Ohmic Drop Effects Research Articles

Mechanism and Kinetics of the Phase Formation and Dissolution of NaxWO3 on a Pt Electrode in a Na2WO4-WO3 Melt.

A comprehensive study concerning the phase formation mechanism and growth/dissolution kinetics of sodium tungsten bronze crystals during the electrolysis of a 0.8Na2WO4-0.2WO3 melt was carried out. The regularities of deposit formation on a Pt(111) working electrode were investigated experimentally using cyclic voltammetry, chronoamperometry, scanning electron microscopy, and X-ray diffraction analysis. Models have been developed to calculate the current response during the formation, growth and dissolution of a two-phase deposit consisting of NaxWO3 and metallic tungsten or two oxide tungsten bronzes with different sodium content. These models consider mass transfer to the electrode and nuclei; chemical and electrochemical reactions with the participation of polytungstate ions, Na+, Na0, and O2-; as well as the ohmic drop effect. The approach was proposed to describe the dissolution of an NaxWO3 crystal with a nonuniform sodium distribution. The fitting of cyclic voltammograms was performed using the Levenberg-Marquardt algorithm. The NaxWO3 formation/growth/dissolution mechanism was determined. Concentration profiles and diffusion coefficients of [WnO3n]-, reaction rate constants, number density of nuclei, and time dependencies of crystal size were calculated. The proposed approaches and models can be used in other systems for the cyclic voltammogram analysis and study of the mechanism and kinetics of electrode processes complicated by phase formation; parallel and sequential electrochemical and chemical reactions; as well as the formation of a deposit characterized by a nonuniform phase and/or chemical composition.

Modeling Strategies Based on Multiple Neural Network Systems Applied for a Monopolar and Bipolar Electrocoagulation

The objective of this research was to evaluate the efficiency of an electrocoagulation system using iron and aluminum electrodes, arranged in both monopolar and bipolar arrangements, for the removal of acid red 18 dye. Experimental and modeling approaches were employed to investigate the system’s performance. The effects of operating parameters: including initial pH (3–9), current density (0.4–5.6 mA cm−2), charge passed (2.16–21.6 C cm−2), and initial dye concentration (50–300 mg l−1) were studied. The results demonstrated that an increase in electric current intensity and passed charge enhanced the removal of COD and dye. However, to minimize energy consumption, these parameters were optimized for different dye concentrations. The monopolar arrangement exhibited favorable performance for the both electrodes, primarily due to reduced ohmic drop effect, although the iron electrode generated sludge with better settling characteristics. The monopolar iron electrode consumed the least energy (38.3 kWh kg−1 COD). Experimental evaluation was conducted to assess the influence of key electrolysis process parameters on dye and COD removal. Additionally, neural network models, employing radial basis function and multilayer perceptron approaches, were utilized to predict system outputs based on initial characteristics (COD and dye) and operation conditions. The neural network models provided accurate predictions, offering practical insights for experimental applications.

Finite Element Modeling of the Combined Faradaic and Electrostatic Contributions to the Voltammetric Response of Monolayer Redox Films.

The voltammetric response of electrodes coated with a redox-active monolayer is computed by finite element simulations based on a generalized model that couples the Butler–Volmer, Nernst–Planck, and Poisson equations. This model represents the most complete treatment of the voltammetric response of a redox film to date and is made accessible to the experimentalist via the use of finite element modeling and a COMSOL-generated report. The model yields a full description of the electric potential and charge distributions across the monolayer and bulk solution, including the potential distribution associated with ohmic resistance. In this way, it is possible to properly account for electrostatic effects at the molecular film/electrolyte interface, which are present due to the changing charge states of the redox head groups as they undergo electron transfer, under both equilibrium and nonequilibrium conditions. Specifically, our numerical simulations significantly extend previous theoretical predictions by including the effects of finite electron-transfer rates (k0) and electrolyte conductivity. Distortion of the voltammetric wave due to ohmic potential drop is shown to be a function of electrolyte concentration and scan rate, in agreement with experimental observations. The commonly used Laviron analysis for the determination of k0 fails to account for ohmic drop effects, which may be non-negligible at high scan rates. This model provides a more accurate alternative for k0 determination at all scan rates. The electric potential and charge distributions across an electrochemically inactive monolayer and electrolyte solution are also simulated as a function of applied potential and are found to agree with the Gouy-Chapman-Stern theory.

Kinetics of electrochemical intercalation of lithium ion into Li[Li0.2Co0.3Mn0.5]O2 electrode from Li2SO4 solution

The kinetics of electrochemical lithium ion intercalation into Li[Li0.2Co0.3Mn0.5]O2 electrode in 2 M Li2SO4 aqueous electrolyte has been studied using two electroanalytical methods, namely, potentiostatic intermittent titration technique (PITT) and galvanostatic intermittent titration technique (GITT). The results are compared with those from nonaqueous electrolytes. Layered, lithium-rich Li[Li0.2Co0.3Mn0.5]O2 cathode material was synthesized by reactions under autogenic pressure at elevated temperature (RAPET) method. The effects of ohmic potential drop and charge-transfer resistance have been considered while predicting the current transients obtained with aqueous electrolyte. For PITT and GITT, we have defined their characteristic time-invariant functions, It1/2 and dE/dt1/2, respectively to present the diffusion time constant τ. Application of different theoretical diffusion models for treating the results obtained by the above-mentioned techniques allowed us to calculate the diffusion coefficient of lithium ions (D) at different potentials (E). The intercalation process is explained by considering the possible attractive interactions of the intercalated species in terms of Frumkin intercalation isotherm. We have observed a strictcorrespondence between the peaks of the intercalation capacitance and the minima in the corresponding log D vs. E curve.

Three infinite series and integral formulations for the voltammetric responses of reversible electrochemical reactions at a rotating disk electrode. Discrimination using the method of lines, and generalization to the integral equation method with finite kinetics, Ohmic potential drop and (pseudo-) capacitive interfacial behaviour

Given the closed-form expression of the mass transfer function pertaining to one-dimensional mass transport conditions in the electrolyte, the voltammetric response of one-step reversible electrochemical (E r ) reactions to a linear potential ramp can be derived first as an infinite series well suited to formal analysis, and then as an integral formulation well suited to numerical calculation. This approach applies in this article to the E r reaction studied on a uniformly accessible Rotating Disk Electrode (RDE). Using three closed-form approximations of the mass transfer function, three infinite series and three integral formulations can be derived for the Faradaic current response versus the electrode potential. Discrimination of the approximating formulations of the voltammetric response can be performed by comparison to benchmark voltammetric data obtained using the numerical method of lines to solve the relevant partial differential equation for diffusion-convection at RDE. Once the best approximating model is selected, based on its voltammetric features, the appropriate mass transfer function can be used together with the inverse Laplace transformation to derive the kernel function involved in the Integral Equation method. Application to the E reaction investigated by cyclic voltammetry on RDE is an illustration example in this article, first disregarding and then taking into account Ohmic drop and double-layer charging effects with capacitive or pseudo-capacitive (constant phase element) interfacial behaviour. • Infinite series formulations of the voltammetric response for the E r reaction on RDE . • Integral formulations of the voltammetric response for the E r reaction on RDE . • Derivation of a new kernel function for the dissolved species reacting on RDE. • Application to the Integral Equation method for cyclic voltammetry on RDE. • Voltammetric modelling of Ohmic drop and double-layer (capacitance/CPE) effects.

AC Voltammetry for Metal Electrodeposition Studies

Analytic solutions are derived for the alternating current voltammetry (ACV) harmonic waveforms of electrodeposition reactions where the surface activity of a metal species is constant. Reversible and quasi- reversible charge transfer kinetics are considered, as well as the effects of ohmic drop and double layer charging. It is shown that ohmic drop produces a characteristic distortion of the waveform envelopes, resulting in the emergence of waveform extrema in the higher harmonics that are distinct from those of the conventional ACV theory of soluble redox couples. A quantitative connection is made between the peak potential of the second harmonic and the electrode reaction parameters. The analytic solutions agree well with measurements of the harmonic waveforms for metal electrodeposition in high temperature molten salts.

Effect of ohmic-drop on electrochemical performance of EDLC fabricated from PVA:dextran:NH4I based polymer blend electrolytes

Proton conducting solid polymer blend electrolytes based on poly(vinyl alcohol)(PVA):dextran that were doped with different quantities of ammonium iodide (NH4I) were prepared. The X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) study were carried out to examine the compatibility of NH4I withPVA:dextran polymers. FTIR spectroscopy assessment was used to establish the presence of a complex formation between the PVA:dextran and added salt through the modification and reduction in the intensity of FTIR bands relevant to the functional groups. The field emission scanning electron microscopy (FESEM) examination was used to assess the channels for proton transport. Electrical impedance spectroscopy (EIS) was used to analyse the samples conductivity behaviour. The sample with 20 wt.% of added salt has shown a high DC conductivity which can be employed in electrochemical devices such as EDLC. It is also demonstrated by the transference number (TNM) and linear sweep voltammetry (LSV) that it is appropriate to use the largest conducting sample for electrochemical device. There was electrochemical stability of the electrolyte sample with voltage sweeping linearly to 1.3 V. It is shown by the outcome of cyclic voltammetry (CV) plot that charge storage at the site of electrode-electrolyte is non-Faradiac. A high drop voltage (Vd=IR) is implied by the usual galvanostatic charge-discharge. The equivalent series resistance (Res) increases as a result of the increase in Vd all the way through the charge-discharge cycle. Specific capacitance (Csp) is nearly constant from the foremost cycle to the 100th cycle, with average of 4.2 F/g.

Alternating current voltammetry of electrode reactions with constant surface activity: Application to electrolysis of molten electrolytes

Analytic solutions are derived for the alternating current voltammetry (ACV) harmonic waveforms of electrodeposition and gas evolution reactions where the surface activity of the reduced species R is constant. Reversible and quasi-reversible charge transfer kinetics are considered, as well as the effects of ohmic drop and double layer charging. It is shown that ohmic drop produces a characteristic distortion of the waveform envelopes, resulting in the emergence of waveform extrema in the higher harmonics that are distinct from those of the conventional ACV theory of soluble redox couples. A quantitative connection is made between the peak potential of the second harmonic and the electrode reaction parameters. The analytic solutions agree well with measurements of the fundamental, second, and third harmonic waveforms of Pb electrodeposition on liquid Pb and of Cl2 evolution on graphite in molten PbCl2-NaCl-KCl at 700 °C.

Determination of Surface Area and Reaction Rate Constant from Cyclic Voltammetry Considering Voltage-Dependence of the CPE Parameters

Due to its advantages such as sensitive response to a reaction and easy implementation, cyclic voltammetry has been used as an electrochemical method for analyzing complex mechanics in electrochemistry. Analytical relationships between peak potentials or peak current magnitudes with scan rates have been developed for different situations [1–3]. However, under most circumstances, the current responses of cyclic voltammetry consist not only the faradaic current generated by the reaction of interest, but also the non-faradaic current from an electrical double layer. Using the analytical relationships without considering ohmic drop and the non-faradaic current leads to incorrect kinetic and thermodynamic information obtained from the cyclic voltammogram interpretation [4]. Therefore, it is important to model the entire voltammogram to characterize the complex mechanism [5]. To model the ohmic drop effect and the non-faradaic current, commercial softwares such as DigiSim© and DigiElch© apply a concept of an electrical circuit with cyclic voltammetry simulation. They use a pure capacitor connecting in parallel to the faradaic impedance to act as the double layer capacitor and a resistance connecting in series to those elements to represent the ohmic resistance. Although it is rarely used in simulating voltammetric responses, a constant phase element (CPE) is widely accepted and used to model the electrical double layer capacitor (EDLC) in electrochemical impedance spectroscopy (EIS) analysis. A model for simulating the cyclic voltammetry responses which combines the effects of ohmic resistance, CPE, mass transfer, and faradaic processes was proposed [6]. The set of equations conventionally used to simulate voltammetric responses such as Fick’s second law and Butler-Volmer equation were used to solve for the faradaic current, while the time-domain response of a CPE was used to describe the non-faradaic current. The total current can be obtained by applying a concept of an electrical circuit. In that study, the CPE parameters were assumed to be constant throughout the potential window and it was proved to be reasonable. However, in some systems, it is known that such an assumption does not hold true [7]. In this study, commercial EDLCs were investigated. The CPE parameters were evaluated using cyclic voltammetry and EIS at several voltages. The cyclic voltammetry experiments were conducted under a narrow voltage window since it was observed that the responses under such a narrow voltage window can be described by the constant CPE parameters. This is understandable since the EIS approach also performed under a narrow voltage window. In cyclic voltammetry approach, the CPE parameters were determined by fitting the obtained currents with analytical solutions, while they were obtained by fitting the R-CPE circuit with the EIS spectra in EIS approach. It was found that the CPE parameters were different at each potential, as shown in Fig. 1a and both methods showed good agreement when compared with each other. These voltage-dependent parameters and the ohmic resistance were applied to the approximate solution proposed in [6] to simulate the entire voltammogram of the selected commercial EDLCs. Fig. 1b clearly shows a reasonable agreement between the numerical results and the experimental data in a case of the voltage-dependent CPE parameters and it is clear that the constant CPE parameters failed to predict the voltammetric responses. This result suggested that it is important to consider voltage-dependence of the CPE parameters in modeling the non-faradaic current and the difference between the response during forward and backward scans of an EDLC can be attributed to this dependency. Since it is essential to correctly predict the entire voltammogram to obtain kinetic and thermodynamic information in a system involving faradaic reaction, using our proposed approach to simulate the non-faradaic current would lead to more accurate prediction of such important information. Acknowledgment The authors would also like to express their gratitude to the Ministry of Education, Culture, Sports, Science and Technology, Japan, for providing financial support under the scholarship program for foreign students.

Anodic stripping voltammetry of silver in the absence of electrolytes: Theory and experiment

This work demonstrates that anodic stripping voltammetry at a microdisc electrode allows the detection of silver in the absence of added supporting electrolyte. The voltammetry however becomes distorted under low-support conditions. A semi-analytical model is developed to investigate the effects of the lack of supporting electrolyte on the voltammetric responses. First, the alteration in the double-layer kinetics is studied. In particular, we show that the classical Frumkin correction cannot explain the distorted shape of the voltammetry and only accounts for no more than ~50% of the shift in the voltammetric peaks. Second, compared to the Frumkin correction, Ohmic drop contributes much more significantly to the shift in the peak potential and the distorted wave shape upon lowering electrolyte concentrations. The Ohmic drop determined on the basis of predicted constriction resistance as expressed by Newman gives a reasonable approximation to the solution resistances, especially at ionic strengths above 200 μM. In deionized water or ultra-low conductivity water with the ionic strength of sub-micromolar level, a discrepancy in the resistances of up to a factor of 25 was observed. This discrepancy likely arises due to the assumptions made in applying the Newman model to the experimental system. Finally, we show that a model which considers simultaneously the electrode kinetics and the ohmic drop effects give a consistent fit with experimental data, when the values of the resistances are appropriately adjusted to account for the inaccuracy in the approximation of constriction resistances.

Effect of Environment Variables and Material Properties on the Limiting Current Density for Oxygen Reduction

For aircraft alloys that have received surface treatments, primers, and topcoats to control self-corrosion due to precipitate phases, high-strength fasteners made of stainless steel or titanium alloys are the most obvious point of attack due to galvanic mismatches between the materials. However, the electrolytes that form from atmospheric processes can differ widely in composition and thickness. Dissolved oxygen concentrations vary due to temperature and salinity changes in the electrolyte. Recent attempts to model atmospheric corrosion have suggested that, at high salt loadings on the surface, electrolytes can form that have water layer thicknesses that appear as bulk electrolytes in terms of oxygen transport1. Experiments using rotating disk electrode (RDE) measurements and polarization curve data,2 have suggested an upper bound of approximately 800 mm on the diffusion layer thickness for oxygen that can be expected to form in a thin film of water. At distances beyond this thickness, convective flow is expected to dominate oxygen transport. In this report, we compare the results of RDE experiments that simulate thin film behavior on different substrate materials in different electrolytes with polarization data obtained under actual electrolyte thin films equilibrated at different relative humidity values, and bulk polarization measurements, as shown in Figure 1, in order to determine the effects of ohmic drop on the measurements and isolate relevant kinetic parameters. Isolating these effects is important in order to accurately model the corrosion currents arising from galvanic couples in which the oxides at which the reduction reactions are occurring can be changing in response to the changes in the electrolyte as the environment changes. Figure 1. Bulk cathodic polarization curves in 0.6M NaCl at 25◦C for Pt, UNS S31600, and UNS R56400. This work was sponsored by the Office of Naval Research, ONR, under grant/contract number N0001418WX00826; the views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy or the U.S. government. References 1 Van den Steen, N., Simillion, H., Dolgikh, O., Terryn, H. & Deconinck, J. An integrated modeling approach for atmospheric corrosion in presence of a varying electrolyte film. Electrochimica Acta 187, 714-723, doi:10.1016/j.electacta.2015.11.010 (2016). 2 Liu, C., Srinivasan, J. & Kelly, R. G. Electrolyte Film Thickness Effects on the Cathodic Current Availability in a Galvanic Couple. J Electrochem Soc 164, C845-C855, doi:10.1149/2.1641713jes (2017). Figure 1

Determination of Effective Surface Area and Reaction Rate Constant By Cyclic Voltammetry Considering Ohmic Resistance and CPE Effects

One of the most commonly used electroanalytical techniques for determining the effective surface area and reaction rate constant in a redox system is linear sweep voltammetry due to its sensitive response to a reaction and easy implementation. In linear sweep voltammetry, a linear potential scan is applied to an immobile electrode submerged in an unstirred analyte. The responses to the applied potential is the total current which consists of the faradaic and non-faradaic currents. The faradaic current is the current generated by the reaction of interest, while the non-faradaic current is generated by an electrical double layer. As the faradaic processes received high attention in modeling [1-3], the non-faradaic current, which can be described by the constant phase element (CPE), received less attention. The conventional approach to evaluate the effective surface area and reaction rate constant is to assume that the non-faradaic current is linear and the ohmic drop is negligible. For a quasireversible reaction, by extrapolating the baseline, the peak current and potential are obtained and applied to the relation of Matsuda and Ayabe [4] and Nicholson’s method [5] to determine for the effective surface area and the reaction rate constant, respectively. However, this conventional approach could lead to an inaccurately estimated peak current and potential since the non-faradaic current is not always linear and the ohmic drop, which results in the peak potential shifting, occasionally cannot be negligible. In addition, the faradaic and non-faradaic currents are coupled through ohmic drop effects. Our research group has successfully developed numerical modeling of voltammetry, including the combined effects of ohmic resistance, CPE, mass transfer, and faradaic processes [6]. In this model, the CPE parameters, i.e. the pseudo-capacitance and the dispersion coefficient, are required as inputs. Generally, the CPE parameters were evaluated by electrochemical impedance spectroscopy (EIS). However, the EIS analysis is time-consuming and requires prior knowledge about the equivalent circuit of a system to obtain the CPE parameters. Furthermore, it measures only impedance of a system at one specific potential in frequency domain, while a voltammetry method is a potentiodynamic measurement, which operates under a wide potential window. Therefore, the semi-theoretical approach is introduced to evaluate the CPE parameters under voltammetry condition. This approach possesses several advantages for evaluating the CPE parameters, such as time-saving, simple, and straightforward. In this study, by considering the effects of ohmic resistance and CPE, an approach for determining the electrode effective surface area and reaction rate constant using linear sweep voltammetry was developed. By fitting the peak current and potential of the numerical simulation to those of the experimental data (see Fig. 1a), the electrode effective surface area and reaction rate constant can be determined. As the non-faradaic current becomes relatively more important at faster scan rates, it is difficult to extrapolate the baseline, as depicted in Fig. 1b. This difficulty could lead to underestimation of the effective surface area since the relation of Matsuda and Ayabe, needs the faradaic peak current as an input. Furthermore, Nicholson’s method, which evaluates the reaction rate constant directly from the peak separation, would underestimate the reaction rate constant if the ohmic drop effect was appreciable. Therefore, owing to the problem that the non-faradaic current and ohmic drop effect occasionally could not be avoided, using our approach that takes them into consideration would lead to more accurate prediction of such parameters. Acknowledgment The authors would also like to express their gratitude to the Ministry of Education, Culture, Sports, Science and Technology, Japan, for providing financial support under the scholarship program for foreign students.

Further investigation of the equivalence of staircase and linear scan voltammograms. II - Effects of electron transfer kinetics, Ohmic drop and double-layer charging

The optimal value of sampling parameter that guarantees the global equivalence of staircase and linear scan voltammograms plotted for electrochemical reactions involving soluble species is re-examined in this second article taking the electron transfer kinetics, the uncompensated resistance and the double-layer charging current into account. The discussion is based on the numerical solution of linear/nonlinear integral-equation-based models. The comparison of voltammetric and staircase voltammetric current data is carried out in terms of the mean squared deviation between voltammograms. Main results concern the dependence of the optimal sampling parameter on the potential step amplitude of staircase signal, and the dimensionless parameters pertaining to kinetic, Ohmic and capacitive effects.

Electroactive Self-Assembled Monolayers Detect Micelle Formation

The interfacial electrochemistry of self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate on gold (FcC12SAu) electrodes is applied to detect the micellization of some common anionic surfactants, sodium n-alkyl sulfates, sodium n-alkyl sulfonates, sodium diamyl sulfosuccinate, and sodium dodecanoate, in aqueous solution by cyclic voltammetry. The apparent formal redox potential (E°'SAM) of the FcC12SAu SAM is used to track changes in the concentration of the unaggregated surfactant anions and determine the critical micelle concentration (cmc). The effect of added salt (NaF) on the sodium alkyl sulfate concentration dependence of E°'SAM is also investigated. Weakly hydrated anions, such as ClO4-, pair with the electrogenerated SAM-bound ferroceniums to neutralize the excess positive charge created at the SAM/electrolyte solution interface and stabilize the oxidized cations. E°'SAM exhibits a Nernstian-type dependence on the anion activity in solution. Aggregation of the surfactant anions into micelles above the cmc causes the free surfactant anion activity to deviate from the molar concentration of added surfactant, resulting in a break in the plot of E°'SAM versus the logarithm of the concentration of anionic surfactant. The concentration at which this deviation occurs is in good agreement with literature or experimentally determined values of the cmc. The effects of Ohmic potential drop, liquid junction potential, and surfactant adsorption behavior on E°'SAM are addressed. Ultimately, the E°'SAM response as a function of the anionic surfactant concentration exhibits the same features reported using potentiometry and surfactant ion-selective electrodes, which provide a direct measure of the free surfactant anion activity, thus making FcC12SAu SAM electrodes useful for the detection of surfactant aggregation and micelle formation.

Conductive Mesoporous Catalytic Films. Current Distortion and Performance Degradation by Dual-Phase Ohmic Drop Effects. Analysis and Remedies

In the active interest aroused by catalysis of electrochemical reactions, particularly molecule activation related to modern energy challenges, mesoporous films deposited on electrodes are often preferred to catalysts homogeneously dispersed in solution. Conduction in the solid portion of the film and in the pores may strongly affect the characteristic catalytic Tafel plots, possibly leading to mechanistic misinterpretation and also degrade the catalytic performances. These ohmic drop effects take place, unlike those classically encountered with a massive electrode immersed in an electrolytic solution, in two different zones of the film, the solid bulk of the film and the pores, that are coupled together by a distributed capacitance and by the faradaic impedance representing the catalytic reaction located at their interface. A transmission line modeling allows the analysis of the capacitance charging responses as a function of only two dimensionless parameters in the framework of linear scan voltammetry: ...

Electrochemical Characterization of Corrosion Products in Leaded Bronze Sculptures Considering Ohmic Drop Effects on Tafel Analysis

AbstractThe characterization of corrosion products in leaded bronze based on the voltammetry of immobilized particles methodology is described. Voltammetric data, supported by Fourier transform infra‐red spectroscopy, field emission scanning electron microscopy‐energy dispersive X‐ray microanalysis (FESEM‐EDX) and scanning electrochemical microscopy (SECM) allow the identification of copper and lead corrosion materials. The mutual influence of such products is modeled upon considering uncompensated ohmic drops in the Tafel analysis of the rising portion of the respective voltammetric signals for their electrochemical reduction.

Spatially Resolved Electrochemical Impedance Measurement in a PEM Fuel Cell Using an Array of Reference Electrodes

Electrochemical impedance spectroscopy (EIS) is a widely used technique for fuel cell characterisation that can be applied in situ with minimal modification to commercial fuel cell hardware [1]. The technique involves applying a small ac stimulus to the system and measuring the response across a wide frequency range, typically 0.1 Hz - 10 kHz . The cell voltage is measured and various techniques are used to deconvolute the contributions from anode, cathode and membrane to the overall impedance. For PEMFCs, the anode component is often assumed to be negligible due to the more rapid kinetics of the hydrogen oxidation reaction compared to the oxygen reduction reaction. However, conventional EIS gives an average impedance across the entire area of the cell and provides no information on local variations that exist due to inhomogeneities in reactant concentration, water distribution and catalyst utilisation. The use of segmented cells to obtain a functional map of fuel cell performance is well established [2,3]. However, deconvolution of anode and cathode components in the impedance response is only possible if the electrode of interest is measured against a stable reference electrode, which is particularly important where air bleeding or fuels other than pure hydrogen are used in the anode. A reference electrode is also necessary for measurements to validate physical-based models used to fit impedance spectra. Recently, an innovative reference electrode was developed at NPL that makes contact with the electrode of interest via a Nafion tube salt bridge inserted through the end plates of the cell [4]. An ionic pathway through the gas diffusion layer is achieved by impregnating with Nafion at the point of contact. This gives a localised measurement of electrode potential and avoids the problems of ohmic drop and edge effects experienced by other reference electrode configurations. A significant advantage of the design is that commercial MEAs can be tested directly, with minimal modification to cell hardware. Using an array of these reference electrodes, a spatial map of electrode potential can be generated, as previously demonstrated during start-up/shut-down [5]. Here we present the application of this reference electrode design to localised EIS measurement without the need for a segmented cell or complicated multiple frequency response analysis (FRA) equipment. The spatial variation in impedance in a commercial prototype single cell was mapped under a range of operating conditions using an array of reference electrodes and a single FRA with auxiliary voltage inputs. The advantages and disadvantages of this technique and the implications for modelling and optimisation of fuel cell design are discussed.

Effect of Ohmic Drop on Accuracy of Reinforcement Corrosion Rate Measured Using Different Setups

Abstract High resistance offered by concrete to the current applied to polarize reinforcing bars embedded in concrete during measurement of reinforcement corrosion rate results in Ohmic drop, which causes inaccuracy in the measured value of the reinforcement corrosion rate. A study on accuracies of a simple developed setup and two other commercially available setups in measuring reinforcement corrosion rate using the linear polarization resistance method with and without Ohmic drop compensation is reported in this paper. The good correlation between electrochemically measured corrosion current density and gravimetrically measured percentage weight loss was taken as an indicator of accuracy of each setup. The developed setup was found to be most accurate in measuring corrosion rate without, as well as with, Ohmic drop compensation. Both commercial setups were found to have reasonably acceptable accuracy without Ohmic drop compensation; however, both were found with unacceptable accuracy with Ohmic drop compensation.

In Situ Mapping of Electrode Potential in a PEMFC

An overview is given of the use of reference electrodes in polymer electrolyte membrane fuel cells (PEMFCs). The two most common types of reference electrode employed in PEMFCs are the reversible hydrogen electrode and the dynamic hydrogen electrode, which have the advantage of minimizing chemical contamination of the fuel cell. Conventional configurations include internal reference electrodes, which are constructed by sandwiching the electrode between two membranes, and external reference electrodes, which are typically connected to the edge of the membrane via a salt bridge. A recent breakthrough is the development of an innovative reference electrode array with a through-plate configuration, which makes possible for the first time accurate mapping of the variation in electrode potential across the active area of an operating PEMFC. This configuration overcomes the major limitations of conventional fuel cell reference electrodes in that ohmic drop, potential distribution and electrode edge effects in the membrane are negated.

Improved Conversion Rates in Drug Screening Applications Using Miniaturized Electrochemical Cells with Frit Channels

This paper reports a novel design of a miniaturized three-electrode electrochemical cell, the purpose of which is aimed at generating drug metabolites with a high conversion efficiency. The working electrode and the counter electrode are placed in two separate channels to isolate the reaction products generated at both electrodes. The novel design includes connecting channels between these two electrode channels to provide a uniform distribution of the current density over the entire working electrode. In addition, the effect of ohmic drop is decreased. Moreover, two flow resistors are included to ensure an equal flow of analyte through both electrode channels. Total conversion of fast reacting ions is achieved at flow rates up to at least 8 μL/min, while the internal chip volume is only 175 nL. Using this electrochemical chip, the metabolism of mitoxantrone is studied by microchip electrospray ionization-mass spectrometry. At an oxidation potential of 700 mV, all known metabolites from direct oxidation are observed. The electrochemical chip performs equally well, compared to a commercially available cell, but at a 30-fold lower flow of reagents.