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

Abstract

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.