V.D. Nithya

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.

Phase and shape dependent electrochemical properties of BiPO4 by PVP assisted hydrothermal method for pseudocapacitors

BiPO4 materials with different shapes are obtained under hydrothermal conditions by simply varying the concentration of PVP (polyvinylpyrrolidone). The prepared materials are characterized using various techniques such as X-Ray Diffraction (XRD), Thermo Gravimetric/Differential Scanning Calorimetry (TG/DSC), Fourier Transform Infrared (FT-IR) spectra, Field Emission Scanning Electron Microscopy (FE-SEM) and Transmission Electron Microscopy (TEM). The XRD with Rietveld analysis infers the formation of a monazite-type monoclinic BiPO4 structure. A transformation from monazite-type into trigonal BiPO4 phase is observed upon increasing the PVP concentration. The FE-SEM images reveal the change of shape from irregular cubic structure to octahedral to spindle upon increasing the concentration of PVP from 0.25 g to 1.0 g and thereafter the shape begins to disintegrate (1.25 g). Similarly, the effect of reaction time on the shape of BiPO4 is also studied. The electrochemical performance of pristine BiPO4 electrode is studied in various aqueous electrolytes such as 1 M LiOH, 1 M NaOH, 1 M KOH, 0.5 M K2SO4 and 1 M KNO3. Overall, the pristine BiPO4 provides a maximum specific capacitance in 1 M KOH electrolyte and a specific capacitance of 89 F g−1 is obtained at 5 mA cm−2. At low PVP concentration, there is an increase in specific capacitance whereas it decreases at high concentrations (1.0 g and 1.25 g). The monazite-type monoclinic structure with rod shape particles exhibits a high specific capacitance (167 F g−1 at 5 mA cm−2) and cycle life compared with other morphologies. The effective role of crystal phase and shape on the electrochemical performance of BiPO4 is elucidated.

Surfactant assisted sonochemical synthesis of Bi2WO6 nanoparticles and their improved electrochemical properties for use in pseudocapacitors

Hexamethylenetetramine (HMT) assisted Bi2WO6 nanoparticles were successfully synthesized by a sonochemical method for use as negative electrodes for pseudocapacitors. The effect of different concentrations of HMT on the structural, morphological, compositional and electrochemical properties was investigated. Detailed structural investigations were performed using Rietveld analysis and the formation of a single phase Bi2WO6 with orthorhombic russellite structure was deduced. The compound formation temperature of Bi2WO6 was analyzed using differential scanning calorimetry. The presence of possible functional groups was revealed by Fourier transform infrared spectroscopy analysis. A decrease in particle size was observed due to the effect of surfactant. The electrochemical performance of the materials was assessed using cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy (EIS). The electrochemical studies showed an enhanced specific capacitance of 708 F g−1 for Bi2WO6 in the presence of 0.01 mol HMT as compared to 304 F g−1 which was obtained without surfactant. The EIS spectral analysis suggested that the surfactant assisted Bi2WO6 has a lower charge transfer resistance (12.4 Ohm for 0.01 mol HMT) compared with the pristine (32.77 Ohm) material. The electrochemical analysis proved the vital role played by HMT in enhancing the specific capacitance of Bi2WO6 material.