Interfacial Water Coordination on Ruthenium Oxide Nanoparticles Confined Within Covalent Organic Framework and Its Effect on Electrochemical Nitrate Reduction

ABSTRACT Electrochemical nitrate reduction reaction (NO ₃ RR) enables sustainable and decentralized ammonia production. Here, we demonstrate how oxygen‐deficient ruthenium oxide (RuO _x ) nanoparticles confined within the imine‐based covalent organic framework (COF) produced from 1,3,5‐tris(4‐aminophenyl)benzene (TAPB) achieve highly efficient nitrate‐to‐ammonia conversion, delivering a high Faradaic efficiency of 99.2% at ‐1.1 V vs Ag/AgCl in a neutral electrolyte. Crystal structure, optical spectra, and electronic state analysis reveal the strong interaction between RuO _x nanoparticles and TAPB‐COF. The confined nanoparticles change the interlayer spacing of TAPB‐COF, which in turn results in the high oxygen‐deficiency of RuO _x . Investigations by in situ optical spectroscopies and ab initio molecular dynamics simulations reveal the occurrence of a repelling effect on water molecules at the surface of hydrophobic TAPB‐COF framework, which contributes to the prevalence of 2‐coordinated water at the surface of RuO _x nanoparticles. This ensures a slow‐down proton transfer kinetics, leading to suppression of the hydrogen generation as an undesired competing process. The upshift of d‐band center and bridge‐site adsorption due to the high oxygen‐deficiency and the shortened Ru−Ru distance of the confined RuO _x nanoparticles contribute to the strengthened bonding of *NO ₂ and *NOH intermediates, which alleviates the nitrite production and accelerates the NO ₃ RR process.