Catalytic electroreduction of CO2 to methanol on cobalt phthalocyanines and the effect of macrocycle functionalization

Converting CO2 to methanol electrochemically (CO2 + 6H+ + 6e-  CH3OH + H2O) is an attractive technology, as methanol is a pivotal chemical compound for the chemical industry. However, producing methanol through electrocatalytic CO2 reduction (CO2RR) remains a challenge, as few efficient electrocatalysts have been reported for this process. [1] Recent studies have shown improved methanol productivity with cobalt phthalocyanines (CoPc) dispersed on carbon nanotubes, leading to faradaic efficiencies towards methanol in the 10-30% range at –1 V vs. the reversible hydrogen electrode (RHE).[2-3] While these results are indeed promising, the reported overpotentials remain high and little has been reported regarding the influence of the chemistry of the macroligand on the electrocatalytic performances. Here we propose to address these shortcomings by functionalizing the reference CoPc with four electron-donating n-butoxy groups. Functionalization of the macroligand leads to a reduction in overpotential: the highest methanol yield is obtained at –1.2 V vs RHE for CoPc, while it is shifted to –1.0 V vs. RHE for (nBuO)4CoPc. Moreover, by carefully controlling the loading of (nBuO)4CoPc on CNTs, the methanol faradaic efficiency increased from 10% to over 15% in our most efficient system, while no such improvement could be achieved on unsubstituted CoPc. We attribute this increase to the possibility of loading a higher amount of (nBuO)4CoPc without molecular stacking at the surface of the CNTs compared to CoPc, which is enabled by the steric hinderance brought by the alkyl chains.This work demonstrates a promising molecular engineering strategy to significantly boost CO2RR performance for methanol production. It paves the way for a new generation of efficient molecular electroacatalysts capable of increasingly higher selectivity and activity for this challenging reaction.The ERANET-ACT3 NEXTCCUS project (ANR-21-ACT3-0001) funding is greatly acknowledged.1 A. Bagger et al., ChemPhysChem 18, 3266, 20172 Y.S. Wu et al., Nature 575, 639, 20193 E. Boutin et al., Angew. Chem. Int. Ed. 58, 16172, 2019