Visualizing Molecular-Scale 3D Distributions of Ionic Liquids in Electric Double-Layer Capacitor by 3D Scanning Force...

Atomic force microscopy (AFM) imaging of ionic liquid (IL) distribution in electric double-layer (EDL) devices has been actively explored to understand the origin of their excellent performance. However, this has been impeded by insufficient resolution or a poor understanding of the mechanisms of 3D IL imaging. Here, we overcome these difficulties using 3D scanning force microscopy (3D-SFM) with variable tip/sample bias voltages for visualizing 3D N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI) distributions on a Au electrode in EDL capacitors. Unlike previous reports, the multilayered vertical IL distribution and lateral molecular arrangements in the first adsorption layer are simultaneously visualized in one 3D image. This has allowed us to find the sample-bias-dependent changes in the molecular stability and thickness of the first IL adsorption layer, suggesting the significant bias dependence of the EDL capacitance. Such bias dependence is also confirmed by our molecular dynamics simulation and electrochemical impedance spectroscopy experiments, demonstrating the capability of 3D-SFM to provide molecular insights into the macroscopic device properties. Detailed comparisons between simulation and experiments also reveal that the 3D-SFM force contrasts mostly represent the distribution of anions having a higher molecular weight, yet the contrast is strongly enhanced by a positive tip bias. This is because the positively (or negatively) charged Au-coated tip is covered with a quasi-solid-state anion (or cation) layer, enhancing (or reducing) the electrostatic repulsion from the anions in the EDL. This counterintuitive finding should reinforce the theoretical basis for 3D IL imaging and help understand the molecular-scale origins of the EDL device performance.