Relating the Ambient Temperature and Ionic Liquid Structure to the Changes of Capacitance Using Molecular Dynamics
Authors: Ers Heigo , Voroshylova Iuliia , Pikma Piret , Ivanistsev Vladislav 
Published: July 2022

Ionic liquids (ILs) are a unique class of electrolytes, which characteristics make them suitable for use in solar cells, supercapacitors, and fuel cells1. Due to the appealing properties such as good electrochemical stability, low vapour pressure, high concentration of ions and the lack of solvent, they have been under intense study since the early 2000s. Although numerous theoretical2,3, computational4,5, and experimental studies6,7 have shed light on the interfacial properties of ILs, which differ noticeably from the aqueous electrolytes, multiple open questions remain. One such problem is how the interfacial capacitance is affected by the ambient temperature, as studies have shown both positive and negative temperature dependences8,9. Understanding the temperature dependence of interfacial capacitance is crucial as it is relevant for the description of energy storage and is one of the few quantities, which can be estimated both experimentally and computationally.
In this study, we combine the density functional theory (DFT) calculations with molecular dynamics (MD) simulations of graphene (Gr) | EMImBF4 IL interface to explain the effect of temperature on capacitance. MD simulations allow us to investigate the probable distribution of ions near the electrode’s surface and relate the changes of ILs structure to the capacitance using the interfacial bilayer model (IBL). We show that the increase of temperature affects the capacitance near the potential of zero charge by attenuating the overscreening without a notable change in the IL interfacial structure. The characteristic peaks and plateaus in the capacitance potential dependence are explained through the concepts of IL layering and saturation of the second IL layer described in the IBL. Using the DFT calculations, we estimate the impact of the quantum capacitance of Gr on the total interfacial capacitance and its temperature dependence. By accounting for the limiting quantum capacitance, the total interfacial capacitance was significantly altered in the case of the Gr electrode, as the effect of the temperature was dampened, and a V-shaped capacitance curve was obtained.
Acknowledgements:
This work was supported by the Estonian Research Council grant PSG249 and by the EU through the European Regional Development Fund under project TK141 (2014-2020.4.01.15-0011). The financial support from FCT/MCTES through the Portuguese national funds, project No. UID/QUI/50006/2021 (LAQV@REQUIMTE) is also acknowledged.
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