Tuning the catalytic activity of graphene nanosheets for oxygen reduction reaction via size and thickness reduction.

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 nanometers were 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.