Operando NMR probing the direct role of vacancy-trapped hydroxyl species in CO₂ electroreduction

Resolving the dynamic role of surface-bound oxygen species during electrocatalytic CO₂ reduction remains a profound challenge due to the complex solid-liquid-gas interface and the need for atomic-scale resolution under operando conditions. We overcome this barrier by integrating operando multi-scale nuclear magnetic resonance (NMR) spectroscopy with complementary techniques (ATR-SEIRAS, DEMS) to directly identify and track the reaction pathway of vacancy-trapped hydroxyls on oxide catalysts (In₂O₃, SnO₂, Bi₂O₃). The results demonstrate that oxygen atoms of vacancy-trapped hydroxyls (¹⁷O-labeled oxygen vacancies) participate in direct oxygen exchange with the key *¹⁶OCH¹⁶O intermediate, forming hydroxylated *¹⁶OCH¹⁷O/*¹⁷OCH¹⁶O. This oxygen-vacancy-site-mediated mechanism (OVSM) exhibits a significantly lower kinetic barrier (0.84 eV) than the conventional adsorbate evolution mechanism (1.86 eV) and maintains optimal adsorption geometry, leading to superior selectivity and yield for formate (HCOO^-). The universality of the OVSM across multiple oxide systems establishes a design strategy for oxygen-deficient electrocatalysts. Furthermore, our multiscale NMR strategy provides a general approach for resolving hydroxyl-driven catalytic dynamics, offering broad implications for advancing aqueous-phase electrocatalysis.