Faradaic vs capacitive charge transfer

Charge can cross the electrode-electrolyte interface by two mechanisms. Capacitive (non-Faradaic) transfer charges and discharges the double layer without any chemical reaction — no electrons cross the interface and no new species are created — and is fully reversible. Faradaic transfer involves an actual electrochemical reaction (oxidation or reduction) in which electrons cross the interface; it may be reversible (for example the valence-state changes in iridium oxide) or irreversible (for example water electrolysis producing gas and pH shifts, or electrode corrosion), the latter being what damages tissue and electrodes.

The distinction is central to safe stimulation: the goal is to inject the required charge using reversible capacitive and reversible-Faradaic processes and to avoid irreversible Faradaic reactions by keeping the electrode potential inside the electrochemical water window. Materials are chosen accordingly — capacitive materials like TiN, pseudocapacitive / reversible-Faradaic materials like activated iridium oxide, and mixed ionic-electronic conductors like PEDOT that offer high reversible charge storage.