Electrical and structural properties of deep eutectic solvents in MXene-based supercapacitors

The development of safer and more sustainable electrolytes is an important step toward improving next-generation energy storage devices. In this work, classical molecular dynamics (MD) simulations are used to investigate how deep eutectic solvents (DES), specifically ethaline and reline, behave when confined within MXene-based supercapacitors with different surface terminations. The calculated differential capacitances clearly depend on the surface chemistry, following the trend Ti 3 C 2 ( OH ) 2 > Ti 3 C 2 O 2 > Ti 3 C 2 F 2 , with values reaching up to ∼ 7.2 µF/cm 2 . These results are comparable to those reported for ionic liquids and conventional electrode materials, reinforcing the potential of DES–MXene systems for energy storage applications. To better understand these differences, we investigated how the electrolytes organize near the electrode surface. The results show that both the position and orientation of DES molecules are influenced by the surface termination. In particular, Ti 3 C 2 ( OH ) 2 allows the molecules to approach the surface more closely. Upon application of voltage, counter-ions accumulate near the electrode, while co-ions are repelled. We also observe that the amount of ethylene glycol at the interface increases with applied potential. At the negative electrode, this is associated with the presence of hydrogen-bond-donor species interacting with the surface. A clear voltage-dependent behavior is observed, that is, at low potentials, counter-ion adsorption is more pronounced, while around 2 V there is an increase in ethylene glycol at the interface, especially for Ti 3 C 2 ( OH ) 2 . At higher potentials, counter-ion accumulation becomes dominant again. Overall, these results show that the charging process in DES-based supercapacitors is not governed solely by electrostatics, but also by how molecules arrange and interact at the interface. This molecular-level understanding helps explain the superior performance of hydroxyl-terminated MXenes and highlights the importance of surface chemistry in designing more efficient energy storage devices.