Self-enhanced localized alkalinity at the encapsulated Cu catalyst for superb electrocatalytic nitrate/nitrite reduction to NH₃ in neutral electrolyte.

Abstract

The electrocatalytic nitrate/nitrite reduction reaction (eNO<i>_x</i>^-RR) to ammonia (NH₃) is thermodynamically more favorable than the eye-catching nitrogen (N₂) electroreduction. To date, the high eNO<i>_x</i>^-RR-to-NH₃ activity is limited to strong alkaline electrolytes but cannot be achieved in economic and sustainable neutral/near-neutral electrolytes. Here, we construct a copper (Cu) catalyst encapsulated inside the hydrophilic hierarchical nitrogen-doped carbon nanocages (Cu@hNCNC). During eNO<i>_x</i>^-RR, the hNCNC shell hinders the diffusion of generated OH^- ions outward, thus creating a self-enhanced local high pH environment around the inside Cu nanoparticles. Consequently, the Cu@hNCNC catalyst exhibits an excellent eNO<i>_x</i>^-RR-to-NH₃ activity in the neutral electrolyte, equivalent to the Cu catalyst immobilized on the outer surface of hNCNC (Cu/hNCNC) in strong alkaline electrolyte, with much better stability for the former. The optimal NH₃ yield rate reaches 4.0 moles per hour per gram with a high Faradaic efficiency of 99.7%. The strong-alkalinity-free advantage facilitates the practicability of Cu@hNCNC catalyst as demonstrated in a coupled plasma-driven N₂ oxidization with eNO<i>_x</i>^-RR-to-NH₃.