“Corrosion engineering”

Working of Potentiostat and its Importance in Corrosion Study

Submitted toProf. Dr. Akhlaq Ahmed

Submitted byMuhammad Naeem

Redg. No.2011-MS-MME-22

Department of Metallurgical and Materials EngineeringUniversity of Engineering & Technology Lahore

1 | P a g e

Working of Potentiostat and its importance in Corrosion study

PotentiostatAn electronic instrument that measures and controls the voltage difference between a

Working Electrode and a Reference Electrode.

It measures the current flow between the Working and Counter Electrodes.

Potential or Voltage (E, sometimes V):

Unit: Volt The Potential is the driving force for the redox reaction.

The potential is related to the thermodynamics of the system:

ΔG = -n F ΔE(negative ΔG is spontaneous)

Potential is always measured versus a Reference Electrode. A positive voltage is oxidative and a negative voltage is reductive.

0 Volts is not nothing! Redox reactions happen at 0 volts that do not happen at +1 volt.

There is no correlation between the thermodynamics of the chemical system and the

kinetics (rate) of the reaction.

2 | P a g e

What is CurrentCurrent (i):

Unit: Ampere

Electron flow is the result of a redox reaction.

Current measures the rate of the reaction (electrons per second).

Zero current is nothing, i.e., if the current is zero, no redox reactions are occurring

(that’s not quite true in corrosion!).

Anodic (oxidation) and cathodic (reduction) currents have different polarity (signs).

Current may be expressed as current or current density.

ElectrodesA Potentiostat works with three electrodes immersed in a conductive electrolyte.

Working Electrode

A sample of the corroding metal being tested.

Reference Electrode

An electrode with a constant electrochemical potential.

Counter Electrode

A current-carrying electrode that completes the cell circuit.

Why does a Potentiostat have three electrodes?We would like to study the electrochemical events taking place at one specific

electrode…the Working Electrode. The use of a three-electrode potentiostat with a

separate Reference and Counter Electrode allows the potential at the WE and the

current at the WE to be measured with little or no “interference” or “contribution” from

the other electrodes.

Pay Special Attention to the Reference Electrode!A Potentiostat needs a low impedance Reference Electrode!

Use large junction reference electrodes

Replace isolation frits regularly

Avoid narrow Luggin Capillaries

Potentiostats are less forgiving of high-impedance Reference Electrodes than pH

Meters!

3 | P a g e

If there’s an problem with the cell, it’s almost always the Reference Electrode!

The Analog Potentiostat

Three Primary Components of a PotentiostatControl Amplifier: Supplies the power to maintain the controlled potential between the

Working and Reference Electrodes.

Electrometer: Measures the potential difference between the Reference and Working

Electrodes.

Current-to-Voltage Converter: Measures the current between the Working and Counter

Electrodes.

4 | P a g e

Basic relationshipsA potentiostat is a control and measuring device. It comprises an electric circuit which

controls the potential across the cell by sensing changes in its resistance, varying

accordingly the current supplied to the system: a higher resistance will result in a

decreased current, while a lower resistance will result in an increased current, in order

to keep the voltage constant as described by Ohm's law.

As a result, the variable system resistance and the controlled current are inversely

proportional

 Is the output electrical current of the potentiostat

 Is the voltage that is kept constant

 Is the electrical resistance that varies.

Principles of OperationSince 1942, when Hickling built the first three electrode potentiostat, [5] substantial

progress has been made to improve the instrument. Hickling's device used a third

electrode, thereference electrode to control the cell potential automatically. Up until the

present day his principle has remained in use. At a glance, a potentiostat measures the

potential difference between the working and the reference electrode, applies the

current through the counter electrode and measures the current as an     voltage drop

over a series resistor (  in Fig. 1).

The control amplifier CA is responsible for maintaining the voltage between the

reference and the working electrode as closely as possible to the voltage of the input

source  . It adjusts its output to automatically control the cell current so that a

5 | P a g e

condition of equilibrium is satisfied. The theory of operation is best understood using the

equations below.

Prior to observing the following equations, one may note that, from an electrical point of

view, the electrochemical cell and the current measurement resistor   may be

regarded as two impedances (Fig. 2).   includes   in series with the interfacial

impedance of the counter electrode and the solution resistance between the counter

and the reference.   represents the interfacial impedance of the working electrode in

series with the solution resistance between the working and the reference electrodes.

Fig. Schematic of a potentiostat, with electrochemical cell replaced by two impedances.

The role of the control amplifier is to amplify the potential difference between the

positive (or noninverting) input and the negative (or inverting) input. This may be

translated mathematically into the following equation:

. (1)

where   is the amplification factor of the CA. At this point the assumption may be made

that a negligible amount of current is flowing through the reference electrode. This

correlates to physical phenomenon since the reference electrode is connected to a high

impedance electrometer. Thus, the cell current may be described in two ways:

, (2)

and

. (3)

6 | P a g e

Combining Eqs. (2) and (3) yields Eq. (4):

 (4)

where   is the fraction of the output voltage of the control amplifier returned to its

negative input; namely the feedback factor:

.

Combining Eqs. (1) and (4) yields Eq. (6):

. (6)

When the quantity     becomes very large with respect to one, Eq. (6) reduces to Eq.

(7), which is one of the negative feedback equations:

. (7)

Eq. (7) proves that the control amplifier works to keep the voltage between the

reference and the working close to the input source voltage.

Significant featuresIn electrochemical experiments the electrodes are the pieces of equipment that comes

in immediate contact with the analyte. For this reason the electrodes are very important

for determining the experimental result. The electrode surface may or may not catalyze

chemical reactions. The size of the electrodes affects the magnitude of the currents

passed which can affect signal to noise. But electrodes are not the only limiting factor

for electrochemical experiments; the potentiostat also has a limited range of operation.

The following are a few significant features that vary between instruments.

Electric potential range (measured and applied): while the potential window is mostly

based on the solvent window the electronics can also limit the possible range.

Accuracy in potential (measured and applied): limits of deviations between the

actual and reported.

7 | P a g e

Range of scan rate: how slow or fast a potential window can be scanned this is most

important for experiments that require high scan rates such as those involving ultra

microelectrodes.

Sample rate: the rate at which potential or voltage can be accurately sampled. This can

be important for experiments that need high scan rates such as those involving ultra

microelectrodes.

File size: a limiting factor can be the file size limit. This would most likely affect the

choice of the potential range swept or the potential sample rate.

Electric current range (measured and applied): the maximum range over which

current can be sampled. Applying large currents is important for experiments that pass

a great deal of current like a large bulk electrolysis. Measuring small currents is

important for experiments that pass small currents like those involving ultra

microelectrodes.

Current resolution: determines the operational range of a specific experiment and the

bit resolution of that data in the current dimension.

Accuracy in current (measured and applied): limits of deviations between the actual

and reported.

Number of working channels: how many working electrodes can the instrument

control. A bipotentiostat is necessary to controlling systems with two working electrodes

like a rotating ring-disk electrode. A polypotentiostat may be important for controlling

some biological experiments with three or more working electrodes.

Footprint: potentiostat include small devices of about 20 x 10 x 5 cm weighing well

under a kilogram or a simple board that can be installed in a desktop computer. A large

bench-top model would be on the order of 50 x 20 x 10 cm and weigh up to or more

than 5 kilograms.

Interface: can the instrument run independently or must they be slaved to a personal

computer.

Sweep generator: can the system apply an analogue sweep or does it use a digital

staircase generator as an approximation. If it does use a digital staircase then the

resolution of the staircase is important.

8 | P a g e

Rotating electrode: can the instrument operate a rotating electrode. This is intrinsic for

experiments that require a rotating disk electrode or rotating ring-disk electrode.

Primary useThis equipment is fundamental to modern electrochemical studies using three electrode

systems for investigations of reaction mechanisms related to redox chemistry and other

chemical phenomena. The dimensions of the resulting data depend on the experiment.

In voltammetry, electric current in amps is plotted against electric potential in voltage. In

a bulk electrolysis total coulombs passed (total electric charge) is plotted against time in

seconds even though the experiment measures electric current (amperes) over time.

This is done to show that the experiment is approaching an expected number of

coulombs.

Most early potentiostats could function independently, providing data output through a

physical data trace. Modern potentiostats are designed to interface with a personal

computer and operate through a dedicated software package. The automated software

allows the user rapidly to shift between experiments and experimental conditions. The

computer allows data to be stored and analyzed more effectively, rapidly, and

accurately than historic methods.

Summary:1. In order to ensure stable potential control of an electrochemical cell with a double

layer capacitance in the boundary layer it is essential to have a potentiostat with phase

compensation. The internal phase angle of the potentiostat must not exceed 90° up to

about the transit frequency.

2. For similar reasons it is necessary that the internal resistance of the power output

stage should not produce additional phase shift up to about the transit frequency.

3. The time constant of the reference electrode /capillary system Tk = Rk Ck should be

kept small compared with the rise time constant of the potentiostat T =1/ωt.

4. The cell resistance Rz and the resistance Rm and Rj should be made as small as

possible in order to achieve a large bandwidth.

9 | P a g e

5. If ωg is smaller than ßo ωt there is a rise in the frequency characteristic which

increases when ωg and ßc ωt are further apart and when the difference between Rp

and Rs increases. This rise causes an overshoot in the response to a step input and

produces damped oscillations as the difference between the two values increases.

10 | P a g e