HER activity of Ni-poly was significantly improved upon the surface modification by Rh (Figure ). The activity of as-prepared electrodes increases in the order Ni-poly < Rh-modified Ni-poly (10 seconds) < Rh-modified Ni-poly (30 seconds). Here we have systematically addressed the effects of surface oxidation of the studied electrodes to obtain deeper insight into the HER mechanism. For pure Ni-poly, surface oxidation causes a progressive increase in activity. Here, the oxidation was performed by potential cycling to increasingly higher anodic potentials as explained. For Ni-poly, the activity (measured as overpotential needed to reach -10 mA cm -2 , here denoted ηHER,10) increased for the potentiodynamic oxidation up to 1.2 V vs. RHE and then dropped slightly when the electrode was exposed to high anodic potentials of 1.45 V vs. RHE (Table ).