Self-powered energy-efficient electrochemical nitrite reduction coupled with sulfion oxidation for ammonia synthesis and sulfur recovery over hierarchical cobalt sulfide nanostructures

Electrochemical nitrite reduction reaction (NO2RR) becomes an attractive approach for hazardous nitrite removal in wastewater and valuable ammonia (NH3) production under ambient condition. However, the overall energy efficiency of NO2RR-involved electrolytic systems frequently limited by anodic half-reaction of oxygen evolution reaction (OER) with slow kinetics. Herein, a thermodynamically favorable sulfion oxidation reaction (SOR) substituted for OER is explored to combine NO2RR to address this issue. The hierarchical Co3S4 nano- structures are developed as a novel electrocatalytic system with dual functionalities of NO2RR and SOR, manifesting the sustainable ammonia production with dramatically reduced energy consumption. In an electrolyte of 1 M NaOH with 0.1 M NaNO2, the Co3S4/NF electrode exhibits the NH3 production rate of 19.2 mg h- 1 cm- 2 with Faradaic efficiency (FE) of 96.9 % at-0.3 V vs RHE, as well as long-lasting durability. When introducing SOR as a substituted anodic reaction, the NO2RR-SOR couples afford a significantly lower cell voltage of 1.13 V than that of NO2RR-OER couples (2.50 V) at 50 mA cm- 2. Inspired by this energy-efficient electrolysis, a proof- of-concept prototype for the production of value-added NH4Cl fertilizer and elemental sulfur powered by Co3S4- based Zn-NO2- battery is proposed.