A High-Performance Sustainable Solid-State Supercapacitor Utilizing Hazelnut Shell-Derived Activated Carbon and Optimized Polymer Blend Electrolyte

In this work, we report the development of a solid-state supercapacitor based on activated carbon derived from hazelnut shells (HZs) and an optimized polymer blend electrolyte. Hazelnut shells, an abundant agricultural waste, were converted into highly porous activated carbon through controlled pyrolysis and chemical activation, yielding a material with a well-developed pore structure suitable for electrochemical energy storage. A flexible polymer electrolyte was prepared using a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/polyethylene glycol (PEG) blend (90:10% (mass)) with potassium chloride (KCl) as an ionic dopant to enhance ionic conductivity. The optimal electrolyte composition containing 15% (mass) KCl exhibited an ionic conductivity of 7.0×10 -7 S·cm -1 at room temperature. Structural and chemical analyses using X-Ray Diffraction and Fourier Transform Infrared Spectroscopy confirmed the amorphous carbon structure and the presence of surface functional groups favorable for ion transport. The intrinsic electrochemical properties of the activated carbon electrodes were first evaluated in a three-electrode configuration using 1 mol·L -1 Na 2 SO 4 aqueous electrolyte, revealing dominant electric double-layer capacitive behavior, low charge-transfer resistance, and a high specific capacitance of 350 F·g -1 at a current density of 0.4 mA·g -1 , with 95% capacitance retention after 4400 cycles. Subsequently, a solid-state symmetric supercapacitor device was fabricated using the optimized polymer electrolyte and identical activated carbon electrodes. The device exhibited a specific capacitance of 320 F·g -1 at a scan rate of 2 mV·s -1 and excellent cycling stability, maintaining 98% capacitance retention after 4000 cycles. The practical applicability of the device was demonstrated by successfully powering a red Light Emitting Diode. This study highlights the potential of combining biomass-derived carbon electrodes with optimized polymer electrolytes for sustainable and high-performance solid-state energy storage applications.