In Situ Clustering of Single-Atom Copper Precatalysts in a Metal-Organic Framework for Efficient Electrocatalytic Nitrate-to-Ammonia Reduction

When serving as a "precatalyst ", metal-organic frameworks (MOF) usually incur uncontrollable framework collapse in electrocatalysis. Herein, we report an anticollapse MOF-supported single-atom Cu precatalyst for electrocatalytic nitrate-to-ammonia reduction reaction (NARR), which can be applied in the rechargeable ammonia energy storage (RAES) technology. In situ X-ray absorption spectroscopy (XAS) revealed the association of the formation of real catalytic sites with the in situ clustering of single-atom Cu during NARR. Notably, the noncollapse MOF can afford the confined space to prevent the excessive aggregation of Cu atoms, leading to uniform ultrasmall nanoclusters (ca. 4 nm). Moreover, it achieves a maximal Faradaic efficiency toward NH3 of 85.5%, a formation rate of NH3 of 66 mu mol h(-1) cm(-2), and a specific activity of 53.43 mg(NH3) h(-1) mg(Cu)(-1) in 5 mM NO3- solution. The specific activity is found to be at least 3.3 times higher than that of other reported Cu-based catalysts. Density function theory (DFT) calculation further confirms the size effect and the host-guest interaction in facilitating the NO3- activation and the reaction energy decrease. Besides, it also exhibits a high selectivity of ammonia-to-nitrate of 93.3%, displaying great potential in RAES technology.