Ester-Based Electrolyte Mixtures for Graphene Supercapacitors: A Molecular Dynamics Investigation

The development of sustainable and high-performance electrolytes is essential for advancing next-generation supercapacitors. In this study, we employed classical molecular dynamics simulations to investigate graphene-based supercapacitors using hydrated ester-based ionic liquids, both in pure form and in mixtures. The models are based in 1-butyl-3-methylimidazolium ([bmim]) cation and acetate ([ace]), benzoate ([bnz]) and propanoate ([prop]) anions. Structural analyses revealed well-defined electric double layers (EDLs) characterized by charge alternation across sublayers and moderate overscreening. Electrostatic potential profiles, obtained from the one-dimensional Poisson equation and corrected by the point of zero charge (PZC), exhibited a nearly linear response with surface charge density, with potential drops (ΔΦ) ranging from ∼1.1 V for [bmim][ace] + [bmim][bnz] to over 2.1 V for [bmim][prop]. Capacitance values confirmed these trends: the highest total capacitances were observed for [bnz]-containing mixtures (up to 2.83 μF/cm² for [bmim][ace] + [bmim][bnz]), while [bmim][prop] and [bmim][ace] showed the lowest (≈2.5 μF/cm²). Energy density calculations highlighted a contrasting behavior: [bmim][prop], despite its lower capacitance, reached the largest gravimetric (4.06 J/g) and volumetric (4.58 J/cm³) energy densities due to its higher total potential difference. However, when normalizing the comparison at a fixed potential difference of 2.5 V, [bnz]-containing electrolytes─particularly [bmim][ace] + [bmim][bnz]─achieved the best performance, with gravimetric and volumetric energy densities of 6.44 J/g and 7.37 J/cm³, respectively. These results emphasize the decisive role of the [bnz] anion in tuning interfacial structure and energy storage, providing valuable guidelines for the rational design of ester-based ionic liquid electrolytes for sustainable supercapacitors.