Amreesh Chandra

Significant Performance Enhancement in Asymmetric Supercapacitors based on Metal Oxides, Carbon nanotubes and Neutral Aqueous Electrolyte.

Amongst the materials being investigated for supercapacitor electrodes, carbon based materials are most investigated. However, pure carbon materials suffer from inherent physical processes which limit the maximum specific energy and power that can be achieved in an energy storage device. Therefore, use of carbon-based composites with suitable nano-materials is attaining prominence. The synergistic effect between the pseudocapacitive nanomaterials (high specific energy) and carbon (high specific power) is expected to deliver the desired improvements. We report the fabrication of high capacitance asymmetric supercapacitor based on electrodes of composites of SnO2 and V2O5 with multiwall carbon nanotubes and neutral 0.5 M Li2SO4 aqueous electrolyte. The advantages of the fabricated asymmetric supercapacitors are compared with the results published in the literature. The widened operating voltage window is due to the higher over-potential of electrolyte decomposition and a large difference in the work functions of the used metal oxides. The charge balanced device returns the specific capacitance of ~198 F g−1 with corresponding specific energy of ~89 Wh kg−1 at 1 A g−1. The proposed composite systems have shown great potential in fabricating high performance supercapacitors.

Enhancing Specific Energy and Power in Asymmetric Supercapacitors - A Synergetic Strategy based on the Use of Redox Additive Electrolytes

The strategy of using redox additive electrolyte in combination with multiwall carbon nanotubes/metal oxide composites leads to a substantial improvements in the specific energy and power of asymmetric supercapacitors (ASCs). When the pure electrolyte is optimally modified with a redox additive viz., KI, ~105% increase in the specific energy is obtained with good cyclic stability over 3,000 charge-discharge cycles and ~14.7% capacitance fade. This increase is a direct consequence of the iodine/iodide redox pairs that strongly modifies the faradaic and non-faradaic type reactions occurring on the surface of the electrodes. Contrary to what is shown in few earlier reports, it is established that indiscriminate increase in the concentration of redox additives will leads to performance loss. Suitable explanations are given based on theoretical laws. The specific energy or power values being reported in the fabricated ASCs are comparable or higher than those reported in ASCs based on toxic acetonitrile or expensive ionic liquids. The paper shows that the use of redox additive is economically favorable strategy for obtaining cost effective and environmentally friendly ASCs.

Evolution of crystallographic phases in the system (Pb 1-x Ca x )TiO 3 : a Rietveld study

X-ray powder diffraction studies on (Pb 1 - x Ca x )TiO 3 ceramic powders revealed the presence of superlattice reflections due to antiphase and inphase tilts of oxygen octahedra for x ≥ 0.421. Rietveld analysis of the powder x-ray diffraction data confirmed that the structure of (Pb 1 - x Ca x )TiO 3 is orthorhombic with Pbnm space group and a - a - c + tilt system for x ≥ 0.421. For compositions with 0 < x ≤ 0.416, the structure was tetragonal, and the tetragonality decreased with increasing Ca 2 + content.

High electrochemical performance in asymmetric supercapacitors using MWCNT/nickel sulfide composite and graphene nanoplatelets as electrodes

The electrochemical performance of asymmetric supercapacitors (ASCs) using MWCNT/NiS and graphene nanoplatelets as the positive and negative electrode, respectively, are reported. Nickel sulfide nanoparticles can be decorated on multiwall carbon nanotubes using a hydrothermal synthesis process, with graphene nanoplatelets obtained via a chemical route. The fabricated ACSs were operated over a potential window of 1.4 V with a specific capacitance of 181 F g−1 observed at 1 A g−1. The ASCs were cycled at 2 A g−1 showing 92% retention of initial capacitance after 1000 cycles.

Nanostructures of Sr2+ doped BiFeO3 multifunctional ceramics with tunable photoluminescence and magnetic properties

Careful tuning of formation (calcination) temperature of Sr(2+) doped BiFeO(3) multiferroic ceramics results in tailorable particle morphologies ranging from spherical to pillar-like. Based on the minimization of Gibbs free energy approach, the dominant homogeneous mechanism for particle growth is suggested. The chemical substitution of a trivalent ion (Bi(3+)) by a divalent ion (Sr(2+)) causes the transformation of certain fraction of Fe(3+) to Fe(4+) and/or the appearance of oxygen vacancies. This has been respectively proved by the analysis of XPS and refinement of neutron diffraction data. Although significant modification in the particle morphology is observed, the crystal unit cell remains rhombohedral with a R3c space group but interesting variations in physical properties are achieved. O-vacancies induced strong and sharp photoluminescence activity in the IR region, similar to ZnO, is reported for the first time. This observation opens up a new application for multiferroic ceramics. SQUID M-H data confirms the straightening of the canted spin structure of BiFeO(3), which in turn results in magnetism similar to ferromagnetic materials. Findings of the magneto-dielectric effect are also discussed to claim the multiferroic nature of the sample.

A new approach for crystallization of copper( ii ) oxide hollow nanostructures with superior catalytic and magnetic response

We report the synthesis of copper(II) oxide hollow nanostructures at ambient pressure and close to room temperature by applying the soft templating effect provided by the confinement of droplets in miniemulsion systems. Particle growth can be explained by considering a mechanism that involves both diffusion and reaction control. The catalytic reduction of p-nitrophenol in aqueous media is used as a model reaction to prove the catalytic activity of the materials: the synthesized hollow structures show nearly 100 times higher rate constants than solid CuO microspheres. The kinetic behavior and the order of the reduction reaction change due to the increase of the surface area of the hollow structures. The synthesis also leads to modification of physical properties such as magnetism.

Effect of pressure on structural, electronic and elastic properties of cubic (Pm3m) SnTiO 3 using first principle calculation

The electronic band structure, density of state and elastic properties of lead-free perovskite oxide SnTiO3 (ST) were investigated by employing first principles calculation using the Density Functional Theory (DFT) within local density approximation (LDA). The energy band gap was calculated from the separation between the Ti 3d (conduction band) and the maximum of O 2p (valence band). This gives an indirect band gap of 2.36 eV. The elastic constants and their pressure dependence were calculated up to 30 GPa and the independent elastic constants (C11, C12, and C44), bulk modules, B were obtained and analyzed. The results showed that SnTiO3 have a mechanical stability in cubic phase (Pm3m).

Tuning porous nanostructures of MnCo2O4 for application in supercapacitors and catalysis

It is shown that spinel type MnCo2O4 can be used as a pure negative electrode in supercapacitors. The electrochemical response can be significantly tuned by forming and modulating the mesoporous structures of these materials. With varying formation temperature, the nature of particle growth mechanism moves from Oswald ripening type to digestive ripening. This leads to increased surface area, porosity and charged states on the particles surface. Thus, making this material useful from application ranging from supercapacitors to catalysis. As a function of changing porosity, specific capacitance could be tuned in the range ∼290 F g−1. Added functionality of the material could be established by proving its excellent catalytic response for reducing p-nitrophenol, a common pollutant found in industrial waste streams. The sample calcined at 800 °C reduced p-nitrophenol in 8 min at 1 mg mL−1 catalyst concentration, while the sample obtained at 400 °C could complete the task only after 16 min. The superior catalytic response have been explained by analyzing the change in zeta potential, which develops on the surface of different porous structures.

Anomalous magnetic behavior below 10 K in YCrO3 nanoparticles obtained under droplet confinement

Nanoparticles of multiferroic YCrO3 synthesized using the droplet confinement of miniemulsions show unusual features in the magnetic properties at low temperatures, which have not been reported before. Below 10 K, there is a sudden increase in the magnetization, and the nature of M–H hysteresis loops changes appreciably. The hysteresis loop shows two contributions, one similar to ferromagnetic and another similar to that expected from antiferromagnetic systems. This behavior can be understood by the formation of elongated grains or mesocrystals. It is remarkable that YCrO3 behaves quite differently from other multiferroic chromates such as ACrO3 (A = In, Sc, Sm).

Nanocrystalline ZnS dispersed in polymer electrolyte (PEO :NH4I): Preparation and electrical conductivity measurements

Abstract Nano-composites of a polymer electrolyte PEO:NH4I (80:20) have been prepared by dispersing nano-size ZnS crystallites in it. The measured band gap of dispersed ZnS is ∼3.9 eV and its particle size as estimated from the XRD linewidths is ∼ 11 nm. Detailed I–V and polarisation studies show that the composite polymer film is a mixed (ionic+electronic) conductor and that the dispersoid ZnS is n-type. The total electrical conductivity Vs composition studies show two peaks at the ZnS concentrations of 4 and 10wt% which can be qualitatively explained on the basis of two-percolation threshold model.