Soft-template assisted morphology tuning of NiMoO₄ for hybrid supercapacitors

The electrochemical performance of electrode materials depends on various physicochemical properties like particle size, morphology, porosity, etc. of materials. Herewith, a facile surfactant assisted hydrothermal route using cetyltrimethylammonium bromide (CTAB) and hexamethylenetetramine (HMT) is used to successfully synthesize nanostructured nickel molybdate (NiMoO₄) with various morphologies. It is found that NiMoO₄ possesses different morphologies when synthesized by using these surfactants. With change in morphology, there is a significant increase in diffusion coefficient for sample synthesized with CTAB. It is interesting to note that NiMoO₄ synthesized by using CTAB exhibit a high specific capacitance of 947 F g ^{− 1} when cycled at 1 A g ^{− 1} in 1 M KOH solution as compared to that of 322 F g ^{− 1} (synthesized without CTAB), which can be ascribed to its flower-like shape with uniform porosity. Both Ni²⁺/Ni³⁺ and Mo⁶⁺/Mo⁴⁺ redox couples are involved in the capacitive characteristic of NiMoO₄, as found from the XPS study. Additionally, a hybrid supercapacitor has been assembled using NiMoO₄ as positive electrode and reduced grapgene oxide (rGO) as negative electrode, which can exhibit 107 F g ^{− 1} with an operating voltage of 1.6 V with excellent cycling stability of maintaining 94 % capacitance retention after 6000 cycles in 1 M KOH solution. The hybrid supercapacitor can possess an energy density of 37.7 Wh kg⁻¹ and a power density of 8 kW kg⁻¹. Thus, this study enables the importance of tuning the morphology to enhance the capacitance property of transition metal oxide based electrode materials for supercapacitor applications.