Multivalent-Cation Effects on the Interfacial Structure, Capacitance, and Energy Storage in Humid [Pyr14][TFSI] Graphene Supercapacitors: An Atomistic Molecular Dynamics Study

Abstract

Water uptake alters the interfacial organization and charge storage response of ionic-liquid supercapacitors, but its effects in the presence of multivalent cations remain poorly understood. Here, atomistic molecular dynamics simulations were used to investigate dry N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, [Pyr14][TFSI], the corresponding humid electrolyte, and humid systems containing Al3+, Ca2+, Mg2+, or Zn2+ at a 1:1 metal-to-water ratio. The electrolytes were confined between oppositely charged graphene electrodes over surface-charge magnitudes of 0.00−0.30 e/nm2. Interfacial organization was characterized using number- and charge-density profiles and species populations within the electric double layer. Projected and differential capacitances were obtained from the charge−potential relationship, and stored energies were compared at a common potential difference of 2.0 V. The principal [Pyr14]+/[TFSI]− layering was broadly preserved, whereas metal-cation accumulation and water redistribution were strongly composition-dependent. Hydration yielded a slightly lower total projected capacitance in the global fit analysis (2.23 ± 0.12 to 2.10 ± 0.10 μF/cm2), although the block-level comparison revealed no statistically significant difference between the dry and humid electrolytes (Welch’s two-sample t-test, p = 0.85). Al3+ yielded a central value closest to the dry-electrolyte reference (2.21 ± 0.16 μF/cm2), while Ca2+and Zn2+ remained close to the metal-free humid system; none of these differences was clearly resolved within the present uncertainty. Mg2+produced the largest reduction, reaching 1.58 ± 0.13 μF/cm2, with lower capacitance at both electrode interfaces. At 2.0 V, hydration decreased the stored energy relative to the dry electrolyte, and Mg2+ produced the largest decrease among the metal-containing systems (26.3%), whereas Al3+ was the only system whose central stored energy exceeded that of the dry electrolyte, with a correspondingly larger block-to-block uncertainty (±0.68 versus ≤0.12 μJ/cm2 for the other humid electrolytes). These results demonstrate that multivalent-cation addition produces composition- and voltage-dependent changes in interfacial screening rather than a uniform recovery of the dry electrolyte response.