Cobalt porphyrin immobilized on the TiO₂ nanotube electrode for CO₂ electroreduction in aqueous solution

Herein we report CO₂ electrochemical reduction reaction (CO₂ERR) on the cobalt tetraphenylporphyrin (CoTPP) modified TiO₂ nanotube (TNT) electrode. It was found the axial coordination of drop-casting solvent to CoTPP and the porphyrin structure are the major factors that have significant effects on the catalytic performance of the electrode. As confirmed by spectrophotometric titration, pyridine has a stronger coordination bond to CoTPP than DMF and THF thus leading to the highest efficiency among the drop-casting solvents tested in the study. Based on the spectrophotometric analysis, possible coordination mechanism between drop-casting solvents and CoTPP is put forward. On the other hand, introduction of –COOMe substituents in phenyl rings of CoTPP weakens the coordination bond between pyridine and CoTPP as clearly evidenced by deuterium NMR spectra, resulting in a detrimental effect on CO₂ERR. Therefore, the manipulation of the coordination environment around the metal center of immobilized catalyst is crucial in designing an efficient electrocatalytic system.