Xindong Wang

In situ polyaniline modified cathode material Li[Li0.2Mn0.54Ni0.13Co0.13]O2 with high rate capacity for lithium ion batteries

Lithium-rich layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 is prepared by a fast co-precipitation method and surface modified with conducting polyaniline (PANI, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt% theoretically) via in situ chemical oxidation polymerization to optimize the electrochemical properties. The uniform PANI layer with a thickness of 5 nm (10 wt%) has been successfully coated on the surface of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 particles, as observed by field-emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). The X-ray powder diffraction (XRD) results show that all the prepared samples have a typical layered hexagonal α-NaFeO2 structure. The PANI layer maintains the integrity of the surface material crystal structure of the Li[Li0.2Mn0.54Ni0.13Co0.13]O2 particles by protecting the electrodes from external erosion during continuous charge–discharge cycles. PANI-coated Li[Li0.2Mn0.54Ni0.13Co0.13]O2 electrodes present excellent electrochemical properties at room temperature. The initial discharge capacity is 313.5 mA h g−1 (0.05 C) with a coulombic efficiency of 89.01% (PANI, 10 wt%), compared with 291.9 mA h g−1 (0.05 C) for the pristine Li[Li0.2Mn0.54Ni0.13Co0.13]O2 with a coulombic efficiency of 81.31% in the potential range 2.0–4.8 V (vs. Li/Li+). The discharge capacity is retained at 282.1 mA h g−1 after 80 cycles at 0.1 C. Moreover, the PANI-coated Li[Li0.2Mn0.54Ni0.13Co0.13]O2 exhibits an excellent high rate capacity of 198.6 mA h g−1 at 10 C. The electrochemical impedance spectra (EIS) measurements reveal that the thin PANI coating layer significantly optimizes the interfacial electrochemical reaction activity by reducing the charge transfer resistance. Moreover, the special H+/Li+ exchange reaction during the proton acid doping procedure also promotes the improvement of the electrochemical performance.

Influence of over-discharge on the lifetime and performance of LiFePO4/graphite batteries

In this study, the degradation of a LiFePO4/graphite battery under an over-discharge process and its effect on further cycling stability are investigated. Batteries are over-discharged to 1.5, 1.0, 0.5 or 0.0 V and then cycled 110 times under over-discharge conditions. The batteries over-discharged to 0.5 and 0.0 V experience serious irreversible capacity losses of 12.56% and 24.88%, respectively. The same batteries lost 7.79 and 24.46% more capacity after they were further subjected to 110 cycles between 3.65 and 2.0 V at 1C/1C, respectively. This shows that a serious loss of active lithium and loss of anode material occur at 0.0 V during both over-discharging and the normal cycling stage. Dissolution and breakdown of solid electrolyte interphase (SEI) films are suggested to be the main reason for degradation under over-discharge at low voltage and further lead to a poor cycling performance. Gas generation can be found on the cycled batteries below 1 V and the gas mainly contains H2, CH4 and C2H6. The structures of LiFePO4 and graphite materials have almost no change according to the results of XRD tests. Half-cell study suggests that almost no irreversible capacity loss occurs at the LiFePO4 cathode, whereas a decline in the capacity is observed at the graphite anode, especially for the batteries over-discharging bellow 1.0 V. Evidence for fierce side reactions at 0.5 and 0.0 V is provided as well, as demonstrated by the developed rich surface chemistry and an significant impedance increase for the aged electrodes.

An oxygen evolution catalyst on an antimony doped tin oxide nanowire structured support for proton exchange membrane liquid water electrolysis

Developing catalysts with high electrocatalytic activity has recently attracted much attention due to the slow reaction kinetics for oxygen evolution reaction (OER) and poor durability under harsh operating environments. Aiming at the enhancement of oxygen electrode kinetics and durability, a facile and scalable electrospinning method is employed to fabricate antimony doped tin oxide nanowires (Sb–SnO2 NWs) as support materials for iridium oxide. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results show that the as-prepared Sb–SnO2 NW is stacked from primary Sb–SnO2 nanoparticles (Sb–SnO2 NPs) with diameters of 15–25 nm and exhibits a uniform porous nanowire structure with diameters in the range of 200–300 nm. The synthesized Sb–SnO2 NW has a BET surface area of 60 m2 g−1 and an electronic conductivity of 0.83 S cm−1. Benefiting from the porous nanowire structure and high electronic conductivity of the Sb–SnO2 NW support, the supported IrO2 catalyst exhibits significant enhancement of mass activity toward OER in acidic electrolytes compared with that of Sb–SnO2 NP supported IrO2 catalyst and pure IrO2. The improved catalytic performance for OER is further confirmed by proton exchange membrane (PEM) electrolyzer tests at 80 °C. A test of such an electrolyzer cell at 450 mA cm−2 shows good durability within a period of up to 646 h.

Pt Nanoclusters Electrodeposited on Single-Walled Carbon Nanotubes for Electrochemical Catalysis

Single-walled carbon nanotube supported Pt (Pt-SWNT) catalysts with uniform Pt nanoclusters were successfully synthesized on the carbon electrode by a pulse electrodeposition technique. The morphology of the catalysts was characterized by scanning electron microscopy analysis. The electrocatalytic activities of the catalysts toward the methanol oxidation reaction and the oxygen reduction reaction were investigated by cyclic voltammetry measurements. In comparison with Pt-XC-72 catalysts prepared by the same method, the Pt-SWNT catalysts exhibited higher electrocatalytic activity for methanol oxidation and oxygen reduction. Electrochemical active surface area and chronoamperometry results indicated that Pt-SWNT catalysts had larger active surfaces. This may be attributed to the high dispersion of Pt catalysts and the unique properties of single-walled carbon nanotube support. The substructure of Pt particles also contributed to the enhanced active surfaces. Furthermore, the electron transition mechanism in the oxygen reduction reaction at the Pt-SWNT electrode is preliminarily discussed using the rotating disk electrode analysis.

Nafion®/SiO2/m-BOT composite membranes for improved direct methanol fuel cell performance

Bentonite (BOT) has excellent hygroscopicity and large specific surface, so it is chosen as a dopant of Nafion® membranes in this paper. By using the sol–gel method, bentonite has been modified by dodecylamine and fixed to the Nafion 212 membrane to prepare a Nafion/SiO2/m-BOT composite membrane. The results of SEM and FT-IR shows that m-BOT is successfully synthesized and bound well with the Nafion 212 membrane. The limiting current density of cathode methanol oxidation indicates that the methanol permeability of the composite membrane is 20.40% lower than that of the Nafion 212 membrane. Although the conductivity of the composite membrane (6.67 × 10−2 S cm−1) declines slightly compared with that of Nafion 212 (9.91 × 10−2 S cm−1), the performance of the cell using the composite membrane (135.17 mW cm−2) is better than the Nafion 212 membrane (118.7 mW cm−2) at 55 °C. Besides, as the anode methanol concentration increases, higher performance is obtained, which indicates that the composite membrane is more suitable for cells running with a high concentration of methanol.

Nanostructured electrocatalytic materials and porous electrodes for direct methanol fuel cells

Direct methanol fuel cells (DMFCs) are promising for use in portable devices because of advantages such as high fuel energy density, low working temperature and low emission of pollutants. Nanotechnology has been used to improve the performance of DMFCs. Catalytic materials composed of small, metallic particles with unique nanostructure supported on carbons or metal oxides have been widely investigated for use in DMFCs. Despite our increased understanding of this type of fuel cell, many challenges still remain. This paper reviews the current developments of nanostructured electrocatalytic materials and porous electrodes for use in DMFCs. In particular, this review focuses on the synthesis and characterization of nanostructured catalysts and supporting materials. Both computational and experimental approaches to optimize mass transportation in porous electrodes of DMFCs, such as theoretical modeling of internal transfer processes and preparation of functional structures in membrane electrode assemblies, are introduced.

Characterization of cathode from LiNixMn2−xO4 nanofibers by electrospinning for Li-ion batteries

Ni substituted LiMn2O4 nanofiber cathode materials (LiNixMn2−xO4, x = 0.2, 0.3, 0.4, 0.5) have been prepared by a combination of electrospinning and sol–gel techniques. The nanofiber cathode materials appear to have a porous “network-like” morphology with nanosized diameters of ∼120 nm and microsized lengths of >5 μm. The structure provides short lithium diffusion paths and a large surface area, facilitating rapid charge–discharge characteristics. The Ni substitution not only mitigated the Jahn–Teller distortion effect but also greatly suppressed the Mn dissolution, which improved the rate capability and cycle performance. The LiNi0.4Mn1.6O4 nanofiber cathode exhibits excellent rate capability and cycle performance both at room temperature and at 55 °C. Thus, the LiNi0.4Mn1.6O4 nanofibers may be a potential cathode material for high rate discharge lithium ion batteries.

Suppression of methanol cross-over in novel composite membranes for direct methanol fuel cells

A series of novel composite membranes was prepared by using poly(vinyl alcohol) (PVA) with polyimide (PI) as base material and 8-trimethoxysilylpropyl glycerine ether-1,3,6-pyrenetrisulfonic acid (TSGEPS) as proton conductor for direct methanol fuel cells (DMFCs). The parameters of membranes, including water sorption, hydrolysis stability, dimensional stability, proton conductivity, and methanol permeability were studied. The proton conductivity of the membranes is in the order of 10–2 S/cm, and the membranes show better resistance to methanol permeability (1.51 × 10–7 cm2 s–1) and better selectivity (20.6 × 104 S cm–3 s) than those of Nafion115 under the same measurement conditions.

Electropolymerization of Ni(salen) on carbon nanotube carrier as a capacitive material by pulse potentiostatic method

The composites of poly[Ni(salen)] and multi-walled carbon nanotube (MWCNT) were synthesized by pulse potentiostatic method. The composites were characterized by field emission scanning electron microscopy, Fourier transform infrared spectra, and electrochemical impedance spectroscopy. The wrapping of carbon nanotubes with poly[Ni(salen)] varied significantly with anodic pulse duration. Variance of structure of poly[Ni(salen)] caused by anodic pulse duration affected the ability of absorption to solvent molecules or solvated ions, which was indicated by ν (C≡N) intensity. The ability to store/release charge of poly[Ni(salen)] caused by redox switching was evaluated in the form of low-frequency capacitance. Correlations of charge-transfer resistance/ionic diffusion resistance with potential and anodic pulse duration were investigated.

Fabrication of highly oriented ZnO nanorod arrays by galvanostatic deposition

Abstract Highly oriented ZnO nanorod arrays were successfully prepared on the indium tin oxide (ITO) substrate using a galvanostatic electrodeposition method. The ITO substrate was pretreated with ZnO nanoparticles via simple low-temperature solution route. The crystallinity, micro- structure of surface, and optical properties of the obtained ZnO were characterized by X-ray diffraction, scanning electron microscope, and transmittance spectrum. The results indicate that the average diameter of ZnO nanorod arrays is about 30 nm, and the narrow size distribution ranges from 20 to 50 nm. The nanorod arrays are growing along [001] direction with an orientation perpendicular to the substrate. When the wavelength of incident is over 380 nm, the ZnO nanorod arrays show a high optical transmission of above 95%. Furthermore, the possible growth mechanism of the nanorod arrays was discussed.