Enhancing Molecular Electrocatalysis of CO₂ Reduction with Pressure‐Tunable CO₂‐Expanded Electrolytes

Abstract

Electrochemical studies of CO2 conversion by molecular catalysts are typically carried out in a narrow range of near‐ambient CO2 pressures wherein low CO2 solubilities in the liquid phase can limit the rate of CO2 reduction. In this study, five‐fold rate enhancements are enabled by pairing CO2‐expanded electrolytes (CXEs), a class of media that accommodate multimolar concentrations of CO2 in organic solvents at modest pressures, with a homogeneous molecular electrocatalyst, [Re(CO)3(bpy)Cl] (1, bpy=2,2′‐bipyridyl). Analysis of cyclic voltammetry data reveals pressure‐tunable rate behavior, with first‐order kinetics at moderate CO2 pressures giving way to zero‐order kinetics at higher pressures. The significant enhancement in the space‐time yield of CO demonstrates that CXEs offer a simple yet powerful strategy for unlocking the intrinsic potential of molecular catalysts by mitigating CO2 solubility limitations commonly encountered in conventional liquid electrolytes. Moreover, our findings reveal that 1, a workhorse molecular catalyst, performs with intrinsic kinetic behavior, which is competitive with fast enzymes under optimal conditions in CXEs.

Organic Electrosynthesis in CO2-eXpanded Electrolytes: Enabling Selective Acetophenone Carboxylation to Atrolatic Acid

“…Gas chromatography analysis of the headspace after bulk electrolysis showed only trace gas-phase products (e.g., H 2 or CO), revealing total faradaic efficiencies that are less than unity (Supporting Information Table S1). Interestingly, we also observed similar faradaic efficiencies for CO 2 reduction to CO in our previous work using the same reactor setup (74% on Au and 70% with Re­(CO) 3 (bpy)­Cl). This was attributed to the use of a single compartment electrochemical cell, which may allow some reduced species formed at the working electrode to be potentially oxidized at the counter electrode (or vice versa) resulting in lower faradaic yields.…”

“…At headspace pressures exceeding 4.2 MPa, we observed an attenuation in the (±)-atrolactic acid production rate (Figure ). Similar to our previous studies on electrochemical CO 2 reduction, − we attribute the decrease in rate to a decrease in the polarity of the electrolyte as the liquid-phase CO 2 concentrations approach that of pure liquid CO 2 . Gas chromatography analysis of the headspace after bulk electrolysis showed only trace gas-phase products (e.g., H 2 or CO), revealing total faradaic efficiencies that are less than unity (Supporting Information Table S1).…”

“…12−14 We refer to these as CO 2 -eXpanded electrolytes because the liquid-phase volume expands with increasing CO 2 pressure (from 1.4−5.5 MPa) due to the dissolution of CO 2 (Supporting Information, Figure S1). In previous work, we observed (1) an order-of-magnitude enhancement of the catalytic current for CO 2 reduction to CO on heterogeneous catalysts (polycrystalline gold and copper), 12,13 (2) a significant enhancement of the catalytic rate for homogeneous (Re(CO) 3 (bpy)Cl (bpy = 2,2 -bipyridyl)) catalysts, 14 In this work, we demonstrate that the favorable properties of CXEs enhance the rate and selectivity of high-carbon content products via electrochemical carboxylation reaction pathways. Electrochemical carboxylation is an alternative approach to produce high-carbon-content carboxylic acids via the electrochemical coupling of CO 2 with organic compounds.…”

“…Recently, we demonstrated that organic solvent-based CO 2 -eXpanded electrolytes (CXEs) can dissolve multimolar amounts of liquid CO 2 at moderate pressures while retaining sufficient supporting electrolyte concentrations to facilitate electrochemistry. − We refer to these as CO 2 -eXpanded electrolytes because the liquid-phase volume expands with increasing CO 2 pressure (from 1.4–5.5 MPa) due to the dissolution of CO 2 (Supporting Information, Figure S1). In previous work, we observed (1) an order-of-magnitude enhancement of the catalytic current for CO 2 reduction to CO on heterogeneous catalysts (polycrystalline gold and copper), , (2) a significant enhancement of the catalytic rate for homogeneous (Re­(CO) 3 (bpy)Cl (bpy = 2,2 -bipyridyl)) catalysts, and (3) a maximum in the electrocatalytic current at intermediate CO 2 pressures of ∼3 MPa.…”

Molecular Simulations of Phase Equilibria and Transport Properties in a Model CO2-Expanded Lithium Perchlorate Electrolyte

“…Carbon-dioxide-expanded liquids (CXLs), a class of solvents in which a significant mole fraction of carbon-dioxide (CO 2 ) is dissolved in an organic solvent under moderate-to-high partial CO 2 partial pressures, have attracted significant and growing interest as catalytic reaction media. − CXLs have been shown experimentally to provide the advantages of both traditional organic solvents and super-critical fluids, while reducing many disadvantages of both media. − Specifically, CXLs have been seen to have pressure-tunable solubility of CO 2 , a volume expansion effect that can result in up to a five-fold reduction in the amount of solvent, as well as a decrease in viscosity and an increase in diffusion. ,− …”