Preparation of thin films and the setup -Electrochemical measurements were conducted with a CompactStat (Ivium Technologies) in a two-compartment electrochemical cell in a 0.1 M HClO4 (Merck, Suprapur, 70 %, diluted by Milli-Q, 18.2 MΩ cm) electrolyte with a conventional three-electrode system. Ag|AgCl was used as a reference and a graphite rod as a counter electrode. The working electrode was a glassy carbon disc embedded in Teflon (Pine Instruments) with a geometric surface area of 0.196 cm 2 . The Ag|AgCl reference was separated from both the working and counter electrode with a salt bridge in order to avoid Cl -ions contamination. Prior to each experiment, the two-compartment electrochemical cell was boiled in Milli-Q water for 1 hour, and the electrode was polished to mirror finish with Al2O3 paste (particle size 0.05 µm, Buehler) on a polishing cloth (Buehler). After polishing, the electrodes were rinsed and ultrasonicated (Ultrasound bath Iskra Sonis 4) in Milli-Q/isopropanol mixture for 5 min. 20 µL of 1 mg mL -1 water-based well-dispersed electrocatalyst ink was pipetted on the glassy carbon electrode completely covering it and dried under ambient conditions. After the drop had dried, 5 µL of Nafion solution (ElectroChem, 5 % aqueous solution) diluted in isopropanol (1:50) was added. The electrode was then mounted on the rotator (Pine Instruments). The Ag|AgCl reference electrode potential against RHE was determined before the start of the experiment.