How to chemically tailor metal-porphyrin-like active sites on carbon nanotubes and graphene for minimal overpotential in the electrochemical oxygen evolution and oxygen reduction reactions

Density functional theory calculations are used to study the energetics of the electrochemical oxygen evolution reaction (OER) of water and the reverse oxygen reduction reaction (ORR) on metal-porphyrin-like centers incorporated into graphene layers or single-walled carbon nanotubes (SWCNTs). The objective is to explore the reductions in computational thermodynamic overpotential that can be achieved relative to catalysis on metal oxide surfaces (OER) or platinum (ORR) by varying the metal center and axial ligand. This permits a degree of simultaneous control over the free energy gap between the lowest energy OH and highest energy OOH intermediates, and the position of the oxo (O) intermediate in this gap. Optimal choice of metal toward the right of the first transition series largely controls the gap. Given a suitable metal such as Fe, the overpotential for OER can be tuned over a range greater than 0.35 V by choice of the axial ligand. For OER occurring within the SWCNTs, a minimum predicted overpotential of 0.35 V is found, very close to the gap-imposed limit of 0.30 V for this system. Similarly, the overpotential of ORR can be tuned over a range more than 0.30 V by selection of the axial ligand. While the calculations necessarily have limited accuracy, the principles should provide a transferable path toward overpotential optimization for the OER and ORR.