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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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Controlled Current Techniques
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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Controlled Current TechniquesApplications
Ex. Oxidation of p-aminophenol (PAP)
CHEM 5390
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Controlled Current TechniquesApplications
Ex. Oxidation of p-aminophenol (PAP)
CHEM 5390
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Controlled Current Techniques
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