Download DICMAPI Seminars 3 rd -4 th December 2014, Naples

Practical impedance
measurement
Michele Curioni
DICMAPI Seminars 3rd-4th December 2014, Naples
Outline
• How potentiostats / frequency response
analysers work
• Issues
– Non stationarity
– Non linearity
• Meaning of Settings
• Choosing the best electrodes configuration
DICMAPI Seminars 3rd-4th December 2014, Naples
Operational Amplifier
IN +
OUT
IN -
The amplifier is a device that outputs a current such as
the voltage difference between IN+ and IN- is 0
DICMAPI Seminars 3rd-4th December 2014, Naples
Current Measurement - Zero
Resistance Ammeter (ZRA)
Rm (RANGE)
IIN
IN +
OUT
IOUT
IN V=IIN x Rm
DICMAPI Seminars 3rd-4th December 2014, Naples
Current Measurement - Zero Resistance
Ammeter (ZRA) with Autorange
Rm3
Rm2
Rm1
IIN
IN +
OUT
IOUT
IN V=IIN x Rm
DICMAPI Seminars 3rd-4th December 2014, Naples
Bandwidth
Cf (Bandwidth)
Rm (RANGE)
IIN
IN +
OUT
IOUT
To data logger
IN V=IIN x Rm
DICMAPI Seminars 3rd-4th December 2014, Naples
Simplified Potentiostat
IN +
OUT
IN -
Control V
Cell
Wk
Ref
Aux
DICMAPI Seminars 3rd-4th December 2014, Naples
Simplified Potentiostat
IN +
0 V between
these two points
OUT
IN -
Control V=1V
1V
Cell
Wk
1V
Ref
Aux
DICMAPI Seminars 3rd-4th December 2014, Naples
Potentiostat with ZRA
Cf (Bandwidth)
Rm (RANGE)
IN +
OUT
IN +
IN -
IN -
Control V
Measure I
Cell
Wk
Ref
Aux
DICMAPI Seminars 3rd-4th December 2014, Naples
Realistic Potentiostat
IN +
OUT
IN -
Control V
Cf (Bandwidth)
IN +
OUT
Rm (RANGE)
IN -
IN +
IN -
V buffer
Measure I
Cell
W
k
Re
f
Aux
DICMAPI Seminars 3rd-4th December 2014, Naples
Bandwidth and Filters
Filters ‘smooth’ the recorded
signal
Cf (Bandwidth)
Rm (RANGE)
I
IN +
IN -
Measure I
RC Filter
Filtered I
Bandwidth determines how
fast the machine responds to
fluctuations
DICMAPI Seminars 3rd-4th December 2014, Naples
Frequency response analyzer (FRA)
FRA physically performs
multiplication and averaging
Multiplier
Integrator
DICMAPI Seminars 3rd-4th December 2014, Naples
Potentiostat+FRA
IN +
OUT
IN -
Control V
Cf (Bandwidth)
IN +
OUT
Rm (RANGE)
IN -
IN +
IN -
V buffer
Measure I
Cell
Ref
Wk
Aux
DICMAPI Seminars 3rd-4th December 2014, Naples
Settings
DICMAPI Seminars 3rd-4th December 2014, Naples
Solartron Modulab
DICMAPI Seminars 3rd-4th December 2014, Naples
Solartron 1280
DICMAPI Seminars 3rd-4th December 2014, Naples
Ivium
DICMAPI Seminars 3rd-4th December 2014, Naples
Measurement Time
• Measurement time is primarily determined by
the lowest value of frequency and averaging
method.
• Secondarily, it is determined by the number of
points per decade acquired
Measurement time is important: if it is necessary to
follow the time evolution of a corrosion process,
the measurement time must be much shorter than
the rate of change.
DICMAPI Seminars 3rd-4th December 2014, Naples
Measurement Time
2 Points per decade
1 Point per decade
Exp
5
4
3
2
1
0
-1
-2
-3
Freq (Hz) Period (s) N of averages
100000 0.00001
3
10000
0.0001
3
1000
0.001
3
100
0.01
3
10
0.1
3
1
1
3
0.1
10
3
0.01
100
3
0.001
1000
3
Time x
Point (s)
0.00003
0.0003
0.003
0.03
0.3
3
30
300
3000
Min Time (sec)
1111.111
Actual
Time(sec)
3333.333
Some machines allow to reduce
averages or points per decade at low
freq.
Exp
Freq (Hz) Period (s) N of averages
5 100000 0.00001
3
4.5 31622.78 3.16E-05
3
4
10000 0.0001
3
3.5 3162.278 0.000316
3
3
1000
0.001
3
2.5 316.2278 0.003162
3
2
100
0.01
3
1.5 31.62278 0.031623
3
1
10
0.1
3
0.5 3.162278 0.316228
3
0
1
1
3
-0.5 0.316228 3.162278
3
-1
0.1
10
3
-1.5 0.031623 31.62278
3
-2
0.01
100
3
-2.5 0.003162 316.2278
3
-3
0.001
1000
3
Min Time (sec)
1462.475
Time x
Point (s)
0.00003
9.49E-05
0.0003
0.000949
0.003
0.009487
0.03
0.094868
0.3
0.948683
3
9.486833
30
94.86833
300
948.6833
3000
Actual
Time(sec)
4387.426
DICMAPI Seminars 3rd-4th December 2014, Naples
Issues During EIS Measurements
DICMAPI Seminars 3rd-4th December 2014, Naples
Non Stationarity (EN)
Electrolyte
Anodic Event
Ref
Cathodic Site
Cathodic Site
V(t)
Active Cathodic Area
Active Anodic Area
I(t)
I(t)
I(t)
External Circuit
DICMAPI Seminars 3rd-4th December 2014, Naples
Electrochemical Noise
DICMAPI Seminars 3rd-4th December 2014, Naples
Corrosion Event and EIS
FRA physically performs
multiplication and averaging
Multiplier
Integrator
DICMAPI Seminars 3rd-4th December 2014, Naples
Corrosion Event and EIS
Applied V
I Response
FRA physically performs
multiplication and averaging
I noise
I measured
DICMAPI Seminars 3rd-4th December 2014, Naples
Corrosion Event and EIS
Applied V
I Response
I noise
I measured
If this signal is multiplied and
integrated the result is
unreliable
DICMAPI Seminars 3rd-4th December 2014, Naples
Corrosion Event and EIS
Applied V
I Response
I noise
I measured
At high frequency the
problem is not substantial
DICMAPI Seminars 3rd-4th December 2014, Naples
Noise and Frequency
Noisy output
OK
For this reason the spectra for corroding system are
acquired starting form HIGH frequency and finishing at
LOW frequencies
DICMAPI Seminars 3rd-4th December 2014, Naples
Non stationarity
DICMAPI Seminars 3rd-4th December 2014, Naples
Non stationarity
DICMAPI Seminars 3rd-4th December 2014, Naples
Non stationarity
DICMAPI Seminars 3rd-4th December 2014, Naples
Non stationarity / Approaches to
improve data
• Increase integration time
– Experiment might become very long, no guarantee of
success if intrinsically too noisy
– Balance increased acquisition with reduced point/decade
• Increase minimum-frequency value
– Simply avoid acquiring noisy part of the spectrum and
keep experiment short
• Increase amplitude
– Might be successful but might introduce corrosion at low
freq and issues of non-linearity
If unsuccessful better to use electrochemical noise
DICMAPI Seminars 3rd-4th December 2014, Naples
Non linearity
Applied V
Linear resp.
Non Linear resp.
Amplitude and Phase are obtained (not
noisy) but they are not correct.
Measured response contains harmonics
DICMAPI Seminars 3rd-4th December 2014, Naples
Non linearity
Behaviour above CANNOT be
obtained by any combination of
DICMAPI Seminars 3rd-4th December 2014, Naples
Approaches to reduce issue
• Reduce amplitude
– Signal to noise ratio decreases
– Might need to increase integration time
• Don’t use impedance/be aware of the
limitation of the result
• Validate results with complementary
approaches
DICMAPI Seminars 3rd-4th December 2014, Naples
Electrodes Configuration
• 3-electrodes configuration
• 2-electrodes configuration
• Modified 2-electrodes configuration
DICMAPI Seminars 3rd-4th December 2014, Naples
Potentiostat + FRA
Pot.stat+FRA
Working
Electrode
Reference 2
or Sense
(not always present)
Reference 1
Counter Electrode or
Auxiliary Electrode
DICMAPI Seminars 3rd-4th December 2014, Naples
3-Electrodes Configuration
Pot.stat+FRA
WE
WE
RE1
RE2
Cell
RE
AUX
CE
By using this configuration:
- A constant DC potential is maintained
between RE and WK, and the AC signal
is superimposed to that DC potential
- The signal is applied symmetrically
with respect to that potential,
regardless of the corrosion processes
on the surface
Consequently:
- There must be confidence that the
corrosion potential does not drift
significantly during the measurement,
otherwise significant faradic current is
passed
- If there is a drift of corrosion potential
as a function of time, such drift is
suppressed by the measurement.
DICMAPI Seminars 3rd-4th December 2014, Naples
Examples of Potential Drift
-1.60
Ecor (VSCE)
-1.65
-1.70
Pure Mg
-1.75
-1.80
-1.85
0
10
20
30
40
Time (min)
50
60
AZ31 – Mg Alloy
AA2024T3
DICMAPI Seminars 3rd-4th December 2014, Naples
Potential drift/measurement time/ DC
polarization (3-elec)
OK
Bad
Bad
Bad
AA2024T3
DICMAPI Seminars 3rd-4th December 2014, Naples
Potential drift/measurement time/ DC
polarization (3-elect)
AA2024T3
Bad
Tragic
DICMAPI Seminars 3rd-4th December 2014, Naples
Possible Solution (3-elect)
Free corrosion
AA2024T3
Impedance Measurement
DICMAPI Seminars 3rd-4th December 2014, Naples
2-Electrodes Configuration
Pot.stat+FRA
WE
WE1
RE1
RE2
Cell
AUX
WE2
By using this configuration:
- A constant DC potential (generally 0 V)
is maintained between WK1 and WK2.
If one uses identical electrodes for
WK1 and WK2, maintaining 0V
between them does not induce
additional corrosion
- The signal is distributed among the
two electrodes, therefore if asymmerty
develops, the impedance of the noncorroding electrode dominates
(electrodes are in series)
Consequently:
- A slow drift in potential is not an issue
- The natural potential drift is not
perturbed by the measurement
- The anodic current on one electrode
equals the cathodic current on the
other
DICMAPI Seminars 3rd-4th December 2014, Naples
2-Electrodes Conf
-
•
-
By using this configuration:
A constant DC potential (generally 0 V)
is maintained between WK1 and WK2. If
one uses identical electrodes for WK1
and WK2, maintaining 0V between
them does not induce additional
corrosion
The signal is distributed among the two
electrodes, therefore if asymmetry
develops, the impedance of the noncorroding electrode dominates
(electrodes are in series)
Consequently:
A slow drift in potential is not an issue
The natural potential drift is not
perturbed by the measurement
The anodic current on one electrode
equals the cathodic current on the
other.
0V
-1.56
Potential vs. SCE / V
•
-
WE 1
WE 2
-1.58
-1.60
-1.62
-1.64
0
2000
4000
6000
8000
10000
12000
Time / s
DICMAPI Seminars 3rd-4th December 2014, Naples
Modified 2-Electrodes Configuration
Pot.stat+FRA
WE
RE1
RE2
WE1
Voltmeter
Cell
AUX
WE2
By using this configuration:
- A constant DC potential (generally 0 V)
is maintained between WK1 and WK2.
If one uses identical electrodes for
WK1 and WK2, maintaining 0V
between them does not induce
additional corrosion
- The signal is distributed among the
two electrodes, therefore if asymmerty
develops, the impedance of the noncorroding electrode dominates
(electrodes are in series)
Consequently:
- A slow drift in potential is not an issue
- The natural potential drift is not
perturbed by the measurement
- The anodic current on one electrode
equals the cathodic current on the
other
DICMAPI Seminars 3rd-4th December 2014, Naples
Potential vs. SCE / V
-1.56
-1.58
-1.60
-1.62
-1.64
0
2000
4000
6000
8000
10000
12000
Time / s
DICMAPI Seminars 3rd-4th December 2014, Naples
Summary
• Potentiostats/FRAs are analogue device based on
operational amplifier.
• It is useful to understand basic functioning principle to
determine settings in different machines
• Non-stationarity and non-linearity can be issues during
impedance measurement.
• Settings can be adjusted to reduce/minimize issues in
some circumstances
• Electrodes configuration is important:
– 3-electrodes configuration imply fixed potential
– 2- electrodes configuration does not imply fixed potential,
but requires the assumption of identical behaviour
DICMAPI Seminars 3rd-4th December 2014, Naples