Functionalized melanin for enhanced energy storage in aqueous and ionic liquid electrolytes

Eumelanin is an interesting functional material for electrochemical applications due to its quinone/hydroquinone redox equilibrium. One major issue in the eumelanin film processing is the lack of solubility in polar solvents. In this study, the influence of functional groups of different polarity and their steric effects (tert-butyloxycarbonyl (Boc) groups giving "Mel-Boc" and nitro groups giving "Mel-NO₂") on the electrochemical properties of eumelanin in Zn coin cell devices is discussed. The derivatives are investigated using structural and surface analysis methods. Mel-Boc gives increased particle size and reduced capacity (0.97 mA h g^-1 at 0.4 A g^-1) in aqueous electrolyte compared to synthetic eumelanin. This indicates the importance of high surface area and metal ion chelation in the polymers. Mel-NO₂ shows improved water-solubility, cycling stability, improved capacity at current densities over 0.1 A g^-1 (19.5 mA h g^-1 at 0.2 A g^-1), and good conductivity in ionic liquid electrolyte devices. Post-density functional theory calculations using a higher-level theoretical approach (meta-GGA level) compared to previous theoretical melanin investigations show the electron withdrawing nitro groups causing a reduced HOMO-LUMO gap potentially being a reason for the improved electrochemical properties.