Ultrasound‐Assisted Deposition of Sepia Melanin and Multiwalled Carbon Nanotubes on Carbon Cloth: Toward Sustainable Surface Engineering for Flexible Supercapacitors

Abstract The rising global demand for energy requires, among others, sustainable energy storage devices. Biosourced redox‐active molecules are interesting for eco‐designed electrochemical energy storage as they increase the energy density of the electrodes adding the Faradaic (redox) storage mechanism to the electrostatic one. The engineering of the electrode surface and electrode surface/molecule interface is key to optimizing storage. Here, (i) electrodes prepared by ultrasound‐assisted modification of carbon cloth in the presence of Sepia melanin, a quinone macromolecule, and multiwalled carbon nanotubes (MWCNTs) and (ii) their use in flexible symmetric electrochemical capacitors assembled with polyvinyl alcohol (PVA)‐based hydrogel electrolyte is reported. Electrodes exhibit an areal capacitance as high as 274 mF cm⁻². Corresponding semi‐solid‐state symmetric supercapacitors feature high energy density of 18 Wh kg⁻¹, power density up to 221 W kg⁻¹ (evaluated at 0.5 A g⁻¹), outstanding cycling stability (100% capacitance retention, and 100% Coulombic efficiency after 10 000 cycles) along with excellent flexibility. This work contributes to the development of sustainable surface engineering approaches for environmentally benign electrochemical energy storage devices.