Understanding the role of functional groups in reduced graphene oxide electrodes modified with aromatic redox-active compounds for supercapacitor applications

Abstract

Reduced graphene oxides (rGOs) are promising electrode materials for redox-enhanced supercapacitors. However, the combined effects of surface chemistry, supramolecular organization, and electronic structure on their performance remain underexplored. In this study, rGOs with varying oxygen contents (24.3, 8.1, and 1.3 wt% for rGO300, rGO700, and rGO1000, respectively) were thermally synthesized and modified by incorporating methylene blue (MB) and indigo carmine (IC), two redox-active aromatic compounds. We show that thermal reduction enhanced electrical conductivity (resistivity decreased from 277.5 to 57.0 Ω cm), but also reduced wettability. Upon adsorption of MB and IC, supramolecular structures (MB@rGO and IC@rGO) were formed, altering the rGOs' thermal stability and interlayer spacing. These compounds were not leached into the electrolyte and significantly influenced the specific capacitance, rate capability, and energy efficiency of the electrodes. IC@rGO300 achieved the highest specific capacitance (245.0 F g⁻¹ at 1 A g⁻¹), while MB@rGO1000 exhibited outstanding performance at high rates, maintaining 76.4 % energy efficiency at 20 A g⁻¹. Despite its lower intrinsic conductivity, rGO300 benefits from a high density of defects and functional groups, enabling strong interactions with the aromatic compounds, resulting in a 3.9-fold capacitance increase, with retention even at 20 A g⁻¹. In contrast, rGO1000 demonstrates faster electron transfer kinetics and more reversible faradaic processes, which lead to enhanced performance via redox mediation. These findings highlight the importance of optimizing interfacial properties between electrodes and redox-active additives for improving the efficiency and charge storage capacity of supercapacitors.