Enhancing charge storage in melanin via sulfur functionalization for supercapacitors applications

Melanins have emerged as promising organic materials for pseudocapacitive energy storage due to their redox-active building blocks, intrinsic biocompatibility, and potential biodegradability. However, their electrochemical performance remains limited, requiring strategies to enhance charge storage capacity and stability. In this work, we investigate the effect of sulfonated functionalization on the electrochemical behavior of melanin-based electrodes. Three distinct sulfonated melanins (SMel, SMel-T, and SMel-P) were synthesized under different conditions, resulting in variations in their sulfonation degree and structural characteristics. XPS and FT-IR confirmed that SMel-P exhibits the highest content of SO₂CH₃ groups, followed by SMel and SMel-T, which in turn influences redox activity and charge storage. Electrochemical analyses revealed that the degree of sulfonation plays a fundamental role in modulating pseudocapacitive behavior, leading to a significant increase in specific capacitance from 166.3 F g −1 to 428.4 F g −1 . Additionally, the presence of carboxylate units was found to enhance electrolyte interaction, further contributing to capacitance improvement. To assess the practical applicability of these materials, an asymmetric supercapacitor was assembled using SMel-P as the positive electrode, achieving an energy density of 30.1 ± 1.5 Wh kg −1 at 511.9 ± 9.2 W kg −1 , with 85 % capacitance retention after 20,000 cycles. These findings demonstrate that both sulfonation and the presence of carboxyl groups are key factors in tuning melanin's electrochemical properties, reinforcing their potential as sustainable, high-performance electrode materials for energy storage applications. • SO₂CH₃ functionalization enhances the electrochemical performance of melanin. • Functionalized melanin electrodes reach up to 428.4 F g −1 of specific capacitance. • The SMel-P//AC asymmetric supercapacitor delivers 30.1 Wh kg −1 at 511.9 W kg −1 . • 85 % capacitance retention after 20,000 GCD cycles ensures long-term stability.