K.S. Hui

3D porous layered double hydroxides grown on graphene as advanced electrochemical pseudocapacitor materials

A 3D hybrid nickel-aluminum layered double hydroxide (NiAl-LDH)–graphene nanosheets (GNS) composite as a supercapacitor material has been fabricated by in situ deposition of LDH nanosheets on graphene oxide (GO) through a liquid phase deposition method. The results reveal that NiAl-LDH homogeneously grew on the surface of GNS as spacers to keep the neighboring sheets separate. Optimum effects could be achieved when feeding ratio, reaction time and temperature are tuned. The obtained porous GNS/NiAl-LDH composite exhibited high-capacitance performance with a specific capacitance of 1255.8 F g−1 at a current density of 1 A g−1 and 755.6 F g−1 at 6 A g−1, respectively. Moreover, the composite exhibited excellent cycling performance with an increase of 6% capacitance compared with the initial capacitance after 1500 cycle tests. Such high specific capacitance, rate capability and exceptional cycling ability of the composite offered great promise in energy storage device applications.

Self-assembled free-standing three-dimensional nickel nanoparticle/graphene aerogel for direct ethanol fuel cells

In this work, we report a low-cost technique for fabrication of a simple three-dimensional (3D) free-standing nickel nanoparticle/graphene aerogel with a graphene sheet network. The 3D composite architecture was formed through the self-assembly aggregation of graphene accompanied by nickel nanoparticle in situ loading on the graphene sheet during the hydrothermal reduction of graphene oxide and Ni ions. The obtained composite architecture was characterized using X-ray diffraction, Fourier transform infrared, and scanning electron microscopy. The electrocatalytic properties of the as-synthesized Ni/graphene aerogel for ethanol oxidation were investigated using cyclic voltammetry and chronoamperometry. A high peak current density of about 6 mA cm−2 for ethanol oxidation was recorded during ethanol oxidation under the test condition of adding 0.1 M ethanol in 0.1 M NaOH solution. This result revealed excellent electrocatalytic activity for ethanol oxidation, which shows great potential for direct application in ethanol fuel cells. This study provides a guide to preparing well-defined sponge-like three-dimensional metallic/graphene nanoarchitectures for fuel cell applications.

3D hierarchical porous graphene aerogel with tunable meso-pores on graphene nanosheets for high-performance energy storage.

New and novel 3D hierarchical porous graphene aerogels (HPGA) with uniform and tunable meso-pores (e.g., 21 and 53 nm) on graphene nanosheets (GNS) were prepared by a hydrothermal self-assembly process and an in-situ carbothermal reaction. The size and distribution of the meso-pores on the individual GNS were uniform and could be tuned by controlling the sizes of the Co3O4 NPs used in the hydrothermal reaction. This unique architecture of HPGA prevents the stacking of GNS and promises more electrochemically active sites that enhance the electrochemical storage level significantly. HPGA, as a lithium-ion battery anode, exhibited superior electrochemical performance, including a high reversible specific capacity of 1100 mAh/g at a current density of 0.1 A/g, outstanding cycling stability and excellent rate performance. Even at a large current density of 20 A/g, the reversible capacity was retained at 300 mAh/g, which is larger than that of most porous carbon-based anodes reported, suggesting it to be a promising candidate for energy storage. The proposed 3D HPGA is expected to provide an important platform that can promote the development of 3D topological porous systems in a range of energy storage and generation fields.

Vertically stacked bilayer CuCo2O4/MnCo2O4 heterostructures on functionalized graphite paper for high-performance electrochemical capacitors

Cobaltite systems with spinel structures are promising cathode materials for next-generation high-performance electrochemical capacitors because of their high electrochemical stability. However, increasing the mass loading of active materials without sacrificing the geometry of the nanostructures remains a challenge. In this study, we propose vertically stacked bilayer spinel heterostructures constructed from hierarchical CuCo2O4/MnCo2O4 on graphite paper as highly capable supercapacitor electrodes. A two-step hydrothermal method with post annealing treatment is used in the preparation of the heterostructures. The CuCo2O4/MnCo2O4 electrode delivers a remarkable specific capacitance of 1434 F g−1 at 0.5 A g−1, considerable high-rate capability (810 F g−1 at 15 A g−1), and an excellent cycling stability, maintaining 81.4% at 10 A g−1 after 5000 cycles. An electrochemical capacitor device operating at 1.6 V is also constructed using CuCo2O4/MnCo2O4 and graphene as positive and negative electrodes, respectively. The device shows a high energy density of 42.1 W h kg−1 at a power density of 400 W kg−1, as well as good cycling stability (88.4% retention after 10 000 cycles). The concept of stacking heteronanostructures can potentially enrich the electrochemical performance of metal oxides for next-generation electrochemical capacitors.

Synthesis, band-gap tuning, structural and optical investigations of Mg doped ZnO nanowires

In the present investigation, synthesis, characterizations and the tuning of the optical band gap (Eg) of ZnO nanowires (NWs) has been successfully achieved by introducing Mg as an intentional impurity with varying concentrations Zn1−xMgxO (x = 0, 5, 10 and 20 at. %). Although the ionic radius of Mg2+ (0.57 A) is close to that of Zn2+ (0.60 A) the crystal structure difference and large lattice mismatch between ZnO (wurtzite, 3.25 A) and MgO (rock salt, 4.22 A) causes phase segregation in Zn1−xMgxO with Mg compositions between 37% < x < 62%. Optical measurements of the as grown and Mg doped ZnO NWs shows the optical bandgap tunability from ∼3.35 eV to 3.65 eV as a function of the Mg content. Rietveld refinement of XRD data for the Mg doped ZnO nanowires confirm the wurtzite structure and a continuous compaction of the lattice (in particular, the c-axis parameter) with increasing Mg content. Strong UV with weak visible emission by PL studies establishes the sensitivity of the nanostructures yield, size and band gap to the intentional impurity. This tunability of the band gap of ZnO NWs with an intentional impurity could eventually be useful for optoelectronic applications.

Structural and optical properties of ZnS thin films deposited by RF magnetron sputtering

Zinc sulfide [ZnS] thin films were deposited on glass substrates using radio frequency magnetron sputtering. The substrate temperature was varied in the range of 100°C to 400°C. The structural and optical properties of ZnS thin films were characterized with X-ray diffraction [XRD], field emission scanning electron microscopy [FESEM], energy dispersive analysis of X-rays and UV-visible transmission spectra. The XRD analyses indicate that ZnS films have zinc blende structures with (111) preferential orientation, whereas the diffraction patterns sharpen with the increase in substrate temperatures. The FESEM data also reveal that the films have nano-size grains with a grain size of approximately 69 nm. The films grown at 350°C exhibit a relatively high transmittance of 80% in the visible region, with an energy band gap of 3.79 eV. These results show that ZnS films are suitable for use as the buffer layer of the Cu(In, Ga)Se2 solar cells.

One-step fabrication of multifunctional composite polyurethane spider-web-like nanofibrous membrane for water purification

A stable silver-doped fly ash/polyurathene (Ag-FA/PU) nanocomposite multifunctional membrane is prepared by a facile one-step electrospinning process using fly ash particles (FAPs). Colloidal solution of PU with FAPs and Ag metal precursor was subjected to fabricate nanocomposite spider-web-like membrane using electrospinning process. Presence of N,N-dimethylformamide (solvent of PU) led to reduce silver nitrate into Ag NPs. Incorporation of Ag NPs and FAPs through electrospun PU fibers is proven through electron microscopy and spectroscopic techniques. Presence of these NPs on PU nanofibers introduces several potential physicochemical properties such as spider-web-like nano-neeting for NPs separation, enhanced absorption capacity to remove carcinogenic arsenic (As) and toxic organic dyes, and antibacterial properties with reduce bio-fouling for membrane filter application. Preliminary observations used for above-mentioned applications for water treatment showed that it will be an economically and environmentally friendly nonwoven matrix for water purification. This simple approach highlights new avenues about the utilization of one pollutant material to control other pollutants in scalable and inexpensive ways.

Facile synthesis of manganese carbonate quantum dots/Ni(HCO3)2–MnCO3 composites as advanced cathode materials for high energy density asymmetric supercapacitors

We have developed a high performance supercapacitor cathode electrode composed of well dispersed MnCO3 quantum dots (QDs, ∼1.2 nm) decorated on nickel hydrogen carbonate–manganese carbonate (Ni(HCO3)2–MnCO3) hedgehog-like shell@needle (MnCO3 QDs/NiH–Mn–CO3) composites directly grown onto a 3D macro-porous nickel foam as a binder-free supercapacitor electrode by a facile and scalable hydrothermal method. The MnCO3 QDs/NiH–Mn–CO3 composite electrode exhibited a remarkable maximum specific capacitance of 2641.3 F g−1 at 3 A g−1 and 1493.3 F g−1 at 15 A g−1. Moreover, the asymmetric supercapacitor with MnCO3 QDs/NiH–Mn–CO3 composites as the positive electrode and graphene as the negative electrode showed an energy density of 58.1 W h kg−1 at a power density of 900 W kg−1 as well as excellent cycling stability with 91.3% retention after 10 000 cycles, which exceeded the energy densities of most previously reported nickel or manganese oxide/hydroxide-based asymmetric supercapacitors. The ultrahigh capacitive performance is attributed to the presence of the high surface area core–shell nanostructure, the well dispersed MnCO3 quantum dots, and the high conductivity of MnCO3 quantum dots as well as the synergetic effect between multiple transition metal ions. The superior supercapacitive performance of the MnCO3 QDs/NiH–Mn–CO3 composites makes them promising cathode materials for high energy density asymmetric supercapacitors.

An architectured TiO2 nanosheet with discrete integrated nanocrystalline subunits and its application in lithium batteries

An anatase TiO2 nanosheet with discrete integrated subunits was successfully synthesized via a rapid annealing treatment, which could be classified as non-equilibrium conditions on an as-prepared hydrogen titanate nanosheet. This unique TiO2 nano-object is uniform in length and width as a “sheet-like” shape which is integrated with numerous discrete nanocrystalline subunits. In contrast with the internal architecture of the TiO2 nanosheets produced under equilibrium and non-equilibrium conditions, the local structure collapse transformation mechanism from the hydrogen titanate nanosheet to the anatase TiO2 nanosheet with internal architecture is discussed. This unique anatase nano-object electrode exhibits high reversible lithium ion storage capacities and superior cyclic capacity retention at a high current rate. Such enhanced lithium storage performance could be attributed to the discrete subunits aggregation allowing efficient Li+ ion diffusion and the interior anisotropy in the nanosheet can be more effective to buffer the volume variation during the lithium insertion/desertion cycle.

Deposition of Pd/graphene aerogel on nickel foam as a binder-free electrode for direct electro-oxidation of methanol and ethanol

We reported a simple and green method to fabricate various palladium (0.8, 2.17, 7.65 wt%) loaded graphene aerogel deposited on nickel foam (Pd/GA/NF) as binder-free direct electrodes for electro-oxidation of methanol and ethanol. L-Ascorbic acid (vitamin C, VC) was used as a reducing agent in the process under a mild temperature of 40 °C. The morphology, chemical composition, and electrochemical performance of the prepared electrodes were characterized by optical microscopy, SEM/EDX, TEM, XRD, XPS, XRF, and cyclic voltammetry (CV), respectively. The XPS results revealed that both graphene oxide and Pd ions were simultaneously reduced by VC. The CV analysis revealed that the 7.65 wt% Pd/GA/NF electrode showed a maximum peak current density of 798.8 A g−1 (forward to backward peak current density ratio (If/Ib) of 3.11), and 874 A g−1 (If/Ib of 2.72) in methanol and ethanol electro-oxidation, respectively. The catalytic performance of the electrodes was enhanced with increasing the Pd loading. The results indicated that the 7.65 wt% Pd/GA/NF electrode exhibited a good electrocatalytic activity and an outstanding stability for alcohol electro-oxidation. The prolong CV scanning study (over 1000 cycles) showed that the 7.65 wt% Pd/GA/NF electrode achieved a better overall performance and stability in ethanol oxidation compared to methanol oxidation. The proposed electrode preparation method has a great potential for preparing various binder-free catalytic electrodes, which would be beneficial to the development of fuel cell application.