I am working on HER and OER. I use Ni foam as working electrode substrate. While running EIS for my sample, I have the impedance spectrum starting from the negative side. i.e. The semi-circle starts at minus values of Re(Z) and ends near 1 ohm or 2 ohms on the positive side. So, I am left only with Rct value. Why does this occur? Also, for what is the Rs value in this case. How should I do IR correction, without knowing Rs?
Kindly help!
Karthick raj AG
I was studying for OER characteristics. I used 1 M KOH as electrolyte and Hg/HgO as reference electrode. We have PARASTAT PMC 1000 potentiostat. For counter, I used Pt mesh. For EIS, I usually perform it in both 0 V vs OC and at overpotential region (some anodic potential vs Ref) as DC. The frequency region was from 1 MHz to 0.1 mHz. Vrms amplitude was 7.07 mV, which is 70.7% of 10 mV.I didn't try with a resistor. But I tried with bare Ni foam (without catalyst loading), bare GCE. I got Randles circuit like behaviour for GCE and a semi-circle for Ni foam. But the semi-circle was in the positive side of Re(Z).
Only for catalyst, I got negative value of Re(Z)
I would suggest to calibrate the instrument with dummy cell before you take any measurements. That way you would know your potentiostat is working good.
Can you show some plots? Did you check the Bode plot as well?
You can also try a dummy cell with similar values as those in your experiments (~1-2 Ohms). You should also include the reference electrode in your dummy cell.
Low impedance values can sometimes give problems to the potentiostat. impedance values
I'll discuss what helped me get the correct one. I started from a slower frequency from 100 kHz, instead of 1 MHz. So, whatever reactions that occur in 1 MHz timescale are omitted and the semicircle that starts in the negative Re(Z) is omitted. Since, I'm studying EIS in overpotential region, I suppose kinetics of the reaction are in the 100 kHz frequency range (and are too fast or slow to study in 1 MHz range, resulting in the negative impedance artefact).
@Alberto Battisel, I haven't studied Bode plots so far. But I have just started to learn how to infer Bode plot. I checked with standard dummy cells, (the values are not similar as in my experiments), and obtained EIS spectrum with good agreement of values.
Dear G. Srividhya,
you might like to implement:
1) a] a somehow lower AC-values of Vrms ~ 2 mV ('amplitude was 7.07 mV')
and you might like to show (at least) 2-more, illuminative, (EIS-) graphs at b] somehow lower DC-values of (absolute values of) VDC polarizations (Voverp., overpotentials) values, e.g. at VDC (=VHER + Voverp. and/or VOER - Voverp.)
2) a] Voverp.=100mV, and at, b] Voverp.=50mV.
Then,
3) re-measure (3rd-EIS), near at the critical[1] Voverp.~ 0 V.
Note: include the 1 MHz range, also, upon using lower AC-values of Vrms.
1. due to mainly, some new phase(s) on WE (-'coating') , either big bubbles, e.g. naked eye-visible or even very small, e.g. invisible-nanobubbles, (new gaseous phases '(s)') bubbles.
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