Enhanced electrostatic potential with high energy and power density of a symmetric and asymmetric solid-state supercapacitor of boron and nitrogen co-doped reduced graphene nanosheets for energy storage devices

The world is currently focused on high energy density and long-term cycling stability electrode materials due to the extinction of carbonaceous fuels. A major challenge is to increase the electrostatic potential window (1.0–1.8 V) to increase the energy density of devices in practical applications. Herein, we synthesized a boron and nitrogen co-doped reduced graphene (BNUG) using boric acid (H3BO3) and urea at different compositions in a two-step process of hydrothermal synthesis followed by pyrolysis in chemical vapour deposition (CVD) at 1000 °C in an Ar atmosphere, which exhibits a high specific capacitance of 325.5 F g−1 at 1 A g−1 with excellent cycling stability, with 102% retention of the capacitance after 10 000 cycles in 1 M H2SO4 electrolyte. The as-synthesized BNUG4 was used to fabricate a flexible symmetric solid-state supercapacitor (SSC) (with BNUG as the +ve and −ve electrode) that delivers a high energy density of 64.75 W h kg−1 at a power density of 900 W kg−1 at 1 A g−1, and an asymmetric solid-state supercapacitor (ASSC) (with BNUG as the +ve and reduced graphene oxide as the −ve electrode) that delivers a high energy density of 55.8 W h kg−1 at a power density of 900 W kg−1 in H2SO4/PVA solid–gel electrolyte. Both the symmetric and asymmetric solid-state supercapacitor exhibit an excellent coulombic efficiency of 99.7%, whereas the SSC shows long-term cycling stability with 104.7% capacitance retention and ASSC shows 99.7% retention after 10 000 cycles. The resistance observed from the equivalent series resistance (ESR) offers additional support for low resistance, exhibiting high specific capacitance. To demonstrate application of the fabricated SSC and ASSC devices, they were successfully used to light-up a 5 mm red-light emitting diode and after recycling a DC motor with a fan was used.