Lanthanide-regulating Ru-O covalency optimizes acidic oxygen evolution electrocatalysis

Precisely modulating the Ru-O covalency in RuO_x for enhanced stability in proton exchange membrane water electrolysis is highly desired. However, transition metals with d-valence electrons, which were doped into or alloyed with RuO_x, are inherently susceptible to the influence of coordination environment, making it challenging to modulate the Ru-O covalency in a precise and continuous manner. Here, we first deduce that the introduction of lanthanide with gradually changing electronic configurations can continuously modulate the Ru-O covalency owing to the shielding effect of 5s/5p orbitals. Theoretical calculations confirm that the durability of Ln-RuO_x following a volcanic trend as a function of Ru-O covalency. Among various Ln-RuO_x, Er-RuO_x is identified as the optimal catalyst and possesses a stability 35.5 times higher than that of RuO₂. Particularly, the Er-RuO_x-based device requires only 1.837 V to reach 3 A cm⁻² and shows a long-term stability at 500 mA cm⁻² for 100 h with a degradation rate of mere 37 μV h⁻¹.