Dr. Marc Madou, UCI, Winter 2012 Class X Electrochemical Impedance Analysis (EIS) Electrochemistry...

Dr. Marc Madou, UCI, Winter 2012 Class X Electrochemical Impedance Analysis (EIS) Electrochemistry MAE-295

Table of Content 2 Introduction Advantages and Disadvantages Impedance Concept and Representation of Complex Impedance Values Nyquist plot or Cole-Cole plot Bode plot Summary Nyquist and Bode Plots Review of Circuits Elements Equivalent Circuit of a Cell Back to Electrochemistry Summary

Introduction In EIS an electrochemical system is perturbed with an alternating current or voltage signal of small magnitude and one observes the way in which the system follows the perturbation at steady state. EIS measures the impedance of a circuit to an applied voltage: Z(t)=E(t)/I(t) When E (or I) is applied as a sinusoidal function in a linear system, the I (or V) response can be represented by a sum of sinusoidal functions with phase shifts. If an equivalent circuit for the system being probed can be constructed, then the resistance or capacitance values for each circuit element can be backed out from Z. 3

Introduction Electrical circuit theory distinguishes between linear and non-linear systems (circuits). Impedance analysis of linear circuits is much easier than analysis of non-linear ones. A linear system... is one that possesses the important property of superposition: If the input consists of the weighted sum of several signals, then the output is simply the superposition, that is, the weighted sum, of the responses of the system to each of the signals. Mathematically, let y 1 (t) be the response of a continuous time system to x 1 (t) ant let y 2 (t) be the output corresponding to the input x 2 (t). Then the system is linear if: 1)The response to x 1 (t) + x 2 (t) is y 1 (t) + y 2 (t) 2) The response to ax 1 (t) is ay 1 (t)... 4

For a potentiostated electrochemical cell, the input is the potential and the output is the current (for a galvanostated cell it is the other way around). Most electrochemical cells are not linear! Doubling the voltage will not necessarily double the current. However, electrochemical systems can be pseudo-linear. When you look at a small enough portion of a cell's current versus voltage curve, it seems to be linear. In normal EIS practice, a small (1 to 10 mV) AC signal is applied to the cell. The signal is small enough to confine you to a pseudo-linear segment of the cell's current versus voltage curve. You do not measure the cell's nonlinear response to the DC potential because in EIS you only measure the cell current at the excitation frequency. Introduction 5

Measure Z( ,V bias ) = V( ) / I( ) 6

Introduction Let us assume we have an electrical element to which we apply an electric field E(t) and get the response I(t), then we can disturb this system at a certain field E with a small perturbation dE and we will get at the current I a small response perturbation dI. In the first approximation, as the perturbation dE is small, the response dI will be a linear response as well. If we plot the applied sinusoidal signal on the X-axis of a graph and the sinusoidal response signal I(t) on the Y-axis, an oval is plotted. This oval is known as a "Lissajous figure". Analysis of Lissajous figures on oscilloscope screens was the accepted method of impedance measurement prior to the availability of lock-in amplifiers (LIAs) and frequency response analyzers (FRAs). 7

Introduction Multiplier: V x ( t) sin( t) & V x ( t) cos( t) Integrator: integrates multiplied signals Display result: a + jb = V sign /V ref Impedance: Z sample = R m (a + jb) But be aware of the input impedance of the FRA! 8

Introduction General schematic V pwr.amp = A k V k A= amplification V work V ref = V pol. + V 3 + V 4 Current-voltage converter provides virtual ground for Work-electrode. Source of inductive effects 9

Introduction 10

Introduction FRA: Frequency Response Analysis Potentiostatic or galvanostatic measurements 11

Advantages and Disadvantages Electrochemical Impedance Spectroscopy (EIS) is also called AC Impedance or just Impedance Spectroscopy. The usefulness of impedance spectroscopy lies in the ability to distinguish the dielectric and electric properties of individual contributions of components under investigation. For example, if the behavior of a coating on a metal when in salt water is required, by the appropriate use of impedance spectroscopy, a value of resistance and capacitance for the coating can be determined through modeling of the electrochemical data. The modeling procedure uses electrical circuits built from components such as resistors and capacitors to represent the electrochemical behavior of the coating and the metal substrate. Changes in the values for the individual components indicate their behavior and performance. Impedance spectroscopy is a non-destructive technique and so can provide time dependent information about the properties but also about ongoing processes such as corrosion or the discharge of batteries and e.g. the electrochemical reactions in fuel cells, batteries or any other electrochemical process. 12

Advantages and Disadvantages Advantages. 1. Useful on high resistance materials such as paints and coatings. 2. Time dependent data is available 3. Non- destructive. 4. Quantitative data available. 5. Use service environments. 6. System in thermodynamic equilibrium 7. Measurement is small perturbation (approximately linear) 8. Different processes have different time constants 9. Large frequency range, Hz to GHz (and up) 10. Generally analytical models available 11.Pre-analysis (subtraction procedure) leads to plausible models and starting values Disadvantages. 1.Rather expensive equipment, 2.Low frequencies difficult to measure 3. Complex data analysis for quantification. 13

AC Circuit Theory and Representation of Complex Impedance Values Concept of complex impedance: from R to Z Ohm's law defines resistance in terms of the ratio between voltage E and current I : The relationship is limited to only one circuit element -- the ideal resistor. An ideal resistor has several simplifying properties: It follows Ohm's Law at all current and voltage levels It's resistance value is independent of frequency. AC current and voltage signals though a resistor are in phase with each other 14

AC Circuit Theory and Representation of Complex Impedance Values In practice circuit elements exhibit much more complex behavior. This forces one to abandon the simple concept of a resistance only. In its place we use impedance, which is a more general circuit parameter (Z instead of R). Like resistance R, impedance Z is a measure of the ability of a circuit to resist the flow of electrical current. Electrochemical impedance is usually measured by applying an AC potential (current) to an electrochemical cell and measuring the current (voltage) through the cell. Suppose that we apply a sinusoidal potential excitation. The response to this potential is an AC current signal, containing the excitation frequency and it's harmonics. This current signal can be analyzed as a sum of sinusoidal functions (for a linear system-see earlier). The current response to a sinusoidal potential will be a sinusoid at the same frequency but shifted in phase. 15

AC Circuit Theory and Representation of Complex Impedance Values The excitation signal, expressed as a function of time, has the form of: E(t) is the potential at time t, E o is the amplitude of the signal, and is the radial frequency. The relationship between radial frequency (expressed in radians/second) and frequency f (expressed in Hertz (1/sec). 16

AC Circuit Theory and Representation of Complex Impedance Values In a linear system, the response signal, the current I(t), is shifted in phase ( ) and has a different amplitude, I 0 : An expression analogous to Ohm's Law allows us to calculate the impedance (=the AC resistance) of the system : The impedance is therefore expressed in terms of a magnitude, Z 0, and a phase shift . This impedance may also be written as a complex function (see next slide) : 17

AC Circuit Theory and Representation of Complex Impedance Values Using Eulers relationship: it is possible to express the impedance as a complex function. The potential is described as: and the current response as: The impedance is then represented as a complex number: E(t) = E 0 exp(i t) 18

AC Circuit Theory and Representation of Complex Impedance Values E(t) = E 0 cos( t), =2 f I(t) = I 0 cos( t- ) Or, if one writes in complex notation: E(t) = E 0 exp(i t) I(t) = I 0 exp(i t - i ) Z(t) = Z 0 exp(i ) = Z 0 (cos + isin ) E/I response for a resistor ( =0) E/I response for a capacitor ( =-90) E/I response for an inductor ( =90) 19

Data Presentation: Nyquist Plot with Impedance Vector The expression for Z( ) is composed of a real and an imaginary part. If the real part is plotted on the X axis and the imaginary part on the Y axis of a chart, we get a "Nyquist plot (also Cole-Cole plot). Notice that in this plot the y-axis is negative and that each point on the Nyquist plot is the impedance Z at one frequency. In the Nyquist plot the impedance is represented as a vector of length |Z|. The angle between this vector and the x-axis is the phase angle . Nyquist plots have one major shortcoming. When you look at any data point on the plot, you cannot tell what frequency was used to record that point. Low frequency data are on the right side of the plot and higher frequencies are on the left. Y =1/Z. A semicircle is characteristic of a single "time constant". Electrochemical Impedance plots often contain several time constants. Often only a portion of one or more of their semicircles is seen. 20

The magnitude of Z and phase angle are given by the following, respectively (with R = real and X c = imaginary, also a and b later in the class): The impedance Z is a kind of a generalized resistance R. The phase angle expresses the balance between capacitive and resistance components in the series circuit. For a pure resistance, =0; for a pure capacitance, = /2; and for mixtures, intermediate phase angles are observed. AC Circuit Theory and Representation of Complex Impedance Values 21

For a pure resistance R, E=IR, and the phase is zero. For a pure capacitance C: Where X c is the capacitive reactance, 1/ C A comparison of R and X c shows that X c must carry the dimensions of a resistance, but the magnitude of X c falls with increasing frequency. Data Presentation: Nyquist Plot with Impedance Vector 22

Take a look at the properties of a capacitor: Charge stored (Coulombs): Change of voltage results in current, I: Alternating voltage (ac): Impedance: Admittance: Data Presentation: Nyquist Plot with Impedance Vector 23

Impedance resistance Admittance conductance: Data Presentation: Nyquist Plot with Impedance Vector Representation of impedance value, Z = a +jb, in the complex plane (see also http://math.tutorvista.com/number-system/absolute-value-of-a-complex- number.html 24

Data Presentation: Nyquist Plot with Impedance Vector What is the impedance of an -R-C- circuit? Admittance? Semi- circle time constant: = RC time constant: = RC 25

The parallel combination of a resistance and a capacitance, start in the admittance representation: Transform to impedance representation: A semicircle in the impedance plane! Data Presentation: Nyquist Plot with Impedance Vector R C 26

Data Presentation: Nyquist Plot with Impedance Vector 1 MHz1 Hz 518 Hz R = 100 k C = 3 nF f max = 1/(6.3x3 10 -9 x10 5 )=530 Hz 27

Data Presentation: Nyquist Plot with Impedance Vector What happens for > ? > : This is best observed in a so-called Bode plot log(Z re ), log(Z im ) vs. log(f ), or log|Z| and phase vs. log(f ) 28

Bode plot (Z re, Z im ) -1 -2 29

Bode, abs(Z), phase 30

The Bode Plot Another popular presentation method is the "Bode plot". The impedance is plotted with log frequency on the x-axis and both the absolute value of the impedance (|Z| =Z 0 ) and phase-shift on the y-axis. The Bode plot for the RC circuit is shown below. Unlike the Nyquist plot, the Bode plot explicitly shows frequency information. 31

Data Presentation: Nyquist Plot with Impedance Vector 32 An electrical layer of a device can often be described by a resistor and capacitor in parallel: Voigt network.

When we plot the real and imaginary components of impedance in the complex plane (Argand diagram), we obtain a semicircle or partial semicircle for each parallel RC Voigt network: Nyquist plot or also Cole-Cole plot. The diameter corresponds to the resistance R. The frequency at the 90 position corresponds to 1/ = 1/RC 33 Data Presentation: Nyquist Plot with Impedance Vector

34 The Randles cell is one of the simplest and most common cell models. It includes a solution resistance, a double layer capacitor and a charge transfer or polarization resistance. In addition to being a useful model in its own right, the Randles cell model is often the starting point for other more complex models. The equivalent circuit for the Randles cell is shown in the Figure. The double layer capacity is in parallel with the impedance due to the charge transfer reaction The Nyquist plot for a Randles cell is always a semicircle. The solution resistance can found by reading the real axis value at the high frequency intercept. This is the intercept near the origin of the plot. This plot was generated assuming that R s = 20 and Rp= 250 . The real axis value at the other (low frequency) intercept is the sum of the polarization resistance and the solution resistance. The diameter of the semicircle is therefore equal to the polarization resistance (in this case 250 ).

Data Presentation: Nyquist Plot with Impedance Vector 35

Summary Nyquist and Bode Plots 38

Review of Circuit Elements Very few electrochemical cells can be modeled using a single equivalent circuit element. Instead, EIS models usually consist of a number of elements in a network. Both serial and parallel combinations of elements occur. Impedances in Series: Impedances in Parallel 39

Review of Circuit Elements EIS data is commonly analyzed by fitting it to an equivalent electrical circuit model. Most of the circuit elements in the model are common electrical elements such as resistors, capacitors, and inductors. To be useful, the elements in the model should have a basis in the physical electrochemistry of the system. As an example, most models contain a resistor that models the cell's solution resistance. Some knowledge of the impedance of the standard circuit components is therefore very important. The Table below lists the common circuit elements, the equation for their current versus voltage relationship, and their impedance: ComponentCurrent Vs.VoltageImpedance resistorE= IRZ = R inductorE = L di/dt Z = i L capacitorI = C dE/dt Z = 1/i C Notice that the impedance of a resistor is independent of frequency and has only a real component. Because there is no imaginary impedance, the current through a resistor is always in phase with the voltage. The impedance of an inductor increases as frequency increases. Inductors have only an imaginary impedance component. As a result, an inductor's current is phase shifted 90 degrees with respect to the voltage. The impedance versus frequency behavior of a capacitor is opposite to that of an inductor. A capacitor's impedance decreases as the frequency is raised. Capacitors also have only an imaginary impedance component. The current through a capacitor is phase shifted - 90 degrees with respect to the voltage. 40

Review of Circuit Elements Resistance and impedance both go up when resistors are combined in series. Now suppose that we connect two 2 F capacitors in series. The total capacitance of the combined capacitors is 1 F Suppose we have a 1 and a 4 resistor is series. The impedance of a resistor is the same as its resistance. We thus calculate the total impedance Z eq : Impedance goes up, but capacitance goes down when capacitors are connected in series. This is a consequence of the inverse relationship between capacitance and impedance. C1C1 C2C2 R1R1 R2R2 41

Resistance: ZR E I 0 Capacitance: Z C 1 I E 90 Review of Circuits Elements 42

In a general sense, we ought to be able to represent the performance of a cell by an equivalent circuit of resistors and capacitors under a given excitation. The elements of equivalent circuit of a cell: double-layer capacitance C d, faradaic impedance Z f, solution resistance R s, charge transfer resistance R ct, Warburg impedance Z w. Equivalent Circuit of a Cell 43

Electrolyte resistance R is often a significant factor in the impedance of an electrochemical cell. A modern 3 electrode potentiostat compensates for the solution resistance between the counter and reference electrodes. However, any solution resistance between the reference electrode and the working electrode must be considered when you model your cell. The resistance of an ionic solution depends on the ionic concentration, type of ions, temperature and the geometry of the area in which current is carried. In a bounded area with area A and length l carrying a uniform current the resistance is defined as: 44 Equivalent Circuit of a Cell: R s and C d

Standard chemical handbooks list values for specific solutions. For other solutions and solid materials, you can calculate from specific ion conductances. The units for are Siemens per meter (S/m). The Siemens is the reciprocal of the ohm, so 1 S = 1/ohm Unfortunately, most electrochemical cells do not have uniform current distribution through a definite electrolyte area. The major problem in calculating solution resistance therefore concerns determination of the current flow path and the geometry of the electrolyte that carries the current. A comprehensive discussion of the approaches used to calculate practical resistances from ionic conductances is beyond the scope of this class. Fortunately, you don't usually calculate solution resistance from ionic conductances. Instead, it is found when you fit a model to experimental EIS data. 45

A Resistance and capacitance in series f is low: f is high: In electrochemical cell: R=R s : solution resistance C=C d : double layer capacitance Electrochemical Impedance Spectroscopy Equivalent Circuit of a Cell: R s and C d 46

A electrical double layer exists at the interface between an electrode and its surrounding electrolyte. This double layer is formed as ions from the solution "stick on" the electrode surface. Charges in the electrode are separated from the charges of these ions. The separation is very small, on the order of angstroms. Charges separated by an insulator form a capacitor. On a bare metal immersed in an electrolyte, you can estimate that there will be approximately 30 F of capacitance for every cm 2 of electrode area. The value of the double layer capacitance depends on many variables including electrode potential, temperature, ionic concentrations, types of ions, oxide layers, electrode roughness, impurity adsorption, etc. Principle of the Electric Double-Layer: Here C electrodes 47 Equivalent Circuit of a Cell: R s and C d

Equivalent Circuit of a Cell: CPE Constant Phase Element (for double layer capacity in real electrochemical cells) Capacitors in EIS experiments often do not behave ideally. Instead, they act like a constant phase element (CPE) as defined below. When this equation describes a capacitor, the constant A = 1/C (the inverse of the capacitance) and the exponent = 1. For a constant phase element, the exponent a is less than one. The "double layer capacitor" on real cells often behaves like a CPE instead of like a capacitor. Several theories have been proposed to account for the non-ideal behavior of the double layer but none has been universally accepted (fractal explanation!). In most cases, you can safely treat as an empirical constant and not worry about its physical basis. 48

Equivalent Circuit of a Cell: CPE Constant Phase Element: Y CPE = Y 0 n {cos(n /2) + j sin(n /2)} n = 1 Capacitance:C = Y 0 n = Warburg: = Y 0 n = 0 Resistance:R = 1/Y 0 n = -1 Inductance:L = 1/Y 0 All other values, fractal? Non-ideal capacitance, n < 1 (between 0.8 and 1?) 49

Equivalent Circuit of a Cell: CPE Deviation from ideal dispersion: Constant Phase Element (CPE), (symbol: Q ) General observations: Semicircle (RC ) depressed vertical spur (C ) inclined Warburg less than 45 n = 1, , 0, -1, ? n = 1, , 0, -1, ? 50

Equivalent Circuit of a Cell: CPE How to explain this non-ideal behaviour? 1980s: Fractal behaviour (Le Mehaut) = fractal dimensionality i.e.: What is the length of the coast line of England? Depends on the size of the measuring stick! Self similarity 51

Equivalent Circuit of a Cell: CPE Fractal line Sierpinski carpet Self similarity! 52

Mixed kinetic and diffusion control C dl or CPE RPRP R ZWZW with 0 n 1 Equivalent Circuit of a Cell 53

Equivalent Circuit of a Cell: R p(or ct) and C d 54 A charge transfer resistance is formed by common kinetically controlled electrochemical reaction Consider a metal substrate in contact with an electrolyte. The metal molecules can electrolytically dissolve into the electrolyte, according to: or more generally: In the forward reaction in the first equation, electrons enter the metal and metal ions diffuse into the electrolyte. Charge is being transferred. This charge transfer reaction has a certain speed. The speed depends on the kind of reaction, the temperature, the concentration of the reaction products and the potential. The general relation between the potential and the current holds: i o = exchange current density C o = concentration of oxidant at the electrode surface C o * = concentration of oxidant in the bulk C R = concentration of reductant at the electrode surface F = Faradays constant T = temperature R = gas constant a = reaction order n = number of electrons involved = overpotential ( E - E 0 )

Equivalent Circuit of a Cell: R p(or ct) and C d The overpotential, , measures the degree of polarization. It is the electrode potential minus the equilibrium potential for the reaction. When the concentration in the bulk is the same as at the electrode surface, C o =C o * and C R =C R *. This simplifies the last equation into: This equation is called the Butler-Volmer equation. It is applicable when the polarization depends only on the charge transfer kinetics. Stirring will minimize diffusion effects and keep the assumptions of C o =C o * and C R =C R * valid. When the overpotential, , is very small and the electrochemical system is at equilibrium, the expression for the charge transfer resistance changes into: From this equation the exchange current i 0 density can be calculated when R ct is known (see R c t in next figure). 55

A resistance and capacitance in parallel (Randles circuit) Z=R s at high frequency Z=R ct +R s at low frequency Equivalent Circuit of a Cell: R p(or ct) and C d 56

Equivalent Circuit of a Cell: R p(or ct) and C d When there are two simple, kinetically controlled reactions occurring, the potential of the cell is again related to the current by the following (known as the Butler-Volmer equation). I is the measured cell current in amps, I corr is the corrosion current in amps, E oc is the open circuit potential in volts, a is the anodic Beta coefficient in volts/decade, c is the cathodic Beta coefficient in volts/decade If we apply a small signal approximation (E-E oc is small) to the buler Volmer equation, we get the following: which introduces a new parameter, R p, the polarization resistance. If the Tafel constants i are known, you can calculate the I corr from R p. The I corr in turn can be used to calculate a corrosion rate. The R p parameter comes again from the Nyquist plot. 57

Equivalent Circuit of a Cell: R p(or ct) and C d 58

Equivalent Circuit of a Cell: Warburg Impedance Diffusion can create an impedance known as the Warburg impedance. This impedance depends on the frequency of the potential perturbation. At high frequencies the Warburg impedance is small since diffusing reactants don't have to move very far. At low frequencies the reactants have to diffuse farther, thereby increasing the Warburg impedance. The equation for the "infinite" Warburg impedance On a Nyquist plot the infinite Warburg impedance appears as a diagonal line with a slope of 0.5. On a Bode plot, the Warburg impedance exhibits a phase shift of 45. In the above equation, is the Warburg coefficient defined as: = radial frequency D O = diffusion coefficient of the oxidant D R = diffusion coefficient of the reductant A = surface area of the electrode n = number of electrons transferred C* = bulk concentration of the diffusing species (moles/cm 3 ) 59

Define impedance in Laplace space! Equivalent Circuit of a Cell: Warburg Impedance Take the Laplace variable, p, complex: p = s + j . Steady state: s 0, which yields the impedance: In solution: 60

Equivalent Circuit of a Cell: Warburg Impedance The former equation of the Warburg impedance is only valid if the diffusion layer has an infinite thickness. Quite often this is not the case. If the diffusion layer is bounded, the impedance at lower frequencies no longer obeys the equation above. Instead, we get the form: with, = Nernst diffusion layer thickness D = an average value of the diffusion coefficients of the diffusing species This more general equation is called the "finite" Warburg. For high frequencies where , or for an infinite thickness of the diffusion layer where d , this equation becomes the infinite Warburg impedance. 61

Equivalent Circuit of a Cell: Warburg Impedance semicircle 45 62

Equivalent Circuit of a Cell: Warburg ImpedanceCoating Capacitor A capacitor is formed when two conducting plates are separated by a non-conducting media, called the dielectric. The value of the capacitance depends on the size of the plates, the distance between the plates and the properties of the dielectric. The relationship is: With, o = electrical permittivity r = relative electrical permittivity A = surface of one plate d = distances between two plates Whereas the electrical permittivity is a physical constant, the relative electrical permittivity depends on the material. Some useful r values are given in the table: Material rr vacuum1 water80.1 ( 20 C ) organic coating 4 - 8 Notice the large difference between the electrical permittivity of water and that of an organic coating. The capacitance of a coated substrate changes as it absorbs water. EIS can be used to measure that change 63

A metal covered with an undamaged coating generally has a very high impedance. The equivalent circuit for such a situation is in the Figure: C R The model includes a resistor (due primarily to the electrolyte) and the coating capacitance in series. A Nyquist plot for this model is shown in the Figure. In making this plot, the following values were assigned: R = 500 (a bit high but realistic for a poorly conductive solution) C = 200 pF (realistic for a 1 cm 2 sample, a 25 m coating, and r = 6 ) f i = 0.1 Hz (lowest scan frequency -- a bit higher than typical) f f = 100 kHz (highest scan frequency) The value of the capacitance cannot be determined from the Nyquist plot. It can be determined by a curve fit or from an examination of the data points. Notice that the intercept of the curve with the real axis gives an estimate of the solution resistance. The highest impedance on this graph is close to 10 10 . This is close to the limit of measurement of most EIS systems Equivalent Circuit of a Cell: Warburg ImpedanceCoating Capacitor 64

Equivalent Circuit of a Cell: Warburg ImpedanceCoating Capacitor The same data from the previous page shown in a Bode plot below. Notice that the capacitance can be estimated from the graph but the solution resistance value does not appear on the chart. Even at 100 kHz, the impedance of the coating is higher than the solution resistance 65

Equivalent Circuit of a Cell: Warburg ImpedanceCoating Capacitor Classification of types of capacitances sourceapproximate value geometric2-20 pF(cm -1 ) grain boundaries1-10 nF(cm -1 ) double layer / space charge0.1-10 F/cm 2 surface charge /adsorbed species0.2 mF/cm 2 (closed) pores1-100 F/cm 3 pseudo capacitances stoichiometry changeslarge !!!! 66

Equivalent Circuit of a Cell: Warburg ImpedancePorous Coating 67

Summary 68

Summary Below we show some common equivalent circuits models. Equations for both the admittance and impedance are given for each element. Equivalent element AdmittanceImpedance R 1/RR C iCiC1/1/i C L 1/i LiLiL W (infinite Warburg) Y 0 (i ) 1/2 1/Y 0 (i ) 1/2 O (finite Warburg) Q (CPE) Y 0 (i ) 1/Y 0 (i ) 69

Summary By using the various Cole-Cole plots we can calculate values of the elements of the equivalent circuit for any applied bias voltage By doing this over a range of bias voltages, we can obtain: the field distribution in the layers of the device (potential divider) and the relative widths of the layers, since C ~ 1/d 70

Note : Data validation Kramers-Kronig relations Kramers-Kronig conditions: causality linearity stability (finiteness) Real and imaginary parts are linked through the K-K transforms: Response only due to input signal Response scales linearly with input signal State of system may not change during measurement 71

Note: Putting K-K in practice Problem: Finite frequency range: extrapolation of dispersion assumption of a model. Relations, Real imaginary: Imaginary real: not a singularity! 72

Dr. Marc Madou, UCI, Winter 2012 Class X Electrochemical Impedance Analysis (EIS) Electrochemistry...

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Transcript of Dr. Marc Madou, UCI, Winter 2012 Class X Electrochemical Impedance Analysis (EIS) Electrochemistry...