Imran Shakir

Three-dimensional holey-graphene/niobia composite architectures for ultrahigh-rate energy storage

As with donuts, the holes matter Improving the density of stored charge and increasing the speed at which it can move through a material are usually opposing objectives. Sun et al. developed a Nb2O5/holey graphene framework composite with tailored porosity. The three-dimensional, hierarchically porous holey graphene acted as a conductive scaffold to support Nb2O5. A high mass loading and improved power capability were reached by tailoring the porosity in the holey graphene backbone with higher charge transport in the composite architecture. The interconnected graphene network provided excellent electron transport, and the hierarchical porous structure in the graphene sheets facilitated rapid ion transport and mitigated diffusion limitations. Science, this issue p. 599 A graphene/Nb2O5 composite shows optimized electron and ion transport. Nanostructured materials have shown extraordinary promise for electrochemical energy storage but are usually limited to electrodes with rather low mass loading (~1 milligram per square centimeter) because of the increasing ion diffusion limitations in thicker electrodes. We report the design of a three-dimensional (3D) holey-graphene/niobia (Nb2O5) composite for ultrahigh-rate energy storage at practical levels of mass loading (>10 milligrams per square centimeter). The highly interconnected graphene network in the 3D architecture provides excellent electron transport properties, and its hierarchical porous structure facilitates rapid ion transport. By systematically tailoring the porosity in the holey graphene backbone, charge transport in the composite architecture is optimized to deliver high areal capacity and high-rate capability at high mass loading, which represents a critical step forward toward practical applications.

Electrospun fibers for tissue engineering, drug delivery, and wound dressing

Electrospinning, a technique well known for fabricating nanoscale fibers, has recently been studied extensively due to its various advantages such as high surface-to-volume ratio, tunable porosity, and ease of surface functionalization. The resulting fibers are extremely useful for applications in the fields of tissue engineering, drug delivery, and wound dressing. Since electrospun fiber mimic extracellular matrix of tissue in terms of scale and morphology, its potential to be used as scaffold is continuously explored by researchers, especially in the field of vascular, nerve, bone, and tendon/ligament tissue engineering. Besides morphology, physical, and chemical properties, electrospun scaffolds are often evaluated through various cell studies. Researchers have adopted approaches such as surface modification and drug loading to enhance the property and function of scaffold. This review gives an overview of some current aspects of various applications of electrospun fibers, particularly in biomedical fields, how researchers have enhanced electrospun fibers with different methods and attempted to overcome the inherent limitation of electrospinning by using novel techniques.

Extracting hydroxyapatite and its precursors from natural resources

Healing of segmental bone defects remain a difficult problem in orthopedic and trauma surgery. One reason for this difficulty is the limited availability of bone material to fill the defect and promote bone growth. Hydroxyapatite (HA) is a synthetic biomaterial, which is chemically similar to the mineral component of bones and hard tissues in mammals and, therefore, it can be used as a filler to replace damaged bone or as a coating on implants to promote bone in-growth into prosthetic implants when used in orthopedic, dental, and maxillofacial applications. HA is a stoichiometric material with a chemical composition of Ca10(PO4)6(OH)2, while a mineral component of bone is a non-stoichiometric HA with trace amounts of ions such as Na+, Zn2+, Mg2+, K+, Si2+, Ba2+, F−, CO32−, etc. This review looks at the progress being made to extract HA and its precursors containing trace amount of beneficial ions from biological resources like animal bones, eggshells, wood, algae, etc. Properties, such as particle size, morphology, stoichiometry, thermal stability, and the presence of trace ions are studied with respect to the starting material and recovery method used. This review also highlights the importance of extracting HA from natural resources and gives future directions to the researcher so that HA extracted from biological resources can be used clinically as a valuable biomaterial.

Porous Fe2O3 Nanoframeworks Encapsulated within Three-Dimensional Graphene as High-Performance Flexible Anode for Lithium-Ion Battery

Integrating nanoscale porous metal oxides into three-dimensional graphene (3DG) with encapsulated structure is a promising route but remains challenging to develop high-performance electrodes for lithium-ion battery. Herein, we design 3DG/metal organic framework composite by an excessive metal-ion-induced combination and spatially confined Ostwald ripening strategy, which can be transformed into 3DG/Fe2O3 aerogel with porous Fe2O3 nanoframeworks well encapsulated within graphene. The hierarchical structure offers highly interpenetrated porous conductive network and intimate contact between graphene and porous Fe2O3 as well as abundant stress buffer nanospace for effective charge transport and robust structural stability during electrochemical processes. The obtained free-standing 3DG/Fe2O3 aerogel was directly used as highly flexible anode upon mechanical pressing for lithium-ion battery and showed an ultrahigh capacity of 1129 mAh/g at 0.2 A/g after 130 cycles and outstanding cycling stability with a capacity retention of 98% after 1200 cycles at 5 A/g, which is the best results that have been reported so far. This study offers a promising route to greatly enhance the electrochemical properties of metal oxides and provides suggestive insights for developing high-performance electrode materials for electrochemical energy storage.

Design and evaluation of novel Zn doped mesoporous TiO2 based anode material for advanced lithium ion batteries

We report Zn doped mesoporous TiO2 microspheres synthesized via a combined sol–gel and solvothermal method exhibiting excellent Li-ion insertion–extraction properties. The specific capacity of Zn doped mesoporous TiO2 microspheres was significantly higher than that of TiO2 nanopowder at high charge–discharge rates. The superior rate performance offered by Zn doped mesoporous TiO2 microspheres may be attributed to enhanced electronic and ionic conductivity, which was achieved by improving the donor density via Zn doping and by providing a well interconnected mesoporous network of nanoparticles for the effective diffusion of Li ions. Zn doped mesoporous TiO2 microspheres showed more than an 87% capacity retention after 100 cycles at a charge–discharge rate of 1 C, demonstrating it to be a promising approach for the development of high-performance Li ion batteries.

Layer by layer assembly of ultrathin V2O5 anchored MWCNTs and graphene on textile fabrics for fabrication of high energy density flexible supercapacitor electrodes

Among transition metal oxides, vanadium oxides have received relatively modest attention for supercapacitor applications. Yet, this material is abundant, relatively inexpensive and offer several oxidation states which can provide a broad range of redox reactions suitable for supercapacitor operation. Electrochemical supercapacitors based on nanostructured vanadium oxide (V₂O₅) suffer from relatively low energy densities as they have low surface area and poor electrical conductivities. To overcome these problems, we developed a layer by layer assembly (LBL) technique in which a graphene layer was alternatively inserted between MWCNT films coated with ultrathin (3 nm) V₂O₅. The insertion of a conductive spacer of graphene between the MWCNT films coated with V₂O₅ not only prevents agglomeration between the MWCNT films but also substantially enhances the specific capacitance by 67%, to as high as ∼2590 F g(-1). Furthermore, the LBL assembled multilayer supercapacitor electrodes exhibited an excellent cycling performance of >97%, capacitance retention over 5000 cycles and a high energy density of 96 W h kg(-1) at a power density of 800 W kg(-1). Our approach clearly offers an exciting opportunity for enhancing the device performance of metal oxide-based electrochemical supercapacitors suitable for next-generation flexible energy storage devices by employing a facile LBL assembly technique.

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.

General synthesis and definitive structural identification of MN 4 C 4 single-atom catalysts with tunable electrocatalytic activities

AbstractSingle-atom catalysts (SACs) have recently attracted broad research interest as they combine the merits of both homogeneous and heterogeneous catalysts. Rational design and synthesis of SACs are of immense significance but have so far been plagued by the lack of a definitive correlation between structure and catalytic properties. Here, we report a general approach to a series of monodispersed atomic transition metals (for example, Fe, Co, Ni) embedded in nitrogen-doped graphene with a common MN4C4 moiety, identified by systematic X-ray absorption fine structure analyses and direct transmission electron microscopy imaging. The unambiguous structure determination allows density functional theoretical prediction of MN4C4 moieties as efficient oxygen evolution catalysts with activities following the trend Ni > Co > Fe, which is confirmed by electrochemical measurements. Determination of atomistic structure and its correlation with catalytic properties represents a critical step towards the rational design and synthesis of precious or nonprecious SACs with exceptional atom utilization efficiency and catalytic activities.Atomically dispersed metal catalysts are of increasing importance in many catalytic processes, but clear structural identification is challenging. Here, a general synthesis of metal (nickel, iron and cobalt) single-atom catalysts on nitrogen-doped graphene allows the authors to identify a common structure and furthermore correlate structure with electrocatalytic activity.

Rational synthesis of carbon shell coated polyaniline/MoS2 monolayer composites for high-performance supercapacitors

Conducting polymers generally show high specific capacitance but suffer from poor rate capability and rapid capacitance decay, which greatly limits their practical applications in supercapacitor electrodes. To this end, many studies have focused on improving the overall capacitive performance by synthesizing nanostructured conducting polymers or by depositing a range of coatings to increase the active surface area exposed to the electrolyte and enhance the charge transport efficiency and structural stability. Despite this, simultaneously achieving high specific capacitance, good rate performance, and long cycle life remains a considerable challenge. Among the various two-dimensional (2D) layered materials, octahedral (1T) phase molybdenum disulfide (MoS2) nanosheets have high electrical conductivity, large specific surface areas, and unique surface chemical characteristics, making them an interesting substrate for the controlled growth of nanostructured conducting polymers. This paper reports the rational synthesis of carbon shell-coated polyaniline (PANI) grown on 1T MoS2 monolayers (MoS2/PANI@C). The composite electrode comprised of MoS2/PANI@C with a ~3 nm carbon shell exhibited a remarkable specific capacitance of up to 678 F·g–1 (1 mV·s–1), superior capacity retention of 80% after 10,000 cycles and good rate performance (81% at 10 mV·s–1) due to the multiple synergic effects between the PANI nanostructure and 1T MoS2 substrates as well as protection by the uniform thin carbon shell. These properties are comparable to the best overall capacitive performance achieved for conducting polymers-based supercapacitor electrodes reported thus far.

One Dimensional Graphitic Carbon Nitrides as Effective Metal-Free Oxygen Reduction Catalysts

To explore the effect of morphology on catalytic properties of graphitic carbon nitride (GCN), we have studied oxygen reduction reaction (ORR) performance of two different morphologies of GCN in alkaline media. Among both, tubular GCN react with dissolved oxygen in the ORR with an onset potential close to commercial Pt/C. Furthermore, the higher stability and excellent methanol tolerance of tubular GCN compared to Pt/C emphasizes its suitability for fuel cells.