**3. Manganese ferrite (MnFe2O4)**

MnFe2O4 based electrodes exhibited good capacitive properties in aqueous electrolyte [53, 66–70]. It is observed that MnFe2O4 stores the charge by pseudo mechanism in aqueous electrolyte [71]. Shin-Liang Kuo et al. [71] have shown that charge storage in MnFe2O4 involves insertion/extraction of proton into/from the lattice at both the Mn- and Fe-ion sites. In that study, MnFe2O4 was synthesized by coprecipitation method for supercapacitor application. For electrochemical characterization, the

#### *Application of Ferrites as Electrodes for Supercapacitor DOI: http://dx.doi.org/10.5772/intechopen.99381*

MnFe2O4 powder was mixed carbon black and PVDF, and then coated on current collector. Electrochemical performance was determined with a three-electrode cell in 1 M KCl aqueous solution. The overall specific capacitance of 63.4 Fg−1 was determined for the electrode and specific capacitance of 115 Fg−1 for MnFe2O4 electrode. Baoyan Wang et al. [53] have studied the effect of surfactants on the electrochemical performances of MnFe2O4 synthesized by solvothermal method. The capacitive performances of MnFe2O4 colloidal nanocrystal cluster was observed to be larger than MnFe2O4 hollow sphere in aqueous LiNO3 electrolyte. In that work, an almost rectangular CV curves were obtained for MnFe2O4 in a potential range of −0.4-1.5 V. Addition of surfactants leads to increase in the capacitance of MnFe2O4 in LiNO3 electrolyte. It may be due to the reduction of interfacial tension between electrode and electrolyte in presence of surfactants with promoting the diffusion of lithium ions. After addition of different surfactants, SDS (Anionic surfactant sodium dodecyl sulphate), Triton-X-100 (non-ionic surfactant p-toctylophenol) and P123 (poly(ethylene glycol)-block-poly(propylene glycol)-blockPoly(ethylene glycol)), the capacitance increased about 36.8%, 22.8% and 12.8%, respectively.

V. Vignesh et al. [69] have reported electrochemical properties of MnFe2O4 spherical nanoparticles (20–50 nm) synthesized by simple and facile coprecipitation method. The capacitor performance was evaluated in different electrolytes, 1 M LiNO3, 1 M Li3PO4 and KOH. The MnFe2O4 nanoparticles showed specific capacitance of 173, 31 and 430 F g−1 in electrolytes of 3.5 M KOH, 1 M LiNO3 and 1 M Li3PO4, respectively. However, excellent rate performance was observed in 3.5 M KOH electrolyte with good retention of capacitance at higher current densities. Supercapacitor with two electrodes of MnFe2O4 nanoparticles exhibited specific capacitance of 245 F g−1, and energy density and power density of 12.6 Wh kg−1 and 1207 W kg−1, respectively in 3.5 M KOH electrolyte.

Further, the electrochemical performances of MnFe2O4 colloidal nanocrystal clusters (CNCs) was investigated in symmetric supercapacitors with different aqueous electrolytes [72]. The specific capacitances of MnFe2O4 electrode was found to be 97.1, 93.9, 74.2 and 47.4 F g−1 in electrolytes 2 M KOH, 2 M NaOH, 2 M LiOH and 2 M Na2SO4, respectively. It was found that MnFe2O4 CNCs exhibited better performance in 6 M KOH electrolyte with the specific capacitance of 152.5 F g−1 and retention of capacitance of about 76% after 2000 cycles. MnFe2O4 colloidal nanocrystal assemblies (CNAs) with size of 420 nm, composed of 16 nm nanoparticles showed specific capacitance of 88.4 Fg−1 calculated at the current density of 0.01 Ag−1 [70]. When the current increased from 0.01 to 2 Ag−1 MnFe2O4 CNAs retained 59.4% capacitance, and 69.2% capacitance after 2000 cycles. The electrochemical performance of MnFe2O4 CNAs was related to the size of primary nanoparticles in the CNAs.

Further improvement in MnFe2O4 based supercapacitor was made by making nanocomposite of MnFe2O4 with grapheme [66, 68, 73–75]. Isara Kotutha et al. [73] have used one-pot hydrothermal approach to prepare rGO/MnFe2O4 nanocomposite. A maximum specific capacitance of 276.9 Fg−1 was determined for the rGO/MnFe2O4 nanocomposite at scan rate of 10 mVs−1 in 6.0 M KOH electrolyte. A flexible supercapacitor of MnFe2O4/graphene using current collectors of flexible graphite sheets has been fabricated [66]. The flexible supercapacitor exhibited specific capacitance of 120 F g−1 at 0.1 A g−1 with retaining 105% capacitance after 5000 cycles.

Larissa H. Nonaka et al. [68] have achieved 195 Fg−1 capacitance for MnFe2O4 nanoparticles on a crumpled graphene sheet at scan rate of 0.5 Ag−1 in 0.05 M KCL electrolyte. A pseudocapacitive behavior was observed in the CV curves which indicates that there is major contribution from MnFe2O4 to the overall capacitance of the hybrid electrode. A larger specific capacitance of 454.8 F g−1 at 0.2 A g−1 was obtained by a ternary MnFe2O4/graphene/polyaniline nanocomposite fabricated by a facile two-step approach. The ternary nanocomposite also exhibited outstanding rate capability about 75.8% capacitance retention at 5 A g−1 and excellent cycling stability, 76.4% retention in capacitance after 5000 cycles. Specific capacitance of 307.2 F g−1 at 0.1 A g−1 has been achieved with symmetric supercapacitor. The device exhibited a maximum energy density of 13.5 W h kg−1.
