High performance supercapacitors based on oil fly ash-derived carbon nanotubes

Supercapacitors (SCs) and batteries have become essential parts for new equipment and transportation systems. Several carbon materials in their bulk and nanostructured forms have been extensively investigated as electrodes for such energy storage devices. However, the reported low performance, high cost, and limited stability remain major challenges. In this study, heavy oil fly ash (HOFA)-derived carbon nanotubes (CNTs) are found to have attractive properties as SC materials. These CNTs are produced by the chemical vapor deposition technique employing pretreated HOFA. The HOFA underwent pretreatment either by probe sonication (Son-CNTs) or sintering (Sin-CNTs). These CNTs exhibit small diameters, zigzagged walls, high surface area, and low production cost making them suitable for energy storage devices. Their energy storage performance is evaluated using three-electrode and two electrode systems. The Sin-CNTs exhibit better specific capacitance compared to Son-CNTs in1M H2SO4, achieving 104.7 F/g at a current density of 0.1 A/g, while Son-CNTs show 69.7 F/g at the same current density. The effect of different factors on the specific capacitance are systematically studied including type of electrolyte, concentration, pH value, and electrode weight. Increasing the electrolyte temperature from room temperature to 90 °C results in an increased specific capacitance to 133.2 F/g at a current density of 1 A/g. Finally, after 5000 cycles of charge-discharge, the Son-CNTs recorded a capacitance retention of 107.8%, while the Sin-CNTs exhibit 98.11%, demonstrating high stability and long-term durability as SCs materials. These results are comparable, and in some cases even superior to those reported for commercial CNTs.