Potential and Current Control

1
Potential and Current Control

2
Control of Potential

• The lowest cost method of potential control is to
  connect the sample to a low resistance electrode
  with a stable potential

3
Control of Potential

• For a variable potential, a voltage source can be
  inserted between the two electrodes

4
Control of Potential

• If the potential is monitored, it can be manually
  adjusted to the desired value

5
Control of Potential

• However, the above techniques suffer from
  difficulties when trying to control potential in
  some systems (e.g. active-passive transitions)
• For these situation the potentiostat provides
  near-ideal control characteristics

6
The Potentiostat - Principle of Operation

• The potentiostat uses a simple amplifier circuit
  to hold the output at the same value as a control
  input

7
The Potentiostat - Principle of Operation

• The amplifier is known as an operational
  amplifier (or Op-Amp), and has the transfer
  function

8
The Potentiostat - Principle of Operation

• The amplifier is known as an operational
  amplifier (or Op-Amp), and has the transfer
  function

9
The Potentiostat - Principle of Operation

• A control voltage is connected to the
  non-inverting input of the amplifier

Vin

_
WE
Note - all voltages are measured with respect to
the working electrode (system ground)
10
The Potentiostat - Principle of Operation

• The reference electrode is connected to the
  inverting input of the amplifier

Vin

Vref
_
RE
WE
11
The Potentiostat - Principle of Operation

• The output of the amplifier is connected to the
  counter electrode

Vin

Vout
CE
Vref
_
RE
WE
12
The Potentiostat - Principle of Operation

• Then the output voltage will be given by
• Vout A(Vin - Vref)

Vin

Vout
CE
Vref
_
RE
WE
13
The Potentiostat - Principle of Operation

• Then the output voltage will be given by
• Vout A(Vin - Vref)
• If the reference electrode voltage is too low,
  this will increase the output voltage in such a
  way as to tend to raise Vref

Vin

Vout
CE
Vref
_
RE
WE
14
The Potentiostat - Practical

• Real potentiostats will have more connections
  and controls to allow for changing the potential,
  applying a potential from an external instrument
  etc.

15
The Potentiostat - Practical
16
The Potentiostat - Practical
The Working Electrode is connected to 0 V of the
power supply. It may be internally connected to
mains earth, or, as here, a separate earth (or
ground) terminal may be provided - this should be
connected to the Working Electrode terminal if
possible
17
The Potentiostat - Practical
The Reference Electrode input is connected to the
inverting input of the amplifier. It is a high
resistance input.
18
The Potentiostat - Practical
There may be a potential output that is derived
from the difference between the reference
electrode and working electrode terminals. It
will have a low output impedance, and the
reference electrode input will not be affected by
loads connected to this output (it is said to be
buffered)
19
The Potentiostat - Practical
The Output from the amplifier and the connection
to the Counter Electrode may be separated on some
potentiostats. This allows a resistor to be
connected between the two terminals, with an
internal buffer amplifier providing the same
voltage at the current output terminals
20
The Potentiostat - Practical
An internal source of controlled voltage allows
the control potential to be set.
21
The Potentiostat - Practical
Some potentiostats have an internal meter to
monitor cell potential or current - beware of the
current ranges, as the resistors are often
damaged by excessive currents
22
The Potentiostat - Practical
Various forms of IR compensation allow for
automatic or semi-automatic correction for the
resistive potential difference between the tip of
the Luggin probe and the specimen surface
23
The Potentiostat - Practical
The overload indicator (where fitted) provides a
warning that the potentiostat is no longer able
to control the potential, because too much
current or too large a potential is required
24
Problems with Potentiostats - 1Oscillation

• The potentiostat relies on negative feedback to
  control the potential

25
Problems with Potentiostats - 1Oscillation

• The potentiostat relies on negative feedback to
  control the potential
• However, delays in the feedback loop due to the
  charging of capacitances can shift the phase of
  an ac signal such that the feedback becomes
  positive

26
Problems with Potentiostats - 1Oscillation

• The potentiostat relies on negative feedback to
  control the potential
• However, delays in the feedback loop due to the
  charging of capacitances can shift the phase of
  an ac signal such that the feedback becomes
  positive
• This causes oscillation, typically in the
  kiloherz region

27
Problems with Potentiostats - 1Oscillation

• A cure for this problem (and a method of
  diagnosis) if to fit a 1 mF capacitor between the
  counter and reference electrode terminals

28
Problems with Potentiostats - 1Oscillation

• A cure for this problem (and a method of
  diagnosis) if to fit a 1 mF capacitor between the
  counter and reference electrode terminals
• This produces a strong negative feedback at high
  frequencies that swamps the oscillation

29
Problems with Potentiostats - 1Oscillation - a
solution

• A cure for this problem (and a method of
  diagnosis) is to fit a 1 ?F capacitor between the
  counter and reference electrode terminals
• This produces a strong negative feedback at high
  frequencies that swamps the oscillation
• Unfortunately, it also slows down the response of
  the potentiostat

30
Problems with Potentiostats - 2Noise Pickup

• The reference electrode input is a high impedance
  point, and is very sensitive to noise pickup
  (most commonly at mains frequency)
• Mains frequency noise is rejected by standard
  digital multimeters, and is often overlooked
• But what does it do to the electrochemistry?

31
Problems with Potentiostats - 2Noise Pickup -
solutions

• Check for noise (or oscillation) with an
  oscilloscope connected between the counter
  electrode and the working electrode (not the
  reference electrode, as the connection of the
  oscilloscope may affect the behaviour, and the
  observed potential at the reference electrode
  will be held constant by the action of the
  potentiostat)

32
Problems with Potentiostats - 2Noise Pickup -
solutions

• Screen the reference electrode with a conductor
  connected to ground (or the working electrode)
• Screen the whole system by mounting it in a
  Faraday Cage (e.g. a metal box)

33
Problems with Potentiostats - 3Electronic Noise

• Some instruments may produce a significant level
  of noise - check by performing an experiment on a
  dummy cell made of electronic components, and
  with similar characteristics to the real cell

34
Problems with Potentiostats - 4Overloading

• If the control of the cell potential requires
  either a higher voltage or a higher current than
  the potentiostat can deliver, then the potential
  will not be controlled
• Watch out for very constant current traces, as
  these are not usual for real systems
• Check the overload indicator if one is fitted to
  the potentiostat

35
Problems with Potentiostats - 4Overloading -
solutions

• If the current is limiting
• use a smaller electrode
• If the cell voltage is limiting
• use a smaller electrode
• use a larger counter electrode
• reduce the spacing between the counter and
  working electrodes
• increase the conductivity of the solution (if
  possible)