N-doped and Fe-, N-codoped carbon: tuning of porous structures for highly efficient oxygen reduction reaction

The various heteroatom-doped carbons have emerged as a type of promising oxygen reduction reaction (ORR) catalysts to replace the expensive Pt-based ones. Normally, the carbon catalysts are constructed into porous structures, with one purpose to increase the specific surface areas and another to provide mass transfer channels, while, although the extensive reports on the porous structures, there is still deficient of a systematic investigation on the influence of detailed porous structures on the ORR performances. In this work, we prepared three different doped carbon materials, N-doped dual-porosity carbon, Fe-, N-codoped dual-porosity carbon, and Fe-, N-codoped macroporous carbon, to study this issue. Although some previous reports indicated that the mesopores may have negative effect to the electrochemical performance due to the complex tortuosity, our work elucidated their important contribution to the additional active sites. Simultaneously, increasing the macropore sizes of the catalysts from 100 to 500 nm would not make distinct performance enhancement, and the macropores with 100-nm sizes can basically satisfy the requirement of mass transfer. These results indeed supply a nice guidance for further design of highly efficient carbon-based ORR catalysts.