First, we investigate the effect of Rh exchange on commercial Ni foam. The results show that the effects of time of exchange by Rh are not as pronounced as in the case of smooth Ni surface (Figure ), and the most pronounced effects are seen after several minutes of exchange by Rh. For clean NF η 10 was found to be -0.26 V, and for 2 and 10 min exchange with Rh it reduces to -0.17, and -0.07 V. EDX analysis showed a monotonous increase of Rh surface concentration, without significant alteration of mesh morphology caused by dissolution in acidic Rh-containing solution (Figure ). Nevertheless, for higher Rh exchange times, we clearly observed the formation of Rh phase on NF surface (Figure , Supplementary Information), which we consider mainly responsible for HER activity, as metallic Rh is much more active for HER than Ni. Also, we must note that it was impossible to perform electrode rotation to remove the H 2 bubbles formed on the surface in the case of NF. Thus the measured activity of Rh-exchanged Ni foam is slightly affected by H 2 bubbles blocking the surface. Still, the activities are better than those reported for pure Rh films prepared on NF using AACVD , while the modification procedure is greatly simplified. We also note that the improvements of the NF HER activity were previously reported for the case of spontaneous deposition of Ru and Pd . However, the activities reported in that work are much lower than those reported here, not only for the modified NF but also for the clean one. For this reason, we also investigated long Rh exchange time, 5 minutes, and obtained a further improvement of HER activity. In this case, η 10 amounted to -0.09 V, and the Tafel slope was -122±5 mV dec -1 . However, after SEM and EDX analysis, it was observed that the surface is significantly eroded (Figure , Supplementary Information), while EDX showed ~20 at.% Rh, in line with clear H UPD peaks after 5 min of exchange with Rh (Figure ). This finding is in line with recorded cyclic voltammograms of Ni-dep electrodes before and after Rh exchange, showing progressive loss of voltammetric response corresponding to Ni 2+ /Ni 3+ transition with the extension of Rh exchange time (Figure ). Such erosion of Ni-dep is likely due to fast corrosion of a highly developed Ni surface, and it was not observed for NF or smooth Ni-poly. We ascribe this to the fact that Ni-poly has a low concentration of highly uncoordinated Ni sites, prone to corrosion, unlike Nidep. Moreover, the surface of NF is also rather smooth, much smoother than that of Ni-dep (Figure , insets, and Figure ). We believe this is why no pronounced dissolution and erosion of NF is seen.