Morphological Effects of Immobilized Re Tricarbonyl Catalysts for Aqueous CO2 Reduction

The renewable-energy-driven electrochemical reduction of CO2 offers a promising strategy for storing excess renewable energy in chemical bonds. Immobilized molecular catalysts are attractive platforms for this transformation due to their tunability, robustness, and compatibility with diverse catalytic environments. In this study, the positional dependence of two immobilized fac-tricarbonylchloro(2,2′-bipyridine)rhenium(I) (ReBpy) complexes is investigated in semiaqueous and fully aqueous media as a function of catalyst film loading. Cyclic voltammetry and controlled potential electrolysis reveal that electropolymerization-induced film growth strongly influences CO2 reduction reaction (CO2RR) activity through morphology-dependent transport effects. Both films exhibit high selectivity for CO production, with Faradaic efficiencies >98% and maximum turnover numbers reaching 327. Notably, tethering at the 4,4′-position produces dense, compact films, whereas tethering at the 5,5′-position yields more porous, multilayered structures that enhance ionic ingress and CO2 transport. However, excessive film growth results in diminished per-site activity due to transport limitations within the catalyst matrix. These findings demonstrate that positional tethering provides a strategy for tuning metallopolymer morphology and transport properties, offering new design principles for heterogenized molecular electrocatalysts for aqueous CO2 reduction.