Ultrahigh volumetric capacitance and cyclic stability of fluorine and nitrogen co-doped carbon microspheres

Abstract

Highly porous nanostructures with large surface areas are typically employed for electrical double-layer capacitors to improve gravimetric energy storage capacity; however, high surface area carbon-based electrodes result in poor volumetric capacitance because of the low packing density of porous materials. Here, we demonstrate ultrahigh volumetric capacitance of 521 F cm(-3) in aqueous electrolytes for non-porous carbon microsphere electrodes co-doped with fluorine and nitrogen synthesized by low-temperature solvothermal route, rivaling expensive RuO2 or MnO2 pseudo-capacitors. The new electrodes also exhibit excellent cyclic stability without capacitance loss after 10,000 cycles in both acidic and basic electrolytes at a high charge current of 5 A g(-1). This work provides a new approach for designing high-performance electrodes with exceptional volumetric capacitance with high mass loadings and charge rates for long-lived electrochemical energy storage systems.

Figure 1. Morphology and elemental distribution of CM-NF electrodes.

(a) low-magnification s.e.m. image of the CM-NFs; (b) high-magnification s.e.m. image showing spherical morphology of CM-NF; (c) a s.e.m. image of CM-NFs, and the corresponding EDS elemental mappings of (d) carbon (red); (e) nitrogen (yellow); and (f) fluorine (purple). Scale bar, 15 μm (a), 1 μm (b) and 2 μm (c–f).

Figure 2. Structural characterization of the carbon microspheres.

(a) XRD patterns and (b) Raman spectra of the CM-NFs and CM-Ns.

Figure 3. Chemical analysis of the carbon microspheres.

(a) XPS survey spectrum of CM-NFs; and (b–d) high-resolution XPS spectra of C 1 s, N 1 s and F 1 s of CM-NFs, respectively.

Figure 4. Electrochemical performance of the carbon microspheres.

(a) galvanostatic charge/discharge curves of CM-NFs samples in 1 M H₂SO₄ solution with different current densities; (b) CV curves of CMs, CM-Ns and CM-NFs samples in 1 M H₂SO₄ solution at a scan rate of 10 mV s⁻¹; (c) corresponding capacity retentions at the current density from 0.1 to 5 A g⁻¹; and (d) stability evaluation of the CM-NF electrodes in 1 M H₂SO₄ solution at a charge current of 5 A g⁻¹ for the 10,000 cycles.

Figure 5. Mass loading and cycling stability.

(a) Comparison of the volumetric capacitance of the CM-NF electrodes at different mass loading tested in a 6 M KOH solution at a current density of 0.1 A g⁻¹ and (b) stability evaluation of the CM-NF electrodes in a 6 M KOH solution at a charge current of 5 A g⁻¹ for 20,000 cycles; (c) Electrochemical impedance spectra (inset: magnified 0–4 Ω region) under the influence of an ac voltage of 5 mV.