<p>The reliance on platinum-based hydrogen catalysts in proton-exchange membrane water electrolysers compromises cost efficiency. Here we develop a non-platinum catalyst composite featuring a ruthenium oxide (RuO₂) core encapsulated by a thin ruthenium phosphide (RuP₂) shell, rapidly synthesized by a flame-assisted process. This core–shell catalyst demonstrates good performance towards hydrogen evolution in acidic media, achieving an overpotential of 16 mV at 10 mA cm⁻² at a low loading of 5.5 wt%. Mechanistic studies and density functional theory show that the RuO₂/RuP₂ core–shell structure optimizes interfacial-water organization, enhancing proton transfer at the reaction interface and boosting hydrogen evolution activity. We further demonstrate the scalable synthesis of this Ru catalyst via colloidal milling, enabling bulk production of this catalyst (tens of grams per batch) under mild conditions. When integrating Ru catalyst prepared at this scale into a practical proton-exchange membrane electrolyser (2 × 100 cm⁻²), it sustains 200 A (1 A cm⁻²) at a moderate cell voltage (1.8 V) with a stability over 1,500 h. Our results suggest a possible cost-effective alternative for proton-exchange membrane electrolysis.</p>