Increasing the capacitance of flexible supercapacitors by adding spongy-like CNTs on their electrodes and application of CNTs to remove oil/microplastics from tap water

Abstract

In this work, spongy-carbon nanotubes were utilized as redox material to enhance the capacitance of graphene-based supercapacitors. Such carbon nanotubes had average diameters of 570 nm and were entangled forming a porous network. Firstly, supercapacitors were constructed only with graphene electrodes and had a capacitance and energy density of 322.12 F g−1 and 44.73 Wh kg−1, respectively. In contrast, supercapacitors made with electrodes containing Carbon nanotubes had a higher capacitance: 500.16 F g−1 and energy density: 69.46 Wh kg−1 (at room temperature). Thus, the capacitance was enhanced by 55% after introducing the spongy-Carbon nanotubes on the supercapacitor electrodes. Raman analysis and X-ray photoelectron spectroscopy measurements were carried out on the supercapacitor-electrodes and found the presence of oxygen groups and oxygen vacancies defects, which worked as redox centers for the charge storage. Furthermore, the devices made with spongy-carbon nanotubes presented high electrochemical stability because their capacitance retention was above 90% after 1000 cycles of charge–discharge. The supercapacitors were also operated at a low temperature of 2 °C (scarcely reported in the literature) and produced a capacitance and energy density of 432.35 F g−1 and 60.04 Wh kg−1, respectively. Interestingly, the maximum output voltage of 0.32 V was obtained in the devices operating at 2 °C. Later, the supercapacitor-electrodes made with spongy-carbon nanotubes were recovered and utilized to remove cooking-oil and polystyrene microplastics from contaminated tap water with maximum adsorption capacity of 11.5 g g−1 and 1.2 g g−1, respectively. Overall, this investigation demonstrated the reuse of SC-electrodes for the elimination of emerging pollutants, which extended the lifetime of spongy-carbon nanotubes and promoted their sustainability.