Synthesis and characterization of benzoyl kappa carrageenan doped sodium perchlorate as highly conductive gel polymer electrolyte

Benzoyl kappa carrageenan (Bz-ƙcar) which is the derivative of ƙ-carrageenan (ƙcar) was successfully synthesized by chemical modification through Friedel-craft acylation reaction. Bz-ƙcar is designed for use as a gel polymer electrolyte in a variety of electrochemical devices. Ƙ-carrageenan is known for its high hydrophilicity due to the number of hydroxyl (OH) groups in its monomer units. In this case, ƙcar has strong interactions with water molecules, as the hydroxyl groups are capable of forming hydrogen bonds with polar water molecules. The presence of moisture in most of the electrochemical devices is not favorable. The interaction of moisture with air/moisture sensitive metal/parts in the devices might have caused an explosion and risked the surroundings. Therefore hydrophobic benzoyl chloride salt was used in the modification of ƙcar to produce Bz-ƙcar with reduced hydrophilicity. The ionic conductivity of Bz-ƙcar was enhanced by the addition of sodium perchlorate, (NaClO4) salt as a charge carrier in the Bz-ƙcar gel electrolyte systems. Several characterizations were performed in order to investigate the physicochemical and electrochemical properties of Bz-ƙcar derivatives and assess their suitability as potential gel polymer electrolytes. The successful substitution of benzoyl molecule into ƙcar polymeric chain was confirmed by the Fourier transform infrared spectroscopy (FTIR) analysis based on the formation of new carbonyl (C=O) and C=C bonds in Bz-ƙcar. The OH band intensity also reduced. Nuclear magnetic resonance (1H-NMR) analysis further proved the benzoylation by the appearance of new multiple resonance peaks at δ= 6.6–9.50 ppm, which belonged to the characteristic signals of protons in the aromatic benzoate group. X-ray diffraction (XRD) analysis showed the reduced degree of crystallinity of the synthesized carrageenan, while elemental analyzer analysis revealed increased percentages of carbon in Bz-ƙcar upon the substitution. The highest degree of substitution obtained was 0.27. Thermogravimetric analysis (TGA) showed lower degradation temperature in the synthesized carrageenan, while water contact angle analysis demonstrated that Bz-ƙcar was less hydrophilic as compared to the pristine ƙcar. Solubility tests showed that Bz-ƙcar was best dissolved in ethylene glycol. The benzoylation also improved the ionic conductivity (σ) of Bz-ƙcar to 3.10 x10-4 S cm-1 at ambient temperature. The modification of Bz-ƙcar was further proven to increase the ionic conductivity with the addition of salt. Gel biopolymer electrolytes based on Bz-ƙcar as polymer host and NaClO4 as ionic dopant was successfully produced. The concentration of NaClO4 was varied from 0.5 to 3.0 wt.% in order to investigate its effects on the chemical interactions and the σ of the electrolytes. Significant changes in the FTIR spectra were detected indicating the chemical interactions between Bz- ƙcar and NaClO4. The electrochemical impedance analysis (EIS) revealed that the σ of the gel electrolytes increased with a higher concentration of NaClO4, suggesting that NaClO4 was an effective charge carrier in the system. The highest σ of the gel electrolyte with the addition of 30% NaClO4 attained at ambient temperature (298 K) was 1.29 x 10-3 S cm-1. The same gel electrolyte was tested for the temperature dependence of conductivity and confirmed is Arrhenian in the studied temperature range and achieved elevated σ of 7.90 x 10-3 S cm-1 at 100 °C. Small values in the activation energy (Ea) were observed in all the electrolytes prepared. Hence, these results suggest that suggest that the Bz-ƙcar/NaClO4 gel electrolyte shows potential to be applied in electrochemical devices.