On starting an experiment we stabilise the system by measuring OCP for an hour whereas for EIS we use the OCP from the instrument itself. What is the significance of measuring the OCP in the beginning when we have to enter the instantaneous potential of the system.
Dear Dr. Garima Chaturvedi,
as said by @Dr.Nicolas Guillet in response to a previous question regarding ocp considerations in EIS measurements, theoretically EIS is based on two hypothesis: linearity of the I vs. E curve and stationnarity (system don't change during the measurement, current variation during EIS measurement is quite null).
Measuring an EIS spectrum takes time (often up to many hours). The system being measured must be at a steady state throughout the time required to measure the EIS spectrum. A common cause of problems in EIS measurements and analysis is drift in the system being measured. In practice a steady state can be difficult to achieve. The system can change through adsorption of solution impurities, growth of an oxide layer, build up of reaction products in solution, coating degradation, or temperature changes, to list just a few factors.
You have to verify the validity of these two hypothesis:
- plot a curve (CV) of the evolution of the current measured vs. the applied voltage for a range of +/- 50 mV vs. OCV. It has to be recorded at different speed, in agreement with the frequency range of your EIS measurement. You could then define the range of linearity of your system: may be 10 mV, may be 50 mV or only 1 mV.
- for stationarity, it's more easy, plot the values of the current recorded during EIS measurement vs. time. If the current changed (significantly), don't lose time to study your spectrum. Your system changed during the measurement and no equivalent circuit can be used to fit with something changing.
These two points must be verified for each measurement before analysis, then, you could understand why you only have sometimes nice results.
For more details, please also see the Gamry Tech Note at: https://www.gamry.com/application-notes/EIS/basics-of-electrochemical-impedance-spectroscopy/
Best regards, Pierluigi Traverso
Pierluigi Traverso Thanks the answer. However , there is a concern regarding calculation of current in EIS. In EIS we only get frequency and impedance value due to which it becomes difficult to get the value of current. FYI i am attaching a file showing the input parameters for EIS.
Dear Garima Chaturvedi ,
the (i -)current measurement is a (response) result, in your[1] P-EIS[2]; the i (and the iDC) is not (pre-)shown in the table of the input parameters, since it is an electrical response parameter (to the electrochemical Potential); it is, just, a simple result in your (P-EIS mode of) EIS.
You can find iDC in a next exp. step, e.g. after a Z-measurement[3], not before an actual, spot, Z-measurement; also, it might, sometimes, be shown during a Z-measurement[3].
So, you will find it, only, AFTER a test[4] Z-spectrum measurement.
1. Your table of the input parameters: 'EIS para meters.png'
2. (P-EIS) Basics of Electrochemical Impedance Spectroscopy https://www.gamry.com/application-notes/EIS/basics-of-electrochemical-impedance-spectroscopy/
3. Also, it is, commonly, recorded in the response (table of the informative) parameters, inside the (recorded results, e.g. the Z-spectrum) file.
4. at least, an initial, test Z-spectrum measurement, based on the actual values (in the table) of the input parameters.
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