The electrochemical nitrite (NO₂⁻) reduction reaction (NO₂RR) is not only a promising strategy to degrade the harmful NO₂⁻ contaminant in the environment but an attractive alternative to the Haber–Bosch process for sustainable ammonia (NH₃) production. It is however hindered by low NO₂⁻-to-NH₃ conversion efficiencies and needs superb catalysts to drive the direct six-electron NO₂RR. Here, we report on developing cobalt nanoparticle decorated biomass Juncus derived carbon as a highly active NO₂RR catalyst with high durability for NH₃ electrosynthesis. This catalyst shows an NH₃ yield of 2.8 ± 0.1 mol h⁻¹ g_Co⁻¹ and a high faradaic efficiency of 96.9 ± 2.1% in alkaline media. The catalytic mechanism is further revealed by density functional theory calculations.