**2. Cobalt ferrite (CoFe2O4)**

CoFe2O4 with high magnetic properties has good mechanical hardness and chemical stability, which make it suitable candidate in magnetic device applications [54]. Apart from above properties, CoFe2O4 has also shown good electrochemical performance in supercapacitor [52, 55–59]. CoFe2O4 nanoflakes synthesized by chemical bath deposition process have been utilized in supercapacitor [52]. The nano flakes were deposited on a stainless steel substrate with thickness of 5.3–7.0 μm. The electrochemical properties were investigated in three electrode system with 1 M NaOH electrolyte. The nano-flakes showed a specific capacitance of 366 F g−1 (interfacial capacitance of 0.110 F cm−2) at scan rate of 5 mVs−1. The nano-flakes electrode could preserve 190 Fg−1 capacitance at scan rate of 100 mV/s−1. At such larger scan rates, the inner active sites of the electrodes might not participate in redox process, resulting in decrease in the capacitance. The CoFe2O4 nano-flakes retained 90.6% capacitance after 1000 cycles.

Pawar et al. [60], have synthesized CoFe2O4 nanoparticles (average size 23 nm) by sol–gel method for supercapacitor application. CoFe2O4 nanoparticles exhibited pseudocapacitive behavior in 1 M KOH electrolyte with a three-electrode system as observed from the CV curves. The specific capacitance determined from galvanostatic charge discharge process were found to be 15 Fg−1 at current density of 0.6 Ag−1. This lower value of the capacitance may be due to the poor electrical conductivity of CoFe2O4 nanoparticles. An improved electrochemical performance was achieved with CoFe2O4/FeOOH nanocomposite synthesized by one-step hydrothermal approach [61]. A mixture of CoFe2O4/FeOOH nanocomposites, carbon black and polyvinylidenefluoride (PVDF) in ratio of 80:10:10 wt% was used as electrode. The electrode of CoFe2O4 with FeOOH exhibited good value of capacitance at larger currents which is essential for a supercapacitor. The electrode showed specific capacitance of 332.4, 319.4, 257.2, 239, 193.1, and 180 F g−1 at the current densities of 0.5, 1, 2, 5, 8, and 10 A g−1, respectively. About 8.7% loss in capacitance was observed after 1000 cycles.

Mesoporous CoFe2O4 thin film also demonstrated good value of specific capacitance of 369 Fg−1 at 2 mV/s−1 in 1 M KOH electrolyte with wide potential window of −1.2 to +0.5 V [62]. The capacitance degraded to 167 Fg−1 upon increasing the scan rate to 100 mV/s−1 . The mesoporous CoFe2O4 film was prepared by a chemical spray pyrolysis technique from the aqueous medium at 475°C substrate temperature. The film was observed to be uniform on the substrate and free from any crack with mesoporous type surface morphology. CoFe2O4 thin film consists of grain with size in the range of nm. The electrode of mesoporous CoFe2O4 film showed power density of 28.74 kWkg−1 with maintaining energy density of 27.14 Whkg−1.

A large specific capacitance of 429 Fg−1 was obtained by CoFe2O4 nanoparticles in 6 M KOH electrolyte at 0.5 Ag−1 [63]. These nanoparticles were prepared by hydrothermal and coprecipitation methods using nitrates, chlorides and acetates precursors with average size from 11 to 26 nm and surface area of ~34 m<sup>2</sup> g−1. The nanoparticles has shown an excellent capacitance retention of 98.8% after 6000 cycles at high current density of 10 Ag−1. The above results indicate that CoFe2O4 nanoparticles with the above morphology may be a promising electrode material for supercapacitor. A composite of reduced graphene oxide and CoFe2O4 (RGO–CoFe2O4) was examined for supercapacitor application [64]. The electrode of RGO–CoFe2O4 showed a specific capacitance of 123.2 F g−1 which is larger than that of individual constituents RGO (89.9 F g−1) and CoFe2O4 (18.7 F g−1) at current density of 5 mA cm−2. However, about 22% loss in capacitance was observed for the electrode RGO–CoFe2O4 after 1000 cycles.

Xiong et al. have developed ternary nanocomposite of cobalt ferrite/graphene/ polyaniline as electrode for high-performance supercapacitor [65]. Hydrothermal method was used to make CoFe2O4 nanoparticles and graphene nanosheets and then polyaniline (PANI) coating was performed on CoFe2O4 by in situ polymerization process. A large specific capacitance of 1133.3 F g−1 at a scan rate of 1 mVs−1 was observed by the hybrid ternary nanocomposite electrode in 1 M KOH electrolyte. 716.4 F g−1 specific capacitance was determined in two electrode system in the same electrolyte and at the same scan rate of 1 mVs−1. The electrode demonstrated long cycle stability about 96% retention of initial capacitance after 5000 cycles. The synergistic effects of three components in the ternary composite improved the electrochemical performance of the electrode. The graphene nanosheets greatly enhance the electron transfer in the electrode and surface area of the electrode, leading to increase in the overall capacitance [8]. In addition, PANI also contributes to the pseudocapacitance of CoFe2O4 nanoparticles.

Cobalt ferrite nanoparticles were also used as negative electrode in an asymmetric supercapacitor with positive electrode of Co(OH)2 and Co2Fe(CN)6 particles [55]. The negative electrode of CoFe2O4 showed a specific capacitance of 758.86 F g−1 at 2 mV s−1 in 1 M KOH electrolyte. Overall, the asymmetric supercapacitor with electrode combination of CoFe2O4ǁAC, CoFe2O4ǁCo(OH)2 and CoFe2O4ǁCo2Fe(CN)6 provided the specific capacitance of 339, 127 and 125 F g−1 at 1 mVs−1 scan rate.
