Capacitive semi-circles should be expected always. This is because of the double layer and or any film capacitance. Inductive loops at low frequencies had been associated to adsorption processes.

I agree with Dr. Omar and would like to add some additional information. At low frequency the ions has sufficient time to interact with surface and may adsorb/desorb (oscillate) at the surface (as a function of frequency) and you may get pseudocapacitive (inductive behavior in impedance spectra) behavior. But there may be other different situations which may bring this feature too

(1) If the applied DC potential is 0 vs OCP

This may arise if you have selected AC potential relatively large (beyond the linear region) around OCP and the charge to or from the surface is no more reversible. In other words, the reversible reaction on the surface is not favorable (either in anodic or in cathodic) in atleast one direction during frequency oscillation at OCP. Under these conditions surface may polarize and electrochemical signals would not truly represent the kinetic activity of the surface.

(2) In case where DC potential > or < 0 vs OCP

The favorable reaction either anodic and cathodic would change the active surface area and may lead to this behavior

So it is not easy to say that this feature is related with adsorption, depending on the surface and applied condition one may predict the associated reasons. Simply, this may be due to adsorption, nature of reversible reactions, polarization effects and/or irreversible (in the given time scale, frequency range). It is therefore i recommend you to revisit your system and applied conditions and after having complete information about physical system and looking into the results (phase shift etc) one may predict the actual mechanism. This is my personal experience and may contradict with others. The experts in this area may give you better feedback.

It would be better if you share the details of your experiment to get a better interpretation of the Nyquist Plot you have observed. Secondly, it is important to mention that the source of inductive loop observed in Nyquist plot, in any electrode system, is still unclear. However, I will try to explain the reason of the inductive loop based on my understand in the field of lithium ion batteries. (The fundamentals will be same)

Basically, inductance is defined as a property of an electrical circuit which produces an electromotive force as a result of a changing current flowing through the same circuit. Therefore, an electrical potential difference applied by the user, creates a local potential difference on the surface of electrode. You might be wondering Why?

I will give you an example of a positive electrode material (LiCoO2) we commonly use in lithium ion batteries. During the extraction of lithium from LiCoO2 there is a concentration potential created on the surface of electrode based on the concentration of lithium inside LiCoO2. (You have regions on electrodes with high and low lithium contents). Now this concentration potential creates an electric field resisting the external applied electrical field causing an inductive loop to appear in Nyquist plot.

So in your case, the use of different additives in metal deposition might have created a local potential on the surface electrode (depending on the properties of your additives). This local potential is producing an electric field which opposes the applied external electric field causing the Nyquist plot to show inductive behaviour.

I hope this help

I recommend the answer provided by Umair. The local charge/field development may cause by different reasons. Even the electrode and active material compatability also creates this, in some cases. Again, the reason may vary in the case of a single active material or in the case of a composite. So you should be careful and keep in mind your experimental conditions before getting in to a fruitful conclusion.

Dear Christiaan Coetzee

your different additives (in metal deposition) decrease the electrolyte's conductivity ?

The additives in your metal deposition cell commonly modify the local potential near your electrode (WE); it is, mainly, the modification of the electrochemical double layer capacitor (EDLC) structural morphology; in quasi-flat simple cases of electrodes' topologies, it can more easily attributed to dielectric properties (thickness, ionic mobility, etc.) .

The metallic ions dedicated for your metal deposition are disturbing (oscillate at low currents) some of the EDLC's dielectric properties followed, also, by other related phenomena[1] (absorbption/adsorbption/sorption). Their possible synergies make it a very complex[2] system.

So, these, low frequency (electromechanical) oscillations of the (WE in metal deposition) EDLC might be a possible[3] reason for your (partial) inductive loops (from a RLC model[5]), at low cell's currents (iWE,CE). Sometimes R-values are very low and the loops are not shown very well[4] (or missing loops), your initial case, without additives.

1. Competition between ionic adsorption and desorption on electrochemical double layer capacitor electrodes in acetonitrile solutions at different currents and temperatures

https://www.sciencedirect.com/science/article/pii/S0378775317313903

2. A three-electrodes' configuration is, always, required in these complex[3] systems.

3. It would be better if you share the details of your experiment to get a better interpretation of the Nyquist Plot you have observed.

4. For small DAQ (data acquisition per Freq. decade) number N of EIS data (N