An ultrasound-assisted approach to bio-derived nanoporous carbons: disclosing a linear relationship between effective micropores and capacitance

Abstract

Ultrasound irradiation is a technique that can induce acoustic cavitation in liquids, leading to a highly interactive mixture of reactants. In pursuit of high-performance and cost-effective supercapacitor electrodes, pore size distributions of carbonaceous materials should be carefully designed. Herein, fruit skins (mango, pitaya and watermelon) are employed as carbon precursors to prepare nanoporous carbons by the ultrasound-assisted method. Large BET specific surface areas of the as-prepared carbons (2700-3000 m² g^-1) are reproducible with pore diameters being concentrated at about 0.8 nm. Among a suite of the bio-derived nanoporous carbons, one reaches a maximum specific capacitance of up to 493 F g^-1 (at 0.5 A g^-1 in 6 M KOH) in the three-electrode system and achieves high energy densities of 27.5 W h kg^-1 (at 180 W kg^-1 in 1 M Na₂SO₄) and 10.9 W h kg^-1 (at 100 W kg^-1 in 6 M KOH) in the two-electrode system. After 5000 continuous charge/discharge cycles, the capacitances maintain 108% in 1 M Na₂SO₄ and 98% in 6 M KOH, exhibiting long working stability. Moreover, such high capacitive performance can be attributed to the optimization of surface areas and pore volumes of the effective micropores (referred to as 0.7-2 nm sized pores). Notably, specific capacitances have been found linearly correlated with surface areas and pore volumes of the effective micropores rather than those of any other sized pore (i.e., <0.7, 2-50 and 0.5-50 nm). Consequently, the fit of electrolyte ions into micropore frameworks should be an important consideration for the rational design of nanopore structures in terms of supercapacitor electrodes.

Scheme 1. Schematic illustration of the preparation of MSNPCs.

Fig. 1. (A and B) TEM images with different magnifications, (C) high-resolution TEM image (inset: the magnified region showing clear lattice fringes) of MSNPC-850.

Fig. 2. XPS results of MSNPCs: (A) wide-scan spectra, (B) C1s spectra, (C) O1s spectra, (D) N1s spectra.

Fig. 3. (A) XRD patterns and (B) Raman spectra of MSNPCs.

Fig. 4. (A) N₂ adsorption–desorption isotherms, (B) pore size distributions calculated by the DFT method, and (C) volume-proportion histograms of each pore size segment (<0.7, 0.7–2 and 2–50 nm) of MSNPCs.

Fig. 5. Electrochemical performance of MSNPCs in 6 M KOH: (A) CV curves of MSNPC-850 at scan rates ranging from 5 to 100 mV s⁻¹, (B) GCD curves of MSNPC-850 at current densities ranging from 0.5 to 20 A g⁻¹, (C) CV curves of MSNPCs at a scan rate of 100 mV s⁻¹, (D) GCD curves of MSNPCs at a current density at 1 A g⁻¹, (E) rate performance of MSNPCs, (F) Nyquist plots of MSNPCs in the EIS measurement.

Fig. 6. MSNPCs, PSNPC-850 and WSNPC-850 in 6 M KOH: (A) dependence of C_sp (at 1 A g⁻¹) on the effective micropore volume, (B) dependence of C_sp (at 0.5–20 A g⁻¹) on the effective micropore volume, (C) dependence of C_sp (at 0.5–20 A g⁻¹) on the effective micropore surface area.

Fig. 7. (A) Dependence of C_sp on volume of <0.7 nm sized pores, (B) dependence of C_sp on surface area of <0.7 nm sized pores, (C) dependence of C_sp on volume of 2–50 nm sized pores, (D) dependence of C_sp on surface area of 2–50 nm sized pores, (E) dependence of C_sp on volume of total pores (0.5–50 nm), (F) dependence of C_sp on surface area of total pores (0.5–50 nm).

Fig. 8. (A) Ragone plots of MSNPC-850‖MSNPC-850 in the different electrolytes: 6 M KOH, 6 M NaOH and 1 M Na₂SO₄, (B) cycling stabilities of MSNPC-850‖MSNPC-850 at the current density of 5 A g⁻¹ in 6 M KOH and 1 M Na₂SO₄.