Thermal Characterization of [C₂Im][NO₃] and Multivalent Nitrate Salts Mixtures

Abstract

Due to their intrinsic properties, the current applicability of ionic liquids is enormous. In particular, their use in electrochemistry is beyond question. Numerous studies on these compounds and their mixtures, especially with lithium salts, focus on their use as electrolytes for batteries and other energy storage devices. This includes thermal energy storage devices, where 4th generation ionic liquids and their derivatives show a huge potential. Nevertheless, considering the uneven availability of the raw materials, such as lithium, research has extended to mixtures of these compounds with other salts of different metals that are more abundant and widely distributed, such as magnesium or aluminum. This work presents a comprehensive thermal characterization, using differential scanning calorimetry and thermogravimetry, of the protic ionic liquid ethylimidazolium nitrate and its mixture with magnesium and aluminum nitrate salts at different concentrations. Additionally, a comparison between these results and previous studies of mixtures of this ionic liquid with lithium nitrate, as well as mixtures of the protic ionic liquid EAN with the same metal salts, was also performed. The results indicated that the salt addition tends to broaden and reduce crystallization and melting peaks, while the glass transition becomes more visible and shifts to higher temperatures with increasing salt concentration. This is due to the disorder generated by the rearrangement of ions in the polar domains, which erodes the hydrogen bond network of the protic ionic liquid. Nevertheless, the thermal stability of the blended samples does not change significantly compared to the bulk ionic liquid.

Keywords

• protic ionic liquids
• solvation paradigm
• beyond lithium metals
• thermal energy storage