SFC-ICP-MS measurements

Electrodes were prepared by suspending commercial Alfa Aesar IrO_x · 2H₂O Premion catalyst powder in a mixture of 87.5% ultrapure water (Merck Milli-Q), 12.5% IPA, and Nafion® perfluorinated resin solution (Sigma Aldrich, 5 wt%). Standard inks had an iridium concentration of 663 µg l⁻¹, a Nafion concentration of 332 µg l⁻¹ and a volume of 1 ml in a 1.5 ml Eppendorf tube. Inks were sonicated for 10 min (4 s pulse, 2 s pause) and dropcasted as 0.2 µl on a freshly polished glassy carbon plate (SIGRADUR G, HTW). The quality and diameter of the dropcasted catalyst spots (Ø of ca. 1.3 mm) was screened by employing Keyence VK-X250 profilometer.

For variations of loading and Nafion content, the concentration of iridium and Nafion in the ink was adjusted to the desired loading and concentration. An IPA: DI water ratio of 12.5:87.5, was employed in all IrO_x inks, accounting for the alcohol content in the Nafion solution.

Electrochemical measurements were carried out with an SFC-ICP-MS²⁹, with the modifications described in ref. ²⁸ in Ar-saturated 0.1 M H₂SO₄ (Merck Suprapur) mixed with ultrapure water. The dropcasted spots, acting here as working electrodes, were located with a top view camera to enable vertical alignment with the SFC (Ø 2 mm). A graphite rod served as counter electrode, whereas a saturated Ag/AgCl electrode (Metrohm) was used as reference electrode. ICP-MS measurements were performed with a NexIon 300 spectrometer (Perkin Elmer), employing a flow rate of 208 µl min^-1 for reference measurements. For the flow rate-dependence studies, flow rates were adjusted by tuning the speed of the ICP-MS peristaltic pump. Daily calibration of the ICP-MS was performed by freshly prepared standard solutions containing Ir (0.5 to 5 µg l⁻¹), and Re (10 µg l⁻¹) as an internal standard. All current and dissolution rates shown in this report have been normalized to the nominal loading of the spots.

For the variation of dissolved iridium in the electrolyte, iridium was electrochemically dissolved in 0.1 M H₂SO₄ by 1000 cyclic voltammograms recorded in a potential range from 0.05 V_RHE–1.5 V_RHE^37,47. The iridium concentration was then determined by ICP-MS. Electrolyte and standards were prepared from the electrolyte with dissolved iridium. A baseline measurement was taken before cell contact.

For the variation of pH, the electrolyte was set to the corresponding H₂SO₄ concentration. To ensure electronic conductivity of the electrolyte in measurements with a pH higher than 1, the total concentration of sulfate ions was set to 0.05 M with K₂SO₄ (99.999% purity, Sigma Aldrich).