Alireza Ostovari Supervisor: Dr. Peikari Advisor: Dr. Shadizadeh Introduction to Electrochemical...

Alireza Ostovari

Supervisor: Dr. Peikari

Advisor: Dr. Shadizadeh

Introduction to Introduction to Electrochemical Impedance Electrochemical Impedance

Spectroscopy (EIS)Spectroscopy (EIS)

№ of Slides: 37

Petroleum University of Technology

Outline

AC Circuit Theory and Representation of Complex Impedance Values

Physical Electrochemistry and Circuit Elements

Common Equivalent Circuit Models

EIS Applications

2



Almost everyone knows about the concept of electrical resistance.

While this is a well known relationship, its use is limited to only one circuit element: the ideal resistor.

ER

I

3



Impedance spectroscopy is an extremely powerful non-destructive investigative technique. Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and measuring the current through the cell.

EZ

I

4


Linearity of Electrochemistry Systems

Definition of a linear system:

If x1→y1 & x2→y2 then

1. x1(t) + x2(t) → y1(t) + y2(t)

2. ax1(t) → ay1(t)

Electrochemical cells are not linear! Doubling the voltage will not necessarily double the current.

5


Linearity of Electrochemistry Systems

In normal EIS practice, a small (1 to 10 mV) AC signal is applied to the cell. With such a small potential signal, the system is pseudo-linear.

6


Complex Voltage and Current

The impedance is then represented as a complex number:

Euler’s relationship: je Cos jSin

0j t

tE E e ( )

0j t

tI I e

00 0( )

0

( )j t

jj t

E eEZ Z e Z Cos jSin

I I e

7


Data Presentation

1. Nyquist Plot:

If the negative of the imaginary part of Zω is plotted versus real part, we get a “Nyquist Plot”.

8


Data Presentation

2.Bode Plot:The impedance is plotted with log frequency on the X-axis and both the absolute values of the impedance (|Z|) and the phase-shift on the Y-axis.

9


Electrical Circuit Elements

ComponentComponent Current vs. Current vs. VoltageVoltage

ImpedanceImpedance

Resistor E=IR Z=R

Capacitor I=C dE/dt Z=1/jωC

Inductor E=L di/dt Z=jωL

10


Serial and Parallel Combinations of Circuit Elements

1 2 3eqZ Z Z Z

1 2 3

1 1 1 1

eqZ Z Z Z

11


Outline




EIS Applications

12


Physical Electrochemistry and Equivalent Circuit Elements

Electrolyte Resistance (Rs)

Any solution resistance between the reference electrode and the working electrode must be considered when you model your cell.

.s

l lR

A A

13



Double Layer Capacitance (Cdl)

An electrical double layer exists on the interface between an electrode and its surrounding electrolyte.

This double layer is formed as ions from the solution "stick on" the electrode surface.

14



Coating Capacitance (Cc)

A capacitor is formed when two conducting plates are separated by a non-conducting media, called the dielectric.

0 rACd

15


Constant Phase Element (CPE)Capacitors in EIS experiments often do not behave ideally.

For a constant phase element, the exponent α is less than one.


1

CPEZC j

16



Polarization Resistance (Rp)

Whenever the potential of an electrode is forced away from its value at open-circuit, that is referred to as “polarizing” the electrode.

When an electrode is polarized, it can cause current to flow through electrochemical reactions that occur at the electrode surface.

17



The open circuit potential ends up at the potential where the cathodic and anodic currents are equal. It is referred to as a “mixed potential”.

2.3. .a c

pcorr a c

RI

18



Charge Transfer Resistance (Rct)

A similar resistance is formed by a single kinetically-controlled electrochemical reaction.

This charge transfer reaction has a certain speed.

nM M ne

19



Diffusion (W)

Diffusion also can create an impedance called a Warburg impedance.

1WZ j

1

21

12

( ), tan 45

( )

20


Outline




EIS Applications

21



Purely Capacitive Coating

A metal covered with an undamaged coating generally has a very high impedance.

Rs

Cc

1R C

jZ Z Z R R

j C C

22



jZ R

C

limZ R

The value of the capacitance cannot be determined from the Nyquist Plot.

23



The capacitance can be estimated from the graph but the solution resistance value does not appear on the chart.

24



Simplified Randles Cell

The simplified Randles cell includes a solution resistance, a double layer capacitor and a charge transfer or polarization resistance.

Rs

Rp

Cdl

25



2 2 2 2 2 2

.

1.

1 1 1

p dl

s

p dl

R C

eq RR C

pp p

s sp pp

Z ZZ Z

Z Z

R R R Cj CR R j

R C R CR j C

lim eq sZ R

lim

0

eq s pZ R R

&

hint : & 1R CZ R Z j C

26



The Nyquist Plot for a simplified Randles cell is always a semicircle.

=0

(real )i

=

-(imag )i

Rs Rs+ Rp

27



0 2 4 6 8 10 12-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

(real)i

(im

ag) i

28



Rundles Cell and Diffusion Control

This circuit models a cell where polarization is due to a combination of kinetic and diffusion processes.

Rs

Rp

Cdl

W

29



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, increasing the Warburg-impedance.

30



45

31


Outline




EIS Applications

32


EIS Applications

Evaluation of Corrosion Inhibitors

2 1

2

% 100p p

p

R RIE

R

33


EIS Applications

0 50 100 150 200 250 300 3500

20

40

60

80

100

120

(real)i

(-im

ag) i

34


EIS Applications

Study the Coating Performance

Coatings may experience degradation during service. For example, water may penetrate into coating to decrease its resistivity.

EIS technique has been used extensively to study the coating performance and corrosion of steel under the coating.

35


EIS Applications

There are a Rundles semicircle in the high frequency range and a line with an approximately 45◦ slope in the low frequency range despite the scattered data measured at low frequencies.

36

Thanks for Your AttentionThanks for Your Attention

Alireza Ostovari Supervisor: Dr. Peikari Advisor: Dr. Shadizadeh Introduction to Electrochemical...

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Transcript of Alireza Ostovari Supervisor: Dr. Peikari Advisor: Dr. Shadizadeh Introduction to Electrochemical...