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).C–H键与二氧化碳（CO₂）的成键反应可选择性生成甲酸根（HCOO–）等产物，是将CO₂排放物转化为碳中和或碳负性可再生能源的关键步骤。本研究表明，羰基铁簇合物[Fe₄N(CO)₁₂]⁻是一种可在水相体系（pH 5~13）中将CO₂选择性还原为甲酸根的电催化剂。在较低的施加过电位（230~440 mV）下，该催化体系可实现4 mA cm⁻²的高电流密度，且甲酸根生成的法拉第效率高达96%。上述性能指标，结合催化剂长达24小时以上的使用寿命，以及使用地球储量丰富的铁作为活性金属，相较于此前报道的均相、非均相甲酸根生成电催化剂，本研究实现了催化性能的显著提升。 我们进一步通过循环伏安法对[Fe₄N(CO)₁₂]⁻的催化机制进行了表征，并对关键反应中间体——还原态氢化物[HFe₄N(CO)₁₂]⁻完成了结构解析。此外，我们利用红外光谱电化学法开展热化学测量，测定了[HFe₄N(CO)₁₂]⁻在乙腈（MeCN）与水相缓冲溶液中的氢化物给体能力（氢亲合性，hydricity）。二者的氢亲合性分别为49 kcal mol⁻¹与15 kcal mol⁻¹，这一结果表明，[HFe₄N(CO)₁₂]⁻与CO₂发生C–H成键反应的驱动力在两种溶剂中差异显著：在乙腈中为+5 kcal mol⁻¹（热力学不利），在水相中则为−8.5 kcal mol⁻¹（热力学有利）。