In this work, we report a direct preparation of a few-walled carbon nanotube (FWCNTs) and NiMgAl composites namely FWCNT-NiMgAl by pyrolysis of waste high-density polyethylene (HDPE) plastic at 800 °C with NiMgAl-layered double hydroxide (LDH) as catalysts. The composite formation is carried out in a single step using our lab-developed pyrolysis reactor. The NiMgAl-LDH catalyst was prepared by co-precipitation method and the FWCNTs were grown on the NiMgAl-LDH catalyst with FWCNT yield of 10% and FWCNT-NiMgAl composite yield of 55% whose quality is determined by Raman ID/IG ratio of 2.57. The average outer and inner diameter of the FWCNT are found to be 5.5 nm and 2.9 nm, respectively, from TEM and 2.92 nm from the outer RBM (radial breathing mode) band, which indicates the formation of a few-walled CNTs. FWCNT-NiMgAl is used for the fabrication of flexible supercapacitor electrodes on a polyethylene terephthalate (PET) sheet which achieved a specific capacitance of 432 Fg− 1 in a wide potential range (ΔV = 2) at a scan rate of 5 mV s− 1 in 2 M KOH electrolyte with a high energy density of 240 Wh kg− 1, whereas NiMgAl displayed a capacitance of 200 Fg− 1 with an energy density of 111 Wh kg− 1. The diffusion-type charge storage mechanism (pseudocapacitance) is found to be dominant with contributions of 73.2% and 69.75% for NiMgAl and FWCNT-NiMgAl, respectively. The highest specific capacitance and energy density are obtained for NiMgAl in 2 M KCl and for FWCNT-NiMgAl in 2 M NaOH electrolytes. However, the largest potential window is observed in KOH electrolyte for both NiMgAl and FWCNT-NiMgAl with value of ΔV = 2 V. The electrode material shows good stability in acidic electrolytes and also shows good capacitive stability at high frequencies maintaining a phase angle of 70°. The present work is a novel approach to fabricate low-cost multifunctional carbon composite nanomaterials and will contribute to the research on low-cost waste-derived CNT composite preparation and its application in flexible energy storage devices.

    Abstract
        Graphical abstract
    1 Introduction
    2 Experimental section
        2.1 Materials
        2.2 Preparation of NiMgAl nanocatalyst and FWCNT-NiMgAl composite
        2.3 Instruments for characterization of FWCNT-NiMgAl composite
        2.4 Electrode modification and energy storage measurement by cyclic voltammetry (CV)
    3 Results and discussion
        3.1 Characterization of NiMgAl-LDH catalyst and FWCNT-NiMgAl composite
        3.2 Supercapacitor application of FWCNT-NiMgAl composite
    4 Conclusion
    Acknowledgements 
    References

• Aunggat Shah(Physics Division, Department of Basic Sciences and Social Sciences, School of Technology, North Eastern Hill University)
• Yuvraj Maphrio Mao(Department of Nanotechnology, School of Technology, North Eastern Hill University)
• L. Robindro Singh(Department of Nanotechnology, School of Technology, North Eastern Hill University)
• Manashjit Gogoi(Department of Biomedical Engineering, School of Technology, North Eastern Hill University)
• Mrityunjoy Mahato(Physics Division, Department of Basic Sciences and Social Sciences, School of Technology, North Eastern Hill University)