Coupling electrocatalytic cathodic nitrate reduction with anodic formaldehyde oxidation at ultra-low potential over Cu2O

Abstract

Electrocatalytic ammonia (NH₃) synthesis from nitrate (NO₃⁻) is a promising alternative to the Haber–Bosch route that requires high energy input and carbon emissions. However, sluggish anodic oxygen evolution reaction (OER) kinetics requires a large overpotential (>1.23 V vs. RHE), severely restricting the electrocatalytic cathodic NO₃⁻ reduction reaction (NO₃⁻RR). Herein, a HCHO oxidation reaction (FOR) was developed to achieve 300 mA cm⁻² at 0.81 V_RHE, which was 1.56 V lower than that of the OER (Pt). A superior HCOOH production rate of 9.64 mmol cm⁻² h⁻¹ was achieved over Cu₂O, approaching the highest performance reported to date. Further investigation indicated that the anodic HCHO oxidation mechanism involves electrocatalytic oxidative dehydrogenation (EOD) and tandem reaction pathways. The tandem reaction features the electrocatalytic oxidation of cubic Cu₂O to orthorhombic Cu(OH)₂ and spontaneous reduction of Cu(OH)₂ to Cu₂O by HCHO. Subsequently, the two-electrode coupling FOR and NO₃⁻RR wherein Cu₂O is utilized at both anode and cathode requires an ultra-low cell voltage of −0.19 V to achieve 10 mA cm⁻² and realizes the high faradaic efficiency of 99.77% for NO₃⁻ conversion to NH₃. This strategy represents a novel transformative system for simultaneously treating pollutants (HCHO and NO₃⁻) and yielding value-added chemicals (HCOOH and NH₃).