Unveiling the transport mechanism of glycerol enhancement in zein-based gel polymer electrolytes for electrochemical double layer capacitor performance

Improving the specific capacitance of electrochemical double layer capacitors (EDLCs) is a critical but challenging research goal. High ionic conductivity electrolytes are essential materials in current EDLC technology. In present work, glycerol was incorporated as a plasticizer into a zein-honey-NH₄NO₃ based gel polymer electrolyte (GPE) to enhance the properties of the electrolytes. The zein-honey-NH₄NO₃ GPE (ZHNG10), containing 10 wt.% glycerol demonstrated the highest ionic conductivity of (1.00 ± 0.79) × 10^–2 S cm⁻¹ at room temperature. ZHNG10 also exhibited the lowest degree of crystallinity (12.195%) and a crystallite size of 2.185 nm, indicating a predominantly amorphous structure that facilitates ion conduction. Morphology analysis revealed a porous structure in ZHNG10, promoting continuous pathways for H⁺ ion movement, with an increase in ions density (n), mobility of ions (µ), and diffusion coefficients (D) upon 10 wt.% glycerol addition. Transference number measurement (TNM), indicated an ion transference number 0.99, showing that ions are the dominant charge carriers. ZHNG10 was electrochemically stable up to 2.99 V, indicating safe of the EDLC up to 2.0 V. The specific capacitance (C_sp) of the single-electrode-based EDLC was 196.61 F g⁻¹ based on cyclic voltammetry (CV) and 345.38 F g⁻¹ based on charge–discharge measurements, with an efficiency of more than 90%.