Pyrolysed Co-N4 Macrocycles on Carbon Supports for the Efficient Electroreduction of CO2
Authors: Hamonnet Johan , Bennington Michael , Brooker Sally , Golovko Vladimir , Marshall Aaron Timothy 
Published: July 2022

Converting CO2 into materials such as chemicals or synfuels is an attractive way to mitigate greenhouse gas emissions while producing value-added products1-4. Even though the intrinsic chemical stability of CO2 significantly limits its reactivity, the use of electrocatalysts allows its efficient reduction in a mild environment and at relatively low overpotentials5,6. Cobalt phthalocyanine (CoPC), cobalt tetra-phenyl porphyrin (CoTPP) and Vitamin B12 (VB12) are well-defined Co-N4 macrocycles whose incorporation into heterogeneous catalyst setups have led to materials with outstanding CO2 electroreduction reaction (CO2ERR) properties, most notably in the selective formation of CO at high current densities7-11.

These Co-N4 macrocycles have also been used to manufacture catalysts efficient for the oxygen reduction reaction (ORR). Notably, the pyrolysis of the macrocyclic precursor supported by carbon materials allowed the fabrication of heterogeneous ORR catalysts with high loadings of catalytically active components, controlled local environments and enhanced stabilities12,13. While these properties could lead to these materials also being efficient catalysts for CO2ERR, to the best of our knowledge, such pyrolyzed catalysts have not been investigated for the CO2 reduction reaction.

In this work, we report materials based on CoPC, CoTPP and VB12, supported by carbon black and pyrolyzed under argon, as efficient catalysts for the CO2ERR. The catalytic precursors were dispersed on carbon powders with ball-milling before being annealed at different temperatures chosen from the positions of the significant steps and plateaus observed by thermogravimetric analysis of the pure precursors. The structures of the resulting materials were analyzed by powder X-ray diffraction and X-ray absorption spectroscopy. The electrochemical behaviour of these materials for the CO2ERR were characterized in CO2-saturated potassium bicarbonate (KHCO3) electrolyte using a custom-made flow cell at a range of potentials, and the products of the reaction were analyzed with gas chromatography, 1H NMR spectroscopy and high-performance liquid chromatography.

The high reactivity of the catalysts for the reduction of CO2 into CO was maintained for all materials treated at temperatures as high as 700 °C, even though the structures of their active sites were drastically different from that of the precursor molecules. The pyrolyzed materials also exhibited a change in correlation between the CO current densities and the KHCO3 concentration of the electrolyte, indicating that the reaction mechanism had changed compared to that of the pristine materials.

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