Fe-N-doped　carbon　materials　(Fe-N-C)　are　promising　candidates　for　oxygen　reduction　reaction　(ORR)　relative　to　Pt-based　catalysts　in　proton　exchange　membrane　fuel　cells　(PEMFCs).　However,　the　intrinsic　contributions　of　Fe-N4　moiety　with　different　chemical/spin　states　(e.g.　D1,　D2,　D3)　to　ORR　are　unclear　since　various　states　coexist　inevitably.　In　the　present　work,　Fe-N-C　core–shell　nanocatalyst　with　single　low-spin　Fe(II)-N4　species　(D1)　is　synthesized　and　identified　with　ex-situ　ultralow　temperature　Mössbauer　spectroscopy　(T　=　1.6　K)　that　could　essentially　differentiate　various　Fe-N4　states　and　invisible　Fe-O　species.　By　quantifying　with　CO-pulse　chemisorption,　site　density　and　turnover　frequency　of　Fe-N-C　catalysts　reach　2.4　×　1019　site　g−1　and　23　e　site−1　s−1　during　the　ORR,　respectively.　Half-wave　potential　(0.915　VRHE)　of　the　Fe-N-C　catalyst　is　more　positive　(approximately　54　mV)　than　that　of　Pt/C.　Moreover,　we　observe　that　the　performance　of　PEMFCs　on　Fe-N-C　almost　achieves　the　2025　target　of　the　US　Department　of　Energy　by　demonstrating　a　current　density　of　1.037　A　cm−2　combined　with　the　peak　power　density　of　0.685　W　cm−2,　suggesting　the　critical　role　of　Fe(II)-N4　site　(D1).　After　500　h　of　running,　PEMFCs　still　deliver　a　power　density　of　1.26　W　cm−2　at　1.0　bar　H2-O2.　An　unexpected　rate-determining　step　is　figured　out　by　isotopic　labelling　experiment　and　theoretical　calculation.　This　work　not　only　offers　valuable　insights　regarding　the　intrinsic　contribution　of　Fe-N4　with　a　single　spin　state　to　alkaline/acidic　ORR,　but　also　provides　great　opportunities　for　developing　high-performance　stable　PEMFCs.　©　2024　Science　Press