Gregory Lui

Multifunctional TiO2–C/MnO2 Core–Double-Shell Nanowire Arrays as High-Performance 3D Electrodes for Lithium Ion Batteries

The unique TiO2-C/MnO2 core-double-shell nanowires are synthesized for the first time using as anode materials for lithium ion batteries (LIBs). They combine both advantages from TiO2 such as excellent cycle stability and MnO2 with high capacity (1230 mA h g(-1)). The additional C interlayer intends to improve the electrical conductivity. The self-supported nanowire arrays grown directly on current-collecting substrates greatly simplify the fabrication processing of electrodes without applying binder and conductive additives. Each nanowire is anchored to the current collector, leading to fast charge transfer. The unique one-dimensional core-double-shell nanowires exhibit enhanced electrochemical performance with a higher discharge/charge capacity, superior rate capability, and longer cycling lifetime.

Graphene-wrapped hierarchical TiO2 nanoflower composites with enhanced photocatalytic performance

Graphene-wrapped titanium dioxide nanoflower composites (G–TiO2) consisting of nanosheets and nanoparticles were synthesized using a two-step solvo/hydrothermal process. Materials were characterized using SEM, TEM, high-resolution TEM (HRTEM), XRD, Raman spectroscopy, and FTIR. Further analysis was performed using Branauer–Emmett–Teller (BET) specific surface area analysis, electrochemical impedance spectroscopy (EIS), UV-Vis spectroscopy, and diffuse reflectance UV-Vis spectroscopy. Photocatalytic activity was determined by the photo-degradation of methylene blue under UV irradiation. Results show that the TiO2 nanoflower exhibits a higher photocatalytic activity than commercial P25 by a factor of 1.49. This is attributed to the highly crystalline, hierarchical nature of the nanoflower structure, which provides improved charge transport and a reduced recombination rate of photo-generated electron–hole pairs. After wrapping with graphene, the G–TiO2 composite can further improve the photocatalytic performance by providing a planar conjugated surface for dye adsorption, by further reducing recombination through accepting electrons from TiO2, and by causing a red shift in light absorption. The highest photocatalytic performance was found using a graphene loading of 5 wt%, which outperforms commercial P25 by a factor of 3.4.

Pomegranate‐Inspired Design of Highly Active and Durable Bifunctional Electrocatalysts for Rechargeable Metal–Air Batteries

Rational design of highly active and durable electrocatalysts for oxygen reactions is critical for rechargeable metal-air batteries. Herein, we report the design and development of composite electrocatalysts based on transition metal oxide nanocrystals embedded in a nitrogen-doped, partially graphitized carbon framework. Benefiting from the unique pomegranate-like architecture, the composite catalysts possess abundant active sites, strong synergetic coupling, enhanced electron transfer, and high efficiencies in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The Co3O4-based composite electrocatalyst exhibited a high half-wave potential of 0.842 V for ORR, and a low overpotential of only 450 mV at the current density of 10 mA cm(-2) for OER. A single-cell zinc-air battery was also fabricated with superior durability, holding great promise in the practical implementation of rechargeable metal-air batteries.

Flexible Rechargeable Zinc‐Air Batteries through Morphological Emulation of Human Hair Array

An electrically rechargeable, nanoarchitectured air electrode that morphologically emulates a human hair array is demonstrated in a zinc-air battery. The hair-like array of mesoporous cobalt oxide nanopetals in nitrogen-doped carbon nanotubes is grown directly on a stainless-steel mesh. This electrode produces both flexibility and improved battery performance, and thus fully manifests the advantages of flexible rechargeable zinc-air batteries in practical applications.

Paper-based all-solid-state flexible micro-supercapacitors with ultra-high rate and rapid frequency response capabilities

Paper-based flexible supercapacitors (SCs) have attracted great attention as they enable the realization of next-generation bendable, light-weight, and environmentally-friendly portable electronics. However, conventional paper-based SCs adopt a sandwich-like structure suffering from poor rate performance, slow frequency response and difficulty in direct integration with other micro-devices. We report here for the first time paper-based all-solid-state flexible planar micro-supercapacitors (MSCs) using poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-CNT/Ag as the electrode material by the inkjet printing technique. The as-fabricated paper-based all-solid-state flexible MSCs deliver the best rate capability among all reported paper-based MSCs/SCs (up to 10 000 mV s−1), fast frequency response (relaxation time constant τ0 = 8.5 ms), high volumetric specific capacitance (23.6 F cm−3) and long cycle stability (92% capacitance retention after 10 000 cycles), which shows a strong dependence on the film thickness and the interdigitated spacing between neighbouring fingers, respectively. Furthermore, the series and parallel connections reveal that the as-prepared paper-based MSCs obey the basic theorem of series and parallel connections of capacitors, respectively. The combination of the simple fabrication technology and excellent performances presented here not only make paper-based all-solid-state flexible MSCs an attractive candidate for powering future flexible portable electronics, but also provide important references for the design and fabrication of other high-performance flexible energy storage devices.

Carbon-Coated Silicon Nanowires on Carbon Fabric as Self-Supported Electrodes for Flexible Lithium-Ion Batteries

A novel self-supported electrode with long cycling life and high mass loading was developed based on carbon-coated Si nanowires grown in situ on highly conductive and flexible carbon fabric substrates through a nickel-catalyzed one-pot atmospheric pressure chemical vapor deposition. The high-quality carbon coated Si nanowires resulted in high reversible specific capacity (∼3500 mA h g-1 at 100 mA g-1), while the three-dimensional electrodes unique architecture leads to a significantly improved robustness and a high degree of electrode stability. An exceptionally long cyclability with a capacity retention of ∼66% over 500 cycles at 1.0 A g-1 was achieved. The controllable high mass loading enables an electrode with extremely high areal capacity of ∼5.0 mA h cm-2. Such a scalable electrode fabrication technology and the high-performance electrodes hold great promise in future practical applications in high energy density lithium-ion batteries.

Self-Supported Single Crystalline H2Ti8O17 Nanoarrays as Integrated Three-Dimensional Anodes for Lithium-Ion Microbatteries

Well-ordered, one-dimensional H2Ti2O5, H2Ti8O17, TiO2-B, and anatase TiO2/TiO2-B nanowire arrays were innovatively designed and directly grown on current collectors as high performance three dimensional (3D) anodes for binder and carbon free lithium ion batteries (LIBs). The prepared thin nanowires exhibited a single crystalline phase with highly uniform morphologies, diameters ranging from 70-80 nm, and lengths of around 15 μm. Specifically, reversible Li insertion and extraction reactions around 1.6-1.8 V with initial intercalation capacities of 326 and 271 mA h g(-1) at a cycling rate of 0.1 C (where 1 C = 335 mA g(-1)) were observed for H2Ti8O17 and TiO2-B nanowire arrays, respectively. Among the four compounds investigated, the H2Ti8O17 nanowire electrode demonstrated optimal cycling stability, delivering a high specific discharge capacity of 157.8 mA h g(-1) with a coulombic efficiency of 100%, even after the 500th cycle at a current rate of 1 C. Furthermore, the H2Ti8O17 nanowire electrode displayed superior rate performance with rechargeable discharge capacities of 127.2, 111.4, 87.2, and 73.5 mA h g(-1) at 5 C, 10 C, 20 C, and 30 C, respectively. These results present the potential opportunity for the development of high-performance LIBs based on nanostructured Ti-based anode materials in terms of high stability and high rate capability.

α-NiS grown on reduced graphene oxide and single-wall carbon nanotubes as electrode materials for high-power supercapacitors

α-NiS nano-particles grown on reduced graphene oxide (rGO) and single-wall carbon nanotubes (SWNTs) as high-performance electrode materials for supercapacitors have been synthesized by a one-step solvothermal method. Compared to rGO, the introduction of SWNTs results in an obvious improvement of the electronic conductivity and keeps the α-phase crystal growth of NiS as well. The obtained NiS/SWNTs composite possesses outstanding electrochemical performance, giving high specific capacitances of 1110 F g−1 and 825 F g−1 at high current densities of 5 A g−1 and 50 A g−1, respectively, along with excellent cycling stability. Compared with rGO, the incorporation of SWNTs is more advantageous not only for preventing the aggregation of NiS particles but also for the construction of a highly conductive network, which allows neighboring NiS particles to achieve excellent high rate capacitances.

Multigrain electrospun nickel doped lithium titanate nanofibers with high power lithium ion storage

Novel in situ nickel doped 1-D lithium titanate nanofibers (Li4Ti5−xNixO12, where x = 0, 0.05 and 0.1) have been successfully synthesized using a facile electrospinning process. Physical characterization reveals that nickel is homogeneously incorporated into the lattice of lithium titanate nanofibers (LTONFs) which significantly improves their properties yielding outstanding electrochemical performance in a lithium ion battery at high power rates and significant reduction in the voltage gap between the oxidation and reduction peaks. A capacity of 190 mA h g−1 has been obtained at 0.2C for the 10% nickel doped nanofibers (Ni-LTONF10), which is higher than the theoretical capacity of pristine lithium titanate (175 mA h g−1) and they also show superior rate capability resulting in 63 mA h g−1 obtained at 50C, which is 20 times higher than that of un-doped pristine LTONFs and lithium titanate nanoparticles (LTONPs). Finally, a hybrid supercapacitor is fabricated using Ni-LTONF10, showing superior energy density at high power density.

Highly Oriented Graphene Sponge Electrode for Ultra High Energy Density Lithium Ion Hybrid Capacitors

Highly oriented rGO sponge (HOG) can be easily synthesized as an effective anode for application in high-capacity lithium ion hybrid capacitors. X-ray diffraction and morphological analyses show that successfully exfoliated rGO sponge on average consists of 4.2 graphene sheets, maintaining its three-dimensional structure with highly oriented morphology even after the thermal reduction procedure. Lithium-ion hybrid capacitors (LIC) are fabricated in this study based on a unique cell configuration which completely eliminates the predoping process of lithium ions. The full-cell LIC consisting of AC/HOG-Li configuration has resulted in remarkably high energy densities of 231.7 and 131.9 Wh kg(-1) obtained at 57 W kg(-1) and 2.8 kW kg(-1). This excellent performance is attributed to the lithium ion diffusivity related to the intercalation reaction of AC/HOG-Li which is 3.6 times higher that of AC/CG-Li. This unique cell design and configuration of LIC presented in this study using HOG as an effective anode is an unprecedented example of performance enhancement and improved energy density of LIC through successful increase in cell operation voltage window.