Hydrophobic molecular assembly at the gas-liquid-solid interface drives highly selective CO2 electromethanation

Abstract

The modularity of molecular catalysts enables the tuning of both active site and peripheral units to maximize functionality, thus rendering them as ideal model systems to explore fundamental concepts in catalysis. Hydrophobicity is often regarded as an undesirable aspect that hinders their dissolution in aqueous electrolytes. In contrast, we modified established Co terpyridine catalysts with hydrophobic perfluorinated alkyl side chains and took advantage of their hydrophobic character by utilizing them not as dissolved species in an aqueous electrolyte but at the gas-liquid-solid interfaces on a gas diffusion electrode (GDE) applied towards the electrochemical reduction of CO2. We found that the self-assembly of these perfluorinated units on the GDE surface results in a catalytic system selective for CH4 production, whereas every other Co terpyridine catalyst reported before was only selective for CO or formate. An array of mechanistic and operando spectroscopic investigations suggests a mechanism in which the pyridine units function as proton shuttles that deliver protons to the dynamic hydrophobic pocket in which CO2 reduction takes place. Finally, optimizing the system by integrating fluorinated carbon nanotubes as a hydrophobic conductive scaffold leads to a Faradaic efficiency for CH4 production above 80% at rates above 10 mA/cm2, thus far unprecedented for a molecular electrocatalytic system.

Electrocatalysis with molecules and molecular assemblies within gas diffusion electrodes

“…77 Co terpyridine catalysts functionalized with peruoroalkyl side chains also exhibited unique reactivity at the gas-liquidsolid interface found within GDEs. 78 While this catalyst produced CO as a solubilized homogeneous species in organic electrolytes, its selectivity shied towards CH 4 when immobilized on GDE surfaces. Analytical and spectroscopic experiments revealed that the ordered nature brought about by the peruoro-peruoro interactions led to a proton shuttling mechanism to hydrogenate the CO 2 R intermediates within a hydrophobic catalytic pocket (Fig.…”

“…74 Proton shuttling enables CO 2 R within a hydrophobic pocket within GDE electrodes (c) and shifts the reaction selectivity to CH 4 , with a selectivity of almost unity (d). 78 Reproduced with permission from the American Chemical Society, copyright 2022.…”

“…While most CO 2 R reactions with the systems above produce CO, with a few notable exceptions, 76,78,97,111 an upcoming challenge would be to develop additional strategies for C 2 (e.g., ethylene or ethanol) or more complex product generation (Fig. 7c).…”