Flexible Solid-State Aqueous Sodium-Ion Capacitor Using Mesoporous Self-Heteroatom-Doped Carbon Electrodes

Abstract

Supercapacitors find numerous applications such as in consumer electronics, peak power demands, capturing surge power, etc. Various types of carbon materials are explored as electrode material. In this work, disordered mesoporous carbon was synthesized from expired medicinal capsule covers by sulfonation and subsequent chemical activation was carried out with KOH. Powder X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, Brunauer–Emmett–Teller studies, and X-ray photoelectron spectroscopy were utilized to characterize the generated carbon which confirmed the disordered carbon had a large surface area with stratified pores. The testing for electrochemical charge storage was conducted on a three-electrode setup comprising an aqueous 1 M NaClO4 electrolyte that exhibited a high 324 F g–1 specific capacitance at 0.4 A g–1, revealing suitability for sodium-ion ultracapacitor. Consequently, CR2032 coin-type sodium-ion capacitors were fabricated using both aqueous and nonaqueous electrolytes. The laboratory prototype capacitor with the aqueous electrolyte delivered 42 Wh kg–1 specific energy at 179 W kg–1 specific power. Besides, the capacitor with the nonaqueous electrolyte showed a remarkable 54.3 Wh kg–1 specific energy at 540 W kg–1 specific power. The Coulombic efficiency exhibited by both devices was about 99% even at the 10000th charge–discharge cycle. The amplified charge storage performance of the disordered carbon was ascribed to the presence of homogeneous mesopores that led to enhanced accessibility of the electrode/electrolyte contact area. According to the results of the power law and Dunn’s approach, the mesoporous carbon electrode was found to have a considerable diffusive charge storing mode involving Na+ intercalation in the sodium-ion nonaqueous ultracapacitor. A green-light-emitting diode could run continuously for 16 min on a single charge using the laboratory prototype CR-2032 coin-type nonaqueous ultracapacitor. Interestingly, a homemade solid-state flexible sodium-ion ultracapacitor was also fabricated that impressively manifested 13.7 Wh kg–1 specific energy at 180 W kg–1 specific power in 180° bent angle.