Unsteady ac voltammetry

[Dieter Britz]

When you begin to apply an ac voltage to an electrochemical cell, It
requires some time before the current response is truly sinusoidal
(I am assuming linearity, i.e. a small ac perturbation). It is easiest
to think of this if the applied ac is a cosine function, which jumps
to some value at t = 0, but the same applies to a sine function.
Anyone know of references to this effect?

[Black Adda]

Post by Dieter Britz
When you begin to apply an ac voltage to an electrochemical cell, It
requires some time before the current response is truly sinusoidal
(I am assuming linearity, i.e. a small ac perturbation). It is easiest
to think of this if the applied ac is a cosine function, which jumps
to some value at t = 0, but the same applies to a sine function.
Anyone know of references to this effect?
--
Dieter Britz (dieterbritz<at>yahoo.com)

My experience is that there is not very much literature available on
this particular subject. I would have thought, though, that it had
something to do with the concentration of reactants at the electrode
surface and the gradients that transport the reactants between the
surface and the bulk solution. You don't say much about the particular
cell yous are studying, but it will take som time for these gradients
to readjust, don't you think?

[Yevgen Barsukov]

Post by Dieter Britz
When you begin to apply an ac voltage to an electrochemical cell, It
requires some time before the current response is truly sinusoidal
(I am assuming linearity, i.e. a small ac perturbation). It is easiest
to think of this if the applied ac is a cosine function, which jumps
to some value at t = 0, but the same applies to a sine function.
Anyone know of references to this effect?

It is quite visible if you do FFT from both original excitation and
response.
You will see some additional frequencies on the response which are not
present
in excitation.
It can be either because system is non-linear or, as in the case you
are discussing,
because it did not reach steady state. In FFT impedance spectroscopy
we usually trow
away the first sine period after signal onset to eliminate this
effect.

However, even the first sine wave or any load onset response can still
be fully
analyzed but using Laplace transform. With Laplace transform there is
no assumption
that sine wave was applied for long time, instead there is explicitly
defined load
onset.

For the simplest case where you know an exact equivalent circuit that
you
are testing, but don't know its parameters, you could fit the
analytical expression
of the Laplace transform to this particular excitation to the response
you are measuring
and find the parameters.

It might be easier to just use a step-function as excitation rather
than a sine-wave,
for such analysis, since response function will be just a sum of
exponents.
If exact circuit is now known, it is possible to use a generic circuit
(say 15 RC elements) since it will give a very good approximation of
any circuit,
find its parameters and than get impedance spectrum from it.

I investigated this measurement method at some point since it is about
twice faster than FFT multisine and cheap to implement with just a
pulse, it is summarized here:
"A novel impedance spectrometer based on carrier function Laplace-
transform of the response to arbitrary excitation "
http://www.scirus.com/srsapp/sciruslink?src=sd&url=http%3A%2F%2Fwww.sciencedirect.com%2Fscience%3F_ob%3DGatewayURL%26_origin%3DScienceSearch%26_method%3DcitationSearch%26_piikey%3DS0022072802012093%26_version%3D1%26_returnURL%3Dhttp%253A%252F%252Fwww.scirus.com%252Fsrsapp%252Fsearch%253Fq%253Dlaplace%252Btransform%252Bimpedance%2526t%253Dall%2526sort%253D0%2526p%253D0%2526drill%253Dyes%26md5%3D50c213ed991292b228915003776339a9

Regards,
Yevgen

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