Enhancing Nitrate Reduction to Ammonia Through Crystal Phase Engineering: Unveiling the Hydrogen Bonding Effect in δ‐FeOOH Electrocatalysis

Crystal phase engineering has emerged as a powerful tool for tailoring the electrocatalytic performance, yet its impact on nitrate reduction to ammonia (NRA) remains largely uncharted territory. Herein, density functional theory (DFT) calculations are performed to unravel the influence of the crystal phase of FeOOH on the adsorption behavior of *NO₃. Inspiringly, FeOOH samples with four distinct crystal phases (δ, γ, α, and β) are successfully synthesized and deployed as electrocatalysts for NRA. Remarkably, among all FeOOH samples, δ-FeOOH demonstrates the superior NRA performance, achieving a NH₃ Faradic efficiency ( FE NH 3 $\rm{FE} _ {\rm{NH_3}}$ ) of 90.2% at -1.0 V versus reversible hydrogen electrode (RHE) and a NH₃ yield rate ( Yield NH 3 $\rm{Yield} _ {\rm{NH_3}}$ ) of 5.73 mg h^-1 cm^-2 at -1.2 V. In-depth experiments and theoretical calculations unveil the existence of hydrogen bonding interaction between δ-FeOOH and *NO_x, which not only enhances the adsorption of *NO_x but also disrupts the linear relationships between the free energy of *NO₃ adsorption and various parameters, including limiting potential, d-band center (ε_d) and transferred charge from FeOOH to *NO₃, ultimately contributing to the exceptional NRA performance.