Sustainable Cu-BTC@CNT Composites Derived from Recycled Polypropylene for High-Performance Supercapacitor Electrodes

Increased interest in the development of new sustainable, highly capable energy storage technologies has produced a necessity to study advanced fabrication of eco-friendly supercapacitor electrodes through the use of hybrid frameworks of sustainable sources. This article details the synthesis of Cu-BTC@CNT composite electrodes by integrating carbon nanotubes from recycled polypropylene (PP) bottles with a Cu-BTC MOF host via a solvothermal method. Raman Spectroscopy confirmed the graphitic multiwalled nature of waste-derived CNTs with a D-band at 1338.0 cm^-1, a G-band at 1582.0 cm^-1, and an I_D/I_G ratio of 1.419. TGA validated the thermal stability of the carbon scaffold and CNT purity at 75%. The structure-property relationship of pristine Cu-BTC MOF, Cu-BTC@CNT, and Cu-BTC@rGO was characterized using SEM, EDX, XRD, FTIR, and Raman spectroscopy. These methods confirmed that CNTs form a conductive network with Cu-BTC MOF. Electrochemical tests in 1 M KOH using CV, GCD, and EIS showed Cu-BTC@CNT provided a high specific capacitance of 265.7 F g^-1 at 10 mV s^-1 and 245 F g^-1 at 1 A g^-1, surpassing Cu-BTC@rGO and pure Cu-BTC. EIS revealed a low charge-transfer resistance of 7.23 Ω, showing efficient conductivity from the CNT network. Capacity retention of 84.7% after 5000 GCD cycles at 3 A g^-1 indicated excellent durability. Trasatti analysis showed a dominant pseudocapacitive contribution of 96.89% from reversible redox reactions at Cu sites. b-values from 0.62 to 0.95, from Dunn's analysis, indicated surface-controlled pseudocapacitance at lower potentials and diffusion-controlled K⁺ intercalation at higher potentials. Postcycling X-ray photoelectron spectroscopy directly validated reversible metal redox transitions and electrochemically induced cation intercalation into the porous framework. This work establishes a sustainable, waste-valorization pathway to high-performance interfacial electrode materials for next-generation energy storage applications.