Ultrahigh energy density battery-type asymmetric supercapacitors: NiMoO₄ nanorod-decorated graphene and graphene/Fe₂O₃ quantum dots

NiMoO₄ has attracted intensive attention as one of the promising ternary metal oxides because of its high specific capacitance and electrical conductivity compared to traditional transition-metal oxides. In this study, NiMoO₄ nanorods uniformly decorated on graphene nanosheets (G-NiMoO₄) are synthesized through a facile hydrothermal method. The prepared G-NiMoO₄ composite exhibits a high specific capacitance of 714 C·g^-1 at 1 A·g^-1 and an excellent rate capability, with a retention ratio of 57.7% even at 100 A·g^-1. An asymmetric supercapacitor (ASC) fabricated with the G-NiMoO₄ composite as the positive electrode and Fe₂O₃ quantum dot-decorated graphene (G-Fe₂O₃-QDs) as the negative electrode delivers an ultrahigh energy density of 130 Wh·kg^-1, which is comparable to those of previously reported aqueous NiMoO₄-based ASCs. Even when the power density reaches 33.6 kW·kg^-1, an energy density of 56 Wh·kg^-1 can be maintained. The ASC device exhibits outstanding cycling stability, with a capacitance retention of 113% after 40, 000 cycles. These results indicate that the G-NiMoO₄ composite is a promising candidate for ASCs with ultrahigh energy density and excellent cycling stability. Moreover, the present work provides an exciting guideline for the future design of high-performance supercapacitors for industrial and consumer applications via the simultaneous use of various pseudocapacitive materials with suitable potential windows as the positive and negative electrodes.