Pinecone-derived porous activated carbon for high performance all-solid-state electrical double layer capacitors fabricated with flexible gel polymer electrolytes

We report a novel configuration of electrical double layer capacitors (EDLCs), fabricated with porous activated carbon electrodes derived from bio-waste pinecone and plastic crystals-based gel polymer electrolytes. Different activated carbons have been produced by activating raw pinecone powder employing different amount of activating agent ZnCl ₂ . Optimization of morphology, structure and porosity parameters of activated carbons has been performed by scanning electron microscopy, x-ray diffraction, Raman and porosity analyses. Mixed (micro- and meso-) porous interiors in activated carbon show optimum characteristics of EDLCs. Gel polymer electrolyte comprising mixture of non-ionic plastic crystal and organic ionic plastic crystal (succinonitrile and 1-ethyl-1-methyl pyrrolidinium bis(trifluoromethyl sulfonyl)imide, respectively) entrapped in poly(vinylidene fluoride-co-hexafluoropropylene) has been found suitable to fabricate EDLCs owing to their flexible nature, high ionic conductivity (σ ∼1.53 × 10 ⁻³ S cm ⁻¹ at room temperature) and wide enough electrochemical stability window, ESW (∼3.1 V versus Ag). Addition of Li-salt in the gel polymer electrolyte improves the electrochemical properties (σ ∼2.87 × 10 ⁻³ S cm ⁻¹ and ESW ∼3.8 V) and found to influence the performance of the EDLCs, particularly, the specific power, significantly. The EDLC cell with Li-salt containing electrolyte offers higher values of specific capacitance, energy and power (∼255 F g ⁻¹ , ∼20 Wh kg ⁻¹ and ∼55.7 kW kg ⁻¹ , respectively) than the cell with electrolyte without Li-salt (∼244 F g ⁻¹ , ∼19 Wh kg ⁻¹ and ∼39.3 kW kg ⁻¹ , respectively). The EDLC (with Li-salt electrolyte) exhibits 96–100% Coulombic efficiency and almost stable specific capacitance for ∼20,000 charge-discharge cycles after only ∼11% initial fading. The capacitor gives stable performance for a wide temperature range from −30 to 90 °C.