Vijayamohanan K. Pillai

Co3O4 Nanorods—Efficient Non-noble Metal Electrocatalyst for Oxygen Evolution at Neutral pH

Hydrogen, as a universal fuel, is expected to play an important role in the sustainable development of energy generation, utilization and storage. In addition, it possesses higher energy density in comparison to other common fuels such as petrol, diesel and LPG. Presently, 95 % of the demand for hydrogen is fulfilled from fossil fuels, mainly through the steam reforming process [1], which emits CO2 as a by-product. Photoelectrochemical water splitting is one of the most attractive ways of utilizing the most important renewable energy source, sunlight, for the production of hydrogen without any carbon footprint [2]. Multijunction photoelectrochemical cells (PEC) have, therefore, been used to achieve a better solar to hydrogen efficiency in alkaline conditions. However, water splitting is thermodynamically an uphill reaction, requiring 237 kJ of energy to split 1 mol of water [3]. The mechanism of water splitting involves many steps associated with hydrogen and oxygen evolution at the cathode and anode, respectively. Among these, oxygen evolution reaction (OER), with its sluggish kinetics, is considered as a bottleneck for oxygen formation, since two water molecules have to come closer in order to form a bond between two oxygen atoms [4]. This is supported by the fact that the rate for oxygen evolution is six orders slower when compared to that for hydrogen evolution [4, 5] over polycrystalline platinum, and hence, more efficient catalysts are needed to improve the overall rate of water splitting to make this route of H2 generation commercially viable. At present, precious metal oxides such as RuO2 and IrO2 [6, 7] are some of the best known catalysts for oxygen evolution in water electrolyzers. However, apart from their high cost, both Ir and Ru sources are sparse on earth’s crust, thereby limiting their practical use, especially, on a large commercial scale. Among these, IrO2 is a better catalyst 2 because RuO2 gets converted into RuO4 during the process of oxygen evolution and hence loses its catalytic activity with time [8]. Doping of iridium in RuO2 has been shown to improve its stability, and the catalyst was found to be more active than IrO2 [9]. Fluorine doping has been shown to improve the catalytic activity of IrO2 for oxygen evolution due to the decrease in the energy of the rate determining step by the p,dhybridization of Ir 5d and F 2p states [10, 11]. Recent research work has been focused on developing nonprecious metal oxides for the oxygen evolution to reduce the capital cost of hydrogen production [12]. In particular, nanostructures of earth-abundant transition metal oxides are shown to have promising catalytic activities for oxygen evolution compared to the precious metal oxides such as RuO2 and IrO2. NiO, Ni(OH)2 and MnO2 nanostructures have shown remarkably low overpotential (~300–320 mV) for oxygen evolution in alkaline medium [13–15]. Spinel oxides such as NiCo2O4, ZnCo2O4 and CoFe2O4 are shown to have better stability and catalytic activity towards oxygen evolution [16–18]. The perovskite oxide Ba0.5Sr0.5Co0.8Fe0.2O3–δ exhibits higher catalytic activity than IrO2, and the enhanced activity has been correlated to the occupancy at the eg state of Co [19]. Most of the currently used catalysts work in alkaline conditions (pH>7) with higher overpotential associated with the oxygen evolution and also at higher temperatures, which corrode the electrode as well as the experimental setup. A * Pattayil A. Joy pa.joy@ncl.res.in

Facile Green Synthesis of BCN Nanosheets as High-Performance Electrode Material for Electrochemical Energy Storage.

Two-dimensional hexagonal boron carbon nitride (BCN) nanosheets (NSs) were synthesized by new approach in which a mixture of glucose and an adduct of boric acid (H3 BO3 ) and urea (NH2 CONH2 ) is heated at 900 °C. The method is green, scalable and gives a high yield of BCN NSs with average size of about 1 μm and thickness of about 13 nm. Structural characterization of the as-synthesized material was carried out by several techniques, and its energy-storage properties were evaluated electrochemically. The material showed excellent capacitive behaviour with a specific capacitance as high as 244 F g(-1) at a current density of 1 A g(-1) . The material retains up to 96 % of its initial capacity after 3000 cycles at a current density of 5 A g(-1) .

A single-step room-temperature electrochemical synthesis of nitrogen-doped graphene nanoribbons from carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs) were transformed into nitrogen-doped graphene/graphitic nanoribbons (N-doped GNRs) in a single-step electrochemical process at room temperature in formamide, which acts as a solvent and a source of nitrogen. The N-doped GNRs, with about 4 at% pyridinic and pyrrolic nitrogens, were characterized by transmission electron microscopy, atomic force microscopy, Raman spectrometry, infra-red spectrophotometry, X-ray photoelectron spectroscopy and cyclic voltammetry. Nitrogen doping can be regulated by varying the applied electric field or the duration of the reaction, wherein the kinetics of decomposition of formamide plays a critical role. N-doped GNRs show enhanced charge storage ability, attributed mainly to the increased surface area resulting from a change in morphology when cylindrical MWCNTs are sequentially unzipped as well as to a possible pseudocapacitance contribution from pyridinic and pyrrolic nitrogens. This unprecedented synthetic route provides a room-temperature method for the production of high quality, N-doped GNRs with specific nitrogen types for a variety of applications.

A Single step electrochemical synthesis of luminescent WS2 quantum dots

Transition-metal dichalcogenide quantum dots (TMDQDs) with few layers are in the forefront of recent research on tailored 2D layered materials owing to their unique band structure. Such quantum dots (QDs) draw wide interest as potential candidates for components in optoelectronic devices. Although a few attempts towards single step synthesis of MoS2 QDs have been demonstrated, limited methods are available for WS2 QDs. Herein, we demonstrate a one-step electrochemical synthesis of luminescent WS2 QDs from their bulk material. This is achieved by a synergistic effect of perchlorate intercalation in non-aqueous electrolyte and the applied electric field. The average size of the WS2 QDs is 3  ±1 nm (N=102) with few layers. The QDs show a higher photoluminescence (PL) quantum efficiency (5 %) and exhibit an excitation wavelength-dependent photoluminescence. This unprecedented electrochemical avenue offers a strategy to synthesize size tunable WS2 nanostructures, which have been systematically investigated by various characterization techniques such as transmission electron microscopy (TEM), photoluminescence and UV/Vis spectroscopies, and X-ray diffraction (XRD). Time-dependent TEM investigations revealed that time plays a vital role in this electrochemical transformation. This electrochemical transformation provides a facile method to obtain WS2 QDs from their bulk counterpart, which is expected to have a greater impact on the design and development of nanostructures derived from 2D materials.

Enhanced nucleation of polypropylene by metal–organic frameworks (MOFs) based on aluminium dicarboxylates: influence of structural features

Metal–organic frameworks (MOFs) based on aluminium dicarboxylates provide a new platform for the enhanced nucleation of isotactic polypropylene (iPP). For instance, aluminium dicarboxylates exhibit a unique butterfly-like structure similar to that of carboxylate-alumoxanes and correlates well with the nucleation characteristics of iPP. A subtle change in the structure of the ligand backbone (fumarate/succinate) does not alter the framework structure despite changing the hydrophilic/hydrophobic character and its subsequent nucleation characteristics. This suggests that the nucleating agent should facilitate favourable interaction with hydrophobic iPP for efficient nucleation. Further, a systematic variation of the alkyl chain length in the Al-dicarboxylate does not change the nucleation efficiency considerably, even though it increases the distance between the octahedral alumina chains in the metal–organic framework, suggesting that the butterfly-like structure present in the framework is a key aspect for nucleation. Finally, the significance of the orientational conformation of the dicarboxylate around the metal centre for the nucleation is confirmed by the poor nucleation efficiency of chromium and zirconium suberate MOFs where the orientation of suberate would be different from that of aluminium suberate due to the difference in the ligation of the carboxylate group. The present work thus provides valuable pathways for developing new nucleating agents based on MOFs with appropriate selection and orientation of the organic linkers around the metal centre.

Polydentate disulfides for enhanced stability of AuNPs and facile nanocavity formation

Stabilization of gold nanoparticles (AuNPs) on a unique polydentate disulfide ligand, poly(DSDMA) synthesized by selective polymerization of bis(methacryloyl hydroxyethyl)disulfide (DSDMA), results in 2.2 nm monodisperse AuNPs, which exhibit Coulomb blockade at room temperature. A higher graft density (1.8 chains nm−2) of the polydentate ligand results in additional stability in DMF up to one year and competitive exchange against DTT (100 mM) up to 72 h. Crosslinking of the polymer remarkably enhances the thermal stability up to 140 °C for 4 h and also during selective etching by methyl iodide which results in nanocapsules as confirmed by TEM and AFM.

Adsorption Kinetics of WS2 Quantum Dots onto a Polycrystalline Gold Surface

In this work, we report the adsorption kinetics of electrochemically synthesized WS2 quantum dots (QDs) (ca. 3 nm) onto a polycrystalline gold electrode. The Langmuir adsorption isotherm approach was employed to explore the temperature and adsorbate concentration dependence of the experimentally calculated equilibrium constant of adsorption ( Keq) and the free energy for adsorption (Δ Gads). Subsequently, we extract other thermodynamic parameters, such as adsorption rate constant ( Kads), desorption rate constant ( Kd), the enthalpy of adsorption (Δ Hads), and the entropy of adsorption (Δ Sads). Our findings indicate that Δ Gads is temperature-dependent and ca. -7.64 ± 0.6 kJ/mol, Δ Hads = -43.72 ± 1.7 kJ/mol, and Δ Sads = -0.126 ± 0.017 kJ/(mol K). These investigations on the contribution of the enthalpic and entropic forces to the total free energy of this system underscore the role of entropic forces on the stability of the WS2 QDs monolayer and provide new thermodynamic insights into other transition-metal dichalcogenide quantum dot (TMDQD) monolayers as well.

Application of functionalized CNT-polymer composite electrolytes for enhanced charge storage in "all solid-state supercapacitors"

The ability of specifically functionalized carbon nanotubes to enhance proton transport in Nafion and polybenzimidazole membranes leading to improvement in the specific capacitance of an all solid-state supercapacitor is demonstrated. Cyclic voltammetry experiments reveal a 25% improvement (185 and 150 F per gram of RuO2 for composite and Nafion membranes respectively) in capacitance by a meager 0.05 wt% addition of sulfonated MWCNTs in Nafion membranes. On the other hand, an addition of 1% phosphonated MWCNTs results in ∼60% improvement in olybenzimidazole (PBI) based composites (from 160 to 260 F g−1). Further, composite membranes based on functionalized MWCNTs show increased cycle life which is attributed to the presence of electrostatically linked network structures due to functional moieties on the side walls of carbon nanotubes that increases the interfacial charge density and integrity of the membrane. The equivalent series resistance for the PBI and PBI phosphonated MWCNT (PBpNT) membranes is 470 and 89 milli ohm respectively suggesting improved proton conductivity with the composite membrane. Charge discharge measurements reveal a capacitance value of 500 F g−1 for PBpNT membrane based supercapacitors even after 1000 cycles of operation. Use of such nanocomposite membranes is expected to dramatically improve the life time as well as performance of supercapacitors which in turn would facilitate deployment in different applications such as hybrid electric vehicles.