Sodium-Containing Spinel NiFe ₂ O ₄ Nanoparticles for Enhanced Supercapacitor Performance

Na-containing NiFe 2 O 4 (Na–NFO) nanoparticles were synthesized under different NaOH-assisted hydrothermal conditions and evaluated as supercapacitor electrode materials. Structural analysis indicates that the presence of Na does not generate detectable secondary phases or disrupt the spinel framework. The synthesized Na-containing samples exhibit comparable particle sizes and overall morphologies, whereas commercial NiFe 2 O 4 (Com-NFO) shows a distinct morphology. Electrochemical measurements show that samples prepared under higher NaOH-assisted hydrothermal conditions exhibit larger current responses and higher specific capacitance, with 40-Na-NFO delivering the highest current response and specific capacitance. Both Com-NFO and 40-Na-NFO retain nearly 100% capacitance after 5000 cycles, while 40-Na-NFO maintains higher capacitance throughout cycling. Although Com-NFO exhibits a more dispersed electrode morphology and a higher capacitive contribution of 98%, it delivers lower capacitance than the Na-containing samples. Kinetic analysis indicates predominantly surface-controlled charge storage for Com-NFO and a mixed charge-storage mechanism for 40-Na-NFO, while capacitive/diffusion-controlled contribution separation further suggests enhanced diffusion-controlled contributions associated with more bulk-involved Faradaic processes in the 40-Na-NFO electrode. These results suggest that NaOH-assisted synthesis and associated Na presence promote more bulk-involved pseudocapacitive charge storage in NiFe 2 O 4 without sacrificing cycling stability.