Flash joule heating-induced synthesis of biowaste-derived metal-free catalysts for nitrate reduction reaction

Nitrate pollution and mounting textile/biomass waste are coupled environmental challenges. This study assesses a flash Joule heating-induced (FJHI) route that converts ball-milled wool/biomass blends into sulfur-doped flash biochar (S@FB) using a layered "biochar-boat/feed" assembly under vacuum. Millisecond pulses (∼3200 °C) drive ultrafast carbonization and in-situ S incorporation, yielding an amorphous, defect-rich, microporous carbon with ∼2-4 nm mesopores and uniformly embedded S/O functionalities. Compared with undoped FB, S@FB shows ∼35 % higher electrochemically active surface area (7.10 vs 5.26 mF cm^-2) despite lower BET area. In 1 M KOH + 0.1 M KNO₃, S@FB delivers -110 mA cm^-2 at -1.0 V vs RHE, achieves a peak Faradaic efficiency of 90.7 % at -0.63 V, and reaches an NH₃ production rate of 259.41 μmol h^-1 cm^-2 at -0.93 V; ¹⁵N labeling confirms NO₃^- as the sole N source. In-situ Raman (-0.33 to -0.73 V) shows no irreversible changes, and 22 h chronoamperometry retains >95 % current with FE (88-93 %). Density-functional theory attributes the activity gain to S-induced local polarization and covalent SN interaction that stabilizes *NOH and lowers the *NO → *NOH barrier (1.512 → 1.148 eV). FJHI thus offers a rapid, waste-agnostic route to stable, metal-free NO₃RR catalysts.