As a benchmark, we used a platinized Pt-poly disk (3 mm in diameter), prepared by potentiodynamic cycling of smooth Pt-poly disk in H 2 PtCl 6 solution, as described in ref. . Using H UPD peaks, the roughness factor (RF) of the used platinized Pt-poly disk was evaluated to 7.9. In addition, the activity of the used Pt-disk was also checked in acidic media (0.5 mol dm -3 H 2 SO 4 solution, Figure , Supplementary Information) using the same HER measurement protocol as in alkaline solution, confirming that a highly active Pt surface is obtained.