Double-layer effects on the Cs ion transfer kinetics at the water/nitrobenzene interface

Double layer effects on the kinetics of the Cs⁺ ion transfer across the water/nitrobenzene interface were studied by an ac polarographic technique. Apparent kinetic parameters agree well with those reported previously. By applying the Frumkin-type correction based on the Gouy-Chapman theory, the non-linear Tafel plots were inferred indicating some dependence of the charge transfer coefficient on the interfacial potential difference. The latter effect can weaken the contribution of the diffuse-layer potentials to the apparent kinetic parameters, thereby accounting in part for the observed negligible change of these parameters with the aqueous electrolyte concentration. The Frumkin-type correction failure at low electrolyte concentrations and high surface charge densities is due to an overestimation of the diffuse/layer potential by the Gouy-Chapman theory, rather than due to the dynamic (Levich) effect of the diffuse double layer.     

The capacitance of the diffuse layer of electric double layer of electrodes in molten salts

Similarly to aqueous electrolytes, the electric double layer of electrodes in molten salts is assumed to be composed of compact and diffuse layers. The charge density of the compact layer, formed as a monolayer of specifically adsorbed anions (primary ionic shell), is calculated as the difference between the charge of the primary ionic shell and the charge removed by the exchange current density. The centre of the specifically adsorbed anions create the inner Helmholtz plane (iHp). The counterions to the specifically adsorbed anions in the primary ionic shell, take place in the numerous neighbouring holes, introduced into the molten salt structure by the melting process, and being subjected to thermal motion, create the diffuse layer. The electrostatically adsorbed metal cations form the outer Helmholtz plane (oHp) with the value of the inner potential equal φ₂. Using the Boltzman and Poissone equations, the equation for the capacitance of the diffuse layer of the metallic electrode in molten salt is derived and tested on some literature experimental results.     

Diffuse layer effects on the current in galvanic cells containing supporting electrolyte

We study the effect of an inert supporting electrolyte on the steady-state ionic current through galvanic cells by solving the full Poisson-Nernst-Planck transport equation coupled to the generalized Frumkin-Butler-Volmer boundary equation for the electrochemical charge transfer at the electrodes. Consequently, the model presented here allows for non-zero space charge densities locally at the electrodes, thus extending the frequently used models based on the local electroneutrality condition by including diffuse layer (DL) effects. This extension is necessary since the DLs determine the ion concentration and electrical field at the reaction planes, which uniquely determine the charge transfer at the electrodes.In this work we present numerical results for systems which contain added inert supporting electrolyte using finite element discretization and compare those with semi-analytical results obtained using singular perturbation theory (limit of negligibly thin DLs). In case of negligibly thin DLs the presence of supporting electrolyte will introduce a limiting current below the classical diffusion-limiting current. Just as for systems without supporting electrolyte, the supporting electrolyte induced limiting current formally does not occur for systems having non-negligibly thin double DLs. For thin, however still finite, double layers this limit can still be seen as a steepening of the polarization curve for current vs. voltage.     

Corrosion of pure magnesium under thin electrolyte layers

The corrosion behavior of pure magnesium was investigated by means of cathodic polarization curve, electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) under aerated and deaerated thin electrolyte layers (TEL) with various thicknesses. Based on shot noise theory and stochastic theory, the EN results were quantitatively analyzed by using the Weibull and Gumbel distribution function, respectively. The results show that the cathodic process of pure magnesium under thin electrolyte layer was dominated by hydrogen reduction. With the decreasing of thin electrolyte layer thickness, cathodic process was retarded slightly while the anodic process was inhibited significantly, which indicated that both the cathodic and anodic process were inhibited in the presence of oxygen. The absence of oxygen decreased the corrosion resistance of pure magnesium in case of thin electrolyte layer. The corrosion was more localized under thin electrolyte layer than that in bulk solution. The results also demonstrate that there exist two kinds of effects for thin electrolyte layer on the corrosion behavior of pure magnesium: (1) the rate of pit initiation was evidently retarded compared to that in bulk solution; (2) the probability of pit growth oppositely increased. The corrosion model of pure magnesium under thin electrolyte layer was suggested in the paper.     

Influence of ionic association on Cs+ adsorption at the mercury electrode from glycols

The specific adsorption of Cs⁺ on the Hg electrode from ethylene glycol and 1,2-propylene glycol has been studied by means of capacity measurements using the method of the mixed electrolyte (LiCl + CsCl) at constant ionic strength. Adsorption has been found to obey a virial isotherm only formally. If account is taken of the effect of the diffuse layer, deviations are observed which increase with concentration, electrode surface charge and amount of specific adsorption. At low surface concentration of specifically adsorbed charge, the adsorption parameters have been found to depend on electrode charge and bulk electrolyte concentration. The observed behaviour is explained in terms of ionic association in the inner layer as a consequence of the tendency to form ion pairs in the solution bulk in the non-aqueous solvents investigated.     

An assessment of the dual capacitor model for the metal—electrolyte double layer

The long-accepted subdivision of the double layer at the metal—electrolyte interface into compact and diffuse regions, resulting in Grahame's construction of a concentration-independent compact layer capacity, is shown to be a consequence of certain unreasonable assumptions and approximations. Theoretical treatments of the double layer involving the orientational behaviour of a layer of water molecules between a charged metal surface and a layer of charge at the outer Helmholtz plane are shown to be quite unrealistic.     

Imposed currents in galvanic cells

We analyze the steady-state behavior of a general mathematical model for reversible galvanic cells, such as redox flow cells, reversible solid oxide fuel cells, and rechargeable batteries. We consider not only operation in the galvanic discharging mode, spontaneously generating a positive current against an external load, but also operation in two modes which require a net input of electrical energy: (i) the electrolytic charging mode, where a negative current is imposed to generate a voltage exceeding the open-circuit voltage, and (ii) the “super-galvanic” discharging mode, where a positive current exceeding the short-circuit current is imposed to generate a negative voltage. Analysis of the various (dis-)charging modes of galvanic cells is important to predict the efficiency of electrical to chemical energy conversion and to provide sensitive tests for experimental validation of fuel cell models. In the model, we consider effects of diffuse charge on electrochemical charge-transfer rates by combining a generalized Frumkin-Butler-Volmer equation for reaction kinetics across the compact Stern layer with the full Poisson-Nernst-Planck transport theory, without assuming local electroneutrality. Since this approach is rare in the literature, we provide a brief historical review. To illustrate the general theory, we present results for a monovalent binary electrolyte, consisting of cations, which react at the electrodes, and non-reactive anions, which are either fixed in space (as in a solid electrolyte) or are mobile (as in a liquid electrolyte). The full model is solved numerically and compared to analytical results in the limit of thin diffuse layers, relative to the membrane thickness. The spatial profiles of the ion concentrations and electrostatic potential reveal a complex dependence on the kinetic parameters and the imposed current, in which the diffuse charge at each electrode and the total membrane charge can have either sign, contrary perhaps to intuition. For thin diffuse layers, simple analytical expressions are presented for galvanic cells valid in all three (dis-)charging modes in the two subsequent limits of the ratio δ of the effective thicknesses of the compact and diffuse layers: (i) the “Helmholtz limit” (δ → ∞) where the compact layer carries the double layer voltage as in standard Butler-Volmer models, and (ii) the opposite “Gouy-Chapman limit” (δ → 0) where the diffuse layer fully determines the charge-transfer kinetics. In these limits, the model predicts both reaction-limited and diffusion-limited currents, which can be surpassed for finite positive values of the compact layer, diffuse layer and membrane thickness.     

The effect of electrolyte anions incorporated into anodic oxide films on the experimental transport numbers of ions

The growth of barrier anodic film is considered theoretically with regard to the migration of three ionic carriers: oxygen and metal ions and electrolyte anions. It is shown that the consideration of anion transport leads to the conclusion that the film grows at three interfaces: the metal/oxide and oxide/electrolyte interfaces and the interface between an oxide layer containing electrolyte anions (contaminated layer) and the oxide layer free of them (“pure” layer). The error in the measured transport numbers of metal and oxygen, which is caused by ignoring a contribution of electrolyte anions to the total charge transport, is maximum in the absence of anion motion.     

Effects of electrolyte pattern on mechanical and electrochemical properties of solid oxide fuel cells

In order to enhance the electrochemical performance and reduce the operation temperature of a conventional electrolyte supported solid oxide fuel cell (SOFC), a three layered electrolyte with various geometry is designed and fabricated. Novel three layered electrolytes comprise a dense and thin scandia alumina stabilized zirconia (ScAlSZ) electrolyte layer sandwiched between two hallow ScAlSZ electrolyte layers each having the same thickness as the support but machined into a filter like architecture in the active region with circular, rectangular and triangular cut off patterns. The percent of thin electrolyte layer in the active region is kept constant as 30% for all designs in order to investigate the effect of pattern geometry on the mechanical properties and the performance of the electrolytes. Single cells based on novel electrolytes are manufactured and electrochemical properties are evaluated. A standard electrolyte and electrolyte supported cell are also fabricated as a base case for comparison. Although the electrolyte having triangular patterns has the highest peak power at all operation temperatures considered, it exhibits the lowest flexural strength.     

Charge transport properties of poly(N-vinylimidazole) containing [Os(N)6]2+/3+ moieties

The rate of charge transport of electrodes modified with osmium containing poly(N-vinylimidazole) has been examined as a function of the nature of the contacting electrolyte solution and of temperature. Heterogeneous electron transfer from the electrode into the polymer film has also been investigated. The charge transport parameters show that the nature of the electrolyte anion and its concentration have a large impact on the polymer morphology. In sulfuric acid the films appear significantly swollen, hydrated, and porous, while in perchlorate-containing solutions they are rather compact. Activation energies for the rate-determining step of charge transport show that, depending on the electrolyte, segmental chain motion or ion movement represents the rate-limiting process.     

Dynamics of double-layer by AC Modulation of the Interfacial Capacitance and Associated Transfer Functions

The Modulation of Interface Capacitance Transfer Function (MICTF) technique in combination with Electrochemical Impedance Spectroscopy (EIS) and derived transfer functions has been developed for the study of double-layer relaxation and adsorption processes. Using a Hg electrode in NaBr and KCl solutions as examples, the MICTF is shown to be well-suited for the investigation of physico-chemical processes occurring inside the diffuse layer. Two time-constants are found and their characteristics are analyzed as a function of the potential of zero charge (PZC), the electrolyte concentration, and the nature of the electrolyte. Interpretation is discussed in the framework of accepted double-layer theories including relaxation in the diffuse double-layer and adsorption processes.     

Charge transport properties of poly(N-vinylimidazole) containing [Os(N)6]2+/3+ moieties

The rate of charge transport of electrodes modified with osmium containing poly(N-vinylimidazole) has been examined as a function of the nature of the contacting electrolyte solution and of temperature. Heterogeneous electron transfer from the electrode into the polymer film has also been investigated. The charge transport parameters show that the nature of the electrolyte anion and its concentration have a large impact on the polymer morphology. In sulfuric acid the films appear significantly swollen, hydrated, and porous, while in perchlorate-containing solutions they are rather compact. Activation energies for the rate-determining step of charge transport show that, depending on the electrolyte, segmental chain motion or ion movement represents the rate-limiting process.     

Total internal reflection second harmonic generation from the interface between two immiscible electrolyte solutions

Total internal reflection second harmonic generation (TIR SHG) is used to investigate the interface between two immiscible electrolyte solutions (ITIES) under potential control. The observed potential dependent second harmonic response is attributed to a resonance enhanced process arising from the tetraphenylborate (TPB⁻) ion. As the aqueous phase is biased positive of the organic phase, a large increase in the SH response is observed and is attributed to the adsorption of the TPB⁻ ion at the interface. The potential dependent SH data allows for the determination of the TPB⁻ anion concentration at the interface. TIR SHG compliments measurements of the electrolyte surface excess and can function as a means of measuring the relative concentration of electrolytic species responsible for the potential drop across the electrochemical double layer.     

Influence of hydrophilicity in micro-porous layer for polymer electrolyte membrane fuel cells

Water management is one of the most important factors for improving the performance in polymer electrolyte membrane fuel cells (PEMFCs). The micro-porous layers (MPLs) in the membrane-electrode assembly provide proper pores and paths for mass transport, thereby allowing for the control of the water balance. In this study, a copolymer containing hydrophilic functional groups is introduced into the binder materials of the MPL instead of a highly hydrophobic binder. When 10 wt.% of the binder is incorporated in the MPL on the cathode side, the best performance is exhibited and the ohmic resistance is decreased. Although the charge transfer resistance at low potential is higher than that of the hydrophobic treated MPL, due to the flooding effects, the charge transfer resistance at high potential becomes smaller. This indicates that excess liquid absorption from the catalyst layer to the hydrophilic MPL occurs more strongly than in the case of the hydrophobic MPL. This may bring about an increase in the accessibility of oxygen to the active sites, because the excess liquid near the catalyst agglomerates is expelled as fast as possible. Consequently, the hydrophilicity control in the MPL has a positive effect on the water management in PEMFCs.     

The interface between a metal and an electrolyte

A discussion is given of the interface between a metal and an electrolyte, which includes both a summary of former results and certain new ideas, in particular an analysis of the electrostatic field in the inner region, of the properties of water there and of surface diffusion. Some of the main results are:

The dielectric constant of the water in the inner region is about fifteen when the field vanishes; this occurs to the anodic side of the electrocapillary maximum (e.c.m.). It drops to a smaller constant value (between three and five) for strong fields in the inner region.

The rise in the capacitance of a mercury electrode in a fluoride electrolyte for anodic polarization is probably due, at any rate partly, to adsorbed hydroxyl ions. Mercury adions on the mercury surface may also play a part.

The assumption of constant field in the inner region breaks down for considerable specific adsorption.

Reactions at the interface, including ion-transfer and electron-transfer processes, are also discussed.

In electrodeposition surface diffusion may be rate-determining in certain circumstances, but not if the individual jump process is slower than passage across the barrier between the outer Helmholtz plane and the metal.     

Membrane-based electrolyte sheets for facile fabrication of flexible dye-sensitized solar cells

New electrolyte sheets based on porous polyethylene membranes for flexible dye-sensitized solar cells have been developed. Ionic liquid electrolytes are accommodated in commercial polyethylene membranes to form the electrolyte sheets. The morphology of membranes and iodine concentrations in ionic liquid are varied. The electrochemical measurement results show that the morphology, pore structure, and iodine concentration affect mass transport in electrolyte sheet, as well as charge transfer between platinum electrode and electrolyte sheet greatly. Based on these electrolyte sheets, lamination method instead of conventional vacuum injection of electrolyte is used to fabricate flexible dye-sensitized solar cells. Optimal device with an open-circuit voltage (V_oc) of 0.63 V, a fill factor of 0.58, and a short-circuit current density (J_sc) of 6.17 mA cm⁻² at an incident light intensity of 100 mW cm⁻² is obtained, which yields a light-to-electricity conversion efficiency of 2.25%.     

The voltammetric responses of nanometer-sized electrodes in weakly supported electrolyte: A theoretical study

The effect of the supporting electrolyte concentration on the interfacial profiles and voltammetric responses of nanometer-sized disk electrodes have been investigated theoretically by combining the Poisson-Nernst-Planck (PNP) theory and Butler-Volmer (BV) equation. The PNP-theory is used to treat the nonlinear couplings of electric field, concentration field and dielectric field at electrochemical interface without the electroneutrality assumption that has been long adopted in various voltammetric theories for macro/microelectrodes. The BV equation is modified by using the Frumkin correction to account for the effect of the diffuse double layer potential on interfacial electron-transfer (ET) rate and by including a distance-dependent ET probability in the expression of rate constant to describe the radial heterogeneity of the ET rate constant at nanometer-sized disk electrodes. The computed voltammetric responses for disk electrodes larger than 200 nm in radii in the absence of the excess of the supporting electrolyte using the present theoretical scheme show reasonable agreements with the predications of the conventional microelectrode voltammetric theory which uses the combined Nernst-Planck equation and electroneutrality equation to describe the mixed electromigration-diffusion mass transport without including the possible effects of the diffuse double layer (Amatore et al. [25]). For electrodes smaller than 200 nm, however, the voltammetric responses predicated by the present theory exhibit significant deviation from the microelectrode theory. It is shown that the deviations are mainly resulted from the overlap between the diffuse double layer and the concentration depletion layer (CDL) at nanoscale electrochemical interfaces in weakly supported media, which will result in the invalidation of the electroneutrality condition in CDL, and from the radial inhomogeneity of ET probability at nanometer-sized disk electrodes.     

Impedance studies of cathode/electrolyte behaviour in SOFC

This paper reports impedance studies of the cathode/electrolyte behaviour in solid oxide fuel cells (SOFC), based on comparative investigation of half-cells with yttria stabilized zirconia (YSZ) electrolyte and different cathode materials: lanthanum strontium manganite (LSM), and composite LSM/YSZ with low ionic conductivity as well as the electron conducting Ag, Pt and Au. For improved impedance data analysis the technique of the differential impedance analysis is applied. It ensures structural and parametric identification without preliminary assumptions about the working model. It is found that despite the low ionic conductivity of LSM, the cathode reaction of the oxide cathode materials is a two-step process including: (i) charge transfer with activation energy of the resistivity E_a increasing with the temperature and (ii) transport of oxygen ions through the bulk of the electrode (rate-limiting stage) with E_a independent on the temperature. For the metal (electron conducting) electrodes, the reaction behaviour is described with one step process with higher E_a at higher temperatures. The activation energy of the electrolyte conductivity decreases with the increase of the temperature. The observed changes in E_a for the electrolyte and the cathode reaction (the charge transfer step for the LSM-based electrodes) appear in the same temperature interval. This interesting coincidence suggests for correlation between the bulk (electrolyte) and surface conduction properties. Approaches for improvement of both the ionic conductivity and the supply with electrons in LSM should be also searched.     

Effect of concentration polarization on the current-voltage characteristics of ion transfer across ities

Current-voltage characteristics of ion transfer across the ITIES was theoretically studied, taking concentration polarization into account through the Nernst-Planck equation in the diffusion boundary layers. In the inner layer transport was modelled using either Butler-Volmer or Nernst-Planck equations. Potential distribution across the ideally polarizable ITIES was calculated from the Poisson-Bolzman equation. The current-voltage curves were of the Butler-Volmer type, but no distinction between the two approaches inside the inner layer could be seen because the permeability of the entire system was determined by the diffusion boundary layers.