Understanding the Roles of Sulfur Doping for Enhancing of Hydrophilicity and Electrochemical Performance of N,S-Codoped Hierarchically Porous Carbon

A hydrophilic N,S-codoped hierarchically porous carbon (NSHPC) was fabricated successfully through a green route, which is through activation of bagasse with thiourea using calcium chloride as activation agent by a one-step process. The as-prepared N,S-codoped carbon with hierarchically porosities can offer merits for ion buffering and storage, and moreover, the doped nitrogen and sulfur in this sample can significantly enhance the hydrophilicity/wettability and pseudocapacitance. Thus, the as-prepared NSHPC material with a moderated specific surface area (SSA, 835.9 m² g^-1) and pore volume (0.68 cm³ g^-1), achieving from calcination the raw sources of bagasse, CaCl₂, and thiourea in a mass ratio of 1:2:2 at 800 °C for 2 h under an inert flow, can exhibit a high mass specific capacitance and areal-normalized capacitance (349.0/41.6, 282.5/33.8, and 203.8/24.3 F g^-1/μF cm^-2 at 1.0 A g^-1 in 6.0 M KOH, 1.0 M H₂SO₄, and Li₂SO₄, respectively), superior rate performance (222.0, 152.0, and 98.8 F g^-1 at 100 A g^-1 in 6.0 M KOH, 1.0 M H₂SO₄ and Li₂SO₄, respectively), and outstanding cycle stability. Moreover, a symmetric capacitor fabricated by the NSHPC material shows high energy and good power densities (with respective 21.7 and 14.4 Wh kg^-1 at 0.4 and 4.0 kW kg^-1) in 1.0 M Li₂SO₄ electrolyte. Also, this as-assembled device displays excellent cycle performance (with no capacitance fading after 10000 cycles at 10.0 A g^-1). Compared with the electrochemical performance of the as-prepared NSHPC sample and the previously synthesized N-doped porous carbon and activation carbon with similar SSA and pore volume, it is suggested that the S-doping in the NSHPC sample plays a positive role in enhancing its rate performance for supercapacitors originated from its incremental hydrophilicity and rich heteroatom content.