Observation of Mercury Underpotential Deposition on an Ir Surface using the Electrochemical Quartz Crystal Microbalance Technique

Development of a technique to measure electrochemical surface area (ECSA) is essential to evaluate the activity and durability of Ir-based catalysts for polymer electrolyte membrane (PEM) water electrolysis. Mercury underpotential deposition (Hg-UPD) on Ir and IrOx in perchloric acid has been reported to be useful for ECSA evaluation. However, previous methods need to be carefully evaluated for reproducibility due to the presence of Hg(II)/Hg(I) redox reactions, and differences in UPD potentials between studies. Here, the electrochemical behavior of an Ir electrode in a mercury nitrate containing solution was investigated using the electrochemical quartz crystal microbalance method. We confirmed that a Hg adatom is reversibly adsorbed and desorbed on an Ir surface at potentials over 0.7 V vs the reversible hydrogen electrode in both perchloric acid and sulfuric acid. These results contribute to the establishment of a reproducible method for ECSA calculations for Ir-based catalysts.