sir i am using Glassy carbon with surface area of 7x10-6m2

Ru(NH3)6Cl3 in 0.2 M Phosphate buffer as supporting electrolyte

Dear  Karhikeyan,

I agree with Christian, if you need a good response, you must describe your system/experiment better, so that even those of us, not really working with electrochemistry, can come with some useful comments/suggestions. Let me put forward some comments regarding EIS method :

1. You have to specify frequency range, applied ac voltage (and dc voltage bias if any) of your EIS experiment. It always helps if you show the typical result graphically (for example complex impedance vs. frequency - log/log plot ).

2. You have to specify your SUT (system under test) and the electrodes. Area of the electrodes and system thickness are essential if one is to come up with any useful comment/suggestion.

3. Some of us do not know what Rct number is. Define it in your question, so that no misunderstandings can occur.

4. EIS analysis 1: It is essential that one measures to low enough frequencies, so that you are sure you have achieved well defined steady state (real part of impedance and/or capacitance does not change with frequency as w->0).

5. EIS analysis 2: Your Rct value of 35 kOhm (?) is apparently some measure of resistance in your system. It seems (see Chrisitan and John) that it is too high (empirical finding). If it is the highest resistance in your system, then the problem is possibly the "bottle neck" problem (electrode-system interface). If it is some fitting R in a equivalent R,C,L network you are trying to fit your data to, then treat this with caution !! Different R,C,L networks give the same quality of fit and often, in electrochemical reactions, such R's have nothing to do with a real resistance, but they model the characteristic time of the reaction R=tau/C (C is the system's relevant capacitance).

6. From my experience with EIS on liquids, the system maximum resistance of 35 kOhm is  EXTREMELY small, typical values are in the region of 10 MOhm-100 GOhm. So here, I must have misunderstood something.

If you let us know the answers to point 1. to  3., I am sure we can help you more

With best regards

Petr

Dear  Karhikeyan,

I agree with Christian, if you need a good response, you must describe your system/experiment better, so that even those of us, not really working with electrochemistry, can come with some useful comments/suggestions. Let me put forward some comments regarding EIS method :

1. You have to specify frequency range, applied ac voltage (and dc voltage bias if any) of your EIS experiment. It always helps if you show the typical result graphically (for example complex impedance vs. frequency - log/log plot ).

2. You have to specify your SUT (system under test) and the electrodes. Area of the electrodes and system thickness are essential if one is to come up with any useful comment/suggestion.

3. Some of us do not know what Rct number is. Define it in your question, so that no misunderstandings can occur.

4. EIS analysis 1: It is essential that one measures to low enough frequencies, so that you are sure you have achieved well defined steady state (real part of impedance and/or capacitance does not change with frequency as w->0).

5. EIS analysis 2: Your Rct value of 35 kOhm (?) is apparently some measure of resistance in your system. It seems (see Chrisitan and John) that it is too high (empirical finding). If it is the highest resistance in your system, then the problem is possibly the "bottle neck" problem (electrode-system interface). If it is some fitting R in a equivalent R,C,L network you are trying to fit your data to, then treat this with caution !! Different R,C,L networks give the same quality of fit and often, in electrochemical reactions, such R's have nothing to do with a real resistance, but they model the characteristic time of the reaction R=tau/C (C is the system's relevant capacitance).

6. From my experience with EIS on liquids, the system maximum resistance of 35 kOhm is  EXTREMELY small, typical values are in the region of 10 MOhm-100 GOhm. So here, I must have misunderstood something.

If you let us know the answers to point 1. to  3., I am sure we can help you more

With best regards

Petr

Dear Karhikeyan and John,

just a couple of comments to John's answer :

1. Thanks for explanations regarding Rs and Rct. Now I can add few more comments:

2. Rs can never stand alone, solution's fast polarisation (Cs) has to be added in parallel with Rs. Any system, conducting, insulating or whatever has as minimum, two physical processes contributing to the electromagnetic response, dc current and fast polarisation ( direct current and displacement current in one of Maxwell equations of classical electrodynamics).

3. Possible Electro-chemical reaction taking place can be modelled in a similar fashion as the effect of a deep level in a semiconductor, as series term, consisting of a resistor (characteristic time) and a capacitor (change in capacitance). This term is set in parallel with Rs and Cs. This trivial situation gets more complicated if the relevant reaction is a part of redox reactions at the electrodes. In this type of empirical fitting of the data, using various equivalent R,C,L circuits of varying topologies, one usually fits arbitrary R's and arbitrary C's to this reaction term and first then one analyses the possible physical relevance and meaning of these terms. 

4. It is an unfortunate and widespread misunderstanding of the role, the pure diffusion processes play in electrochemistry. They can never go alone (Warburg term). When electrical response in a material is measured for example by  EIS, the total current (proportional to the gradient of the electrochemical potential) ALWAYS consists of two terms, the drift term(proportional to the applied electrical field) and the diffusion term (proportional to the concentration gradient of the mobile charge in question). With diffusion term only(Warburg analysis, leading to Admittance ~ w^-1/2 frequency dependence ), there would never occur final steady state of the system(constant current through the entire system as time->infinity).

5. There is a number of very important general electrical  response characteristics, common to solid and liquid systems.  One of them is the finding that there are almost always two spatial regions contributing to the overall response, the bulk region ( generaly at higher frequencies - the solution itself) and the electrode-solution interface (lower frequencies - in electrochemistry generally named as "Helmohltz" and "diffusion" regions) .

6. Some of the general aspects of the more fundamental analysis of the electrical response in materials can be found in U.S. patent n. 5 627 479, May 6, 1997.

I hope this will help on the way.

With best regards

Petr

Dear Karhikeyan and John,

just a couple of comments to John's answer :

1. Thanks for explanations regarding Rs and Rct. Now I can add few more comments:

2. Rs can never stand alone, solution's fast polarisation (Cs) has to be added in parallel with Rs. Any system, conducting, insulating or whatever has as minimum, two physical processes contributing to the electromagnetic response, dc current and fast polarisation ( direct current and displacement current in one of Maxwell equations of classical electrodynamics).

3. Possible Electro-chemical reaction taking place can be modelled in a similar fashion as the effect of a deep level in a semiconductor, as series term, consisting of a resistor (characteristic time) and a capacitor (change in capacitance). This term is set in parallel with Rs and Cs. This trivial situation gets more complicated if the relevant reaction is a part of redox reactions at the electrodes. In this type of empirical fitting of the data, using various equivalent R,C,L circuits of varying topologies, one usually fits arbitrary R's and arbitrary C's to this reaction term and first then one analyses the possible physical relevance and meaning of these terms. 

4. It is an unfortunate and widespread misunderstanding of the role, the pure diffusion processes play in electrochemistry. They can never go alone (Warburg term). When electrical response in a material is measured for example by  EIS, the total current (proportional to the gradient of the electrochemical potential) ALWAYS consists of two terms, the drift term(proportional to the applied electrical field) and the diffusion term (proportional to the concentration gradient of the mobile charge in question). With diffusion term only(Warburg analysis, leading to Admittance ~ w^-1/2 frequency dependence ), there would never occur final steady state of the system(constant current through the entire system as time->infinity).

5. There is a number of very important general electrical  response characteristics, common to solid and liquid systems.  One of them is the finding that there are almost always two spatial regions contributing to the overall response, the bulk region ( generaly at higher frequencies - the solution itself) and the electrode-solution interface (lower frequencies - in electrochemistry generally named as "Helmohltz" and "diffusion" regions) .

6. Some of the general aspects of the more fundamental analysis of the electrical response in materials can be found in U.S. patent n. 5 627 479, May 6, 1997.

I hope this will help on the way.

With best regards

Petr