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

C-H bond formation with CO, to selectively form products such as formate (HCOO-) is an important step in harnessing CO2, emissions as a carbon-neutral or carbonnegative renewable energy source. In this report, we show that the iron carbonyl cluster, [Fe4N(CO),(2)](-), is an electrocatalyst for the selective reduction of CO, to formate in water (pH 5 13). With low applied overpotential (230-440 mV), formate is produced with a high current density of 4 rnA cm(-2) and 96% Faradaic efficiency. These metrics, combined with the long lifetime of the catalyst (>24 h), and the use of the Earthabundant 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),2](-). In addition, thermochemical measurements performed using infrared spectroelectrochemistry provided measures of the hydride donor ability (hydricity) for [HFe4N(CO)(2)](-) in both MeCN and aqueous buffered solution. These are 49 and IS kcal me-1, respectively, and show that the driving force for C-H bond formation with CO, by [HFe4N(CO)(12)](-) is very different in the two solvents: +5 kcal mo1-1 in MeCN (unfavorable) and 8.5 kcal moll in water (favorable).