controlled current techniques 1 part

7/31/2019 Controlled Current Techniques 1 Part

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Controlled Current Techniques


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Basic Terminology

Electrolyte: Medium that contain free ions making it electrically conductive

Typically ionic solutions, but molten

and solid electrolytes too exist

Solute dissociates into ions, and its

tendency to dissociate governs the

strength of the electrolyte

Electrolyte drinks contain Na, K

replenish body waters

Electrode: A electric conductor through which electric current is passed.

A collector or emitter of electric charge.

Used in forms of plates, rods, wires, etc.

Metals like Cu, Ag, Pb, Zn and nonmetals like Carbon


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1. Working Electrode:

At which prinicple electrochemical reaction (oxidation/reduction) takes place

Cathodic/Anodic depending on the nature of the reaction

Behavior (Potential) is studied with respect to a standard reference electrode

2. Counter/Auxiliary Electrode:

Closes the current circuit in the cell

Works as complimentary to WE

Surface area larger in order not to limit

process at WE

Supplies the current required by WE

Current carrying electrode that completes

cell circuit

Prone to corrosion - safe

3. Reference Electrode:

Provides fixed potential that doesn't vary

during operation and against which other potentials are measured.


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Current (WE CE) is the controlling parameter

and potential (WE RE) is measured with time

Also called Chronopotentiometry

Techniques differentiated based on the

pattern the current is controlled


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Potentiostat:

Electrical instrument that controls and measures voltage.

Accurately control the potential between WE and CE

Galvanostat:

Electrical instrument that controls and measures the current flow

through electrolytic cell

Capable of maintaining constant current flow even when under load

variations itself Controls current flow between WE and CE

Consists of a high voltage source producing a constant voltage V with a

resistor Rx connected in series .

To maintain almost constant current through load , this resistor shall be

much higher than the load resistor Rload

=

+


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Types of Controlled Current Techniques:

Constant current CP

CP with linearly increasing

current

Current reversalCP

CyclicCP


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(1) First consider constant-current chronopotentiometry for

anthracene (An). The steady current, i, applied to the

electrode causes the (An) to be reduced at a constant rate to

the anion radical An- .

The potential of the electrode varies with time as the An/An-

concentration ratio changes at the electrode surface.

The process can be regarded as a titration of the An in the vicinityof the electrode by the continuous flux of electrons, resulting in

an E-t curve like that obtained forapotentiometric titration .

The time after application of the constant current till this

potential transition occurrence is called the transition time, .

The shape and location of the E-t curve is governed by the

reversibility, or the heterogeneous rate constant, of the electrode

reaction.


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(2) apply a current that varies as a known function of time (e.g., i

=t, a current ramp). this technique, called programmed

current chronopotentiometry .

(3) If The current is reversed after some time (current reversal

chronopotentiometry) at, or before the transition time, the

An formed during the forward step will start oxidizing.

The potential will move in a positive direction as the An/Anconcentration ratio increases.

When the Anconcentraon falls to zero at the electrode

surface, a potential transition toward positive potentials

occurs, and a reverse transition time can be measured.

(4) if the current continuously reversed at each transition. it is

resulting in cyclic chronopotentiometry .


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Mathematics of Semi-infinite Linear Diffusion:

- Consider simple electron transfer reaction occurring at an electrode where semi-infinitelinear diffusion applies:

+

- Boundary conditions involving the concentration gradient allows the diffusion problem to besolved without reference to the rate of electron transfer reaction, in contrast with theconcentration-potential boundary conditions required for controlled potential methods.

- On applying Laplace transform yields,

- These integrals forms are convenient for solving controlled current problems


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Concentration profile of O and R various values oft/ during constant current electrolysis are

shown in graph.

- The measured value ofat known ican be used to determine C0* or D0. A

lack of constancy of the transition time constant, i1/2/C*, with ior C0*

indicates complications to the electrode reaction from coupled

homogeneous chemical reactions, adsorption etc.

- This equation is known as Nernst equation

Constant Current Electrolysis Sand Equation


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Response Function:

=

Any response R is a function of the input (controlled current, ) and the system

properties S.

For the semi infinite linear diffusion scenario, response function at x=0 can be given as

Reversal Techniques:

i. Consider a situation where current i is supplied for a time t1. At t=t1, the

direction of current is reversed toi.

Response function at x = 0 thus turns out to be


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If2 is the time at which CR at the R electrode falls to 0, relation between t1 and 2can

be derived as follows:

At t = t1 + 2, CR = 0, which gives


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Multicomponent systems:

Consider a system containing two reducible substances O1, O21 + 1

1

1 + 2 2

Response functions can be written individually as:

Adding up both equations and rearranging,

From 0 < t < t1, i2 = 0 (due to insufficient negative potential), during which O1 reduces

alone,


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t > t1, C1(0,s) = 0,

At t = 1 + 2, C2(0,s) = 0. So, for a constant current = ,

Applying Laplace Inverse,


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Derivative Method:

By rather straightforword in instrumentalapproach the deriative of the

chronopotonetiogram that is a curve of dE/dt

vs. t

While finding from the maximum of the

derative curve is possible . for the Nernstian process occur at t= 4 /9


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Determination of the by this approach is free from

problem of double layer charging because it

evaluated at a position in the curve before the

transition time region where an appreciablecharging current contribution exist

However the large charging current contribution at

salt of the chronopotentiogram still contributes

The derative approach does suffer from need for

knowledge about the degree of thr reversibility of

the electrode reaction and it is the reversible

knowing the


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Controlled Current TechniquesApplications

EC mechanisms are especially amenable to

study by this technique.

Ex. Oxidation of p-aminophenol (PAP)

CHEM 5390


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CHEM 5390


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CHEM 5390


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Cyclic Chronopotentiometry

Current is continually reversed at potentials

corresponding to the forward and reverse

transition times.

Not used much.

CHEM 5390


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Charge Step (Coulostatic) Methods

In the charge step (or coulostatic) method a very short-

duration(eg., 0.1 to 1 sec) current pulse is applied to the

cell, and the variation of the electrode potential with

time after the pulse is recorded.

The current-pulse length is chosen to be sufficiently shortthat it only causes charging of the electrical double layer.

The charge can be injected by discharging a small

capacitor across the electrochemical cell as shown in fig.

or with a pulse generator connected to the cell by acapacitor or switching diodes.


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Cinj is charged by the voltage source, Vinj

Capacitor is charged by the amount:

Eg. Vinj=10V and Cinj=10-9 F

q= Cinj Vinj

q=0.01C


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The time required for this charge injection willdepend on the cell resistance, R.

This injected charge causes the potential of theelectrode to deviate from its original value Eeq toa value E(t = 0),

The charge on Cd now discharges through thefaradaic impedence, and the open circuitpotential moves back towards asdecreases to zero.

If no faradaic reaction reaction is possible, Cdremains charged and the potential will not decay.


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Large Steps-Coulostatic Analysis

Consider the application of a charge step

sufficiently large that the potential changes

from Eeq to a value, E(t=0)

Assumption: double layer capacity, Cd is

independent of potential in this region.


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is linear with a zero intercept.

This method has been suggested for thedetermination of small concentrations ofelectroactive materials, but not been widelyapplied because it requires recording of E-t curve.


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Application of charge-Step Methods

Advantages in the study of electrode

reactions, since the measurement is made at

open circuit with no net external current flow

Measurements in highly resistive area can be

made

controlled current techniques 1 part

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