Iron-nitrogen-carbon　materials　have　been　known　as　the　most　promising　non-noble　metal　catalyst　for　proton-exchange　membrane　fuel　cells　(PEMFCs),　but　the　genuine　active　sites　for　oxygen　reduction　reaction　(ORR)　are　still　arguable.　Herein,　by　the　thorough　density　functional　theory　investigations,　we　un...