Enhancing Molecular Electrocatalysis of CO2 Reduction with Pressure‐Tunable CO2‐Expanded Electrolytes

• Published in: ChemSusChem 2020-12, Vol.13 (23), p.6338-6345
• Main Authors: Sconyers, David J., Shaughnessy, Charles I., Lee, Hyun‐Jin, Subramaniam, Bala, Leonard, Kevin C., Blakemore, James D.
• Format: Article
• Language: English
• Subjects: Carbon dioxide; catalysis; Catalysts; CO2 expanded liquids; Electrocatalysts; electrochemistry; Electrolytes; Kinetics; Liquid phases; redox chemistry; renewable energy; Solubility
• ISSN: 1864-5631; 1864-564X
• DOI: 10.1002/cssc.202000390

Electrochemical studies of CO2 conversion by molecular catalysts are typically carried out in a narrow range of near‐ambient CO2 pressures wherein low CO2 solubilities in the liquid phase can limit the rate of CO2 reduction. In this study, five‐fold rate enhancements are enabled by pairing CO2‐expanded electrolytes (CXEs), a class of media that accommodate multimolar concentrations of CO2 in organic solvents at modest pressures, with a homogeneous molecular electrocatalyst, [Re(CO)3(bpy)Cl] (1, bpy=2,2′‐bipyridyl). Analysis of cyclic voltammetry data reveals pressure‐tunable rate behavior, with first‐order kinetics at moderate CO2 pressures giving way to zero‐order kinetics at higher pressures. The significant enhancement in the space‐time yield of CO demonstrates that CXEs offer a simple yet powerful strategy for unlocking the intrinsic potential of molecular catalysts by mitigating CO2 solubility limitations commonly encountered in conventional liquid electrolytes. Moreover, our findings reveal that 1, a workhorse molecular catalyst, performs with intrinsic kinetic behavior, which is competitive with fast enzymes under optimal conditions in CXEs. Don't starve: Studies of CO2 reduction by molecular electrocatalysis are often plagued by substrate starvation at the electrode surface, complicating the understanding of the intrinsic catalyst performance. Here, CO2‐rich media generated at modest pressures in acetonitrile are shown to improve the performance of a workhorse [Re(CO)3] molecular catalyst for CO2 reduction. Working at CO2 pressures slightly above 1 atm is shown to afford significant rate enhancements.