Rudra Kumar

3D urchin-shaped Ni3(VO4)2 hollow nanospheres for high-performance asymmetric supercapacitor applications

3D urchin-shaped Ni3(VO4)2 hollow nanospheres were synthesized by a facile, template free, hydrothermal method. The size of the urchin-shaped Ni3(VO4)2 hollow nanospheres was ∼500 nm and they were composed of ∼10 nm thick sheet-like building block units. The morphological evolution was sensitive to alkaline media and ∼50 nm nanoparticles were formed when liquid ammonia was replaced by sodium hydroxide. The formation of [Ni(NH3)6]2+ complex ions (hexaamminenickel(II) ions) and subsequent slow release of nickel ions to the growing crystal seem to have resulted in the formation of hollow urchin-shaped nanostructures. The electrochemical supercapacitor properties of these two nanostructures were investigated and it was found that the urchin-shaped nanospheres exhibited better performance than the nanoparticles in all respects. The as-fabricated porous urchin-shaped Ni3(VO4)2 nanosphere electrode exhibited a specific capacity of 402.8 C g−1 at 1 A g−1 with enhanced rate capability and an excellent capacity retention of 88% after 1000 cycles. An asymmetric supercapacitor was fabricated using Ni3(VO4)2 nanospheres as the cathode and activated carbon (AC) as the anode and the electrochemical properties were studied at various scan rates in the potential range of 0.0–1.6 V. The as-fabricated asymmetric supercapacitor (Ni3(VO4)2//AC) achieved a high specific capacity (114 C g−1), energy density (25.3 W h kg−1) and power density (240 W kg−1). Moreover, this asymmetric supercapacitor displayed an excellent life cycle with 92% specific capacity retention after 1000 consecutive charge–discharge cycles. The impressive electrochemical performance of the Ni3(VO4)2 nanospheres, owing to their large surface area, pore volume and 3D structure, makes them a promising candidate for the future high energy storage systems.

Nongassing long-lasting electro-osmotic pump with polyaniline-wrapped aminated graphene electrodes.

An efficient nongassing electro-osmotic pump (EOP) with long-lasting electrodes and exceptionally stable operation is developed by using novel flow-through polyaniline (PANI)-wrapped aminated graphene (NH2-G) electrodes. The NH2-G/PANI electrode combines the excellent oxidation/reduction capacity of PANI with the exceptional conductivity and inertness of NH2-G. The flow rate varies linearly with voltage but is highly dependent on the electrode composition. The flow rates at a potential of 5 V for pristine NH2-G and PANI electrodes are 71 and 100 μL min(-1) cm(-2), respectively, which increase substantially by the use of NH2-G/PANI electrode. It increased from 125 to 182 μL min(-1) cm(-2) as the fraction of aniline increased from 66.63 to 90.90%. The maximum flux obtained is 40 μL min(-1) V(-1) cm(-2) with NH2-G/PANI-90.9 electrodes. The assembled EOP remained exceptionally stable until the electrode columbic capacity was fully utilized. The prototype shown here delivered 8.0 μL/min at a constant applied voltage of 2 V for over 7 h of continuous operation. The best EOP produces a maximum stall pressure of 3.5 kPa at 3 V. These characteristics make it suitable for a variety of microfluidic/device applications.

Facile synthesis of Cu2O microstructures and their morphology dependent electrochemical supercapacitor properties

In this study, Cu2O microcubes and microspheres were synthesized using facile hydrothermal methods by manipulating the synthesis parameters. The Cu2O microcubes (∼2 μm in diameter) were formed in presence of formic acid, whereas hierarchical Cu2O microspheres (∼5 μm in diameter) were formed in acetic acid. Transmission electron microscopy (TEM) confirmed the formation of single crystalline microcubes and polycrystalline microspheres. The possible growth mechanism suggested that microcubes were formed due to the cubic crystal structure of Cu2O and the formation kinetics, whereas microspheres were formed due to the orientational attachment of nuclei with similar aggregation velocities along every direction. The electrochemical properties of the Cu2O microcubes and microspheres were investigated to understand the role of the morphology on the supercapacitor properties. The Cu2O microcubes exhibited a higher specific capacitance, better rate capability and cycling stability as compared to microspheres, although the particle size and pore size were larger and surface area was lower. The specific capacitance of the Cu2O microcubes and microspheres were 660 and 516 F g−1, respectively, at a 1 A g−1 current density. The Cu2O microcubes showed 80% specific capacitance retention at a 5 A g−1 current density after 1000 cycles. The single crystalline nature and the presence of a smaller number of grain boundaries in the microcubes compared to the microspheres resulted in an increase in conductivity and an increase in capacitance. The results showed that the Cu2O microcubes can be a promising electrode material for high performance supercapacitors.

Free-standing NiV2S4 nanosheet arrays on a 3D Ni framework via an anion exchange reaction as a novel electrode for asymmetric supercapacitor applications

NiV2S4 nanosheet arrays are grown on Ni foam by an anion exchange reaction using Ni3(VO4)2 nanosheet arrays as a template. The height and thickness of Ni3(VO4)2 nanosheet arrays are ∼200 and ∼10 nm, respectively, and the thickness increases with increasing reaction time. The shape and size of NiV2S4 nanosheet arrays remain unchanged after the anion exchange reaction of Ni3(VO4)2 nanosheet arrays. The NiV2S4 nanosheet array exhibits better electrochemical performance than the Ni3(VO4)2 nanosheet arrays. The NiV2S4 nanosheet array based electrode exhibited a specific capacity of 639 C g−1 at 2 mA cm−2 with enhanced rate capability and an excellent capacity retention of 90.7% at 30 mA cm−2 current density after 2000 cycles. The better electrochemical performance of the sulfide based electrode as compared to the oxide one is related to low electronegativity and large size of sulfur, which creates a robust structure and better transport properties. An asymmetric supercapacitor is fabricated, using NiV2S4 nanosheet arrays as the cathode and activated carbon (AC) as the anode, which shows high specific capacity (206 C g−1), energy density (45.1 W h kg−1) and power density (240 W kg−1) at 0.3 A g−1. 90.7% specific capacity retention after 1000 consecutive charge–discharge cycles makes it a promising candidate for future high energy storage systems.

Nickel tungstate–graphene nanocomposite for simultaneous electrochemical detection of heavy metal ions with application to complex aqueous media

We show for the first time that a composite of carbon and binary transition metal oxide, in the form of a reduced graphene oxide and nickel tungstate (RGO/NiWO4) nanocomposite, is an effective material for electrochemical heavy metal ions detection. The multivalent electronic states of this composite show well-defined peaks of Cd(II), Pb(II), Cu(II) and Hg(II) during simultaneous detection, which is otherwise not observed for NiWO4 NPs and RGO sheets. Simultaneous and selective detection of heavy metal ions in drinking water as well as in complex aqueous media such as carbonated drinks, milk and fruit juices has been successfully demonstrated. Differential pulse anodic stripping voltammetric (DPASV) method was adopted for detection because it partially suppresses the background current and improves signal which leads to a low limit of detection (LOD) when compared to linear sweep voltammetry (LSV). LOD for Cd(II), Pb(II), Cu(II) and Hg(II) ions were found to be 4.7 × 10−10 M, 3.8 × 10−10 M, 4.4 × 10−10 M and 2.8 × 10−10 M for individual detection and 1.0 × 10−10, 1.8 × 10−10, 2.3 × 10−10 and 2.8 × 10−10 M, for simultaneous detection, respectively. The effect of deposition time and deposition potential on the sensing parameter was studied in acetate buffer (pH = 5.0). The better sensitivity with the high capacitive current along with individual and simultaneous electrochemical detection of RGO/NiWO4 nanocomposite is mainly attributed to its large surface area, good electronic conductivity, and better electron transport properties which lead to better catalytic response towards the heavy metal ions detection.

Free-standing Ni3(VO4)2 Nanosheet Arrays on Aminated r-GO Sheets for Supercapacitor Applications

Ni3(VO4)2 nanosheet arrays are grown on aminated r-GO sheets through a facile hydrothermal route. The growth of Ni3(VO4)2 nanosheet arrays on aminated r-GO sheets (NiV@r-GO nanocomposites) is confirmed by XRD, FESEM, TEM, XPS, and Raman spectroscopy. The length, height and thickness of the Ni3(VO4)2 nanosheet arrays are ∼100, ∼20 and ∼10 nm, respectively. The density of the Ni3(VO4)2 nanosheet arrays on the r-GO sheets is directly related to the concentration of precursor used. The generation of mesoporosity during synthesis originates from the high surface area in the NiV@r-GO nanocomposites. The high surface area causes the mesoporous NiV@r-GO nanocomposite based electrode to exhibit a specific capacity of 170 C g−1 at 0.5 A g−1, which is approximately three times higher than that of aminated r-GO sheets. The capacity retention of the NiV@r-GO nanocomposite is 97.2% at 0.5 A g−1 current density after 1400 cycles. Other than the faradic redox reaction, the superior electrochemical performance of NiV@r-GO nanocomposites over the aminated r-GO sheet based electrode is related to the three-dimensional (3D) morphology as an effect of the Ni3(VO4)2 nanosheet array, which results in a higher surface area, more active sites and greater electrolyte penetration in the electrode.

A polyaniline wrapped aminated graphene composite on nickel foam as three-dimensional electrodes for enzymatic microfuel cells

Three dimensional flow-through electrodes made of Ni foam coated with aminated graphene and polyaniline composites are studied for enzymatic glucose based microfuel cells. The composites of aminated graphene and polyaniline were prepared by in situ aniline polymerization with aminated graphene in different ratios. The prepared composites were characterized using XRD, FTIR and SEM. Pristine aminated graphene, polyaniline and various ratios of both were dip-coated on Ni foam and tested as a three-dimensional electrode in enzymatic microfuel cells. The enzyme was physically adsorbed and covalently bonded using a cross-linker on the electrode surface and in some cases; the electrode surface was functionalized before covalent attachment of enzyme and tested for an anode half-cell. The electrochemical studies demonstrated that the composite performed better than the pristine and was more promising when it was functionalized and enzymes where covalently bonded to the electrode surface. The maximum power density was observed as 118 μW cm−2 for the composite.

Micropatterned arrays of functional materials by self-organized dewetting of ultrathin polymer films combined with electrodeposition

Large area ordered dewetting offers a new approach to in situ mask generation for selective decoration with functional materials for bio-sensing and lab-on-chip applications, as demonstrated in this work with zinc oxide, copper oxide and nickel hydroxide. The ZnO structures were grown within the pores of partially dewetted polystyrene (PS) films on gold-coated surfaces by electrochemical deposition using the zinc nitrate hexahydrate precursor. The morphology of these ZnO structures is governed by the spatial confinement, i.e., membrane thickness and diameter of the mask opening and the deposition time of ZnO. Vertical and lateral confinement offered by a masking film is found to greatly affect the assembly of the growth. The morphology of microstructures and their evolution are studied to understand the process development. This work establishes controlled dewetting as a route for spatially defined growth of functional materials such as ZnO micro/nano structures which further provide strong binding sites for other functional molecules such as thiol-modified DNA. Ordered arrays of these functional microstructures are deposited over large areas by combining dewetting with a simple lithographic technique, Capillary Force Lithography (CFL). We explore liquid immersion dewetting of a thin polymer film as a lithographic method to template spatially defined growth of zinc oxide (ZnO) micro and nano-structures in the form of hierarchically organized ZnO nano-crystals. The methodology of large area patterning for selective deposition/growth promises to be a facile approach to micro/nano fabrication.