Electrocatalytic CO₂ Reduction with a Homogeneous Catalyst in Ionic Liquid: High Catalytic Activity at Low Overpotential

Abstract

We describe a new strategy for enhancing the efficiency of electrocatalytic CO2 reduction with a homogeneous catalyst, using a room-temperature ionic liquid as both the solvent and electrolyte. The electrochemical behavior of fac-ReCl(2,2'-bipyridine)(CO)3 in neat 1-ethyl-3-methylimidazolium tetracyanoborate ([emim][TCB]) was compared with that in acetonitrile containing 0.1 M [Bu4N][PF6]. Two separate one-electron reductions occur in acetonitrile (-1.74 and -2.11 V vs Fc(+/0)), with a modest catalytic current appearing at the second reduction wave under CO2. However, in [emim][TCB], a two-electron reduction wave appears at -1.66 V, resulting in a ∼0.45 V lower overpotential for catalytic reduction of CO2 to CO. Furthermore, the apparent CO2 reduction rate constant, kapp, in [emim][TCB] exceeds that in acetonitrile by over one order of magnitude (kapp = 4000 vs 100 M(-1) s(-1)) at 25 ± 3 °C. Supported by time-resolved infrared measurements, a mechanism is proposed in which an interaction between [emim](+) and the two-electron reduced catalyst results in rapid dissociation of chloride and a decrease in the activation energy for CO2 reduction.

Electrocatalysis of CO2 Reduction in Brush Polymer Ion Gels

“…These data contrast with a previous report studying a non-coordinating anion that generates a single reduction wave. 5 Collectively, these observations support the interpretation that ligand exchange is the underlying cause of multiple reduction processes, which is more probable for the complex in ionic liquid with a stronger coordinating anion. If the total current is used to find the diffusion coefficient for a two-electron transfer to all electroactive species, D = 1.8 × 10…”

Thermodynamic Aspects of Electrocatalytic CO2 Reduction in Acetonitrile and with an Ionic Liquid as Solvent or Electrolyte

“…43 We found that when neat [emim]-[TCB] (TCB = tetracyanoborate) is used as both the solvent and electrolyte for the electrocatalytic reduction of CO 2 to CO with the homogeneous catalyst, fac-ReCl(bpy)(CO) 3 (bpy =2,2′-bipyridine), the onset potential for catalytic CO 2 reduction is reduced by ∼0.45 V, compared to when CH 3 CN is used as a solvent with tetrabutylammonium hexafluorophosphate ([TBA][PF 6 ]) supporting electrolyte. 43 Importantly, we found that the rate constant for the CO 2 reduction reaction also increased by a factor of ∼40× in [emim][TCB] compared to in CH 3 CN.…”

“…43 We found that when neat [emim]-[TCB] (TCB = tetracyanoborate) is used as both the solvent and electrolyte for the electrocatalytic reduction of CO 2 to CO with the homogeneous catalyst, fac-ReCl(bpy)(CO) 3 (bpy =2,2′-bipyridine), the onset potential for catalytic CO 2 reduction is reduced by ∼0.45 V, compared to when CH 3 CN is used as a solvent with tetrabutylammonium hexafluorophosphate ([TBA][PF 6 ]) supporting electrolyte. 43 Importantly, we found that the rate constant for the CO 2 reduction reaction also increased by a factor of ∼40× in [emim][TCB] compared to in CH 3 CN. It is clear that the enhancement effect invoked by the imidazolium-based IL is different from enhancement effects previously reported in the presence of alkali or alkaline-earth metals; that is, these mono-or divalent metal cations can either improve the catalytic rate constant 44,45 or reduce the catalytic onset potential, 46 while the IL can do both simultaneously.…”

“…It is clear that the enhancement effect invoked by the imidazolium-based IL is different from enhancement effects previously reported in the presence of alkali or alkaline-earth metals; that is, these mono-or divalent metal cations can either improve the catalytic rate constant 44,45 or reduce the catalytic onset potential, 46 while the IL can do both simultaneously. 43 At this stage, the precise mechanisms of electrochemical CO 2 reduction reactions in the presence of ILs have not been fully clarified. In many cases, even the overall reaction has not been elucidated, making it difficult to determine which equilibrium potential should be used in order to deduce an overpotential for a given electrochemical system.…”

Roles of ion pairing on electroreduction of carbon dioxide based on imidazolium-based salts

“…Imidazolium-based salts, composed merely of ions, are a kind of novel medium for many chemical processes. In recent years, increasing attempts have been made to use imidazolium-based salts as the solvents, electrolytes and catalysts for CO 2 electroreduction [8][9][10][11][12]. In the salt 1-butyl-3-methyimidazolium tetrafluoroborate (BMIMBF 4 ), CO 2 was reduced on Ag electrode at À2.0 V (vs. Ag wire) at 50 C [13].…”