines their application effect. In this paper, the ferroelectric, polarization-switching current, and strain properties of Mn-doped 0.75Pb(Mg_1/3Nb_2/3)O₃-0.25PbTiO₃ ceramics were studied in fresh state, aged state, and poled state, respectively. Compared with the symmetric polarization-electric-field (P-E) hysteresis loops, current-density-electric-field (J-E) curves, and bipolar electric-field-induced strain (S-E) curves in fresh state samples, asymmetric P-E loops, J-E curves, and bipolar S-E curves were obtained in poled state samples. Well-aged-state samples exhibit double hysteresis P-E loop, four peaks J-E curves, and symmetric S-E curves without negative strain. The symmetry-conforming short-range order (SC-SRO) principle of point defects and internal electric field E_i is employed to clarify the different phenomenon of three states. Results indicated that randomly oriented defect polarization P_D in aged samples can reverse the spontaneous polarization P_S back and result in the double hysteresis P-E loop and four peaks J-E curves. The oriented P_D and resulting E_i in poled-state samples will lead to the asymmetric loops and strain memory effect.     

The influence of the tunnel probability on the anodic oxygen evolution and other redox reactions at oxide covered platinum electrodes

Potentiostatic and galvanostatic pulse measurements were carried out to investigate the anodic oxygen evolution at platinum electrodes in 1N H₂SO₄ in dependence on the oxide layer thickness d and the electrode potential ε. The thickness d (1·5–10 Å) was obtained from cathodic charging curves. Further, the temperature dependence (0°–81°C) was evaluated from Bowden's measurements. Summarizing, the current i_o2 follows the relation, log i = A - (E⁰_a - αFη)/2·3 RT - d/d_o. The experimental activation energy E_o = E^o_a = αFη decreases linearly with increasing overvoltage η. The linear decrease of log i with increasing d, which is given by the term d/d_o, is correlated to the probability of the quantum mechanical tunnel transition of the electron from adsorbed ions, OH^?_ad or O^2?_ad respectively, through the oxide layer to the metal. Similar effects of the oxide layer thickness on the current density were observed in the case of the oxygen evolution at iridium, the CO-oxidation on platinum, and the reduction of Cl^? and Ce⁴⁺ at platinum. In these cases a rate determining electron transfer through the oxide layer is also assumed.     

Growth,corrosion and capacity of copper oxide films investigated by pulse techniques

A new computer supported pulse technique has been applied to investigate the potentiostatic formation of copper oxides at pH 9 in dependence on the formation potential E_f and the formation time t_f. The direct measurement of anodic current densities i₊, charges Q₊ and potentiodynamic reduction spectra i₋ (E) yield the partial current densities of oxide formation i_ox and corrosion i_corr in dependence on E_f and t_f. These measurements were carried out over 8 orders of magnitude of t_f. In short periods of time a species CuO_x is formed which separates into Cu₂O and CuO with increasing potential and total charge Q. The growth of the Cu₂O as well as the CuO layer follow a high field mechanism up to a potential drop Δφ_ox ≈ 0.5 V for each only. Further potential differences must occur in other phases or at the interphase oxide/electrolyte. At the beginning of a potentiostatic transient i_corr is proportional to i_ox,t whereas i_corr dominates when quasi-stationary conditions are approached. The electrode capacity C was measured simultaneously during the oxide formation. The analysis shows that the changing electronic properties of the oxides formed have a large impact on C. Therefore, the simple condenser formula cannot be applied but limited potential drops within each oxide must be taken into account too.     

AC and DC studies of the pitting corrosion of Al in perchlorate solutions

The pitting corrosion behaviour of Al in aerated neutral sodium perchlorate solutions was investigated by potentiodynamic, cyclic voltammetry, galvanostatic, potentiostatic and electrochemical impedance spectroscopy (EIS) techniques, complemented by ex situ scanning electron microscopy (SEM) examinations of the electrode surface. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al₂O₃ on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film by ClO₄⁻ ions. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allow the pitting potential (E_pit) and the repassivation potential (E_rp) to be determined. E_pit decreases with increase in ClO₄⁻ concentration, but increases with increase in potential scan rate. Potentiostatic measurements showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involve pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation (t_i⁻¹) increases with increase in ClO₄⁻ concentration and applied step anodic potential (E_s,a). EIS measurements showed that at E_s,a < E_pit, a charge-transfer semicircle is obtained. This semicircle is followed by a Warburg diffusion tail at E_s,a > E_pit. An attempt is made to compare the values of E_pit and E_rp obtained through different methods and to determine the factors influencing these values in each particular method.     

The automation of electrode kinetics — III. The dissolution of Mg in Cl−, F− and OH− containing aqueous solutions

The dissolution of Mg in Cl^?, F^?, and OH^? containing aqueous solutions has been investigated over a large potential range, from far into H₂ evolution at ? 3000 mV (sce) to the passive state at ? 1400 mV (sce). The steady state current-potential and the impedance have been measured using a new automatic electrochemical measurement system. An equivalent circuit and a least squares fitting procedure have been used to analyse the data. The resulting C_dl-E, θ-E, σ_Mg-E, R_w,-E, and the primary i-E and Z(ω)-E data are compared for each anion, and reveal details of the electrode kinetics of these complex dissolution reactions.     

Strategies for the determination of the convective-diffusion limiting current from steady state linear sweep voltammetry

The limiting current is an important parameter for the characterization of mass transport in electrochemical systems operating under convective-diffusion control. Four methods to determine the limiting current from current (I) vs. potential (E) plots are considered. Strategies to determine the limiting current values include: (1) direct measurement from I vs. E curves, (2) estimation from the current value at E _L = ΔE/2 where ΔE is the length of the limiting current plateau, (3) evaluation of the first derivative dI/dE in the I vs. E curve and (4) from plots of E/I vs. I ⁻¹. The electrode reactions chosen to demonstrate the different strategies are: Cu(II) → Cu(I) and Cu(I) → Cu(0) in 1.5 mol dm⁻³ NaCl (pH 2) at a platinum rotating disc electrode and in 1 mol dm⁻³ NaOH at a 60 ppi reticulated vitreous carbon electrode (RVC).     

Early-age hydration of fresh concrete monitored by non-contact electrical resistivity measurement

This paper reports the investigations on the early hydration process of fresh concrete using a non-contact electrical resistivity measurement. The bulk resistivity development (ρ(t)-t curves) and the pore solution resistivity development (ρ₀(t)-t curves) of fresh concrete, with and without fly ash replacement, were obtained for the period from casting to the age of 48 h. The porosity development (?(t)-t curves) was obtained from the ρ(t)-t and ρ₀(t)-t evolution curves with the calculated initial porosity and the measured porosity at 1 day by mercury intrusion porosimetry (MIP). The degree of hydration (α(t)-t) of the cement-based materials was then obtained based on the porosity change with time, which was caused by the increase of hydration products with a lower density as compared to that of the original cement in the hydration system. The comparison of the degree of hydration predicted from the porosities (?(t)-t) and obtained from the experimental measurement of non-evaporable water showed a good agreement of the two.The setting and hardening behavior of fresh concrete was characterized by the characteristic points on the ρ(t)-t and dρ(t)/dt-t curves. The retarded hydration caused by the fly ash incorporation could be identified by the characteristic points.     

Some oxide catalysts for the anodic evolution of chlorine: reaction mechanism and catalytic activity

The chlorine evolution reaction was investigated on several oxide electrodes. The active site on the electrode surface is concluded to be the metal site, based on the dependence of the anodic current on pH of the electrolyte. The reaction mechanism can be determined by the d-electron configuration in the oxide; ie the reaction proceeds by the Volmer-Heyrovsky mechanism on the electrode containing the transition metal cation with partially filledt_2g orbitals and empty e_g orbitals, whereas the Volmer-Tafel mechanism on the electrode containing the transition metal cation with just or half filled t_2g orbitals and partially filled e_g orbitals.     

Thin film deposition of metal oxides in resistance switching devices: electrode material dependence of resistance switching in manganite films

The electric-pulse-induced resistance switching in layered structures composed of polycrystalline Pr_1−xCa_xMnO₃ (PCMO) sandwiched between Pt bottom electrode and top electrodes of various metals (metal/PCMO/Pt) was studied by direct current current–voltage (I-V) measurements and alternating current impedance spectroscopy. The I-V characteristics showed nonlinear, asymmetric, and hysteretic behavior in PCMO-based devices with top electrode of Al, Ni, and Ag, while no hysteretic behavior was observed in Au/PCMO/Pt devices. The PCMO-based devices with hysteretic I-V curves exhibited an electric-pulse-induced resistance switching between high and low resistance states. Impedance spectroscopy was employed to study the origin of the resistance switching. From comparison of the impedance spectra between the high and low resistance states, the resistance switching in the PCMO-based devices was mainly due to the resistance change in the interface between the film and the electrode. The electronic properties of the devices showed stronger correlation with the oxidation Gibbs free energy than with the work function of the electrode metal, which suggests that the interface impedance is due to an interfacial oxide layer of the electrode metal. The interface component observed by impedance spectroscopy in the Al/PCMO/Pt device might be due to Al oxide layer formed by oxidation of Al top electrode. It is considered that the interfacial oxide layer plays a dominant role in the bipolar resistance switching in manganite film-based devices.     

Investigations of nonlinear processes by transients: kinetics of dissolution of inhibitor layers on copper

The dissolution of inhibitor layers of AHT (3-amino-5-heptyl-1,2,4-triazole) on copper wires in 0.5m H₂SO₄ was studied by potentiostatic current and capacity transients in a time range from 10 µs to 1000 s. The potential dependent transients show three characteristic regions corresponding to different transfer controlled processes. For short periods an almost constant current of inhibited corrosion through a metastable inhibitor layer is observed. In the second region the dissolution of this layer causes a strong increase in copper corrosion, which was taken as a direct measure of the desorption process. The electrode capacity,C, increases correspondingly. The dissolution itself is not accompanied by a notable charge transfer and, hence, cannot directly be monitored. The last region is dominated by an almost constant active copper dissolution current. The layer dissolution in the second region starts with an inhomogeneous nucleation process. Initially, holes are generated stochastically in the AHT layer and form nuclei of corrosion pits on the copper surface. Based on this model the complete transients can be calculated.Notation b _free Tafel slope of an uncovered copper electrode - b _nucl potential dependence of pit formation (Equation 23) - b ₃ b ₃ = dU/d log (di _pits/dt ³) - C electrode capacity - D dielectric constant of the layer - D ₀ dielectric constant of the vacuum - d layer thickness - F Faraday constant - I ₁ time dependent corrosion current of a single pit - i total current density - i _free corrosion current density of an uncovered copper electrode - i _inh current density of an inhibited copper electrode - i _pits corrosion current density of n pits - i _0v extrapolated value ofI _free at 0 V vs SHE - n time dependent number of pits - n _max maximum number of pits - r time dependent radius of a single pit - S total area of all pits t time - U potential against SHE - U _nucl potential of hole formation - V molar volume - z charge number - degree of coverage of inhibitor This paper is dedicated to Professor Brian E. Conway on the occasion of his 65th birthday, and in recognition of his outstanding contribution to electrochemistry.     

Applications of non-equilibrium thermodynamics to the theory of overvoltage

An unambiguous definition of the different overpotential terms is possible based on the theory of non-equilibrium thermodynamics. The overpotentials are a direct consequence of the thermodynamic irreversibility of the processes taking place in an electrochemical system, and the irreversible entropy production of a process occurring at a finite rate is a quantitative measure of this irreversibility. Therefore, the overpotential terms can be directly defined in terms of the entropy production rate of the process in question.

, with η_i = (T/I)(dS_i/dt). Two examples are given: the separation of ohmic and diffusion overpotentials and the separation of the charge transfer and diffusion overpotentials.