Thermodynamic targeting of electrocatalytic CO₂ reduction: advantages, limitations, and insights for catalyst design

Abstract

Herein is reported the electrocatalytic reduction of CO₂ with the complex [Ni(bis-NHC)(dmpe)]²⁺ (1) (bis-NHC = 1,l':3,3'-bis(1,3-propanediyl)dibenzimidazolin-2,2'-diylidene; dmpe = 1,2-bis(dimethylphosphino)ethane). The hydricity of 1 was previously benchmarked to be , equating to a driving force of a minimum of ∼3.4 kcal mol^-1 for hydride transfer to CO₂. While hydride transfer to CO₂ is thermodynamically favorable, electrocatalytic and infrared spectroelectrochemical (IR-SEC) experiments reveal that hydride transfer is blocked by direct reactivity with CO₂ in the reduced, Ni(0) state of the catalyst, yielding CO via reductive disproportionation (2CO₂ + 2e^- = CO + CO₃^2-) and concomitant catalyst degradation. Although thermodynamic scaling relationships provide guidance in catalyst targeting, the findings herein illustrate the fundamental kinetic challenges in balancing substrate reactivity and selectivity in the design of CO₂ reduction electrocatalysts. Advantages and limitations of this scaling relationship as well as approaches by which divergence from it may be achieved are discussed, which provides insight on important parameters for future catalyst design.