The clean, polished Rh disk measurements showed rather poor HER activity with η 10 more negative than -0.4 V and the corresponding Tafel slope of -140 mV dec -1 (Figure ). Even though the Ni-poly disk was polished to a mirror finish, the roughness factor was somewhat higher than 1. By plotting the currents measured at -0.85 vs. SCE as the function of the potential scan rate and using the value of specific capacitance of 20 μF cm -2 (for metallic surfaces ), the roughness factor was determined to be 2.8. According to the benchmarking by McCrory et al. , η 10 for the Ni surface with roughness factor 20±10 was approximately -0.25 V. For the exchange with Rh up to 20 s, we could not see any significant changes in recorded cyclic voltammograms (Figure , Supplementary Information), but obtained a significant increase of HER activity. However, for 30 s of exchange with Rh and more, cyclic voltammetry of Rh-exchanged Ni-poly disk shows H UPD regions characteristic for Rh and practically unchanged regions corresponding to Ni 2+ /Ni 3+ oxidation (Figure ). For 15, 30, and 45 s of exchange with Rh, η 10 amounted to -168, -120, and -110 mV, without significant change of the Tafel slope . These results align with the unchanged surface roughness, evidenced by 3D surface reconstruction using SEM, and a low percentage of Rh on the surface for short Rh-exchange times, with a very uniform distribution. These issues will be discussed later on. We have used platinized Pt-poly as a reference to Rh-modified Ni-poly surfaces. In ref. η 10 for Pt was reported to amount (-100±20) mV for smooth Pt-poly surface (RF 6±2), and ~ -40 mV