Jianan WANG1, Hua LI1, Rob ATKIN1
1The University of Western Australia, Perth, Australia
The potential-dependent dynamics of ionic liquids (ILs) at electrode/IL interfaces is vital for energy storage applications such as lithium-ion batteries and supercapacitors. In this work, video-rate atomic force microscopy (VR-AFM) is used to probe the dynamics of 1-butyl-3-methylimidazolium dicyanamide (BMIM DCA) and 1-hexyl-3-methylimidazolium dicyanamide (HMIM DCA), near a graphite electrode surface as a function of potential at frame rates around 0.5 frames per second.
Figure 1 presents frames of the AFM videos capturing the near-surface IL nanostructures. The bright and dark regions are the polar and apolar domains, respectively, of the nanostructures about 1 nm above the electrode. The interfacial nanostructures of both ILs move extremely slowly compared to bulk ions; diffusion coefficients at equilibrium are of the order of 0.01 nm2/s. Diffusion is ion- and potential-dependent. At negative potentials, the near surface nanostructure of BMIM DCA is slower than at OCP, whereas HMIM DCA moves faster at negative potentials than at OCP and positive potentials. When the graphite surface potential is switched from negative to positive, a sudden change in the direction of the nanostructure motion is observed. Figure 1 shows an example for BMIM DCA where the nanostructure features associated with the polar domain (circled in yellow) move sharply upwards as the potential is switched.