Steering dynamic active hydrogen with sulfur-deficient Co9S8 nanosheets for nitrite-to-ammonia electroreduction at ultralow energy consumption

Electrocatalytic nitrite (NO 2 − ) reduction reaction (NO 2 RR) has emerged as a promising sustainable route for ammonia (NH 3 ) synthesis. The reaction efficiency critically depends on maintaining adequate active hydrogen (*H) supply to facilitate the hydrogenation process. Herein, we employ sulfur vacancy (V S )-mediated defect engineering on cobalt sulfides (Co 9 S 8 ) to regulate *H dynamics and enhance NO 2 RR performance. The V S -induced charge delocalization effect shifts the d-band center of Co-sites closer to the Fermi level, strengthening *H adsorption and activation. The Co 9 S 8 /CF-V S catalyst demonstrates exceptional performance, achieving an NH 3 production rate of 8.13 mg h −1 cm −2 with a Faradaic efficiency of 97.27% at −0.3 V vs reversible hydrogen electrode (RHE). The underlying mechanism reveals that the exceptional water dissociation capability and substantial surface *H enrichment collectively facilitate NO 2 RR kinetic. Coupling the thermodynamically favorable sulfion oxidation reaction (SOR) with NO 2 RR achieves an ultra-low energy consumption of 6.3 kWh kg −1 NH 3 at 50 mA cm −2 . A proof-of-concept Zn-NO 2 − battery self-powered electrolysis system utilizing the bifunctional Co 9 S 8 /CF-V S is proposed for simultaneous ammonia production and sulfur recovery. • Sulfur-deficient Co 9 S 8 nanosheets is facilely fabricated. • Co 9 S 8 /CF-V S demonstrates exceptional performance for NO 2 − -to-NH 3 conversion. • Dynamic active hydrogen plays critical role for nitrite hydrogenation. • SOR is introduced to couple with NO 2 RR for energy-saving NH 3 synthesis. • Value-added NH 4 Cl and sulfur are produced.