Aqueous CO2 electroreduction with amine-decorated manganese bipyridine complexes immobilized on carbon nanotubes

Abstract

Developing selective and durable catalysts is essential for CO₂ conversion into value-added chemicals and fuels. Hybrid catalysts, formed by anchoring molecular catalysts onto electrode surfaces, retain the advantages of molecular catalysis while enabling reactions in benign aqueous conditions. In this work, a series of manganese bipyridine complexes adorned with varying numbers of diethylamine pendants (zero to four) were immobilized on multi-walled carbon nanotubes (MWCNT) to serve as hybrid catalysts for electrochemical CO₂ conversion. The effects of applied potential and catalyst surface loading were thoroughly investigated. At an optimal potential of −0.67 V vs. reversible hydrogen electrode, the catalysts with one and two amine pendants demonstrated excellent performance in converting CO₂ to HCOOH, achieving faradaic efficiencies of 63% and 72%, respectively. Furthermore, the catalyst with two diethylamine pendants exhibited a notable partial current density of 13.4 mA cm⁻² for HCOOH generation at this specific potential. A turnover number of 20 430 was achieved over 8 h of operation at this potential. Using surface-sensitive attenuated total reflection infrared spectroelectrochemistry, a manganese dimer species was identified in the manganese bipyridine complex without diethylamine pendants, while catalyst leaching was observed for that with diethylamine pendants, providing mechanistic insight into their selectivity. Compared to the homogeneous performance of these four complexes in acetonitrile, this work highlights the effectiveness of molecular catalysts for CO₂ conversion in aqueous conditions while maintaining consistent selectivity when paired with MWCNT.