Single-atom catalysts (SACs) have opened up unprecedented possibilities for expediting oxygen reduction reaction (ORR) kinetics owing to their ultrahigh intrinsic activities. However, precisely controlling over the atomically dispersed metal-N_x sites on carbon support while fulfilling the utmost utilization of metal atoms remain the key obstacles. Here, atomically distributed Co-N₄ sites anchored on N-doped carbon nanofibers aerogel (Co SAs/NCNA) is controllably attained through a direct pyrolysis of metal-chelated cellulose nanofibers (TOCNFs-Cd²⁺/Co²⁺) hydrogel precursor. The usage of Cd salt assists the assembly of cross-linked aerogel, creates a large number of interior micropores and defects, and favors the physical isolation of Co atoms. The hierarchically porous biomass carbon aerogel (2265.1 m²/g) offers an advantageous platform to facilitate accessibility of the catalytic centers, also renders rapid mass diffusion and electron-transfer paths throughout its 3D architecture. Notably, Co SAs/NCNA affords a paramount ORR activity and respectable durability when integrated into zinc-air battery devices.