Different Pathways for CO₂ Electrocatalytic Reduction by Confined CoTMPyP in Electrodeposited Reduced Graphene Oxide

Cobalt [5,10,15,20-(tetra-N-methyl-4-pyridyl) porphyrin] (CoTMPyP) was immobilized in reduced graphene oxide (rGO) films by a one-step electroreduction of a self-assembled emulsion of GO and CoTMPyP. These coatings showed remarkable stability for CO2 reduction to both CO and HCOO–, obtained in the gas and liquid electrolyte phases, respectively, with faradaic efficiencies of 45 and 24.3%, respectively, at −0.7 V vs RHE. The unexpected high production of formate and detection of H2 leads to the conclusion that a (Co hydride)porphyrin intermediate is involved in the process. It is suggested that carboxyl groups in the rGO coatings act as proton shuttles and promote the formation of such an intermediate. This species is stable enough in inner layers of these coatings to react with CO2 and produce formate, while it readily reacts with H2O (or H+) in outer layers facing the electrolyte to produce H2. These conclusions seem to be consistent with the effect of applied potential on the faradaic efficiencies of the products obtained for coatings consisting of CoTMPyP and GO with low and high carboxylic contents (∼2 and 5%, respectively). Effective catalytic CO2 reduction by electrodeposited graphene-porphyrin systems and the possibility to control its conversion to useful products both in the gas and liquid phases allow for considering the use of such composite materials in energy conversion devices.