Heterostructural Graphene Quantum Dot/MnO₂ Nanosheets toward High‐Potential Window Electrodes for High‐Performance Supercapacitors

The potential window of aqueous supercapacitors is limited by the theoretical value (≈1.23 V) and is usually lower than ≈1 V, which hinders further improvements for energy density. Here, a simple and scalable method is developed to fabricate unique graphene quantum dot (GQD)/MnO₂ heterostructural electrodes to extend the potential window to 0-1.3 V for high-performance aqueous supercapacitor. The GQD/MnO₂ heterostructural electrode is fabricated by GQDs in situ formed on the surface of MnO₂ nanosheet arrays with good interface bonding by the formation of Mn-O-C bonds. Further, it is interesting to find that the potential window can be extended to 1.3 V by a potential drop in the built-in electric field of the GQD/MnO₂ heterostructural region. Additionally, the specific capacitance up to 1170 F g⁻¹ at a scan rate of 5 mV s⁻¹ (1094 F g⁻¹ at 0-1 V) and cycle performance (92.7%@10 000 cycles) between 0 and 1.3 V are observed. A 2.3 V aqueous GQD/MnO₂-3//nitrogen-doped graphene ASC is assembled, which exhibits the high energy density of 118 Wh kg⁻¹ at the power density of 923 W kg⁻¹. This work opens new opportunities for developing high-voltage aqueous supercapacitors using in situ formed heterostructures to further increase energy density.