*Article* **Morphological Observation of LiCl Deliquescence in PDMS-Based Composite Foams**

**Emanuela Mastronardo 1,\*, Elpida Piperopoulos 1,2, Davide Palamara 1, Andrea Frazzica <sup>2</sup> and Luigi Calabrese 1,2,\***


**Abstract:** The LiCl-based heat storage system exhibits a high-energy density, making it an attractive and one of the most investigated candidates for low-temperature heat storage applications. Nevertheless, lithium chloride, due to its hygroscopic nature, incurs the phenomenon of deliquescence, which causes some operational challenges, such as agglomeration, corrosion, and swelling problems during hydration/dehydration cycles. Here, we propose a composite material based on silicone vapor-permeable foam filled with the salt hydrate, hereafter named LiCl-PDMS, aiming at confining the salt in a matrix to prevent deliquescence-related issues but without inhibiting the vapour flow. In particular, the structural and morphological modification during hydration/dehydration cycles is investigated on the composite foam, which is prepared with a salt content of 40 wt.%. A characterization protocol coupling temperature scanned X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM) analysis is established. The operando conditions of the dehydration/hydration cycle were reproduced while structural and morphological characterizations were performed, allowing for the evaluation of the interaction between the salt and the water vapor environment in the confined silicon matrix. The material energy density was also measured with a customized coupled thermogravimetric/differential scanning calorimetric analysis (TG/DSC). The results show an effective embedding of the material, which limits the salt solution release when overhydrated. Additionally, the flexibility of the matrix allows for the volume shrinkage/expansion of the salt caused by the cyclic dehydration/hydration reactions without any damages to the foam structure. The LiCl-PDMS foam has an energy density of 1854 kJ/kg or 323 kWh/m3, thus making it a competitive candidate among other LiCl salt hydrate composites.

**Keywords:** lithium chloride hydrate; composite foam; deliquescence; thermochemical storage; in situ characterization

#### **1. Introduction**

The energy transition is a necessary path toward the transformation of the worldwide energy sector from fossil-based to zero carbon by 2050, which is fundamental to reduce energy-related CO2 emissions to limit climate change [1]. One of the main pillars for the energy transition is the global spread of the use of solar energy, being basically endless and free [2]. However, its diurnal nature, weather conditions, and seasonality might make solar energy unavailable when required. This time lag between energy supply and demand is a critical point to be addressed for renewable solar energy development and replacement of fossil-based energy. For this aim, thermal energy storage (TES) is an efficient and effective means to store solar energy when in excess, thus shifting the peak load demand into off-peak hours.

Among TES technologies, the thermochemical one exploits materials with a large reaction enthalpy and reversibility. Inorganic salt hydrate (MnAm·XH2O) is a widely used

**Citation:** Mastronardo, E.; Piperopoulos, E.; Palamara, D.; Frazzica, A.; Calabrese, L. Morphological Observation of LiCl Deliquescence in PDMS-Based Composite Foams. *Appl. Sci.* **2022**, *12*, 1510. https://doi.org/10.3390/ app12031510

Academic Editor: Andrea Dorigato

Received: 21 December 2021 Accepted: 28 January 2022 Published: 30 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

class and one of the most promising investigated classes of materials for thermochemical heat storage [3,4], which is based on the following storage reaction:

$$\text{NH}\_3\text{A}\_{\text{m}}\cdot\text{nH}\_2\text{O} + \text{heat} \leftrightarrow \text{M}\_\text{n}\text{A}\_{\text{m}}\cdot(\text{n}-\text{x})\text{H}\_2\text{O} + \text{xH}\_2\text{O}$$

Heat is transferred from a selected source to the material and the dehydration reaction takes place (storage step). The heat is stored for as long as the salt is in the dehydrated form. When heat is required, water is made accessible to the salt, the reversible hydration takes place, and heat is released (release step). One of the main advantages of this technology is, indeed, the possibility to control the heat release by controlling the water vapor accessibility to the dehydrated salt, thus making the heat discharge available and controllable ondemand [5].

Due to this great potential, several pure salt hydrates and composite systems with different salts were investigated: e.g., LiBr [6], MgSO4 [7], CaCl2 [8], SrCl2 [9], and LiCl [10]. Among them, LiCl is one of the salts that raised more interest for low/medium temperature (i.e., below 120 ◦C) TES application [11]. Indeed, the high-energy density of the LiCl-based system makes it an attractive candidate from the heat storage capacity cost perspective, thus resulting in one of the materials with a lower cost per kWh stored (~6 €/kWh) [12]. Nevertheless, lithium chloride is one of the most hygroscopic salts known and it incurs the phenomenon of deliquescence.

Deliquescence is a first-order phase transformation of the solid to a saturated solution, which occurs at a specific relative humidity (RH) inherent to the properties of the solid and the temperature [13]. When this RH is reached, the aqueous solution is the thermodynamically favored phase and dissolution begins, whereby liquid patterns or patches of solution form on the surface. As the humidity further increases, these patches merge and form a thin liquid film, which gradually thickens; the entire solid particle dissolves and transforms into a solution drop; and its radius abruptly grows. In a closed environment, a saturated solution of lithium chloride will form an equilibrium at a relative humidity of about 12.4% (20 ◦C). The reverse process to deliquescence is called efflorescence. Efflorescence is the process of the crystallization and expulsion of water from the crystallized material when the decreasing humidity reaches another threshold value, called efflorescence relative humidity (ERH).

Several issues arise due to the deliquescence phenomenon in TES technologies. The liquid film that forms on the surface of the salt crystal inhibits the rehydration reaction (e.g., in the case of the LiCl, LiBr, and CaCl2) [14,15]. In addition, the sorbate mass transfer into the system will be hindered, causing issues such as high-pressure drops and ultimately system failure, but also corrosion issues arise due to the dripping of the salt solution to other metal components of the systems.

Accordingly, several efforts were carried out to implement LiCl-based TES technologies and, more specifically, from the material perspective. The general idea is to embed the salt into porous matrices (such as carbon foams, expanded natural graphite, zeolite, vermiculite, silica gel, etc.) to prevent deliquescence and improve water transport into the materials [8,16,17].

Zhang et al. developed a LiCl-based composite on mesoporous alumina [18]. The material showed a TES density of up to 1040 J/g. However, due to the low pore volume of the alumina, only ~15 wt.% of the salt could be loaded in the matrix, thus limiting the possibility of further increasing TES density. Multi-walled carbon nanotubes (MWCNTs), due to their extremely large and variable pore volume, were used as a matrix to host LiCl salt [19]. This composite, with a 44 wt.% content of LiCl salt, achieved a heat storage capacity of 1.6–1.7 kJ/g. More recently, a LiCl/vermiculite, with ~59 wt.% of LiCl, was developed by Grekova and coworkers [20]. The heat storage capacity of this composite is 1.8–2.6 kJ/g (224–253 Wh/m3). It has to be pointed out that, intrinsically, the porous structure of natural mineral vermiculite is non-reproducible and so is the composite. Additionally, vermiculite suffers from hydrothermal structural stability issues that are critical for the technology's long-term durability. Yu et al. developed silica gel-LiCl composites by varying

the amount of embedded salt between 10 and 40 wt.% [21]. An optimum compromise between the heat storage capacity and material stability was identified in the composition with 30 wt.% of embedded salt. For this composition, a heat storage capacity of 480 J/g was determined. The presence of salt deposited onto the external surface of the composite has clearly a detrimental effect. This issue was addressed specifically on mesoporous silica gel-LiCl composites with a post-treatment based on an adsorption phase followed by a slow desorption phase, which allowed the salt to move from the surface into the pores, likely due to a capillary effect [22].

The investigated matrices, being rigid structures, could limit the mechanical stability of the material over cycling due to the consecutive expansion and contraction of the salt grains during the hydration and dehydration steps. In this regard, the use of flexible polymeric macro-porous foam as matrices for salts was proposed in literature for other types of salts, e.g., MgSO4·7H2O [23,24]. The flexibility of these foams can accommodate the expansion and shrinkage of the salt hydrate volume during the hydration and dehydration reactions, thus enhancing the mechanical stability of the composite over the cycles. In addition, the foam should embed the salt, thus retaining the deliquescence that inevitably occurs due to the operating conditions.

In situ studies on TES materials can certainly provide an in-depth understanding of the reaction mechanism and materials changes during the charging/discharging operations, thus leading to valuable information for the advancement of TES materials. Here, we investigate the structural and morphological modification during hydration/dehydration cycles on a LiCl-silicon foam (LiCl-PDMS) with a salt content of 40 wt.%. Specifically, a characterization protocol coupling temperature scanned X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM) analysis was proposed. The operando conditions of the dehydration/hydration cycle were reproduced while structural and morphological characterizations were performed, allowing for the evaluation of the interaction between the salt and the water vapor environment in the confined silicon matrix.

#### **2. Materials and Methods**

#### *2.1. LiCl-PDMS Foam Synthesis and Characterization*

For this study, a silicone foam based on a poly(methylhydrosiloxane) (PMHS) and silanol-terminated polydimethylsiloxane (PDMS) mixture, filled with 40 wt.%. of the LiCl salt final weight, was used. The macro-porous composite foam was prepared by a dehydrogenative coupling reaction between hydroxyl functional materials and hydride functional siloxanes in the presence of a metal salt catalyst (bubbling agent), inducing the hydrogen evolution according to the procedure reported by Calabrese et al. [25]. Briefly, the PDMS and PMHS monomers (supplied by Gelest Inc., Morrisville, PA, USA) used as a monomer and hardener, respectively, were mixed together (PDMS:PMHS = 2:1). Then, LiCl·H2O salt (>99.9%, Sigma Aldrich, St. Louis, MO, USA) was added under vigorous mixing until a homogenously and well-dispersed slurry was obtained. Anhydrous denatured ethanol (Sigma Aldrich, St. Louis, MO, USA) was added (~8 wt.%) to better mix and homogenize the slurry. A tin salt catalyst (bis(2-ethylhexanoate)tin, Gelest Inc., Morrisville, PA, USA) was added (~8 wt.%) in order to activate the reaction. Finally, the foaming process was carried out by placing the slurry in a cylindrical mold at 60 ◦C for 24 h and the dehydrogenative coupling reaction between the slurry constituents took place. Specifically, the hydroxyl and hydride functional groups in PDMS and PMHS, respectively, reacted, forming a siloxane link, namely Si-O-Si, that gradually led to a tri-dimensional rubber-like silicone network. Hydrogen is also a reaction by-product that acts as a foaming agent.

The homogeneity and void distribution of the LiCl-PDMS foam were analyzed on the material cross-sectional area at 50× magnification by using a 3D optical digital microscope, specifically HK-8700 (Hirox, Tokyo, Japan).

#### *2.2. Hydration/Dehydration Cycle through Thermogravimetric Dynamic Vapor Sorption System*

A complete hydration/dehydration cycle in a controlled (temperature, RH) and measurable (mass change) environment was performed through a thermogravimetric dynamic vapor sorption system (DVS Vacuum Surface Measurement Systems). The system consists of a micro-balance (precision of ±0.1 μg) and a water vapor pressure flow controller placed in the measuring chamber. Before the test, the sample was dehydrated at 150 ◦C under vacuum for 2 h. The hydration/dehydration cycle was performed in isothermal mode at 30 ◦C and varying the RH from 0 to 90%.

#### *2.3. In-Situ Characterization of LiCl-PDMS Foam*

In situ X-ray diffraction (XRD, D8 Advance Bruker diffractometer Bragg-Brentano theta-2theta configuration, Cu Kα, 40 V, 40 mA) was carried out on the LiCl-PDMS foam while dehydrating. The diffractometer was equipped with a heating chamber (HTK 1200N, Anton Paar) that enabled the material dehydration reaction from r.T. up to 80 ◦C under a nitrogen flow. A heating/cooling rate of 10◦C/min was used. Each scan was acquired in the 2θ range of 10–80◦, with a step size of 0.010◦ in 0.1 s, immediately after reaching the target temperature and after 30 min of holding time to allow for material equilibration. After the heating step (dehydration), the material was cooled to r.T. under nitrogen flow and XRD patterns were collected immediately after reaching the set temperature as well as after 30 min of holding time. Additionally, after cooling, the material was left for 1 h under atmospheric conditions to rehydrate and XRD was collected. Lattice parameters and the phase fraction of hydrate and anhydrous LiCl salt were evaluated by Rietveld refinement using the GSAS-II Crystallography Data Analysis Software [26].

Morphological observation of LiCl-PDMS foam while hydrating was carried out by an environmental scanning electron microscope (ESEM, FEI Quanta 450) operating with an accelerating voltage of 8 kV. Initially, the material was dehydrated in oven at 80 ◦C for 12 h and then placed in the ESM chamber for acquiring the micrographs under controlled water vapor atmosphere. The relative humidity (RH) was varied between 0 and 90% by tuning the temperature and water vapor pressure in the ranges of 5–40 ◦C and 10–800 Pa. Specifically, the micrographs were acquired after an equilibration time of 30 min under isothermal conditions (40 ◦C), varying the water vapor pressure from 10 Pa to 800 Pa (0.1–10.9% of relative humidity), and then under isobaric conditions (800 Pa), varying the chamber temperature from 40 ◦C to 5 ◦C (10.9–91.3% of relative humidity). The analysis was concluded, restoring the initial conditions. For clarity, a scheme of the ESEM analysis cycle conditions is shown in Figure 1.

**Figure 1.** Schematic representation of environmental scanning electron microscopy (ESEM) analysis cycle conditions.

#### *2.4. Hydration Heat Capacity Measurement*

The hydration enthalpy, namely the heat release capacity, was evaluated through a modified coupled TG/DSC apparatus (Setaram LabsysEvo) that enables measurements under saturated vapour working conditions, as described elsewhere in literature [27,28]. Briefly, the system was equipped with a glass evaporator whose temperature was controlled by an external thermo-cryostat. The entire system was placed in a thermostatic box that allows for preventing condensation on the internal surfaces of the circuit or in the measuring chamber, and the chamber and evaporator pressures were continuously monitored. According to the testing procedure, ~16 mg of the material was placed into the measuring chamber of the TG/DSC apparatus and degassed at 150 ◦C for 12 h under vacuum (~10–3 mbar) to completely dehydrate the sample. Subsequently, the sample temperature was cooled down to the initial adsorption temperature (80 ◦C) and water vapor (vapor pressure of 12 mbar) was streamed in the measuring chamber. The sample temperature was decreased to the final discharging temperature of 35 ◦C, corresponding to an RH of ~22%, after which it was isothermally held for 140 min. This results in a sample mass gain due to the water uptake. Hence, the heat involved in the hydration reaction can be estimated from the integration of the DSC signal.

#### **3. Results**

#### *3.1. LiCl-PDMS Foam Optical Analysis*

The as-synthesized LiCl-PDMS foam has a measured apparent density (ρfoam) and solid density (ρsolid) of 0.628 and 1.094 g/cm3, respectively, and thus a porosity of 42%, calculated as:

$$P(\%) = 1 - \frac{\rho\_{\text{foam}}}{\rho\_{\text{solid}}} \cdot 100 \tag{1}$$

The apparent density was calculated as the sample weight to volume ratio, while the solid density was calculated, applying the mixture rule, by using the constituent content in the composite foam. A 3D optical image of the cross-sectional area of the LiCl-PDMS foam at 50× magnification is shown in Figure 2. The foam exhibited both open and closed porosities caused by the foaming process occurring during the synthesis. The porosities' size broadly varied from 1 to 50 mm and it can be argued that the larger porosities were likely formed due to the coalescence of several bubbles. The LiCl salt appeared to be distributed on the surface of the open porosities but was also embedded by the matrix. The optical analysis was carried out under environmental conditions and this allowed the salt to absorb the humidity present in the atmosphere and hydrate. From the inset of Figure 2, some drops are clearly visible on the surface of the foam, likely due to the deliquescence phenomenon. Indeed, the open porosities present on the surface of the foam allow for the exposure of LiCl to the humidity, thus causing the deliquescence phenomenon.

#### *3.2. In-Situ X-ray Diffraction of LiCl-PDMS Foam during Dehydration Reaction*

We report, in Figure 3, the crystal structure evolution of LiCl-PDMS foam while dehydrating through in situ XRD measurements. The XRD patterns code, the experimental conditions, the refined lattice parameters, and the volume and phase fraction are reported in Table 1.

It is well known from literature that LiCl, besides anhydrous, exists in another four solid hydrate forms with one, two, three, and five water molecules, and that these salts are extremely hygroscopic and soluble in water. As inferred from XRD analysis in Figure 2 (pattern (a)), at the initial conditions, namely r.T. and when exposed to air, the LiCl in the PDMS foam is completely amorphous (Figure 3 (pattern [a])), thus indicating that the salt is overhydrated.

**Figure 2.** Three-dimensional optical image of (**a**) cross-section area of LiCl-PDMS foam at 50× magnification. (**b**) Higher magnification of a portion of the foam.

**Figure 3.** In situ XRD analysis of LiCl-PDMS foam under dry inert atmosphere while heating with measurement temperature as indicated (◦C).

After 30 min under a dry N2 flow, lithium chloride was monohydrated (PDF 04-013- 8884) and exhibited a tetragonal structure (P42/nmc; Figure 3 (pattern [b])). After 1 h under the same conditions, the material began to dehydrate (Figure 3 (pattern [c])) and the cubic (Fm-3m) anhydrous phase (PDF 00-004-0664) was present in ~3.7% of the material fraction, as calculated from the Rietveld refinement. As the temperature increased to 50 ◦C, the most intense reflections indicative of the cubic structure (indexed (111) and (200)) began to be more clearly distinguishable (Figure 3 (pattern [d])), while the ones indicative of the tetragonal structure (indexed (202) and (220)) significantly decreased in intensity. From the phase fraction evolution in Table 1, it is clear that most of the dehydration reaction occurs during the isothermal holding step at 50 ◦C (Figure 3 (pattern [e])). Indeed, after

reaching this temperature, the fraction of the anhydrous phase increased by about 60%. At 80 ◦C, the material was fully dehydrated (Figure 3 (pattern [f])), and a slight increase in the lattice parameters was observed after the isothermal hold at 80 ◦C for 30 min (Figure 3 (pattern [g]), Table 1). As lithium chloride transformed from monohydrate to anhydrous, it underwent a significant volume contraction, reduced by ~70% of its initial volume.

**Table 1.** In situ XRD pattern code, experimental conditions, refined lattice parameters, volume, and phase fraction.


After the complete dehydration reaction, the temperature in the XRD chamber was decreased to r.T. and the collected XRD pattern is reported in Figure 4. The analysis after cooling at r.T. (Figure 4, patterns (h) and (i)) showed the persistence of the anhydrous phase obviously due to the lack of humidity in the chamber. In order to rehydrate the material, the LiCl-PDMS foam was exposed to air at r.T. for 1 h. The collected XRD pattern (Figure 4, pattern (j)) showed a completely amorphous structure due to the overhydration of the LiCl salt. This, as expected, is indicative of the complete reaction reversibility.

**Figure 4.** In situ XRD analysis of LiCl-PDMS foam under dry inert atmosphere after cooling at r.T.

#### *3.3. Hydration/Dehydration Thermochemical Behavior*

The LiCl-PDMS foam thermochemical behavior during a dehydration/hydration cycle was characterized through a thermogravimetric dynamic vapor system under isothermal conditions at 30 ◦C by varying the chamber RH in the range of 0–90% and the results are shown in Figure 5. The mass change was normalized to the salt hydrate content (40%) in the foam.

**Figure 5.** Water vapor hydration/dehydration isotherms (DVS) of the LiCl composite foams at 30 ◦C.

While increasing the RH in the chamber, the hydration conditions became more favorable for the reaction to proceed and the mass uptake progressively increased. More specifically, the material exhibited a small water uptake at low partial pressure (below 1% RH). At intermediate RH values, between 5 and 25%, a quasi-linear water uptake from ~17 to ~143 wt.% was observed. Finally, above an RH of 25%, a notable mass uptake from ~143 up to 550 wt.% was observed. Taking into account the hydration reaction of LiCl salt, which is

$$\text{LiCl}\_{(s)} + \text{H}\_2\text{O}\_{(g)} \rightleftharpoons \text{LiCl} \cdot \text{H}\_2\text{O}\_{(s)}$$

and the molecular mass of both reagents and products (MLiCl = 42.39 g/mol, MLiCl·H2O = 60.41 g/mol), a theoretical mass uptake of 42.39% is expected when LiCl converts into its monohydrated phase. Hence, it is evident from the gravimetric profile that the full conversion of LiCl into its monohydrated phase occurs at an RH of between 10 and 15%. Above this value, the material is overhydrated and it reaches up to ~13 water molecules. The dehydrated LiCl can exist in the solid form at a low temperature below 20 ◦C. Hence, the water intake over the monohydrated threshold at the experimental temperature of 30 ◦C could be caused by the capillary condensation mechanism and the deliquescence phenomenon is expected to take place.

A hysteresis was observable between the dehydration and hydration reactions in the RH range of 0–25%, while above this RH value, the hydration and dehydration profiles almost perfectly overlapped. In general, the presence of hysteresis between the sorption and desorption isotherms indicates that the water diffusion through the material structure is slower as the lattice re-arranges upon hydration [29]. This can be applied to LiCl salt, indeed, as previously observed from in situ XRD analysis (Figures 3 and 4), wherein the

material transformed from a completely amorphous structure to a more ordered one when it converts progressively from the (over)hydrated to the mono as well as dehydrated phase.

In Figure 5, we report the RH values adopted for the ESEM conditions that will be discussed in the following Section 3.3.

#### *3.4. Morphological Characterization of LiCl-PDMS Foam during Hydration Reaction*

An environmental SEM analysis was conducted on the LiCl-PDMS foam simulating the hydration/dehydration reaction conditions. This approach allowed for visually evaluating the morphological modification while hydrating and dehydrating the LiCl salt embedded in the porous silicon matrix. The dehydration and hydration conditions were obtained with varying temperature and water vapor pressure in the ranges of 5–40 ◦C and 10–800 Pa, respectively, in order to vary the chamber RH at which the material was exposed.

Initially, the material was fully dehydrated and the RH during the analysis was increased from 0.1% to 91.3% in order to favor the hydration reaction. Afterwards, the RH was decreased to 0.1% to favor the LiCl salt dehydration reaction. Hence, the experiment reproduced the operating conditions of a whole hydration/dehydration cycle. In Figure 6, we report the micrographs at the selected pressure and temperature conditions in the climatic chamber of the microscope. The area selected for the analysis is an inner part of the foam and is indicative of the overall morphology.

At an RH of 0.1%, according to the DVS analysis, the LiCl salt in the PDMS foam should be in its anhydrous phase, and from SEM micrographs on the foam surface, some asperities (1) and bubbles (2) are visible (Figure 6a). It is likely that the asperities consist of loose salt agglomerates, while the bubbles, with a smooth surface and different sizes, might embed the salt in the foam matrix, thus functioning as a covering layer.

By increasing the RH to 5.4%, no visible morphological modifications could be detected (Figure 6b). Nevertheless, at such an RH value, the beginning of the hydration was expected from the hydration profile, with a water mass uptake of ~17 wt.%. As the RH increased to 10.9%, the hydration reaction was more promoted and the bubbles began to swell (Figure 6c), and their volume increased as the RH increased clearly due to the progressive acquisition of more water molecules (Figure 6d,e), in agreement with the DVS analysis.

At an RH of 25.1%, the conversion of LiCl into its monohydrate phase was complete (Figure 5), and above this RH value, the deliquescence phenomenon was expected to occur. The bubbles doubled and tripled their volume at an RH of 91.3% (as indicated by point 3 in Figure 6e). The asperities (red circles in Figure 6e) increased their volume as well as the RH increased, assuming a roundish shape typical of a drop, thus likely indicative of the deliquescence phenomenon. No detachment of the salt from the foam was observable because of the volume expansion. Nevertheless, the loose salt (asperities) appeared to be slightly agglomerated. Additionally, no damages were evident on the PDMS foam where the salt was embedded (bubbles), thus indicating the ability of the material to easily expand and contract as a consequence of the hydration/dehydration cycles and to both effectively protect and retain the embedded salt hydrate.

As final step of the ESEM analysis, the environmental chamber conditions were restored to the initial ones, that is, an RH of 0.1%. Under these conditions, the dehydration reaction was induced and the hydration/dehydration cycle was completed. It could be clearly observed that the material morphology was almost unmodified, that is, no damages due to the retained water and volume expansion/contraction were present, thus preserving the reusability of the foam. The presence of incomplete dehydrated salts cannot be exclued. Remarkably, no defects or cracks were detected, confirming the effectiveness of this approach for obtaining a durable, flexible, and porous-sorbents composite.

#### *3.5. Energy Storage/Release Density*

To estimate the foam energy storage/release density, the heat release while hydrating was measured with a coupled TG/DSC analysis under pure water vapor atmosphere to simulate the typical operation of a TES system working with a closed cycle. The TG/DSC profile of LiCl-PDMS foam under the temperature drop of 80–35 ◦C at 12 mbar of water vapor pressure (10 ◦C of evaporation temperature) is shown in Figure 7a. The material began to uptake water at ~60 ◦C, reaching a plateau at 35 ◦C, that is, under 12 mbar of water vapor pressure, it corresponded to an RH of ~22%. Under this condition, the total percentage of the normalized mass gain was ~170%, corresponding to ~4 water molecules, which is in good agreement with the DVS analysis (see Figure 5). Hence, the material was overhydrated. From the integration of the heat flow profile, the LiCl-PDMS foam energy density associated with this water uptake was estimated to be ~1854 kJ/kgfoam (or ~323 kWh/m3). From the deconvolution of the heat flow signal, as shown in Figure 7b, it is evident that four thermal events (exothermic) occurred while the material hydrated. The mass uptake related to the first peak (~40%) is very close to the one occurring when LiCl converted into its monohydrated phase (42.39%), while the second one began at a temperature of 45 ◦C, that is, under 12 mbar of water vapor pressure, it corresponded to an RH of 12.5%, very close to the value at which a saturated solution of lithium chloride will form an equilibrium (12.4%). Similarly to the DVS analysis, the remaining two peaks could be associated with the water intake over the monohydrated threshold, which can likely be caused by the capillary condensation mechanism in the foam.

**Figure 7.** *Cont.*

**Figure 7.** (**a**) TG/DSC profile of LiCl-PDMS foam under the temperature drop of 80–35 ◦C at 12 mbar of water vapor pressure, corresponding to 10 ◦C of evaporation temperature. (**b**) Deconvolution of DSC profile.

#### **4. Final Remarks**

From the morphological analysis, no defects or cracks were detected, confirming the effectiveness of this approach for obtaining a durable, flexible, and porous-sorbents composite. Nevertheless, it has to be pointed out that LiCl is not completely embedded by the PDMS foam but, rather, it is partially deposited on the external foam surface. Critically, this superficial salt could affect the overall TES performances due to possible corrosion processes. Hence, this issue should be addressed. Additionally, the material stability at long aging cycles is a key point for thermochemical energy storage applications and will be addressed in the following studies.

With respect to some of the most recently investigated LiCl-based composites (see Table 2), the LiCl-PDMS foam exhibits larger heat storage/release capacity and energy density under both similar testing conditions and similar LiCl content. Hence, it can be considered a competitive candidate among other LiCl salt hydrate composites.



#### **5. Conclusions**

Here, we investigated a composite material based on silicone vapor-permeable foam filled with 40 wt.% of LiCl (LiCl-PDMS), aiming at confining the salt in a matrix to prevent deliquescence-related issues but without inhibiting the vapour flow. Specifically, we studied the structural and morphological modification of the composite foam during the hydration/dehydration cycles through in situ XRD and ESEM analysis. In situ XRD enabled the material dehydration reaction from room temperature up to 80 ◦C, while the environmental SEM analysis simulated the hydration/dehydration reaction conditions, thus allowing for the evaluation of the interaction between the salt and the water vapor environment in the confined silicon matrix. The results show an effective embedding of the material, which limits the salt solution release when overhydrated. Additionally, the flexibility of the matrix allows for the volume shrinkage/expansion of the salt caused by the cyclic dehydration/hydration reactions without any damages to the foam structure. Indeed, as lithium chloride transformed from monohydrate to anhydrous, a significant volume contraction was observable (~70% of its initial volume). The LiCl-PDMS foam energy density was measured with a customized coupled thermogravimetric/differential scanning calorimetric analysis (TG/DSC) during the hydration phase and the estimated energy density value was ~1854 kJ/kg (or ~323 kWh/m3), thus making it a competitive candidate among other LiCl salt hydrate composites.

**Author Contributions:** Conceptualization, L.C. and E.P.; investigation, E.P., A.F., D.P. and E.M.; data curation, E.M. and D.P.; validation, L.C., E.P., A.F. and E.M.; writing—original draft preparation, E.M.; writing—review and editing, E.M., E.P. and L.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research study was funded by the Italian Ministry of University and Research (MUR), program PON R&I 2014/2020—Avviso n. 1735 del 13 July 2017—PNR 2015/2020, under project "NAUSICA—NAvi efficienti tramite l'Utilizzo di Soluzioni tecnologiche Innovative e low CArbon", CUP: B45F21000680005.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**


### *Article* **Thermal Stability of Ionic Liquids: Effect of Metals**

**Francesca Nardelli 1,2, Emilia Bramanti 2, Alessandro Lavacchi 3, Silvia Pizzanelli 2,4, Beatrice Campanella 2, Claudia Forte 2,4, Enrico Berretti <sup>3</sup> and Angelo Freni 2,\***


**Abstract:** We investigated the thermal stability and corrosion effects of a promising ionic liquid (IL) to be employed as an advanced heat transfer fluid in solar thermal energy applications. Degradation tests were performed on IL samples kept in contact with various metals (steel, copper and brass) at 200 ◦C for different time lengths. Structural characterization of fresh and aged IL samples was carried out by high-resolution magic angle spinning nuclear magnetic resonance and Fourier transform infrared spectroscopic analyses, while headspace gas chromatography–mass spectrometry was employed to evaluate the release of volatile organic compounds. The combination of the abovementioned techniques effectively allowed the occurrence of degradation processes due to aging to be verified.

**Keywords:** ionic liquids; heat storage; thermal stability; HRMAS NMR; FTIR

**1. Introduction**

Ionic liquids (ILs) are a group of compounds that are attracting increasing interest in many fields of application, thanks to the possibility of combining different anions and cations, thus allowing the design of new materials with optimal chemical–physical properties for specific applications, especially in the energy sector [1,2]. In particular, ILs are suggested as promising working fluids in solar energy technologies, thanks to their high heat capacity, low melting point and relatively high density in the typical operating conditions of solar thermal energy systems [3–6]. Further attractive features of ILs are the high chemical stability, non-flammability, and the low impact on the environment and on health; this feature derives from their negligible vapor pressure, which limits their release in the atmosphere [7]. Given the wide application potential, evaluation of ILs' thermal stability is fundamental for their implementation in solar energy systems as working fluids [8–10].

Most of the thermal stability studies available in the literature are based on dynamic thermogravimetric (TG) analyses [11,12]. However, several experimental parameters, such as sample mass, pre-treatment conditions, heating rate and testing atmosphere (inert gas or open air), can affect measurement consistency [13]; therefore, TG analysis appears to be more appropriate for comparative thermal stability studies [14], and certainly cannot provide a deep insight into the modifications of the IL structure due to thermal stress. Another issue of relevance is metal corrosion in the presence of ILs. In fact, several R&D activities in the field have focused on the investigation of the corrosion behavior of different metals in contact with ILs, and on the evaluation of the release of volatile compounds during operation in solar thermal devices and processes [15–19].

**Citation:** Nardelli, F.; Bramanti, E.; Lavacchi, A.; Pizzanelli, S.; Campanella, B.; Forte, C.; Berretti, E.; Freni, A. Thermal Stability of Ionic Liquids: Effect of Metals. *Appl. Sci.* **2022**, *12*, 1652. https://doi.org/ 10.3390/app12031652

Academic Editors: Angela Malara and Frontera Patrizia

Received: 5 January 2022 Accepted: 1 February 2022 Published: 4 February 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

To address the previously mentioned issues, in this paper, we present a multi-technique approach to identify possible degradation products of a promising IL subjected to thermal aging in the absence or presence of different metals. Specifically, we have characterized the ionic liquid N-tributyl-N-methylammonium bis(trifluoromethanesulfonyl)imide ([TBuMA][NTF2]) after thermal treatment at T = 200 ◦C for 4, 24 and 168 h in contact with AISI 304 steel, copper or brass, as well as in the absence of metals. The temperature used for aging was selected because it is the standard operating temperature of common diathermic oils used as heat transfer fluids. This specific ammonium-based IL compound was chosen for its potential application as a heat transfer fluid [5,12], and for the stability of the anion [20], which has been reported to withstand temperatures up to 400 ◦C by thermogravimetry. However, to the best of our knowledge, there is no indication of the thermal stability of NTF2 coupled with a quaternary ammonium salt. The characterization of the degraded IL was carried out by high-resolution magic angle spinning nuclear magnetic resonance (HRMAS NMR) and Fourier transform infrared (FTIR) spectroscopies, while headspace gas chromatography–mass spectrometry (HS-GC-MS) was employed to estimate the concentration of volatile compounds produced. HRMAS NMR is suitable for the characterization of highly viscous liquids. In this tecnique, the use of magic angle spinning allows highly resolved spectra to be obtained, which is not feasible using standard solution NMR spectroscopy, due to the presence of residual interactions and magnetic susceptibility issues [21].

#### **2. Materials and Methods**

#### *2.1. Materials*

The ionic liquid N-tributyl-N-methylammonium bis(trifluoromethanesulfonyl)imide (C15H30F6N2O4S2), CAS number 405514-94-5; MW 480.53, Figure 1a, was purchased from Solvionic (Am3408a). Purity of the IL was 99.9%.

**Figure 1.** (**a**) Structure of the ionic liquid [TBuMA][NTF2]. (**b**) In sequence, from left to right, samples of the ionic liquid in the presence of steel, copper and brass metal plates and with no metal plate. The same samples were heated at 200 ◦C for 4 h (**c**), 24 h (**d**) and 168 h (**e**).

The degradation procedure was performed as described in the following. Six milliliters of [TBuMA][NTF2] was heated in an oven at 200 ◦C for 7 days with or without a steel, copper or brass metal plate (2 × 2 cm). At selected times (4, 24 and 168 h; Figure 1b–e, respectively), 1 mL of IL was sampled for the analyses (FTIR, HS-GC-MS and HRMAS NMR spectroscopy).

Table 1 summarizes the thermally treated samples analyzed and the code used throughout the text. The code of the initial non-heated sample is B.



#### *2.2. FTIR Spectroscopy*

Infrared spectra were recorded in reflectance mode by using a Perkin–Elmer Frontiers FTIR Spectrophotometer, equipped with a universal attenuated total reflectance (ATR) accessory and a triglycine sulphate TGS detector. Three replicates (3–5 μL of IL for each measurement) were performed after background acquisition. For each sample, 32 scans were recorded, averaged and Fourier transformed to produce a spectrum with a nominal resolution of 4 cm<sup>−</sup>1.

#### *2.3. HS-GC-MS Analysis*

HS-GC-MS analyses were performed using an Agilent 6850 gas chromatograph, equipped with a split/splitless injector, in combination with an Agilent 5975c mass spectrometer. A CTC CombiPAL autosampler was employed for HS sampling. Vials with 1 g of sample were incubated at 80 ◦C for 15 min. A 0.5 mL HS volume was then sampled (gas-tight syringe held at 85 ◦C) and injected into the GC. The syringe was then flushed with helium. The inlet liner (internal diameter of 1 mm) was held at 200 ◦C and the injection was performed in splitless mode. Compounds were separated on a polar column (DB-WAX ultra-inert; length: 30 m; stationary phase: bonded polyethylene glycol; 0.25 mm inner diameter; 0.50 μm coating) using the following temperature program: 10 min at 30 ◦C, then increased by 5 ◦C/min to 60 ◦C (held for 2 min) followed by an increase of 10 ◦C/min to 240 ◦C (held for 9 min). The temperature of the transfer line was set at 250 ◦C. After GC separation, compounds were ionized in positive EI, and the acquisition was performed in full scan mode. Spectral identification was performed when the spectra and the NIST spectral mass library (NIST 05) combined with our in-house library matched with a spectral similarity >90%. Results are reported as relative intensity (counts).

#### *2.4. HRMAS NMR Spectroscopy*

NMR spectra were acquired on a Bruker AVANCE NEO NMR Spectrometer, working at a 1H Larmor frequency of 500.13 and 125.77 MHz for 1H and 13C nuclei, respectively, and using an HRMAS probe. All samples were spun at 6 kHz. The samples were dissolved in DMSO-d6 (99.7% deuterated, Sigma) (1:1 volume ratio) to provide the lock signal and to reduce their viscosity, thus facilitating their insertion in the rotors; TMS was added to each mixture for 1H spectral referencing. Following this, 50 μL of each mixture was transferred to an HRMAS rotor for NMR analysis. 1H spectra were acquired on all samples using a relaxation delay of 1 s and several scans ranging from 128 to 1000 depending on the sample. 13C spectra were acquired on samples B, 1B168, 2B168 and 4B168 using the Bruker *zg30pg* pulse sequence for NOE enhancement of carbon nuclei signals. A relaxation delay of 2 s was used and 4k scans were accumulated. One-dimensional (1D) selective 1H total correlation spectroscopy (TOCSY) and two-dimensional (2D) 1H-13C heteronuclear single quantum coherence (HSQC) experiments were also performed on sample 4B168. For the TOCSY experiments, the Bruker *seldigpzs* pulse sequence was used, with a Gaussian shaped 180◦ pulse (Bruker pulse shape: Gaus1\_180r.1000) for selective excitation, a relaxation delay of 1 s, and a mixing time of 80 ms. 1H–13C HSQC were obtained by employing the Bruker

*hsqcetgpsisp2.2* pulse sequence, with a relaxation delay of 1 s. For all experiments, a 1H 90◦ pulse of 7 μs and a 13C 90◦ pulse of 12 μs were used. All experiments were performed at 298 K.

#### **3. Results and Discussion**

Figure 1 shows that, after 4 h, all the samples displayed a brown color, indicating that the thermal treatment degrades [TBuMA][NTF2]. The color was more intense in the presence of metals, particularly steel and copper, and became darker with longer heating times. To understand the decomposition pathway(s), FTIR and HRMAS NMR experiments were performed on all the samples. Figure 2 shows the ATR-FTIR spectra of [TBuMA][NTF2] after 4, 24 and 168 h (samples 4B4, 4B24 and 4B168) of thermal treatment without metal plates. The spectrum of untreated IL (sample B) is also reported for comparison.

**Figure 2.** Representative ATR-FTIR spectra of B, 4B4, 4B24 and 4B168 samples in the 3400–2650 and <sup>1740</sup>−600 cm−<sup>1</sup> regions.

As far as the IL anion is concerned, the strong absorptions at 1347 and 1177 cm−<sup>1</sup> are attributed to asymmetric and symmetric S=O stretching vibrations, respectively, the band at 1052 cm−<sup>1</sup> to asymmetric C–F stretching, the band at 739 cm−<sup>1</sup> to asymmetric S–N stretching, and the band at 614 cm−<sup>1</sup> to S=O scissoring. Specific TBuMA cation signals are expected at 1134 cm<sup>−</sup>1, ascribable to symmetric C–N stretching, around 1470 cm−<sup>1</sup> due to methyl and methylene C–H bending, and at 1465 and 1378 cm−<sup>1</sup> due to C–H scissoring and methyl rocking, respectively. The spectra of all the samples are basically identical, except for slight differences in the 3250–3000 cm−<sup>1</sup> region (Figure 3), suggesting that the anion is not affected by the thermal treatment, and indicating a major involvement of the cation in the thermal degradation. Inspection of this region highlights that the original structure of the TBuMA cation, characterized by the large band at 3348 cm−<sup>1</sup> and the shoulder at 3040 cm<sup>−</sup>1, due to ammonium absorptions, is modified after thermal treatment. The most significant changes are the decrease in the band at 3348 cm−<sup>1</sup> and the increase in the peak in the region between 3200 and 3100 cm<sup>−</sup>1, both in the presence and absence of metals. This peak has been assigned to the medium intensity band of unsaturated hydrogen stretches (C=C–H) [22], and suggests the formation of alkenes.

**Figure 3.** ATR-FTIR spectra of all samples in the 3400–2650 cm−<sup>1</sup> region.

Figure 4 shows the trend of the area of the band at 3153 cm−<sup>1</sup> (3208–3103 cm−<sup>1</sup> baseline points) of the IL spectra with or without metal plates, as a function of the duration of the thermal treatment.

**Figure 4.** Area of the peak at 3153 cm−<sup>1</sup> in the IL spectra after 4, 24 and 168 h of thermal treatment with or without metal plates as a function of incubation time.

Further insight on the degradation process was gained from HRMAS NMR spectroscopy. The comparison of the 1H NMR spectrum of the original compound [TBuMA][NTF2] with the spectra recorded on [TBuMA][NTF2] after heating for 4, 24, and 168 h (samples 4B4, 4B24, and 4B168) reveals that the signals of the original cation remain dominant, even at the longest heating time. Low-intensity peaks, due to the degradation products, appear in the samples subjected to thermal treatment, and their intensity tends to increase with the heating time. Figure 5 shows the NMR spectra of samples B (traces a and b) and 4B168 (traces c and d). Four new signals, resonating at 9.16, 8.31, 2.75 and 2.57 ppm, appear in the latter. Additional signals of lower intensity appear in the region between 5 and 6 ppm upon heating. Complete characterization of the degradation compounds was accomplished by the analysis of 1H, 1D selective 1H TOCSY, 13C and 2D 1H–13C HSQC spectra of 4B168 (Figures S1 and S2). The assignments of 1H and 13C NMR signals are reported in Tables S1 and S2.

**Scheme 1.** Decomposition pathways of cation TBuMA. The labeling on each compound is used for the assignment of the NMR signals.

**Figure 5.** 1H HRMAS NMR spectra of [TBuMA][NTF2] (**a**,**b**) and 4B168 (**c**,**d**); in (**b**,**d**), the vertical scale used in (**a**,**c**) is expanded by the factors reported on each spectral region. Selected signals of the degradation compounds are labeled as "Hi,j", with i representing the atom number and j indicating the degradation product (Scheme 1). Signals of water protons and residual protons of deuterated DMSO are marked with asterisks.

The dominant thermal degradation products are N-dibutyl-N-methylammonium (1) and N-butyl-N-methylammonium (2), as outlined in Scheme 1; these compounds are compatible with the Hoffman elimination of one or two alkyl chains from the original cation [23]. For these compounds, the signals due to the hydrogen atoms labeled as H1 and H2 in Scheme 1 are clearly observable. Additional signals, characterized by lower

intensities, were assigned to 1-butene (Scheme 1, compound 3), in agreement with the hypothesized degradation pathway. These assignments are indicated in Figure 5, where the signals are labeled as "Hi,j", with i representing the atom number and j the degradation product. The amount of compounds 1 and 2 was determined from the 1H spectra of all the samples, with respect to the amount of the original cation B, using the integrals of the H2,1 and H2,2 signals. Figure 6 shows the values of these integrals as a function of the heating time, where the intensity of the corresponding signal of the non-degraded ionic liquid, occurring at 2.97 ppm, is arbitrarily set to 100. It was, thus, found that, even at the longest heating time, about six molecules of compound 1 and two molecules of compound 2 were present every 100 molecules of ionic liquid cation. This estimate agrees with that obtained using H1,1 and H1,2 signals. Interestingly, the concentration of compound 2 increased at a slower rate than compound 1, in agreement with the fact that the formation of compound 2 requires the preliminary formation of compound 1. Moreover, the amount of compound 3 was always much lower than that expected on the basis of the stoichiometry of the degradation pathways, as clearly evident from the large scaling factor necessary to visualize the signals of this compound (Figure 5d). This can be explained by the volatility of 1-butene or its further degradation/reactions. The presence of 1-butene is compatible with the alkene absorption band between 3200 and 3100 cm−<sup>1</sup> detected by FTIR (3090 cm−<sup>1</sup> in the FTIR spectrum of butene in the vapor gas phase). Unfortunately, the absence of specific absorption lines prevented the detection of compounds 1 and 2 in the FTIR spectra.

**Figure 6.** Intensity of H2,1 (black squares) and H2,2 (red circles) signals as a function of the heating time, as obtained from the 1H HRMAS NMR spectra of B, 4B4, 4B24 and 4B168 (filled symbols) and of B, 1B4, 1B24 and 1B168 (empty symbols). The intensity value is relative to that of H2 protons of the non-degraded ionic liquid, which was set to 100 in all spectra.

The 1H NMR spectra recorded after heating for up to 168 h in the presence of metal plates do not show significant differences with respect to the 4B168 spectrum (Figure 7). The main signals of the NMR spectrum of 4B168 are those of the original ionic liquid. For both steel and copper, the spectral lines are broadened, with steel inducing larger line broadening of the signals compared to copper. This broadening, not observed in the case of brass, is probably due to the presence of dissolved paramagnetic metal ions resulting from corrosion of the metal/alloy. Figure 8 shows that the signals of the degradation compounds 1, 2 and 3 are also present in the spectra of samples 1B168 and 3B168, whereas, in the case of 2B168, only sharp signals, due to compound 3, are observed. In the latter case, if compounds 1 and 2 are present, their concentration is below detection limits. However, the occurrence of 1-butene signals suggests that the elimination reactions sketched in Scheme 1 also take place in the presence of copper. The absence of the H1,1 and H1,2 signals could be explained by hypothesizing that the ammonium compounds 1 and 2 release H+ and the amine formed coordinates to a copper ion. A similar mechanism has been suggested to rationalize the extraction of Cu2+ ions from aqueous solutions using protic ammonium

ionic liquids [24]. However, due to the large linewidth of the signals, it was not possible to confirm this hypothesis.

**Figure 7.** 1H HRMAS NMR spectra of 4B168 (**a**), 1B168 (**b**), 2B168 (**c**), and 3B168 (**d**).

**Figure 8.** Expansions of 1H HRMAS NMR spectra of 4B168 (**a**), 1B168 (**b**), 2B168 (**c**), and 3B168 (**d**). Expansion factors are reported for each region. Selected signals of the degradation compounds are labeled using the same notation as that applied in Figure 5. Water (3.3–3.6 ppm) and residual DMSOd5 (2.50 ppm) signals are marked with asterisks.

For the samples degraded in the presence of steel, the kinetics of formation of compounds 1 and 2 were monitored. Figure 6 shows the trends of the intensity of the H2,1 and

H2,2 signals as a function of heating time in the samples degraded in the presence and absence of metals. The kinetics of formation of compound 2 from compound 1 seems to be accelerated by steel.

HS-GCMS analysis was performed on the 168 h aged samples, with the aim of observing possible volatile degradation compounds in the most extreme condition. Figure 9 shows the peak area of nine main compounds identified in samples B, 4B168, 1B168, 2B168 and 3B168; these are cyclohexane, butanal, ethyl acetate, tert-butanol, methyl-vinyl ketone, N,-N-dimethylformamide, 2-ethyl-1-hexanol and benzothiazole. However, 1-butene was not detected, likely because it was lost in the pre-analytical phase, considering its high volatility, or because of its oxidation. The relevant result is, indeed, the release of butanal in the 3B168 sample, i.e., in the IL treated with copper. We can hypothesize that copper and copper particles may act as catalyzers for the further reaction of butene. Recently, several authors have reported the remarkable long-term stability and high selectivity towards alkenes of Cu nanoparticles as a promising alternative to replace precious-metal-based catalysts in selective hydrogenation [25]. It must be pointed out that even the concentration of butanal was too low to be detected by FTIR and NMR. These volatiles probably result from minor side reactions occurring during the degradation process.

**Figure 9.** Relative intensity (counts) of 9 main compounds identified in samples B, 4B168, 1B168, 2B168 and 3B168 samples by HS-GCMS analysis.

#### **4. Conclusions**

The structural characterization of the ionic liquid [TBuMA][NTF2], both fresh and after thermal treatment, with or without different metals, was carried out by HRMAS NMR and FTIR spectroscopic analyses, while HS-GC-MS was employed to reveal the formation of volatile compounds. The degradation products of [TBuMA][NTF2] were characterized after thermal treatment at 200 ◦C for 4, 24 and 168 h in contact with AISI 304 steel, copper or brass, and without metals as a comparison. The combination of the above-mentioned techniques evidenced the occurrence of degradation processes of the cation. The data suggested a degradation mechanism compatible with the Hoffman elimination of one or two alkyl chains from the cation, with 1-butene being one of the degradation products after thermal treatment, both in the absence or presence of metal plates. The proposed multitechnique approach was revealed to be suitable for the characterization of the degradation compounds of [TBuMA][NTF2] after thermal treatment in the presence of metals, thus proving to be a promising method for the selection of IL compounds that possess high stability and a suitable lifetime to meet the durability requirements of commercial and industrial solar thermal applications.

**Supplementary Materials:** The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/app12031652/s1: Figure S1: comparison of 1H HRMAS spectrum of 4B168 (a,d) with 1D selective 1H TOCSY obtained by irradiating H1,1 (9.16 ppm, b), H1,2 (8.31 ppm, c) and H1a,3 (5.13 ppm, e).; Figure S2: 1H–13C HSQC spectrum of 4B168; Table S1: assignment of 1H and 13C NMR signals of the ionic liquid B; Table S2: assignment of 1H and 13C NMR signals of the degradation compounds 1, 2 and 3.

**Author Contributions:** Conceptualization, A.F., S.P., E.B. (Emilia Bramanti) and A.L.; methodology, A.F., S.P., E.B. (Emilia Bramanti) and A.L.; investigation, F.N., S.P., E.B. (Emilia Bramanti), B.C., A.L. and E.B. (Enrico Berretti); writing—original draft preparation, A.F., S.P., E.B. (Emilia Bramanti); writing—review and editing, F.N., C.F., A.L., E.B. (Emilia Bramanti), S.P.; supervision, A.F., C.F.; funding acquisition, A.F., A.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the FELIX project (Fotonica ed Elettronica Integrate per l'Industria, project code no. 6455) and project MIUR Cluster CTN02\_00018 «Energia» Codice progetto CTN02\_00018\_10016852 "NeMESi".

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

