Entropy Drives Accelerated Ion Diffusion upon Carbon Dioxide Expansion of Electrolytes.

Carbon dioxide-expanded liquids, organic solvents with high concentrations of soluble carbon dioxide (CO2) at mild pressures, have gained attention as green catalytic media due to their improved properties over traditional solvents. More recently, carbon dioxide-expanded electrolytes (CXEs) have demonstrated improved reaction rates in the electrochemical reduction of CO2, by increasing the rate of delivery of CO2 to the electrode while maintaining facile charge transport. However, recent studies indicate that the limiting behavior of CXEs at higher CO2 pressures is a decline in solution conductivity due to reduced polarity, leading to poorer charge screening and greater ion pairing. In this article, we employ molecular dynamics simulations to investigate the energetic driving forces behind the diffusive properties of an acetonitrile and tetrapropylammonium hexafluorophosphate (TPrAPF6) CXE with increasing CO2 concentration. Our results indicate that entropy drives solvent and electrolyte diffusion with increasing CO2 pressure. The activation energy of ion diffusion increases with higher concentrations of CO2, indicating that increasing the temperature may improve solution conductivity in these systems. This trend in the activation energies is traced to stronger cation-anion Coulombic interactions due to weaker solvent screening at high CO2 concentrations, suggesting that the choice of ion may provide a route to diminish this effect.

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

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.

Distinguishing the mechanism of electrochemical carboxylation in CO2 eXpanded Electrolytes.

We shed light on the mechanism and rate-determining steps of the electrochemical carboxylation of acetophenone as a function of CO2 concentration by using a robust finite element analysis model that incorporates each reaction step. Specifically, we show that the first electrochemical reduction of acetophenone is followed by the homogeneous chemical addition of CO2. The electrochemical reduction of the acetophenone-CO2 adduct is more facile than that of acetophenone, resulting in an Electrochemical-Chemical-Electrochemical (ECE) reaction pathway that appears as a single voltammetric wave. These modeling results provide new fundamental insights into the complex microenvironment in CO2-rich media that produces an optimum electrochemical carboxylation rate as a function of CO2 pressure.

Electrochemical properties and C-H bond oxidation activity of [Ru(tpy)(pyalk)Cl]+ and [Ru(tpy)(pyalk)(OH)].

[Ru(tpy)(pyalk)Cl]Cl (pyalk = 2-(2'-pyridyl)-2-propanol) was synthesized and characterized crystallographically and electrochemically. Upon dissolution in water and acetonitrile, [Ru(tpy)(pyalk)Cl]Cl was found to form [Ru(tpy)(pyalk)Cl]+ and [Ru(tpy)(pyalk)(OH)]+, respectively. The Ru(ii/iii) couple of [Ru(tpy)(pyalk)Cl]+ was found to be relatively low compared to that of other Ru complexes in acetonitrile, but the Ru(iii/iv) couple was not significantly different than other Ru complexes bearing anionic ligands. Pourbaix diagrams were generated for [Ru(tpy)(phpy)(OH2)]+ (phpy = 2-phenylpyridine) and [Ru(tpy)(pyalk)(OH)]+ in water, and it was found that [Ru(tpy)(pyalk)(OH)]+ has a lower Ru(ii/iii) potential than [Ru(tpy)(phpy)(OH2)]+ under neutral to alkaline pH. [Ru(tpy)(pyalk)(OH)]+ was found to catalyze C-H bond hydroxylation of secondary alkanes and epoxidation of alkenes using cerium(iv) ammonium nitrate as the primary oxidant.