shows Tafel slopes that are close but usually larger than -120 mV dec -1 (absolute values). Deviation from the theoretical value of -120 mV dec -1 can be ascribed to incomplete correction for the iR drop (see Section 2.1) and the problems with removing H 2 bubbles (despite electrode rotation during the measurements). Thus, although there it is quite inconvincible to determine the mechanism of HER solely based on the Tafel slope, there is enough reliable literature data to conclude that Heyrovski reaction is the rate-determining step (RDS) on Ni-poly surface, at least at higher HER overpotentials where we performed Tafel analysis. The same reference claims that, at lower HER overpotentials, the Tafel reaction is the RDS . However, more recent findings suggest that surface modifications by Ni-oxy-hydroxides enhance HER kinetics through H 2 O dissociation where H ads ends up on the metal surface. At the same time, OH ads is governed by the oxy-hydroxide phase (and ultimately released back to the solution) . In the original interpretation, this means that the rate of Volmer reaction is increased, allowing fast recombination of H ads and formation of molecular H 2 via the Tafel step. rate is increased, one can also expect that HER proceeds through with the Heyrovsky reaction as the RDS will be enhanced. Actually, the Ni surface binds H ads strongly, so there is no reason to expect that the Volmer reaction (formation of H ads ) will be RDS, in agreement with ref. . Nevertheless, the HER activity significantly decreases if the Rh-exchanged surface is exposed to the same oxidation protocol (Figure ). Rh surface binds H ads weaker than Ni surface, but, still, ΔG ads (H) is negative (Figure ). However, the Rh|Ni interface conversion to Rh|Ni-oxy-hydroxide interface hinders HER.