A sustainable Iron (Fe), Nitrogen (N) co-doped high performance Fe-N-x/C electrocatalyst for oxygen reduction reaction (ORR) is synthesized simply based on nitric acid oxidation of cheap carbon black. The obtained optimal nonprecious metal electrocatalyst shows high ORR performance in both alkaline and acidic conditions and possesses appreciable performance/price ratio due to its low cost. Furthermore, the structure-activity relationship of different active sites on Fe-N-x/C is revealed systematically: Fe-N-4/2-C > Fe-4-N-C > N-C >> Fe-4-C <= C, from both experimental and theoretical points of view. (C) 2015 Elsevier B.V. All rights reserved.