K. R. V. Subramanian

Electrospun α-Fe2O3 nanostructures for supercapacitor applications

Herein, we report the facile synthesis of two α-Fe2O3 nanostructures with different morphologies via an electrospinning technique using ferric acetyl acetonate as a precursor and polyvinyl acetate and polyvinyl pyrrolidone as the respective polymers. The as-electrospun metal oxide–polymer composite fibers were sintered at 500 °C to obtain two distinct nanostructures, denoted as nanograins and porous fibers throughout this manuscript. These crystalline nanostructures were characterized using powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX) and transmission electron microscopy (TEM). The characterization results elucidated the predominance of hematite (α-Fe2O3) with particle sizes of 21 and 53 nm, for the respective nanostructures. Electrophoretic deposition was carried out in order to fabricate thin film electrodes, which were then subjected to electrochemical analysis. Electrochemical characterization revealed that both of the fabricated electrodes exhibited excellent performance in 1 M LiOH electrolyte with specific capacitance values of 256 and 102 F g−1 for the porous fiber and nanograin structures, respectively, at a scan rate of 1 mV s−1 and excellent capacitance retention, even after 3000 cycles, thus making them promising electrode materials for energy storage devices.

Electrical and optical properties of electrospun TiO2-graphene composite nanofibers and its application as DSSC photo-anodes

The present study reports the electrospinning of TiO2-graphene composite nanofibers to develop conductive nano-fiber mats using polyvinylpyrrolidone as a carrier solution. This carrier solution was sublimated at 450 °C to attain a complete conducting continuous nanofibrous network. It was observed during the annealing that as the graphene content was increased to 1 wt% the continuous fiber morphology was lost. Annealing did not have any impact on the fiber diameter (∼150 nm) or morphology as the graphene content was maintained between 0.0–0.7 wt%. The surface porosity of these samples was found be in the range of 45–48%. The presence of graphene in TiO2 nanofibers was confirmed using Raman spectroscopy. Photoluminescence spectroscopy showed excitonic intensity to be lower in graphene-TiO2 samples indicating that the recombination of photo-induced electrons and holes in TiO2 can be effectively inhibited in the composite nanofibers. Fluorescence spectroscopy was used to confirm this phenomenon where blue and quenched emissions were observed for the electrospun TiO2 nanofibers and composite fibers, respectively. Conductivity measurements showed the mean specific conductance values obtained for TiO2-graphene composites to be about two times higher values than that of the electrospun TiO2 fibers. Assembling these TiO2-graphene fiber composites as photoanodes in dye sensitized solar cells, an efficiency of 7.6% was attained.

Ultra fine MnO2 nanowire based high performance thin film rechargeable electrodes: Effect of surface morphology, electrolytes and concentrations

The present study demonstrates a novel approach by which titanium foils coated with MnO2 nanowires can be processed into a high surface area electrode for rechargeable energy storage applications. A detailed study has been performed to elucidate how surface morphology and redox reaction behaviors underlying these electrodes impact the cyclic and capacitive behavior of the electrode. These nanowires were synthesized hydrothermally and exhibited an aspect ratio in the order of 102. BET analysis revealed that these MnO2 nanowires show a high surface area of 44 m2 g−1. From the analysis of the relevant electrochemical parameters, an intrinsic correlation between the capacitance, internal resistance and the surface morphology has been deduced and explained on the basis of relative contributions from the faradic properties of the MnO2 in different electrolytes. Depending on the type of surface morphology incorporated, these thin film nanowire electrodes exhibited specific mass capacitance value as high as 1050 F g−1 and 750 F g−1 measured from cyclic voltammetry and charge–discharge curves respectively. It has been shown that electrodes based on such nanowires can allow significant room for improvement in the cyclic stability of a hybrid supercapacitor/battery system. Further, a working model supercapacitor in cylindrical form is also shown exhibiting a capacitance of 10 F.

Fabrication and performance evaluation of button cell supercapacitors based on MnO2 nanowire/carbon nanobead electrodes

The present study provides in detail experimental results on the synthesis and characterization of carbonized MnO2 nanowires for fabricating large surface area, high power and energy density rechargeable electrodes for supercapacitor/battery applications. High aspect ratio MnO2 nanowires carbonized with camphoric carbon composed of nanobeads were utilized for this purpose. The graphitic nature of these nanobeads was confirmed through Raman spectroscopy and X-ray photoelectron spectroscopy, where a predominance of sp2 hybridization was observed. The relative contributions of the carbon nanobeads’ influence on the capacitive and diffusion controlled processes underlying these thin film electrodes have been mathematically modelled. The electrodes were fabricated into thin films (thickness ∼30 μm) by co-electrophoresis onto titanium foils exhibiting a surface area of ∼50 m2 g−1. CHN analyses revealed an electrophoretic co-deposition of carbon nanobeads along with MnO2 nanowires onto the titanium foils. From the electrochemical studies, an intrinsic correlation between overall specific capacitance, electrode internal resistance and its conductivity has been defined and explained in different electrolyte systems. These electrodes exhibited specific mass capacitance values as high as 1200 ± 18 F g−1. High cyclic stability was observed at the end of 10 000 cycles, which was attributed to the low oxide dissolution of ∼0.02 ppm in the electrolyte as measured by inductively coupled plasma atomic emission spectroscopy studies. Further, a working model of a button cell was also studied based on these thin film electrodes, which exhibited a capacitance of ∼1.2 F. These thin film electrodes exhibited an energy density of 96 Wh kg−1 and a peak power density of 32 kW kg−1.

An effective route to produce few-layer graphene using combinatorial ball milling and strong aqueous exfoliants

In this paper, a simple, cost effective, and scalable process for production of few-layer graphene is reported by combining ball milling with exfoliants. The graphene was derived from low-cost graphite, which was subjected to high-energy ball milling in an aqueous medium containing a strong exfoliant (1-pyrenecarboxylic acid) and a common solvent methanol. Such a combinatorial approach has not been used before. At a fixed concentration of 1-pyrenecarboxylic acid, the extent of exfoliation was found to be strongly dependent upon the energy input from the ball milling process (expressed as number of hours of milling) and the solvent used. The graphene produced had the distinctive Raman signature, x-ray diffraction crystallinity, scanning electron microscopic image features, transmission electron microscopic images, and high conductivity values (6.7 × 103 S m−1) in 4-probe electrical measurements all of which compared reasonably with typical values achieved for few-layer graphene. Application of the few-laye...

Lithium-ion storage performance of camphoric carbon wrapped NiS nano/micro-hybrids

Camphoric carbon wrapped NiS powders have been profitably exploited to fabricate high surface area electrodes for Li storage. The NiS morphology showed a network of interconnected nanoscale units with rod like profiles which terminated into needle-like apexes spanning diameters of about 50–80 nm. These particles were pyrolyzed using a camphoric solution to form a carbon sheath wrapping. These carbon functionalized NiS powders were processed into high-surface-area cathodes for a fully functional coin cell unit. A detailed study was performed to elucidate the effect of carbon content on the performance of these coin cells. BET surface area analysis revealed that these carbon sheathed NiS could exhibit a high surface area of 32 m2 g−1 compared to pristine powders which exhibited surface area values of 20 m2 g−1. From the analysis of relevant electrochemical parameters, an intrinsic correlation between the specific capacity, internal resistance and temperature has been deduced. Relative contributions of capacitive and diffusion-controlled processes underlying these thin-film electrodes have been mathematically modeled. These thin-film electrodes exhibited specific capacity values as high as 500 mA h g−1 as determined from charge discharge curves. The present study shows that this functional material can provide the advantages of simple processing technique, low cost, and scalability.

Cycling Performance of NanocrystallineLiMn2O4Thin Films via Electrophoresis

The present study demonstrates a novel approach by which titanium foils coated with LiMn2O4 nanocrystals can be processed into a high-surface-area electrode for rechargeable batteries. A detailed study has been performed to elucidate how surface morphology and redox reaction behaviors underlying these electrodes impact the cyclic and capacity behavior. These nanocrystals were synthesized by in situ sintering and exhibited a uniform size of ∼55 nm. A direct deposition technique based on electrophoresis is employed to coat LiMn2O4 nanocrystals onto titanium substrates. From the analysis of the relevant electrochemical parameters, an intrinsic correlation between the cyclability and particle size has been deduced and explained in accordance with the Li intercalation/deintercalation process. Depending on the particle size incorporated on these electrodes, it is seen that in terms of capacitance fading, for nanoparticles cyclability is better than their micron-sized counterparts. It has been shown that electrodes based on such nanocrystalline thin film system can allow significant room for improvement in the cyclic performance at the electrode/electrolyte interface.

Facile synthesis of ultrafine TiO2 nanowires with large aspect ratio and its photoactivity

Abstract In the present study, ultrafine TiO2 nanowires (∼80 nm in diameter) exhibiting large aspect ratio in the order of 103 were synthesized hydrothermally. Phase and morphological analysis of the nanowires was carried out using X-ray diffractometry, X-ray photoelectron spectroscopy and scanning electron microscopy. High resolution transmission electron microscopy revealed the wire exhibiting growth in (101). A Tauc plot derived from UV analysis showed the average band gap values for nanowires to be less than for nanoparticles of similar diameter. It was observed that nanowires exhibited a high degree of photoactivity in an eosin-based dye system which was found to be 20 – 30 % more than that of nanoparticles. This high photoactivity in nanowires was attributed to the longer charge retention which was observed during lifetime measurements, resulting in easy radical formation and dye degradation. Lifetime measurements on the nanowires showed the recombination time to be 54 ns as compared to 43 ns for nanoparticles.

Cycling Performance of Nanocrystalline LiMn2O4 Thin Films via Electrophoresis

The present study demonstrates a novel approach by which titanium foils coated with LiMn2O4 nanocrystals can be processed into a high-surface-area electrode for rechargeable batteries. A detailed study has been performed to elucidate how surface morphology and redox reaction behaviors underlying these electrodes impact the cyclic and capacity behavior. These nanocrystals were synthesized by in situ sintering and exhibited a uniform size of ∼55 nm. A direct deposition technique based on electrophoresis is employed to coat LiMn2O4 nanocrystals onto titanium substrates. From the analysis of the relevant electrochemical parameters, an intrinsic correlation between the cyclability and particle size has been deduced and explained in accordance with the Li intercalation/deintercalation process. Depending on the particle size incorporated on these electrodes, it is seen that in terms of capacitance fading, for nanoparticles cyclability is better than their micron-sized counterparts. It has been shown that electrodes based on such nanocrystalline thin film system can allow significant room for improvement in the cyclic performance at the electrode/electrolyte interface.

Synthesis and Characterization of Electrophoretically Deposited Nanostructured LiCoPO4 for Rechargeable Lithium Ion Batteries

Nanosized LiCoPO4 (LCP) was prepared using a simple sol-gel method. For the first time, electrophoretic deposition process was employed to fabricate a LiCoPO4 cathode material in order to improve the electrochemical performance. The prepared powder was deposited on titanium plate by electrophoretic deposition and their electrochemical properties were studied. The electrochemical properties were analyzed by using cyclic voltagramm studies, impedance studies, and charge/discharge tests. The thickness of the prepared cathode material was found to be 11-12 µm by using scanning electron microscope. The initial specific capacity and the charge transfer resistance (Rct) of the prepared cathode was 103 mAh/g and 851 Ω, respectively. The charge/discharge profiles showed moderate columbic efficiency of 70%.