Mechanistic Basis of Conductivity in Carbon Dioxide-Expanded Electrolytes: A Joint Experimental–Theoretical Study

Abstract

Electrolyte conductivity contributes to the efficiency of devices for electrochemical conversion of carbon dioxide (CO2) into useful chemicals, but the effect of the dissolution of CO2 gas on conductivity has received little attention. Here, we report a joint experimental–theoretical study of the properties of acetonitrile-based CO2-expanded electrolytes (CXEs) that contain high concentrations of CO2 (up to 12 M), achieved by CO2 pressurization. Cyclic voltammetry data and paired simulations show that high concentrations of dissolved CO2 do not impede the kinetics of outer-sphere electron transfer but decrease the solution conductivity at higher pressures. In contrast with conventional behaviors, Jones reactor-based measurements of conductivity show a nonmonotonic dependence on CO2 pressure: a plateau region of constant conductivity up to ca. 4 M CO2 and a region showing reduced conductivity at higher [CO2]. Molecular dynamics simulations reveal that while the intrinsic ionic strength decreases as [CO2] increases, there is a concomitant increase in ionic mobility upon CO2 addition that contributes to stable solution conductivities up to 4 M CO2. Taken together, these results shed light on the mechanisms underpinning electrolyte conductivity in the presence of CO2 and reveal that the dissolution of CO2, although nonpolar by nature, can be leveraged to improve mass transport rates, a result of fundamental and practical significance that could impact the design of next-generation systems for CO2 conversion. Additionally, these results show that conditions in which ample CO2 is available at the electrode surface are achievable without sacrificing the conductivity needed to reach high electrocatalytic currents.

Entropy Drives Accelerated Ion Diffusion upon Carbon Dioxide Expansion of Electrolytes

“…On the other hand, D normalC O 2 increases in a more rapid, nonlinear fashion and both ionic species first show an initial increase as the solution becomes more dilute before plateauing. These increased mobilities coincide with a decrease in the shear viscosity of the solution as more CO 2 is added, as we have previously shown . In this context, it is important to note that the CO 2 concentration in the system is also increasing nonlinearly, indicating that the CO 2 mobility is enhanced by the greater fraction of like molecules in the solution.…”

“…Recently, Piskulich and Laird conducted molecular dynamics (MD) simulations on an acetonitrile (MeCN)-based CXE with lithium perchlorate salt to assess the impact of electrolyte concentration on CO 2 expansion and transport properties of the CXE . In our recent work, we combined experimental and theoretical approaches to quantify solution conductivity with CO 2 expansion, addressing a crucial limitation of CXEs: as more nonpolar CO 2 molecules are added, the solution conductivity diminishes . We obtained excellent agreement between the experimentally measured solution conductivity and values obtained from MD simulations for a MeCN-based CXE with a tetraalkylammonium hexafluorophosphate electrolyte.…”

“…At 3.4 bar, the simulation box has a volume of ∼83,000 Å 3 and contains just seven CO 2 molecules, while at 51.0 bar, the volume has increased nearly threefold and contains 1613 CO 2 molecules (see Table S5). At the highest pressure, the CXE is much more nonpolar, and the electrolytes are just below their solubility limit . In understanding and designing such CXE systems, it is critical to understand the driving forces that underlie their tunability via CO 2 pressure.…”

“…More recently, these approaches have been expanded to applications in electrochemistry through the development of CO 2 -expanded electrolytes (CXEs). CXEs can support CO 2 concentrations of up to 15 M at relatively mild pressures of 0.7–55.1 bar, making them highly relevant for CO 2 conversion processes. This is particularly advantageous for the electrochemical reduction of CO 2 , where it removes the, often significant, limitations of CO 2 delivery to the electrode. , While conventional CO 2 reduction in aqueous media requires close to ambient operating conditions (at which CO 2 concentration is 0.034 M) to avoid acidification, − CXEs overcome this limitation and achieve higher CO 2 reduction rates .…”

“…To develop design principles for CXEs to overcome the limitations of solution conductivity at high CO 2 concentrations, a comprehensive, molecular-level understanding of the factors influencing the dynamic properties is needed. In the present work, we address this issue by using our previously developed CXE model to investigate the energetics behind the ion-pairing behaviors that lead to decreased solution conductivity, with the goal of designing CXEs with electrolytes that minimize these interactions. More broadly, a fundamental study of the properties of CXEs with increasing CO 2 pressures is valuable for the further development and implementation of these media.…”