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

Herein, we report the chemical synthesis of NiFe 2 O 4 thin films forming nanosheet-, nanoflower-, and nanofeather-like morphologies using NiCl 2 ·6H 2 O, Ni(NO 3 ) 2 ·6H 2 O, and NiSO 4 ·6H 2 O 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 NiFe 2 O 4 nanosheets revealed a maximum surface area of 47 m 2 g –1, which was higher than those of nanoflowers and nanofeathers (25 and 11 m 2 g –1 ). Similarly, the supercapacitive properties of the individual NiFe 2 O 4 nanosheet-based electrode demonstrated maximum specific capacitance of 1139 F g –1, which is found to be better than that of NiFe 2 O 4 nanoflowers (677 F g –1 ) and nanofeathers (435 F g –1 ) in 6 M KOH electrolyte. Furthermore, the symmetric device fabricated using NiFe 2 O 4 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 .