▎ 摘　　要

Herein, the high potential window of 2.5 V (from -1.35 to +1.15 V) is achieved using nitrogen-doped reduced graphene oxide (N-rGO) as an electrode in neutral 0.5 m K2SO4 electrolyte after running 150 electrochemical cyclic voltammetry (CV) cycles. During these CV cycles, K+ ions are increasingly adsorbed at the active sites of the electrode, restricting the recombination of nascent hydrogen to generate dihydrogen (H-2), thereby enhancing the negative potential stability up to -1.35 V. As-synthesized N-rGO obtained by effective functionalization of carbon is used to fabricate high-voltage (2.5 V) symmetric supercapacitors (N-rGO//N-rGO) in aqueous neutral electrolyte (0.5 m K2SO4), providing a high energy density of 128 Wh kg(-1) at a power density of 813 W kg(-1) with superior cyclic stability. The formation of a tandem device by connecting three as-designed symmetric supercapacitor cells in series increases the output voltage to 7.5 V, which exhibits long-term cyclability up to 10 000 cycles, thus making it sustainable for energy-storage applications. This system demonstrates the highest cell voltage for a carbon-based aqueous symmetric supercapacitor with a high energy density.