H2 production at positive potentials with a stable and bifunctional Ru0-RuO2Cy electrocatalyst

• Carbon-coated metallic Ru⁰ and ruthenium oxycarbonate (RuO 2 Cy) materials were synthesized via a mild and green hydrothermal method. • Fine-tuning of the synthesis conditions enables modulation of the concentration of ruthenium species • High activity toward the Hydrogen Evolution Reaction is achieved (37 mV in acid and 7 mV in base vs. RHE to reach a current density of 10 mA cm −2 ). • Hydrogen production at positive potentials is confirmed by DEMS and attributed to local pH changes. • Negligible deactivation is observed in chronopotentiometry tests (40 h at 10 mA cm −2 ) in both acidic and basic electrolytes. Developing highly efficient and earth-abundant catalysts for the hydrogen evolution reaction (HER) is essential to produce hydrogen (H 2 ) as a sustainable energy vector, providing an alternative to fossil fuels. Promoting HER in basic media remains challenging due to the lower proton availability compared to acidic conditions. Herein, we report the synthesis of partially carbon-coated, ruthenium-based materials with carefully controlled synthesis parameters that modulate the concentrations of metallic Ru⁰ and ruthenium oxycarbonate (RuO 2 C y ). These materials exhibit impressive electrocatalytic performance in HER, with overpotentials required to reach a current density of 10 mA cm −2 (versus RHE) as low as 37 mV in acidic and 7 mV in basic media. The high efficiency observed in basic media was further characterized using differential electrochemical mass spectrometry (DEMS) and in situ Raman spectroscopy, revealing H 2 production at positive potentials. This phenomenon may result from local pH changes facilitated by efficient water deprotonation on ruthenium species with intermediate oxidation states. In addition to high activity, the most efficient material demonstrated excellent stability, with no significant deactivation during chronopotentiometry experiments at 10 mA cm −2 for 40 h. This work underscores the importance of precise material design in developing effective electrocatalysts for HER.