D. H. Nagaraju

Two-dimensional heterostructures of V2O5 and reduced graphene oxide as electrodes for high energy density asymmetric supercapacitors

In this article, we report the synthesis of electrode materials based on two-dimensional (2D) heterostructures of V2O5 nanosheets (V2O5 NS) and reduced graphene oxide (rGO) electrodes for asymmetric supercapacitor applications. Specifically, the 2D V2O5 and rGO/V2O5 nanosheet electrodes showed a specific capacitance of 253 F g−1 and 635 F g−1, respectively at a current density of 1 A g−1. The capacitance of the heterostructures is almost 2.5 times higher than the 2D V2O5 nanosheets alone. The corresponding energy density of 39 Wh kg−1 and 79.5 Wh kg−1 were achieved for the two electrodes at a power density of 900 W kg−1 in an asymmetric supercapacitor configuration. The energy and power density using the nanosheet heterostructure are, to our knowledge, higher than any of those that were previously reported for asymmetric supercapacitors using V2O5 electrodes.

In situ reduction and functionalization of graphene oxide with L-cysteine for simultaneous electrochemical determination of cadmium(II), lead(II), copper(II), and mercury(II) ions

One pot reduction and functionalization of graphene oxide (GO) with L-cysteine (L-cys-rGO) at the edges and basal planes of the carbon layers are presented. The L-cys-rGO was characterized by X-ray diffraction studies (XRD), X-ray photoelectron spectroscopy (XPS), attenuated infrared spectroscopy (ATIR), and Raman spectroscopy. The surface morphology was studied by scanning electron microscopy (SEM) and transmittance electron microscopy (TEM). The L-cys-rGO was further utilized for the simultaneous electrochemical quantification of environmentally harmful metal ions such as, Cd2+, Pb2+, Cu2+ and Hg2+. Detection limits obtained for these metal ions were 0.366, 0.416, 0.261 and 1.113 μg L−1 respectively. The linear range obtained for Cd2+, Cu2+ and Hg2+ was 0.4 to 2.0 μM and for Pb2+ was 0.4 to 1.2 μM. The detection limits were found to be less than the World Health Organization (WHO) limits. The developed protocol was applied for the determination of the above metal ions in various environmental samples and the results obtained were validated by atomic absorption spectroscopy (AAS).

Surface Passivation of MoO3 Nanorods by Atomic Layer Deposition toward High Rate Durable Li Ion Battery Anodes

We demonstrate an effective strategy to overcome the degradation of MoO3 nanorod anodes in lithium (Li) ion batteries at high-rate cycling. This is achieved by conformal nanoscale surface passivation of the MoO3 nanorods by HfO2 using atomic layer deposition (ALD). At high current density such as 1500 mA/g, the specific capacity of HfO2-coated MoO3 electrodes is 68% higher than that of bare MoO3 electrodes after 50 charge/discharge cycles. After 50 charge/discharge cycles, HfO2-coated MoO3 electrodes exhibited specific capacity of 657 mAh/g; on the other hand, bare MoO3 showed only 460 mAh/g. Furthermore, we observed that HfO2-coated MoO3 electrodes tend to stabilize faster than bare MoO3 electrodes because nanoscale HfO2 layer prevents structural degradation of MoO3 nanorods. Additionally, the growth temperature of MoO3 nanorods and the effect of HfO2 layer thickness was studied and found to be important parameters for optimum battery performance. The growth temperature defines the microstructural features and HfO2 layer thickness defines the diffusion coefficient of Li-ions through the passivation layer to the active material. Furthermore, ex situ high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction were carried out to explain the capacity retention mechanism after HfO2 coating.

Nanostructured binary and ternary metal sulfides: synthesis methods and their application in energy conversion and storage devices

Metal sulfides, known as being analogous to metal oxides, have emerged as a new class of materials for energy conversion and/or storage applications due to their low cost and high electrochemical activity. They have shown fascinating properties such as excellent redox reversibility, conductivity, and capacitance. Further, binary metal sulfides have gained enormous attention due to their large redox reaction sites and high electrical conductivity compared to metal sulfides. Recently, use of binary metal sulfides as electrode materials for various applications such as lithium-ion batteries (LIBs), supercapacitors (SC), and solar cells has been extensively studied by various research groups and this review critically overviews the strategies and advances made towards attaining high performances from these sulfides in an effort to provide leads for scale-up and to find long term solutions for energy and environment crisis. Finally, challenges in achieving superior performances and the future scope of research for metal sulfide based energy materials have been outlined.

Hydrothermal synthesis of 2D MoS2 nanosheets for electrocatalytic hydrogen evolution reaction

Nanostructured molybdenum disulfide (MoS2) is a very promising catalyst for producing molecular hydrogen by electrochemical methods. Herein, we have designed and synthesized highly electocatalytically active 2D MoS2 nanosheets (NS) from molybdenum trioxide (MoO3) by a facile hydrothermal method and have compared their electrocatalytic activities for hydrogen evolution reaction (HER). The electrochemical characterization was performed using linear sweep voltammetry (LSV) in acidic medium. The MoS2 NS show a HER onset potential at about 80 mV vs. reversible hydrogen electrode (RHE) which is much lower than MoO3 (300 mV). The MoS2 NS and MoO3 show a current density of 25 mA cm−2 and 0.3 mA cm−2, respectively at an overpotential of 280 mV vs. RHE. The MoS2 NS showed an 83 times higher current density when compared to MoO3. The Tafel slopes of the MoS2 NS and MoO3 were about 90 mV per dec and 110 mV per dec respectively. This suggests that MoS2 NS are a better electrocatalyst when compared to MoO3 and follow the Volmer–Heyrovsky mechanism for HER.

Electrochemical Synthesis of Thiol-Monolayer-Protected Clusters of Gold

We have synthesized for the first time thiol-monolayer-protected gold nanoparticles (MPCs) by the process of electrochemical dissolution of gold in an ethanol-water mixture. The MPCs have been formed both with and without NaBH4 as a reducing agent. The well-dispersed thiol-capped MPCs were characterized by UV-visible absorption and transmission electron microscopy (TEM) studies.

Electroforming free resistive switching memory in two-dimensional VOx nanosheets

We report two-dimensional VOx nanosheets containing multi-oxidation states (V5+, V4+, and V3+), prepared by a hydrothermal process for potential applications in resistive switching devices. The experimental results demonstrate a highly reproducible, electroforming-free, low SET bias bipolar resistive switching memory performance with endurance for more than 100 cycles maintaining OFF/ON ratio of ∼60 times. These devices show better memory performance as compared to previously reported VOx thin film based devices. The memory mechanism in VOx is proposed to be originated from the migration of oxygen vacancies/ions, an influence of the bottom electrode and existence of multi-oxidation states.

Novel synthetic approach for 1,4-dihydroxyanthraquinone and the development of its lithiated salts as anode materials for aqueous rechargeable lithium-ion batteries

The use of organic electrode materials in the field of lithium ion batteries is becoming a keen interest for the present generation of scientists. Here we are reporting a novel method of synthesis for electrode materials using a combination of sono-chemical and thermal methods. The advantages of organic active materials in lithium ion batteries are of core interest in this study. Structural confirmations are provided by FT-IR, 1H NMR, MALDI-TOF mass spectroscopy and powder XRD data. The electrochemical properties of lithiated-1,4-dihydroxyanthraquinone were studied using electrochemical techniques such as cyclic voltammetry, galvanostatic cyclic potential limitation and potentiostatic electrochemical impedance spectroscopy. Satisfactory results towards the stability of the active species in aqueous media, reasonable discharge capacity with a 0.9 V average voltage and agreeable cycling performance during the charge–discharge process with reproducibility are achieved. For the construction of the full cell, the anode material was coupled with LiNi1/3Co1/3Mn1/3O2 as a cathode material.

Nanostructured mesoporous materials for lithium-ion battery applications

The Energy crisis happens to be one of the greatest challenges we are facing today. In this view, much effort has been made in developing new, cost effective, environmentally friendly energy conversion and storage devices. The performance of such devices is fundamentally related to material properties. Hence, innovative materials engineering is important in solving the energy crisis problem. One such innovation in materials engineering is porous materials for energy storage. Porous electrode materials for lithium-ion batteries (LIBs) offer a high degree of electrolyte-electrode wettability, thus enhancing the electrochemical activity within the material. Among the porous materials, mesoporous materials draw special attention, owing to shorter diffusion lengths for Li+ and electronic movement. Nanostructured mesoporous materials also offer better packing density compared to their nanostructured counterparts such as nanopowders, nanowires, nanotubes etc., thus opening a window for developing electrode materials with high volumetric energy densities. This would directly translate into a scenario of building batteries which are much lighter than todays commercial LIBs. In this article, the authors present a simple, soft template approach for preparing both cathode and anode materials with high packing density for LIBs. The impact of porosity on the electrochemical storage performance is highlighted.