Transcript echem

Electrochemistry
Introduction
This presentation will introduce:
- Cyclic Voltammetry
- Current vs potential graphs (cyclic voltammograms)
- Randles-Sevcik equation
- Potential step experiments and the Cottrell equation
- How to use the potentiostat software
Cyclic Voltammetry: is an electrochemical technique used to measure the current
that develops in an electrochemical cell when the voltage is varied.
• The voltage applied is controlled by a potentiometer (you have to programme
the potentiometer).
• The potential is swept through a defined potential range over time (as shown by
section 1 of the graph opposite).
• This linear increase is known as ramping.
• The scan rate is the time over which the potential is changing before returning to
the original voltage, i.e. the time to complete one cycle.
• The laptop will record current (I/amps) readings at intervals over the scan.
• The data obtained is plotted as current against voltage and known as a cyclic
voltammogram. An example is shown and explained on the next slide.
1
Current vs Potential Graph:
Explained
Potential ≡ Voltage
The characteristic shape of a voltammogram for a solution phase redox couple is explained
by the effect of diffusion: at potentials beyond the peak potential the electroactive species
at the surface of the electrode is depleted and the kinetics of the reaction (the current) is
limited by the rate of diffusion of the species to the electrode surface from the bulk of the
solution. Hence the current shows a peak and then a decrease before leveling out slightly.
As time increases, the
potential increases, (shown
by the ramping on the graph
in slide 2 and on the x-axis of
this graph). The current in
region A remains unchanged
C
B
A
D
F
E
Zero
Current
line
Current vs Potential Graph:
Explained
Region B shows increasing current as the potential difference is increased. This is due to the
oxidation of
the species in solution. The current in this region is limited by the transfer of electrons
between the species and the electrode.
C
In region C the current then
B
peaks before dropping off as the rate of
reaction is limited by the
diffusion rate of the species in solution.
Region D shows the potential
difference being decreased. As the
potential difference decreases, the
current decreases because the reaction
rate decreases. In Region E the species
produced in the forward scan is being
reduced.
A
D
F
E
Zero
Current
line
Region F shows the reaction being limited by the diffusion of the oxidised species to the
electrode.
In this way, the Current vs Potential Graph can be read with time in a clockwise direction even
though time is not a feature of the graph.
Current vs Potential Graph: Uses
• The graph can be used to determine the number of moles (of copper in this
case) that are reduced/oxidised at the electrode during the reaction.
• Find the area under the curve of the cyclic voltammogram (CV)
• Divide the area by the scan rate this will give the charge (Q/As) that is associated
with the electron transfer in a reaction.
• The charge is then directly related to the number of moles by the equation:
Q = nFm
where:
Q is the charge (C)
n is the number of electrons transferred in the reaction
F the Faraday constant (96485 C mol-1)
m the number of moles (mol)
Remember 1 Coulomb is 1 Amp second
C = As
The Randles-Sevcik equation
The Randles-Sevcik equation relates the peak current, ip, (measured as the
distance from the baseline to the peak) to the scan rate allowing determination
of the diffusion coefficient. The equation is written as:
ip = (2.69 x 105)n3/2AcD1/2𝜈1/2 (eq.1)
Where: ip is the peak current (A)
n is the number of electrons transferred in the reaction
A is the area of the electrode (m2)
c is the concentration of the solution used (mol m-3)
D is the diffusion coefficient (m2 s-1)
𝝂 is the scan rate (V s-1)
The constant has units of A s V-1/2 mol-1
The tendency observed is that the faster the scan rate the larger the current.
The equation needs to be rearranged to determine the diffusion coefficient.
Potential step
During potential step experiments the current is recorded as a function of time
(Amperometric detection). The current results from the oxidation or reduction of
ions in a solution. Any species that can be oxidized or reduced can be detected in
this method.
The simplest form of amperometric detection is direct current (DC) amperometry,
which will be used in this experiment:
A potential is applied between two electorodes and the current changes as an
electroactive species is oxidized or reduced.
The data obtained from amperometric detection is a second way of determining
the diffusion coefficient, this time through the use of the Cottrell equation:
where:
𝒏𝑭𝑨𝑫𝟏/𝟐 𝒄
i = 𝟏/𝟐 𝟏/𝟐
𝝅 𝒕
i is the current (in A),
n is again the number of electrons,
F the Faraday constant (96484 C mol-1)
A area of the cell (m3)
D the diffusion coefficient (m2 s-1)
c the concentration of the solution (mol m-3)
By plotting a graph of i versus 1/√t it is possible to find D.
What the software display values mean
when programming the potentiometer:
E = Potential
Shows the amount of
time that the electrode
is allowed to ‘settle’
before the experiment
starts.
The potential at the start.
The lowest Potential that the
potentiostat will scan between.
The highest potential that the
potentiostat will scan between.
The size of the intervals
(in volts) made between
each data point.
The change in voltage per
second during the scan.
The number of scans that the
potentiostat will make.