(51a) In-Situ X-Ray Absorption Spectroscopy of Titanium/Titanium Hydride Electrode Evolution Under Electrochemical Nitrate Reduction Reaction Conditions

The conversion of wastewater nitrate to ammonia can simultaneously remediate widespread nitrogen pollution and electrify ammonia manufacturing. The electrochemical nitrate reduction reaction (NO₃RR) has been actively studied because of its potential to circularize nitrogen management. Titanium (Ti), an inexpensive and abundant metal, has been identified as a robust NO₃RR electrocatalyst. We have demonstrated with ex situ X-ray spectroscopy that water-stable titanium hydride (TiH₂) forms after NO₃RR. In this work, we employed in situ XAS measurements to systematically characterize the evolution of Ti electrodes under a variety of NO3RR and hydrogen evolution reaction (HER) conditions.

To illustrate the dynamic evolution of Ti electrodes under reaction conditions, we employed in situ XAS and studied effects of reaction duration, applied potential, electrolyte pH, and electrolyte anion composition. We found that the atomic percentage of TiH₂ in the electrode near-surface region consistently increased with increasingly negative NO₃RR potential and duration, while metallic Ti exhibited the opposite trend. Density functional theory (DFT) calculations predicted that the H* surface coverage increases with increasingly acidic electrolyte pH, and we confirmed this trend experimentally. Under the same applied potential of –0.4 V vs. RHE, TiH₂ content was 57%, 45%, and 0% in electrolytes with bulk pH values of 1, 5, and 13, respectively. Furthermore, we observed that nitrate hinders the formation of TiH₂. Under the same applied potential of –1.2 V vs. RHE, the transition from Ti to TiH₂ was completed at 20 min in the absence of nitrate, and only 60% with 10 mM nitrate.

Ti electrode reconstructs to TiH₂ under NO₃RR and HER reaction conditions, not only competing with desired reactions but also modifying the electrocatalyst properties. This work underscores the importance of characterizing the dynamic restructuring of electrocatalysts under reaction conditions and furthers understanding of electrolyte effects in NO₃RR.