Electrochemical Methods for Assessing Kinetic Factors in the Reduction of CO₂ to Formate: Implications for Improving Electrocatalyst Design

Abstract

Thermochemical insights are often employed in the rationalization of reactivity and in the design of electrocatalysts for CO2 reduction reactions targeting C–H bond-containing products. This work identifies experimental methods for assessing kinetic aspects of reactivity. These methods are illustrated using [Fe4N­(CO)12]−, which produces formate from CO2 at −1.2 V versus SCE in either a MeCN/H2O solvent (95:5) or pH 6.5 buffered water. Elementary rates for each reaction step are identified along with the rate-determining step (RDS) as C–H bond formation. Transition state kinetics were determined from an Eyring analysis for the rate-determining C–H bond formation step using temperature-dependent electrochemical measurements. A lower measured ΔG ⧧ (298 K, 12.3 ± 0.1 kcal mol–1) in a pH 6.5 aqueous solution, compared with a ΔG ⧧(298 K) of 15.0 ± 0.1 kcal mol–1 in a MeCN/H2O solvent (95:5), correlates with faster observed reaction rates and provides a kinetic rationalization for the solvent-dependent chemistry. Taken together, the experimentally determined kinetic insights highlight that enhancement of the local concentration of CO2 at catalyst–hydride sites should be a primary focus of ongoing catalyst design. This will both enhance reaction rates and increase selectivity for C–H bond formation over competing H–H bond formation, because that step is fast in H2 evolution reactions.

Transition metal-based catalysts for the electrochemical CO2reduction: from atoms and molecules to nanostructured materials

“…The product distribution could be tuned in a controlled manner by modulating the applied potential, affording syngas mixtures with a CO : H 2 ratio of 1 : 1 or 1 : 2 at more negative potentials, likely due to the partial contribution of the less CO 2 RR selective Ni I/0 wave. 261 The majority of the non-heme Fe electrocatalysts studied for CO 2 RR displayed high or moderate faradaic efficiencies for formate production, including a highly selective iron carbonyl cluster catalyst 69,262,263 and Fe complexes with bipyridinecontaining Schiff base ligands, 264,265 phenanthroline derivatives 266 or macrocyclic nitrogen platforms. 260 Recently, a series of Fe II complexes supported by polydentate bipyridyl-based platforms bearing different functional groups in the second coordination sphere, were explored for CO 2 RR.…”

Controlling Hydrogen Evolution during Photoreduction of CO2 to Formic Acid Using [Rh(R-bpy)(Cp*)Cl]+ Catalysts: A Structure–Activity Study

“…[18][19][20][25][26][27][28][29][30] Product selectivity in CO 2 reduction is one of the most important issues to be addressed. [31][32][33][34][35][36] While in general molecular catalysis is limited to 2-electron reduction leading to formic acid and/or carbon monoxide, a major competing reaction is the reduction of protons into dihydrogen. 10,37 Optimization of the catalytic systems thus requires a better understanding of the factors which would limit the hydrogen evolution reaction.…”

“…37,38 Wellknown examples are Ru-polypyridyl, Mn(bpy)(CO) 3 and Rh(bpy=2,2′-bipyridine)(Cp*=pentamethylcyclopentadienyl) complexes, followed recently by Co(diphosphine)(Cp) complexes 30 and Fe-carbonyl clusters. [32][33][34][35] These systems most often give a mixture of HCOOH and H 2 , but the control of their selectivity has not been addressed in details.…”

Cobalt Carbonyl Clusters Enable Independent Control of Two Proton Transfer Rates in the Mechanism for Hydrogen Evolution

“…Equation 3 applies to the determination of rate constants for stoichiometric chemical steps immediately following an ET event under purely kinetic conditions. However, the method has also proven valuable to investigate the rate of chemical steps in more complicated mechanistic pathways (such as ECEC or ECCE catalysis, Scheme 2), [16,28,29,37] when the substrate concentration is kept low so that no catalytic turnover is observed on the timescale of the CV experiment. To minimize turnover we collected CV's at varied scan rate, 0.1 to 0.9 Vs À 1 , on solutions containing low concentrations of AnsdH + , 0.2 mM, and of 1 2À , 0.15 mM, and the 1 3À /4À redox couple remained reversible in this study.…”

“…[3,15] The reactivity of molecular electrocatalysts has also been shown to change with reaction conditions: two studies demonstrated that water as a solvent stabilizes transition states for hydride transfer to CO 2 and in those cases HER was effectively suppressed. [16,17,18] MCCs are atomically defined nanomaterials and this enables their precise characterization using molecular tools to understand elementary reaction steps and physical properties while harnessing many of the desirable reactivity characteristics of heterogeneous or nanomaterial electrocatalysts. Recent work demonstrated that the many metal-metal bonds in a cobalt carbonyl cluster statistically enhance PT rates to afford an overall turnover frequency (TOF) for HER of k obs = TOF = 1 × 10 8 s À 1 using [Co 13 C 2 (CO) 24 ] 4À (2 4À ) as catalyst with an applied potential of À 0.86 V vs. SCE (η = 760 mV and k obs is the observed rate constant).…”