Molecular dynamics simulations were conducted to study the structure and differential capacitance of the electric double layer capacitor by varying electrode topography. Studying the dependence of structure and differential capacitance (DC) on electrode potential is a central point of electric double layer (EDL) models, from the early Gouy-Chapman approximation to the more recent Kornyshev formulation or Lamperski treatment. Although the influence on DC of temperature, density, polarisability and dispersion interactions of room temperature ionic liquid (RTIL) electrolyte is becoming more understood, the effect of electrode topography on DC remains unclear and perhaps controversial. Moreover, the variability of DC results for poorly prepared polycrystalline electrodes may indicate a strong dependence of EDL structure and DC on the topography of electrode surface.

In this work, we employ molecular dynamics simulations to prove that the crystallographic orientation and the surface roughness of electrode could intensely alter the shape of DC curve as a function of electrode potential, especially near the potential of zero charge (PZC). Specifically, we simulated a RTIL electrolyte, consisting of 1-ethyl-3-methyl imidazolium (EMIM ⁺), bis-fluorosulfonyl imide (FSI^-), and Li+ (in a molar ratio of 24:31:7), in contact with two types of graphite electrodes: one is graphite with atomically flat basal plane and the other is graphite with edge orientation.

This work demonstrates that the capacitor with atomically flat basal plan graphite generates a camel-shaped DC curve; while, the capacitor with edge orientation graphite engenders a bell-shaped DC curve. Furthermore, the capacitor with edge orientation graphite displays a significantly larger DC (almost double) than the capacitor with atomically flat basal plane graphite near PZC. Additionally, these findings coincide with the results of numerous recent experiments. Thus, we conclude that the crystallographic orientation and surface roughness will modify the EDL structure and hence, vary the DC curve of the EDLC.