Origin of the Volcano Trend in the ORR Activity of N-Doped Graphene Supported on Carbon Substrates: The Dominant Role of Work Function Differences.

N-doped graphene (NG) is a promising electrocatalyst for the oxygen reduction reaction (ORR) at the cathodes of Polymer Electrolyte Fuel Cells (PEFCs). The NG catalyst is generally supported on an electrode substrate composed of carbon materials. However, the influence of the carbon substrate on the ORR performance of the NG catalyst has not been systematically explored. In this study, we have systematically investigated the impact of the carbon substrate on the ORR activity of the NG catalyst using first-principles calculations based on density functional theory. Specifically, we have examined the ORR activity of the NG catalyst on graphene-based substrates with a van der Waals (vdW) interface. It has been revealed that the difference in work function between the substrate and the NG catalyst dominates the ORR activity; the maximum electrode potential (UMax) depends linearly on the work function difference. Such a linear relationship is derived from the fact that the net charge of the NG catalyst scales linearly with the work function of the substrate. Furthermore, UMax was predicted to show a volcano trend with respect to the work function difference. This is because the reaction step that determines UMax switches due to the change in the free energies of reaction intermediates by charge transfer across a vdW gap, indicating an optimal work function difference that maximizes the ORR activity of the NG catalyst supported on graphene-based substrates. The establishment of experimental techniques to control the work function difference at the substrate/NG catalyst interface will be key to achieving superior catalytic performance.