Leela Mohana Reddy Arava

Electrocatalytic Polysulfide Traps for Controlling Redox Shuttle Process of Li-S Batteries.

Stabilizing the polysulfide shuttle while ensuring high sulfur loading holds the key to realizing high theoretical energy of lithium-sulfur (Li-S) batteries. Herein, we present an electrocatalysis approach to demonstrate preferential adsorption of a soluble polysulfide species, formed during discharge process, toward the catalyst anchored sites of graphene and their efficient transformation to long-chain polysulfides in the subsequent redox process. Uniform dispersion of catalyst nanoparticles on graphene layers has shown a 40% enhancement in the specific capacity over pristine graphene and stability over 100 cycles with a Coulombic efficiency of 99.3% at a current rate of 0.2 C. Interaction between electrocatalyst and polysulfides has been evaluated by conducting X-ray photoelectron spectroscopy and electron microscopy studies at various electrochemical conditions.

Graphene as an atomically thin interface for growth of vertically aligned carbon nanotubes.

Growth of vertically aligned carbon nanotube (CNT) forests is highly sensitive to the nature of the substrate. This constraint narrows the range of available materials to just a few oxide-based dielectrics and presents a major obstacle for applications. Using a suspended monolayer, we show here that graphene is an excellent conductive substrate for CNT forest growth. Furthermore, graphene is shown to intermediate growth on key substrates, such as Cu, Pt, and diamond, which had not previously been compatible with nanotube forest growth. We find that growth depends on the degree of crystallinity of graphene and is best on mono- or few-layer graphene. The synergistic effects of graphene are revealed by its endurance after CNT growth and low contact resistances between the nanotubes and Cu. Our results establish graphene as a unique interface that extends the class of substrate materials for CNT growth and opens up important new prospects for applications.

High temperature electrical energy storage: advances, challenges, and frontiers

With the ongoing global effort to reduce greenhouse gas emission and dependence on oil, electrical energy storage (EES) devices such as Li-ion batteries and supercapacitors have become ubiquitous. Today, EES devices are entering the broader energy use arena and playing key roles in energy storage, transfer, and delivery within, for example, electric vehicles, large-scale grid storage, and sensors located in harsh environmental conditions, where performance at temperatures greater than 25 °C are required. The safety and high temperature durability are as critical or more so than other essential characteristics (e.g., capacity, energy and power density) for safe power output and long lifespan. Consequently, significant efforts are underway to design, fabricate, and evaluate EES devices along with characterization of device performance limitations such as thermal runaway and aging. Energy storage under extreme conditions is limited by the material properties of electrolytes, electrodes, and their synergetic interactions, and thus significant opportunities exist for chemical advancements and technological improvements. In this review, we present a comprehensive analysis of different applications associated with high temperature use (40-200 °C), recent advances in the development of reformulated or novel materials (including ionic liquids, solid polymer electrolytes, ceramics, and Si, LiFePO4, and LiMn2O4 electrodes) with high thermal stability, and their demonstrative use in EES devices. Finally, we present a critical overview of the limitations of current high temperature systems and evaluate the future outlook of high temperature batteries with well-controlled safety, high energy/power density, and operation over a wide temperature range.

Power from nature: designing green battery materials from electroactive quinone derivatives and organic polymers

Current lithium ion battery technologies suffer from challenges derived from the eco-toxicity, costliness, and energetic inefficiency of contemporary inorganic materials used in these devices. Small organic molecules containing polycyclic aromatic moieties and polar functional groups have recently been presented as attractive electron donors that bind lithium and other small metal ions. This has endowed them with the potential to replace traditional inorganic electrodes consisting of metal composites. A family of naturally occurring carbonyl compounds, or quinones, have been of particular interest to the scientific community. However, they themselves have been plagued by issues of low voltages, poor conductivity, and capacity fading due to solubility in common polar electrolytes. Herein, we review a number of theoretical and experimental solutions to this problem, which include the use of heterocyclic derivatives, polymers, and conductive supramolecular carbon frameworks as electrochemical property enhancers, or stabilizers, of potential organic electrodes. This review focuses on the benign synthesis, current status, and future direction of organic battery materials with the aim of developing sustainable energy storage systems to meet the demands of a greener future.

Quasi-Solid Electrolytes for High Temperature Lithium Ion Batteries.

Rechargeable batteries capable of operating at high temperatures have significant use in various targeted applications. Expanding the thermal stability of current lithium ion batteries requires replacing the electrolyte and separators with stable alternatives. Since solid-state electrolytes do not have a good electrode interface, we report here the development of a new class of quasi-solid-state electrolytes, which have the structural stability of a solid and the wettability of a liquid. Microflakes of clay particles drenched in a solution of lithiated room temperature ionic liquid forming a quasi-solid system has been demonstrated to have structural stability until 355 °C. With an ionic conductivity of ∼3.35 mS cm(-1), the composite electrolyte has been shown to deliver stable electrochemical performance at 120 °C, and a rechargeable lithium battery with Li4Ti5O12 electrode has been tested to deliver reliable capacity for over several cycles of charge-discharge.

Ionic Liquid–Organic Carbonate Electrolyte Blends To Stabilize Silicon Electrodes for Extending Lithium Ion Battery Operability to 100 °C

Fabrication of lithium-ion batteries that operate from room temperature to elevated temperatures entails development and subsequent identification of electrolytes and electrodes. Room temperature ionic liquids (RTILs) can address the thermal stability issues, but their poor ionic conductivity at room temperature and compatibility with traditional graphite anodes limit their practical application. To address these challenges, we evaluated novel high energy density three-dimensional nano-silicon electrodes paired with 1-methyl-1-propylpiperidinium bis(trifluoromethanesulfonyl)imide (Pip) ionic liquid/propylene carbonate (PC)/LiTFSI electrolytes. We observed that addition of PC had no detrimental effects on the thermal stability and flammability of the reported electrolytes, while largely improving the transport properties at lower temperatures. Detailed investigation of the electrochemical properties of silicon half-cells as a function of PC content, temperature, and current rates reveal that capacity increases with PC content and temperature and decreases with increased current rates. For example, addition of 20% PC led to a drastic improvement in capacity as observed for the Si electrodes at 25 °C, with stability over 100 charge/discharge cycles. At 100 °C, the capacity further increases by 3-4 times to 0.52 mA h cm(-2) (2230 mA h g(-1)) with minimal loss during cycling.

Graphene-decorated graphite–sulfur composite as a high-tap-density electrode for Li–S batteries

Establishing the efficient electronic conductivity of a sulfur cathode without compromising the volumetric energy density and confining dissolved polysulfides within the cathode of the cell are first-order research priorities in the area of Li–S batteries. The emerging nanotechnology-based approaches, especially the use of porous nanocarbon in the formation of the sulfur electrode, stand to negatively affect the volumetric energy due to the low tap-density of C–S cathodes. In order to address these issues, we study the effects of the porosity and density of different carbons such as graphite, graphene and graphite–graphene hybrids on the overall volumetric capacity of the electrode. Although graphene–sulfur (GS) and graphene-decorated graphite–sulfur (GGS) electrodes show similar gravimetric capacities (~1050 mA h g−1), the GGS electrode exhibits a high volumetric capacity (745 mA h cm−3) without compromising the electrochemical stability over 50 cycles. Furthermore, an excellent cycle stability of the GGS electrode over 100 cycles is achieved by coating a thin layer of poly(methyl methacrylate) (PMMA) on the GGS electrode. Maintaining a high tap-density along with porosity is key in achieving high volumetric capacity in C–S cathodes.

Utilization of solar energy for continuous bioethanol production for energy applications

The focus of the present research is to develop energy-efficient, sustainable, and continuous-flow bioethanol production based on solar energy. Solid-state fermentation of glucose was performed in a specially designed solar-energy-driven continuous flow reactor. Aqueous glucose solutions of 10 and 20 wt% were fed into the reactor bed containing bakers yeast (Saccharomyces cerevisiae), resulting in 4.7 and 8.7 wt% ethanol yields, respectively. The bioethanol produced was separated from the yeast bed soon after its formation by an evaporation–condensation process. High ethanol yields (91.2 and 85.5% of the theoretical yield, respectively) indicate the atom-efficiency of the process. No loss in the activity of yeast was observed even after two months of continuous operation of the reactor. The current study demonstrates an energy-efficient methodology for bioethanol production utilizing solar energy. The bioethanol obtained (8.7 wt%, ca. 2 M) was further tested in alkaline-acid direct ethanol fuel cells operated at 303 K, resulting in a power density value of 330 mW cm−2 at a modest open circuit voltage value of 1.65 V (65.5% voltage efficiency).

Solar‐Energy Driven Simultaneous Saccharification and Fermentation of Starch to Bioethanol for Fuel‐Cell Applications

A solar reactor was designed to perform the conversion of starch to ethanol in a single step. An aqueous starch solution (5 wt %) was fed into the reactor bed charged with Bakers yeast (Saccharomyces cerevisiae) and amylase, resulting in approximately 2.5 wt % ethanol collected daily (ca. 25 mL day(-1) ). A significant amount of ethanol (38 g) was collected over 63 days, corresponding to 84 % of the theoretical yield. The production of ethanol without additional energy input highlights the significance of this new process. The ethanol produced was also demonstrated as a potential fuel for direct ethanol fuel cells. Additionally, the secondary metabolite glycerol was fully reduced to a value-added product 1,3-propanediol, which is the first example of a fungal strain (Bakers yeast) converting glycerol in situ to 1,3-propanediol.

Facile synthesis of electrocatalytically active NbS2 nanoflakes for an enhanced hydrogen evolution reaction (HER)

We report a simple ambient pressure annealing technique for the synthesis of ultrathin niobium disulfide (NbS2) nanoflakes. The structure, morphology and composition of the as-synthesized NbS2 flakes are well characterized using various microscopic and spectroscopic techniques. The synthesized two-dimensional layered NbS2 is in stoichiometric proportion, and has a single crystal 3R-NbS2 polymorph structure with semiconducting behavior and has abundant catalytic defect sites. In this paper, the hydrogen evolution reaction (HER) activity of the NbS2 nanoflakes/rGO composite having dense exposed basal planes with improved conductivity is explored, and it is found to be a good HER catalyst in terms of low onset potential, low Tafel slope and high exchange current density.