Proton exchange membrane water electrolysis (PEMWE) is a key technology for sustainable hydrogen production; however, its efficiency is limited by the sluggish kinetics and high overpotential of the anodic oxygen evolution reaction (OER). Although RuO₂ offers a cost-effective alternative to scarce IrO₂-based catalysts, its application is impeded by a fundamental trade-off between activity and stability under acidic conditions. Herein, we incorporate Hafnium (Hf) into the RuO₂ lattice to modulate the Ru oxidation state and oxygen vacancy concentration. The introduction of Hf suppresses Ru overoxidation, while controlled generation of oxygen vacancies minimizes lattice oxygen participation. The optimized Hf_0.1Ru_0.9O₂ catalyst exhibits a low overpotential of 187 mV at 10 mA·cm⁻² and outstanding durability, maintaining performance for 1500 h in 0.5 M H₂SO₄. Notably, a practical PEMWE device employing this catalyst achieves stable operation for over 600 h at 500 mA·cm⁻². A combination of in-situ differential electrochemical mass spectrometry (DEMS) and operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) reveal that Hf_0.1Ru_0.9O₂ facilitates oxygen evolution primarily through a multiple-pathway mechanism dominated by the adsorbate evolution mechanism (AEM) and the oxide pathway mechanism (OPM), with effectively suppressed lattice oxygen-mediated mechanism (LOM). These findings establish a new design principle for the development of durable acidic OER electrocatalysts.