Designing Ultrasmall Carbon Nanospheres with Tailored Sizes and Textural Properties for High-Rate High-Energy Supercapacitors

The present work demonstrates the efficient design of ultrasmall porous carbon nanospheres with tailored sizes (5-40 nm in diameter) and optimized intrasphere textural properties for high-rate high-energy super-capacitor application. The carbon nanospheres are synthesized via a miniemulsion polymerization technique followed by KOH activation. It is shown that dual-step activation renders enlarged intrasphere micropores/ mesopores, facilitating enhanced ion transports. Meanwhile, a decrease in nanosphere size from 40 to 5 nm significantly improves the rate performance, demonstrating the pronounced size effects due to enhanced intrasphere ion transport. The optimum dual-step-activated carbon nanosphere sample with an average sphere size of 5 nm, ACNS5-2, shows the high specific capacitances along with outstanding high-rate capabilities in both aqueous (272 F g(-1) at 0.5 A g(-1) and 81.6% of retention at 200 A g(-1)) and EMIMBF4 (223 F g(-1) at 0.5 A g(-1) and 67.2% of retention at 100 A g(-1)) electrolytes in symmetrical two-electrode cells. In EMIMBF4, ACNS5-2 displays a high energy density of 48 Wh kg(-1) at a high power density of 14 kW kg(-1), suggesting excellent energy storage efficiency. Moreover, the performance of ACNS5-2 competes well with or is superior to some best-performing porous carbon-based materials reported in the literature for supercapacitor applications even at lowered temperatures (at -20 degrees C: 150 F g(-1) at 0.5 A g(-1) with a capacitance retention of 64% at 10 A g(-1)) and high mass loading (8 mg cm(-2): 205 F g(-1) at 0.5 A g(-1) with a capacitance retention of 64.5% at 20 A g(-1)). Our results, combined with structure-performance relationships, offer valuable guidelines for the rational design of carbon nanomaterials of optimum supercapacitive performances.

Automated summary

The research demonstrates the efficient design of ultrasmall porous carbon nanospheres with tailored sizes and optimized intrasphere textural properties for high-rate high-energy super-capacitor application. Dual-step activation enlarges intrasphere micropores/mesopores, facilitating enhanced ion transports, and decreasing nanosphere size significantly improves the rate performance due to enhanced intrasphere ion transport. The optimum dual-step-activated carbon nanosphere sample shows high specific capacitances and outstanding high-rate capabilities in both aqueous and EMIMBF4 electrolytes.