Balanced NO_<i>x</i>^- and Proton Adsorption for Efficient Electrocatalytic NO_<i>x</i>^- to NH₃ Conversion.

Abstract

Electrocatalytic nitrate (NO₃^-)/nitrite (NO₂^-) reduction reaction (eNO_<i>x</i>^-RR) to ammonia under ambient conditions presents a green and promising alternative to the Haber-Bosch process. Practically available NO_<i>x</i>^- sources, such as wastewater or plasma-enabled nitrogen oxidation reaction (p-NOR), typically have low NO_<i>x</i>^- concentrations. Hence, electrocatalyst engineering is important for practical eNO_<i>x</i>^-RR to obtain both high NH₃ Faradaic efficiency (FE) and high yield rate. Herein, we designed balanced NO_<i>x</i>^- and proton adsorption by properly introducing Cu sites into the Fe/Fe₂O₃ electrocatalyst. During the eNO_<i>x</i>^-RR process, the H adsorption is balanced, and the good NO_<i>x</i>^- affinity is maintained. As a consequence, the designed Cu-Fe/Fe₂O₃ catalyst exhibits promising performance, with an average NH₃ FE of ∼98% and an average NH₃ yield rate of 15.66 mg h^-1 cm^-2 under the low NO₃^- concentration (32.3 mM) of typical industrial wastewater at an applied potential of -0.6 V versus reversible hydrogen electrode (RHE). With low-power direct current p-NOR generated NO_<i>x</i>^- (23.5 mM) in KOH electrolyte, the Cu-Fe/Fe₂O₃ catalyst achieves an FE of ∼99% and a yield rate of 15.1 mg h^-1 cm^-2 for NH₃ production at -0.5 V (vs RHE). The performance achieved in this study exceeds industrialization targets for NH₃ production by exploiting two available low-concentration NO_<i>x</i>^- sources.