In-situ synthesis of N, S co-doped electrode material by an environmentally benign method towards designing of high voltage (3.0 V) binder-free sustainable aqueous symmetric supercapacitor

In aqueous electrolyte based symmetric supercapacitor, the decomposition of water to evolve hydrogen and oxygen occurs in relatively lower potential window, thereby, limits its cell voltage and needs to be modified so as to achieve high energy density. This manuscript reports a novel greener approach for an in-situ synthesis of N, S co-doped reduced graphene oxide (N, S co-doped rGO-10/20) nanohybrids as porous electrode material for designing a binder-free and up-scaled mass-loaded (5.0 mg cm−2) high voltage (3.0 V) aqueous symmetric supercapacitor in water-in-salt (WIS), 17 m NaClO4 electrolyte. A fairly rich co-doping/integration of heteroatoms (N (7.2 %) and S (1.5 %)) into the rGO matrix takes place through the formation of C-N, C-S and C-SOx-C bonds. The N, S co-doped rGO-20 nanohybrids in three-electrode system shows the pseudocapacitive behavior with the electrochemical stability of potential window (ESPW) of 3.0 V. In two electrodes set-up, the optimized aqueous symmetric supercapacitor cell (SSC) in WIS shows the superb cell voltage at 3.0 V (highest reported to-date), a fairly high areal energy density@areal power density (0.256 (mWh cm−2)@5.239 (mW cm−2)) at 0.7 A/g with excellent capacitance retention (93.7 %) and Coulombic efficiency (99.3 %) after 20 k cycles. Its energy storage capability is further validated by illumination of 125 white LEDs and 91 green LEDs upon lighting a single SSC for 50 s each and forming the tandem device with high cyclic stability (102 %). Under open circuit, SSC exhibits self-discharge to 2.4 V without any further appreciable discharge till 24 h.