Nitrogen-doped graphene/carbon nanotube self-assembly for efficient oxygen reduction reaction in acid media

• Graphene/CNT self-assembly was prepared for oxygen reduction reactions.
• Presence of CNTs assists transfer of electrons and reactants to active sites.
• The assembly shows higher ORR performance than that of graphene or CNTs.
• The catalyst displays high stability with an enhanced 4-electron pathway.

Graphene, a two-dimensional layer structure of sp2-hybridized carbon, has garnered a great deal of attention as a promising material in electrochemistry. However, graphene has strong direction-dependent transport properties and is easily restacked to graphite; further development of graphene technology should thus be pursued for applications related to electrochemistry. Herein, a graphene/CNT self-assembly (GCA) was synthesized through the electrostatic interaction between graphene and CNTs, and was applied as a catalyst for oxygen reduction reactions (ORRs) in acid media after modification with N-doping. We demonstrated that the assembly with CNTs effectively increases the electric conductivity and hinders restacking of graphene layers, inducing facile transfer of electrons through CNTs and of reactants (e.g. oxygen and protons) through the interspace of graphene layers. The construction of highways for electrons and reactants on graphene layers resulted in 0.91 V onset potential and 2.13 mA/mg ORR activity at 0.75 V in acid media, representing significantly improved performance compared with that of catalysts derived from only graphene (0.86 V, 0.34 mA/mg) or CNTs (0.80 V, 0.02 mA/mg). In addition, the N-modified GCA shows much higher durability than that of only graphene, CNT or commercial Pt/C catalysts in severe operation conditions, with low production of peroxide in ORRs.

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