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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