First Principles Investigation of Covalently Immobilised Metalloporphyrins on Carbon Supports for the Electrochemical Reduction of CO2

Abstract

Metal-organic molecular catalysts used for CO2 electroreduction (CO2ERR) have recently been shown to exhibit remarkable CO2ERR performance (particularly when immobilised). One of the main challenges, however, is determining accurate entropy values of molecules in the aqueous-phase. We perform a bench marking study of different solvation entropy calculation methods for a set of 56 molecules ranging in size from H2 to tetrabutylammonium (C16H36N). Attention is paid to the cavity packing parameters which are important for calculating accurate cavity formation entropies. It is found that our developed approach provides the more accurate solvation entropy values. We also study the impact that intrinsic defects in graphene and carbon nanotube supports have on the CO2ERR reaction performance of cobalt-centred phthalocyanine (CoPc) and tetraphenylporphyrin (CoTPP) catalysts. We find that up-right immobilised CoPc on a Stone-Wales defect, and CoTPP on an octagon-pentagon line defect, respectively have the most favourable reaction pathways. The pyridine linker immobilising CoPc (via its Co active site) and the oxygen linker immobilising CoTPP are also studied with the CoPc-pyridine system demonstrating superior CO2ERR reaction pathway performance compared to all other systems considered. Finally, we perform a systematic study of the immobilisation of CoPc via its Co active site for eight different atomic-style linkers on a carbon nanotube support. The bonding between the linker and the Co active site can change the d-state ordering, shifting components of the dz2 states, which facilitate CO2 adsorption, to higher energies making them unoccupied and close to the Fermi level. This was found to be the case for the NH, S and PH linkers which exhibited the most favourable reaction free energy pathways compared to all other linker systems considered. We therefore propose that the NH, S and PH linkers would be promising candidates for future experimental studies.