Uncompensated Solution Resistance Research Articles

A study of the hydrophobic properties of alkanethiol self-assembled monolayers prepared in different solvents

We have measured the interfacial capacitance of an alkanethiol self-assembled monolayer (SAM) and its barrier properties adsorbed in different non-aqueous solvents by means of electrochemical impedance spectroscopy and cyclic voltammetry. Our impedance results show that the permeability of the SAM depends to a large extent on the solvent used as the adsorption medium. Monolayers of alkanethiol are highly impermeable when formed in solvents like hexane and chloroform, compared to those formed in solvents like ethanol, DMF, acetonitrile, hexadecane and toluene. Our studies lead us to believe that a thin hydrophobic layer separates the alkanethiol SAM and the aqueous electrolyte when the monolayer is prepared in hexane and chloroform. This layer acts as a series capacitor to the dielectric film of the alkanethiol monolayer. This tends to lower the effective interfacial capacitance and also enhances the uncompensated solution resistance of the monolayer coated gold electrode. Our experimental results may indicate a large-scale depletion of water density as predicted by the theory of Lum Chandler and Weeks (LCW theory, J. Phys. Chem. B 103 (1999) 4570), or the presence of nanobubbles or cavities at the highly hydrophobic interface.

Determination of the ionic resistance in a porous electrode using chronocoulometry

The purpose of the work is to establish a simplified method, which is easy to use and yet sufficiently accurate for the determination of ionic resistance in porous electrodes. The equation of the chronocoulometric transient following a potential step is deduced for porous electrodes in the double layer region based on a one-dimensional transmission line model. According to the result of numerical calculations, the internal ionic resistance is R p=2.550 t 0.69/ C total−2.986 R e where t 0.69 is the time of 69% response on the charge–time curve. The total double layer capacitance ( C total) can be obtained from the equation C total= Q/ η 0, where Q is the plateau charge on the chronocoulometric curve and η 0 is the amplitude of the potential step. R e is the uncompensated solution resistance between the Luggin tip and the front surface of the porous electrode, and R e= η 0/ I(0) where I(0) is the current at the beginning of the potential step. The theoretical error of the approximate equation is below 0.7%. The validity of the proposed method is well supported by the experimental data obtained for a porous silver electrode in 1 and 6 mol l −1 KOH solutions.

Simplified methods for determining the ionic resistance in a porous electrode using linear voltammetry

Two simplified methods are proposed for determining the internal ionic resistance of a porous electrode using linear voltammetry in the double layer region. With the 70% current response time t 0.7 on linear voltammogram and the independently measured uncompensated solution resistance R e, the internal ionic resistance of a porous electrode can be calculated using the equation R p=2.483 t 0.7/ C total−2.989 R e, where C total is the total capacitance of the porous electrode and can be deduced from the plateau current on the voltammogram. When R p is not much smaller than R e, it is also possible to estimate the ionic resistance according to the curve shape of the linear voltammogram without knowing R e.

Evaluation of uncompensated solution resistance for electrodes with spherical-cap geometry

Typical custom and commercial hanging mercury drop electrodes have the geometry of a spherical cap formed by the plane of the lower surface of the electrode holder cutting off the top of the drop. To conduct accurate quantitative measurements by voltammetry, it is necessary to be able to account for the effects of solution resistance, Ru. A method of determining the solution resistance is proposed and tested. The method involves making measurements of a test reaction (in this case, oxidation of ferrocene) by cyclic voltammetry at scan rates where resistance effects are important and at more than one concentration. When the data are analyzed by digital simulation, it is found that only one value of Ru will provide adequate matches between simulation and experiment at all concentrations. An approximate equation has been derived that allows calculation of Ru from the dimensions of the spherical-cap electrode and the solution resistivity. The calculated values of Ru for electrodes of three different sizes agreed well with the measured values. Error analysis was performed for a particular measurement, the determination of the standard heterogeneous electron-transfer rate constant, ks, by cyclic voltammetry, and it was shown that uncertainty in Ru puts an upper limit of about 1 cm/s for the determination of ks with the hanging mercury drop electrodes used in this study.

Capacitance of a metal/liquid interface during anion adsorption: phase-selective measurement in the presence of d.c. voltage sweep and finite solution resistance

Anion adsorption plays a critical role in pitting corrosion of metals in aqueous media. Frequently, this adsorption occurs as a non-faradaic process, without any electron transfer across the interface. Such processes cannot be studied efficiently by using standard electrochemical methods such as linear sweep voltammetry (LSV). Phase-selective measurement of the interfacial differential capacitance is necessary in such cases. However, when accompanied by LSV, even these latter measurements can be affected severely by the uncompensated solution resistance. In this paper, we describe a relatively simple phase-selective impedance method where both the solution resistance and the differential capacitance can be measured in situ during the d.c. voltage scan of LSV. We apply this technique to study the adsorption properties of ClO4−, Cl− and SCN− on a polycrystalline Au electrode. The results indicate that this method can be standardized easily for the analysis of adsorption isotherms involving similar systems. Copyright © 1999 John Wiley & Sons, Ltd.

A unified approach to trace analysis and evaluation of electrode kinetics with fast Fourier transform electrochemical instrumentation

Abstract A microcomputer-based form of Fourier transform instrumentation, which yields potential and frequency-dependent current, admittance and impedance data with excellent signal-to-background ratio over a wide frequency range, is shown to provide state-of-the-art performance in both analytical and kinetic applications of voltammetry. The limit of detection of 5×10−8 to 10−7 M found for the determination of cadmium and lead at a hanging mercury drop electrode, when applying a multi-frequency excitation signal, is equivalent to that reported with single-frequency or time-domain ac, pulse and square-wave polarographs. The highly effective discrimination against the charging current, which is required for trace analysis, can be achieved only by correction of the real part of the admittance data for the uncompensated solution resistance (a parameter readily obtained from impedance analysis of the data). Furthermore, excellent agreement with literature data is achieved when calculating the physico-chemical parameters associated with the cadmium(II) reduction process in different media. The ability to use the same instrument to undertake analytical determinations and characterise the electrode kinetics represents an ideal method for confirming the fidelity of a voltammetric analysis.

Design of liquid|liquid electrochemical cells

Abstract Electrochemical cell designs for liquid|liquid studies have been briefly summarised. Experiments using reference Luggin capillaries mounted on ball and socket joints showed that the lateral separation between the Luggin tips on either side of the liquid|liquid interface had a negligible effect upon the solution resistance. However, the vertical separation, especially of the Luggin tip on the organic side, did have a significant effect upon the uncompensated solution resistance. Even small amounts of uncompensated solution resistance will lead to significant peak separations which might be mistaken for kinetic limitations. Correct compensation of the solution resistance is essential for liquid|liquid systems. In many cell designs for liquid|liquid systems, the counter electrode for the organic phase is actually in an aqueous solution which is connected to the organic phase by a porous glass frit. The electrical connection between the two phases the relies upon then transfer of a common ion. If the size of the glass frit is too small, the transfer of the common ion can be limited and this in turn will lead to current limitations in the overall cell currents. This results in a characteristic ‘clippung’ of the current in a cyclic voltammogram, which cannot be traced to faults in the electronic instrumentation. To avoid this problem, the frit must be at least twice the size of the interface. A new electrochemical cell design for liquid|liquid studies which does not possess a Luggin capillary for the organic phase is reported. This Luggin-less cell will exhibit lower impedances for the reference electrode arm than the more conventional reference arms involving Luggin capillaries.

Aperiodic equivalent circuit for charge permeable thin-layer cells of symmetric, asymmetric types

In this paper, some of the various predictions of the original BBB (Barker–Brumleve–Buck) circuit, primarily the high frequency migration (geometric), and the low frequency diffusion–migration sub-circuit units, are illustrated using a newer procedure that was not available in our laboratory in 1980: SPICE circuit simulation. The aperiodic Rs and Cs generate the low-frequency Warburg characteristics. The Poisson string generates the high frequency geometric cell capacitor in parallel with the unperturbed-concentration ac resistance. The well known two feature impedance plane plot arises, for reversible interfacial ion and electron transport, from this simple circuit. The usual three feature impedance plane plots can follow for slow, potential-dependent ion and/or electron transfers with the usual linearized activation kinetics, by addition of parallel activation resistance and double layer capacitance networks for each ion requiring activation to cross the interfaces. Likewise the uncompensated solution resistance gives a fourth feature that is not often observed experimentally except as a non-zero real intercept at highest frequencies. The theory and the basic two-feature circuit of nearly 20 years contains most of Albery's recent contributions, as well as much more advanced analysis. The new analysis using SPICE shows the scope and limits of classical transmission lines compared with the more general BBB circuits.

The effects of uncompensated solution resistance and rate of the homogeneous electron transfer reaction on electrochemiluminescence transients

In double-step electrochemiluminescence experiments, the luminescence emission transients contain interesting information on the electrode reactions involved, the coupled mass transport and the homogeneous annihilation reaction. It was found that the start of luminescence emission, following the second potential step, occurs only after a delay of the order of 100 μs. The effect of Förster-type energy transfer from the electronically excited molecules to the metal electrode is insignificant under the experimental conditions considered. Rather, it is shown that this delay is caused by the IRu drop, which is closely related to the time constant RuCDL, where Ru is the uncompensated electrolyte resistance and CDL is the double-layer capacitance. ECL transients generated with the digital simulation technique using this information agree with the experimental transients. The rate constant of the homogeneous annihilation reaction is derived from such fits.

Use of Multiple Electrodes To Provide Uniform Potential Distribution during Controlled Potential Electrolysis

In practice, the size of a single working electrode in controlled potential electrolysis cells can be extended in only two dimensions while maintaining uniform potential distribution. A previously published theoretical model predicted that working electrode dimensions in controlled potential electrolysis cells could be effectively extended in three directions while maintaining form potential distribution by using several working electrodes interconnected with appropriately chosen external resistors. A controlled potential electrolysis cell utilizing three working electrodes placed at successively increasing distances along the current path from the counter electrode has been designed and tested. The electrodes were connected in either series or parallel with external compensating resistors calculated according to theory. Nearly identical interfacial potentials were observed for all three electrodes, with dramatic reduction of the potential errors observed without the use of the external resistors. It was further demonstrated that if optimum placement of the reference electrode did not pertain, then the multiple electrode/external resistance network behaved as a single electrode operating with a single uncompensated solution resistance. This should facilitate the use of conventional positive feedback techniques to mimimize potential control errors. Because changes in cell geometry or solution require different sets of compensating resistors, two operational amplifier-based circuits which eliminate this inconvenience were designed and tested. The circuits employ comparison of either individual electrode currents or potentials and feedback control to maintain these at identical value

The Crank-Nicolson technique—an efficient algorithm for the simulation of electrode processes at macro- and microelectrodes

The basic concepts of a simulation program for electrochemical problems using the Crank-Nicolson technique are developed. The algorithm allows the inclusion of homogeneous reactions and the effect of uncompensated cell resistance and double-layer capacity in parallel. A general model is derived for most electrode geometries, which may be described by a one-dimensional grid. The technique is applied to simulations of cyclic voltammograms involving pure charge transfer and coupled chemical reactions, at planar and spherical (micro)electrodes, for multi-species systems, finite diffusion problems, IR drop phenomena and even multi-sweep experiments. The CNA in connection with implicit boundary conditions allows the construction of fast simulation programs of high accuracy and stability. Well-thought-out implementations allow a wide field of mechanisms to be investigated by one program. The narrow band matrices used for each species and the expansion control mechanism keep down the memory space needed and also allow multi-sweep simulations. A measure of accuracy is the mass sum at each space element as well as the mass sum of the whole system because the mass retention condition is not used in the program. The implementation of homogeneous reactions is possible in different ways, whereby the true CNA treatment allows very fast kinetics. Uncompensated solution resistance and double-layer capacity are easily introduced. The use of a general formulation of Fick's second law allows the simulation of different electrode geometries simply by exchanging the grid subroutine. By this means, an expanding grid becomes accessible, which is very useful in simulations requiring small space steps.

Square-wave voltammetry applied to the totally irreversible reduction of adsorbate

The totally irreversible reduction of adsorbed material is studied by square-wave voltammetry. The model compound chosen is midazolam, 8-chloro-6-(2-fluorophenyl)-1-methyl-4 H-imidazo[1,5- a][1,4]benzodiazepine. Experimental voltammograms are compared directly with the exact numerical solution for the boundary value problem via COOL, a numerical method yielding estimates of transfer coefficient and rate constant for the process. The agreement between experimental and calculated voltammograms is excellent over a wide of experimental conditions. Artifacts of measurement such as uncompensated solution resistance are readily detected. The limitations of previous empirical descriptions of the response are shown by comparison with these results.

Kinetics of the electroreduction of Co(salen) in DMSO

The main problem with kinetic measurements in cyclic voltammetry is connected with the increase or the ohmic drop IR S , where R S is the uncompensated solution resistance, as the sweep rate increases. Although modern potentiostats allow some compensation or R S , total compensation is not possible. The presence or the uncompensated resistance causes nonlinearity or the potential sweep and leads to an increase in the separation or peak potentials, which may result in erroneous kinetic parameters. The development or ultramicroelectrodes has increased the possibilities or cyclic voltammetry at high sweep rates

Error Analysis of the Polarization‐Resistance Technique for Corrosion‐Rate Measurements

The polarization‐resistance measurement is a simple and rather popular technique for the determination of corrosion rates from electrochemical measurements, and the characteristics and limitations of this technique have been discussed in many publications. In this work, the error caused by the neglect of mass transport is examined, since this limitation, somewhat surprisingly, has not been considered earlier. It can be shown that, for the idealized polarization‐resistance technique (i.e., when the slope of the polarization curve is taken at ), the percent error in the corrosion current density due to the neglect of mass transport is: . The error increases with increasing and with increasing ; the error is always negative, and, in extreme cases, it can be very large. This is a dangerous situation because the true corrosion rate can be much larger than the calculated rate. However, for small values of , which is often the case for practical corrosion reactions, the error can be negligible. For the practical polarization‐resistance technique (i.e., when the slope of the polarization curve is measured with a finite value), the error due to the neglect of mass transport and the linearization error are combined. However, these errors are not simply additive because the linearization error itself is a function of the extent of the mass transport. The effect of several variables is discussed. The opportunity is also taken to reevaluate some other limitations of the polarization‐resistance technique, namely, the errors caused by (i) linearization of the current potential equation, (ii) incorrect estimates of the Tafel slopes, (iii) neglect of reverse partial‐reactions, and (iv) neglect of uncompensated solution resistance. These limitations have been discussed before, but our representations are more generalized, especially in that the effect of all pertinent variables is expressed in one single graph.

Fuel Cell Cathode Studies in Aqueous K 2 CO 3 and KOH

The galvanostatic steady‐state performance of PTFE‐bonded Pt‐on‐carbon gas diffusion electrodes (GDEs) has been measured with variable oxygen partial pressures in and . The catalyst in the porous electrodes was characterized with cyclic voltammetry in a special cell with low uncompensated solution resistance. Cyclic voltammograms yielded measurements of the wetted areas of carbon and Pt and the local electrolyte composition. Corrections for differences in the area of Pt wetted by the electrolyte collapsed oxygen reduction (OR) performance curves for several different electrodes into a single curve of log‐specific current density vs. potential. Comparisons with kinetic results suggest that OR on carbon contributes to the high current densities in at high overpotentials. In electrolyte, current density was found to be proportional to the square root of both wetted Pt area and pressure, indicating that mass‐transfer rates were not limited by transport. Hysteresis measurements and cyclic voltammetry were used to identify slow transport of OH− ion away from the reaction site as the limiting phenomenon. The generally lower wettability and pH rise in the carbonate‐containing electrodes resulted in lower performance when compared with . The results suggest that modern GDEs, which have been optimized primarily for efficient transfer, are not adequate for use in aqueous carbonate electrolytes. Modifications of the GDE structure to provide for more rapid OH− transport will be necessary to obtain good performance with aqueous carbonate electrolytes.

Electrochemical cell design and experimental procedures for measuring electrode reaction kinetics at low and variable temperatures

AbstractAn electrochemical cell is described for measuring electrode reaction kinetics at low and variable temperatures in nonaqueous solvents. The cell contains a working electrode of conventional size (at which the standard heterogeneous electron transfer rate constant is determined by cyclic voltammetry) and a microelectrode (at which the diffusion coefficient is measured by steady‐state microelectrode voltammetry). Uncompensated solution resistance is minimized by a combination of close placement of the reference electrode salt bridge tip and the working electrode surface and instrumental positive feedback. Cell performance is evaluated by an examination of ferrocene oxidation and manganese(III) (tetraphenylporphyrin) chloride reduction at temperatures between 229 and 298 K. Comparison of the responses from these reversible and quasi‐reversible redox couples in the same solution demonstrates that the effects of uncompensated solution resistance are absent from the experimental measurements and that increases in the cyclic voltammetric peak potential separation with sweep rate are due entirely to the kinetics of the Mn(III)/Mn(II) electrode reaction.

Effect of uncompensated resistance on large-amplitude chronoamperometric experiments

The influence of uncompensated solution resistance in large-amplitude potential step chronoamperometry involving reversible redox systems was investigated. By means of additional external resistors, the system-imminent resistance could be deliberately changed. The observed current-transients show significant deviations from the ideal “Cottrell” behaviour. It is shown by means of digital simulation experiments that already small uncompensated resistance values give rise to distorted current-transients particularly in systems with high charging or faradaic currents. This effect manifests itself in particular in electrochemical reactions on polymer-coated electrodes.

Chronoamperometry of polypyrrole: migration of counterions and effect of uncompensated solution resistance

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTChronoamperometry of polypyrrole: migration of counterions and effect of uncompensated solution resistanceChris D. Paulse and Peter G. PickupCite this: J. Phys. Chem. 1988, 92, 24, 7002–7006Publication Date (Print):December 1, 1988Publication History Published online1 May 2002Published inissue 1 December 1988https://doi.org/10.1021/j100335a032RIGHTS & PERMISSIONSArticle Views647Altmetric-Citations82LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (651 KB) Get e-Alerts

Extension of dc transient techniques to reactions with a potential dependent charge transfer coefficient

The evaluation of data from dc transient measurements invariably involves comparison of the experimental results with the predictions of a theoretical model. In the past, these models were usually based on the Butler—Volmer equation, with the assumption that the charge transfer coefficient (α) is constant. The Marcus theory of charge transfer predicts a linear variation of α with electrode potential; therefore, it is important to examine the problems in the use of the dc relaxation techniques caused by a potential dependent α, and to examine possible ways to detect and to overcome these problems. An error analysis was carried out, and it was concluded that the dc transient techniques cannot be used for the measurement of kinetics of electrode reactions if the charge transfer coefficient of the reaction is potential dependent and the potential excursion during the transient is large enough to cause an appreciable variation of α, unless the potential dependence of α is explicitly included in the data evaluation model. While this effect is self explanatory for those transient techniques where the potential is not the controlled variable ( eg, galvanostatic and coulostatic techniques), large errors can also occur for the potentiostatic techniques (where the potential is the controlled variable) if the reaction is fast and the uncompensated solution resistance is large. It was also shown that an all numerical data evaluation method based on multidimensional, non-linear, least squares curve fitting calculations coupled with a numerical differential equation solver routine can extend the usefulness of the dc relaxation techniques to reactions having a potential dependent charge transfer coefficient provided a functional dependence of α on the potential can be included in the data evaluation model.

Effects of solution resistance and double layer capacity on the faradaic polarization of an electrochemical cell

The basis of a method which allows one to investigate the kinetics of corrosion processes in systems exhibiting an uncompensated solution resistance and whose anodic and cathodic reactions have current—voltage behaviors described by the Butler—Volmer relation is presented.