Recycling waste for energy: Reusing methyl orange dye-adsorbed polythiophene/Fe3O4 nanorods/reduced graphene oxide nanocomposite as a supercapacitor electrode

This study presents a sustainable waste-to-wealth approach by repurposing methyl orange (MO) dye-adsorbed polythiophene/iron oxide/reduced graphene oxide nanocomposite (PTh/Fe₃O₄/RGO) for dual functionality in wastewater treatment and energy storage. PTh/Fe₃O₄/RGO nanocomposite was initially employed as an effective adsorbent to remove harmful MO dye from wastewater. After successful adsorption, the dye-saturated PTh/Fe₃O₄/RGO was repurposed as a supercapacitor electrode to investigate its energy storage capabilities. Electrochemical tests before MO adsorption revealed that the pristine PTh/Fe₃O₄/RGO electrode demonstrated promising performance with a specific capacitance of 467.3 Fg⁻¹ at 1 Ag⁻¹ and excellent cyclic stability of 95.3% retention over 5000 Galvanostatic charge-discharge (GCD) cycles. Further, the maximum adsorption of MO by the PTh/Fe₃O₄/RGO was achieved under optimal conditions: pH 3, 2.5 gL⁻¹ dosage, 120 min contact time, and 50 mgL⁻¹ MO concentration. The adsorption behaviour was well-explained by the Langmuir isotherm, pseudo-second-order kinetics, and Dubinin-Radushkevich (D-R) isotherm, indicating a physical adsorption process with a monolayer capacity of 151.34 mgg⁻¹. Thermodynamic analysis, with a positive enthalpy (ΔH°) and negative Gibbs free energy (ΔG°), confirmed that the process is endothermic and spontaneous. Desorption studies showed that 88.72% of the MO could be desorbed in the first cycle, with effective regeneration up to six cycles using NaOH. Post-adsorption, the material still retained significant supercapacitor properties, with a specific capacitance of 380.9 Fg⁻¹ at 1 Ag⁻¹ and 83.2% cyclic stability over 5000 GCD cycles. This study demonstrates a circular, sustainable approach that integrates waste treatment with energy storage, highlighting the potential of reusing materials for multifunctional applications.