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-Nx　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.　©　2022　Elsevier　Inc.