Electrochemical Quartz Crystal Microbalance Study of Underpotential Deposition of Mercury on Iridium Metal and Iridium Oxide

To evaluate the properties of an Ir‐based catalyst (IrOx), which is the anode in polymer electrolyte membrane‐type water electrolysis, a new method for estimating the electrochemical surface area (ECSA) is required. ECSA evaluation using the underpotential deposition of mercury (Hg‐UPD) can be applied to metals and oxides. However, research regarding Hg‐UPD is needed because several Hg(II)/Hg(I) redox reactions occur simultaneously, and direct evidence of Hg‐UPD on IrOx has not been reported. We previously confirmed the Hg‐UPD phenomenon on Ir polycrystalline films using the electrochemical quartz crystal microbalance (EQCM) method. However, measurements on IrOx electrodes were not performed. In this study, Ir and IrOx electrodes are prepared by the sputtering method, and their Hg deposition behavior observed by EQCM. We quantitatively confirm Hg‐UPD on Ir metal surfaces and provide new evidence of Hg‐UPD on the IrOx surface. These results contribute to establishing the ECSA evaluation of various Ir‐based catalysts