Flexible energy storage devices are the cornerstone to the development of future-generation electronics such as flexible displays on phones, smart bands, laptops, and televisions. The advancement of flexible supercapacitors has turned into an essential task because supercapacitors are designed with the rewards of optimum power and energy density. Owing to the dual function as an electrical double-layer capacitor and a pseudocapacitor, heteroatom-doped graphene is presumed to be a promising electrode material for supercapacitor applications. Herein, we report p-toluenesulfonic acid as the precursor to the formation of sulfur-doped graphene by supercritical fluid-aided processing. Both the existence and nature of S doping in graphene were confirmed with the elemental and X-ray photoelectron spectroscopy techniques. Full cell analysis indicated that the energy density achieved using hydroquinone (HQ) as a redox additive in 1 M H2SO4 solution was found to be 27 W h/kg, which is twice that of an aqueous solution of 1 M H2SO4 (13 W h/kg). To extend the application of the symmetric cell, a flexible device using polyvinyl alcohol (PVA)/HQ/H2SO4 is fabricated. A 3-fold increase in energy density is observed for the flexible solid-state single device using PVA/HQ/H2SO4 (E = 21.3 W h/kg) when compared with PVA/H2SO4 as an electrolyte (E = 7.7 W h/kg).