Sulfur Doping: Unique Strategy To Improve the Supercapacitive Performance of Carbon Nano-onions

Abstract

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.

Heteroatom‐Doped and Oxygen‐Functionalized Nanocarbons for High‐Performance Supercapacitors

“…[162] S-doped carbon nano-onions with 3.57 at% S-content and 2.86 at% O-content show the water wetting angle of 48°. [150] Generally, a higher amount of sulfur compared to oxygen is evidenced from the XPS results of the S-doped nanocarbons. [153,156,158,160,161] On contrary, the increase in O-content from 5.8 to 12.4 wt% along with sulfoxide and sulfone group introduction is hard to avoid under realistic experimental conditions.…”

“…[160] In addition, the positive charge introduced by the thiophene-S group to the neighboring carbon atoms attracts the electrolyte ions and hence an improvement in charge-storage capacity is observed. [150,151]…”

“…Reproduced with permission. [150] Copyright 2019, American Chemical Society. Dependency of specific capacitance of sulfur-doped nanoporous carbons modified by graphene on E) DC conductivity and F) amount of groups (oxygen or sulfur).…”

“…[118,216] Surprisingly, the higher gravimetric capacitance of doped nanocarbons is observed in Na 2 SO 4 than in H 2 SO 4 . [150,159,163] Moreover, it has been seen in a few reports that the charge-storage performance of doped nanocarbons is explored either in an acidic or basic aqueous electrolyte with respect to the reference electrode while the device performance is demonstrated in a neutral electrolyte. [86,215,280,281] The use of pH-neutral electrolyte along with the doped nanocarbons allows operation within the relatively higher potential window (≥2 V) and this behavior is ascribed to the high overpotential for hydrogen evolution (0.6 V).…”

“…[193] N-doped and S-doped nanocarbons can be used as a negative electrode. [150,292] B-doped nanocarbons also could be effective for asymmetric supercapacitor whereas P-doped CNTs are better suited for symmetric supercapacitors. [173] Although Figure 20.…”