Molecular Understanding of Bilayer Structural Transition of CO₂ at the Ionic Liquid-Carbon Nanotube Electrode Interface

Abstract

In this paper, the atomistic-level descriptions of the electric double layer (EDL) formation of a CO2/ionic liquid (IL) on the carbon nanotube electrode are revealed by molecular dynamics simulations. The interfacial microenvironment and the orientation distribution depending on the pore radius and external potential are investigated. Throughout the evolution process, three different stages can be recognized by the variations of the coordination number and density distribution of cations. For small pore sizes, the interfacial structure is initially dominated by the nonelectrical force caused by the confinement effect; therefore, a similar density distribution of cation–anion ion pairs is observed in the vicinity of the electrode. With the increase of external potential, the cation [Bmim+] aggregates near the electrode, and the ion pairs are gradually separated by the CO2 molecule. Meanwhile, the coordination microenvironment of [Bmim+] is gradually occupied by the combination of [Bmim+] and CO2, and a steady EDL structure is formed. Once the external potential exceeds the critical value, the spontaneous charge separation phenomena can be determined by the interfacial density. With the decrease of pore radius, the critical potential for the transition becomes earlier. The EDL formation under potential polarization is associated with the simultaneous rearrangement of the orientation and mobility of [Bmim+] and CO2. The mobility and the orientation distribution determined from the MD simulations coincide well with the transition process. Once the transition is initiated, the orientation of CO2 changes from disordered to parallel to the electrode, while the imidazole ring of [Bmim+] is gradually oriented preferentially perpendicular to the electrode surface. On the other hand, the self-diffusion coefficient begins to decrease when structural transition is initiated. Once the transition process is completed, the value increases again, which reaffirms the microstructural evolution of EDL.