Chemical bath synthesis of NiCo₂O₄ nanoflowers with nanorods like thin film for flexible supercapacitor application-effect of urea concentration on structural conversion

In a simple chemical bath deposition, the flexible NiCo₂O₄ thin films with different nanostructures such as nanoflakes, nanoflowers with the surface composed of nanorods have been synthesized on stainless steel by changing the urea concentration of precursor. This study showed that the concentration of urea has a crucial role in the structural conversion of nanostructures. Here, the NiCo₂O₄ nanoflowers with nanorods like thin film demonstrated superior supercapacitive performance exhibiting a maximum specific capacitance of 702 F g⁻¹ ascribed to the higher electronic conductivity of NiCo₂O₄ and lower ionic diffusion resistance due to nanoflowers with nanorods like mixed morphology. In further, the higher concentration of urea during synthesis leads to disorientation and overgrowth of the NiCo₂O₄ nanoflowers which deteriorated its supercapacitive performance. The two flexible solid state symmetric supercapacitor devices have been successfully fabricated using polyvinyl alcohol–KOH and polyvinyl alcohol-LiClO₄ gel electrolytes. In comparison, the device with polyvinyl alcohol-LiClO₄ gel exhibited superior performance exhibiting a maximum specific capacitance of 132 F g⁻¹ with an energy density of 18.52 Wh kg⁻¹ and a power density of 3.13 kW kg⁻¹.