Construction of a metal–organic tetrahedral cage for boosting selective electrocatalytic reduction of nitrite to ammonium

Electrocatalytic reduction of nitrite to ammonium (NO₂RR) presents a promising approach for removing harmful NO₂^- and efficiently synthesizing ammonium but still faces difficulties, due to the complex six-electron transfer reaction, the formation of various by-products and competition from the hydrogen evolution reaction (HER). Herein, we constructed a metal-organic tetrahedral cage H1 (FeII4L₆) with a large inner cavity and Fe metal centers surrounded by hydrazide groups that act as hydrogen bond acceptor sites. This enables the cage to efficiently recognise nitrite and thermodynamically activate it under electrocatalytic conditions for its efficient reduction to ammonium. The kinetic experiments demonstrated that the catalytic process followed the Michaelis-Menten mechanism, which further verified the key role of host-guest interactions in the mimetic activation of nitrite and the enzyme-like catalytic behavior.