3D Network of Sepia Melanin and N- and, S-Doped Graphitic Carbon Quantum Dots for Sustainable Electrochemical Capacitors

Organic electrode materials operating in aqueous electrolytes offer the opportunity to avoid toxic, critical, and expensive materials for electrochemical energy storage. When deposited on carbon current collectors, redox active organic materials add faradaic to electrostatic capacitance contribution to the electrodes. Here, a 3D network electrode material is reported upon, based on sepia melanin, a quinone macromolecule, and nitrogen- and sulfur-doped graphitic carbon quantum dots (N,S GCQDs) designed to achieve good electronic conductivity and electrolyte wettability. The effect of various undoped and doped carbon quantum dots is also investigated, synthesized from acetic acid and sucrose instead of graphite, on the electrochemical performance of sepia melanin. Sepia/N,S GCQD shows optimum areal capacitance (≈180 mF cm−2) that is about twice as high as sepia (≈77 mF cm−2) with lower charge transfer resistance (1 ohm for sepia/N,S GCQDs compared to 10 ohms for sepia). The sepia/N,S GCQD symmetric supercapacitor in 0.5 m Na2SO4(aq) exhibits promising capacitance retention ≈92% after 10 000 cycles at 5 A g−1, 100% coulombic efficiency, 11 µW h cm−2 and 102 mW cm−2 maximum energy and power densities. The work paves the way for stable and potentially biodegradable supercapacitor electrode materials for environmentally benign electrochemical energy storage.