Why is the electrolyte ion not solvated?

Why do you think that ions are not solvated? In aqueous electrolyte, the charged ions are always solvated.

why is this happening?

Its not easy to explain the phenomena in few lines. Briefly, when the Fermi level of the metal (electrode) matches with the electrolyte's oxidation/reduction energy level, the electron transfer takes place.

Why do we get additional capacitance?

If charge transfers/Faradic redox occur in the potential range you are looking for the capacitance studies, the addition capacitance would be found, called as pseudocapacitance.

Thanks Sujoy!

For redox reaction, I see the specifically adsorbed ions are not solvated in many texts (Or I might have not understood it properly- please see the picture in the link)

Why do we get additional capacitance or Pseudocapacitance?- Looking for a profound insight!

https://en.wikipedia.org/wiki/Pseudocapacitance

Welcome Deepak,

I suggest you not to follow the wikipedia to understand the basic. Metal/electrolyte interface is one of the basic chapters in electrochemistry, comprises three different models and difficult to discuss here elaborately.

When the metal ion specifically adsorbed on the electrode surface, then solvated molecules reside around the metal ions instead of the interface and that is depicted in the figure you have attached.

Eventually, we do not get any additional capacitance from pseudocapacitor. People often subtract the faradic current and extract the double layer current from the CV curves for calculating the capacitance in the supercapacitor.

Hi Sujoy,

Thanks for the answer! I don't see any Solvated molecules around the specifically adsorbed ions! (There are solvated ions only in the direction of the diffuse layer).

Actually my main question would be "Will there be tunneling of electrons through the electrode- during redox?!"

Dear Sujoy Sarkar

Could you suggest any book for detailed discussion and clear the concept of pseudocapacitor. 

Dear Deepak,

first the answer to your direct question "Will there be tunnelling of electrons through the electrode during redox reaction ?":

1. If there is a finite dc current (steady state when time->infinity!), there will be (must be) a TRANSFER of electrons from the electrode to the electrolyte (neutralising the respective relevant ions that are piling up at the electrode), but it will not be necessarily quantum mechanical tunnelling. The reason for this is that the width of this tunnelling barrier is too large (~of the order of the diffusion layer width !!). So, the transfer will be "classical" over the barrier rather than through the barrier.

And now I have some further comments, hopefully useful to you:

2. If there is NO ELECTRON TRANSFER from(to) the respective electrodes, then your system is characterised as the one with completely blocking electrodes and the concentration of the solvated ions within the bulk of your electrolyte will diminish. This will be seen as the increase of the electrolyte bulk resistance as the system "relaxes" towards a steady state. This steady state is though characterised (in case of completely blocking electrodes) by ZERO final current through the entire system !!

3. I have checked the Wikipedia figure and it looks to me very illustrative and as a good starting point. But be aware, the  "structure" of depicted layers is highly controversial, these are just the possible hypothetical cases, requiring a detailed thermodynamical/quantum mechanical calculations. It is for example my experience that the so called Helmholtz double layer is in fact only a single layer (the inner Helmholtz layer), where the species are "physisorbed"(perhaps chemisorbed ?) on the metal electrode surface and can not rotate. For example, if the electrolyte is some aqueous solution, this inner layer would be formed by water (solvent) molecules with dielectric constant more than factor ten less than the usual eps(H2O)= 80 (rotational contribution to the polarisation->zero). What is depicted as a second Helmholtz layer is something that we have never seen in our electrical impedance data and I am afraid that it is a fallacy that has stuck to the electrochemistry of the metal electrode-electrolyte interface from historical times when the proper experimental characterization (EIS) has not been available. In some cases (I would guess some large solvated ions) it may be that the ions can stick to the inner Helmholtz layer and be quasi-immobile, but I doubt it.  The important fact is that as soon as they are mobile, their concentration at the inner Helmholtz plane is just given by the solution of the Poison equation (more or  less).

4. The ions to be reduced through redox reaction DO NOT HAVE TO BE right at the metal electrode . Once the electron is transferred from the metal electrode to the electrolyte, it can easily diffuse (hop) extra 2-3 Ångstroms to the ions at the inner Helmholtz plane.

5. There are no PSEUDICAPACITORS. Either we measure some capacitors or we do not. There is only one electrical response of anything to the external electrical perturbation and that response is governed by equations of classical electrodynamics (Maxwell equations). Nothing more, nothing else.

Well, I do hope this will help you on the way.

With best regards

Petr

Thanks Petr