**4. Zinc ferrite (ZnFe2O4)**

Zn ferrite is widely used electrode material in supercapacitor because of its nontoxic nature, strong redox process, good chemical stability and high storage capacity of 2600 F g−1 [76–81]. Furthermore, the morphology of ZnFe2O4 can also be tuned such as nanoparticles [78, 82], nanorods [83] and nano-flakes [78, 80], offering large surface area for charge storage. M. M. Vadiyar et al. [84] have reported an empirical relationship between surface wettability and charge storing capacity for ZnFe2O4 nano-flake thin films. Different electrolytes, 1 M KOH, NaOH, LiOH and their combinations were chosen for this study. All the CV curves for ZnFe2O4 nanoflake recorded in the above electrolytes exhibited pseudocapacitive behavior in the scan range of 0.0 to −1.3 V. ZnFe2O4 nano-flakes exhibited larger area under the CV curves in 1 M KOH due to small hydrated K<sup>+</sup> and its fast intercalation and deintercalation on the electrode surface. This is found with good agreement with the smaller contact angle value of 12o and larger surface energy of 71 mJ m−2.

ZnFe2O4 thin film synthesized by successive ionic layer adsorption and reaction (SILAR) method has shown good value of capacitance specific of 471 Fg−1 in aqueous electrolyte of 1 M NaOH at a scan rate of 5 mVs−1 [85]. The synthesized thin film of ZnFe2O4 was also used in solid-state symmetric supercapacitor which exhibited specific capacitance of 32 Fg−1 in voltage window of 1.0 V. A power density of 277 Wkg−1 with energy density of 4.47 Whkg−1 was achieved with ZnFe2O4 thin film based supercapacitor. A specific capacitance of 615 F g−1 has been achieved for binder free ZnFe2O4 thin films at current density of 3 mA cm−2 [79]. The porous ZnFe2O4 thin film was tested in asymmetric supercapacitor as a negative electrode with positive electrode of Mn3O4. The device showed a specific capacitance of 81 F g−1 with energy and power density of 28 Wh kg−1 and 7.97 kW kg−1, respectively. 74% retention was observed in capacitance after 3000 cycles.

ZnFe2O4 nanoparticles (size 20–30 nm) synthesized by combustion method was used for supercapacitor application [86]. The electrode showed a large maximum specific capacitance of 1235 F g−1 calculated at 1 mA cm−2. The electrochemical performance of ZnFe2O4 material was also demonstrated in an asymmetric supercapacitor as negative electrode and nickel hydroxide as positive electrode. The device exhibited voltage window of 1.7 V with specific capacitance of 179 Fg−1 calculated at 2 mVs−1. 3-D aligned ZnFe2O4 nano-flakes on flexible stainless steel mesh substrate have shown promising results as electrode in asymmetric supercapacitor with Ni(OH)2 [87]. The asymmetric device exhibited large value of specific capacitance of 1625 F g−1 at 1 mA cm−2 with 97% retention in capacitance after 8000 cycles.

ZnFe2O4 microspheres synthesized by solvothermal approach demonstrated a specific capacitance of 131 Fg−1 [81]. The electrode of ZnFe2O4 microspheres could retain 92% capacitance after 1000 cycles. ZnFe2O4 anchored on multiwalled carbon nanotubes (CNT) yielded a high specific capacity of 217 mAh g−1 at 5 mV s−1 [88]. A solid-state symmetric supercapacitor with ZnFe2O4-CNT exhibited a highest specific energy of 12.80 Wh kg−1 and a specific power of 377.86 W kg−1.

M. M. Vadiya et al. [78] have developed self-assembled ZnFe2O4 nanoflakes@ ZnFe2O4/C nanoparticles heterostructure electrode for high performance

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

supercapacitor application. The hybrid electrode showed very high value of specific capacitance of 1884 Fg−1 determined at a current density of 5 mA cm−2. A flexible asymmetric supercapacitor was also designed using ZnFe2O4 nano-flakes@ ZnFe2O4/C nanoparticles heterostructure as a negative electrode and reduced graphene oxide as a positive electrode. A specific capacitance of 347 F g−1 was achieved from the supercapacitor. The asymmetric supercapacitor exhibited an energy density of 81 Wh kg−1and power density of 3.9 kW kg−1. Only 2% loss in the capacitance was observed after 35000 cycles.

ZnFe2O4 nanoparticles were dispersed on nitrogen-doped reduced graphene for supercapacitor application [82]. The reduction of graphitic oxide, the doping of nitrogen to graphene and dispersion of ZnFe2O4 nanoparticles were achieved in a single process. The structure of ZnFe2O4/NRG exhibited a specific capacitance of 244 Fg−1 calculated at 0.5 Ag−1. The electrode has also demonstrated good rate capability with retention of 131.5 Fg−1 capacitance at 10 Ag−1. ZnFe2O4/ NRG retained 83.8% capacitance after 5000 cycles. In this type of electrode, the graphene sheets provide high exposure of active sites for redox process and high dispersion of nanoparticles resulting good capacitive performance of the electrode [89, 90].

ZnFe2O4 nanorodes with rGO showed a specific capacitance of 1419 Fg−1 at scan rate of 10 mVs−1 in 2 M KOH solution. The electrode demonstrated good retention of capacitance about 93% after 5000 cycles. The improved electrochemical performance is due to the large surface area offered by rGO and good electrical conductivity [83]. The porous nano-flakes-ZnFe2O4 thin films demonstrated a larger capacitance of 768 Fg−1 at current density 5 mA cm−2 with energy density of 106 Wh kg−1 and power density of 18 kW kg−1 [76]. The electrode had good cycle stability about 88% retention of capacitance after 5000 cycles.
