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.