An Iron Electrocatalyst for Selective Reduction of CO₂ to Formate in Water: Including Thermochemical Insights

C–H bond formation with CO₂ to selectively form products such as formate (HCOO^–) is an important step in harnessing CO₂ emissions as a carbon-neutral or carbon-negative renewable energy source. In this report, we show that the iron carbonyl cluster, [Fe₄N­(CO)₁₂]⁻, is an electrocatalyst for the selective reduction of CO₂ to formate in water (pH 5–13). With low applied overpotential (230–440 mV), formate is produced with a high current density of 4 mA cm^–2 and 96% Faradaic efficiency. These metrics, combined with the long lifetime of the catalyst (>24 h), and the use of the Earth-abundant material iron, are advances in catalyst performance relative to previously reported homogeneous and heterogeneous formate-producing electrocatalysts. We further characterized the mechanism of catalysis by [Fe₄N­(CO)₁₂]⁻ using cyclic voltammetry, and structurally characterized a key reaction intermediate, the reduced hydride [HFe₄N­(CO)₁₂]⁻. In addition, thermochemical measurements performed using infrared spectroelectrochemistry provided measures of the hydride donor ability (hydricity) for [HFe₄N­(CO)₁₂]⁻ in both MeCN and aqueous buffered solution. These are 49 and 15 kcal mol^–1, respectively, and show that the driving force for C–H bond formation with CO₂ by [HFe₄N­(CO)₁₂]⁻ is very different in the two solvents: +5 kcal mol^–1 in MeCN (unfavorable) and −8.5 kcal mol^–1 in water (favorable)