Molecular dynamics simulations of novel electrolytes based on mixtures of protic and aprotic ionic liquids at the electrochemical interface: Structure and capacitance of the electric double layer

Mixtures of alkylammonium-based protic ionic liquids and alkylmethylimidazolium-based aprotic ionic liquids near neutral and charged graphene interfaces were studied by means of molecular dynamics simulations. Effects previously observed for bulk mixtures were found to have a pronounced impact on the liquid's interfacial structure. Hydrogen bonding effects were found to affect the composition of the electric double layer (EDL). The lateral structure of the innermost layer of the EDL was found to be rich, showing transitions from solid-like to more fluid configurations, in which low mobility or high mobility anions give rise to hexagonal or striped patterns, respectively. Finally, two-dimensional nanoscale segregation, into polar/apolar domains but also into protic/aprotic domains, was seen at the graphene-liquid interface, resulting in a highly heterogeneous interfacial structure. The integral EDL capacitance vs. electrode potential was seen to nonlinearly increase with protic ionic liquid concentration due to the rearrangements of the three-dimensional ionic density distribution of species with different charge geometries close to the electrode.