Cellulose-based polymer electrolyte derived from waste coconut husk: Residual lignin as a natural plasticiser / Chua Kai Ying

Chua , Kai Ying (2021) Cellulose-based polymer electrolyte derived from waste coconut husk: Residual lignin as a natural plasticiser / Chua Kai Ying. Masters thesis, Universiti Malaya.

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Abstract

Polymer electrolyte (PE) finds many applications in the fabrication of electrochemical devices such as batteries, solar cells, supercapacitors, and electrochromic windows. The usage of cellulose from the waste of coconut husk is one alternative in using natural polymer as a green electrolyte. However, due to its high crystallinity, the usage of plasticiser is advantageous to reduce the crystallinity of cellulose. Lignin which co-exists with cellulose in the cell walls of the plants could act as a natural plasticiser in preparing a good polymer electrolyte. In this work, cellulose was isolated from waste coconut husk (Cocos nucifera), modified into water-soluble carboxymethylcellulose (CMC) and then made into a film by solvent-casting method. The chemical compositions of the cellulose fibre after each chemical treatment were analysed, and the cellulose derivatives were characterised by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD), thermogravimetric analysis (TGA) and electrochemical impedance spectroscopy (EIS). The cellulose fibre was found to have major removal of hemicellulose and lignin with one-time alkali treatment followed by twice bleaching treatments. The thermal degradation of all cellulose fibre samples was found to decompose at around 220 C and continued up to 400 C. Slight increase of degradation temperature was observed due to the chemical treatment of fibre which indicates that major removal of lignin and hemicellulose has been achieved for CH-A-2B sample. It was observed that lignin imparts amorphous character to the polymer and its presence was confirmed by a peak at 1509 cm-1. Then, the introduction of carboxymethyl group onto the cellulose backbone was confirmed by the appearance of three new FTIR peaks at 1320, 1416, and 1590 cm-1. The presence of lignin enhanced the organo-solubility of the CMC as proven by the results of viscosity and solubility tests. In addition, XRD analyses revealed that CMC prepared from cellulose with single bleaching treatment possessed lower crystallinity compared to that of double bleaching treatment. Besides, the crystallinity index was found to increase with the increasing crystallite size due to the chemical treatment onto the fibre, however, the parameters of carboxymethylation reaction does not affect the crystallite size significantly. The ionic conductivity of CMC-based polymer electrolyte (PE) film fabricated with an addition of various lithium and ammonium salts has been indicated through EIS analysis. Besides, the presence of lignin as a function of its conductivity,  was studied by comparing CH-A-1B and CH-A-2B CMC with several different carboxymethylation conditions. The highest conductivity was obtained from CH-A-1B-C:38:5 with a  = 4.82 × 10-4 mS cm-1. It was found that the electrochemical properties of the polymer electrolyte were influenced by the extent of carboxymethyl substitution, degree of crystallinity and lignin content. Moreover, PE films with various NH4NO3 salt contents have been fabricated and the highest conductivity  = 4.15 × 10-3 mS cm-1 was achieved by the PE where double carboxymethylation product CH-1-1B-dC:38:05 as host incorporated with 35 wt.% of NH4NO3 salt. Besides, dielectric study showed that the fabricated PE(s) obeyed non-Debye behaviour and were to be thermally activated obeying the Arrhenius behaviour.

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