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 (CoS_x) 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, CoS_x 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 CoS_x 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 CoS_x as a promising platform for sustainable nitrogen and sulfur management, offering a scalable and energy-efficient approach for ammonia production and wastewater treatment.