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

The electrocapacitive behavior of MnFeO-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 MnFeO colloidal nanocrystal assemblies (CNAs) was in the range of 230–950 nm, formed by the in situ self-assembly of primary MnFeO nanoparticles with different sizes. Electrochemical measurements showed that the electrochemical performances of MnFeO-based supercapacitors were related to the structure of MnFeO CNAs. MnFeO 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 MnFeO 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 MnFeO CNAs was analyzed and discussed based on the experimental data.