Secondary-Sphere Effects in Molecular Electrocatalytic CO2 Reduction

Abstract

The generation of fuels and value-added chemicals from carbon dioxide (CO₂) using electrocatalysis is a promising approach to the eventual large-scale utilization of intermittent renewable energy sources. To mediate kinetically and thermodynamically challenging transformations of CO₂, early reports of molecular catalysts focused primarily on precious metal centers. However, through careful ligand design, earth-abundant first-row transition metals have also demonstrated activity and selectivity for electrocatalytic CO₂ reduction. A particularly effective and promising approach for enhancement of reaction rates and efficiencies of molecular electrocatalysts for CO₂ reduction is the modulation of the secondary coordination sphere of the active site. In practice, this has been achieved through the mimicry of enzyme structures: incorporating pendent Brønsted acid/base sites, charged residues, sterically hindered environments, and bimetallic active sites have all proved to be valid strategies for iterative optimization. Herein, the development of secondary-sphere strategies to facilitate rapid and selective CO₂ reduction is reviewed with an in-depth examination of the classic [Fe(tetraphenylporphyrin)]⁺, [Ni(cyclam)]²⁺, Mn(bpy)(CO)₃X, and Re(bpy)(CO)₃X (X = solvent or halide) systems, including relevant highlights from other recently developed ligand platforms.

Keywords: CO2; electrocatalysis; inorganic; molecular; secondary-sphere.