Transcript Intro to Electrochemistry Powerpoint

INTRODUCTION TO ELECTROCHEMICAL
CELLS AND BASIC ELECTROANALYTICAL
MEASUREMENTS
ANDREA MARDEGAN
JAN 17th 2013
Although we would like to measure electrochemical observables
(current, voltage, …) associated with a single working electrode, we
can’t.
We must always couple our working electrode to a second
electrode in order to make a measurement. These two electrodes
comprise an electrochemical cell.
(ideal) Reference electrode…
1. It has a well defined and invariant potential. That is – no matter
how much current we drow from this electrode, its potential must
not vary
2. It has zero impedance. It imposes no resistive load on our cell.
3. It does not contaminate our solution. That is , it is not a source of
undesidered ions in our electrochemical cell.
THE PROBLEM
… but to do that, we need a
second electrode in the
solution to complete the circuit
We wish to control the
potential of this working
electrode…
For example, let’s say both electrodes are platimum…
At open circuit, no potential is applied between them…
And we don’t know this potential
Now we apply +0.8 V to the WE
The potential of both electrodes changes and NOT SYMMETRICALLY.
THE SOLUTION
Substitute the platinum RE with a SCE
Reference electrode…
1. It has a well defined and invariant potential. That is – no matter how much current we
drow from this electrode, its potential must not vary
2. It has zero impedance. That is, it imposes no resistive load on our cell.
3. It does not contaminate our solution. That is , it is not a source of undesidered ions in
our electrochemical cell.
3-electrode cell…
Let’s measure the current that flows as we chage the voltage of the
platinum electrode
SUPPORTING ELECTROLYTE: an inert salt added to impart conductivity to the solution
BACKGROUND LIMITS: the two limits at which the solvent + supporting electrolyte begin to
react at WE
POLARIZABLE ELECTRODE: an electrode operating within a potential range in which no
Faradaic electrochemistry occurs.
Polarization…
A red-ox process that takes place at a WE can be considered as a sequence of 3 steps:
-Mass transfer from the solution to the electrode surface
-Reation on the electrode surface
-Tranfer of the product from the electrode surface to the bulk solution.
Ox  ne  Re d
Mass transfer regimes…
•
Diffusion: in order to minimize the
differences in concentration
(concentration gradients).
•
Migration: movement of ionic species
under an eletric field
•
Convection : agitation, rotation, thermal
gradients (external agents…)
Linear sweep voltammetry
The potential is scanned linearly from a
value where no reaction occurs to a more
negative potential:
E-
Ox  ne  Re d
t
The current will increase
when the potential is closed
to E° (standard potential of a
red-ox couple).
Then a current decay
(cottrellian decay) is caused
by the lack of Ox species
closed to the electrode
surface.
Cyclic voltammetry
E-
The potential is scanned linearly
until a certain value (usually more
than 90 mV after E peak) and then
back to the starting potential
Ox  ne  Re d
Re d  Ox  ne
Red, generated in the first scan
and being in the proximity of
the electrode surface, is reoxidixed to Ox.
t
Cyclic voltammetry
B
C
A
D
Randles-Sevcik equation:


3
2
1
2
I p  2.69 105 n ACD v
1
2
A:
B:
C:
D:
Cox= max; CRed= 0
Cox= CRed
Cox= 0; CRed= max
Cox= CRed
Redox Cycling through Inter Digitated
electrodes Arrays(IDA)
•Two Working electrodes :Generator (Anode) and
Collector (Cathode) electrodes in close proximity
such that the adjacent regions with concentration
gradients overlap
•The redox couple may redox cycle multiple times
before they diffuse out into the bulk solution.
•This behavior results in an amplified signal thereby
lowering the lower limit of detection (LOD)
significantly (upto pM)
.
•Single mode: Cyclic Potential sweep across Generator, Collector not
connected.
•Dual Mode: Constant reduction potential applied across collector while
potential is swept linearly across generator
Redox Cycling as electrode width and gap
Redox Cycling as electrode height