On the Parasitic Surface Adsorption of Pyrrolidinium and Phosphonium-Based Ionic Liquid Preventing Accurate Differential Capacitance Measurements
Authors: Fraysse Kilian , Huang Lixu , Li Hua , Atkin Rob , Padua Agilio , Armand Michel , Howlett Patrick C. , Zhang Mingjie 
Published: December 2024

Ionic Liquids (ILs) have received a substantial interest from the scientific community over the past few decades1, in particular, because of their possible use as electrolytes for battery technology. This wide excitement stems from their known thermal stability and non-volatile nature2 (as opposed to more traditional organic carbonate-based electrolytes) which has led the study of IL to fall under the category of green chemistry3.
In a similar fashion to aqueous-based electrolytes, whose bulk properties are fundamentally different from their properties near charged interfaces (giving rise to an Electrical Double Layer, EDL), ILs and IL-based electrolytes also exhibit a distinct interfacial structuring near charged surfaces consisting of alternating layers of cations and anions4 extending into the liquid over several nanometers5. This interfacial structuring (i.e., EDL) initially formed at the electrode/electrolyte interface defines the charge transfer mechanism6. It is also believed to play a critical role in the formation of the Solid Electrolyte Interphase (SEI)7,8. The latter, via its morphological, mechanical and chemical properties directly governs the performance of the battery9.
Electrochemical Impedance Spectroscopy (EIS) is an attractive technique for the indirect study of the EDL at charged interfaces, via the so-called differential capacitance measurement. The latter giving an indication of the ionic density near the interface10. Differential capacitance measurements are, however, difficult to interpret and compare, because there is no real consensus as to how the measurement needs to be performed. To add to the difficulty, some ionic liquids are known to display a hysteresis process, inducing measurement variabilities.
In this study, we propose to investigate the structure of the EDL of pure IL (trimethyl isobutyl phosphonium bis(fluorosulfonyl)imide, P111i4FSI and N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide, C3mpyrFSI) as well as IL-based electrolytes composed of 42mol% of NaFSI in C3mpyrFSI and/or P111i4FSI using both a differential capacitance and AFM approach. Using a well-established method for the measurement of the differential capacitance in IL and IL-based electrolytes, we show that all the studied systems display important hysteresis properties (in particular, the pyrrolidinium-based ones). Via a series of differential capacitance as well as AFM experiments, we further demonstrate that the hysteresis is a result of adsorption processes on the surface of the electrode. This parasitic adsorption leads inaccuracies in traditional differential capacitance measurements. We therefore propose an alternative method for the measurement of the differential capacitance which allows to minimize the impact of adsorption.
This new understanding of differential capacitance measurements has important methodological implications. By providing guidelines for accurately probing the differential capacitance of ILs and IL-based electrolytes, in particular, pyrrolidinium and phosphonium-based ones via a single, reliable method, this report may well help pave the way to easier comparisons and interpretations.

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