Fe nanoparticle entrained in tubular carbon nanofiber as an effective electrode material for metal–air batteries: A fundamental reason

Fe nanoparticle entrained in tubular carbon nanofiber (TCNF) as an effective electrode material for metal–air batteries was successfully prepared. TEM and STM investigations revealed that during impregnation Fe precursor ions penetrated into TCNF tubes through factures between the structural units of TCNF, which were absent in commercial carbon nanotube (CCNT). Thus the location of Fe nanoparticles depended on carbon materials: they were in the TCNF tubes and on the surfaces of CCNT. During repeated voltammetric cycles, the Fe nanoparticles in the TCNF tubes evolved to elongated oxygen-lean Fe nanoparticles, while those on CCNT did to oxygen-rich Fe oxide dendrites. Voltammograms of the modified Fe nanoparticles revealed that a higher overpotential was needed for dendrite formation. On the other hand, impedance spectroscopy also disclosed that the mass transfer of soluble Fe species was much faster in the one-dimensional TCNF tubes than in the three-dimensional open spaces between individual CCNTs. Thus, a low overpotential of the redox process of the elongated Fe nanoparticles and the fast mass transfer of soluble Fe species in the TCNF tubes resulted in an increased amount of Fe involved in charging/discharging process, which accounted for the higher efficiency of Fe nanoparticles on TCNF in Fe-air battery system.