Eumelanin-inspired nanomaterials in electrochemical energy storage devices: A review

• The preparation methods and chemical structures of the eumelanin-inspired nanomaterials are introduced. • The current works of eumelanin-inspired nanomaterials in energy storage are summarized from the perspective of mechanism. • The applications of eumelanin-inspired nanomaterials in rechargeable batteries and supercapacitors are introduced. • The challenges and future perspectives focus on eumelanin-inspired nanomaterials in order to provide a reference for advanced energy storage applications. At present, rechargeable batteries are researched due to the increasing demand for electricity. For these energy storage devices, their energy density or power density are highly dependent on electrode materials including organic, inorganic and their composite substances. Organic electrode materials with structural diversity and tunability begin to attract attention, because it can better adapt to different sizes of cations. As a kind of organic electrode material, eumelanin-inspired nanomaterials (eumelanin-inspired nanomaterials mean natural eumelanin and artificial synthetic eumelanin-PDA in this review) have been widely used in rechargeable batteries and supercapacitors (SCs) including anodes, cathodes, separators and electrolytes. Eumelanin-inspired nanomaterials can mainly use to gain/lose electrons with reversible bonding of metal ions in order to form an electric current, and the performance of energy storage devices can be improved due to its many advantages, such as nanometer size, adhesion and hydrophilia. However, the in-depth summarization of the current work from the perspective of energy storage mechanisms is still lacking at present. In this review, the preparation methods and energy storage mechanisms of eumelanin-inspired nanomaterials are systematically summarized. The applications and current challenges of eumelanin-inspired nanomaterials on rechargeable batteries and SCs have been introduced, and the future perspectives in the rational fabrication of higher performance energy storage devices by the eumelanin-inspired nanomaterials have been proposed.