MOF-Derived Hollow Cage Ni_xCo_{3− x}O₄ and Their Synergy with Graphene for Outstanding Supercapacitors

Highly optimized nickel cobalt mixed oxide has been derived from zeolite imidazole frameworks. While the pure cobalt oxide gives only 178.7 F g⁻¹ as the specific capacitance at a current density of 1 A g⁻¹, the optimized Ni:Co 1:1 has given an extremely high and unprecedented specific capacitance of 1931 F g⁻¹ at a current density of 1 A g⁻¹, with a capacitance retention of 69.5% after 5000 cycles in a three electrode test. This optimized Ni:Co 1:1 mixed oxide is further used to make a composite of nickel cobalt mixed oxide/graphene 3D hydrogel for enhancing the electrochemical performance by virtue of a continuous and porous graphene conductive network. The electrode made from GNi:Co 1:1 successfully achieves an even higher specific capacitance of 2870.8 F g⁻¹ at 1 A g⁻¹ and also shows a significant improvement in the cyclic stability with 81% capacitance retention after 5000 cycles. An asymmetric supercapacitor is also assembled using a pure graphene 3D hydrogel as the negative electrode and the GNi:Co 1:1 as the positive electrode. With a potential window of 1.5 V and binder free electrodes, the capacitor gives a high specific energy density of 50.2 Wh kg⁻¹ at a high power density of 750 W kg⁻¹.