**5. Nickel ferrite (NiFe2O4)**

Among various metal ferrites, NiFe2O4 is one of the promising ferrite material for supercapacitor application. Several studies are available on NiFe2O4 and their nanocomposite as electrode in supercapacitor [46, 91–94]. NiFe2O4 particles with submicron-sized synthesized by the molten salt process demonstrated a specific capacitance of 18.5 F g−1 at a scan rate of 10 mV/s [95]. NiFe2O4 nanospheres showed a specific capacitance of 122 F g−1 at current density of 8.0 Ag−1 [26]. The results showed that the capacitance is increased with increasing the KOH concentration. NiFe2O4 nanospheres could store specific energy of 16.9 Wh kg−1 at a high current density of 8.0 Ag−1.

Nagesh Kumar et al. [96] have synthesized mesoporous NiFe2O4 nanoparticles (size 10–15 nm) by one step hydrothermal method. The mesopores were distributed homogeneously on the surface of NiFe2O4 particle. A surface area of 148 m2 g−1 was calculated for the mesoporous NiFe2O4 nanoparticles. It exhibited high value of specific capacitance of 1040 Fg−1 at 1 Ag−1 in a three-electrode configuration with 2 M KOH electrolyte. However, 30% loss in capacitance was observed for NiFe2O4 nanoparticles after 500 cycles. NiFe2O4 synthesized by combustion route showed a specific capacitance of 454 Fg−1 with good cycle stability for 1000 chargingdischarging cycles [46].

To improve the capacitive properties, graphene based materials have been mixed with NiFe2O4 [97–99]. Soam et al. [98] obtained a specific capacitance of 207 Fg−1 from ferrite/graphene nanocomposite in 1 M Na2SO4 electrolyte (**Figure 3**). This value of capacitance was observed about 4 times greater than NiFe2O4 electrode. NiFe2O4 with graphene nanosheets exhibited a stable capacitance about 95% over 1000 cycles. Numerous pores in the electrode might be responsible for improved

#### **Figure 3.**

*(a) TEM images of graphene/NiFe2O4 nanocomposite. NiFe2O4 nanoparticles have good contact with graphene, providing fast charge transportation within the electrode (b) CV curves of graphene and NiFe2O4 recorded at 5 mVs−1. The nanocomposite of graphene/NiFe2O4 exhibited larger area under CV curve, indicating better charge storage capacity than NiFe2O4. (c) CV curves at different scan rates for graphene/NiFe2O4 nanocomposite. (d) Capacitance versus scan rate for the graphene/NiFe2O4 nanocomposite, the electrode exhibited specific capacitance in the range of 207-30 Fg−1 at scan rates of 5-100 mVs−1 and (e) charging/discharging curves with constant current of 0.5 mA. (f) Cycle stability test performed over 1000 cycles [98].*

electrochemical performance of NiFe2O4. Zhuo Wang et al. [100] have studied the reduce graphene oxide–NiFe2O4 nanocomposites for supercapacitor application. rGO-NiFe2O4 nanocomposites were prepared by hydrothermal process with varying the pH value of solution (8, 10, 12 and 14). rGO-NiFe2O4 synthesized with pH value of 10 exhibited the largest surface area of 459.6 m<sup>2</sup> g−1. A specific capacitance of 218.47 Fg−1 was achieved for the rGO-NiFe2O4 (pH -10) at 5 mV/s−1, which is the largest among all the samples.

Addition of conducting network of PANI to NiFe2O4 improved the electrochemical performance of PANI-NiFe2O4 nanocomposite electrode [101]. A specific capacitance of 448 Fg−1 was achieved with PANI-NiFe2O4. The electrode showed 80% retention in the capacitance after 1000 cycles at the rate of 10 mAcm−2. A composite of mesoporous NiFe2O4 with multiwall carbon nanotubes (MWCNTs) prepared via hexamethylene tetramine (HMT) assisted one pot hydrothermal process exhibited large value of specific capacitance, 1291 F g−1 determined at 1 A g−1 [92]. The electrode showed capacitance retention of 81% over 500 charge–discharge cycles in 2 M KOH electrolyte. The asymmetric device with NiFe2O4/CNT nanocomposite as cathode and N-doped graphene as anode demonstrated a specific capacitance of 66 F g−1 with energy density of 23 W h kg−1 and power density of 872 W kg−1.

NiFe2O4 nanoparticles grown on a flexible carbon cloth substrate via hydrothermal method demonstrated a high capacitance 1135.5 F g−1 in 1 M H2SO4 electrolyte and 922.6 F g−1 in 6 M KOH electrolyte with current density of 2 mAcm−2 [93]. The large capacitance can be attributed to the conductive 3D network of carbon cloth and large surface area for NiFe2O4 nanoparticles. The binder free electrode of NiFe2O4 nanocone forest on carbon textile (NFO-CT) exhibited specific capacitance of 697 F g−1 calculated by CV at scan rate of 5 mV s−1 [94]. Further, a solid state supercapacitor of NFO-CT also demonstrated good value of capacitance of 584 F g−1 at 5 mV s−1. Moreover, the device showed good cycle stability with 93.57% capacitance retention over 10,000 cycles. These results indicate that NFO-CT may be a promising candidate for high performance supercapacitor. The capacitance of NiFe2O4 was also observed to be dependent on the synthesis process [91]. The NiFe2O4 synthesized by combustion, polyol-mediated and sol–gel methods have different morphology and consequently different value of capacitance. A high specific capacitance value of 97.5 Fg−1 was obtained from sol–gel synthesized method. The size of grains and pores are smaller for sol–gel synthesized NiFe2O4 which could be the reason for better value of capacitance.

1D NiFe2O4/graphene composites prepared via hydrothermal process exhibited specific capacitance of 481.3 F g−1 at a current density of 0.1 A g−1 [97]. The 1D NiFe2O4/graphene electrode maintained 298.2 F g−1 capacitance upon increasing the current density to 10 A g−1. The electrode demonstrated outstanding cycle stability over 10000 cycles (about 1% degradation in capacitance). On the other hand, 40% loss of capacitance was observed for NiFe2O4 electrode (125 to 75 F g−1). The excellent electrochemical performance of NiFe2O4/graphene composites electrode is due to the conducting network of graphene and large number of redox active site from NiFe2O4. Ternary nitrogen-doped graphene/nickel ferrite/polyaniline (NGNP) nanocomposite showed specific capacitance of 645.0 F g−1 at 1 mV s−1 [99]. In a two-electrode symmetric system, the energy density and power density were determined to be 92.7 W h kg−1 and 110.8 W kg−1, respectively. About 90% retention in capacitance was seen after 10,000 cycles. The electrochemical behavior of NGNP is improved due to combined effects of EDLC and pseudocapacitor.
