Low-Cost Nanomaterials for High-Performance Polymer Electrolyte Fuel Cells (PEMFCs) - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 367

Fig. 6 a Cyclic voltammograms (oxidation) of MWCNTs in the potential window from 0.1 to
0.7 V versus MMS in 0.5 M H 2 SO 4 using glassy carbon electrode at 100 mV/s scan rate.
b Cyclic voltammograms (reduction) of MWCNTs in the potential window from -0.1 to
-0.75 V versus MMS in 0.5 M H 2 SO 4 at 100 mV/s scan rate. Regions marked with a star
indicate the potentials at which the CNTs have been selectively oxidized or reduced (adapted
from Ref. [ 99 ] with permission from American Chemical Society)

precursors during the growth of nanotubes [ 100 - 104 ] or by annealing pre-oxidized
nanotubes in NH 3 at elevated temperatures [ 105 ]. The resulting catalysts exhibit
superior ORR activity in alkaline electrolytes [ 103 , 105 , 106 ] but very low activity
in acid electrolytes. Till date, majority of the nanotube-based ORR catalysts have
exhibited inferior activities compared with those formed with carbon black and
platinum/carbon in acidic solutions, due to availability of the relatively few cata-
lytic sites formed on the CNTs. One of the possible approaches to enhancing ORR
activity is to enrich defects and functional groups onto the CNTs by increasing the
number of catalytic sites. However, severe oxidation conditions could lead to
the loss of the structural integrity as well as their electrical conductivity which are
desirable for faster charge transport during electrocatalysis. Taken into account of
the above facts it is essential to identify a suitable protocol to afford abundant
catalytic sites on CNTs while retaining the structure and electrical conductivity for
producing advanced ORR electrocatalysts.
In connection with the above, Li et al. [ 107 ] developed a new type of ORR
electrocatalyst based on few-walled (two to three walls) carbon nanotube-graphene
(NT-G) complexes. They identified a unique oxidation condition to produce
abundant defects on the outer walls of the CNTs through partial unzipping of the
outer walls and the formation of large amounts of nanoscale graphene sheets,
attached to the intact inner walls of the nanotubes. The edge- and defect-rich
graphene sheets facilitate the formation of catalytic sites for ORR on annealing in
NH 3 . Iron impurities and nitrogen doping are found to be responsible for the high
ORR activity of the resulting NT-G complex catalyst. Indeed, in acidic solutions the
catalyst exhibits high ORR activity and superior stability, and in alkaline solutions
its ORR activity closely approaches that of platinum. They have also employed
annular dark-field (ADF) imaging and electron energy loss (EELS) spectrum
imaging in aberration-corrected scanning transmission electron microscopy