Quantifying the Specific Activity of Graphene Edges and Unveiling the Roles of Topological Defects for Oxygen Reduction Reaction

Abstract

Graphene edges and topological defects are two important active sites in metal-free carbon-based electrocatalysts for various energy-related reactions such as the oxygen reduction reaction (ORR). However, quantitative measurement and exploration of the atomic origin of the activities of these defects remain challenging, largely due to the structural complexity and heterogeneity of carbon-based electrocatalysts. Their ORR selectivity toward either the two-electron pathway producing hydrogen peroxide or the four-electron pathway leading to water also remains controversial. Herein, we fabricate well-defined model electrocatalysts via chemical vapor deposition of nanocrystalline graphene domains with tunable edge densities. The ORR activities are determined to be dominated by the two-electron pathway producing hydrogen peroxide and scale linearly with the edge density, enabling the quantification of the specific activity per unit edge length. Using atomic-resolution differential phase contrast scanning transmission electron microscopy, we identified the favorable formation of topological defects at the edges, with significant fluctuations in localized electron states, providing an important atomic origin for their enhanced ORR activity. This was corroborated by the deliberate introduction of topological defects by Ar plasma treatment, leading to two-dimensional amorphous carbon and significantly enhanced two-electron ORR activity.