Tiara Ni Clusters for Electrocatalytic Nitrate Reduction to Ammonia with 97% Faradaic Efficiency

The electroreduction of nitrate (NO₃^-) for sustainable ammonia (NH₃) production has recently emerged as a green process to solve water contamination and produce valuable chemicals. In this study, we developed Ni₆@CuFe-LDH composites comprising tiara Ni₆(SC₂H₄COOH)₁₂ (Ni₆) clusters anchored on the edges of 2D CuFe-LDH (LDH: layered double hydroxides) nanosheets via electrostatic interactions. The Ni₆@CuFe-LDH catalyst exhibits high electrochemical performance in nitrate reduction reaction (NO₃RR). Specifically, the Ni₆@CuFe-LDH gives rise to an excellent faradaic efficiency of ∼97%, significantly surpassing the ∼73% FE of the pristine CuFe-LDH, with the NH₃ productivity (0.91 mmol mg^-1 h^-1) being similar to that of the CuFe-LDH. Mechanistic studies reveal that the superior electrocatalysis of Ni₆-based catalysts is primarily due to the synergistic interaction between Ni₆ clusters and CuFe-LDH, which alters the rate-determining step (RDS) of the desorption of *NH₃ species (for CuFe-LDH) to the *NO₃ → *NO₂ step (for Ni₆@CuFe-LDH); this is corroborated by the control experiments of NO₂RR, in situ Raman and infrared spectroscopies, and computational approaches. In all, these efforts push forward the NO₃RR research to study the structure-property relationships from the micro/nano-level to the precise atomic-level.