Electrochemical properties of colloidal nanocrystal assemblies of manganese ferrite as the electrode materials for supercapacitors

The electrocapacitive behavior of MnFe₂O₄-based supercapacitors has been studied by a series of electrochemical techniques, including circle voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy. The size of the used MnFe₂O₄ colloidal nanocrystal assemblies (CNAs) was in the range of 230–950 nm, formed by the in situ self-assembly of primary MnFe₂O₄ nanoparticles with different sizes. Electrochemical measurements showed that the electrochemical performances of MnFe₂O₄-based supercapacitors were related to the structure of MnFe₂O₄ CNAs. MnFe₂O₄ CNAs with the size of 420 nm, composed of 16 nm nanoparticles, displayed the highest capacitance of about 88.4 F/g at the current density of 0.01 A/g, which, respectively, decreased to 55.8 and 20.2 F/g for CNAs with size of 230 and 950 nm, assembled by 21 and 43 nm nanoparticles. Electrochemical stability data showed that 420 nm MnFe₂O₄ CNAs had the best capacitance retention of 59.4% with the current density increased from 0.01 to 2 A/g and the best capacitance retention of 69.2% after 2000 cycles among all the samples under the current density of 0.2 A/g. The structure–property relationship of MnFe₂O₄ CNAs was analyzed and discussed based on the experimental data.