Chitosan and Ionic Liquid Based Solid Polymer Electrolytes: The Anion Alkyl Chain Length Effect

It has been long recognized that polymer electrolytes (PEs) are potentially interesting as electrolytes for solid-state electrochemical devices ^1-3. It is also recognized that the most important properties of electrolyte solutions are non-volatility and high ionic conductivity. If the former property is easily fulfilled because polymers do not vaporize, only decompose at higher temperatures, the latter one is the major drawback that is delaying their application in commercial devices. To overcome this, the solvent-free polymer electrolytes (SPEs) community has been paying great attention to ionic liquids (ILs). Since ILs are composed only of ions, and show very high ionic conductivity, non-volatility and non-flammability ⁴.

Nowadays, safety, environmental issues also pose a significant concern. Because of that, some research groups have been turning their attention to natural polymers, due to their biodegradability, low toxicity and extraction from renewable sources, as fast growing plants and animals ⁵. Among the extensive range of the existing natural polymers is chitosan. Chitosan is obtained by deacetylation of its parent polymer chitin, a polysaccharide widely distributed in nature (e.g. crustaceans, insects and certain fungi) ⁶.

This work describes the results of the characterization of polymer electrolytes using chitosan matrix doped with ionic liquids series 1-ethyl-3-methylimidazolium dialkylphosphate [C₂mim][C_nPO₄], 1-ethyl-3-methylimidazolium alkylsulfonate [C₂mim][C_nSO₃] and 1-ethyl-3-methylimidazolium alkylsulfate [C₂mim][C_nSO₄]. This work studies the effect of increasing the alkyl chain length of the anion on the thermal, morphological and electrochemical properties of these SPEs.

The samples have been characterized by thermal analysis (TGA and DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), polarized optical microscopy (POM), complex impedance spectroscopy and cyclic voltammetry.

Acknowledgements

This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/QUI/UI0686/2011 and PEST-OE/EQB/LA0004/2011, projects PTDC/CTM-NAN/121274/2010 and PTDC/QUI-QUI/117340/2010, grant SRFH/BD/90366/2012 (R.L.) and Investigador FCT grant (J.M.S.S.E.).

References

1P.V. Wright, Br. Poly. J. 7 (1975) 319–327.

2 M. Armand, M. T. Duclot, J.M. Chabagno, In Proceedings of the Second International Meeting on Solid State Electrolytes; St. Andrews, Scotland, Sept 20_22, 1978; 1978; Extended Abstract 6.5.

3F.M. Gray, in Solid Polymer Electrolytes: Fundamentals and Technological Applications, VCH Publishers, Inc., New York, 1991.

4 H. Ohno, Electrochemical Aspects of Ionic Liquids, 2nd ed. John Wiley & Sons, Inc., New Jersey, 2011.

5R. Leones, F. Sentanin, L.C. Rodrigues, I.M. Marrucho, J.M.S.S. Esperança, A. Pawlicka, M.M. Silva, eXPRESS Polymer Letters 6 (12) (2012) 1007.

6 R.A.A. Muzzarelli, Natural chelating polymers: alginic acid, chitin and chitosan, Pergamon Press Oxford, New York, 1973.