Electrodeposition of iridium onto glassy carbon and platinum electrodes

The mechanism for the electrodeposition of iridium onto glassy carbon and platinum substrates has been investigated. Iridium coatings were characterized by X-ray photoelectron microscopy to determine their chemical compositions and the morphologies of deposits were observed by scanning electron microscopy. The deposition of iridium on glassy carbon electrodes requires, in a first stage, the formation of Ir surface sites. These sites, generated by reduction of Ir³⁺ ions for large overpotential, allow the adsorption of H atoms which act as the reducing agent for Ir³⁺ ions. In contrast, on platinum, Ir electrodeposition occurs readily due to a high H_ads surface coverage. The optimal H_ads surface coverage for the electrodeposition of Ir on Pt is close to 0.5. Electrochemical quartz crystal microbalance measurements demonstrated that the Faradaic efficiency of Ir deposition on Pt is very low (0.2–1.7%).