The protocol for the measurements consisted of deep cathodic sweeps for accessing HER activity (three cycles at 20 mV s -1 between 0 and -0.35 V vs. RHE) separated by electrode cycling to progressively higher anodic potential (three cycles at 50 mV s -1 between 0 and 0.8, 1.0, and 1.2 V vs. RHE). HER overpotentials were estimated from the anodic part of the third cycle. This part of the protocol ended with three wide cyclic voltammograms (50 mV s -1 between 0 and 1.45 V vs. RHE), after which one cycle between -0.35 and 1.65 V vs. RHE was done (at 20 mV s -1 ). During the HER measurements and the last cyclic voltammogram, the electrode was rotated at 1800 rpm in order to remove any H2 or O2 bubbles formed on the surface, which could block the electrode surface.