Developing catalytic materials for oxygen reduction reactions (ORR) with high performance and low cost has been one of the major challenges for large-scale applications of fuel cells. We demonstrated a high-yield " shape fixing via salt recrystallization" method to efficiently synthesize N-doped carbon nanomaterials with a high density of active sites, as an ORR catalyst. The process of shape-fixing involves pouring a supersaturated sodium chloride (NaCl) solution onto a 3D polyaniline (PANI) carbon-based polymer in a beaker, which results in the water evaporating and NaCl recrystallizing around the PANI until the PANI is fully covered by crystals. The NaCl crystal functions as a fully sealed nanoreactor which facilitates the N incorporation and graphitization. The gas from gasification in such a closed nanoreactor produces a large quantity of pores in resultant samples, and the carbonized PANI retains its original 3D shape due to previously being shape-fixed by the NaCl crystal. This makes it possible for the active sites (activated by planar pyridinic and pyrrolic N atoms with sp² hybrid orbitals, which only appear at the outside fringe of a graphene sheet) to appear in large quantities on the edges of numerous pores. Such favorable catalyst structure leads to the availability of efficient mass transport pathways and a high utilization of the active sites in fuel cell. It results in an excellent ORR activity with a half-wave potential only 58 mV behind that of Pt/C in an acidic medium. The proton-exchange membrane fuel cells with the CPANI-Fe-NaCl-catalyzed cathode outputs a peak power of 600 mW cm^-2, which is among the best for non-precious metal catalysts reported so far for the ORR. Additionally, no other method is comparable to this invention, in terms of its high yield per batch of N-doped carbon catalysts. The “shape fixing via salt recrystallization” method represents a powerful approach for the preparation of high-performance carbon nanostructures by confined pyrolysis of nanopolymers and, more importantly, provides insight into the design of advanced PEMFC cathode catalysts.