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

Area of Science:

• Electrochemistry
• Materials Science
• Polymer Chemistry

Background:

• Eumelanin possesses a useful redox equilibrium for electrochemical applications.
• A key challenge is eumelanin's poor solubility in polar solvents, hindering film processing.
• Functionalization offers a route to tune eumelanin's properties.

Purpose of the Study:

• To investigate the effect of tert-butyloxycarbonyl (Boc) and nitro functional groups on eumelanin's electrochemical properties.
• To understand how polarity and steric effects influence performance in Zn coin cell devices.
• To correlate structural modifications with electrochemical behavior and theoretical calculations.

Main Methods:

• Synthesis of functionalized eumelanin derivatives (Mel-Boc, Mel-NO2).
• Structural and surface analysis of the synthesized materials.
• Electrochemical testing in Zn coin cell devices with aqueous and ionic liquid electrolytes.
• Post-density functional theory (DFT) calculations (meta-GGA level).

Main Results:

• Mel-Boc exhibited larger particle size and lower capacity compared to synthetic eumelanin.
• Mel-NO2 demonstrated improved water-solubility, cycling stability, and higher capacity at elevated current densities.
• Mel-NO2 showed good conductivity in ionic liquid electrolytes.
• DFT calculations indicated that electron-withdrawing nitro groups reduce the HOMO-LUMO gap, potentially enhancing electrochemical properties.

Conclusions:

• Functional group modification is crucial for optimizing eumelanin's electrochemical applications.
• Mel-NO2 presents a promising derivative for improved performance in electrochemical devices due to enhanced solubility and conductivity.
• Surface area and metal ion chelation are important factors for capacity, while electronic structure modifications influence conductivity and stability.