K. Vijaya Sankar

Electrochemical performances of CoFe2O4 nanoparticles and a rGO based asymmetric supercapacitor

CoFe2O4 nanoparticles were prepared using a polyethylene glycol (PEG) assisted solution combustion method. The X-ray diffraction pattern, Fourier transform infrared and Raman spectra revealed the single phase formation of CoFe2O4 particles. Transmission electron microscopy (TEM) images revealed nanosized particles less than 10 nm in size. The calculated voltammetry specific capacitance of the CoFe2O4 electrode was 195 F g−1 at 1 mV s−1. The Powers law suggests the capacitive mechanism is dominant over an intercalation mechanism, while the maximum number of charges accommodated in the inner surface of the electrode, is given by the Trasatti plot. The fabricated rGO based hybrid supercapacitor (CoFe2O4‖rGO) provides a good specific capacitance (38 F g−1) and energy density (12.14 W h kg−1) at 3 mA with good cycle life, and the serially connected asymmetric supercapacitor device powers the light emitting diode for 10 minutes.

Studies on the electrochemical intercalation/de-intercalation mechanism of NiMn2O4 for high stable pseudocapacitor electrodes

Sub-micron sized polyhedral shaped NiMn2O4 particles were successfully prepared by a glycine assisted solution combustion method. The phase purity and the presence of functional groups in NiMn2O4 were revealed through X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR), respectively. The formation of polyhedral shaped particles was inferred by field emission scanning electron microscopy (FE-SEM). The negative temperature coefficient of resistance (NTCR) behaviour of NiMn2O4 was observed using a solid state impedance analyser in the measured temperature range between 30 and 180 °C. Further, electrochemical studies revealed that NiMn2O4 stores the charge through intercalation rather than by a capacitive mechanism. The electrode stores 91% of the specific capacitance by intercalation and 9% by a capacitive mechanism. Also, NiMn2O4 possesses a specific capacitance of 202 F g−1 at 0.5 mA cm−2 in 1 M Na2SO4 electrolyte and exhibits excellent cyclic stability over 15 000 cycles. Similarly, the fabricated asymmetric device (FeVO4‖NiMn2O4) also delivers good specific capacitance (50 F g−1 at 1 mV s−1) and cyclic stability.

Nitrogen-doped reduced graphene oxide and aniline based redox additive electrolyte for a flexible supercapacitor

Nitrogen-doped reduced graphene oxide (N-rGO) with a layered structure was prepared by a simple hydrothermal method. Structural and elemental analysis revealed the successful doping of N atoms into the carbon sites of graphene. The presence of wrinkles and folds in the transmission electron microscope images provided evidence for the excellent flexibility of the prepared N-rGO. Additionally, the curved edges further illustrated the implantation of N atoms in the carbon sites of graphene. The unreduced oxygen functional groups and N atoms provided additional pseudocapacitance to the N-rGO. The flexible N-rGO fiber supercapacitor exhibited a 5.7 times higher specific capacitance with the aniline additive than without the additive. The maximum specific capacitance was 2.02 F m−1 at 5 mV s−1. Finally, a serially connected device was integrated with a commercially available solar cell and used to power a light emitting diode in different flexible modes, after charging up to 2.2 V. All these results demonstrate that the fabricated device can be a suitable energy storage device for various flexible energy storage applications.