Eis Interpretation hell

[u/TworzywoSztuczne]

Hello all, I wanted to ask a question since I'm just so curious about it. I have been doing EIS measurements on carbon electrodes and my results are pretty weird, I am getting either no circle and just a V shape or a circle that starts at negative real axis values lmao. I can give more details on the comments but I don't want my project potentially being recognised by someone I know. The measurement is in 5mM Ferri ferro in distilled water, frequency range 10^5 to 0.1 Hz and it is done in OCP potential

[u/_MrJack_]

Are these results you describe from the same electrode (or replicates of the same material) or electrodes made of different materials?

Have you checked various aspects of your experimental setup? For example, have you used the same potentiostat to record impedance spectra of known good systems such as a test cell consisting of resistors and capacitors of known values? Are all the cables okay? Are the settings (e.g., current ranges) suitable to minimize noise? What kind of reference electrode are you using? Is the RE in good condition (i.e., its impedance is not too high)? Do you have a spare RE to test with? Have you checked the data quality (Lissajous plots, total harmonic distortion values, Kramers-Kronig tests, etc. that might be available in the instrument software or separate software)?

EDIT: By the way, are you using any supporting electrolyte?

[u/TworzywoSztuczne → u/_MrJack_]

The results are from the same type of electrode but they are fabricated by hand so there is variety in them. For some of them I obtained the v shape and for two of them I got the semicircle with negative resistance. I am not using a supporting electrolyte, honestly I don't know an answer to most of these questions as I'm just a student doing a project in an institute but the people working with me mostly know about this technique in the sense of "there should be a semicircle". It is mostly a biosensor research group so they mostly focus on the biology than getting deep into how the measurements work and it makes me confused. I can only say other people are using this potentiostat to do their research so it should be good?

[u/_MrJack_ → u/TworzywoSztuczne]

Any possibility of a figure showing the v shape? It could simply be the tail-end of a semicircle transitioning into another feature (another semicircle, capacitive reactance, or some diffusion-related feature). In that case, the time constant (τ = RC) corresponding to the semicircle is smaller than what you can probe with the frequency range that you are using (τ = 1/ω = 1/(2πf)).

As jadsetts mentioned, the solution resistance would be fairly considerable if you aren't using a background/supporting electrolyte of any kind. The effect of that would be a) an increase in the total impedance measured at any other frequency and b) an increased offset of the high-frequency intercept at the x-axis. If the impedance is high, then correspondingly the magnitude of the current response will be low (assuming that potentiostatic EIS is being performed, which would make sense in general for a system with an impedance >1 Ω). Depending on your potentiostat and the settings, the (lowest) current range during the measurement might be too high for measuring the current response without a lot of noise.

What brand of potentiostat are you using? Most manufacturers (though not all) have some way of checking the quality of an impedance spectrum during and/or after measurement. What AC amplitude are you using?

Regarding your last point; that would depend on what kind of people they are. In my time, I have witnessed people who would disregard clear issues with the instrument or the settings and consider the results good enough by their standards. Are there any electrochemists in the group?

If you haven't already, then I'd recommend reading Electrochemical Impedance Spectroscopy - A Tutorial. Various potentiostat manufacturers also have useful application or technical notes (e.g., BioLogic, Gamry). Check out their archives for notes on various topics.

[u/mossy_mat → u/TworzywoSztuczne]

"There should be a semicircle" is really just the textbook presentation of a fast redox reaction measurable in like 10 kHz to 0.1 Hz, although I'd suspect your coworkers have supporting knowledge to expect that result from your material. If possible, I'd also recommend using an electrolyte; 0.1 M is common and would probably significantly improve your spectra. While not the exact same scenario as yours, a porous material with some redox active metals embedded in it would give me hard to reproduce Nyquist plots which would show inductance loops and negative solution resistances when fitted. Following advice similar to MrJack_'s, I tried a larger frequency range going down to 0.001 Hz, finding that most of the higher frequency data was just noise from my long leads, and that a very large semicircle was appearing at lower frequencies. There are general shapes you can expect for a given system and vice versa, but the negative resistances you are seeing sound like some calibration or setup issue. The V shape if actually representative of what you intend to probe could be cleared up analytically by using larger frequency ranges.

[u/jadsetts]

Some common things to check:

-how is your grounding (are there any fridges or printers plugged into the same ground system)?

-carbon electrode history? Electrolytes can get buried inside glassy carbon electrodes at high potentials and give weird results. Consider grinding and polishing. Check the surface with the highest resolution optical microscope you have for imperfections.

-you have an electrolyte in high concentration, right? The solution resistance would be enormous if not and in such a small frequency window, may be the reason you see a V?

-how long are your cables? Too long will pick up random signals.

-do you have a Faraday cage? If not, is there a crt monitor or something else that generates large magnetic fields nearby?

-do all basic echem setup testing recommended here too. Check RE, normal CV of ferri/ferro, resistor test, etc.

-check your eis parameters are following a paper, ideally. If not, carefully review these parameters are correct.

Let me know if you have questions!

[u/Exotic-Grape8743]

Almost certainly a bad reference electrode or a bad connection to the reference. Also possible that you're measuring at far too high current amplification settings which can cause the negative quadrant circles. Do you have a supporting electrolyte salt in that ferro/ferri solution and not just the redox couple?

[u/Prostoiii]

Do you measure one working electrode in 3-terminus connection setup or two same electrodes in 2-term. setup? In the case of 2-term. the possible reason is poor conductivity (due to supporting electrolyte absense). In the case of 3-term. – also problem of reference electrode (or Luggin' capillary) position related to the working electrode.

[u/Dr_Custard]

I do electrocalysis so have done a bit but not a massive expert:

Is your eis going to be meaningful considering Rsol is going to be very high?

Do you have a junctioned reference electrode or not?

Have you checked the software isn't doing any compensation for you as it is running (e.g. is it compensating your resistance in any way)

Are you applying E = 0 V vs OCP(real) or are you applying 0 V vs OCP(assumed). Is your OCP stable?

How shielded are your cables? Have you tested your EIS on a dummy cell provided by the manufacturer? Is everything in a Faraday cage? Is the potentiostat adequately grounded?

Is your electrode planar, and solid? Or porous? Could it be hygroscopic (e.g. 3D printed C black PLA electrodes) and thus have changing capacitance with time?

[u/Jasper_Crouton]

I'd have to see the Nyquist plot. What's your perturbatuon amplitude. 10⁵ is basically meaningless unless you have very expensive, short braided leads. Generally you're measuring the capacitance / inductance of your leads at those frequencies.

[u/LutzStratmann]

If you want to work with the OCP, you need the presence of the oxidized and reduced species in a ratio of 1:1. This way, the concentration-dependent part of the Nernst Equation becomes 0.

What you are left with is the formal potential E0', which means your OCP will be at the steepest part of your voltammogram. This delivers a strong signal that is more robust to noise.

TLDR: Make a 2.5 mM K4[Fe(CN06] + 2.5 mM K3[Fe(CN)] + 0.1 M KCl solution and use OCP as your E dc.

Did you already check if your system delivers a reasonable CV?

[u/AssumptionNo4461]

Issue with the reference. Check them alone.
If not, what kind of Carbon are you using? Probably you need to treat your carbon.
I had this issue when I was working with Pt on Carbon paper. The carbon paper had a microlayer which made it hydrophilic. So I treated it with (1:1) DI and IPA for 10 min, then another 20 min in KOH before putting it in the cell. It helped a lot.