Electrochemical reduction of nitrate to ammonia on ultra-stable amorphous Co–P electrocatalyst

Abstract

Electrocatalytic reduction of nitrate (NO₃⁻) to ammonia (NH₃) is garnering increasing interest due to its potential to reduce CO₂ emissions as a substitute for the Haber-Bosch process, while also mitigating NO₃⁻ pollution. However, it remains a challenge to achieve a current density exceeding 300 mA cm⁻² while maintaining the stability of catalysts. Additionally, the anodic oxygen evolution reaction, characterized by slow kinetics and high energy barriers, severely impedes the widespread adoption of NH₃ formation from NO₃⁻ reduction. Therefore, in this study, we introduce amorphous phosphorus-doped cobalt catalysts (Co–P@NF) prepared via a facile electrodeposition process for efficient NO₃⁻ reduction and hydrazine oxidation. The incorporation of phosphorus in Co–P@NF facilitates electron migration from phosphorus to cobalt, enhancing *H provision for efficient hydrogenation of the intermediate *NO₂⁻. This results in a current density of 2 A cm⁻² at −0.3 V, with a faradaic efficiency for NH₃ of 91% in an electrolyte containing 1 M NO₃⁻. Moreover, the Co–P@NF catalyst exhibits remarkable long-term stability, maintaining an NH₃ faradaic efficiency exceeding 90% and a current density of 799 mA cm⁻² after 82 hours of electrolysis. Furthermore, Co–P@NF displays high catalytic activity in promoting the rate-determining step of hydrazine oxidation, from *N₂H₂ to *N₂H. The incorporation of the HzOR (hydrazine oxidation reaction)-assisted NO₃⁻RR (nitrate reduction reaction) unit significantly reduces the cell voltage to 0.34 V at 300 mA cm⁻².