High Energy Quasi‐Solid‐State Supercapacitors Totally Derived from Alginate Hydrogel

Abstract Utilizing sustainable and low‐cost resources to achieve high‐energy supercapacitors (SCs) remains a significant challenge. Herein, we propose a strategy to design high‐energy quasi‐solid‐state SCs, where electrode materials, binder, and electrolyte are entirely derived from sodium alginate (SA). N‐doped porous carbon (NPC) with well‐developed hierarchical pores and high nitrogen content is synthesized via the direct in‐situ carbonization of Ca 2+ −crosslinked alginate hydrogel with urea. The resulting distribution of mesopores and micropores in NPC facilitates ions transport and adsorption and ensures high electric‐double‐layer capacitance, while its high nitrogen‐doping provides substantial pseudo‐capacitance. In addition, the use of SA as a binder significantly improves water wettability and lowers charge transfer resistance, further enhancing ion accessibility and capacitance of the carbon electrode. The tough hydrogel electrolyte, combined with the interpenetrating alginate and polyacrylamide networks, exhibits enhanced mechanical strength, water retention, and ionic conductivity. Consequently, the as‐fabricated all‐in‐one alginate‐based quasi‐solid‐state SC delivers an outstanding energy density of 20.2 Wh kg −1 at 112.5 W kg −1 and exceptional cycling stability of 95.9% over 10 000 cycles at 10 A g −1 . This innovative design highlights the value‐added use of biomass from both a material engineering and device integration perspective, paving the way for the manufacture of high‐energy quasi‐solid‐state SCs.