Volumetric capacitance (mixed ionic-electronic conductor)

In a conventional metal electrode, charge accumulates only at the two-dimensional electrode-electrolyte interface (the Helmholtz double layer), so capacitance scales with geometric area. In a mixed ionic-electronic conductor (MIEC) such as PEDOT:PSS, ions permeate the entire hydrated volume of the film while electronic (hole) transport proceeds along the polymer backbone, so charge is stored throughout the three-dimensional bulk. Capacitance therefore scales with volume rather than area, which is the origin of the term volumetric capacitance.

This decoupling of electrochemical performance from footprint is the single most important reason conducting polymers, and to a lesser degree porous carbons, can shrink electrodes toward single-cell dimensions without the impedance penalty a bare metal would incur. It also underlies the organic electrochemical transistor, whose channel current is modulated by ions entering the bulk. The practical limit is ionic transport kinetics: thicker films gain capacitance but slow their frequency response, so there is a bandwidth-capacitance tradeoff.