Enhanced Supercapacitor Performance via Double Network Hydrogel Modified with Self-Assembled Metal Nanoparticles

Double-network (DN) hydrogels have emerged as promising candidates for flexible supercapacitor applications due to their exceptional mechanical toughness and flexibility. However, their practical use has been limited by poor electrolyte retention and low ionic conductivity, which hinder capacitive performance. In this study, we present a novel DN hydrogel modified through the self-assembly of metal nanoparticles to address these limitations. The hydrogel was synthesized via a combination of physical and chemical cross-linking, followed by in situ reduction of incorporated metal ions to form nanoparticles within the network. The resulting nanocomposite hydrogel exhibited enhanced electrolyte swelling capacity and improved ionic conductivity and maintained robust mechanical flexibility. A flexible supercapacitor (FSC) was fabricated using this modified DN hydrogel as both the electrode and solid-state electrolyte, with activated carbon nanosheets (ACNSs) derived from banana leaves serving as the active material. The ACNSs adhered effectively to the hydrogel matrix with the aid of a polyvinylidene fluoride (PVDF) binder. Electrochemical performance was evaluated in a symmetric two-electrode configuration using 0.5 M sodium sulfate aqueous solution as the supporting electrolyte. The device achieved a high areal specific capacitance (C_sp) of 1361 mF cm^-2, an energy density (E) of 23 mWh cm^-2, and a power density (P) of 700 mW cm^-2. Furthermore, the FSC demonstrated excellent cyclic stability, retaining 94% of its initial Coulombic efficiency after 500 cycles. These findings highlight the potential of metal nanoparticle-enriched DN hydrogels as multifunctional materials for next-generation integrated supercapacitors.