(393b) Formation of Nanoscale Ionic Liquid Clusters to Dictate the Electrostatic Screening Length Via Molecular Dynamics

Ionic liquids (ILs), such as 1-ethyl-3-methylimidazolium tetrafluoroborate (EMim BF₄), have been scrutinized as viable electrolytes for electrochemical reactions due to their highly tunable cation/anion properties, intrinsic conductivity, wide electrochemical stability window, low volatility, and strong electrostatic interactions [1-3]. When mixed with organic solvents, ILs have been proposed to form nanoscale clusters that lead to unexpected effects on electrostatic screening. For example, previous experimental studies have identified an apparent nonmonotonic variation in the Debye screening length with IL concentration in contrast to conventional expectations from Debye-Hückel theory [4]. Similarly, recent experimental studies of electrocatalytic CO₂ reduction have identified a similar nonmonotonic dependence of electrocatalytic activity on IL concentration in IL-solvent mixtures, which may be related to nanoscale cluster formation. However, the molecular structure of ILs mixed with organic solvents remains poorly understood. Inhibiting mechanistic interpretation of these behaviors and prevents the rational design of ILs [5-7]. Molecular dynamics (MD) simulations can address this problem by modeling the dynamic formation of IL nanostructures that is difficult to observe via experiments.

In this work, we performed all-atom MD simulations to quantify concentration-dependent changes in EMIM-BF₄ structure by modeling very dilute concentrations (0.001 mol fraction) to concentrated IL solutions (0.25, 0.5, 1 mol fraction) in acetonitrile. These MD simulations allow us to index individual ions and trace their movements as they form nanoclusters of different charge. Simulation analysis revealed a nonmonotonic trend in the both the formation of neutral clusters (i.e. clusters with no net charge) and mobile charge carriers (i.e. individual ions and clusters with a non-zero charge) with IL concentration, in broad agreement with nonmonotonic trends in electrostatic properties observed experimentally [7]. Notably, the average number of neutral clusters and mobile charged clusters reach a maximum that coincides with a maximum current density in previously performed electrocatalytic studies [7]. We further varied the solvent chemical identity ranging from a low dielectric constant (acetone), intermediate (dimethyl sulfoxide), and high (propylene carbonate) to predict changes in cluster behavior. Next, we introduce a silver electrode to the IL to observe the impact of the interface on the IL nanostructure. Together, these simulations provide molecular details regarding how these three tuning knobs (concentration, solvent, electrode) affect the formation of nanoscale IL clusters that dictate the electric double layer and potentially the electrocatalytic activity based on comparisons to experiments [7].

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[2] K. Motobayashi, Y. Maeno, and K. Ikeda, "In Situ Spectroscopic Characterization of an Intermediate of CO2 Electroreduction on a Au Electrode in Room-Temperature Ionic Liquids," The Journal of Physical Chemistry C, vol. 126, no. 29, pp. 11981-11986, 2022.

[3] B. A. Rosen et al., "In Situ Spectroscopic Examination of a Low Overpotential Pathway for Carbon Dioxide Conversion to Carbon Monoxide," The Journal of Physical Chemistry C, vol. 116, no. 29, pp. 15307-15312, 2012.

[4] A. M. Smith, A. A. Lee, and S. Perkin, "The Electrostatic Screening Length in Concentrated Electrolytes Increases with Concentration," The Journal of Physical Chemistry Letters, vol. 7, no. 12, pp. 2157-2163, 2016.

[5] B. A. Rosen et al., "Ionic Liquid-Mediated Selective Conversion of CO2 to CO at Low Overpotentials," (in English), Science, Article vol. 334, no. 6056, pp. 643-644, Nov 2011, doi: 10.1126/science.1209786.

[6] M. A. Gebbie et al., "Long range electrostatic forces in ionic liquids," Chemical Communications, vol. 53, no. 7, pp. 1214-1224, 2017.

[7] B. Liu, W. Guo, and M. A. Gebbie, "Tuning Ionic Screening To Accelerate Electrochemical CO2 Reduction in Ionic Liquid Electrolytes," ACS Catalysis, vol. 12, no. 15, pp. 9706-9716, 2022.