Fast-Charging High-Energy Battery-Supercapacitor Hybrid: Anodic Reduced Graphene Oxide-Vanadium(IV) Oxide Sheet-on-Sheet Heterostructure.

The battery-supercapacitor hybrid (BSH) device has potential applications in energy storage and can be a remedy for low-power batteries and low-energy supercapacitors. Although several studies have investigated electrode materials (particularly for a battery-type anode material) and design for BSHs, the energy density and power density are insufficient (far from the levels required for practical applications). Herein, a hierarchical vanadium(IV) oxide on reduced graphene oxide (rGO@VO2) heterostructure as an anode and activated carbon on carbon cloth (AC@CC) as a cathode are proposed for fabricating an advanced BSH. The mixed valency of V ions inside the as-prepared VO2 matrix (V3+ and V4+) facilitates redox reactions at a low potential, giving rise to rGO@VO2 as a typical anode with a working potential of 0.01-3 V (vs Li/Li+). The sheet-on-sheet heterostructured rGO@VO2 yields a high specific capacity of 1214 mAh g-1 at 0.1 A g-1 after 120 cycles, with a high rate capability and stability. The rGO@VO2//AC@CC BSH device exhibits a maximum gravimetric energy density of 126.7 Wh kg-1 and a maximum gravimetric power density of ∼10 000 W kg-1 within a working voltage range of 1-4 V. Moreover, it exhibits fast charging times of 5 and 834 s with energy densities of 15.6 and 82 Wh kg -1, respectively.

Redox-Driven Route for Widening Voltage Window in Asymmetric Supercapacitor.

Although aqueous asymmetric supercapacitors are promising technologies because of their high-energy density and enhanced safety, their voltage window is still limited by the narrow stability window of water. Redox reactions at suitable electrodes near the water splitting potential can increase the working potential. Here, we demonstrate a kinetic approach for expanding the voltage window of aqueous asymmetric supercapacitors using in situ activated Mn3O4 and VO2 electrodes. The underlying mechanism indicates a specific potential of ∼1 V vs Ag/AgCl for the oxidation of Mn4+-to-Mn7+ at the positive electrode and ∼ -0.8 V vs Ag/AgCl for the reduction of V3+-to-V2+ at the negative electrode, which limits oxygen and hydrogen evolution reactions, respectively. The as-fabricated aqueous asymmetric supercapacitor exhibited a working voltage of 2.2 V with a high-energy density of 42.7 Wh/kg and a power density of ∼1.1 kW/kg. This mechanism improves the voltage window and energy and power densities.

Directional growth of Ag nanorod from polymeric silver cyanide: A potential substrate for concentration dependent SERS signal enhancement leading to melamine detection.

Attention has been directed to prepare exclusive one-dimensional silver nanostructure from the linear inorganic polymer AgCN. Successive color change from yellow to orange, to red and finally to green reflects the evolution of high yielding Ag nanorods (NRs) from well-known -[Ag-CN]- chains of polymeric AgCN at room temperature. The parental 1D morphology of AgCN is retained within the as-synthesized Ag NRs. So we could successfully exploit the Ag NR for surface-enhanced Raman scattering (SERS) studies for sensing a popular milk adulterant melamine down to picomolar level. We observed interesting concentration dependent selective SERS band enhancement of melamine. The enhanced ~1327cm-1 SERS signal intensity at lower concentration (10-9 and 10-12M) of melamine speaks for the preferential participation of -C-N of melamine molecule with Ag surface. On the other hand, '-NH2' group together with ring 'N' participation of melamine molecule onto Ag surface suggested an adsorptive stance at higher (10-3-10-7M) concentration range. Thus the binding modes of the molecule at the Ag surface justify its fluxional behavior.

Micelle confined mechanistic pathway for 4-nitrophenol reduction.

The model 4-nitrophenol reduction has been carried out by different groups in presence of metallic or even non-metallic catalyst elaborating different mechanistic aspects. In the present investigation, we have thoroughly studied the hydrogenation of 4-nitrophenol in a completely metal free homogeneous condition. The introduction of a non-fluorescent probe unequivocally generates a fluorescent molecule that indirectly justifies the anion radical stabilization in the micelle. The reduction mechanism under metal-free condition was proposed and the concept of stabilization of anion radical transition state of 4-nitrophenol at the positively charged Stern layer of anionic micelle was established. The plausible reduction mechanism has also enlightened the graphene-like conducting property of Stern layer of the homogeneous micellar system. Furthermore, the confinement effect for catalysis has also been authenticated by supporting experimental evidences. The borrowed concept of catalysis in confinement drives the catalytic study to a new era of catalysis.

Advance Aqueous Asymmetric Supercapacitor Based on Large 2D NiCo2O4 Nanostructures and the rGO@Fe3O4 Composite.

NiCo2O4 nanostructure is a widely studied pseudocapacitor material because of its high specific capacitance value. Most of the time, the thickness of the nanostructure inhibits the electrode material from whole-body participation and causes sluggish charge transportation. These phenomena directly interfere with the electrochemical performance of the electrode, such as specific capacitance value, stability, energy density, and so forth. Here, two different thin two-dimensional morphologies (nanosheet and nanoplate) of the NiCo2O4 nanocomposite with a large lateral size are reported using ammonia as a hydrolyzing agent. The large size and flat surface of the as-synthesized materials offer enormous active sites during the electrochemical reaction, and the thin wall makes the ion penetration and transportation very effective and facile. Therefore, the NiCo2O4 nanosheet and nanoplate structures exhibited high specific capacitance values of 1540 and 1333 F/g, respectively, with excellent rate and good cycling stability. Here also, two different advance aqueous asymmetric supercapacitors have been reported utilizing two NiCo2O4 nanostructure materials as positive electrodes and the rGO@Fe3O4 composite as a negative electrode, which exhibited excellent rate and high specific energy without sacrificing the specific power. We also studied the electrochemical activity of the rGO@Fe3O4 composite at different compositions.

2D materials for renewable energy storage devices: Outlook and challenges.

Scientists are looking for cost-effective, clean and durable alternative energy devices. Superior charge storage devices can easily meet the demands of our daily needs. In this respect, a material with suitable dimensions for charge storage devices has been considered to be very important. Improved performance of charge storage devices has been derived from whole-body participation and the best are from 2D materials, which provide a viable and acceptable solution.

Metal Bromide Controlled Interfacial Aromatization Reaction for Shape-Selective Synthesis of Palladium Nanostructures with Efficient Catalytic Performances.

Herein, the effect of diverse metal bromides for the shape evolution of palladium nanostructures (Pd NS) has been demonstrated. Aromaticity-driven reduction of bromopalladate(II) is optimized to reproducibly obtain different Pd NS at the water/organic layer interface. In this soft interfacial strategy, a redox potential driven reaction has been performed, forming the thermodynamically more stable (>10(4) -fold) PdBr4 (2-) precursor from PdCl4 (2-) by adding extra metal bromides. In the process, the reductant, Hantzsch dihydropyridine ester (DHPE), is aromatized. Interestingly, alkali metal bromides devoid of coordination propensity exclusively evolve Pd nanowires (Pd NWs), whereas in the case of transition metal bromides the metal ions engage the 'N' donor of DHPE at the interface, making the redox reaction sluggish. Hence, controlled Pd nanoparticles growth is observed, which evolves Pd broccolis (Pd NBRs) and Pd nanorods (Pd NRs) at the interface in the presence of NiBr2 and CuBr2 , respectively, in the aqueous solution. Thus, the effect of diverse metal bromides in the reaction mixture for tailor-made growth of the various Pd NS is reported. Among the as-synthesized materials, the Pd NWs stand to be superior catalysts and their efficiency is almost 6 and 2.5 times higher than commercial 20 % Pd/C in the electrooxidation of ethanol and Cr(VI) reduction reaction by formic acid, respectively.

Suitable Morphology Makes CoSn(OH)6 Nanostructure a Superior Electrochemical Pseudocapacitor.

Morphology of a material with different facet, edge, kink, etc., generally influences the rate of a catalytic reaction.1,2 Herein, we account for the importance of altered morphology of a nanomaterial for a supercapacitor device and employed CoSn(OH)6 as an electrode material. Suitable fabrication of a stable aqueous asymmetric supercapacitor (AAS) using metal hydroxide as positive electrode can be beneficial if the high energy density is derived without sacrificing the power density. Here we have synthesized an uncommon hierarchical mesoporous nanostructured (HNS) CoSn(OH)6 to fabricate a pseudocapacitor. In this endeavor, NH3 is found to be a well-suited hydrolyzing agent for the synthesis.3 Serendipitously, HNS was transformed into favored cubic nanostructure (CNS) in NaOH solution. In solution, NaOH acts as a structure directing as well as an etching agent. Both the samples (HNS & CNS) were used as pseudocapacitor electrodes in KOH electrolyte independently, which is reported for the first time. The HNS exhibits very high specific capacitance value (2545 F/g at 2.5 A/g specific current) with better cyclic durability over CNS sample (851 F/g at 2.5 A/g specific current). To examine the real cell application, we used HNS sample as the positive electrode material with the activated carbon (AC) as the negative electrode material for the development of an aqueous asymmetric supercapacitor (AAS). The as-fabricated AAS exhibited very high specific capacitance value of 713 F/g at a specific current of 1.5 A/g and retained 92% specific capacitance value even after 10 000 charge-discharge cycles. A maximum energy density of 63.5 Wh kg(-1) and a maximum power density of 5277 W kg(-1) were ascertained from the as-fabricated AAS, HNS CoSn(OH)6//AC.

Fabrication of Nitrogen-Doped Mesoporous-Carbon-Coated Palladium Nanoparticles: An Intriguing Electrocatalyst for Methanol and Formic Acid Oxidation.

Inspired by the attractive catalytic properties of palladium and the inert nature of carbon supports in catalysis, a concise and simple methodology for in situ nitrogen-doped mesoporous-carbon-supported palladium nanoparticles (Pd/N-C) has been developed by carbonizing a palladium dimethylglyoximate complex. The as-synthesized Pd/N-C has been exfoliated as a fuel cell catalyst by studying the electro-oxidation of methanol and formic acid. The material synthesized at 400 °C,namely, Pd/N-C-400,exhibitssuperior mass activity and stability among catalysts synthesized under different carbonization temperaturesbetween300 and 500 °C. The unique 1D porous structure in Pd/N-C-400 helps better electron transport at the electrode surface, which eventually leads to about five times better catalytic activity and about two times higher stability than that of commercial Pd/C. Thus, our designed sacrificial metal-organic templatedirected pathway becomes a promising technique for Pd/N-C synthesis with superior catalytic performances.

Redox mediated synthesis of hierarchical Bi2O3/MnO2 nanoflowers: a non-enzymatic hydrogen peroxide electrochemical sensor.

Uniform hierarchical Bi2O3/MnO2 nanoflowers (BM NFs) are fabricated via a reaction strategy by combining redox reaction and hydrothermal treatment. This wet chemical method reports for the first time a one pot synthesis of Bi2O3/MnO2 nanoflowers via a thermodynamically allowed galvanic reaction between Bi(0) and KMnO4 in aqueous solution under modified hydrothermal (MHT) conditions. The Bi2O3/MnO2 NF composites are then applied as a catalyst for electrochemical hydrogen peroxide detection. Exceedingly high H2O2 detection sensitivity (0.914 µA µM(-1) cm(-2)) lies in a wide linear range of 0.2-290 µM and the detection limit goes down to 0.05 µM (S/N = 3) for non-enzymatic detection of H2O2 in solution. This prototype sensor demonstrates an admirable analytical performance considering its long-term stability, good reproducibility and acceptable selectivity against common interfering species. The employment of the stable nanocomposite for real sample analysis makes it a deliverable for H2O2 sensing.

Liquor ammonia mediated V(V) insertion in thin Co3O4 sheets for improved pseudocapacitors with high energy density and high specific capacitance value.

Ultrathin 2D Co3O4 and Co3V2O8 nanosheets have been produced from our modified hydrothermal technique (MHT). Both the materials have been proved to be extraordinary electrode materials for pseudocapacitors. The neat nanosheets of Co3O4 and Co3V2O8 exhibit a record specific capacitance value of 1256 F g(-1) and 4194 F g(-1) at 1 A g(-1) current density, respectively.

Redox-Mediated Synthesis of a Fe3O4-MnO2 Nanocomposite for Dye Adsorption and Pseudocapacitance.

A logically chosen redox reaction of submerged Fe(0) in an aqueous KMnO4 solution has been reported. The template-free reaction conditions produced gram amounts of a hierarchical flowerlike Fe3O4-MnO2 nanocomposite. More precisely, freshly prepared Fe(0) nanoparticles were prepared from air-free hot water under submerged conditions using a door magnet. The black Fe(0) particles were oxidized in water quantitatively by KMnO4 in the solution phase and the nanocomposite was prepared. The material has been used as a dye adsorbent and the representative cationic dye uptake, recovery, and recycling of the dye becomes easy owing to the ferromagnetic properties and surface negative charge of the material. The nanocomposite also showed a higher specific capacitance (327 F g(-1) at 10 mV s(-1)) than the reported values of pure MnO2 and Fe3O4. The material exhibited a high energy density as well as a high power density, and remained stable even after a large number of charge-discharge cycles.

Solid-state transformation of single precursor vanadium complex nanostructures to V2O5 and VO2: catalytic activity of V2O5 for oxidative coupling of 2-naphthol.

A vanadium complex, [(C5H5N)2V2O3·H2O], of different morphologies has been obtained via a modified hydrothermal procedure using pyridine and VOSO4 salt as the starting material. The evolved [(C5H5N)2V2O3·H2O] nanobelts are of 50-200 nm in width and of a length up to several millimeters. At higher temperatures (600 °C), the solid [(C5H5N)2V2O3·H2O] nanostructures are converted to vanadium pentoxide (V2O5) and vanadium dioxide (VO2) when heated in air and nitrogen atmosphere, respectively. During growth, the mechanism of the evolution of octahedra, truncated octahedra, and hollow truncated octahedra of [(C5H5N)2V2O3·H2O] are reported for the first time. These types of well-structured morphology are also isolated while V2O5 and VO2 are evolved. The as-grown belt-like and octahedral morphologies of [(C5H5N)2V2O3·H2O] are retained during the solid-state transformation, suggesting a route to evolve crystalline nanomaterials. Again, the morphological evolution of the [(C5H5N)2V2O3·H2O] nanostructures has been examined to be pyridine and precursor vanadyl sulfate (VS) concentration dependent. Thus, we are able to isolate truncated octahedra as an intermediate during the formation of [(C5H5N)2V2O3·H2O] nanobelts and nanoflowers with a high pyridine (Py) concentration. Interestingly, longer reaction times successively featured the transformation of truncated octahedra into nanobelts. Nanobelt evolution is not observed at low pyridine concentrations. However, the formation of octahedral morphology takes place at low pyridine concentration. All of the nanostructures were critically examined and characterized thoroughly by various physical techniques to ascertain their purity, structure and composition. An interesting, thermodynamically stable, single crystalline product from DMF soluble [(C5H5N)2V2O3·H2O] has been characterized, which indirectly supports the composition of [(C5H5N)2V2O3·H2O]. Selectively, vanadium pentoxide nanobelts have been found to be an efficient catalyst for the oxidative coupling of 2-naphthol to binaphthols under a molecular oxygen atmosphere.

Mesoporous gold and palladium nanoleaves from liquid-liquid interface: enhanced catalytic activity of the palladium analogue toward hydrazine-assisted room-temperature 4-nitrophenol reduction.

The importance of an interfacial reaction to obtain mesoporous leafy nanostructures of gold and palladium has been reported. A new synthetic strategy involving 1,4-dihydropyridine ester (DHPE) as a potential reducing agent performs exceptionally well for the desired morphologies of both the noble metals at room temperature. The DHPE in turn transforms into its oxidized aromatic form. The as-synthesized gold leaves exhibit high surface-enhanced Raman scattering activity with rhodamine 6G (R6G) due to their hyperbranched structure. It is worthwhile that as-synthesized porous architectures of palladium support the room-temperature hydrogenation of 4-nitrophenol (4-NP) by hydrazine hydrate (N2H4·H2O), reported for the first time. Furthermore, MPL exhibits exceptionally good catalytic activity toward electrooxidation of formic acid. Therefore, an aromaticity driven synthetic technique achieves a rationale to design leafy nanostructures of noble metals from the liquid-liquid interface for multifaceted applications.

Arsenate stabilized Cu2O nanoparticle catalyst for one-electron transfer reversible reaction.

The befitting capping capabilities of AsO4(3-) provide a stable Cu2O nanocatalyst from a galvanic reaction between a Cu(II) precursor salt and As(0) nanoparticles. This stable Cu2O hydrosol appears to be a suitable catalyst for the one-electron transfer reversible redox reaction between Eosin Y and NaBH4. The progress of the reaction relates to three different kinetic stages. In the presence of the new catalyst the reversible redox reaction of Eosin Y in air shows a periodic color change providing a new crowd-pleasing demonstration, i.e. a "clock reaction".

Intrinsic peroxidase-like activity of mesoporous nickel oxide for selective cysteine sensing.

Mesoporous nickel oxide nanoflowers (NiO NFs) can be easily synthesized by a two-step synthetic procedure based on modified hydrothermal (MHT) treatment of nickel acetate and ethanol amine in water followed by thermal decomposition at 350 °C for 4 h. After thermal treatment, the porosity is increased by 18% with retention of parental nickel hydroxide size. In this study, for the first time, a new catalytic application of NiO NFs has been revealed in terms of peroxidase-like activity where colorless 3,3',5,5' tetramethylbenzidine (TMB) is oxidized to blue color product in the presence of H2O2 at room temperature. Comparative study confirms that mesoporous NiO NFs exhibit superior catalytic activity to the parent analogues, i.e. Ni(OH)2 or bulk NiO. This intrinsic peroxidase-like activity from an easily synthesized inorganic nanomaterial provides an alternative to horseradish peroxidase (HRP) enzyme. The lower Michaelis constant (Km) value indicates that the catalyst NiO NFs bind efficiently to the test substrate, i.e. TMB. Interestingly, the NiO NFs-catalyzed TMB oxidation, i.e. blue color formation, has been found to be selectively and successively inhibited by a variable amount of cysteine among a set of 21 congeners. Thus our adopted simple, low-cost and novel colorimetric assay stands to be a highly efficient approach for selective detection of cysteine with a limit of detection (LOD) value of ∼1.1 µM using a simple UV-vis spectrophotometer. The proposed method also exhibits outstanding selectivity and accuracy for N-acetyl cysteine (an analogue of cysteine) estimation in real pharmaceutical samples.

Pure inorganic gel: a new host with tremendous sorption capability.

In aqueous medium at room temperature Ag(I) forms a metallogel with vanadate ions which doesn't have any carbon in the gel skeleton. The serendipitously discovered inorganic gel, which involves a reasonably large volume of water, is capable of sequestering gas, dye and toxic pollutants from water bodies.