(Invited) Ferrocene-Modified Electroactive Ionic Liquids to Prevent Self-Discharge of Redox-Active Electrolyte Capacitors
Authors: Rochefort Dominic , Xie Han Jin , Gélinas Bruno 
Published: February 2020

Introduction: Redox-active electrolyte supercapacitors (RESC) are obtained by dissolving electroactive species in the electrolyte as a mean to increase the energy density of carbon-based capacitors.[1-3] RESCs are most often using aqueous electrolytes due to a higher solubility of redox molecules in water compared to organic solvents. For the same reason, ionic liquids (ILs) have not been extensively studied as electrolyte in RESCs despite the fact that they could present the advantage of an increased maximum operating voltage. So far, a compromise must be done between fast transport and high solubility of the redox molecule (obtained in aqueous electrolytes) and a high cell voltage (obtained with organic solvents and ionic liquids). RESCs may benefit from the development of electroactive electrolytes specifically formulated for this purpose, and which should i) allow high concentrations in redox species, ii) present a high maximum voltage, and iii)include a self-discharge suppression mechanism. This work demonstrates that electroactive ionic liquids, in which a redox moiety is covalently linked to one of the ions, may be used to meet these challenges.

Experimental: Electroactive ILs were obtained by modifying the common ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [EMIm][NTf2], with ferrocene. The ferrocene redox unit was linked either to the cation ([FcEIm][NTf2]) or the anion ([EMIm][FcNTf]) (Fig. 1). The ionic liquids were then used as the electrolyte in symmetrical cells using activated carbon (80 wt.%) as the active material. The cells were studied to evaluate the energy density and self-discharge to evaluate the effect of the ionic liquid structure on performance.

Results and discussion: Cyclic voltammetry was used in first place to evaluate the response of the cells containing an electrolyte composed of 80 wt.% of either [EMIm][NTf2], [FcEIm][NTf2] or [EMIm][FcNTf] ILs in acetonitrile. The oxidation and reduction of the ferrocenyl unit on the IL results in the presence of peaks on the CV curves (Fig. 1). These faradaic reactions increase the amount of energy stored in the device by accumulating charges in the electrolyte which is a the heart of redox-active electrolyte supercapacitors. Galvanostatic charge-discharge (GCD) experiments were carried out to evaluate the energy density of the devices employing the electroactive ionic liquids. The gravimetric specific energy values, based on total weight of both electrodes for the RESCs presented here, are (at I = 2 mA) 13.2 and 9.0 Wh kg-1 for [EMIm][FcNTf] and [FcEIm][NTf2], respectively. These values represent a significant increase from equivalent double-layer devices using either the unmodified ionic liquid [EMIm][NTf2] (Wg = 7.2 Wh kg-1) or the conventional electrolyte 1 M TBAP in CH3CN (Wg = 6.4 Wh kg-1).
Self-discharge (SDC) represents a major issue with RESCs because a shuttle discharge mechanism (reduction oxidized species at the negative electrode) adds up to the diffusion-based SDC.[4] The profiles of individual electrodes demonstrate that after an initial decrease due to ohmic drop, the potential of the positive electrode remained constant for three hours. SDC suppression in RESC based on redox IL with the Fc-modified anion is explained by the formation of a film at the positive electrode upon oxidation of [FcNTf]- to [Fc+NTf-]0that prevent the shuttling of the charged species to the negative electrode.

Conclusions: The functionalization of an ionic liquid with ferrocene led to high concentrations of redox moieties in the electrolyte (2.4 M) and a large maximum operating voltage (2.5 V). An energy density of up to 13.2 Wh per kg (both electrodes) was obtained which represents an 83 % increase vs. the unmodified ionic liquid. When the ionic liquid’s anion is modified with ferrocene, the self-discharge at the positive electrode is fully suppressed due to the deposition of a film on the electrode. While the rate dependency of the specific energy and stability could be improved, these findings suggest that electroactive ionic liquids are particularly well-suited for use in redox-active electrolyte supercapacitors.

References

1) E. Frackowiak et al. Chemsuschem, 5 (2012) 1181-1185; 2) S. Roldan et al. Angew. Chem. Int. Ed., 50 (2011) 1699-1701; 3) S.T. Senthilkumar et al. J. Mater. Chem. A, 1 (2013) 12386-12394; 4) L.B. Chen et al. Energy Environ. Sci., 7 (2014) 1750-1759.

Figure 1