Nitrogen-Deficient ORR Active Sites Formation by Iron-Assisted Water Vapor Activation of Electrospun Carbon Nanofibers

Abstract

Fe- and N-modified carbon nanofibers (Fe-CNF) were synthesized via electrospinning and pyrolysis as electrocatalysts for oxygen reduction reaction (ORR). In order to increase the exposed surface area with the active sites buried inside Fe-CNF, we attempted water vapor activation for Fe-CNF and observed a substantial improvement of ORR activity up to the comparable level with Pt/C. Unlike what was expected, however, water vapor activation did not significantly increase the specific surface area of Fe-CNF; instead, it induced a depletion of surface N content, which makes it difficult to explain the improved ORR activity with the increase of surface area with N-based active sites. In water vapor activation, the chemical phase of embedded particles is changed from Fe3C to Fe3O4 and nitrogen-free Fe- and C-based ORR active sites were exposed, which seemed to be related with hierarchical macro/mesopore structure and graphitic edge defects. This study demonstrates a facile activation method for better ORR activity of Fe-modified CNF and suggests a potential relationship of surface carbon structure with the catalytic activity toward ORR rather than the type and concentration of N in Fe-CNF, which should be investigated further. © 2016 American Chemical Society.