Li Hua Chen

Design of new anode materials based on hierarchical, three dimensional ordered macro-mesoporous TiO2 for high performance lithium ion batteries

As anode materials for lithium ion batteries, two three dimensionally ordered macroporous TiO2, one with disordered inter-particle mesopores formed by the aggregation of nanoparticles (3DOM) and another with inner-particle mesopores generated by a surfactant templating strategy (3DOMM), have been synthesized using poly(styrene-methyl methacrylate-3-sulfopropyl methacrylate potassium) (P(St-MMA-SPMAP)) spheres as a hard template and their electrochemical properties are compared. SEM and TEM observations reveal that both 3DOM TiO2 and 3DOMM TiO2 have well-ordered macropores and interconnected macropore walls with a regular periodicity. 3DOMM TiO2 demonstrates a specific surface area of 139 m2 g−1, which is higher than that of 3DOM TiO2 (99 m2 g−1) due to the smaller crystallite size and inner-particle mesopores. The electrolyte adsorption results show that both 3DOM TiO2 and 3DOMM TiO2 have similar adsorption capacities despite a difference in the surface area. Electrochemical impendence spectroscopy analysis shows that 3DOMM TiO2 has a lower charge transfer resistance and faster Li+ diffusion coefficient than 3DOM TiO2. Moreover, both 3DOM TiO2 and 3DOMM TiO2 possess excellent initial capacity of 248 mA h g−1 and 235 mA h g−1 at 0.2 C and 208 mA h g−1 and 202 mA h g−1 at 1 C, respectively. The reversibility study demonstrates that the 3DOMM TiO2 displays higher cycling capacity, superior rate behavior and higher Coulombic efficiency because the higher surface area provides more active sites and the presence of the inner-particle mesopores in the walls of macropores serve as a bicontinuous transport path and affords a shorter path length for diffusion of Li ions compared with the 3DOM TiO2 and its crystallite aggregated mesopores. The reversible capacity of 106 mA h g−1 observed for the 3DOMM TiO2 can be retained after 200 charge–discharge cycles at a relatively high current rate of 4 C. This cycle stability performance can be equally attributed to the crystallite size and inner-particle mesopores in the 3DOMM TiO2. Moreover, the existence of a bicontinuous porous structure in the 3DOMM TiO2 can further enhance the lithium insertion/extraction capacity at high rates. We believe that this study can shed light on the 3DOMM structure as a promising material for highly enhanced performance in lithium ion batteries.

Tracing the slow photon effect in a ZnO inverse opal film for photocatalytic activity enhancement

Highly ordered, dense and continuous ZnO inverse opal (ZnO-IO) films with different air sphere sizes have been successfully prepared by a metal salt-based sol–gel infiltration method and used to prove and demonstrate the slow photon effect occurrence to enhance the photocatalytic activity. The obtained ZnO-IO films have a pure wurzite structure with similar crystallite size according to the XRD experiments and show very ordered macroporous structures from SEM and TEM analyses. The ZnO-IO films present a photoinduced surface wettability conversion phenomenon and the wettability of the ZnO film can be tuned from superhydrophobicity to hydrophilicity after UV-vis irradiation. Compared with the ZnO film without inverse opal structure, both ZnO-IO films demonstrate highly enhanced photocatalytic activities due to the hierarchically macro–mesoporous structure and particularly the slow photon enhanced light absorption. The synergy of the slow photon effect and hierarchically porous structure of inverse opal itself results in the highest photocatalytic activity at an incident light angle of θ = 40°. Moreover, our results suggest that the slow photon effect occurring at the red edge of PBG exhibits a higher photocatalytic reaction rate than that at the blue edge of PBG. The extraordinary enhancement of the photocatalytic activity via changing the incident light angle reveals that the slow photon effect does take place and further dramatically enhance the photocatalytic activity of ZnO-IO films. This work may open an exciting door to all the fields related to light absorption, such as solar cells, and optical and electro-optical devices.

Unique walnut-shaped porous MnO2/C nanospheres with enhanced reaction kinetics for lithium storage with high capacity and superior rate capability

This work is realized in the frame of a program for Changjiang Scholars and Innovative Research Team (IRT_15R52) of Chinese Ministry of Education. B. L. Su acknowledges the Chinese Central Government for an “Expert of the State” position in the Program of the “Thousand Talents” and a Life Membership at the Clare Hall, Cambridge and the financial support of the Department of Chemistry, University of Cambridge. Y. Li acknowledges Hubei Provincial Department of Education for the “Chutian Scholar” program. T. Hasan acknowledges funding from the Royal Academy of Engineering (Graphlex) and an Impact Acceleration Award (GRASS). This work is also financially supported by the National Science Foundation for Young Scholars of China (No. 21301133 and 51302204), International Science & Technology Cooperation Program of China (2015DFE52870) and and Self-determined and Innovative Research Funds of the SKLWUT (2015‐ZD‐7). The authors also would like to thank Dr. Bin-Jie Wang from Shanghai Nanoport (FEI, Shanghai) for TEM analysis, and thank Hang Ping from Wuhan University of Technology for the TGA/DSC tests.

Engineering 3D bicontinuous hierarchically macro-mesoporous LiFePO4/C nanocomposite for lithium storage with high rate capability and long cycle stability

A highly crystalline three dimensional (3D) bicontinuous hierarchically macro-mesoporous LiFePO4/C nanocomposite constructed by nanoparticles in the range of 50~100 nm via a rapid microwave assisted solvothermal process followed by carbon coating have been synthesized as cathode material for high performance lithium-ion batteries. The abundant 3D macropores allow better penetration of electrolyte to promote Li+ diffusion, the mesopores provide more electrochemical reaction sites and the carbon layers outside LiFePO4 nanoparticles increase the electrical conductivity, thus ultimately facilitating reverse reaction of Fe3+ to Fe2+ and alleviating electrode polarization. In addition, the particle size in nanoscale can provide short diffusion lengths for the Li+ intercalation-deintercalation. As a result, the 3D macro-mesoporous nanosized LiFePO4/C electrode exhibits excellent rate capability (129.1u2009mA h/g at 2 C; 110.9u2009mA h/g at 10 C) and cycling stability (87.2% capacity retention at 2 C after 1000 cycles, 76.3% at 5 C after 500 cycles and 87.8% at 10 C after 500 cycles, respectively), which are much better than many reported LiFePO4/C structures. Our demonstration here offers the opportunity to develop nanoscaled hierarchically porous LiFePO4/C structures for high performance lithium-ion batteries through microwave assisted solvothermal method.

Tunable macro-mesoporous ZnO nanostructures for highly sensitive ethanol and acetone gas sensors

Tunable macro–mesoporous ZnO (M/m-ZnO) nanostructures with a wurtzite hexagonal structure have been successfully synthesized using polymer colloids as a hard template and 20–40 nm ZnO nanoparticles as a precursor via controlling the ratios of colloids and ZnO nanoparticles. The as-prepared macro–mesoporous ZnO nanostructures are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Gas sensing performance is carried out for ethanol and acetone at different temperatures and concentrations. The gas sensing results show that the tunable M/m-ZnO nanostructures exhibit excellent gas sensing performances because the hierarchical macro–mesopores provide a large contacting surface area for electrons, oxygen and target gas molecules, offer smooth transport channels for target gas diffusion and finally enhance the gas molecular diffusion kinetics. The M/m-ZnO-600 nm demonstrates the best performance for ethanol and acetone detection. In addition, the sensor based on M/m-ZnO-600 nm gives obvious tendencious selectivity and a good repeatability and long-term stability to acetone at the optimum temperature of 300 °C. This work suggests that the macro–mesoporous ZnO is a potential material for advanced gas sensing.

Hollow Cu2O microspheres with two active {111} and {110} facets for highly selective adsorption and photodegradation of anionic dye

Hollow Cu2O microspheres (0.7 to 4 μm in diameter) with two active {111} and {110} facets have been prepared in water/ethylene glycol (H2O/EG) solution via a fast hydrothermal route in only 1 h. Due to the dangling “Cu” atoms in the highly active {111} and {110} facets, the microspheres demonstrate preferential selective adsorption and photodegradation for negatively charged methyl orange (MO), comparing to cationic rhodamine B (RhB) and neutral phenol. The 0.7 μm hollow Cu2O microspheres show the best adsorption capacity and photodegradation performance for MO removal: 49% MO can be adsorbed in 60 min and 99.8% MO can be fully removed under visible light illumination in 80 min, owing to the two active {110} and {111} facets and hollow structure. To exactly evaluate the photocatalytic efficiency, a new methodology is proposed by deducting the adsorption effect. The results show that in spite of 99.2% MO is removed from the solution under visible light illumination in 60 min, 14% MO is still adsorbed on the catalyst, which can be totally removed under further 20 min illumination. Our synthesis strategy presents a new opportunity for the preparation of hollow structures with high active facets. And the proposed accurate evaluation methodology may be extended to other photocatalysts with high adsorption capability for organic pollutants.

Insight into the positive effect of porous hierarchy in S/C cathodes on the electrochemical performance of Li-S batteries

Hierarchically porous carbon-supported sulfur material is widely used as a cathode for Li-S batteries because its large surface area and rich porous system provide high sulfur loading, resulting in high specific capacity with stable charge/discharge cycling performance. However, limited attention has been paid to whether the structure of hierarchical porous system affects the final electrochemical performance of Li-S/C batteries. Herein, we present hierarchically structured carbon (WSAC) with varied amounts of mesopores and micropores as a sulfur container for Li-S batteries. It is found that the relationship between electrochemical performance and percentage of microporous volume obeys a volcano distribution, which indicates that the volume percentage of microporous in the meso-microporous structure could be suitably tuned to achieve the desired electrochemical performance. Such S/hierarchically meso-microporous carbon (i.e., WSAC-8) with moderate microporous volume percentage (68.3%) shows high initial specific capacity (1375 mA h g-1) and stable charge/discharge performance (942.6 mA h g-1 after 200 cycles at 0.5C). In particular, WSAC-8 also presented superior capacity behavior at high rate capability, with final capacity as high as 800.1 and 758.7 mA h g-1 for 1C and 2C, respectively.