Boosting CO2-to-CO Evolution Using a Bimetallic Diketopyrrolopyrrole Tethered Rhenium Bipyridine Catalyst

The use of homogenous 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 homogenous species, the bimetallic system effectively doubles the rate of CO2-to-CO conversion in the reaction-diffusion layer achieving a TOFCO = 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 hr-1), and longevity (active for up to 5 hours) for CO2-to-CO evolution from aqueous electrolyte.