OPTIMISATION OF CALCIUM ION BASED POLYMER ELECTROLYTES UTILIZING 2-HYDROXYETHYL CELLULOSE WITH CALCIUM NITRATE OR CALCIUM METABORATE FOR APPLICATION IN ELECTRICAL DOUBLE LAYER CAPACITORS

Abstract

This project focuses on the development of calcium-ion-conducting polymer electrolytes (PEs) for electric double-layer capacitor (EDLC) applications. A series of 2-hydroxyethyl cellulose (2-HEC)-based polymer electrolytes was fabricated via a simple solution casting method, incorporating calcium nitrate [Ca(NO₃)₂] or calcium metaborate (CaB₂O₄), the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF₄), and glycerol (GLY) as a plasticizer. PE films prepared from the samples were sandwiched between two activated carbon electrodes and subjected to electrochemical characterization. Structural analyses (XRD and FTIR) confirmed that the incorporation of salts, glycerol, and ionic liquids decreased crystallinity, increased amorphousness, and promoted strong Ca²⁺–polymer coordination, thereby facilitating ion transport. Electrochemical Impedance Spectroscopy (EIS) revealed significant conductivity improvements in CN0.3, achieving 2.58 × 10⁻⁴ S cm⁻¹ at room temperature, while CN0.3IL5, containing ([BMIM]BF₄), delivered the highest conductivity of 7.02 × 10⁻⁴ S cm⁻¹ with a cation transference number close to unity (t⁺ ≈ 0.99). Plasticization with glycerol further reduced activation energy to 0.15 eV and enhanced conductivity to 5.07 × 10⁻⁴ S cm⁻¹. For CaB₂O₄ systems, optimized loading at 27.3 wt.% yielded 1.7 × 10⁻⁶ S cm⁻¹, while modified samples B5GLY7 and B5IL8 reached 5.0 × 10⁻⁵ and 5.32 × 10⁻⁵ S cm⁻¹, respectively. Linear Sweep Voltammetry (LSV) showed wide electrochemical stability windows (4.4 – 4.6 V), supporting high-voltage operation. Galvanostatic Charge–Discharge (GCD) measurements demonstrated remarkable improvements in capacitive behavior. For CN0.3, specific capacitance increased from 3.41 to 45.35 F g⁻¹ (0–1 V) and from 4.22 to 50.54 F g⁻¹ (0–1.5 V). Meanwhile, form CN0.3IL5 the specific capacitance rose from 12.71 F g⁻¹ to 197.4 F g⁻¹ at(0–1 V) from 17.47 F g⁻¹ to 324.5 F g⁻¹ (0–1.5 V) Similarly, form CN0.3GLY5 of 11.03 F g⁻¹ to 182.9 F g⁻¹ (1.0 V) 15.34 F g⁻¹ to 203.7 F g-1at (1.5 V). Comparable enhancements were also observed for CaB₂O₄-based electrolytes, specific capacitance increased of B5 from 1.51 to 16 F g⁻¹ (0–1 V) and 1.79 to 18.2 F g⁻¹ (0–1.5 V). Meanwhile, form B5GLY7 the specific capacitance rose from increase from 4.3 F g⁻¹ to 95.14 F g⁻¹ (0–1 V) and from 7.22 F g⁻¹ to 149.86 F g⁻¹ (0–1.5 V) Similarly, form B5IL8 of from 6.14 to 125.12 F g⁻¹ (0–1 V)11.34 to 201.83 F g⁻¹at (0–1.5 V) revealing enhanced charge storage performance across an extended potential window. The rational design of flexible Ca²⁺-conducting polymer electrolytes incorporating salts, glycerol, and ionic liquids provides a promising platform for next-generation, cost-effective, and sustainable EDLCs with high ionic conductivity, wide stability, and superior charge storage capabilities.