Syed Khalid

Microwave Assisted Synthesis of Porous NiCo2O4 Microspheres: Application as High Performance Asymmetric and Symmetric Supercapacitors with Large Areal Capacitance.

Large areal capacitance is essentially required to integrate the energy storage devices at the microscale electronic appliances. Energy storage devices based on metal oxides are mostly fabricated with low mass loading per unit area which demonstrated low areal capacitance. It is still a challenge to fabricate supercapacitor devices of porous metal oxides with large areal capacitance. Herein we report microwave method followed by a pyrolysis of the as-prepared precursor is used to synthesize porous nickel cobaltite microspheres. Porous NiCo2O4 microspheres are capable to deliver large areal capacitance due to their high specific surface area and small crystallite size. The facile strategy is successfully demonstrated to fabricate aqueous-based asymmetric & symmetric supercapacitor devices of porous NiCo2O4 microspheres with high mass loading of electroactive materials. The asymmetric & symmetric devices exhibit maximum areal capacitance and energy density of 380 mF cm−2 & 19.1 Wh Kg−1 and 194 mF cm−2 & 4.5 Wh Kg−1 (based on total mass loading of 6.25 & 6.0 mg) respectively at current density of 1 mA cm−2. The successful fabrication of symmetric device also indicates that NiCo2O4 can also be used as the negative electrode material for futuristic asymmetric devices.

A novel Z-scheme WO3/CdWO4 photocatalyst with enhanced visible-light photocatalytic activity for the degradation of organic pollutants

A novel Z-scheme WO3/CdWO4 photocatalyst has been fabricated with sheet-like tungsten trioxide (WO3) hybridized by rod-like cadmium tungstate (CdWO4) via a hydrothermal and chemisorption method. The as-synthesized WO3/CdWO4 photocatalyst exhibited enhanced photocatalytic efficiency for the degradation of different organic dyes under visible light irradiation. It was found that the photocatalytic performance of the composite WO3/CdWO4 was much higher than that of either WO3 or CdWO4 for the degradation of each organic dye. The highest activity of the composite was recorded for the degradation of MB which was about 7 times greater than pure CdWO4 and 2.3 times that of pure WO3. The enhanced performance of the photocatalyst was mainly attributed to the increased surface area and introduction of WO3 into the composite sample, which can induce higher adsorption activity for organic dyes and increased electron–hole separation at the interface between two semiconductors by establishing an inner electric field.

Bifunctional catalysts of Co3O4@GCN tubular nanostructured (TNS) hybrids for oxygen and hydrogen evolution reactions

Catalysts for oxygen and hydrogen evolution reactions (OER/HER) are at the heart of renewable green energy sources such as water splitting. Although incredible efforts have been made to develop efficient catalysts for OER and HER, great challenges still remain in the development of bifunctional catalysts. Here, we report a novel hybrid of Co3O4 embedded in tubular nanostructures of graphitic carbon nitride (GCN) and synthesized through a facile, large-scale chemical method at low temperature. Strong synergistic effects between Co3O4 and GCN resulted in excellent performance as a bifunctional catalyst for OER and HER. The high surface area, unique tubular nanostructure, and composition of the hybrid made all redox sites easily available for catalysis and provided faster ionic and electronic conduction. The Co3O4@GCN tubular nanostructured (TNS) hybrid exhibited the lowest overpotential (0.12 V) and excellent current density (147 mA/cm2) in OER, better than benchmarks IrO2 and RuO2, and with superior durability in alkaline media. Furthermore, the Co3O4@GCN TNS hybrid demonstrated excellent performance in HER, with a much lower onset and overpotential, and a stable current density. It is expected that the Co3O4@GCN TNS hybrid developed in this study will be an attractive alternative to noble metals catalysts in large scale water splitting and fuel cells.

Synthesis of CuS flowers exhibiting versatile photo-catalyst response

Hierarchically structured covellite copper sulfide (CuS) microflowers composed of nanosheets have been successfully fabricated via a one-pot sonochemical process, using copper sulfate and thiourea aqueous solution as precursors in the presence of citric acid, without any prefabricated template. Large-scaled architectures are homogeneous and quite separately displaced and assembled by pure hexagonal single-crystalline CuS nanosheets, having thickness within 20 nm. The as obtained hierarchical CuS structures possess rather high surface area and unique double pore size distributions measured from N2 adsorption isotherms. Moreover, a possible growth mechanism for the CuS hierarchical architectures is proposed on the basis of temporal evolution controlled experiments. Most importantly, these hierarchically structured CuS catalysts showed highly efficient and versatile photo-catalytic activities as well as excellent recyclability in degrading highly concentrated dye aqueous solutions of methylene blue (MB), rhodamine B (RhB) and their mixed solution (MB + RhB) with the help of hydrogen peroxide (H2O2) under natural light irradiation, suggesting a promising application in wastewater purification.

Microwave assisted synthesis of mesoporous NiCo2O4 nanosheets as electrode material for advanced flexible supercapacitors

Mesoporous nickel cobaltite (NiCo2O4) nanosheets are synthesized using a cost effective, ultra fast and environmentally friendly microwave assisted heating method followed by a post-calcination process of the as-prepared precursors. XRD, XPS, BET, SEM, TEM and HRTEM methods are used to characterize the nanosheets. The as-prepared nanosheets with a thickness of around 2 nm possess many interparticle mesopores. The nanosheets have a mesoporous structure, high specific surface area (111.15 m2 g−1), large pore volume (0.3033 cm3 g−1) and narrow pore size distribution (2.25–10 nm). A flexible supercapacitor working electrode of the mesoporous NiCo2O4 nanosheets is prepared on carbon cloth. Cyclic voltammetry, chronopotentiometry and impedance spectroscopy measurements are used to investigate the electrochemical performance of the as-prepared mesoporous NiCo2O4 nanosheet/carbon cloth electrode. The mesoporous NiCo2O4 nanosheets exhibit specific capacitances of 292.5 and 200 F g−1 in 2 M KOH aqueous electrolyte at current densities of 1 and 8 A g−1 respectively. The cyclic performance indicates excellent capacitance retention of 94.5% after 2000 cycles at a current density of 3 A g−1. The excellent cyclic stability can be attributed to the mesoporous nature, high specific surface area, large pore volume and narrow pore distribution of the nanosheets. The synthesized mesoporous NiCo2O4 nanosheets using a microwave method are proved to be excellent electrode material for advanced flexible supercapacitors.

Chrysanthemum-like TiO2 nanostructures with exceptional reversible capacity and high coulombic efficiency for lithium storage

The rational design of hierarchically structured materials is of great significance for developing energy-storage devices. Herein, a novel and uniform chrysanthemum-like TiO2 nanostructure built by well-defined surface-folded nanorods (CLNR-TiO2) has been fabricated by a facile, effective and template-free synthetic method. By only changing the solvent in the reaction another two different morphologies have been obtained, including flower-like structures built by nanoplates (FLNP-TiO2) and microspheres (Microsphere-TiO2). As chrysanthemum-like nanostructures allow efficient Li+ ion diffusion, as well as have better structural stability, CLNR-TiO2 exhibits the best lithium storage performances among the three samples. A superior capacity up to 309.3 mA h g−1 is achieved at a current rate of 0.5 C (1 C = 170 mA g−1) for the first cycle with a high coulombic efficiency of 93.4% and it is significant for practical applications. At a high current rate of 5 C, a high reversible capacity of 198.3 mA h g−1 is obtained in 100 cycles (92% of capacity retention) with excellent rate capacity, high coulombic efficiency and good cycling stability.

A high performance solid state asymmetric supercapacitor device based upon NiCo2O4 nanosheets//MnO2 microspheres

A high performance solid state asymmetric supercapacitor (SSASCs) device is successfully fabricated by combining NiCo2O4 as positive and MnO2 as negative electrode materials. Herein, we also report a facile strategy to synthesize mesoporous layered NiCo2O4 nanosheets and 3D hierarchical MnO2 microspheres by a simple microwave heating method. Both materials exhibit excellent electrochemical performance due to their unique morphological features along with nanocrystallite size, high specific surface area, narrow pore size distribution and large pore volume. The SSASCs device operates within the potential window of 1.5 V and exhibits high volumetric capacity and energy density of 0.954 mA h cm−3 (2.3 F cm−3) and 0.715 mW h cm−3 at 1 mA cm−2 respectively. The device also demonstrates excellent cyclic stability with capacity retention of 83% by the end of 10 000 cycles at a current density of 2 mA cm−2. This work constitutes the first demonstration of using 3D hierarchical MnO2 microspheres as a high energy negative electrode for a SSASCs device. A SSASCs device with high volumetric capacity and energy density has significant potential applications in portable electronics and electrical vehicles.

A facile one-step fabrication of novel WO3/Fe2(WO4)3·10.7H2O porous microplates with remarkable photocatalytic activities

Highly efficient visible-light-driven WO3/Fe2(WO4)3·10.7H2O porous microplates have been fabricated for the first time by a simple one-step hydrothermal method using sodium tungstate and iron chloride as precursors. The photocatalyst was characterized by employing X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-VIS absorption, photoluminescence (PL), and Fourier transform infrared spectroscopy (FTIR). From the FESEM and TEM analysis, the size of as-prepared porous microplates was estimated to be ca. 2 μm. The as-prepared sample maintained a high specific surface area (96.906 m2 g−1) and showed remarkably high photocatalytic performance (k = 0.06771 min−1) for the degradation of methyl orange (MO) under visible light irradiation. The improved performance of the product was found to be 3.3 times greater than pure WO3 and about 6.7 times greater than that of P25 TiO2. The enhanced optical absorption in the visible region, large specific surface area, suitable band position and efficient separation of photogenerated electron–hole pairs at the heterojunction interface between the two materials were the key factors involved in the enhancement of the photocatalytic activity.

Energetic interpenetrating polymer network based on orthogonal azido–alkyne click and polyurethane for potential solid propellant

High energetic propellants with synergistic mechanical strength are the prerequisites for aerospace industry and missile technology; though glycidyl azide polymer (GAP) is a renowned and a promising energetic polymer which shows poor mechanical and low-temperature properties. In order to overcome these problems, a novel energetic interpenetrating polymer network (IPN) of acyl-terminated glycidyl azide polymer (Acyl-GAP) and hydroxyl terminated polybutadiene (HTPB) is effectively synthesized and characterized via an “in situ” polymerization by triazole and urethane curing system respectively. Acyl-GAP and dimethyl 2,2-di(prop-2-ynyl)malonate (DDPM) have been synthesized and well characterized by using FT-IR, 1H NMR, 13C NMR and GPC. The maximum tensile strength ∼5.26 MPa and elongation 318% are achieved with HTPB-PU/Acyl-GAP triazole in 50 : 50 weight ratios. The solvent resistance properties have been investigated by the equilibrium swelling method and the glass transition temperature (Tg), morphology and thermal stability are evaluated by DSC, SEM and TGA-DTG respectively. Thus, HTPB-PU/Acyl-GAP triazole is a futuristic binder for the composite solid propellant.

Energetic hybrid polymer network (EHPN) through facile sequential polyurethane curation based on the reactivity differences between glycidyl azide polymer and hydroxyl terminated polybutadiene

To improve the thermo-mechanical properties of glycidyl azide polymer (GAP) and hydroxyl terminated polybutadiene (HTPB) based propellants, a facile sequential polymerization approach has been conducted to prepare an energetic hybrid polymer network (EHPN) through stepwise curation. The detailed curing conditions for the EHPN formation were determined using an in situ FTIR kinetic study. The effect of curing ratio (NCO/OH) on the mechanical properties of the polyurethane networks of GAP and HTPB was investigated, wherein hexamethylene diisocyanate biuret trimer (Desmodur N100) and isophorone diisocyanate (IPDI) were used as mixed curative agents. A series of EHPNs were prepared by varying the relative weight ratios of GAP and HTPB with a single poly-isocyanate mixed curing system (IPDI/N100). A remarkable mechanical strength of up to 5.83 MPa and an elongation at break of 359% were achieved with a 50:50 weight ratio of GAP to HTPB, which is the maximum mechanical strength reported thus far for a binder system of GAP and HTPB, which has a thermally more stable cross-linked network. The thermal properties of the as-synthesized PU networks of GAP, HTPB and GAP–HTPB EHPNs with different weight ratios were characterized using the DMA and DSC techniques. Thermal degradation behavior and morphological studies were also investigated with TGA-DTG and scanning electron microscopy (SEM), respectively. The facile sequential polyurethane curation polymerization technique can be potentially used for advanced solid composite propellants.