Agricultural-Waste-Driven Innovation in Energy Storage : Tailoring Coconut-Husk Hydroxyethyl Cellulose/Alginate Networks Integrated with Calcium Triflate into Bio-Inspired Polymer Electrolytes

Heng, Song Fong (2025) Agricultural-Waste-Driven Innovation in Energy Storage : Tailoring Coconut-Husk Hydroxyethyl Cellulose/Alginate Networks Integrated with Calcium Triflate into Bio-Inspired Polymer Electrolytes. Final Year Project (Bachelor), Tunku Abdul Rahman of Management and Technology.

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Abstract

This research aims to derive water-soluble hydroxyethylcellulose (HEC) from the coconut husk waste, optimize the HEC-Calcium triflate [Ca(OTf)2] composition and evaluate the impact of integration of sodium alginate (SA) on electrochemical performances of electrical double layer capacitor (EDLC). HEC was synthesised through etherification with ethylene glycol and characterized with Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), Proton Nuclear Magnetic Resonance Spectroscopy (1H NMR), Field Emission Scanning Electron Microscopy (FE-SEM), Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). Two systems of polymer electrolyte have been investigated which are HEC-Ca(OTf)2 solid polymer electrolytes in System 1 and HEC-Ca(OTf)2-SA hydrogel polymer electrolytes in System 2. All HEC-based polymer electrolytes were prepared by solution casting method. The structural and thermal properties of synthesized HEC-based polymer electrolytes were elucidated by ATR-FTIR, DSC, TGA and X-ray Diffraction (XRD). Meanwhile, the electrochemical properties of synthesized HEC-based polymer electrolytes were evaluated by Electrochemical Impedance Spectroscopy (EIS), Linear Sweep Voltammetry (LSV) and transference number measurement, while the electrochemical properties of fabricated EDLC were assessed through Cyclic Voltammetry (CV), low frequency EIS and Galvanostatic Charge-Discharge Performance (GCD). The synthesized HEC exhibited a reduction in glass transition temperature (Tg) from 178.42 ℃ to 82.40 ℃ and characteristic hydroxyethyl peaks at 3.1 to 3.3 ppm has been observed in 1H NMR which confirmed the successful hydroxyethylation of extracted cellulose. The optimal HCA60 (60 wt. % HEC: 40 % Ca(OTf)2) achieved the maximum ionic conductivity of (6.44±4.18) × 10-5 S cm-1 with the highest percentage of free ions of 57.99 %and the widest of working potential window of 3.85 V. The incorporation 35 wt.% of sodium alginate in HSA65 (the most conducting hydrogel polymer electrolyte) drastically increased the ionic conductivity to (9.78±0.02) × 10-3S cm-1 and extended the working potential window to 4.43 V. Addition of sodium alginate reduced the Tg of HSA65 to -20.12 ℃ and further improved the salt dissociation of free ions up to 67.50 %. The highest conducting polymer electrolytes from both systems were employed in EDLC fabrication. The incorporation of sodium alginate remarkably enhanced the specific capacitance about 99% from 11.01 F/g (HCA60) to 21.89 F/g (HSA 65) and energy density about 124% from 1.22 W h kg-1 (HCA60) to 2.73 W h kg-1 (HSA65) However, the GCD results revealed 37 % capacity retention after 1500 cycles, attributed to water loss and electrolyte depletion, indicating the need for stability optimization in further studies.