Study of reversible electrode processes with unsymmetrical cyclic reciprocal derivative chronopotentiometry

In this paper, the theoretical study of cyclic reciprocal derivative chronopotentiometry (CRDCP) for totally reversible electrode processes with symmetrical and unsymmetrical programmed currents is presented. The main viewpoints are: (1) for symmetrical programmed current, the amplitudes of the successive currents are the same, i.e. I(t) = (−1)ⁱ⁺¹I₀, whereas the transition times of each current step, τ_i, have different values. The properties of the dt/dE-E curves corresponding to each current cycle also differ a lot. (2) For unsymmetrical programmed current proposed in this work for the first time, the applied current successively reversed at each transition time steps to different amplitude. The use of this unique programmed current is advantageous versus symmetrical programmed current since the transition times obtained are equal to each other anticipatively, and the dt/dE-E curves also coincide with each other for the successive cycles. In this case, the results obtained in CRDCP are almost similar to those of cyclic voltammetry (CV). The characteristic parameters obtained in the dt/dE-E curves in both cases are quantitatively related to the species concentrations adjacent to the electrode surface and afford simple diagnosis criteria to characterize the reversibility of electrode processes. Properties of reversible electrode processes have also been further studied through the use of more than one current cycle.     

Study of charge transfer processes in a surface confined redox system by means of differential staircase voltacoulommetry

A new multipotential pulse technique called differential staircase voltacoulommetry (DSVC) is developed to study charge transfer processes which take place in an electroactive monolayer. We have demonstrated that the faradaic capacitances obtained from DSVC ((ΔQ/ΔE)-E curves) are identical to those obtained in cyclic voltammetry ((I/v)-E curves) when the pulse amplitude fulfils ΔE ? RT/(nF), independently of the reversibility of the electrode process.DSVC presents the advantage of allowing a simpler and more effective correction of the non-faradaic components of the response than does cyclic voltammetry (CV), even considering that the non-faradaic interfacial capacitance changes with the redox state of the adsorbed species.Moreover, DSVC leads to a peak shaped response which is easily characterisable for reversible processes whereas other multipulse techniques lead to null currents in such a way that it is necessary to use short time pulses in order to transform the response to quasi-reversible, which is much more difficult to analyse and which will present a greater distortion, due to non-faradaic effects.Methods for calculating the surface excess and the thermodynamic and kinetic parameters of the electroactive adsorbates are proposed, experimentally tested and compared with those of CV.     

Potentiodynamic and potentiostatic deposition of polyaniline on stainless steel: Electrochemical and structural studies for a potential application to corrosion control

Polyaniline (PAn) films were electrodeposited on stainless steel 304 (SS) from 0.5 M H₂SO₄ solution containing 0.1 M aniline (An) by using potentiodynamic and potentiostatic techniques. In particular, PAn films were prepared as follows: (i) by cyclic potential sweep (CPS) deposition upon varying the upper potential limit (E_l) of the polymerization potential region between 0.8 and 1.1 V, while the lower potential limit was equal to −0.2 V; (ii) by potentiostatic deposition upon varying the applied potential (E_appl) between 0.8 and 1.1 V. The potential sweep rate (dE/dt) was also varied for the An polymerization during the CPS deposition. Variation of the E_l, dE/dt and E_appl affects the PAn growth leading to films of different electrochemical and structural properties. The electrochemical properties of the PAn were examined by using cyclic voltammetry. Scanning electron microscopy was used to reveal the structure and morphology of the PAn films. Monitoring the open circuit potential (E_OC) of the PAn-coated SS electrode in 0.5 M H₂SO₄ shows that the SS remains in the passive state. PAn films can also provide protection to SS in chloride-containing 0.5 M H₂SO₄ for the studied period of time, although pits were detected during prolonged immersion. The protection efficiency seems to be related with the parameters E_l, dE/dt and E_appl varied during polymerization. The mechanism of the SS protection provided by the PAn coatings is discussed in terms of the active role of PAn in corrosive environments.     

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.     

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.     

Hydrogen evolution on conducting polymer electrodes in acidic media

Hydrogen evolution reaction (HER) was studied on polyaniline (PAn), polypyrrole (PPy) and on aniline/pyrrole (PAn–PPy) copolymer in acidic solutions. The cathodic Tafel slopes (b_c) and exchange current densities (j₀) were calculated from Tafel curves obtained in solutions of X M H₂SO₄ (X = 0.1, 0.2, 0.3, 0.4 and 0.5 M). Activation energies (E_a) were determined. The E_a-values were found to be ca. 26 for PAn, 36.5 for PPy, 40.6 for PAn–PPy and 20.6 kJ mol⁻¹for Pt.     

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.     

The reduction of dispersed indigo by cathodically formed 1,2,4-trihydroxynaphthalene

The indirect cathodic reduction of the vat dye indigo (C.I. Vat Blue 1) by cathodically reduced Lawsone (2-hydroxy-1,4-naphthoquinone; C.I. Natural Orange 6) was studied in aq. solution at different pH values. Cyclic voltammetry and spectroelectrochemistry were used to investigate the electrochemical behavior of 2-hydroxy-1,4-naphthoquinone at a hanging mercury drop electrode. The cathodic peak potential (E_p)_d measured at 0.1 mM lawsone solution at a scan rate of 50 mV s^?1 changed from ?425 mV at pH 7, to ?730 mV at pH 11.5 and ?750 mV at pH 13 (vs. Ag/AgCl, 3 M KCl). Particularly at pH values of 8–9 and 11.5–13 voltammograms indicated successful, indirect cathodic reduction of the dye in which the cathodically reduced 2-hydroxy-1,4-naphthoquinone acted as soluble mediator. The linear relationship obtained for (I_p)_d vs. v^1/2 is indicative of a diffusion-controlled electrode reaction mechanism. In the presence of dispersed indigo, the overall cathode reaction is similar to the E_cat process with continuous regeneration of the electroactive species. Spectrochemical experiments were used to prove the indirect cathodic reaction of dispersed vat dyes by 2-hydroxy-1,4-naphthoquinone.     

Influence of halides on the dissolution and passivation behavior of AZ91D magnesium alloy in aqueous solutions

The electrochemical behavior of AZ91D in various aqueous sodium halide solutions was investigated using open-circuit potential (E_oc), potentiodynamic polarization and ac impedance (EIS) techniques. Generally, the results reveal that during immersion a protective layer of a salt film is formed on the alloy surface whose passivation performance depends on the halide nature, its concentration and temperature. E_oc shifts positively with time until attaining a steady (E_st) value, which becomes less noble with increasing concentration or temperature of the test solution. At any given conditions, self-passivation was found to be favored in the order F⁻ > I⁻ > Br⁻ > Cl⁻. This sequence is consistent with that for surface film resistance (R_T) and its relative thickness (1/C_T). Nevertheless, in F⁻ medium each of the above parameters increases with [F⁻] up to a critical value of 0.3 M then decreases. Increasing concentration above 0.3 M induces large change in the microstructure of the outermost layer of the fluorinated extremely protective film and depassivation behavior predominates. In Br⁻ and I⁻ solutions, as well as the lower Cl⁻ concentrations (≤0.01 M), AZ91D exhibits pseudo-passive state over the polarization range from the corrosion potential (E_corr) to the knee point (E_pt) in the anodic scan, at which passivity breakdown occurs with rapid increase in the anodic current and hydrogen gas reaction. At Cl⁻ concentrations >0.01 M the negative difference effect (NDE) occurs under cathodic polarization where E_pt lies negative to E_corr. Addition of F⁻ to the Cl⁻ solution can induce large changes in the behavior of AZ91D. Equal concentration addition (1:1) produces the highest propensity of the surface to form passivating layer that can afford better protection.     

A new gold(III)-aminoethyl imidazolium aurate salt as precursor for nanosized Au electrocatalysts

In this work the novel, robust, air- and moisture-stable gold(III)-aminoethyl imidazolium aurate salt [Cl₃AuNH₂(CH₂)₂ImMe][AuCl₄] (1) has been employed as precursor for the electrosynthesis of gold nanoparticles (AuNPs) onto ITO electrodes in 0.5 mol dm⁻³ H₂SO₄ aqueous solution without the use of additional additives and/or stabilizers. The electrode termed ILNH₂-Au_NPs200 prepared at a fixed electrodeposition time (t_d) of 200 s was characterized with AFM, SEM, XRD diffraction as well as cyclic voltammetry (CVs) and compared with an electrode in which the AuNPs have been electrodeposited from KAuCl₄ in the presence of KI as additive at the same t_d (Au_NPs200). The effect of the t_d (i.e. 50 s or 500 s; electrodes ILNH₂-Au_NPs50 and ILNH₂-Au_NPs500, respectively) was also investigated together with the electrocatalytic activity towards methanol oxidation in alkaline medium of all the prepared electrodes. The comparison with Au_NPs obtained with KI as additive shows that the presence of the amino-functionalized imidazolium moiety although not relevant in relation to the particle size, favours the metal electrodeposition process and plays an important role in the enhancement of the following parameters: (i) surface coverage (S.C.), (ii) electroactive particle coverage Φ_p %, (iii) electroactive surface area σ_red(exp)/σ_red(theor) and (iv) catalytic efficiency.     

Study of phase composition of Zn-Ni alloy electrodeposited in acetate-chloride electrolyte at a temperature of 50 °C

The phase composition of Zn-Ni alloys electrodeposited under potentiodynamic and galvanostatic conditions in acetate-chloride plating solution with the ratio [Zn⁺²]/[Ni⁺²] = 1:12.8 at 50 °C has been investigated by the potentiodynamic stripping and XRD methods. It has been determined that two anodic peaks, which can be attributed to oxidation of the certain phases of Zn-Ni alloy emerged in potentiodynamic stripping curves (PDC) at E < 0 V and at E > 0 V. The Zn-Ni alloy phase that oxidized at E > 0 V was obtained by using cyclic voltammetry with the partial potentiodynamic stripping. This phase is ∼7 at.% Zn solid solution in Ni. The composition of Zn-Ni alloy phase that oxidized at E < 0 is ∼41.0 at.% Zn and ∼59.0 at.% Ni. The ratio of these phases in the alloys depends on the cathodic current density (i_c).     

Investigation of the surface properties and the hydrogen evolution reaction, HER, at thermal rhodium oxide electrodes

A systematic investigation was conducted of the surface properties and the HER at electrodes of nominal composition Ti/Rh_xTi_(1−x)O_y prepared by thermal decomposition (T_cal: 500 °C; t_cal: 2 h; O₂ flux: 5 dm³ min⁻¹) from salt precursor solutions dissolved in 6.0 mol dm⁻³ HNO₃. Films were characterized ex situ by SEM, EDX, XPS and XRD and in situ by open circuit potential measurements and CV. The electrochemical behaviour was investigated by CV as function of the anodic, E_λ,a, and cathodic, E_λ,c, switching potentials showing the Rh surface oxidation states strongly depend on these experimental variables. Surface Rh-sites are reduced to metallic rhodium in the cathodic potential region while higher oxidation states (I-III) are formed at more positive potentials (E ≥ 0.5 V/RHE). Hydrogen adsorption and desorption peaks as well as a short double layer charging region are observed at intermediate potential values. The HER was investigated by Tafel coefficients and reaction order with respect to H⁺ as function of nominal Rh-content.     

Effect of chloride ions on the corrosion behaviour of steel in 0.1 M citrate

The corrosion behaviour of steel was studied in aerated near neutral citrate solutions without and with various concentrations of NaCl. The potentiodynamic anodic polarization curve in 0.1 M citrate solution exhibits four anodic peaks A1, A2, A3 and A4 prior to the oxygen evolution reaction. Addition of Cl⁻ ions to the solution enhances the four peaks currents, specially A3, which is followed by pitting corrosion. The negative going scans of the cyclic voltammograms show two anodic reactivation peaks A5 and A6 and one cathodic plateau P1. A diffusion controlled process in the potential range of A1, A2 and P1 was detected by RDE experiments. The potentiostatic current time transients, at different concentrations of NaCl and applied potentials E_a > A3, were studied. The pit nucleation rate (t_i⁻¹) is found to increase with increasing the concentration of NaCl and the applied anodic potential. The impedance spectra exhibit four different behaviours depending on the potential range used. They were fitted with a single time constant circuit at E_a < −700 mV. However, at −700 mV < E_a < −480 mV, they were fitted with a circuit with two time constants. At E_a > −480 mV, the second semicircle is replaced by negative polarization resistance which is disappeared at E_a > −300 mV. The electrode impedance was found to decrease with the applied potential.     

Low temperature synthesis and dielectric, ferroelectric and piezoelectric study of microwave sintered BaTiO3 ceramics

Barium titanate (BaTiO₃/BT) ferroelectric system was synthesized in single perovskite phase at low temperature by using powders derived from modified solid state reaction (MSSR) and sintered by microwave (MW) processing routes. Conventional calcination temperature was optimized at 900 °C for 4 h. MW sintering of BT samples was carried out at 1100 °C for 30 min to get dense (98% density) ceramics. Room temperature (RT) dielectric constant (?_r) and dielectric loss (tan δ) at 1 kHz frequency of MW sintered BT samples was found to be ∼2500 and 0.03, respectively. Saturated polarization vs. electric field (P-E) loops with remnant polarization (P_r) ∼6 μC/cm² and coercive field (E_c) ∼1.45 kV/cm confirmed the ferroelectric nature of MW sintered BT samples. Piezoelectric coefficient from strain vs. electric field (S-E) loops study was found to be 335 pm/V.     

Electrochemical behaviour of titanium in H2SO4-MnSO4 electrolytes

The corrosion of titanium in H₂SO₄ electrolytes (0.001-1.0 M) at temperatures from ambient to 98 °C has been investigated using steady-state polarization measurements. Four distinct regions of behaviour were identified, namely active corrosion, the active-passive transition, passive region and the dielectric breakdown region. The active corrosion and active-passive transition were characterized by anodic peak current (i_m) and voltage (E_m), which in turn were found to vary with the experimental conditions, i.e., d(log?(im))/dpH=−0.8±0.1 and dE_m/dpH which was −71 mV at 98 °C, −58 mV at 80 °C and −28 mV at 60 °C. The activation energy for titanium corrosion, determined from temperature studies, was found to be 67.7 kJ mol⁻¹ in 0.1 M H₂SO₄ and 56.7 kJ mol⁻¹ in 1.0 M H₂SO₄. The dielectric breakdown voltage (E_d) of the passive TiO₂ film was found to vary depending on how much TiO₂ was present. The inclusion of Mn²⁺ into the H₂SO₄ electrolyte, as is done during the commercial electrodeposition of manganese dioxide, resulted in a decrease in titanium corrosion current, possibly due to Mn²⁺ adsorption limiting electrolyte access to the substrate.     

Electrooxidation of sulfide by cobalt pentacyanonitrosylferrate film on glassy carbon electrode by cyclic voltammetry

A glassy carbon (GC) electrode was modified with cobalt pentacyanonitrosylferrate (CoPCNF) film. Cyclic voltammetry (CV) of the CoPCNF onto the GC (CoPCNF/GC) shows a redox couple (Fe^III/Fe^II) with a standard potential (E^0′) of 580 mV. The current ratio I_pa/I_pc remains almost 1, and a peak separation (ΔE_p) of 106 mV is observed in 0.5 M KNO₃ as the supporting electrolyte. Anodic peak currents were found to be linearly proportional to the scan rate between 10 and 200 mV s⁻¹, indicating an adsorption-controlled process. The redox couple of the CoPCNF film presents an electrocatalytic response to sulfide in aqueous solution. The analytical curve was linear in the concentration range of 7.5 × 10⁻⁵ to 7.7 × 10⁻⁴ M with a detection limit of 4.6 × 10⁻⁵ M for sulfide ions in 0.5 M KNO₃ solution.     

Mechanism of formation and growth of sunflower-shaped imperfections in anodic oxide films on niobium

Anodizing of niobium has been investigated to develop niobium solid electrolytic capacitors. Chemically polished niobium specimens were anodized in a diluted phosphoric acid solution, initially galvanostatically at i_a = 4 A m⁻² up to E_a = 100 V, and then potentiostatically at E_a = 100 V for t_pa = 43.2 ks. During the galvanostatic anodizing, the anode potential increased almost linearly with time, while, during potentiostatic anodizing, the anodic current decreased up to t_pa = 3.6 ks, and then increased slowly before decreasing again after t_pa = 30.0 ks. Images of FE-SEM and in situ AFM showed that nuclei of imperfections were formed at the ridge of cell structures before t_pa = 3.6 ks. After formation, the imperfection nuclei grew, showing cracking and rolling-up of the anodic oxide film, and crystalline oxide was formed at the center of imperfections after t_pa = 3.6 ks. The growth of imperfections caused increases in the anodic current between t_pa = 3.6 and 30.0 ks. Long-term anodizing caused a coalescence of the imperfections, leading to decreases in the anodic current after t_pa = 30.0 ks. As the imperfections grew, the dielectric dispersion of the anodic oxide films became serious, showing a bias voltage dependence of the parallel equivalent capacitance, C_p, and a dielectric dissipation factor, tan δ. The mechanism of formation and growth of the imperfections, and the correlation between the structure and dielectric properties of anodic oxide films is discussed.     

Cluster and discriminant analysis of electrochemical noise data

In a typical pitting system of Q235 low carbon steel in 0.50 mol/L NaHCO₃ + NaCl solutions, a new method is presented to analyze electrochemical noise (EN) data, including potential (E) and current (I) noise, and identify its corresponding pitting states. The proposed method is based on cluster analysis (CA) and discriminant analysis (DA). Firstly, E and log|I| were determined as the variables for clustering. Then, data points (E, log|I|) of the EN groups from different pitting states were classified by CA to two clusters, which relate to the metastable state (Cluster 1) and stable state (Cluster 2), respectively. When a group of (E, log|I|) data points were dispersed stochastically into two clusters, it relates to the intermediate state that was defined to describe the transformation from the metastable pitting to stable pitting. Based on the obtained clustering results, a discriminant function(s) was established to discriminate the ungrouped EN data from the similar pitting processes and thus its corresponding pitting state could be determined by the cluster distribution result. The validity of the cluster/discriminant analysis has been proved in the studied pitting system.     

Electrochemical study of indoor atmospheric corrosion layers formed on ancient iron artefacts

Several indoor atmospheric corrosion layers (0-800 years old) were selected from different localisations in France. Each sample was scrapped from its iron substrate. The resulting powder was mixed with graphite in appropriate proportions and the mixture was pressed onto a stainless steel grid to constitute a composite electrode. The electrochemical responses of the different samples were recorded under galvanostatic regulation, in a near-neutral pH-buffered NaCl solution at 25 °C. The E-t reduction curves allowed the determination of two characteristic parameters, E_τ/2, the potential value obtained at half the transition time, and Q_τ, the coulombic charge obtained at the end of the reduction. The diminution of E_τ/2 and Q_τ with the age of the corrosion layer showed that the “reduction reactivity” decreases with time, suggesting a progressive stabilisation of the corrosion layer.In a second part of the work, we synthesised several common ferric or ferrous/ferric products (goethite, lepidocrocite, magnetite, maghemite, ferrihydrite) and compared their reduction responses (product alone or mixture of 2 or 3 products) to those of corrosion samples.