As can be seen, Pt-poly has a low HER overpotential and a high OER overpotential, while pure Nipoly behaves the opposite, with a high HER overpotential and lower OER overpotential. However, Rhmodified Ni-poly combines both low HER overpotential and low OER overpotential, while it is also characterized by low values of HER and OER Tafel slopes (Table ). Considering the OER catalyst overview provided by Plevová et al. , the Rh-modified Ni-poly electrodes fall in the high activity group of OER electrocatalysts presented so far, surpassing more than 70% of them. The values of HER and OER overpotentials, together with the calculated Tafel slopes, were used to estimate water decomposition voltage at 1000 mA cm -2 (Table , UiR-free). We note that these are iR-free water decomposition voltages to the extent to which we were able to remove iR drop from the cyclic voltammograms presented in Figure (left). Based on the obtained results, the Rh-modified Ni-poly electrodes offer water decomposition voltages below 2 V at a high current density of 1000 mA cm -2 (geometric), which is 20% lower compared to the water decomposition voltage on pure Ni electrodes estimated here.