Balancing active hydrogen and NO₂⁻ for interfacial tandem nitrate electrocatalysis to ammonia

Electrocatalytic reduction of nitrate to ammonia can transform nitrate-polluted sewage into value-added chemical products through an eco-friendly method. It is generally accepted that nitrate is first reduced to nitrite followed by a cascade reaction of hydrogenation of nitrite to ammonia. However, the imbalance between excessive accumulation of nitrite ions and insufficient supply of adsorbed active hydrogen (H_ad) results in a slow ammonia production rate with low yield. Herein, we developed heterostructured Ni@Cu_2-xSe nanoarrays grown on copper foam as tandem catalyst to solve the above challenge. Electron paramagnetic resonance and theoretical calculation proved the synergistic effect of Cu_2-xSe and Ni in the tandem reaction, in which Cu_2-xSe preferentially promoted the reduction of NO₃⁻ to NO₂⁻, while Ni mainly provided sufficient H_ad due to the enhanced H₂O dissociation for the subsequent hydrogenation. Therefore, Ni@Cu_2-xSe heterostructures exhibited outstanding ammonia yield of 17.2 mg h⁻¹ cm⁻² with a Faraday efficiency of 91.6 % for ammonia (FE_NH3) at −0.4 V vs RHE. This research provides valuable guidance toward balancing NO₂⁻ and H_ad by rationally constructing heterostructures, which is helpful for the development of highly selective and tandem catalysts toward multi-step reactions.