Boosting CO2-to-CO evolution using a bimetallic diketopyrrolopyrrole tethered rhenium bipyridine catalyst

Abstract

The use of homogeneous electro- and photo-catalysis involving molecular catalysts offers valuable insight into reaction mechanisms as it relates to the structure–function of these tunable systems. However, supported molecular catalysts (i.e., hybrid electrodes) are multiplexed and not fully understood with regards to specific support–catalyst interactions. Even so, it still remains that catalyst activity for CO₂ electroreduction can be tuned by modifying specific functional groups to achieve performance enhancement. Herein, a series of derivatized [Re(bpy)(CO)₃Cl] catalysts were prepared with molecular structures having variability in both the number of Re-centers and π-conjugated diketopyrrolopyrrole (DPP) units. While tethering [Re(bpy)(CO)₃Cl] to the DPP unit had a negligible effect on molecular electro- and photo-catalyst properties in organic solvent, the DPP chromophore enabled facile coupling of two [Re(bpy)(CO)₃Cl] moieties. As a homogeneous species, the bimetallic system effectively doubles the rate of CO₂-to-CO conversion in the reaction–diffusion layer achieving a TOF_CO = 1000 s⁻¹ and FEco% = 98% for up to 6 hours of electrolysis as the two catalytic centers act independently. Immobilization onto carbon hybrid electrodes was found to evolve H₂, where the ratio of CO : H₂ produced during electrolysis depended on both the molecular structure of the catalyst and the additive(s) to the carbon surface used to suppress the hydrogen evolution reaction (HER). Introducing a commercial DPP-based polymer and/or colloid imprinted carbons (CICs) into carbon paper favours CO evolution from the catalyst by suppressing the HER at carbon and by dispersing the molecular catalyst across a larger more wettable surface to mitigate mass transport limitations. Once again, the bimetallic catalyst has the highest activity in comparison to the monometallic analogues, with a selectivity (FEco% = 53%), activity (TOF = 39 h⁻¹), and longevity (active for up to 5 hours) for CO₂-to-CO evolution from aqueous electrolyte.