High Electrochemical Performance of 2.5 V Aqueous Symmetric Supercapacitor Based on Nitrogen‐Doped Reduced Graphene Oxide

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 K 2 SO 4 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 K 2 SO 4 ), 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.