Xiao-Xia Liu

One-pot synthesis of magnetically separable ordered mesoporous carbon

Ordered mesoporous carbon materials with magnetic frameworks have been synthesized via a “one-pot” block-copolymer self-assembly strategy associated with a direct carbonization process from resol, ferric citrate and triblock copolymer F127. The effects of iron loading on framework, pore features and magnetic properties of the resultant mesostructured maghemite/carbon composites were investigated by SAXS, WXRD, TEM, N2 sorption, TG and magnetometer measurements. The results show that the mesoporous nanocomposites with a low γ-Fe2O3 content (such as 9.0 wt%) possess an ordered 2-D hexagonal (p6mm) structure, uniform mesopores (∼4.0 nm), high surface areas (up to 590 m2/g) and pore volumes (up to 0.48 cm3/g). Maghemite nanocrystals with a small particle size (∼9.3 nm) are confined in the matrix of amorphous carbon frameworks. With the increase in γ-Fe2O3 content, the surface area and pore volume of the nanocomposites decrease. The particle size of the γ-Fe2O3nanocrystals increases up to 13.1 nm. The iron oxide particles can extend from the carbon walls into mesopore channels, and hence bring a rough pore surface and gradually break down the mesoscopic regularity. The maghemite/carbon nanocomposites exhibit excellent superparamagnetic behaviors. The saturation magnetization strength can be easily adjusted from 2.5 to 12.1 emu/g by increasing the content of γ-Fe2O3. Further H2O2oxidation treatment of the magnetic nanocomposites endows plenty of oxygen-containing functional groups on the carbon surface, which improves their hydrophilic properties efficiently. The γ-Fe2O3 particles, embedding into the carbon matrix, show high stability during the H2O2oxidation process. Such modified nanocomposites with hydrophilic and magnetic framework show evidently improved adsorption properties of water and fuchsin base dye molecules in water and an easy separation procedure.

Electrochemical Codeposition of Vanadium Oxide and Polypyrrole for High-Performance Supercapacitor with High Working Voltage

Electrochemical codeposition of vanadium oxide (V2O5) and polypyrrole (PPy) is conducted from vanadyl sulfate (VOSO4) and pyrrole in their aqueous solution to get V2O5-PPy composite, during which one-dimensional growth of polypyrrole (PPy) is directed. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) are used to characterize the composite, while scanning electron microscopy (SEM) is used to investigate their morphologies. Cyclic voltammetry (CV), chronopotentiometry (CP) for galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) are used to study electrochemical activities and pseudocapacitive properties of the composite. The influences of VOSO4 to pyrrole ratio in the electro-codeposition solution on morphologies and pseudocapacitive properties of the composite are discussed. Due to the organic-inorganic synergistic effect, V2O5-PPy composite exhibits good charge-storage properties in a large potential window from -1.4 to 0.6 V vs SCE, with a specific capacitance of 412 F/g at 4.5 mA/cm(2). A model supercapacitor assembled by using the V2O5-PPy composite as the electrode materials displays a high operating voltage of 2 V and so a high energy density of 82 Wh/kg (at the power density of 800 W/kg).

Electrodeposition of vanadium oxide–polyaniline composite nanowire electrodes for high energy density supercapacitors

To meet the increasing demand for high energy density supercapacitors, it is crucial to develop positive and negative electrodes with comparable energy density. Previous studies have primarily focused on the development of positive electrodes, while negative electrodes are relatively less explored. Here we report an electro-codeposition method to synthesize a high performance negative electrode composed of a vanadium oxide (V2O5) and polyaniline (PANI) composite. Scanning electron microscopy revealed that the composite film is composed of one-dimensional polymer chains. Energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) confirmed successful incorporation of V2O5 into PANI chains. Significantly, the V2O5–PANI composite nanowires exhibited a wide potential window of 1.6 V (between −0.9 and 0.7 V vs. SCE) and a maximum specific capacitance of 443 F g−1 (664.5 mF cm−2). The flexible symmetric supercapacitor assembled with this composite film yielded a maximum energy density of 69.2 W h kg−1 at a power density of 720 W kg−1, and a maximum power density of 7200 W kg−1 at an energy density of 33.0 W h kg−1. These values are substantially higher than those of other pure V2O5 or PANI based supercapacitors. Moreover, the assembled symmetric supercapacitor device showed an excellent stability with 92% capacitance retention after 5000 cycles. The capability of synthesizing high performance composite electrodes using the electro-codeposition method could open up new opportunities for high energy density supercapacitors.

Ordered Polypyrrole Nanowire Arrays Grown on a Carbon Cloth Substrate for a High-Performance Pseudocapacitor Electrode

Highly aligned nanoarchitecture arrays directly grown on conducting substrates open up a new direction to accelerate Faradaic reactions for charge storage as well as address dead volume limitations for high-performance pseudocapacitor electrodes. Here we reported the electrochemical fabrication of well-ordered polypyrrole (PPy) nanowire arrays (NWAs) on surfaces of carbon fibers in an untreated carbon cloth to construct hierarchical structures constituted by the three-dimensional conductive carbon fiber skeleton and the atop well-ordered electroactive polymer nanowires. The morphologies, wetting behaviors, and charge-storage performances of the polymer were investigated by scanning electron microscopy, transmission electron microscopy, contact-angle measurement, cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. The well-ordered PPy NWA electrode exhibited a high specific capacitance of 699 F/g at 1 A/g with excellent rate capability, and 92.4% and 81.5% of its capacitance could be retained at 10 and 20 A/g, respectively. An extremely high energy density of 164.07 Wh/kg could be achieved by the PPy NWAs at a power density of 0.65 kW/kg. It also displayed a quite high energy density of 133.79 Wh/kg at a high power density of 13 kW/kg. The assembled symmetric supercapacitor of PPy NWAs//PPy NWAs also exhibited excellent rate capability, and only 19% of its energy density decreased when the power density increased 20 times from 0.65 to 13 kW/kg.

Morphology and Doping Engineering of Sn-Doped Hematite Nanowire Photoanodes

High-temperature activation has been commonly used to boost the photoelectrochemical (PEC) performance of hematite nanowires for water oxidation, by inducing Sn diffusion from fluorine-doped tin oxide (FTO) substrate into hematite. Yet, hematite nanowires thermally annealed at high temperature suffer from two major drawbacks that negatively affect their performance. First, the structural deformation reduces light absorption capability of nanowire. Second, this passive doping method leads to nonuniform distribution of Sn dopant in nanowire and limits the Sn doping concentration. Both factors impair the electrochemical properties of hematite nanowire. Here we demonstrate a silica encapsulation method that is able to simultaneously retain the hematite nanowire morphology even after high-temperature calcination at 800 °C and improve the concentration and uniformity of dopant distribution along the nanowire growth axis. The capability of retaining nanowire morphology allows tuning the nanowire length for optimal light absorption. Uniform distribution of Sn doping enhances the donor density and charge transport of hematite nanowire. The morphology and doping engineered hematite nanowire photoanode decorated with a cobalt oxide-based oxygen evolution reaction (OER) catalyst achieves an outstanding photocurrent density of 2.2 mA cm-2 at 0.23 V vs Ag/AgCl. This work provides important insights on how the morphology and doping uniformity of hematite photoanodes affect their PEC performance.

Hybrid material based on a coordination-complex-modified polyoxometalate nanorod (CC/POMNR) and PPy: a new visible light activated and highly efficient photocatalyst

To improve the photocatalytic activity of a coordination-complex-modified polyoxometalate (CC/POM), a new type of hybrid material (abbreviated as PPy/CC/POMNR) was fabricated by the combination of its nanorod (CC/POMNR) and polypyrrole (PPy) via a facile in situ chemical oxidation polymerization process under the initiation of ammonium persulfate (APS). Under the irradiation of visible light, PPy/CC/POMNR exhibited higher photocatalytic activity compared to CC/POMNR, PPy, and their mechanically blended products formed on the degradation of Rhodamine B (RhB). Optical and electrochemical tests showed that the enhancement of photocatalytic performance can be attributed to the high separation efficiency of the photogenerated electrons and holes on the interface of CC/POMNBs and PPy, which results from the synergistic effect between them. Furthermore, the influence of the concentration ratio between pyrrole (Py) and APS on the morphology, conductivity, and photocatalytic properties of the PPy/CC/POMNR is discussed and the optimal condition to fabricate a hybrid material with high efficiency was determined. These results suggest that the hybrid of CC/POMNR and PPy would be a feasible strategy to enhance the photocatalytic activity of CC/POMNR.

Integration of nickel–cobalt double hydroxide nanosheets and polypyrrole films with functionalized partially exfoliated graphite for asymmetric supercapacitors with improved rate capability

High-rate asymmetric supercapacitors (ASCs) made of abundant and low-cost electrode materials and operating in safe aqueous electrolytes can be attractive for electrochemical energy storage. Here, we design a new type of ASC by using pseudo-capacitive nanomaterials, Ni–Co double hydroxide (Ni–Co DH) nanosheets and polypyrrole (PPy) films, for the cathode and anode, respectively, which were integrated with a functionalized partially exfoliated graphite (FEG) current collector. Benefiting from the “super highway” for fast electron/ion transportation in hybrid systems, the as-prepared electrodes exhibit superior rate capability (2442 and 2039 F g−1 at 1 and 50 A g−1, with 83.5% retention for Ni–Co DH; 560 and 441 F g−1 at 1 and 50 A g−1, with 79% retention for PPy). The assembled ASC displays a high specific capacitance (261 F g−1 at 1 A g−1) and excellent rate capability; 77% of its initial capacitance can be retained when the current density increases 30 times from 1 to 30 A g−1. An energy density of 61.3 W h kg−1 can be achieved by the ASC at 0.65 kW kg−1. Even at an ultra-high power density of 19.5 kW kg−1, the ASC can still deliver a high energy density of 47.2 W h kg−1. Through careful control of charges which can be stored in the anode and cathode, the cycling stability of the ASC is much improved, and 91% capacitance retention can be achieved after 5000 charge/discharge cycles. These features demonstrate a new avenue for developing high-performance pseudo-capacitive electrodes and rational assembly strategies for high power/energy density charge storage devices with good cycling stability.

Analysis of ribbed-strip rolling by rigid-viscoplastic FEM

The rolling of ribbed-strips is analysed in the paper using the slightly compressible rigid-viscoplastic finite element method. The energy functional and the rigid-plastic finite element equations for the ribbed-strip rolling process are derived. The simulation results are compared with those obtained by experiments and found to be in good agreement with the latter.

Amorphous Mixed-Valence Vanadium Oxide/Exfoliated Carbon Cloth Structure Shows a Record High Cycling Stability

Previous studies show that vanadium oxides suffer from severe capacity loss during cycling in the liquid electrolyte, which has hindered their applications in electrochemical energy storage. The electrochemical instability is mainly due to chemical dissolution and structural pulverization of vanadium oxides during charge/discharge cyclings. In this study the authors demonstrate that amorphous mixed-valence vanadium oxide deposited on exfoliated carbon cloth (CC) can address these two limitations simultaneously. The results suggest that tuning the V4+ /V5+ ratio of vanadium oxide can efficiently suppress the dissolution of the active materials. The oxygen-functionalized carbon shell on exfoliated CC can bind strongly with VO x via the formation of Cuf8ffOuf8ffV bonding, which retains the electrode integrity and suppresses the structural degradation of the oxide during charging/discharging. The uptake of structural water during charging and discharging processes also plays an important role in activating the electrode material. The amorphous mixed-valence vanadium oxide without any protective coating exhibits record-high cycling stability in the aqueous electrolyte with no capacitive decay in 100 000 cycles. This work provides new insights on stabilizing vanadium oxide, which is critical for the development of vanadium oxide based energy storage devices.

Encapsulation of polyaniline in 3-D interconnected mesopores of silica KIT-6.

Composite material PANI/KIT-6, with polyaniline (PANI) chains encapsulated in the 3-D interconnected pore channels of mesoporous silica, KIT-6, has been synthesized via a gas-phase method. The composite formation and the presence of PANI inside the pore channels of KIT-6 were evidenced by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), small-angle X-ray scatter (SAXS), transmission electron microscopy (TEM), and N(2) adsorption-desorption isotherms. The PANI/KIT-6 composite showed good electrical conductivity (2.4x10(-3) S/cm) due to the formation of 3-D networks of PANI inside the 3-D interconnected channels of KIT-6. The resistance of PANI/KIT-6 composite at different relative humidities (RH) was investigated. An essentially linear relationship between the relative resistance of the composite and the relative humidity of the environment was found from 11.3% to 97.3% RH.