Nitrogen- and sulfur-enriched porous carbon from waste watermelon seeds for high-energy, high-temperature green ultracapacitors

Abstract

Electrochemical ultracapacitors exhibiting high energy output and an ultra-long cycle life, utilizing green and sustainable materials, are of paramount importance for next-generation applications. Developing an ultracapacitor that has high output energy under high power conditions in a high-voltage non-aqueous electrolyte and maintaining a long cycle life is an ongoing challenge. Herein, we utilize watermelon seeds, a bio-waste from watermelons, for use in high-voltage, high-energy, and high-power ultracapacitors in a sodium ion-based non-aqueous electrolyte. The as-synthesized hierarchically porous, high surface area carbon is surface-engineered with a large quantity of nitrogen and sulfur heteroatoms to give a high specific capacitance of ∼252 F g⁻¹ at 0.5 A g⁻¹ and 90 F g⁻¹ at 30 A g⁻¹. An ultra-high stability of ∼90% even after 150 000 cycles (10 A g⁻¹) with 100% coulombic efficiency is achieved at room temperature (25 °C), equivalent to an ultra-low energy loss of ∼0.0667% per 1000 cycles. Furthermore, the porous carbon demonstrates remarkable stability even at high temperature (55 °C) for 100 000 cycles (10 A g⁻¹), ensuring the safety of the device and enabling it to outperform graphene-based materials. A maximum energy of ∼79 W h kg⁻¹ and a maximum power of 22.5 kW kg⁻¹ with an energy retention of ∼28.2 W h kg⁻¹ was attained. The results provide new insights that will be of use in the development of high-performance, green ultracapacitors for advanced energy storage systems.