AC impedance theory—the transmission line

We now turn to the movement of charge between the polymer centres. In our particular case, the redox reaction for the trimer species we consider below is 

This is termed a Nernst potential, as the expression is simply the Nernst equation for the one electron redox couple. Again, this driving potential tells us about the combination of the diffusional and migrational terms. 

In AC impedance, where a small amplitude potential perturbation is applied, the driving potentials can be linearised, giving 

The redox reaction of the coat requires the storage of charge, and therefore a capacitance. It is the capacitance which ensures electroneutrality in the system and it is given by [1] 

CD is the Donnan capacitance, which tells us about the amount of charge in the pores accumulated by the potential difference between ED and E, the potential in the pores. When only counterion is present, CD will be given by 

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