**1. Introduction**

The production of sustainable energy is one of the most relevant issues of current times. The increase in polluting emissions and global warming have emphasized the impropriety of fossil fuels, favoring the development of methods to produce clean energy from renewable sources. However, as most of them have a discontinuous nature, storage and management systems are required to secure the generated energy. In this context, the design of efficient and sustainable energy storage devices and systems is therefore a key point to adequately support the energy transition from fossil to renewable sources.

In the field of energy storage, supercapacitors (SCs), and in particular the electrochemical double layer capacitors (EDLCs), have become of great interest thanks to their complementary performance compared to batteries, such as lithium-ion batteries (LIBs). Indeed, EDLCs generally have moderate energy density (generally < 10 Whkg−1) but provide high power (up to 10 kWkg<sup>−</sup>1) thanks to a fast charge/discharge mechanism, and they also have a significantly high number of life cycles (>>100,000 cycles).

The properties of an EDLC are defined by its electrostatic energy storage mechanism and by its components. These devices consist of two electrodes with a high surface area electrically isolated by a porous dielectric separator. The electrodes include a metal collector (usually aluminum), on which a layer of high porosity active material (usually activated carbon and carbon black) is cast. The electrodes and the dielectric separator are

**Citation:** Melchiorre, M.; Esposito, R.; Di Serio, M.; Abbate, G.; Lampasi, A.; Balducci, A.; Ruffo, F. Lactic Acid-Based Solvents for Sustainable EDLC Electrolytes. *Energies* **2021**, *14*, 4250. https://doi.org/10.3390/ en14144250

Academic Editor: Haolin Tang

Received: 22 June 2021 Accepted: 11 July 2021 Published: 14 July 2021

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soaked with a conductive medium, typically an electrolyte consisting of a solvent and a conducting salt [1].

In EDLC devices, the electrolyte is a key component, as the charges are accumulated at the electrode/electrolyte interface (EEI) and the specific energy (Esp) and the specific power (Psp) can be expressed as [2]:

$$\mathcal{E}\_{\text{SP}} = \frac{\mathcal{C}\_{\text{SP}} \mathbf{V}^2}{2} \tag{1}$$

$$\mathbf{P\_{sp}} = \frac{\mathbf{V^2}}{4\mathbf{m\_{cell}}\mathbf{R\_{cell}}} \tag{2}$$

In the equations, Csp is the specific capacitance, V is the applied potential, mcell is the mass of electrode active materials, and Rcell is the internal resistance of the device [3].

The capacitance is mostly determined by the properties of the electrode (available surface, pore distribution, accessibility of the pores) and by the EEI, while the applicable potential and the internal resistance of the device strongly depend on the nature and characteristics of the electrolyte. Hence, aprotic polar organic solvents are often favored over water, as this is subject to redox activity for potentials between 1.0 V and 1.2 V.

A class of water-containing electrolytes capable of overcoming this limit is that of water-in-salt electrolytes (WiS) [4]. In this type of electrolyte, the water molecules strongly interact with the ions present at very high concentrations, thus increasing the electrochemical stability compared to the classic salt in-water (SiW) solutions [5]. However, WiS with lithium bis(trifluoromethane)sulfonimide (LiTFSI) presents problems related to the internal resistance of the device due to modest conductivity (21 m LiTFSI 8.2 mScm<sup>−</sup>1) and high viscosity (21 m LiTFSI 30.2 mm2s−1) [6]. Furthermore, a rational design of the active material porosity is essential to achieve high performance [7]. In addition, the development of new technologies should be oriented towards lithium-free devices, given the progressive saturation of its production sites [8], and with cheap and user-friendly electrolytes [9]. To extend the applicable potential window using water as an electrolytic component, Hughson et al. recently reported in a communication the use of water–oil microemulsions in the presence of surfactants at a potential of 2.7 V [10]. However, the high internal resistance recorded (26 Ohm) compromises its concrete application.

Cell voltages of 2.7–3.0 V can be regularly achieved with non-aqueous electrolyte. Moreover, organic solvents can be used in wider temperature ranges than aqueous electrolytes. On the other hand, the electrical conductivity of non-aqueous electrolytes is often significantly lower than that of water, and this contributes to increasing their internal resistance. For these reasons, commercial EDLCs supercapacitors contain acetonitrile (ACN) or propylene carbonate (PC) based electrolytes, but these solvents have limitations for high-voltage applications and risks related to their handling, and are both obtained from fossil feedstock. The need to address these issues represents the driving-force that pushes the scientific community towards the search for new electrolytes.

For a long time, huge efforts have been made to increase the EDLCs performances [11–13], and only more recently to also improve safety and ecotoxicological profiles of the used electrolytes. Previous results and new perspectives towards new electrolytes have been described in recent reviews [14–17].

Among the non-aqueous electrolytes, a category of wide interest consists in aprotic ionic liquids (AILs) [18], and more recently also protic liquids (PILs) [19]. These electrolytes are highly attractive for electrochemical applications due to their stability and safety as non-flammable substances [20,21]. However, their efficiency as electrolytes is strongly affected by the electrodes porosity, resulting in a low energy efficiency in combination with electrodes with a high content of micropores [22]. Furthermore, the high cost of these electrolytes hampered their commercial applications in solvent-free conditions.

A similar approach to that of WiS electrolytes was reported by Stettner et al. [23], using electrolytes based on protic ionic liquids (PILs) with additions of water (1–3.8%). Despite a considerable improvement of transport properties, the operating voltage of EDLC containing these electrolytes is lower than that of AIL-based EDLCs (from 1.8 V to 2.2 V).

Recent advances regarding electrolytes based on organic solvents concern the study of nitriles, in linear aliphatic chains (glutaronitrile GTN, adiponitrile ADN), branched chains (2-methylglutaronitrile 2MGN) and as functional groups present in methyl esters (3-cyanopropionic acid methyl ester CPAME) [24]. These solvents reach very high operational potentials (3.5 V), making high-voltage applications possibly able to address the need to increase the EDLC's specific energy. However, these substances have high acute toxicity (some even fatal in case of inhalation) and health hazards, and therefore they do not seem suitable for common commercial applications.

To increase the performance of the classic ACN-based electrolytic solutions, binary mixtures with other organic solvents have recently been studied. When mixed with dibutyl carbonate (10%−33% v)**,** a net increase in performance at low temperatures (up to −60 ◦C) was reported [25], while in combination with ethylisopropylsulfone (75–50%) operational potentials of 3.0 V were reached [26].

Recently investigated nitrile-free organic solvents are 1,2-butylene carbonate (BC) [27], tetramethoxyglyoxal (TMG) and tetraethoxyglyoxal (TEG) [28]. A combination of BC with Pyr14BF4 provided a potential window of 3.1 V, while modest results were achieved with TMG and TEG due to their relatively high viscosity.

From the point of view of the sustainability of EDLCs, many efforts have been made to obtain active carbonaceous materials and binders from biomass [29], but this approach is still lacking for the development of non-aqueous electrolytes. In fact, recent papers do not highlight the origin of the investigated electrolytes despite the design of innovative aprotic polar solvents from renewable sources is a topic of great and current interest [30].

Our research group has long been involved in the study of the catalytic conversion of biomass [31–34] and in the valorizations of bio-based molecules derived from renewable feedstock [35–37] to produce innovative materials that beneficially replace the traditional ones.

This approach has recently been oriented towards the design of innovative solvents, and, in this manuscript, we report the synthesis of different lactic acid ketals and the investigation on their properties as solvents for electrolytes in symmetrical EDLCs. Lactic acid is a bio-based chemical platform industrially prepared through bacterial fermentation of carbohydrates [38] and is widely used to produce biodegradable polymers or as starting feedstock for green routes to bulk chemicals productions [39].

The synthesized solvents have the common structure of 2,2-R,R'-5-methyl-1,3-dioxolan-4-one (DOLOs), which is a chemical platform that allows to selectively evaluate the effect of small structural variations on the electrolyte properties (Figure 1).

**Figure 1.** Lewis structure of 2,2-R,R -5-methyl-1,3-dioxolan-4-one.

These compounds are already used as precursors of sustainable polymers [40,41], but to the best of our knowledge lactic acid derived DOLOs have never been used as solvents in the field of energy storage. The solvent 5-methyl-1,3-dioxolan-4-one displayed performances competitive (Csp 14.2 Fg−1, Esp 13.4 Whkg−<sup>1</sup> and Psp 22.5 kWkg−1) with commercial solvents, such as propylene carbonate, and therefore it represents an example of what can be defined as a "Non-Aqueous Sustainable Electrolyte" (NASE).
