Enhanced CO₂ electroreduction using graphene-CoTMPyP coatings

The sharply rising level of atmospheric CO₂ is one of the largest environmental concerns facing our civilization today. CO₂ is the most significant greenhouse gas that has major effects on the environment such as global warming and ocean acidification. The conversion of CO₂ back to useful compounds is a critical goal that would restore balance to earth’s atmosphere. CO₂ reduction is possible through chemical catalysis, electrochemistry, photochemistry and biological processes. Chemical catalytic processes generally operate at high temperatures and pressures which lead to high energy cost. Electrochemical methods, however, operate at ambient conditions which offer a simple and effective route for CO₂ reduction.

The electrocatalytic capabilities toward CO₂ reduction of some cobalt porphyrins have been reported in the literature, although at considerable overpotentials and low current densities. The present work aims to assemble an electrocatalytic system composed of cobalt porphyrins and graphene derivatives. Spectroscopic, microscopic and electrochemical methods are used to analyze the interactions occurring between such porphyrins and graphene derivatives, and their effect on CO₂ reduction. Such self-assembled systems are formed between 5,10,15,20-Tetrakis(1-methyl-4-pyridinio) porphyrin (CoTMPyP) and graphene oxide (GO). This combination was electrodeposited on electrode surfaces, from aqueous solutions, using concessive cyclic voltammetry scans. Cross-section TEM-EELS images found dense distribution of CoTMPyP between the graphene sheets throughout the coating.

This arrangement of dense cobalt porphyrins between large conducting graphene sheets leads to enhanced electrocatalytic for CO₂ reduction vs. water reduction (1.35 and 0.27 mA/cm², respectively, at -1.2V vs. Ag/AgCl), as examined in aqueous 0.1 M Na₂CO₃ solution at pH 11.5. This enhanced electrocatalytic activity leads to the formation of CO in the gas phase and formate in the liquid phase with a faradaic efficiency of 36% for CO, 21% for formate and 22% for H₂.