Superior capacitive performance of spherical MnV₂O₆·2H₂O-graphene nanosheet hybrid as electrode material for asymmetric supercapacitor

The evolution of advanced supercapacitors depends highly on the design and synthesis of electrodes with rational morphology and structure. In this paper, a facile methanol-assisted solvothermal method is reported for the synthesis of spherical hydrated manganese vanadate (MnV₂O₆·2H₂O)-reduced graphene oxide (rGO) nanosheets hybrid. In particular, the utilization of methanol can not only change the isotropy growth of MnV₂O₆ but also suppresses the phase-transformation process (from orthorhombic to monoclinic) by preventing the deintercalation of crystal water. Remarkably, the MnV₂O₆·2H₂O exhibits a superior specific capacity of 1678.5 F g⁻¹ which is nearly four times more than that of MnV₂O₆ (410.2 F g⁻¹) at 2 A g⁻¹. After integrating with graphene, MnV₂O₆·2H₂O spheres are uniformly encapsulated by graphene sheets with a size reduction from 1 μm to 200 nm. And a “GO-assisted Ostwald ripen-splitting” mechanism is proposed to explain the formation of MnV₂O₆·2H₂O-rGO. Consequently, the MnV₂O₆·2H₂O-rGO hybrid exhibits significantly improved conductivity, enhanced specific capacity (1976.3 F g⁻¹ at 2 A g⁻¹) and good rate capability (1422.5 F g⁻¹ at 10 A g⁻¹ and 1155.5 F g⁻¹ at 20 A g⁻¹) as well as excellent cycling stability (94.2% capacitance retention after 10,000 cycles). Moreover, the assembled asymmetric supercapacitor using MnV₂O₆·2H₂O-rGO (the positive electrode) and activated carbon (the negative electrode) shows a superior energy density of 51.5 Wh⋅kg⁻¹ at the power density of 849.2 W kg⁻¹.