Free-standing nickel molybdate/cobalt phosphate heterostructure for supercapacitor

Heterostructured materials possess a compelling combination of characteristics that render them highly appealing as electrode materials for supercapacitors. Recently, there has been increased attention on these materials as potential electrodes for use in supercapacitors. However, synthesis of heterostructured materials directly on the conducting support is challenging. The application of polymer binders to deposit active material on conductive support leads to increased charge transfer resistance, which reduces the energy storage capacity. Therefore, it is crucial to strategically design and create highly active free-standing heterostructured electrode for superior supercapacitive charge storage. In this work, a straightforward hydrothermal method is employed to fabricate free-standing nickel molybdate (NMO) with a rod-like nanostructure, onto which 2D nanosheets of cobalt phosphate (CP) are coated using the successive ionic layer adsorption and reaction (SILAR) method. The NMO exhibits a triclinic crystal structure, while CP shows an amorphous structure, together forming a crystalline-amorphous heterostructure. The self-supported nanorods of NMO and micro sheets of CP addresses challenges resulting from the use of a binder and exposes a significant surface area of the electrode to the electrolyte, enhancing charge storage. The electrochemical performances of these materials as cathodes for supercapacitors are subsequently investigated in a 2 M KOH electrolyte. The NMO/CP electrode exhibits a higher areal capacitance of 8.56 F cm⁻² and lower charge transfer resistance of 20.03 Ω cm⁻² compared to NMO (5.58 F cm⁻², 25.12 Ω cm⁻²) and CP (4.3 F cm⁻², 27.31 Ω cm⁻²) electrodes, indicating enhanced ionic charge transfer kinetics and improved performance in the NMO/CP system. The NMO/CP electrode demonstrates remarkable cycling performance, maintaining a 98% coulombic efficiency and 60% of initial capacitance over 5000 cycles at a constant current density of 16 mA cm⁻², showcasing its excellent durability. This study highlights the significant potential of the free-standing NMO/CP heterostructure as an efficient electrode for supercapacitors, particularly in Faradaic pseudocapacitive storage.