A Hierarchical Interconnected Nanosheet Structure of Porous δ-MnO₂ on Graphite Paper as Cathode with a Broad Potential Window for NaNO₃ Aqueous Electrolyte Supercapacitors

The potential window for aqueous supercapacitor cathodes greatly depends on the electrochemical stability of water, and thus is generally limited to ∼1.2 V. Herein, a 1.4 V potential window (vs Ag/AgCl) in the 5 M NaNO3 aqueous electrolyte for porous δ-MnO2 with a hierarchical interconnected nanosheet structure grown on electrochemically roughened graphite paper by electrodeposition is reported. A specific capacitance of ∼407.6 F g–1 is delivered at 1 A g–1, and capacitance retention up to ∼90.7% is achieved after 5000 cycles at 8 A g–1. The aqueous asymmetric supercapacitors of 2.4 V are assembled with the configuration of δ-MnO2//activated carbon, and an energy density (E) of ∼38.4 Wh kg–1 is obtained at 599.7 W kg–1 (even at 12 kW kg–1, E of ∼21.7 Wh kg–1 is still delivered). Moreover, the asymmetric supercapacitors exhibit good rate and cycling performance. Thanks to the simple preparation for the electrode materials/structures and high device performance, it is believed that this work provides valuable contributions to developing aqueous supercapacitors delivering a broad working voltage window and thus an elevated energy density.