  An immobile charged species provides a charged medium for transport of charge
carriers that is exploited in many applications, such as permselective
membranes, doped semiconductors, biological ion channels, as well as porous
media and microchannels with surface charges. In this paper, we theoretically
study the electrochemical impedance of electrodiffusion in a charged medium by
employing the Nernst-Planck equation and the electroneutrality condition with a
background charge density. The impedance response is obtained under different
dc bias conditions, extending above the diffusion-limiting bias. We find a
transition in the impedance behavior around the diffusion-limiting bias, and
present an analytical approximation for a weakly charged medium under an
overlimiting bias.
