Catalytic Performance and Near-Surface X-ray Characterization of Titanium Hydride Electrodes for the Electrochemical Nitrate Reduction Reaction.

The electrochemical nitrate reduction reaction (NO ₃ RR) on titanium introduces significant surface reconstruction and forms titanium hydride (TiH _x , 0 < x ≤ 2). With ex situ grazing-incidence X-ray diffraction (GIXRD) and X-ray absorption spectroscopy (XAS), we demonstrated near-surface TiH ₂ enrichment with increasing NO ₃ RR applied potential and duration. This quantitative relationship facilitated electrochemical treatment of Ti to form TiH ₂ /Ti electrodes for use in NO ₃ RR, thereby decoupling hydride formation from NO ₃ RR performance. A wide range of NO ₃ RR activity and selectivity on TiH ₂ /Ti electrodes between -0.4 and -1.0 V _RHE was observed and analyzed with density functional theory (DFT) calculations on TiH ₂ (111). This work underscores the importance of relating NO ₃ RR performance with near-surface electrode structure to advance catalyst design and operation.