Double Layer Capacity Research Articles

AC impedance modelling study on porous electrodes of proton exchange membrane fuel cells using an agglomerate model

A one-dimensional model of the PEM fuel cell cathode is developed to analyse ac impedance spectra and polarisation curves. The porous gas diffusion electrode is assumed to consist of a network of dispersed catalyst (Pt/C) forming spherically shaped agglomerated zones that are filled with electrolyte. The coupled differential equation system describes: ternary gas diffusion in the backing ( O 2 , N 2 , water vapour), Fickian diffusion and Tafel kinetics for the oxygen reduction reaction (ORR) inside the agglomerates, proton migration with ohmic losses and double-layer charging in the electrode. Measurements are made of a temperature-controlled fuel cell with a geometric area of 1.4 cm × 1.4 cm. Lateral homogeneity is ensured by using a high stoichiometry of λ min . The model predicts the behaviour of measured polarisation curves and impedance spectra. It is found that a better humidification of the electrode leads to a higher volumetric double-layer capacity. The catalyst layer resistance shows the same behaviour depending on the humidification as the membrane resistance. Model parameters, e.g. Tafel slope, ionic resistance and agglomerate radius are varied. A sensitivity analysis of the model parameters is conducted.

Estimation of non-ionic, surface-active substances in aqueous solutions by means of the Controlled Growth Mercury Electrode

In this paper, a quick and simple tensammetric method of estimation of non-ionic surfactants (NS) in aqueous solutions is proposed. The method makes use of the variation in the differential capacity of double layer in relation to the time of accumulation ( C d– t acc) of non-ionic surfactants at the hanging mercury drop electrode, generated by a single, very quick opening of the valve. Under such conditions, the capacity current measured at the potential of maximum adsorption diminishes with accumulation time of non-ionic surfactants. The proposed method, which was verified for model surfactant (Triton X-100), may also be applied in the determination of other NS. Modifications in construction of the CGME electrode and its improved metrological parameters played an important role in the presented procedure. In addition, other measurements were performed using standard electrochemical techniques, whereby current–time and differential capacity–potential curves were recorded. Satisfactory results were obtained in the determination of Triton X-100 in the range from 0.05 to 20 mg L −1 (R.S.D. = 6%, recovery = 94–103%, r = 0.999, DL = 0.15 mg L −1). Applicability of the method was presented using the water samples from Białka and Dunajec rivers, from which NS were removed by addition of fumed silica.

Differential Capacity of the Double-Layer Formed at a Solid Electrode (Pt, Au)/Ionic Liquid Interface

The differential capacity at the electrode (Pt, Au)/ionic liquid interface of 18 ionic liquids (ILs), was measured applying chronoamperometry. The measurements were done by a two electrode system. The double layer capacity at the Pt/IL and Au/IL interface was 1 - 8 μF/cm2. The capacity, estimated from the impedance measurements, was approximately constant within a potential range of ca. 3 V.

The influence of nonpolar organics adsorption on the electrochemical behaviour of powdered activated carbon electrodes in aqueous electrolytes

Adsorption and electrochemical studies were carried out on three activated carbon samples first oxidized, then heat-treated under vacuum (at 180, 500 and 900 °C). The investigations were performed with aqueous electrolyte (Na 2HPO 4 and H 3PO 4) solutions containing selected nonpolar organics (benzene and n-hexane). Adsorption measurements were carried out on solution with a wide range of organics concentration (up to saturation point). Cyclovoltammetric curves of powdered electrodes prepared from the activated carbon samples were recorded for the organics in saturated solutions. The electric double layer capacities of the anodic and cathodic parts were estimated, and the surface anodic and cathodic charge was calculated both in absence and presence of organics in the electrochemical systems. The relative surface charge (in relation to systems without organics) was found to decrease with a reduction in the concentration of surface oxygen-containing groups. Other physicochemical parameters characterizing the degree of surface oxidation (total oxygen concentration, primary water adsorption centres) were also taken into consideration. The correlation between adsorption capacity towards the nonpolar organic compounds (obtained from adsorption isotherms) and change of surface charge was analyzed.

Performance of templated mesoporous carbons in supercapacitors

By analogy with other types of carbons, templated mesoporous carbons (TMCs) can be used as supercapacitors. Their contribution arises essentially from the double layer capacity formed on their surface, which corresponds to 0.14 F m −2 in aqueous electrolytes such as H 2SO 4 and KOH and 0.06 F m −2 for the aprotic medium (C 2H 5) 4NBF 4 in CH 3CN. In the case of a series of 27 TMCs, it appears that the effective surface area determined by independent techniques can be as high as 1500–1600 m 2 g −1, and therefore exceeds the value obtained for many activated carbons (typically 900–1300 m 2 g −1). On the other hand, the relatively low amount of surface oxygen in the present TMCs, as opposed to activated carbons, reduces the contribution of pseudo-capacitance effects and limits the gravimetric capacitance to 200–220 F g −1 for aqueous electrolytes. In the case of non-aqueous electrolyte, it rarely exceeds 100 F g −1. It is also shown that the average mesopore diameter of these TMCs does not improve significantly the ionic mobility compared with typical activated carbons of pore-widths above 1.0–1.3 nm. This study suggests that activated carbons remain the more promising candidates for supercapacitors with high performances.

Influence of the electrolyte composition on the kinetics of the oxygen evolution reaction and ozone production processes

The influence of electrolyte composition (pure sulphuric or perchloric acid and their mixtures) on the kinetics of the oxygen evolution reaction (OER) and the electrochemical ozone production (EOP) on beta-PbO2 electrodes was investigated by electrochemical impedance spectroscopy and quasi-stationary polarisation curves. Analysis of the impedance spectra revealed a dependency of the electric double layer capacity and the adsorption pseudocapacitance, due to reaction intermediates, on electrolyte composition. The pseudocapacitance is a function of electrode potential and electrolyte composition, indicating the surface coverage by the reaction intermediates depends on these parameters. Tafel slope data support primary water discharge as rate determining step of the OER and EOP electrode processes. EOP current efficiency data (phiEOP) show pure electrolytes lead to maximum O3 production when compared to mixed electrolytes. According to the theoretical analysis of the electrode mechanism, this behaviour is a consequence of the reduction in surface concentration of the active centres leading to EOP caused by anion adsorption.

The Termodynamic Model of Open-Circuit Potential for Electroless Deposition of Ni on Silicon

This investigation first proposed that open-circuit potential vs time (OCP-) is a novel nonlinear potential step controlled by redox reaction and nucleation. A novel thermodynamic model of OCP- is obtained for electroless deposition of Ni on Pd-activated p-type silicon(100). Thermodynamic transient properties such as capacity of double layer , surface charge density , interfacial tension γ, and resistance of chemical reaction are calculated. Results show that the variation of OCP is dependant on the interfacial double layer. The breakdown and rearrangement of the double layer causes surface charge density step, which triggers momentary nucleation of Ni on the substrate. Results also show that reductant pulses would be an effective way to speed nuclei growth.

PAN/SAN/SWNT ternary composite: Pore size control and electrochemical supercapacitor behavior

Single wall carbon nanotubes (SWNT) act as a compatabilizer for polyacrylonitrile (PAN)/styrene–acrylonitrile (SAN) copolymer blends. Carbonization of PAN/SAN/SWNT blend films results in pore widths in the range of 1–200nm, while carbonized PAN/SAN blend films resulted in pores with typical width of 1–10μm. Electrochemical supercapacitor behavior of the carbonized PAN/SAN/SWNT films was characterized using 6M KOH electrolyte. Surface area and pore size distribution were analyzed using nitrogen gas adsorption and the BET and DFT theories. Double layer capacity of the carbonized PAN/SAN/SWNT films was as high as 205μF/cm2 based on the BET surface area.

Impedance measurements at thin polyaniline films – the influence of film morphology

AC impedance measurements of thin 100-nm polyaniline films electrosynthesized in various acidic electrolyte solutions known to support different film morphologies are reported. By carefully defining experimental parameters obtained data could be satisfactorily explained invoking a simple equivalent circuit assuming a double layer capacity assigned to the polymer/solution interface, a pseudocapacity related to the internal redox capacity of the film, a film and a solution resistance and could be related to film morphologies. The data can be related to structural/morphological features of PANI films in close agreement with generally accepted changes of PANI films as a function of the degree of oxidation and type of anion present during electropolymerization.

Model treatment of double layer charging in electroactive polymer films with two kinds of charge carriers

To calculate such phenomenological parameters of the polymer film impedance theory, as charge transfer resistances and interfacial capacities, a well-known model of homogeneous electroactive films with two kinds of charge carriers is used. General representations of the electrochemical kinetics are applied to describe the injection processes of charge carriers into the films. The exchange current densities (i.e. inverse values of the charge transfer resistances) and the interfacial capacities are calculated as functions of the electrode potential and the bathing electrolyte solution concentration for films with both cationic and anionic nature of counter-ions. In both cases, the exchange current density of electrons as a function of the electrode potential may have an extremum or be monotonous, depending on particular values of the partition coefficient of electrons, while the capacity of the substrate/film interface is a monotonic function of the potential. On the contrary, the exchange current density of counter-ions is a monotonic function of the electrode potential, whereas the capacity of the film/solution interface might depend on the potential in a non-monotone way. The obtained results can be useful for a quantitative treatment of experimental data on the charge transfer resistance and the double layer capacity often observed in the Nyquist plots of the polymer film impedance. Some restrictions of the model used are also discussed.

Adsorption of N, N′-dimethylthiourea at a mercury/aqueous solution of NaClO 4 interface; dependence on the supporting electrolyte concentration

Adsorption of N, N′-dimethylthiourea (DMTU) on mercury electrode from 0.1, 1 and 5 M NaClO 4 was studied as the function of electrode charge density and adsorbate bulk concentration. In the study, the experimental data obtained from the measurements of differential capacity of double layer were used, the measurements of zero charge potential and surface tension at the zero charge potential. In each system studied the values of the relative surface excess increase with an increase of the concentration of N, N′-dimethylthiourea and NaClO 4. The adsorption parameters were obtained from the Frumkin, virial and modified Flory–Huggins isotherms. It was found that the values of free adsorption energy, interactions constants and integral capacity depends on the supporting electrolyte concentration. The strength of the surface bond formed between N, N′-dimethylthiourea and the electrode surface and the influence of water present on the electrode surface in the obtained results of calculations were discussed.

The nitrate reduction process: A way for increasing interfacial pH

This paper is devoted to the electrochemical study of the nitrate reduction in aqueous media. The reduction process allows the production of hydroxyl ions at the electrode surface, increasing the local pH. This process provides a useful tool for the precipitation of metallic oxide/hydroxide such as ZnO. When the interfacial pH was high enough, the chemical equilibrium was shifted towards the crystallization. The nitrate reduction was carried out in the presence of dissolved oxygen. The investigated potential range corresponded to the mixed reduction of oxygen and nitrate where both reactions produced OH− and increased the interfacial pH. In this case, it was shown that the pH could reach a value higher than 12 in 1 M KNO3 instead of 10.4 in the presence of only oxygen. Thus, nitrate reduction was useful for the precipitation occurring at pH higher than 10.4. The electrochemical mechanism was studied by electrochemical impedance spectroscopy in a potential range corresponding to the first step of the oxygen reduction. The stationary current–voltage curve did not highlight any current due to the nitrate reduction. The impedance diagrams revealed two capacitive loops characterizing the reduction process of oxygen, namely the charge transfer process at high frequencies and the diffusion process at low frequencies according to the expectations. In the lowest frequency domain, an inductive loop was observed with an amplitude depending on the potential. It was shown that this response was due to the presence of adsorbed species which blocked a part of the active surface for the oxygen reduction. These adsorbed species came from nitrate reduction even if the stationary current of this reaction was negligible in the corresponding potential range. The decrease of the double layer capacity confirmed this approach.

The effect of surface attachment on ligand binding: studying the association of Mg2+, Ca2+and Sr2+by 1-thioglycerol and 1,4-dithiothreitol monolayers

The difference in the heterogeneous binding of Mg(2+), Ca(2+) and Sr(2+) ions by 1-thioglycerol (TG) and 1,4-dithiothreitol (DTT) spontaneously adsorbed monolayers on Au has been studied following the changes in the double layer capacity. A mathematical treatment, based on calculating the electrochemical potential difference at the monolayer-electrolyte interface, has followed our recent work which dealt with the acid-base equilibrium at the interface as a means of calculating the pK of ionizable SAMs and their binding with Cd(2+). Experimentally, spontaneously adsorbed monolayers of TG and DTT were assembled on Au surfaces and studied by impedance spectroscopy and alternating current voltammetry (ACV). The capacity was measured for each of the modified surfaces at increasing concentrations of the divalent metal ions separately. The goal of this study has been to examine the effect of metal ion binding by similar ligands that are differently attached onto the surface. TG and DTT monolayers differ in their flexibility, which is a result of their attachment to the surface through one and two arms, respectively. The general trend of the apparent heterogeneous association constants of the divalent metal ions, which were calculated from the capacity measurements, was substantially different from the classical Irving-Williams series that is applicable to homogeneous systems. This difference could be nicely explained by the reduction of the degree of freedom and flexibility of the attached ligands.

Adsorption of Cytosine on a Mercury Electrode

The electrosorption behavior of cytosine at the mercury electrode/acetic buffer of pH 4 and 5 interfaces was determined from the double-layer differential capacity measurements extrapolated to zero frequency. Solutions of cytosine were prepared to cover the range from 1 × 10-4 to 6 × 10-3 mol dm-3. Adsorption of cytosine was described by the adsorption isotherms constants derived from the surface pressure data as a function of electrode charge density and bulk concentration. The obtained values of the relative surface excesses Γ′ were higher in the acetic buffer of pH 4 than of pH 5. Maximum of cytosine adsorption in the mentioned buffers was at -581 and -551 mV, respectively. The values of the standard Gibbs energy ∆G° obtained from the Frumkin isotherm were higher in the buffer of pH 4 than of pH 5. The values of the interaction parameter A indicated weaker repulsive interaction between adsorbed molecules of cytosine in the former buffer. The adsorption parameters obtained from the virial isotherm confirmed corresponding parameters obtained from the Frumkin isotherm. The dependences of ΦM-2 on the relative surface excess at a constant charge density were analyzed in order to calculate the electrostatic parameters of the inner layer.

Sobre la impedancia faradaica de la disolución electroquímica del níquel

Consecutive impedance spectra for nickel electrodes in acid medium have been obtained. The first recorded spectrum is fitted to an equivalent circuit with three time constants, if one consider a partially or completely passivated electrode. The second registered spectrum is characteristic to a corrosion kinetic controlled by the transport across a finite layer thickness. The microscopic images show an increase in the irregularity of the electrode surface after the second spectrum is registered. Also, the double layer capacity is considerably increased.

Thermodynamic approach to the double layer capacity of a Pt(1 1 1) electrode in perchloric acid solutions

A thermodynamic method based on the work done by Frumkin and Petrii [A.N. Frumkin, O.A. Petrii, Electrochim. Acta 20 (1975) 347], to calculate the so-called double layer capacity for a Pt(1 1 1) electrode is proposed. The analysis requires careful measurement of the total charge density versus potential curves for a series of solutions with composition (0.1 − x) M KClO 4 + x M HClO 4. A method in which the total charge densities are determined by integration of cyclic voltammograms recorded in solutions with or without chloride is described. Following this procedure the double layer capacity curves were calculated. The double layer capacity curves displayed three peaks that were tentatively assigned to the solvent reorientation, onset of OH adsorption and completion of the OH adlayer. In the hydrogen adsorption region, the double layer capacity values were 14 ± 5 μF/cm 2, in good agreement with previous estimates reported in the literature by using other approaches.

EIS study of niobium films sputtered at different target–substrate angles

Nb films have been magnetron sputtered onto quartz sheets oriented with respect to the target at angles varying between 0° and 90°, with 15° steps. Impedance plots have been obtained by contacting these films with aqueous Na 2SO 4, either at the open circuit potential or at a potential where Nb is covered by an anodic passive Nb 2O 5 film. As the target–substrate angle θ increases, the shape of the impedance plots changes from that of a smooth electrode to that of a porous one, characterised in the high frequency range by a straight line forming a 45° angle with the real axis. The surface roughness of the Nb deposits, calculated from their double layer capacity, is low and constant at low θ, significantly increases at θ = 45°, goes through a maximum in the range 60–75° and drops at θ = 90°. AFM surface profiling confirms this trend, but estimates a lower surface roughness. Attempts to obtain Nb deposits with a surface roughness less strongly dependent on θ have been made by performing the depositions under pulsed conditions or by heating the substrates at 400–600 °C. Heating at the higher temperature was a fairly effective method for decreasing the roughness of deposits formed at large θ.

Simultaneous EIS and in situ microscope observation on a partially blocked electrode application to scale electrodeposition

A novel technique allowed EIS and microscopical observation of the interface between electrode and solution to be coupled in situ. The electrochemical reduction of oxygen was investigated by using this device when the electrode surface was progressively covered by an insulating layer of scale deposit. It was shown that two time domains can be distinguished from the initial time of the experiment corresponding to a bare electrode up to a totally blocked surface. In the first part of the experiment, EIS showed one time constant which characterizes the charge transfer process of water reduction. When the percentage of coverage reached more than 80%, this latter electrochemical contribution to EIS was negligible and the reduction process of oxygen was highlighted by two time constants observed from impedance diagrams. The high frequency response characterized the charge transfer process and the low frequency loop identified the diffusion process of oxygen. An electrochemical model was proposed to explain the shape of the impedance spectra plotted with respect to the time of coverage. The good correlation between the experimental and the fitted model curves led to the change versus time of the different parameters of the equivalent circuit. It notably showed that the double layer capacity was directly proportional to the active surface measured by the microscope observation until the surface was totally covered, according to the expectations in the experimental conditions used here.

The adsorption of thiourea on the mercury from neutral and acidic solutions of perchlorates

The adsorption of thiourea at the interface the mercury/1 mol dm −3 NaClO 4 and the mercury/1 mol dm −3 HClO 4 was studied in the function of the electrode potential and the adsorbate concentration. In the study was used, the experimental data obtained from the measurement of differential capacity of double layer, the measurement of zero charge potential, and surface tension at the zero charge potential. The information concerning thiourea adsorption was obtained from values of relative surface excess, free adsorption energy, interactions constants and the electrostatic parameters of the inner layer. It was found out that protonized thiourea molecules from HClO 4 solutions adsorb better at the interface the mercury/ ClO 4 - . They are also characterized by stronger intermolecular interactions in comparison with thiourea molecules from NaClO 4 solutions adsorbing on the mercury.

Impedance measurements of low temperature mesophase carbon fibers during lithium insertion/extraction

Surface modifications with three different procedures were performed on an integrated felt of mesophase carbon fiber (MCF) fired at 600 °C. The surface modification procedures were (1) heating in acetylene black powder under a low pressure of oxygen (MOT), (2) vacuum deposition of Ag film on the fiber surface and (3) vacuum deposition of Ag film on the fiber, followed by heating under low pressure of oxygen. The performance of Li insertion/extraction reaction was found improved by any of these modification treatments. The cause of the improvement was examined mainly with impedance spectroscopy analysis (EIS). EIS spectra revealed that both the film resistance and the charge-transfer resistance were decreased remarkably by these surface modifications. Among the three modification procedures, MOT revealed the best improving effect. By comparing the EIS data with the results obtained from the double layer capacity measurement, we concluded that the major cause of the increase in the reaction rate is the activation of the reaction site.