Facile hydrothermal synthesis of cubic spinel AB ₂ O ₄ type MnFe ₂ O ₄ nanocrystallites and their electrochemical performance

Cubic spinel MnFe ₂ O ₄ nanoparticles were synthesized using a simple hydrothermal method followed by post-annealing. The effects of the reaction temperature on the crystallinity, morphology, and electrochemical performance were studied. The reaction temperature played an important role in the synthesis of highly crystalline MnFe ₂ O ₄ nanoparticles. At low reaction temperatures (<160 °C), the synthesized product contained a secondary inactive Fe ₂ O ₃ phase as well as MnFe ₂ O ₄ nanoparticles. In contrast, pure MnFe ₂ O ₄ nanoparticles were obtained at temperatures above 180 °C. Furthermore, the crystallinity of the MnFe ₂ O ₄ nanoparticles was enhanced significantly by increasing the reaction temperature to 200 °C. The cubic spinel MnFe ₂ O ₄ nanoparticles synthesized at 200 °C delivered a maximum specific capacitance of 282.4 F g ⁻¹ at a current density of 0.5 A g ⁻¹ in a 2 M aqueous KOH solution, and exhibited long-term cyclic stability of 85.8% capacitance retention after 2000 cycles. This was attributed to the cubic spinel ferrite nanocrystallite particles not only providing the more active sites for OH ⁻ ion diffusion but also reducing the path lengths for OH ⁻ ion diffusion. These results show that the synthesized MnFe ₂ O ₄ nanoparticles are promising candidates for pseudocapacitors and other electrochemical applications.