Transcript 04__Electrochemistry1

Electrochemistry: Introduction
Electrochemistry at your finger tips
Part 1: Electron transfer
reactions, Electrodes, Cells,
Potentiostat
Electrochemistry studies electron transfer processes
(reduction – oxidation reactions) – they are always coupled
Electrochemistry studies redox processes on interfaces:
electron-conductive metal / ion conductive liquid
electrolyte
solution
(ionic
conductivity)
Electron
transfer
interface
metal (electron
conductivity)
Electrical conductivity of electrodes and wires originates
from free moving electrons in the solid phase
Electrodes can be made of:
1. Metals: gold, silver, cupper,
mercury, etc.
2. Carbon materials: glassy
carbon, pyrographite, etc.
3. Semiconductive materials
(usually highly doped – ntype): indium tin oxide (ITO)
Solutions used in electrochemical processes should
conduct electrical current – they should be electrolytes
This movie shows
various solutions with
conductive and nonconductive properties
Electrical conductivity in solutions depends on the
presence of ions originating from dissociated molecules
Electrochemical reactions are always interfacial –
if the initial reactant are dissolved in a solution,
the process should include diffusional steps
Various kinds of electrochemical processes are
shown in the following schemes:
Diffusion of Fe3+
Diffusion of Fe2+
A single electron transfer reaction:
1) Diffusion of the initial substrate from the bulk solution to the electrode surface
2) Electron transfer process at the interface
3) Diffusion of the product from the electrode surface to the bulk solution
http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l1html/intro01.html
Diffusion of Cu2+
Metal deposition on the electrode:
1)
Diffusion of the metal cations from the bulk solution to the
electrode surface
2)
Reduction of the metal cations
3)
Deposition of metal atoms on the electrode surface
http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l1html/intro01.html
Diffusion of the
initial substrate
Diffusion of the
final products
Electron transfer followed by chemical reaction:
1)
Diffusion of the initial substrate from the bulk solution to the electrode surface
2)
Electron transfer and formation of unstable intermediate product
3)
Following chemical reactions (decomposition of the intermediate product)
4)
Diffusion of final products from the electrode surface to the bulk solution
http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l1html/intro01.html
Electron transfer between the electrode and the
redox substrate is a potential dependent process
Representation of the Fermi-Level in a metal
at three different applied voltages
http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l1html/intro01.html
Schematic representation of the reduction of
a species (O) in solution
http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l1html/intro01.html
Animation of the reduction of a species (O) in solution
http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l1html/intro01.html
What is the Connection between electrochemical
Potential (E0Cell) and free energy (DG0)?
Reduction Potentials can be used to predict whether a chemical reaction is spontaneous
Gibbs free energy reflects the spontaneity of a chemical reaction
DG0 = -n●F●E0cell
n●F●E0cell= R●T●ln ( Keq)
For non-standard conditions:
The Nernst equation: Ecell = E0 – (RT/nF)lnQ
0
cell
E
E E
0
ox
0
red
DG= -nf●E
For a single applied voltage the free energy profiles appear qualitatively to be the
same as corresponding chemical processes. However if we now plot a series of
these free energy profiles as a function of voltage it is apparent that the plots alter as
a function of the voltage. It is important to note that the left handside of the figure
corresponding to the free energy of R is invariant with voltage, whereas the right
handside ( O + e) shows a strong dependence. At voltage V1 the formation of the
species O is thermodynamically favoured. However as we move through the voltages
to V6 the formation of R becomes the thermodynamically favored product. This can
be explained in terms of the Fermi level diagrams noted earlier, as the voltage is
altered the Fermi level is raised (or lowered) changing the energy state of the
electrons.
http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l2html/l2kin.html
An electrical potential of a metal electrode immersed into a
solution containing redox species comes to the equilibrium
with the chemical system:
E - equilibrium potential
Electrode
Electrolyte solution
-
+
-
+
+
-
Oxidized redox species
Reduced redox species
Electrolyte cations and anions
U - applied voltage
Electrode 1
1V
Electrode 2
Electrolyte solution
I - current
E1 - potential
+0.45 V
E2 - potential
-0.45 V
R - electrolyte resistance
I x R - lost voltage in the solution
Problems:
0.1 V
U = E1 - E2 + I x R
1) Both electrodes are polarized; 2) Both potential are not known
U - applied voltage
1V
Electrode 1
Electrode 2
(large)
(small)
Electrolyte solution
I - current
i = I/S current
density
E1 - potential
+0.05 V
E2 - potential
-0.85 V
R - electrolyte resistance
I x R - lost voltage in the solution
Problem:
0.1 V
U = E1 - E2 + I x R
The potential shift depends on the current density.
Only working electrode is polarized, but its potential is not known.
U - applied voltage
Electrode 1
Electrode 2
(large)
(small)
Electrolyte solution
I - current
Small polarization of
a counter electrode
Large polarization of
a working electrode
Long distance
for the bullet
Short distance
for the gun
In order to know the exact potential of the
working electrode, we need to use a reference
electrode.
Reference electrodes are not simply metal
conductors – they are chemical redox systems
maintaining stable standard potentials.
Standard Hydrogen Electrode (SHE):
theoretically very important - practically never used
SHE = 0 volts by definition
http://courses.cm.utexas.edu/archive/Summer2004/CHs305/Murray/LectureNotes/redox.html
Saturated Calomel Electrode (SCE)
Silver/Silver Chloride Electrode (Ag/AgCl/Cl-sat)
are the most frequently used reference
electrodes
SCE
AgCl + e-
Ag+ + ClAg/AgCl/Cl-sat
Luggin capillary is a connector between a reference
electrode and a solution near a working electrode
The Luggin capillary in a laboratory cell is made from glass or plastic. It is generally filled with
the background electrolyte solution. The Luggin holds the reference electrode. The tip of the
Luggin capillary near the working electrode is open to the test solution. The reference
electrode senses the solution potential at this open tip. Note that the Luggin tip is significantly
smaller than the reference electrode itself. The Luggin capillary allows sensing of the solution
potential close to the working electrode without the adverse effects that occur when the large
reference electrode is placed near the working electrode.
A Luggin capillary can be used to bring the potential measuring
point in close proximity to a working electrode under investigation.
Such a device can be made of any material provided it is inert to the
electrolytic environment. It basically consists of a bent tube with a
large enough opening to accommodate a reference electrode and a
usually much smaller opening only large enough to insure
diffusional movement of the electrolyte. The device minimizes any
iR drop in the electrolyte associated with the passage of current in
an electrochemical cell.
R
Reference
electrode
Potentiostat
C W
Working electrode
Counter
electrode
I - current
CW - applied voltage
CW - measured current
RW - measured voltage, providing feedback
Problems:
1) Disconnection of C or W - no voltage, no current
2) Disconnection of R - very large voltage and current between W & C
Potentiostat is the most
important part of any
electrochemical analyzer
Hans Wenking
Wenking´s`potentiostat scheme
• A control voltage is fed into the control input CI. This control voltage
forces a current through the counter electrode exactly as high as to
achieve the wanted potential difference between working electrode
and reference electrode. The control voltage may be produced by
the internal potential control source of the potentiostat, or by an
external signal generator, e.g. a ramp generator or a sine wave
generator. While for a constant potential the internal potential control
source is sufficient, time-dependent signals are to be fed from
external sources (except our scanning potentiostats that also have
built-in voltage scanners).
Cathodic and anodic
processes can be controlled
by the applied potential
using a potentiostat
reference working
counter
working
cathodic process
working
anodic process
http://www-biol.paisley.ac.uk/marco/Enzyme_Electrode/Chapter1/electrodes_and_cell.gif
Electrochemical cells and electrode configurations
An electrochemical cell must consist of at least two electrodes and one
electrolyte. An electrode may be considered to be an interface at which the
mechanism of charge transfer changes between electronic (movement of
electrons) and ionic movement of ions. An electrolyte is a medium through
which charge transfer can take place by the movement of ions.
In a cell used for electroanalytical measurements there are always three
electrodes:
The first of the three electrodes is the indicating electrode also known as the
test or working electrode. This is the electrode at which the electrochemical
phenomena being investigated takes place.
The second functional electrode is the reference electrode. This is the electrode
whose potential is constant enough that it can be taken as the reference
standard against which the potentials of the other electrodes present in the cell
can be measured.
The final functional electrode is the counter or auxiliary electrode which serves
as a source or sink for electrons so that current can be passed from the
external circuit through the cell. In general, neither its true potential nor current
is ever measured or known.
http://www-biol.paisley.ac.uk/marco/Enzyme_Electrode/Chapter1/Ferrocene_animated_CV1.htm
Recommended textbooks
on electrochemistry:
Electrochemical Methods : Fundamentals and
Applications, by A.J. Bard and Larry R. Faulkner
Analytical Electrochemistry,
by Joseph Wang
Broadening Electrochemical Horizons:
Principles and Illustration of
Voltammetric and Related Techniques,
Edited by Alan Bond