Madhavi Srinivasan

Cobalt Sulfide Nanosheet/Graphene/Carbon Nanotube Nanocomposites as Flexible Electrodes for Hydrogen Evolution

Flexible three-dimensional (3D) nanoarchitectures have received tremendous interest recently because of their potential applications in wearable electronics, roll-up displays, and other devices. The design and fabrication of a flexible and robust electrode based on cobalt sulfide/reduced graphene oxide/carbon nanotube (CoS2 /RGO-CNT) nanocomposites are reported. An efficient hydrothermal process combined with vacuum filtration was used to synthesize such composite architecture, which was then embedded in a porous CNT network. This conductive and robust film is evaluated as electrocatalyst for the hydrogen evolution reaction. The synergistic effect of CoS2 , graphene, and CNTs leads to unique CoS2 /RGO-CNT nanoarchitectures, the HER activity of which is among the highest for non-noble metal electrocatalysts, showing 10 mA cm(-2) current density at about 142 mV overpotentials and a high electrochemical stability.

Recent developments in electrode materials for sodium-ion batteries

The rapid consumption of non-renewable resources has resulted in an ever-increasing problem of CO2 emissions that has motivated people for investigating the harvesting of energy from renewable alternatives (e.g. solar and wind). Efficient electrochemical energy storage devices play a crucial role in storing harvested energies in our daily lives. For example, rechargeable batteries can store energy generated by solar cells during the daytime and release it during night-time. In particular, lithium-ion batteries (LIBs) have received considerable attention ever since their early commercialization in 1990s. However, with initiatives by several governments to build large-scale energy grids to store energy for cities, problems such as the high cost and limited availability of lithium starts to become major issues. Sodium, which also belongs to Group 1 of the periodic table, has comparable electrochemical properties to Lithium, and more importantly it is considerably more accessible than lithium. Nonetheless, research into sodium-ion batteries (NIBs) is currently still in its infancy compared to LIBs, although great leaps and bounds have been made recently in terms of research and development into this technology. Here in this review, we summarize the recent advancements made, also covering the prospective materials for both the battery cathode and anode. Additionally, opinions on possible solutions through correlating trends in recent papers will be suggested.

Synthesis and electrochemical properties of electrospun V2O5 nanofibers as supercapacitor electrodes

Vanadium pentoxide (V2O5) nanofibers (VNF) were synthesized through a simple electrospinning method, and their application as supercapacitor electrodes demonstrated. The effect of annealing temperature on the microstructure and morphology of VNF was investigated systematically through scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) surface area measurements. Electrochemical properties of the synthesized products as electrodes in a supercapacitor device were studied using cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy in aqueous electrolyte of different pH and also in an organic electrolyte. The highest specific capacitance was achieved for VNF annealed at 400 °C, which yielded 190 F g−1 in aqueous electrolyte (2 M KCl) and 250 F g−1 in organic electrolyte (1 M LiClO4 in PC) with promising energy density of 5 Wh kg−1 and 78 Wh kg−1 respectively.

The facile synthesis of hierarchical porous flower-like NiCo2O4 with superior lithium storage properties

In this work, we demonstrate the facile fabrication of 3-dimensional (3D) hierarchical porous flower-like NiCo2O4 and its application as an anode material in high-performance lithium ion batteries (LIBs). The uniform flower-like NiCo2O4 is built from porous nanoplates with thicknesses of approximately 25 nm. A detailed investigation reveals that PVP plays an important role, not only in controlling the formation of the delicate hierarchical flower-like structure, but also in creating the uniform pores of each nanoplate. Furthermore, a possible formation mechanism for this unique structure is proposed based on the experimental results. As a virtue of its beneficial structural features, the as-prepared NiCo2O4 exhibits an enhanced lithium storage capacity and excellent cycling stability (∼939 mA h g−1 at 100 mA g−1 after 60 cycles). This remarkable electrochemical performance can be attributed to the hierarchical structure and sufficient void space within the surface of the nanoplates, which effectively increases the contact area between the active materials and the electrolyte, reducing the Li+ diffusion pathway and buffering the volume change during cycling.

Electrospun Porous NiCo2O4 Nanotubes as Advanced Electrodes for Electrochemical Capacitors

Novel, porous NiCo2O4 nanotubes (NCO-NTs) are prepared by a single-spinneret electrospinning technique followed by calcination in air. The obtained NCO-NTs display a one-dimensional architecture with a porous structure and hollow interiors. The effect of precursor concentration on the morphologies of the products is investigated. Due to their unique structure, the prepared NCO-NT electrode exhibits a high specific capacitance (1647 F g(-1) at 1 A g(-1)), excellent rate capability (77.3 % capacity retention at 25 A g(-1)), and outstanding cycling stability (6.4 % loss after 3000 cycles), which indicates it has great potential for high-performance electrochemical capacitors. The desirable enhanced capacitive performance of NCO-NTs can be attributed to the relatively large specific surface area of these porous and hollow one-dimensional nanostructures.

Fabrication of spinel one-dimensional architectures by single-spinneret electrospinning for energy storage applications.

A facile and general method is developed to fabricate one-dimensional (1D) spinel composite oxides with complex architectures by using a facile single-spinneret electrospinning technique. It is found that precursor polymers and heating rates could control the structures of the products, such as 1D solid, nanotube and tube-in-tubes structures. Especially, the tube-in-tube structures have been successfully fabricated for various mixed metal oxide, including CoMn2O4, NiCo2O4, CoFe2O4, NiMn2O4 and ZnMn2O4. Benefiting from the unique structure features, the tube-in-tube hollow nanostructures possess superior electrochemical performances in asymmetric supercapacitors and Li-O2 batteries.

1D hollow α-Fe2O3 electrospun nanofibers as high performance anode material for lithium ion batteries

Hollow-structured α-Fe2O3 nanofibers were successfully synthesized by a simple electrospinning technique using iron acetylacetonate (Fe(acac3)) and polyvinylpyrrolidone (PVP) precursor. Fe (acac)3–PVP composite fibers were calcined at high temperature to form an interconnected 1D hollow-structure of α-Fe2O3 nanofibers. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) were employed to characterize α-Fe2O3 hollow fibers. Based on the characterization results, a formation mechanism for electrospun α-Fe2O3 hollow fibers is proposed. Electrochemical measurements showed that the hollow-structure of α-Fe2O3 nanofibers played an important role in improving the electrode cycle stability and rate capability in lithium ion batteries. The α-Fe2O3 hollow fiber anodes exhibit a high reversible capacity of 1293 mA h g−1 at a current density of 60 mA g−1 (0.06 C) with excellent cycle stability and rate capability. Based on our study this high performance is attributed to the interconnected hollow-structure of large aspect ratio α-Fe2O3 nanofibers, which makes them a potential candidate for lithium ion batteries.

Nanoweb anodes composed of one-dimensional, high aspect ratio, size tunable electrospun ZnFe2O4 nanofibers for lithium ion batteries

Nanowebs consisting of interwoven ZnFe2O4 nanofibers are synthesized by a simple electrospinning technique, to be employed as an environmentally friendly anode in lithium ion batteries. Effect of precursor viscosity on the growth mechanism of electrospun ZnFe2O4 nanofibers (ZFO-NF) and ZnFe2O4 nanorods (ZFO-NR) is studied by microscopy and diffraction techniques. Structural characterization by powder X-ray diffraction, FESEM and HRTEM studies evaluates the single phase nature of ZnFe2O4, which consists of 11(3) nm nanocrystals that self-agglomerate to form nanofibers after thermal treatment. FESEM micrographs depict the self-assembly of electrospun ZnFe2O4 nanofibers into intertwined porous nanowebs with a continuous framework. Benefitting from the one-dimensional functional nanostructured architecture, the application of electrospun nanowebs with ZnFe2O4 nanofiber (ZFO-NF) anodes in lithium ion batteries exhibits excellent cyclability and retains a reversible capacity of 733(10) mAh g−1 up to 30 cycles at 60 mA g−1 as compared to ZnFe2O4 nanorods (ZFO-NR) with a capacity of ∼200 mAh g−1. In addition, the importance of providing electronic wiring during lithiation/delithiation, especially in prolonged cycling, is emphasized.

Printable photo-supercapacitor using single-walled carbon nanotubes

A printable, all solid-state photo-supercapacitor (PSC) incorporating both organic photovoltaic (OPV) and supercapacitor (SC) functions has been demonstrated utilizing a single-walled carbon nanotube network, enabling a thinner (< 0.6 mm) and lighter (< 1 g) device architecture, which leads to a 43% reduction in device internal resistance as compared to external wire connected OPVs and SCs.

TiO2/AC Composites for Synergistic Adsorption-Photocatalysis Processes: Present Challenges and Further Developments for Water Treatment and Reclamation

Titanium dioxide supported on activated carbon, or TiO2/AC composite, exhibits bifunctionality of adsorption and photocatalysis in synergism. The authors review the TiO2/AC synthesis techniques, characteristics, and performances in removing organic pollutants in water. Practical issues pertinent to applications of the TiO2/AC composite in water treatment and reclamation are discussed. These include dispersing the particles and recovering from the product water, UV introduction and attenuation in the photoreactor, long-term photostability and mechanical stability of the composite, potential TiO2 deactivation by the organic and inorganic matrices, assessment of intermediates and byproducts, and regeneration techniques for the exhausted or fouled TiO2/AC. Coupling with a membrane separation process to recover and regenerate TiO2/AC in various continuous flow-through system configurations is proposed. There are also possible integrations of TiO2/AC treatment systems with other treatment processes that may result in effective pollutant removal with reduced energy and chemical cost. Future developments including incorporation of solar energy are proposed. A modified TiO2/AC that can be photoexcited by solar light has been developed by the authors, and its performance in adsorbing and photocatalytic degradation (PCD) of bisphenol-A is presented.