Metal Precursor Dependent Synthesis of NiFe₂O₄ Thin Films for High-Performance Flexible Symmetric Supercapacitor

Herein, we report the chemical synthesis of NiFe₂O₄ thin films forming nanosheet-, nanoflower-, and nanofeather-like morphologies using NiCl₂·6H₂O, Ni(NO₃)₂·6H₂O, and NiSO₄·6H₂O nickel salt precursors, respectively, while using the same iron salt precursor. A nanostructure formation mechanism is proposed in detail using coordination chemistry theory. Interestingly, nanostructures of NiFe₂O₄ nanosheets revealed a maximum surface area of 47 m² g^-1, which was higher than those of nanoflowers and nanofeathers (25 and 11 m² g^-1). Similarly, the supercapacitive properties of the individual NiFe₂O₄ nanosheet-based electrode demonstrated maximum specific capacitance of 1139 F g^-1, which is found to be better than that of NiFe₂O₄ nanoflowers (677 F g^-1) and nanofeathers (435 F g^-1) in 6 M KOH electrolyte. Furthermore, the symmetric device fabricated using NiFe₂O₄ nanosheet electrodes and PVA-KOH solid gel electrolyte shows higher specific capacitance of 236 F g^-1 with 98% retention after 7000 cycles and higher specific energy density of 47 Wh kg^-1 at a specific power of 333 W kg^-1.