Enhancing acidic oxygen evolution reaction with a highly stable Eu-doped RuO2 electrocatalyst.

Ruthenium oxides (RuO2) are attractive alternatives for the oxygen evolution reaction (OER), but their stability in acidic environments is compromised by Ru oxidation and dissociation. Here, we leverage rare-earth (RE) europium (Eu) doping to engineer an Eu-RuO2 catalyst, exploiting the unique 4f orbital properties of Eu to modulate RE(f)-O(p)-Ru(d) orbital coupling for enhanced OER performance in acidic solution. The Eu-RuO2 catalyst exhibits a low overpotential of 195 mV, achieves 10 mA cm-2 current density, and exhibits outstanding stability for 2800 hours in 0.5 M H2SO4. In acidic PEM-WE devices, our Eu-RuO2 catalyst achieves a high current density of 1000 mA cm-2 at 1.67 V cell voltage and sustains operation at 1000 mA cm-2 at 60 °C in normal ambiance with negligible degradation for 300 hours. Density functional theory (DFT) calculations and in situ X-ray absorption spectroscopy (XAS) reveal that the 4f buffer band of Eu donates electrons to stabilize Ru-O covalency via 4f-2p-3d gradient orbital coupling, suppressing Ru oxidation and dissociation during acidic OER. Attenuated total reflection-surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and DFT further show that this orbital coupling optimizes oxygen intermediate adsorption, lowering reaction barriers and stabilizing the acidic OER. Importantly, ATR-SEIRAS and DFT also confirm that Eu doping enhances the oxide path mechanism (OPM), evidenced by an O-O vibrational peak at low overpotential and a reduced rate-determining barrier. This kinetic advantage of the OPM, together with the effective suppression of Ru-O covalency loss, collectively explains the exceptional activity and long-term stability of the Eu-RuO2 catalyst under acidic conditions.