Iridium oxide electrode

Iridium oxide, particularly activated iridium oxide film (AIROF) grown electrochemically from iridium metal, or sputtered iridium oxide film (SIROF), is a workhorse electrode material for both recording and stimulation because it stores and injects charge through a fast, reversible Faradaic reaction — the Ir(3+)/Ir(4+) valence transition — accompanied by proton exchange with the electrolyte. This pseudocapacitive mechanism gives a high reversible charge injection capacity (roughly 1 to 5 mC/cm^2 depending on activation, pulse width, and bias) while keeping the interface within the safe potential window.

Because the charge transfer is Faradaic rather than purely capacitive, iridium oxide's usable charge depends strongly on pulse timing and on any DC bias that keeps the oxide in the reactive state, and its performance can drift with activation state and chronic use. It nonetheless remains a standard for microstimulation electrodes (including intracortical microstimulation arrays) where high charge density from a small area is required.