The recent advancement in flexible/bendable displays such as phones, smart bands, laptops, and televisions demands flexible energy storage devices. Heteroatom-doped graphene electrodes are promising candidates for flexible energystorage applications because of its dual nature of electrochemical double-layer capacitance and pseudocapacitance. The low energy density of supercapacitors limits their potential use and attempts are in progress to overcome this issue. Redox-additive electrolytes are presumed to be one of the elite tactics to augment the energy density of the supercapacitor device. Here, the sodium molybdate (Na 2 MoO 4 )-incorporated polymer gel electrolyte has been demonstrated to enhance the energy density of a flexible supercapacitor. The supercapacitor electrode with Na 2 MoO 4 /H 2 SO 4 (714 F/g) acquired ∼2.4-fold greater specific capacitance than with H 2 SO 4 (300 F/g) in a threeelectrode configuration. Moreover, the energy density of symmetric supercapacitor with Na 2 MoO 4 /H 2 SO 4 (23 Wh/kg) is ∼2-fold higher compared with H 2 SO 4 electrolyte (13 Wh/kg). The fabricated flexible supercapacitor device with sodium molybdate/ poly(vinyl alcohol)/sulfuric acid (Na 2 MoO 4 /PVA/H 2 SO 4 ) gel electrolyte showed 3.4-fold higher energy density (E = 24 Wh/kg) compared to a device with PVA/H 2 SO 4 gel electrolyte (E = 7 Wh/kg). The increase in energy density has been observed after the incorporation of Na 2 MoO 4 , which is due to the faradaic reaction between the MoO 4 2− ions in the redox-additive and H + ions in the H 2 SO 4 electrolyte.

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).

Herein, we have used ammonium vanadate (NH4VO3) as a redox-additive for the first time to boost the energy density of a flexible supercapacitor. Surprisingly, a significant enhancement in the specific capacitance (C sp) is attained after the addition of NH4VO3 with H2SO4 (658 F g–1) when compared with H2SO4 (375 F g–1). In addition, the energy density conquered with NH4VO3/H2SO4 (32 Wh kg–1) is 4.5-fold greater than H2SO4 electrolyte at 3 A g–1 (7 Wh kg–1). Moreover, the flexible solid-state supercapacitor (FSSC) device with ammonium vanadate/poly­(vinyl alcohol)/sulfuric acid (NH4VO3/PVA/H2SO4) gel electrolyte shows a better electrochemical performance while compared with PVA/H2SO4. The electrochemical performance of the working electrode enhanced after the addition of NH4VO3 with conventional H2SO4 due to the redox reaction between the VO3 – and H+ ions.