Experimental Studies of the Capacitive-Potential Relationship for Ionic Liquid – Electrode Interfaces
Authors: Lucio Anthony J , Zhang Jie , Bond Alan M , Shaw Scott K 
Published: February 2020

The electrochemical double layer (EDL) in ionic liquid (IL) solvents plays an important role in myriad energy storage and conversion applications. A clear description of molecular organization within this interfacial region requires a multipronged approach. This presentation will discuss studies of the IL – electrode interface using AC voltammetry, electrochemical impedance spectroscopy (EIS), and vibrational spectroscopy covering slow (re)orientation / movement of ILs within the EDL as it relates to capacitive hysteresis, and make critical comparisons of capacitance vs. potential relationships to theoretical predictions.

ILs at an electrified interface have been shown to exhibit capacitive hysteresis depending on the applied potential scan direction (e.g. cathodic or anodic), the extent of which depends on electrode surface material and the measurement technique employed.1-3 Recent work in our group examined several aprotic and protic ILs on metallic, carbon, and semiconducting electrodes demonstrating that capacitive hysteresis is not necessarily a uniform feature of the IL electrified interface. Our studies use large amplitude Fourier transformed alternating current (FT-AC) voltammetry, which demonstrates little (<10 %) hysteresis in the resulting capacitance vs. potential curve respective to the electrochemical potential scan direction. This highlights the benefits of utilizing large amplitude FT-AC voltammetry to probe capacitive currents in ILs.

Additional work employs vibrational spectroscopy to study the evolution of the IL EDL under an applied potential. This data provides a vantage point to discuss capacitive hysteresis and how it could relate to molecular (re)orientation in ILs. Our preliminary data show that the IL vibrational profile shows small changes when responding to a potential step of 2 V within the EDL potential window. However, the vibrational profile exhibits noticeable hysteresis in IR peak intensities when undergoing positive- and negative-going applied potential scan directions further demonstrating the slow response times in ILs.

It has been suggested that slow (re)orientation of ILs within the EDL could be the mechanism behind the observed hysteresis and that these processes are likely a common feature of the IL – electrode interface.4-5 Depending on the electrochemical technique employed the effective ‘scan rate’ can vary from less than one minute (e.g. FT-AC voltammetry) to several hours (e.g. EIS) when acquiring capacitance data over a range of potentials. Our work using broadband EIS to study the 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate – polycrystalline gold interface demonstrates over a 2 V window that capacitive hysteresis is absent based on several potential scan directions employed compared to Au(111) which exhibits this phenomenon.1 Further this work spanning from 1 MHz to 0.01 Hz suggests the slow process (visible in the complex capacitance plane plot as an arc in the low frequency region) is also absent on the polycrystalline gold electrode compared to the Au(111) single-crystal surface.1 Our work with large amplitude FT-AC voltammetry suggests three possible scenarios to explain contradictions with previous reports: (1) large amplitude FT-AC voltammetry scans the potential window cycle fast enough (~100 mV s-1) relative to slow movement of IL EDL ions to not capture any hysteresis processes, (2) the large perturbation amplitude employed overcomes the energetic barrier for interfacial IL ion movement to exhibit hysteresis processes or (3) capacitive hysteresis and slow (re)orientation are not necessarily common features of the IL – electrode interface.

Capacitance curvature in published scientific literature show a wide range of results, even for similar electrochemical systems. Additionally, it has proved challenging to unite theoretical predictions with experimental observations for IL capacitance responses. ILs are concentrated in the sense that they contain no molecular solvent and one viewpoint is they are composed entirely of dissociated ions. However, recent reports challenge this view to suggest they are not well dissociated and behave as dilute electrolytes. Our work examining the 1-butyl-3-methylimidazolum tetrafluoroborate – polycrystalline gold interface corroborates the latter viewpoint. Furthermore, our studies of the IL – semiconductor electrode interface demonstrate a capacitance response analogous to dilute aqueous electrolytes at a semiconductor electrode. Specifically, we report relatively featureless, ‘rising’ capacitance curvature that shows minimal dependence on the nature of the IL (i.e. protic vs. aprotic). Capacitance-potential curvature suggests at the ensemble behavior of the EDL but coupling potential control with spectroscopy can provide a powerful method of probing interfacial chemistry.

It is anticipated that this presentation will bring to light some basic questions of IL behavior at an electrified interface.

References:

Druschler et al. J. Phys. Chem. C (2011) 115, 6802

Alam et al. J. Phys. Chem. C (2011) 115, 19797

Gore et al. Electrochem. Commun. (2010) 12, 1340

Uysal et al. J. Phys. Chem. C (2014) 118, 569

Yasui et al. Electrochem. Commun. (2010) 12, 1479