Recently,
enhancement of the energy density of a supercapacitor
is restricted by the inferior capacitance of negative electrodes,
which impedes the commercial development of high-performance symmetric
and asymmetric supercapacitors. This article introduces the in situ
bulk-quantity synthesis of hydrophilic, porous, graphitic sulfur-doped
carbon nano-onions (S-CNO) using a facile flame-pyrolysis technique
and evaluated its potential applications as a high-performance supercapacitor
electrode in a symmetric device configuration. The high-surface wettability
in the as-prepared state enables the formation of highly suspended
active conducting material S-CNO ink, which eliminates the routine
use of binders for the electrode preparation. The as-prepared S-CNO
displayed encouraging features for electrochemical energy storage
applications with a high specific surface area (950 m2 g–1), ordered mesoporous structure (∼3.9 nm),
high S-content (∼3.6 at. %), and substantial electronic conductivity,
as indicated by the ∼80% sp2 graphitic carbon content.
The in situ sulfur incorporation into the carbon framework of the
CNO resulted in a high-polarized surface with well-distributed reversible
pseudosites, increasing the electrode–electrolyte interaction
and improving the overall conductivity. The S-CNOs showed a specific
capacitance of 305 F g–1, an energy density of 10.6
W h kg–1, and a power density of 1004 W kg–1 at an applied current density of 2 A g–1 in a
symmetrical two-electrode cell configuration, which is approximately
three times higher than that of the pristine CNO-based device in a
similar electrochemical testing environment. Even at 11 A g–1, the S-CNO||S-CNO device rendered an energy density (6.1 W h kg–1) at a deliverable power density of 5.5 kW kg–1, indicating a very good rate capability and power
management during peak power delivery applications. Furthermore, it
showed a high degree of electrochemical reversibility with excellent
cycling stability, retaining ∼95% of its initial capacitance
after more than 10 000 repetitive charge–discharge cycles
at an applied current density of 5 A g–1.
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