Electrochemical Flow Cell Setup -The setup and measurement guidelines were established as part of the previous work (Scheme 1) . Briefly, the working and counter electrode in the electrochemical flow cell (EFC) were glassy carbon discs (3 mm diameter) embedded into PEEK material (BASi). The discs were aligned in series; the counter electrode was placed first and the working electrode second in the direction of the electrolyte flow. The sample was deposited on the electrode by drop casting 5 µL drop of the ultrasonically homogenized catalyst ink (1 mg mL -1 ). Such preparation resulted in the electrocatalyst loading of 5 µg for all electrocatalysts. In addition, in order to increase the surface area of the counter electrode, 5 µL drop of Ketjen Black EC300J suspension (1 mg mL -1 ) was deposited on the glassy carbon counter electrode. After the drop had dried, 5 µL of Nafion solution (ElectroChem, 5% aqueous solution) diluted in isopropanol (1:50) was added. The Ag|AgCl reference electrode potential against RHE was determined before the start of the experiment. The housing of the cell was made from PEEK material and the design was modelled after a commercial cross-flow cell (BASi, MF-1092, cross-flow cell). The volume of the cell was established with a home-made silicon gasket with 1.0 mm thickness and 1.5 cm 2 ellipsoidal cut. The carrier solution (0.1 M HClO4) was pumped through the cell at a constant flow of 400 µL min -1 . Two glass syringes using Luer Lock connection to a PTFE tubing, two syringe pumps (WPI AL1000-220Z) and a diagonal 4-way flow valve (Idex, V-100D) were used to enable a continuous flow of the solution.