Steady-state Current-potential Curves Research Articles

Redox mediation at poly(o-aminophenol) coated electrodes: mechanistic diagnosis from steady state polarization curves

In this work, the mediated reduction of Fe(CN)63- and Fe3+ in poly(o-aminophenol) coated electrodes is analyzed by means of a diagnosis diagram based on the features of the steady state current-potential curves. This analysis allows to identify the current determining process and to reproduce the experimental characteristics of the polarization curve from the relevant kinetic and thermodynamic parameters with a minimum amount of experimental measurements.

Redox mediation at poly(o-aminophenol) coated electrodes: Mechanistic diagnosis from steady state polarization curves

<p class="PaperAbstract"><span lang="EN-US">In this work, the mediated reduction and oxidation of Fe(CN)<sub>6</sub><sup>3-/4-</sup> and Fe<sup>3+ </sup>in poly(o-aminophenol) coated electrodes is analyzed by means of diagnosis diagram based on the features of steady state current-potential curves. This analysis allows to identify the current determining process and to reproduce the experimental characteristics of the polarization curve from the relevant kinetic and thermodynamical parameters with a minimum amount of experimental measurements. </span></p>

Transitions between classes of neuronal excitability and bifurcations induced by autapse

Neuronal excitabilities behave as the basic and important dynamics related to the transitions between firing and resting states, and are characterized by distinct bifurcation types and spiking frequency responses. Switches between class I and II excitabilities induced by modulations outside the neuron (for example, modulation to M-type potassium current) have been one of the most concerning issues in both electrophysiology and nonlinear dynamics. In the present paper, we identified switches between 2 classes of excitability and firing frequency responses when an autapse, which widely exists in real nervous systems and plays important roles via self-feedback, is introduced into the Morris-Lecar (ML) model neuron. The transition from class I to class II excitability and from class II to class I spiking frequency responses were respectively induced by the inhibitory and excitatory autapse, which are characterized by changes of bifurcations, frequency responses, steady-state current-potential curves, and nullclines. Furthermore, we identified codimension-1 and -2 bifurcations and the characteristics of the current-potential curve that determine the transitions. Our results presented a comprehensive relationship between 2 classes of neuronal excitability/spiking characterized by different types of bifurcations, along with a novel possible function of autapse or self-feedback control on modulating neuronal excitability.

Effect of Added Graphitized Carbon Black on Both Performance and Durability of Pt/Nb-SnO2 Cathodes for PEFCs

The development of cathode catalysts that have both high activity for the oxygen reduction reaction (ORR) and high durability is an important subject for the application of PEFCs in fuel cell vehicles. At present, Pt nanoparticle catalysts supported on carbon black (Pt/CB) are typically being used in PEFC cathodes. While CB supports are essential for the high performance of PEFCs because of their large surface area, high electrical conductivity and well-developed pore structure, they corrode seriously during startup and shutdown of the fuel cell. Graphitized carbon black (GCB), which has a high degree of graphitization, has been used to mitigate such corrosion. According to our previous studies, the high stability of a Pt catalyst supported on GCB (Pt/GCB) was confirmed by use of simulated startup/shutdown evaluation.1 Electrochemical catalysts for PEFCs are required to have both electrically conductive paths and gas diffusion paths in order to form effective catalyst layers (CL). Therefore well-developed pore structures are needed in addition to high conductivity. We have developed Pt catalysts supported on doped SnO2 (Pt/Sb-SnO2, Nb-SnO2, and Ta-SnO2) with fused aggregate structures, which are similar to that of CB, constructed by the fusion of nearest-neighbor particles to form a branched structure.2,3 The Pt/doped SnO2 catalyst provides high electronic conductivity due to the low grain boundary resistance and also has a large micropore volume. Due to the effect of the doped SnO2 with fused aggregate structure, the high performance of these cathodes, which was superior to that of Pt/CB, was confirmed by use of single cell measurements.4,5 In this research,6 we report a detailed investigation of a single cell using Pt/Nb-SnO2 with the fused aggregate structure under actual operating conditions. For instance, we confirmed that the steady-state current-potential (I–E) curves of membrane-electrode assemblies (MEAs) were strongly dependent on the humidity condition of the supplied gases. We also investigated the effects of the addition of GCB into the Pt/Nb-SnO2cathode for the improvement of the cell performance. In Fig.1, the single cell performances using Pt/Nb-SnO2 with/without GCB as function of relative humidity were compared with that of a cell using Pt/GCB at 80◦C and hydrogen/air under 1 atm. The Pt-mass specific power of the Pt/Nb-SnO2 cathode under low humidity conditions was superior to that of the Pt/GCB cathode, because the hydrophilic SnO2 support helped to increase the proton conductivity of the ionomer, which led to high Pt effectiveness. The addition of GCB to the Pt/Nb-SnO2 cathode improved the cell performance under high humidity, and the Pt-mass specific power value reached more than 10 kW gPt −1. The improved performance was attributed to the formation of gas diffusion paths due to the addition of hydrophobic GCB. Fig. 2 shows that the Pt/Nb-SnO2 cathodes, with/without GCB, had greater durability than that of the Pt/GCB cathode after 60,000 cycles of potential sweep cycle evaluation (1.0-1.5 V, 0.5 V s-1). Fig. 3 shows transmission electron microscopic (TEM) images of the Pt/Nb-SnO2 CL and Pt/GCB CL both before and after durability evaluation. The Pt nanoparticle size on the Pt/Nb-SnO2 surface increased to 4.9 ± 0.8 nm (after 60000 cycles) from 3.0 ± 0.6 nm (initial state) in diameter during durability evaluation, becoming spherical in shape (Fig. 3(a) and 3(b)). In contrast, it can be observed that the Pt particles on GCB formed elongated clusters, most likely due to the aggregation of spheres, after 60000 cycles in Fig. 3(d). Moreover, we confirmed that the Pt (111) lattice planes were parallel to those of SnO2 (110), and that the Pt nanoparticles were well oriented on the Nb-SnO2 surface in the initial state, as shown in Fig. 4. We consider that such interaction between the Pt nanoparticles and the Nb-SnO2 support could also have suppressed the migration of the Pt nanoparticles during the durability evaluation. We conclude that the Pt/Nb-SnO2 cathodes, both with and without GCB, exhibited outstanding durability during the startup / shutdown potential sweep evaluation in PEFCs.6 Acknowledgement This research was supported by funds for the “Research on Nanotechnology for High Performance Fuel Cells” (HiPer-FC) project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, and the JSPS KAKENHI grant Number B24350093. References M. Uchida et al., Physical Chemistry Chemical Physics, 15, 11236 (2012). K. Kakinuma et al., Electrochimica Acta, 56, 2881 (2011). Y. Senoo et al., RSC Advances, 4, 32180 (2014). K. Kakinuma et al., Electrochimica Acta, 110, 316 (2013). Y. Senoo et al., Electrochemistry Communications, 51, 37 (2015). Y. Chino et al., Journal of the Electrochemical Society, 162, F736 (2015). Figure 1

Coupling between dissolution and passivation revisited – Kinetic parameters of anodic oxidation of titanium alloys in a fluoride-containing electrolyte

In the present paper, the anodic film growth on Ti alloys in water- and fluoride-containing ethylene glycol electrolyte were investigated using electrochemical and surface analytical techniques. Steady-state current–potential curves and electrochemical impedance spectra as depending on potential and fluoride content pointed to two parallel reaction pathways – film growth/dissolution and titanium dissolution through the film. The chemical composition of the anodic films was estimated by X-ray photoelectron spectroscopy (XPS). XPS analyses revealed the presence of a non-stoichiometric oxide containing mainly Ti4+ and a certain amount of Ti3+, with a certain degree of hydroxylation. Mo and Al were found to be enriched in the oxides, and evidence of transpassive oxidation/dissolution of Mo was found. A kinetic model of the process is proposed and its parameters are estimated by quantitative comparison with the current–potential and EIS data. The main features of the XPS data, namely film thickness, concentration of Ti(III) and fluoride as depending on potential are also reproduced by the model.

Electrochemical behaviour of ferrocene in the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate, EMIMBF4, at 298 K

Electrochemical oxidation of ferrocene (Fc) at platinum and glassy carbon electrodes in the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate, EMIMBF4, has been studied by cyclic voltammetry (CV), convolutive potential sweep voltammetry (CPSV), chronopotentiometry (CP) and chronoamperometry (CA).On both electrodes, the electro-oxidation of ferrocene to ferrocenium (Fc+) takes place in a quasi-reversible electrochemical mode. The intrinsic rate constant of charge transfer, k0, and the charge transfer coefficient, α, have been calculated for the first time in the mentioned ionic liquid, by simulation of the cyclic voltammograms, logarithmic analysis of the convoluted curves and from the steady-state current potential curves by applying the Gauss–Newton non-linear square method. The diffusion coefficient of Fc has been also calculated by CV, CPSV, CP and CA. A two-way analysis of variance, ANOVA, has shown that with a 95% confidence level, there are no significant differences between the diffusion coefficients obtained with both electrodes by the different techniques.

Interfacial and bulk processes during oxide growth on titanium in ethylene glycol-based electrolytes

Anodic oxidation has proven to be a promising route for the growth of self-ordering oxide nanotubes on Ti, the best results being obtained in ethylene glycol (EG)-based electrolytes with the addition of fluoride and small amounts of water. In the present paper, emphasis is put on the investigation of barrier film growth and dissolution on Ti in EG electrolytes with the addition of H2O (0.3–2.4 M) and NH4F (0.015–0.17 M) using electrochemical and surface analytical techniques. Steady-state current–potential curves and electrochemical impedance spectra as depending on potential (−0.1/5.0 V vs. AgCl/Ag), water and fluoride content have been registered. In addition, the chemical composition of the surface of the oxides obtained at 0.1 and 1.0 V has been estimated by X-ray photoelectron spectroscopy (XPS). XPS analysis revealed the presence of a non-stoichiometric oxide containing mainly Ti4+ and a certain amount of Ti3+, with a certain degree of hydroxylation. Estimates of the total thickness of the oxide from the XPS data using a dual layer model are also presented. A kinetic model of the process is advanced to quantitatively interpret the electrochemical and surface analytical results.

Electrode reactions of several dissolved hydroquinones on a polyaniline-modified electrode and what occurs in the polyaniline

A conductive polymer polyaniline (PANI) was easily prepared on the electrode surface as a stable film by the electropolymerization of aniline in an acidic solution. In McIlvaine's buffer solution whose pH exceeded 4, no redox current of the dissolved species was observed on the PANI-modified electrode because PANI acted as an electric insulator. Even in a less acidic solution, the redox current of the dissolved hydroquinone was evident. On the other hand, in the potential region greater than 0.7V vs. Ag/AgCl, PANI acted as an electric insulator. Even in the electroinactive potential region, the redox current of the dissolved p-dimethoxybenzene was evident. These two electrochemical responses of the PANI film-modified electrode are caused by processes in which hydroquinone and p-dimethoxybenzene selectively permeate through the PANI film to reach the electrode substrate. The analyses of the rotating disk electrode (RDE) voltammograms using a PANI-modified RDE and steady-state current-potential curves using a PANI pellet electrode revealed that hydroquinone and p-dimethoxybenzene were concentrated in the PANI during the electro-oxidation. Furthermore, the concentration of hydroquinone and p-dimethoxybenzene increased the electric conductivity of the PANI from 6.50×10−7 to 2.73×10−3S/cm for hydroquinone and from 0.64 to 1.92S/cm for p-dimethoxybenzene.

Influence of fluoride content on the barrier layer formation and titanium dissolution in ethylene glycol–water electrolytes

In the present paper, the initial stages of anodic film growth on Ti in water-containing ethylene glycol electrolyte with the addition of fluoride (0.015–0.17M) were investigated using electrochemical and surface analytical techniques. Steady-state current–potential curves and electrochemical impedance spectra as depending on potential and fluoride content point to two parallel reaction pathways – film growth/dissolution and titanium dissolution through the film. The chemical composition of the anodic films in electrolytes with different fluoride content was estimated by X-ray photoelectron spectroscopy (XPS). XPS analyses revealed the presence of a non-stoichiometric oxide containing mainly Ti4+ and a certain amount of Ti3+, with a certain degree of hydroxylation. A kinetic model of the process is proposed and its parameters are estimated by quantitative comparison with the current–potential and EIS data. The apparent reaction orders of the individual steps with respect to fluoride are also estimated. The main features of the XPS data are also reproduced by the model.

Electrochemical Response to Dissolved Hydroquinone on Insulating Polyaniline-modified Electrode in Less Acidic Solution and Enhancement of the Conductivity of Polyaniline

A conductive polymer, polyaniline (PANI), was easily prepared on an electrode surface as a stable film by the electropolymerization of aniline in acidic solution. In McIlvain’s buffer solution with a pH that exceeded 4, no redox current by the dissolved species was observed on the PANI-modified electrode because PANI acted as an electric insulator. However, in a less acidic solution, the redox current of the dissolved hydroquinone was evident because hydroquinone selectively permeates through the PANI film to reach the electrode substrate. The results of steady-state current-potential curves for the oxidation of hydroquinone using a PANI pellet electrode revealed that hydroquinone was concentrated in the PANI during the electro-oxidation. Furthermore, the concentration of hydroquinone increased the electric conductivity of the PANI from 6.50×10-7 S/cm to 2.73×10-3 S/cm.

Electrochemical impedance spectroscopy of fully hydrated Nafion membranes at high and low hydrogen partial pressures

The proton transport mechanism in fully hydrated Nafion 117 membranes was examined via electrochemical impedance spectroscopy (EIS) and steady-state current–potential measurements both in a symmetric H 2, Pt|Nafion|Pt, H 2 cell and in a H 2, Pt|Nafion|Pt, air PEM fuel cell with hydrogen partial pressure values, P H 2 , varied between 0.5 kPa and 100 kPa. In agreement with recent studies it is found that for low P H 2 values the steady-state current–potential curves exhibit bistability and regions of positive slope. In these regions the Nyquist plots are found to exhibit negative real part impedance with a large imaginary component, while the Bode plots show a pronounced negative phase shift. These observations are consistent with the mechanism involving two parallel routes of proton conduction in fully hydrated Nafion membranes, one due to proton migration in the aqueous phase, the other due to proton transfer, probably involving tunneling, between adjacent sulfonate groups in narrow pores. The former mechanism dominates at high P H 2 values and the latter dominates in the low P H 2 region where the real part of the impedance is negative.

Transient and steady-state model of cobalt deposition in borate-sulfate solutions

A model for cobalt deposition on a cobalt rotating disk in borate-sulfate solutions is fit to experimental impedance spectroscopy (EIS) data obtained at different CoSO 4 concentrations and electrode potentials and steady state polarization curves obtained in the same solutions. The model also accounts for the evolution of hydrogen (HER), adsorption–desorption of boric acid and the transport of dissolved species to and from the electrode. The kinetic parameters are estimated by minimization of the sum-of-squares error between the measurements and model predictions using a genetic algorithm. The model could not be satisfactorily fit to all of the EIS spectra in this study with a unique set of kinetic parameters. A single set of parameters could fit the spectra well over the range of potentials at a given CoSO 4 concentration, but the parameters obtained vary with the CoSO 4 concentration. Conversely, the steady-state current–potential curves obtained at the same CoSO 4 concentrations are very well fit with a single set of parameters. Model analysis indicates that the Co(II) reduction rate is controlled by the first step involving the formation of the adsorbed intermediate Co(I) ads. The situation is less clear for the HER where both the first and second steps appear to influence its rate.

Electrochemical consideration on the optimum pH of bilirubin oxidase

Steady-state current–potential curves were obtained for the direct electron transfer (DET) of bilirubin oxidase (BOD) at a highly oriented pyrolytic graphite electrode, and the theoretical analysis based on nonlinear regression enabled us to determine the formal redox potential ( E°′) of BOD in a wide pH range of 2.0 to 8.5. Cyclic voltammetric measurements were also performed for substrates, including p-phenylenediamine (PPD), o-aminophenol (OAP), and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and their E°′ values or the anodic peak potentials (for OAP) were determined at various pH values. The difference in the redox potentials between BOD and substrates (Δ E°′) showed a maximum at pH 6.5 to 8.0, pH 6.5 to 8.0, and pH 3.5 to 4.5 for PPD, OAP, and ABTS, respectively. These pH ranges should be thermodynamically most favorable for the electron transfer between BOD and the respective substrates. In practice, the pH ranges showing a maximum Δ E°′ corresponded well with the optimum pH values for the O 2 reduction activity of BOD: pH 6.5 to 7.5, pH 8.0 to 8.5, and pH 4.0 for PPD, OAP, and ABTS, respectively. Thus, it was suggested that Δ E°′ should be one of the primary factors determining the activity of BOD with the substrates.

Cathodic behaviour of europium (III) on glassy carbon, electrochemical formation of Al4Eu, and oxoacidity reactions in the eutectic LiCl–KCl

Abstract The electrochemical redox process Eu(III)/Eu(II) in the temperature range 673–823 K on glassy carbon was studied by using transient electrochemical techniques. The results showed that the electroreduction of Eu(III) to Eu(II) is quasi-reversible. Accurate values of the intrinsic rate constant of charge transfer, k 0 , as well as of the charge transfer coefficient, α , have been calculated by simulation of the cyclic voltammograms and from the steady-state current potential curves by applying the pit-mapping, Gauss–Newton non-linear square and simplex methods. The diffusion coefficients of Eu(III) and Eu(II) ions have also been calculated. The validity of the Arrhenius law was verified by plotting the variation of the logarithm of the diffusion coefficients vs 1/ T . For the extraction of europium from molten chlorides, the use of a reactive electrode made of aluminium leading to an Al 4 Eu alloy seems to be a pertinent route. Potentiometric titrations of Eu(III) solutions with oxide donors, using a ytria stabilized zirconia electrode “YSZE” as a pO 2− indicator electrode, have shown the formation of europium oxychloride and its solubility product has been determined at 723 K (p k s (EuOCl) = 8.4 ± 0.3).

Thermally prepared Ti/RhO x electrodes IV: O 2 evolution in acid solution

Ti/RhO x layers were prepared by thermal decomposition of Rh chloride hydrate at 400–600 °C. Oxide electrodes were characterized electrochemically by cyclic voltammetry and steady-state current–potential curves in aqueous H 2SO 4. Integration of voltammetric curves gave the surface charge which is proportional to the surface concentration of active sites. A maximum was observed for a calcination temperature around 430 °C. A hump is visible close to 500 °C, indicating a second transition. O 2 evolution was found to take place on Ti/RhO x electrodes with a Tafel slope close to 80 mV and a reaction order with respect to H + slightly <|−1|. The mechanism did not depend appreciably on the calcination temperature. The layer of RhO x was found to resist to O 2 evolution, as indicated by the stability of the voltammetric charge in the course of the experiments. The activity is dominated by surface area effects, true electrocatalytic factors being marginal.

Corrosion mechanisms of steel concrete moulds in the presence of a demoulding agent

The behaviour of E24 mild steel, the constituent steel of most moulds for concrete, was studied by voltamperometry and electrochemical impedance spectroscopy in two electrolytes: (i) a filtered cement solution and (ii) a homogeneous mortar without large air cavities. Experiments were carried out with and without Aquadem®, a demoulding agent in aqueous phase. In the filtered cement solution, the E24 steel is passivated and the passivation mechanism is totally controlled by the anodic process, not by oxygen reduction. Whatever the experimental conditions (O2 concentration, rotation speed, immersion time), the corrosion current, i cor, is equal to the anodic plateau current and is of the order of 0.8 μA cm−2. Therefore i cor can be directly assessed from the steady state current–potential curves. The E24 steel in contact with a homogeneous mortar without large air cavities is passivated as well as in the filtered cement solution. For both electrolytes, the results are independent of the presence or absence of Aquadem®. Therefore the pitting corrosion observed in service conditions does not arise from the presence of a solid phase in the electrolyte but may result from the heterogeneity of concrete created by air cavities.

Oxygen evolution electrocatalysis in alkaline medium at thin Mn xCo 3- xO4 (0 ≤ x ≤ 1) spinel films on glass / SnO 2: F prepared by spray pyrolysis

Films of a series of manganese-cobalt mixed valency spinel oxides of theoretical formula Mn xCo 3-xO 4, (x = 0; 0.25; 0.5; 0.75 and 1), have been prepared by spray pyrolysis at 150 °C on conductive glass in order to relate solid state and surface chemistry to electrocatalytical activity towards the oxygen evolution reaction (OER). X-ray diffraction (XRD) and IR spectroscopy characterized their bulk crystalline properties. Both techniques have confirmed that all oxides belong to the spinel family. Their surface properties were characterized by XPS yielding the Co 3+ / Co 2+ surface ratio. The kinetics of the OER was investigated in 1 M KOH by means of steady state current-potential curves, obtained in the potentiostatic mode and by reaction order determination. The order of reaction ranging from 1.0 to 1.2, and the Tafel slopes ranging from 67 to 69 mV dec −1 can be tentatively interpreted by a mechanism involving a Temkin type of electrosorption of OH intermediates associated with an electrochemical rate-determining step. Replacing progressively Co by Mn, the electrodes were increasingly electrocatalytically less active, surface Co 3+ ions being the active sites, and making Co 3O 4 the most active in the series.

Voltammetric studies of the Au|YSZ interface at temperatures between 300 and 600°C

Abstract Electrochemical processes occurring at the gold|yttria stabilised zirconia interface were studied by cyclic voltammetry and steady-state current potential curves in different mixtures of oxygen and nitrogen at temperatures between 300 and 600°C. Point and paste electrodes were employed. The equilibrium of the oxygen electrode is not established at temperatures below 450°C any more. Contrary to Pt electrodes, the electrochemical formation and reduction of an oxide are reflected by the shape of cyclic voltammograms. However, gold oxide is not formed from O 2 in the gas phase.

Electrochemical characterization of the {(TPA)Mn(III)O2Mn(IV)(TPA)}2(SO4)3 complex

Abstract Thin-layer electrochemistry and steady-state current-potential curves were used to study the electrochemical behavior of the {(TPA)Mn(III)O2Mn(IV)(TPA)}2(SO4)3 complex in an aqueous medium and forecast its role in a catalytic reaction using the usual oxidants. By studying the concentration and pH effects it is possible to assess the complex's stability domain, on the one hand, and to characterize the oxidation state of the two manganese atoms and construct a potential-pH diagram including the various equilibria between the manganese species on the other.

Electrochemical behaviour of magnesium ions in the equimolar CaCl 2NaCl mixture at 550 °C

The electrochemical behaviour of magnesium ions was studied in molten CaCl 2NaCl at 550 °C by cyclic voltammetry, chronoamperometry and chronopotentiometry. We observed that the electrode process of the reduction of Mg(II) on a tungsten electrode is quasireversible. The values of the transfer coefficient, α, and the charge-transfer rate constant, k 0, were calculated by means of the logarithmic analysis of the convoluted curves. Those values were compared with the ones obtained from the steady-state current-potential curves. On the other hand, the diffusion coefficient of the Mg(II) ions, has also been determined by different electrochemical techniques, giving values ranging from 7.2 to 9.9 × 10 −6 cm 2 s −1.