MOF-derived CoSx as a bifunctional electrocatalyst for efficient sulfide oxidation and coupled ammonia synthesis

Abstract

The sustainable production of ammonia (NH3) and the remediation of environmental pollutants such as nitrate (NO3-) and sulfide (S2-) require energy- and cost-efficient solutions. Herein, we develop a metal-organic-framework (MOF)-derived cobalt sulfide (CoSx) electrocatalyst with stable and bifunctional activity for the sulfide oxidation reaction (SOR) and nitrate reduction reaction (NO3RR), enabling simultaneous pollutant removal and NH3 production. The catalyst achieves 50 mA cm-2 at 0.46 V vs. RHE for the SOR, reducing energy demand by 1.23 V compared to the oxygen evolution reaction (OER), while maintaining 150 hours of stability in a SOR half-cell test at 0.8 V vs. RHE. For the NO3RR, CoSx achieves a faradaic efficiency of 93.4% at -0.5 V vs. RHE, with a high NH3 production rate of 9.1 mg h-1 cm-2. In a paired SOR-NO3RR flow electrolyzer, the bifunctional CoSx operated at 1.13 V to achieve 50 mA cm-2, whereas coupling the NO3RR with the OER required 2.52 V at the same current density, highlighting significant energy savings and long-term stability over 50 hours. To assess its practical applicability, a real biogas sample was captured, and the resulting solution was oxidized, with the anodic SOR coupled to the NO3RR, leading to complete sulfide removal, 90% NO3- reduction, and 87% of the converted nitrogen forming NH3. These findings establish MOF-derived CoSx as a promising platform for sustainable nitrogen and sulfur management, offering a scalable and energy-efficient approach for ammonia production and wastewater treatment.