Density functional theory calculations of catalytic properties. (a) Surface Pourbaix diagrams and the configuration of Cu-N2C2 and Cu-N2C2-O. The values of onset potential are determined self-consistently. (b) Free energy for four-electron ORR on Cu-N2C2 and Cu-N2C2-O at zero electrode potential and onset electrode potential with RHE. (c) Simulated polarization curve on Pt(111), Cu-N2C2, and Cu-N2C2-O at 1600 rpm rotation speed.