Properties of a Co-Porphyrin 2D Covalent Monolayer on Au(111) for the Oxygen Reduction and Evolution Reaction

Metal-porphyrins are well-known and highly active catalysts for electrochemical reactions such as the oxygen reduction and evolution reactions. Herein, we investigate and quantify the catalytic effects of covalently coupling a network of Co-porphyrins on a Au(111) surface, examining advantages in structural rigidity for the porphyrin-based frameworks and possible cooperative catalytic effects between active single-atom Co species within the network. Monolayers of covalent organic frameworks based on Co-porphyrins are synthesized via an Ullmann-type coupling reaction and characterized using scanning tunneling microscopy, X-ray spectroscopy, and voltammetry. Our findings reveal that for the oxygen reduction reaction, covalent coupling of the Co-porphyrins does not lead to measurable quantitative changes in electrocatalytic activity compared with uncoupled systems. In both cases after oxygen reduction reaction the Co-coordination to N in the porphyrin remains intact in spectroscopic studies, but atomic resolution images reveal that improved structural stability is gained for the Co-porphyrin network, attributed to suppressed dissolution of the porphyrins. At oxidizing potentials relevant for the oxygen evolution reactions, however, covalent coupling does not enhance stability, and the porphyrins decompose, resulting in the formation of Co oxide. Our study indicates that electrode materials functionalized with porphyrin-based covalent organic frameworks lead to stabilized catalysts without affecting their high activity for the oxygen reduction reaction.