N. Padmanathan

Controlled growth of spinel NiCo2O4 nanostructures on carbon cloth as a superior electrode for supercapacitors

In this study, the morphology conversion of bimetallic NiCo2O4 nanostructures on carbon fiber cloth (CFC) was achieved via a simple hydrothermal approach with different precursor salts. As expected, the surface morphology has been successfully driven by varying the precursor. Typical NiCo2O4 nanowall-networks and porous nanoflake microstructures have been grown when using the nitrate and chloride precursors respectively. As an advantage of their unique structural features, they have shown different electrochemical activity towards supercapacitor applications. The as-grown NiCo2O4 nanowall-network structure delivers a maximum capacitance of 1225 F g−1 at a high current density of 5 A g−1 and excellent durability. However, a limited specific capacitance of only 844 F g−1 at a current density of 1 A g−1 was achieved for NiCo2O4 nanoflakes. This variation in the electrochemical features such as specific capacitance, rate capability and cyclic stability is mainly due to their structural discrepancies which have been driven by the precursors during the growth process. From this investigation it can be concluded that precursors with different anions also greatly influence the growth kinetics of metal oxide nanostructures. In this case, we suggest that the directly grown NiCo2O4@CFC with the desired microstructure will be a potential electrode for next generation flexible supercapacitors.

Shape controlled synthesis of CeO2 nanostructures for high performance supercapacitor electrodes

We report CeO2 nanostructures with the desired shape synthesized via a simple hydrothermal approach without any specific structure directing agent for supercapacitor applications. Well-distributed hexagonal nanoplates and nanorods were designed with large exposed surfaces under controlled hydrothermal conditions. As an advantage of this favorable shape and structural features, both of these nanostructures exhibit large specific capacitance values and excellent rate capabilities. Unique carbon supported CeO2 nanorod microstructures have shown a high capacitance of 644 F g−1 at 0.5 A g−1. Furthermore, they can deliver up to 400 F g−1 at a high current density of 20 A g−1. This is the first report of a CeO2 nanostructure for use as a supercapacitor electrode with a high rate capability. The notable electrochemical performance can be attributed to its large exposed surface, reasonable electronic conductivity, and the carbon support which increases the electrode/electrolyte contact significantly. The observed specific capacitance values are much higher and comparable with the other transition metal oxide electrodes. The present study suggests that the shape controlled CeO2 nanostructure will be an alternative high performance electrode for next generation supercapacitors.

Supercapattery Based on Binder-Free Co3(PO4)2·8H2O Multilayer Nano/Microflakes on Nickel Foam

A binder-free cobalt phosphate hydrate (Co3(PO4)2·8H2O) multilayer nano/microflake structure is synthesized on nickel foam (NF) via a facile hydrothermal process. Four different concentrations (2.5, 5, 10, and 20 mM) of Co2+ and PO4-3 were used to obtain different mass loading of cobalt phosphate on the nickel foam. The Co3(PO4)2·8H2O modified NF electrode (2.5 mM) shows a maximum specific capacity of 868.3 C g-1 (capacitance of 1578.7 F g-1) at a current density of 5 mA cm-2 and remains as high as 566.3 C g-1 (1029.5 F g-1) at 50 mA cm-2 in 1 M NaOH. A supercapattery assembled using Co3(PO4)2·8H2O/NF as the positive electrode and activated carbon/NF as the negative electrode delivers a gravimetric capacitance of 111.2 F g-1 (volumetric capacitance of 4.44 F cm-3). Furthermore, the device offers a high specific energy of 29.29 Wh kg-1 (energy density of 1.17 mWh cm-3) and a specific power of 4687 W kg-1 (power density of 187.5 mW cm-3).

Ultra-fast rate capability of a symmetric supercapacitor with a hierarchical Co3O4 nanowire/nanoflower hybrid structure in non-aqueous electrolyte

A free standing Co3O4 nanowire/nanoflower hybrid structure on flexible carbon fibre cloth (CFC) was designed via a facile hydrothermal approach followed by thermal treatment in air. The Co3O4 hybrid structure on CFC showed interesting electrochemical performance in both alkaline and organic electrolytes when used as electrodes for symmetric supercapacitors. Compared to conventional alkaline electrolytes, the fabricated symmetric cell in organic electrolyte has delivered a high rate and cyclic performance. A supercapacitor made from this hierarchical hybrid architecture showed a maximum specific capacitance of 4.8 mF cm−2 at a constant density of 3 mA cm−2 in organic electrolyte. In terms of energy and power, the symmetric supercapacitor conveyed an energy density of 4.2 mW h cm−3 with a power density of 1260 mW cm−3. Also, the device exhibited reasonable tolerance for mechanical deformation under bended conditions demonstrating the flexibility of the materials. The impressive electrochemical activity is mainly attributed to their high surface area (60.3 m2 g−1) resulting from their nano/mesoporous structure; reasonable electrical conductivity resulted from binder-free and intimate metal oxide/substrate integration and superior flexibility of the carbon fibre cloth. Thereby, it was concluded that the direct growth of the Co3O4 nanostructure on CFC is a promising electrode for the advanced flexible energy storage devices regardless of the electrolyte.

Design of hydrophobic polydimethylsiloxane and polybenzoxazine hybrids for interlayer low k dielectrics

The continual development of the microelectronic industry demands the most desirable low dielectric materials with high thermal stability. In this work, to design the low k materials, the following methods have been utilized viz., (i) benzoxazine monomers from aromatic and aliphatic diamines, (ii) variation of the chain length of nonpolar aliphatic diamines [(CH2)2, (CH2)4, (CH2)6], and (iii) an incorporation of polydimethylsiloxane (PDMS). The variation in chemical structure and dielectric properties of modified polybenzoxazine (PBz) have been investigated by these phenomenological approaches. High resolution transmission electron microscopy (HRTEM) images clearly indicate the layer-by-layer arrangement of hybrid PDMS–PBz matrices. Frequency dependent dielectric spectra ascertain the decreased trend in the value of dielectric constant as well as dielectric loss. It is found that 1,6-diaminohexane–polydimethylsiloxane–polybenzoxazine (DAH–PDMS–PBz) hybrid matrix exhibits the lowest dielectric constant of 2.42 at 1 MHz. An interesting structural feature which led to reducing the dielectric constant were high volume fraction of intramolecular hydrogen bonding, nonpolar aliphatic long chain, and the existence of a distinct layer-by-layer arrangement in the PDMS–PBz matrix. In addition, the presence of thermally stable Si and flexible Si–O–Si linkages enhanced the thermal stability and flexibility of thermosetting polybenzoxazine hybrid matrices. From these observations it can be concluded that the structural modification can be achieved by using the above methods to obtain low k hybrid materials.

Effect of ZnO, SiO2 dual shells on CeO2 hybrid core–shell nanostructures and their structural, optical and magnetic properties

This work has investigated the structural, optical and magnetic properties of ceria-based core–shell nanostructures synthesized by a simple and effective precipitation method. Formation of the core CeO2 and shells of ZnO as well as SiO2 over the core is confirmed by the X-ray diffraction (XRD) pattern, Fourier transform infra-red spectroscopy (FTIR), field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). The optical properties of the core–shell nanostructures are determined by UV-visible spectroscopy and photoluminescence (PL) studies. From XRD results it can be seen that the core CeO2 exhibits cubic fluorite and hexagonal ZnO shell structures. A core–shell nanostructure with complete coating of SiO2 on CeO2@ZnO is evidenced by FTIR. UV-visible spectra show broad absorption maxima in the UV region and also show a shift in the band edge for core–shell nanostructures which demonstrate the electronic structural modification in the core–shell nanoparticles. Magnetic studies show room temperature ferromagnetic order in the low field region with significant coercivity, which may be associated with defect-related ferromagnetic exchange in the core–shell nanostructures.

Multifunctional Nickel Phosphate Nano/Microflakes 3D Electrode for Electrochemical Energy Storage, Nonenzymatic Glucose, and Sweat pH Sensors

Multifunctional, low-cost electrodes and catalysts are desirable for next-generation electrochemical energy-storage and sensor applications. In this study, we demonstrate the fabrication of Ni3(PO4)2·8H2O nano/microflakes layer on nickel foam (NF) by a facile one-pot hydrothermal approach and investigate this electrode for multiple applications, including sweat-based glucose and pH sensor as well as hybrid energy-storage device, e.g., supercapattery. The electrode displays a specific capacity of 301.8 mAh g-1 (1552 F g-1) at an applied current of 5 mA cm-2 and can retain 84% of its initial capacity after 10 000 cycles. Furthermore, the supercapattery composed of Ni3(PO4)2·8H2O/NF as positive electrode and activated carbon as negative electrode can offer a high specific energy of 33.4 Wh kg-1 with the power of 165.5 W kg-1. As an electrocatalyst for nonenzymatic glucose sensor, Ni3(PO4)2·8H2O/NF shows an exceptional sensitivity (24.39 mA mM-1cm-2) with a low detection limit of 97 nM (S/N = 3). Moreover, as a sweat-based pH sensor, the electrode is capable of detecting human sweat pH values ranging from 4 to 7. Therefore, this three-dimensional nanoporous Ni3(PO4)2·8H2O/NF electrode, due to its excellent electrochemical performance, can be successfully applied in electrochemical energy-storage and biosensor applications.

Investigation on structural and magneto-optical properties of electrospun Co-doped SnO2 hollow nanofibers

Pure and Co-doped SnO2 hollow nanofibers were successfully synthesized by electrospinning technique. The structures of as spun and hollow nanofibers were characterized with X-ray diffraction (XRD) and scanning electron microscope (SEM). We observe from the SEM images the formation of well-defined hollow nanofiber network with coarse morphology consisting of fine nanograins. The optical properties of Co doped SnO2 nanofiber was studied with the UV–Vis spectroscopy and photoluminescence. The strong emission band at violet-blue region for all the samples with different intensities confirm the presence of defects related luminescent centres in Co-doped SnO2 nanofibers. The magnetic measurement reveals that Co-doped SnO2 hollow nanofibers show room temperature ferromagnetism at the low field strengths. This may be due to the exchange interaction between the local magnetic moment of Co2+ ions and bound magnetic polarons (BMPs) of SnO2.