  Currently, the fundamental factors that control the oxygen reduction reaction
(ORR) activity of graphene itself, in particular the dependence of the ORR
activity on the number of exposed edge sites remain elusive, mainly due to
limited synthesis routes of achieving small size graphene. In this work, the
synthesis of low oxygen content (< 2.5 +/-0.2 at %), few layer graphene
nanosheets with lateral dimensions smaller than a few hundred nm was achieved
using a combination of ionic liquid assisted grinding of high purity graphite
coupled with sequential centrifugation. We show for the first time, that the
graphene nanosheets possessing a plethora of edges exhibited considerably
higher electron transfer numbers compared to the thicker graphene
nanoplatelets. This enhanced ORR activity was accomplished by successfully
exploiting the plethora of edges of the nanosized graphene as well as the
efficient electron communication between the active edge sites and the
electrode substrate. The graphene nanosheets were characterized by an onset
potential of -0.13 V vs. Ag/AgCl and a current density of -3.85 mA/cm2 at -1 V,
which represent the best ORR performance ever achieved from an undoped carbon
based catalyst. This work demonstrates how low oxygen content nanosized
graphene synthesized by a simple route can considerably impact the ORR
catalytic activity and hence it is of significance in designing and optimizing
advanced metal-free ORR electrocatalysts.
