EIS measurements in the diagnosis of the environment within a PEMFC stack

The diagnosis of flooding or drying is an important factor in improving the reliability and durability of PEMFCs. Indeed, flooding and drying lead to a reduction in the output voltage delivered by the fuel cell. Among the various diagnostic techniques, electrochemical impedance spectroscopy (EIS) is a powerful tool. In this study, EIS measurements were carried out on a 500 W stack comprised of 16 elementary cells. Electrochemical impedance spectra were recorded either on each cell or on the stack. Use of an electrical equivalent circuit model revealed that flooding mainly affects the transfer resistance and the cathodic Warburg impedance. A diagnostic tool based on these two components is proposed.     

Pitting corrosion studies of super austenitic stainless steels in natural sea water using dynamic electrochemical impedance spectroscopy

Potentiodynamic anodic polarisation and dynamic electrochemical impedance spectroscopic (DEIS) measurements were carried out on type 316L stainless steel (SS), alloys 33 and 24 in natural sea water environment in order to assess pitting corrosion resistance. The results revealed that the pitting corrosion resistance was higher in the case of alloys 33 and 24 than 316L SS; due to the higher contents of nitrogen, chromium and molybdenum. DEIS measurements were performed over a wide range of potentials covering the corrosion potential, passive region, breakdown region and dissolution region. It was shown that the impedance measurements in potentiodynamic conditions allow simultaneous investigation of changes in passive layer structure. The impedance spectra of various potential regions were also discussed. The Nyquist plots were fitted using non-linear least-square (NLSS) method for different potential regions.     

Mass transfer modelling for GS heavy water plants: Part 1: Point efficiency on GS sieve trays

A model that takes into account the gas-phase and liquid-phase resistance to mass transfer has been developed, where the overall mass transfer coefficient (K_OGa) is expressed as a function of the equivalent Sauter-mean bubble diameter. This parameter was back calculated from mass transfer measurements made at a pilot plant on single pass sieve trays of 0.311 m diameter. Hydraulic parameters were measured for these trays as well. Mean bubble diameters were then correlated as a function of active area F-factor and dispersion height for various tray geometries, and these correlations are used to predict point efficiencies on production plant trays up to 8.5 m in diameter.     

Study on Resistance Switching Properties of Na0.5Bi0.5TiO3 Thin Films Using Impedance Spectroscopy

The Na_0.5Bi_0.5TiO₃ (NBT) thin films sandwiched between Au electrodes and fluorine-doped tin oxide (FTO) conducting glass were deposited using a sol–gel method. Based on electrochemical workstation measurements, reproducible resistance switching characteristics and negative differential resistances were obtained at room temperature. A local impedance spectroscopy measurement of Au/NBT was performed to reveal the interface-related electrical characteristics. The DC-bias-dependent impedance spectra suggested the occurrence of charge and mass transfer at the interface of the Au/NBT/FTO device. It was proposed that the first and the second ionization of oxygen vacancies are responsible for the conduction in the low- and high-resistance states, respectively. The experimental results showed high potential for nonvolatile memory applications in NBT thin films.     

Use of impedance measurements for the determination of the instant rate of metal corrosion

The various methods of evaluating corrosion rate and charge transfer resistance have been critically reviewed on the basis of the recent developments on the measurement and interpretation of faradaic impedances. It is demonstrated that the entity which is most accurately correlated with corrosion rate is the transfer resistance, the limit of the faradaic impedance at infinite frequency. In the case of iron, with and without inhibitor (propargylic alcohol), it has been ascertained, under various experimental conditions, that the measurement of this resistance constitutes nowadays the best electrochemical test for corrosion, and allows thea priori calculation of corrosion rate.This paper constitutes a chapter of H. Takenouti's University thesis, which was presented in Paris on June 25, 1971 [1]     

Mass transfer in trickle-bed reactors at low reynolds number

Mass transfer rates were determined in a 3.4 cm i.d. trickle-bed reactor in the absence of reaction by absorption measurements and in presence of reaction. Gas flow rates were varied from 0-100 l/h and liquid flow rates from 0-1.5 l/h. The catalyst particles were crushed to an average diameter of 0.054 and 0.09 cm. Mass transfer coefficients remained unaffected by change in gas flow rate but increased with liquid rate. The data from absorption measurements were evaluated with predictions based upon plug-flow and axial dispersion model. Mass transfer coefficients were found greater in case of axial dispersion model than that of plug-flow model specially at low Reynolds number (Re₁ < 1).Hydrogenation of α-methylstyrene to cumene using a Pd/Al₂O₃ catalyst was taken as a model reaction. Intrinsic kinetic studies were made in a laboratory-stirred-autoclave. Mass transfer coefficients were determined using these intrinsic kinetic data from the process kinetic measurements in trickle-bed reactor. Mass transfer coefficients under reaction conditions were found to be considerably higher than those obtained by absorption measurements.Correlations were suggested for predicting mass transfer coefficients at low Reynolds number.The gas to liquid mass transfer coefficients for lower gas and liquid flow rates were determined in a laboratory trickle-bed reactor. The effect of axial dispersion on mass transfer was considered in order to evaluate the experimental data. Three correlations were formulated to calculate the mass transfer coefficients, which included the effect of liquid loading, particle size and the properties of the reacting substances. The gas flow rate influences the gas to liquid mass transfer only in the region of low gas velocities. In the additional investigations of gas to liquid mass transfer without reaction in trickle-bed reactor, the mass transfer coefficients were determined under reaction conditions and the intrinsic kinetics was studied in a laboratory scale stirred autoclave with suspended catalyst. A few correlations are formulated for the mass transfer coefficients. A comparison with the gas-liquid mass transfer coefficient obtained by absorption measurements showed considerable deviations, which were illustrated phenomenologically.     

A.c.-impedance spectroscopy study of oxygen reduction at Nafion® coated gas-diffusion electrodes in sulphuric acid: Teflon loading and methanol cross-over effects

The kinetics of oxygen reduction have been studied at Nafion®-coated dual layer gas-diffusion electrodes at 60 °C in 2.5 M H₂SO₄. A.c. impedance and steady state galvanostatic measurements have been carried out on electrodes containing various PTFE loadings. A.c. impedance spectroscopy supplied information on charge transfer resistance, series resistance and double layer capacitance of electrodes during the oxygen reduction process. These parameters, together with those derived by d.c. measurements (i.e. Tafel slopes, charge transfer coefficient, etc.), allowed a better identification of the features governing the electrode efficiency and the reduction mechanism. The best electrode was found to be one containing 30% PTFE; this showed both the lowest charge transfer resistance and the lowest overpotential for oxygen reduction. This electrode also showed lower overpotential than the same uncoated electrode at low current densities. The poisoning effect by methanol has been also investigated. Upon addition of methanol to the solution, no change was observed in the reaction mechanism at low overpotentials. In contrast, a significant variation of the Tafel slope was observed at high overpotentials. This behaviour is interpreted in terms of the variation of Pt-O bond strength as a function of electrode potential.     

Investigation of hydrogen adsorption-absorption on iron by EIS

This work concerns the interaction between hydrogen and iron in the cathodic potential region. It was motivated by the need for a better understanding of the hydrogen insertion mechanism in metals. Electrochemical deposits of iron with various thicknesses were carried out on a gold substrate and they were characterized by impedance measurements under hydrogen evolution conditions.The processes occurring at the surface and within the iron films deposited on a gold electrode were studied using various electrochemical techniques. The impedance and voltammetric behaviours were strongly dependent on the film thickness. The main result of this study is that the charge transfer resistance increases with the film thickness. A model of the adsorption-absorption of hydrogen into iron films was proposed. It considered two types of absorbed hydrogen in a sublayer richer in hydrogen than the bulk metal. There, the hydrogen transported in the film coexists with another one coming from a direct absorption mechanism and “trapped” in some sites. The two absorbed hydrogen are reversibly exchanged. The interaction of hydrogen with palladium and iron was compared. It was concluded that for iron, the insertion of hydrogen results from a competition between a one-step direct hydrogen absorption mechanism and the classical two-step indirect penetration in the metal via the adsorbed hydrogen. At the end, a possible explanation of the deep penetration of the direct hydrogen absorption in the metal, detected by the impedance analysis, is proposed. It is based on the imperfections of numerous first layers of the deposited metals on polycrystalline gold.     

Mass transfer accompanied by chemical reaction at the surface of a single droplet

Mass transfer accompanied by chemical reaction at the surface of a droplet in the free-rise period has been studied. Theoretical analysis was carried out on the basis of two typical models; one is the stagnant spherical drop model proposed by Newman, and the other is the model which accounts for the turbulent circulation in a droplet proposed by Handlos and Baron. Diffusion equations for these models were numerically solved under the boundary conditions which accounts for the chemical reaction at the drop surface. Two cases were discussed. One is the case in which mass transfer resistance in the continuous phase can be ignored, and the other is the case in which it should be accounted for. In the former case, it was found that the mass transfer rate increased with an increase of the initial concentration of reaction component in a droplet when the reaction order is greater than unity.The reverse case occurs when the reaction order is smaller than unity. In the latter case, it was revealed that the mass transfer rate increased with an increase of the initial concentration of the dispersed phase component when it is small but decreased when it is large.On the basis of these results of the theoretical analysis, the extraction rate of acetic acid by Amberlite LA-2, secondary long-chain alkylamine, was examined. As a result of the examination, the experimental results were fairly well explained by the model which takes account of the interfacial reaction of first order with respect to either acetic acid or amine.     

Dynamics of physical adsorption for porous grains with a complex internal structure

The dynamics of physical adsorption for a porous medium consisting of porous grains having a complex internal structure is analysed. The analysis is based on a combined theoretical model describing the mass transfer of an adsorbate inside a porous adsorbent grain with a complex internal structure. The model takes into account the effect of the dead-end transport channels (pores) inside the porous adsorbent grain on the nature of the mass transfer of a substance. For frontal adsorption dynamics with convex isotherms equations have been developed for calculating the width of the zone corresponding to intensive mass exchange in the adsorber (the width of the stationary front). Equations were also obtained for calculating the times of relaxations due to different mechanisms of interphase mass transfer inside a porous grain with a complex internal structure and to mass transfer in the intergrain space of the porous medium. It is shown that a strong interaction can take place between the mass transfer within the zones of a porous grain and the dead-end transport channels. When this occurs the width of the stationary front and the relaxation times characteristic of mass transfer inside the porous grain are in quasilinear dependence on the geometrical dimension of the porous grain and the geometrical dimension of the microporous zone inside the porous grain. These dimensions are: radius R₀ in the case of a porous grain of an adsorbent having a spherical form and radius R₀ in the case of a microporous spherical zone. An analysis of the frontal dynamic curves and the measurement of the coefficients of diffusion inside the microporous zone by other physical methods (e.g., n.m.r. measurements) show a lack of coincidence of the values of the diffusion coefficients. The possible cause of this non-coincidence is indicated.     

Approximations for unsteady state diffusion and adsorption with mass transfer resistance in both phases

A new approximate kinetic model, valid for both long and short duration of transient mass (or heat) transfer processes, was proposed. The presented model may be used in the cases of transient diffusion in bodies in the shape of infinite slab, infinite cylinder or sphere. The case where mass transfer resistance is both inside and outside a body (adsorbent pellet) has been considered. In the presented model Laplace transform for diffusion equation is used, the solution is transformed to the form of continued fractions and the resulting expression is truncated to obtain such a number of terms that ensures the required accuracy. This concept, which was presented by J. Lee and D.H. Kim (Chemical Engineering Journal, 2012) for systems without external mass transfer resistance, in this work has been extended on systems where such resistance exists. On the basis of calculations done in this work it was shown that values obtained with the approximate model are almost identical to values derived from the exact model. It is worth mentioning that such consistency refers to all considered shapes regardless of the magnitude of external mass or heat transfer resistance. The model can be used for modelling and design of cyclic adsorption–desorption processes.     

Analysis of PEM fuel cell stacks using an empirical current–voltage equation

An empirical equation was developed to describe the electrode processes (activation, ohmic and mass-transfer) of PEMFC stacks over the entire current range. The potential–current and power–current curves of a strip PEMFC stack were fitted with the empirical equation under a variety of experimental humidity, temperature and stack length conditions. The concept of mass transfer impedance was defined mathematically in the present research. For the strip PEMFC stack, mass transfer impedance was only important at high currents. With decreasing humidity the mass transfer impedance increased considerably. With increasing temperature or stack cell number the mass transfer impedance increased only slightly.     

Simultaneous heat and mass transfer for multicomponent distillation in a wetted-wall column

A simultaneous heat and mass transfer model was applied to multicomponent distillation in a wetted-wall column. Experiments were carried out, using the benzene-toluene-ethylbenzene ternary system, in a wetted-wall column of 2.2 cm i.d. and 100 cm long equipped with special probes designed for simultaneous liquid sampling and temperature measurements. The experimental results show that the bulk liquid phase was saturated, indicating no resistance to mass transfer in the liquid film. Confirmation of the liquid phase saturation was made through a comparison of the experimentally measured liquid temperature with the calculated bubble point temperature. The average deviation between the measured and calculated temperatures was found to be 0.26°C.Individual mass transfer rates were evaluated locally by measuring compositions and temperatures as functions of column height and were compared to theoretical predictions using the exact film model solution of the Maxwell-Stefan multicomponent equations developed by Krishna and Standart. The comparison shows good agreement with average deviations of 15.76% for benzene, 23.09% for toluene and 23.23% for ethylbenzene.     

Impedance spectroscopy analysis of glucose electro-oxidation on Ni-modified glassy carbon electrode

The electro-oxidation of glucose on nickel-modified glassy carbon electrode (GC/Ni) in a 1 M NaOH solution at different concentration of glucose was studied by the method of ac-impedance spectroscopy. In low concentration of glucose (<7 mM) two semicircles in the first quadrant of a Nyquist diagram were observed corresponding to charge transfer resistance and adsorption of intermediates. In higher concentration of glucose (>10 mM) negative resistance is observed in impedance plots as signified by semicircles terminating in the third quadrant. The impedance data in high concentration of glucose show different behavior at different applied anodic potential. The influence of the electrode potential on impedance pattern in high concentration of glucose is studied and a mathematical model was put forward to quantitative account for the impedance behavior of glucose oxidation. At potentials higher than 0.525 V/Ag-AgCl, a pseudoinductive behavior is observed but at higher than 0.4 V/Ag-AgCl, impedance patterns terminate in the second and third quadrants. The conditions required for this behavior are delineated with the use of the impedance model.     

Faradaic impedance to analyze charge recombination in photoelectrode of dye-sensitized solar cell

Charge transfer on the photoelectrode of dye sensitized solar cell (DSC) is composed of the various processes as follows: photoexcitation of electron in the dye; electron injection from the excited dye into the conduction band of oxide semiconductor; electron diffusion in the oxide semiconductor, reaction between oxidized dye and iodide ions (I⁻); and charge recombination. The charge recombination is one of the main factors that limit the efficiency of photoelectric conversion at photoelectrode. It is well known that 4-tert-butyl pyridine (TBP) is the useful additive in order to suppress the charge recombination. The authors fabricated the DSC with two photoelectrode materials such as titanium dioxide (TiO₂) and zinc oxide (ZnO), and analyzed the charge recombination by using electrochemical impedance spectroscopy. Theoretical equation of Faradaic impedance was derived from the reaction model of photoelectorde, and the charge transfer resistance of the photoelectrode was defined from the Faradaic impedance. In addition, the influence of the charge recombination on the impedance spectrum of the photoelectrode was discussed by the comparison between simulated and experimental results.     

Vacuum freeze-drying kinetics and modelling of a liquid in a vial

Freeze-drying experiments were carried out for milk and water in a vial, and the influence of the heating plate temperature was examined. A simple simulation based on a 'uniformly retreating ice front' type model was performed. A numerical model enables the changes in both water content and temperature of these products to be described satisfactorily during the experiment. Three transfer parameters (water vapour diffusivity in the dry layer, the external mass transfer coefficient, and the contact resistance to the heat transfer) were determined. The contact resistance between the product and the heating plate is the most important resistance to heat transfer, even controlling the dehydration kinetics.     

Oxygen as a tracer for measurements of steady and turbulent flows

The reduction of dissolved oxygen has been studied over a wide conductivity range for use in steady or nonsteady hydrodynamic measurements. Mass transfer fluctuations can be analysed statistically to obtain the power spectra of hydrodynamic fluctuations. This requires a consideration of the proton reduction, which prevents diffusion from limiting the current. The transfer function for deducing the hydrodynamic spectra from mass transfer spectra includes not only transport effects but also kinetic effects which account for the finite rate of the reaction. The experimental study was performed using electrohydrodynamic (EHD) impedance.     

Nano-structured Ni(II)-curcumin modified glassy carbon electrode for electrocatalytic oxidation of fructose

A nano-structured Ni(II)-curcumin (curcumin: 1,7-bis[4-hydroxy-3-methoxyphenyl]-1,6-heptadiene-3,5-dione) film is electrodeposited on a glassy carbon electrode in alkaline solution. The morphology of polyNi(II)-curcumin (NC) was investigated by scanning electron microscopy (SEM). The SEM results show NC has a nano-globular structure in the range 20-50 nm. Using cyclic voltammetry, linear sweep voltammetry, chronoamperometry, steady-state polarization measurements and electrochemical impedance spectroscopy (EIS) showed that the nano-structure NC film acts as an efficient material for the electrocatalytic oxidation of fructose. According to the voltammetric studies, the increase in the anodic peak current and subsequent decrease in the corresponding cathodic current, fructose was oxidized on the electrode surface via an electrocatalytic mechanism. The EIS results show that the charge-transfer resistance has as a function of fructose concentration, time interval and applied potential. The increase in the fructose concentration and time interval in fructose solution results in enhanced charge transfer resistance in Nyquist plots. The EIS results indicate that fructose electrooxidation at various potentials shows different impedance behaviors. At lower potentials, a semicircle is observed in the first quadrant of impedance plot. With further increase of the potential, a transition of the semicircle from the first to the second quadrant occurs. Also, the results obtained show that the rate of fructose electrooxidation depends on concentration of OH⁻. Electron transfer coefficient, diffusion coefficient and rate constant of the electrocatalytic oxidation reaction are obtained. The modified electrode was used as a sensor for determination of fructose with a good dynamic range and a low detection limit.     

On the electrochemical flow measurements using carbon-based screen-printed electrodiffusion probes

In this article, we have studied the possibility of using carbon-based screen-printed probes for electrochemical flow and mass flux measurements. Such probes have, up to now, been mainly used as biosensors to study in vivo reactions. Our study shows that screen-printed sensors allow the measurement of mass flux and mean wallp shear stress with good accuracy and high reproducibility. The existence of a micro-porous layer covering the surface of screen-printed electrodes has been revealed by means of the impedance measurements and has been confirmed by Scanning Electron Microscopy. This layer influences the statistical characteristics of the turbulent limiting diffusion current and should be taken into account as an additional transfer function between the current and the velocity fluctuations. The use of screen-printed sensors opens new possibilities in the field of electrochemical flow diagnostics. The advantage of this new manufacturing technology lies in the possibility of serial mass production at very competitive prices (disposable sensors) combined with the possibility of the manufacture of segmented electrodes or matrix of electrodes. These new possibilities can be used for various industrial applications as well as for scientific studies on near-wall turbulent mass transfer. Furthermore, it is possible to envisage the development of a hybrid sensor (electrodiffusional/biochemical) allowing the study of in situ biochemical reactions in a flow.