Phase-Bode plot: shift of the maximum toward higher frequencies

[Francesca Capitanio]

Hi all,

Thanks for the opportunity to discuss with you about EIS.

I refer to the book “Impedance Spectroscopy” written by Prof.
Macdonald (2nd edition), at page 511. It is observed for the ORR on
silver GDE that the maximum in the phase Bode plot shifts toward
higher frequencies at more cathodic potentials. Do you know what this
shift means? Or better, why the phase shifts towards higher
frequencies? Someone suggests me that it is connected with the
vibrational state of the molecules but I do not know.

In my experiments performed in sulphuric acid solution and in fuel
cell as well, under different operative conditions, I observed this
shift toward high frequencies. From the notes of Prof. Lasia, I know
the relation between the frequency of the maximum and the kinetic
parameters (charge-transfer resistance, electrolyte resistance, double-
layer capacitance). So, it comes quite natural to me to state “the
shift is due to an improved support of reactant and higher proton
conduction in the porous electrode”. Since I haven't found much in
the literature, I don’t know if this is obvious or, on the contrary, I
am going too far in my statement.


Thanks for your help.
Best regards

Francesca

[Mohsen]

Hi Francesca,

One reason that causes the shift in the maximum frequency of Bode plot towards higher frequencies may be due to decrease of double layer capacitance, as I saw in simulating the impedance response of a simple, one time constant circuit with progressive decrease of double layer capacitance and keeping other parameters constant.

Regards,

Mohsen

[Mohsen]

On Tuesday, May 1, 2012 9:46:41 PM UTC+4:30, Francesca Capitanio wrote:

[Mohsen]

[Mohsen]

On Friday, June 1, 2012 10:22:05 PM UTC+4:30, Mohsen wrote:

[Francesca Capitanio]

Hi Mohsen,

Thanks for your kind answer.

I agree with you, the shift may be due to decrease of the double-layer
capacitance. However, for what concern electrochemical processes, and
the shifts observed at the increasing of the overpotential, I will say
that the charge-transfer resistance is responsible for it. It just
makes more sense to me (physical meaning).

Anyway, I am trying to understand if the shift can be explained
without the simulation. Once the simulation is done, it is easy to
observe the shift at the decreasing of the capacitance/resistance.
In other words, can we look at the problem from the other side and
justify the decrease of the capacitance (or resistance) starting from
the shift? That is the reason because I asked if someone knows the
physical meaning of that shift.

Thanks a lot for your help Mohsen.
Best regards,

Francesca

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