High Entropy Oxides—A Cost-Effective Catalyst for the Growth of High Yield Carbon Nanotubes and Their Energy Applications

This report anticipates a thorough strategy for the utilization of high entropy oxide (HEO) nanoparticles (1) as a cost-effective catalyst for the growth of high yield carbon nanotubes (CNTs), resulting in HEO–CNT nanocomposites, and (2) the implementation of HEO–CNT nanocomposites for energy applications such as electrochemical capacitors (ECs). In the first step, HEO nanoparticles were synthesized by a simple sol–gel autocombustion method and then the as-synthesized HEO nanoparticles were ground and used as the catalyst for the growth of CNTs by chemical vapor deposition technique. The as-grown CNTs (HEO–CNT nanocomposite) exhibited unexpectedly high yield, a superior specific surface area of ∼151 m2 g–1, and encapsulation and diffusion of the catalyst throughout the HEO–CNT nanocomposite, providing remarkably high mechanical strength, which make them a promising candidate for energy applications. To study the electrochemical activity of the HEO–CNT nanocomposite, half-cell and full-cell ECs were assembled in different electrolytes. Stupendously, a complete 100% capacitance retention and a Coulombic efficiency up to 15 000 cycles were realized for the HEO–CNT nanocomposite-based full-cell EC assembled in the polyvinyl alcohol/H2SO4 hydrogel electrolyte. Additionally, a high specific capacitance value of 286.0 F g–1 at a scan rate of 10 mV s–1 for the HEO–CNT nanocomposite-based full-cell EC assembled in the [BMIM][TFSI] electrolyte with a wide potential window of 2.5 V is reported. Also, high energy density and power density of ∼217 W h kg–1 and ∼24 521 W kg–1, respectively, are reported. Furthermore, the HEO–CNT nanocomposite-based full-cell EC assembled in the [BMIM][TFSI] electrolyte can successfully light up a red light-emitting diode, demonstrating great potential of the HEO–CNT nanocomposite in the various energy applications.