Oxygen reduction and evolution at single-metal active sites: Comparison between functionalized graphitic materials and protoporphyrins

► Square-planar complexes in carbon networks are active ORR/OER electrocatalysts. ► The oxidation state of the metal atom in these complexes is normally + 2. ► There is an intrinsic ORR/OER overpotential due to inappropriate adsorption. ► This overpotential may be reduced by hydrogen bonds and charge transfer.