P. Ragupathy

Facile synthesis of hollow sphere amorphous MnO2: the formation mechanism, morphology and effect of a bivalent cation-containing electrolyte on its supercapacitive behavior

Nearly X-ray amorphous hollow sphere manganese oxides (hollow sphere MnO2) have been synthesized by a carboxylic acid-mediated system containing KMnO4 and Na2S2O4 under ambient conditions for supercapacitor applications. The product was characterized by powder XRD, Raman spectroscopy and thermal analysis. SEM and TEM were used to investigate the morphology of MnO2. The as-prepared MnO2 was X-ray amorphous and had particles in the size range 0.1–1 μm. A mechanism has been proposed for the formation of hollow sphere structures in the micro-emulsion medium. Upon annealing the sample at temperatures greater than 500 °C, the amorphous MnO2 transforms into Mn2O3. Cyclic voltammetry and galvanostatic charge–discharge cycling were used to evaluate the electrochemical performance. The initial discharge capacities were found to be 283 and 188 F g−1 in 0.1 M Ca(NO3)2 and 0.1 M Na2SO4, respectively, at a current density of 0.5 mA cm−2. The higher specific capacitance in the electrolyte with a bivalent cation is attributed to the reduction of two Mn4+ to Mn3+ by each of the bivalent cations present in the electrolyte.

Tunable hierarchical TiO2 nanostructures by controlled annealing of electrospun fibers: formation mechanism, morphology, crystallographic phase and photoelectrochemical performance analysis

Highly crystalline hierarchical TiO2 nanostructures of morphology ranging from one-dimensional regular fibers, hollow tubes, porous rods and spindles were achieved from electrospun TiO2/composite fibers by annealing at temperatures ranging from 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C, with a ramp rate of 5 °C min−1, and at a pressure of 1 mbar. Crystallographic structure, crystallite size, surface morphology and surface area of annealed TiO2 nanostructures were analysed by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Brunauer–Emmett–Teller (BET) method. The analysis of post-annealing process on electrospun TiO2 nanofibers showed an orderly change in the crystallographic phase transformation with corresponding change in their surface morphologies. XRD and HRTEM analysis confirmed the phase transformation of highly crystalline anatase phase to rutile with crystallite size varied from 11 nm to 36 nm upon tuning the annealing temperature. Interestingly, TiO2 nanostructures annealed at 700 °C showed the formation of biphasic TiO2 hollow tubes with stoichiometry phase compositions of 45.74% anatase and 54.25% rutile. A possible formation mechanism was proposed based on series of temperature-dependent experiments. To evaluate the potential use of these TiO2 nanostructures, dye sensitized solar cell (DSSC) was fabricated using the post-annealed TiO2 nanostructures as photoanode. A higher conversion efficiency (η) of 4.56% with a short circuit current (Jsc) of 8.61 mA cm−2 was observed for highly ordered porous anatase TiO2 nanorods obtained upon annealing at 500 °C under simulated AM1.5 G (100 mW cm−2), confirming that surface area of TiO2 resulted out of porous structure played dominant role.

On the large capacitance of nitrogen doped graphene derived by a facile route

Recent research activities on graphene have identified doping of foreign atoms into the honeycomb lattice as a facile route to tailor its bandgap. Moreover, the presence of foreign atoms can act as defective centres in the basal plane, and these centres can enhance the electrochemical activities of the surface of graphene. Here, we report a facile synthetic approach towards the bulk synthesis of nitrogen doped graphene (N-Graphene) from graphene oxide using a hydrothermal process, with significant control over the extent of N-doping. The electrochemical activeness of N-Graphene (with 4.5 atomic% of nitrogen) is studied by conducting supercapacitor measurements. N-Graphene exhibits a remarkably high specific capacitance of 459 Fg−1 at a current density of 1 mA cm−2 in an electrolyte of 1 M H2SO4 with a high cycle stability compared to that of pristine graphene, which has a specific capacitance of 190 Fg−1. The structural destabilisation of graphene in higher pH/high amount alkaline treatment is demonstrated, and hence optimization of the amount of reagents is necessary in developing a graphene based high performance electronic or electrochemical devices.

Remarkable capacitive behavior of a Co3O4–polyindole composite as electrode material for supercapacitor applications

In this paper, we demonstrate a single step synthesis of cobalt oxide – conducting polyindole (Co3O4–Pind) composites by in-situ cathodic electrodeposition. The structural and morphological changes of the as-prepared Co3O4–Pind composites have been investigated using various techniques such as powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman analysis and X-ray photoelectron spectroscopy (XPS). Very interestingly, polyindole decoration over Co3O4 results in concomitant change in morphology leading to substantial improvement in the supercapacitor behavior. The electrochemical performance of Co3O4–Pind has been investigated by cyclic voltammetry, galvanostatic charge–discharge cycling and impedance analysis. The specific capacitance (SC) of Pind decorated Co3O4 is found to be 1805 F g−1 at a current density of 2 A g−1 with excellent rate capability (SC: 1625 F g−1 at a high current density of 25 A g−1) and cycling stability. This remarkable supercapacitive performance of the Co3O4–Pind composite is mainly attributed to the synergism that evolved between Co3O4 and Pind. More importantly, these electrodes are free from binders and conductive carbon which have significant impact over the gravimetric energy density of the devices.

Shape-selective formation of MnWO4 nanomaterials on a DNA scaffold: magnetic, catalytic and supercapacitor studies

A new route for the aqueous phase synthesis of single crystalline, shape-selective, magnetic MnWO4 nanomaterials on a DNA scaffold has been reported. The synthesis was done by the reaction of MnCl2·4H2O with Na2WO4 in DNA within five minutes of microwave heating. The process exclusively generates wire-like, flake-like and rice-like morphology just by tuning the DNA to Mn(II) salt and WO42− ion concentration and changing other reaction parameters. The field-cooled (FC) and zero-field-cooled (ZFC) magnetization study reveals that the flake-like structure shows the highest magnetization at 5 K compared to that of the wire-like and rice-like structures. The potential of the shape-selective MnWO4 nanomaterials has been tested in two different applications, firstly in a catalysis study for the decomposition of toxic KMnO4 and secondly in electrochemical supercapacitor applications. It was found that the MnWO4 nanomaterials showed different specific capacitance (SC) values for the various shapes and the order of the SC values is: wire-like > flake-like > rice-like. The highest SC of 34 F g−1 was observed for MnWO4 having wire-like shape. The yields of the products with uniform shapes have been found to be significantly high and the synthesized materials are stable for more than six months under ambient conditions. The present work will find a new platform for the generation of other mixed oxides using bio-molecules as scaffolds at low temperature and in short time scales. Moreover, the synthesized material might be useful for other potential applications in the fields of catalysis, sensors, energy storage materials and so on.

Octahedral high voltage LiNi0.5Mn1.5O4 spinel cathode: enhanced capacity retention of hybrid aqueous capacitors with nitrogen doped graphene

A new type of aqueous hybrid supercapacitor has been constructed using electrospun octahedral high voltage LiNi0.5Mn1.5O4 spinel as the cathode material and nitrogen doped graphene (NDG) as the anode material. The structural and morphological changes of the products are investigated by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM) and thermogravimetric analysis (TGA/DTA). The XRD pattern indicates that the LiNi0.5Mn1.5O4 spinel is highly crystalline in nature. The electrochemical performance of LiNi0.5Mn1.5O4 and NDG was evaluated by cyclic voltammetry, galvanostatic constant current charge–discharge cycling and impedance analysis in 3 M LiNO3. The specific capacitance of the LiNi0.5Mn1.5O4/NDG asymmetric hybrid supercapacitor (HSC) cell was found to be 72 F g−1 at a current density of 0.5 mA cm−2. The HSC delivered a maximum energy density of 15 W h kg−1 at a power density of 110 W kg−1. Furthermore, it was shown that an expanded voltage window of 1.3 V could be achieved when combining a composite LiNi0.5Mn1.5O4 (cathode) and NDG (anode) in a charge balanced asymmetric capacitor. Moreover, the HSC exhibits remarkable capacity retention upon cycling indicating the significant impact on the morphology of LiNi0.5Mn1.5O4 and NDG.

Zinc–bromine hybrid flow battery: effect of zinc utilization and performance characteristics

In order to achieve maximum efficiency and long lifetime of a zinc–bromine flow battery (ZBB), the deposition and dissolution of zinc during the charging and discharging processes, respectively, need to be in balance. In view of this, the percentage utilization of zinc during the discharge process was investigated in a zinc–bromine redox flow cell through a potentio/galvanodynamic polarization test and electrochemical impedance spectroscopy. The cell employed carbon–plastic composite electrodes and 98% pure zinc bromide electrolyte solution. The zinc–bromine cells were charged at various current densities of 10, 20 and 30 mA cm−2, and the deposited Zn during charging was compared with the dissolved zinc during the discharge process and found to be 39, 41 and 39%, respectively. A 10% increase in the zinc utilization factor was observed, and this resulted in a 17% increase in the Faradaic efficiency when 99.9% pure zinc bromide electrolyte solution was used. At the same time, 50% utilization of zinc resulted in a further 20% increase in Faradaic efficiency in a cell with non-porous graphite electrodes and 98% pure zinc bromide salt solution. To qualify the nature of the Zn deposit, XRD analysis was carried out along with other spectral studies. The basal plane of zinc (002) was observed at a lower intensity peak, whereas the plane (101) showed preferential growth in the (101) direction. From these spectra, grain size and texture coefficient (TC) were also calculated in order to realize better Zn utilization in a zinc–bromine hybrid flow cell.

Understanding the role of manganese valence in 4 V spinel cathodes for lithium-ion batteries: a systematic investigation

In order to understand the influence of manganese valence on electrochemical performance of spinel manganese oxide cathodes, a systematic investigation of doubly substituted LiMn2−x−yZnxTiyO4 (0.05 ≤ x ≤ 0.25, 0.05 ≤ y ≤ 0.2) spinel compositions has been reported. The synthesized substituted spinel oxides are correlated to the initial manganese valence, observed capacity, ratio of observed to theoretical capacity, capacity loss, degree of manganese dissolution, and irreversible capacity (IRC) loss. The capacity retention and initial observed capacity are found to depend on the manganese valence for the fixed amount of manganese content in the material. The obtained chemical and electrochemical data reveal that the extraction of lithium becomes more difficult with decreasing manganese valence. This phenomenon can be explained by the perturbation of the Mn–Mn interaction across the shared edges, which reduces the ability to extract lithium from spinel cathodes. These investigations lead to finding the optimum doped compositions that can offer the combination of long cycle life, high capacity, and high rate capability.

Enhancing the Sequential Conversion‐Alloying Reaction of Mixed Sn–S Hybrid Anode for Efficient Sodium Storage by a Carbon Healed Graphene Oxide

To date, the possible depletion of lithium resources has become relevant, giving rise to the interest in Na-ion batteries (NIBs) as promising alternatives to Li-ion batteries. While extensive investigations have examined various transition metal oxides and chalcogenides as anode materials for NIBs, few of these have been able to utilize their high specific capacity in sodium-based systems because of their irreversibility in a charge/discharge process. Here, the mixed Sn-S nanocomposites uniformly distributed on reduced graphene oxide are prepared via a facile hydrothermal synthesis and a unique carbothermal reduction process, producing ultrafine nanoparticle with the size of 2 nm. These nanocomposites are experimentally confirmed to overcome the intrinsic drawbacks of tin sulfides such as large volume change and sluggish diffusion kinetics, demonstrating an outstanding electrochemical performance: an excellent specific capacity of 1230 mAh g-1 , and an impressive rate capability (445 mAh g-1 at 5000 mA g-1 ). The electrochemical behavior of a sequential conversion-alloying reaction for the anode materials is investigated, revealing both the structural transition and the chemical state in the discharge/charge process. Comprehension of the reaction mechanism for the mixed Sn-S/rGO hybrid nanocomposites makes it a promising electrode material and provides a new approach for the Na-ion battery anodes.

Understanding the role of oxygen ion (O2−) activity in 1-D crystal growth of rutile TiO2 in molten salts

Controlled synthesis of nanostructured materials using facile and easily scalable synthesis techniques is highly attractive for large-scale production of nanomaterials. In this regard, molten salt synthesis is a well-established technique for large-scale production of nanostructured materials. Few reports have demonstrated the applicability of the molten salt technique for high-aspect-ratio one-dimensional rutile TiO2 synthesis. However, the crystal growth mechanism of 1-D TiO2 in the molten salt is not well understood. Here, various sets of experiments have been delivered to investigate 1-D rutile TiO2 crystal growth starting from anatase TiO2 precursors in molten NaCl with the presence of various inorganic oxy-additives. It was found that the oxygen ion (O2−) activity of the molten salt matrix, which can be controlled by oxy-additives, is the decisive factor for the formation of the 1-D structure. The (NaPO3)6 additive, which reduces the O2− activity of the molten salt matrix, increased the solubility of anatase TiO2. The increased solubility facilitates the easy mobility of ions to the growth site where the crystallographic surface energy is high. The natural tendency to minimize the total surface energy causes the crystal to grow in a particular direction, which eventually leads to 1-D rutile TiO2 nanoparticles.