CoxZn1-x ZIF-derived binary Co3O4/ZnO wrapped by 3D reduced graphene oxide for asymmetric supercapacitor: Comparison of pure and heat-treated bimetallic MOF

Sodalite zeolitic imidazolate frameworks containing Co (ZIF-67) and Zn (ZIF-8) as well as three hybrid MOFs (Co0.75Zn0.25, Co(0.5)Zna(0.5) and C0.25Zn0.75-ZIFs) were successfully synthesized at room temperature under aqueous conditions within 24 h. These MOFs (ZIF1 to 5) were used as template for the synthesis of nanoporous metal oxide (pure CO3O4, three hybrids of CO3O4/ZnO with different ratios of CO3O4 and ZnO, as well as pure ZnO; named as HTZIF1 to 5, respectively). X-ray photoelectron spectroscopy and energy dispersive X-ray analysis were applied to obtain stoichiometric ratios of Co/Zn in pure and heat-treated MOFs. Performances of the reduced graphene oxide/active material composites were investigated by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. Amongst the two groups (rGO/ZIFn and rGO/HTZIFn) of electrodes, the rGO/ZIF2 and rGO/HTZIF3 exhibited favorable specific capacitances of 163 and 204 F g(-1), respectively, at 1 A g(-1). The rGO/HTZIF3 electrode demonstrated a 1.6 V potential window in hydrous electrolyte when it was used in an asymmetric supercapacitor device with rGO nanosheets as anode electrode, which led to achieve a high specific energy of 12.4 W h kg(-1). Furthermore, the asymmetric supercapacitor device provided a maximum specific power of 8500 W kg(-1) and a specific energy of 12.4 W h kg(-1) with a large working potential of 1.6 V. In addition, 87% of its capacity was kept after 2000 cycles at a current density of 3 A g(-1). This indicates that the heat-treated ZIF electrode can be suggested as a promising candidate for future applications in renewable energy storage.