Surface oxidation of Ru(10-10) and Pt–Ru/Ru(10-10) electrocatalysts and its effect on the hydrogen evolution reaction

• On Ru(10-10), HER current increases as a function of UPL • On Pt–Ru/Ru(10-10), HER current decreases as a function of UPL • Cycling to UPLs above 1.1 V RHE triggers surface oxidation and growth of RuO 2 layer • Cycling to UPLs above 1.3 V RHE leads to dealloying of Pt–Ru surface alloy The relationship between surface composition, oxidation state, and electrocatalytic activity is central to the knowledge-based design of efficient electrocatalysts. In this work, we employ a model electrochemistry approach combining the preparation and characterization of well-defined Ru(10-10) and Pt 0.2 Ru 0.8 /Ru(10-10) surfaces by synchrotron radiation photoelectron spectroscopy (SRPES), cyclic voltammetry (CV), and differential electrochemical mass spectrometry (DEMS). The combined study allowed us to establish the presence of adsorbed oxygen species on both surfaces after cycling to upper potential limits (UPLs) of 0.5 and 0.9 V RHE , the onset of surface oxidation at 1.1 V RHE , and formation of RuO 2 layer at 1.3 V RHE . We found that Pt–Ru surface alloying significantly enhances HER activity compared to bare Ru(10-10); however, the effect of oxidation is strongly surface-dependent. Increasing the UPL promotes HER activity on Ru(10-10), consistent with a beneficial role of oxygen species. In contrast, HER activity on Pt 0.2 Ru 0.8 /Ru(10-10) decreases with increasing UPL up to 1.1 V RHE , indicating a detrimental effect of adsorbed oxygen, likely due to steric blocking or electronic modification of Pt–Ru active sites. At higher UPLs (1.3 V RHE ), partial dealloying and the formation of ultrasmall Pt aggregates restore high HER activity. These findings identify a critical balance between oxidation and alloy structure in determining HER performance and highlight the importance of dynamic surface transformations in bimetallic catalysts.