Graphene Defects as Active Catalytic Sites that are Superior to Platinum Catalysts in Electrochemical Nitrate Reduction

We report that graphene defects serve as active catalytic sites for the electrochemical nitrate reduction reaction (NRR). A defective graphene (without foreign elements) electrode exhibits an onset potential of 0.95 V versus Ag/AgCl for the electrochemical NRR in 5m HNO3, which is 200 mV more positive than that exhibited by a platinum electrode. In addition, although the cathodic current of the platinum electrode during nitrate reduction decreases by 70% upon the addition of 100 mm methanol, the graphene electrode does not show any poisoning effects. The NRR on the graphene electrode shows autocatalytic behavior, including an induction period prior to the onset of the cathodic current. Moreover, the reduction reaction is accelerated by the addition of nitrite ions, and this indicates that nitrite is a key intermediate species in the autocatalytic mechanism. Density functional theory calculations reveal that a nitrosonium ion, which is an essential electrochemically active intermediate for the NRR, can strongly adsorb on the defect sites of a graphene sheet with electronic interaction, which indicates that the defect site can act as the active center for the NRR.