Subramani Surendran

Effect of pH on the sonochemical synthesis of BiPO4 nanostructures and its electrochemical properties for pseudocapacitors.

Using sonochemical method, BiPO4 nanocrystals were prepared at different pH conditions (pH-1, 3, 5, 7, 9 & 12) for the possible applications of pseudocapacitor electrodes. The prepared BiPO4 nanocrystals belong to monoclinic structure with P21 space group. The SEM image revealed that the particles changed from irregular coarse shape into rod like structure (pH-1 to 7) which finally collapsed into irregular aggregates (pH-9 to pH-12). The observed spot patterns from SAED inferred the polycrystalline nature of the material. The electrochemical performance of the synthesized BiPO4 in various ultrasound irradiation conditions such as irradiation time (30min, 1h, 2h and 3h) and ultrasonication power (40%, 50%, 60% and 70% of instrumental power) was studied. A maximum specific capacitance of 1052F/g (pH-7 at 2mV/s) was observed for the BiPO4 prepared in the ultrasonication reaction condition of 2h with 60% power. Also the obtained specific capacitance was high compared with the conventional precipitation method (623F/g at 2mV/s) that revealed the prominence of sonication method. Similarly, BiPO4 prepared at pH-7 delivered a maximum specific capacitance of 302F/g at 2mA/cm(2) calculated from galvanostatic charge-discharge (GCD) method than the other pH conditions. However, the cycling stability of BiPO4 (pH-7) was not appreciable even for 200 cycles. So, attempts were taken to enhance the cycling stability of the material by employing various carbon matrices such as acetylene black, activated carbon and MWCNT instead of carbon black during electrode preparation. BiPO4 material with activated carbon delivered good capacitance retention compared with other carbon matrices. This enhanced electrochemical performance of BiPO4 (pH-7) using activated carbon matrix inferred that it could be utilized as efficient negative electrode material for pseudocapacitors.

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.

Facile hydrothermal synthesis of carbon-coated cobalt ferrite spherical nanoparticles as a potential negative electrode for flexible supercapattery

Battery type electrodes would replace the currently available pseudocapacitive electrodes by the cause of high energy density and long discharge time. In this regard, battery type carbon coated CoFe2O4 spherical nanoparticles is prepared by the facile hydrothermal method and tested as the possible negative electrode for supercapattery applications. The phase purity, electronic states of elements, and the presence of carbon is inferred through various sophisticated techniques. The calculated surface area of CoFe2O4 and carbon coated CoFe2O4 are found to be 9 and 26 m2 g-1, respectively. The morphological analysis confirms the formation of uniform CoFe2O4 nanospheres (∼25 nm) with a thin layer of carbon coating (∼2 nm). The amorphous carbon coating over CoFe2O4 nanosphere is identified via high-resolution transmission electron microscope. The observed peak and plateau regions in the cyclic voltammogram and galvanostatic charge/discharge curves reveals the battery-type charge storage behaviour of the material. The carbon coated CoFe2O4 delivers the maximum length capacitance of 9.9 F m-1 at 1 mV s-1 with a useful lifespan over 5000 cycles. The electrochemical impedance spectroscopy reveals that the carbon-coated CoFe2O4 delivers the low charge transfer resistance than CoFe2O4. Further, the fabricated supercapattery provides the energy density of 160 × 10-8 Wh cm-1 at a power density of 67.2 μW cm-1. As well as, the device shows 93% of coulombic efficiency and 75% of the specific capacitance retention over 11,000 cycles. Overall, it is believed that the carbon-coated CoFe2O4 can serve as a good candidate for flexible supercapatteries.