Targeting the Efficiency and Selectivity Challenges for Molecular Electrocatalysts that Reduce CO2 to Formic Acid

Currently, energy generation is one of the major global concerns for sustainable development. The large consumption of non-renewable fossil fuels has caused high atmospheric CO₂ concentrations, contributing to global warming as well as future energy shortages.¹ The conversion of atmospheric CO₂, a harmful greenhouse gas, to an energy source could mitigate both energy shortages as well as global warming.¹ Non-toxic HCOOH is one such energy source that can be easily transported.^2-4 However, CO₂ is a thermodynamically stable molecule that requires a large excess of energy to activate, and CO₂ reduction to HCOOH competes with CO formation and H₂ evolution.^5,6 For this reason, the efficiency and selectivity of CO₂ reduction to HCOOH is challenging. Studies have shown that the required energy to reduce CO₂ is significantly lower with the assistance of a proton source.⁷ The CO₂ binding mode and metal-hydride (M-H) intermediate are important to the selectivity of formate over CO and H₂, respectively.^5,6 The proposed CO₂ reduction at the active site of [NiFe] CO dehydrogenase suggests that the secondary coordination sphere can stabilize the CO₂-adduct to assist in CO₂ reduction. In addition, the Fe-S cluster can buffer electron transfer, minimizing energy consumption from geometry re-arrangement.¹ Therefore, this talk will discuss two efficient and selective molecular electrocatalysts that reduce CO₂ to formic acid under mild conditions. The Fe-based electrocatalyst, [Fe₄N(CO)₁₂]^-, focuses on the effects of M-H on the selectivity, and the Co-based electrocatalyst, [Co(P^Cy₂N^Bn₂)I]⁺, focuses on the effects of the secondary coordination sphere on the efficiency.^8,9 These results provide directions for the design of an ideal electrocatalyst: 1) Well-delocalized structure. 2) Secondary coordination sphere. 3) M-H-CO₂ binding mode. 4) Ligand substituents.^2,8