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.
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