Abstract
In the present work we report on the development of a novel and sustainable electrolyte based on chitin. Chitin biopolymer was carboxymethylated in simple, mild, and green conditions in order to fine-tune the final properties of the electrolyte. To this end, chitin was modified for various reaction times, while the molar ratio of the reagents, e.g., sodium hydroxide and monochloroacetic acid, was maintained fixed. The resulting chitin derivatives were characterized using various techniques. Under optimized conditions, modified chitin derivatives exhibiting a distinct degree of carboxymethylation and acetylation were obtained. Structural features, morphology, and properties are discussed in relation to the chemical structure of the chitin derivatives. For electrolyte applications, the ionic conductivity increased by three magnitudes from 10−9 S·cm−1 for unmodified chitin to 10−6 S·cm−1 for modified chitin with the highest degree of acetylation. Interestingly, the chitin derivatives formed free-standing films with and without the addition of up to 60% of ionic liquid, the ionic conductivity of the obtained solid electrolyte system reaching the value of 10−3 S·cm−1.
Highlights
Polymeric electrolytes are a class of polymeric materials widely used as an ionic conductive component in electrochemical devices
Ionic liquids (ILs) are molten salt or organic liquid salts, which possess properties distinct from conventional salts (e.g., NaCl) such as their ability to function as a solvent at room temperature, or their strong intermolecular [ion-ion] interactions that do not exist for other salts melted at high temperatures
The presence of the 1640 cm−1 peak representing the amide I in the Fourier Transformed Infra-Red (FTIR) spectra in CMChit_15, CMChit_30, and CMChit_45 derivatives confirms the conservation of N-Acetyl-D-glucosamine units of the native chitin structure, the degree of carboxymethylation is increasing, which demonstrates the appropriateness of the chosen reaction conditions to control the carboxymethylation of chitin while preserving the acetyl groups in the glucosamine units as much as possible [42]
Summary
Polymeric electrolytes are a class of polymeric materials widely used as an ionic conductive component in electrochemical devices. The structure of chitin is very similar to cellulose and comprises N-Acetyl-d-glucosamine units which are associated with β-type (1–4) links [18] This biopolymer has specific characteristics, such as its high molar mass [19] and low degree of deacetylation [20], as well as various crystalline states [21], that distinguish it from other natural polymers. The first studies on the carboxymethylation of chitin followed a protocol in which the reaction was performed in 40% sodium hydroxide solution at −20 ◦ C for 12 h [32], in recent studies, the conditions were optimized and less concentrated NaOH solutions (e.g., 8 wt.% NaOH in 4 wt.% urea) were used at −20 ◦ C for 36 has a first step and 15 ◦ C for another 72 h [33] Even under these conditions, the reaction remains complicated, as the risk of converting the chitin to chitosan is very high and the reaction time is very long. The new chitin derivatives were tested as SPE either alone or blended, for the first time to our knowledge, with ILs
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