nhua Yu

Direct Reduction of Graphene Oxide by Ni Foam as a High-Capacitance Supercapacitor Electrode

Three dimensional reduced graphene oxide (RGO)/Ni foam composites are prepared by a facile approach without using harmful reducing agents. Graphene oxide is reduced by Ni foam directly in its aqueous suspension at pH 2 at room temperature, and the resultant RGO sheets simultaneously assemble around the pillars of the Ni foam. The RGO/Ni foam composite is used as a binder-free supercapacitor electrode and exhibits high electrochemical properties. Its areal capacitance is easily tuned by varying the reduction time for different RGO loadings. When the reduction time increases from 3 to 15 days, the areal capacitance of the composite increases from 26.0 to 136.8 mF cm(-2) at 0.5 mA cm(-2). Temperature is proven to be a key factor in influencing the reduction efficiency. The composite prepared by 5 h reduction at 70 °C exhibits even better electrochemical properties than its counterpart prepared by 15 day reduction at ambient temperature. The 5 h RGO/Ni foam composite shows an areal capacitance of 206.7 mF cm(-2) at 0.5 mA cm(-2) and good rate performance and cycling stability with areal capacitance retention of 97.4% after 10000 cycles at 3 mA cm(-2). Further extending the reduction time to 9 h at 70 °C, the composite shows a high areal capacitance of 323 mF cm(-2) at 0.5 mA cm(-2). Moreover, the good rate performance and cycling stability are still maintained.

Preparation of PAN-based carbon nanofibers by hot-stretching

Partially aligned and oriented polyacrylonitrile (PAN) nanofibers were electrospun from PAN solution in dimethylformamide (DMF) for the preparation of carbon nanofibers. The as-spun polyacrylonitrile nanofibers were hot-stretched by weighing metal in a temperature controlled oven to improve its crystallinity and molecular orientation. Then they were stabilized at 250°C under stress, and carbonized at 1000°C in N2 atmosphere by fixing the length of the stabilized nanofibers to convert them into carbon nanofibers. The result showed that the average diameter of the carbon nanofibers was 140 nm. The degree of crystallinity of the stretched fibers confirmed by X-ray diffraction analysis was enhanced 4-fold in comparison with that of as-spun fibers. The improved fiber alignment and crystallinity resulted in the increased modulus and tensile strength of the nanofibers as much as 5.7-fold and 4.7-fold, respectively. The modulus and tensile strength the of carbon nanofiber increased up to 2243 ± 120 MPa and 170 ± 15.8 GPa, respectively. Thus, the hot-stretched nanofiber can be used as a potential precursor to produce high-performance carbon composites.

Effects of electrolytes on the capacitive behavior of nitrogen/phosphorus co-doped nonporous carbon nanofibers: an insight into the role of phosphorus groups

The capacitive properties of nitrogen/phosphorus co-doped nonporous carbon nanofibers and nitrogen doped nonporous carbon nanofibers are comprehensively and comparatively investigated in different aqueous electrolytes in order to identify the role of phosphorus groups in improving the capacitive performance of carbon. The introduction of phosphorus groups is favourable for the adsorption of electrolyte ions onto the carbon surface, especially protons, and thus greatly enhances the electric double layer capacitance.

Carbon nanotube@layered nickel silicate coaxial nanocables as excellent anode materials for lithium and sodium storage

Layered nickel silicate provides massive interlayer space similar to graphite for the insertion and extraction of lithium ions and sodium ions; however, the poor electrical conductivity limits its electrochemical applications in energy storage devices. Herein, carbon nanotube@layered nickel silicate (CNT@NiSiOx) coaxial nanocables with flexible nickel silicate nanosheets grown on conductive carbon nanotubes (CNTs) are synthesized by a mild hydrothermal method. CNTs serve as conductive cables to improve the electron transfer performance of nickel silicate nanosheets, resulting in reduced contact and charge-transfer resistances. In addition to a high specific surface area, short ion diffusion distance and good electrical conductivity, one-dimensional coaxial nanocables have a stable structure to sustain volume change and avoid structure destruction during the charge–discharge process. As an anode material for lithium storage, the first cycle charge capacity of the CNT@NiSiOx nanocables reaches 770 mA h g−1 with the first cycle coulombic efficiency as high as 71.5%. Even after 50 cycles, the charge capacity still reaches 489 mA h g−1 at a current density of 50 mA g−1, which is nearly 87% and 360% higher than those of the NiSiOx/CNT mixture and nickel silicate nanotube, respectively. As anode materials for sodium storage, the coaxial nanocables exhibit a high initial charge capacity of 576 mA h g−1, which even retains 213 mA h g−1 at 20 mA g−1 after 16 cycles.

Enhanced lithium storage capability of a dual-phase Li4Ti5O12–TiO2–carbon nanofiber anode with interfacial pseudocapacitive effect

Hydrothermal treatments of electrospun titanium dioxide/carbon nanofibers (TiO2/CNFs) in LiOH solution were performed in a temperature range of 130–190 °C, and then followed by a thermal treatment at 600 °C in N2 atmosphere. The changes in morphologies, microstructures and compositions as well as the electrochemical performances with hydrothermal temperatures were investigated for all samples. The morphological and compositional characterizations showed that the surfaces of the CNFs-matrix were covered by numerous nanoparticles with size distributions of 25–100 nm. For the sample hydrothermally treated at 150 °C (denoted as LC-150), these nanoparticles (∼25 nm) were composed of well-crystalline spinel Li4Ti5O12 and anatase TiO2 with abundant phase interfaces and grain boundaries, which can induce the interfacial pseudocapacitive effect. Therefore, the as-prepared dual-phase structured Li4Ti5O12–TiO2–CNFs sample as a binder-free anode for lithium-ion batteries (LIBs) presented a greatly enhanced reversible capacity (203.8 mA h g−1 at 100 mA g−1 after 200 cycles) and a favored rate capability (114.3 mA h g−1 at 2000 mA g−1) compared with the single-phase Li4Ti5O12–CNFs sample.

Self-improving anodes for lithium-ion batteries: continuous interlamellar spacing expansion induced capacity increase in polydopamine-derived nitrogen-doped carbon tubes during cycling

Polydopamine-derived nitrogen-doped carbon has attracted tremendous attention owing to its huge success in improving the electrochemical properties of high-capacity lithium-storage materials (such as S, Si and Sn). In this study, we first demonstrate its excellent and durable lithium-storage capability by designing a one-dimensional hollow structure through a template-assisted method. The polydopamine-derived nitrogen-doped carbon tubes show high specific capacity, excellent rate capability and robust durability. Interestingly, the capacity gradually increased from 587 to 1103 mA h g−1 upon the 500th cycle at 500 mA g−1. The self-improvement in capacity stems from the continuous interlamellar spacing expansion of the graphene-like carbon layers as confirmed by HR-TEM. Our work offers a new insight into the electrochemical behaviour of polydopamine-derived carbon and is beneficial for its future utilization in high-performance lithium-ion battery electrode materials.

Hygrothermal Aging on Pultruded Carbon Fiber/Vinyl Ester Resin Composite for Sucker Rod Application

The effects of hygrothermal aging on thermal and mechanical properties, and on the microstructure of pultruded unidirectional carbon fiber/vinyl ester resin (CF/VE) composites were investigated. The composite specimens were immersed in distilled water and salt solution at 95 and 65 C for more than 1000 h and the moisture absorption was recorded. The diffusion rate of moisture into the composite was found to fit Fickian’s diffusion model. SEM observation revealed that the fiber-matrix interface was weakened due to the invasion of moisture leading to debonding. Three-point flexural and interlaminar shear tests showed that both flexural strength and interlaminar shear strength (ILSS) of the composite specimens deteriorated to some extent after hygrothermal aging, while stiffness was less affected. Dynamic mechanical thermal analysis (DMTA) results indicated that the glass transition temperature of the matrix underwent complicated changes, which was attributed to the combinational effects of plasticization and the formation of hydrogen bonds in the systems.

High-Performance Li-Ion Capacitor Based on an Activated Carbon Cathode and Well-Dispersed Ultrafine TiO2 Nanoparticles Embedded in Mesoporous Carbon Nanofibers Anode

A novel Li-ion capacitor based on an activated carbon cathode and a well-dispersed ultrafine TiO2 nanoparticles embedded in mesoporous carbon nanofibers (TiO2@PCNFs) anode was reported. A series of TiO2@PCNFs anode materials were prepared via a scalable electrospinning method followed by carbonization and a postetching method. The size of TiO2 nanoparticles and the mesoporous structure of the TiO2@PCNFs were tuned by varying amounts of tetraethyl orthosilicate (TEOS) to increase the energy density and power density of the LIC significantly. Such a subtle designed LIC displayed a high energy density of 67.4 Wh kg-1 at a power density of 75 W kg-1. Meanwhile, even when the power density was increased to 5 kW kg-1, the energy density can still maintain 27.5 Wh kg-1. Moreover, the LIC displayed a high capacitance retention of 80.5% after 10000 cycles at 10 A g-1. The outstanding electrochemical performance can be contributed to the synergistic effect of the well-dispersed ultrafine TiO2 nanoparticles, the abundant mesoporous structure, and the conductive carbon networks.

Amorphous Cu-added/SnOx/CNFs composite webs as anode materials with superior lithium-ion storage capability

Amorphous Cu-added tin oxides/carbon nanofiber (Cu-added SnOx/CNFs) composite webs used for lithium-ion battery anode materials are prepared by an electrospinning technique and subsequent thermal treatment. The Cu-doped SnOx particles are uniformly distributed in the CNFs and maintain the original morphology after long-term cycling. In a controlled experiment, SnOx-20%Cu/CNFs with an atomic ratio of Cu : Sn = 0.2 shows the highest electrochemical performance with a high reversible capacity of 743 mA h g−1 at a current density of 200 mA g−1 and an excellent rate capacity of 347 mA h g−1 at 5 A g−1. Moreover, the composite electrode exhibits an outstanding long-term cycling performance at 2 A g−1 even after 1000 cycles. The superior reversible lithium-ion storage capability is attributed to the uniform dispersion of the Cu2O and ultrafine SnOx particles in CNFs as well as the Cu-addition effects such as promoting electron transport and Li+ diffusion, preventing Sn from aggregation during cycling, and improving the reversibility of Sn back to SnOx in the recharge process.

Hierarchical flower-like TiO2/MPCNFs as a free-standing anode with superior cycling reversibility and rate capability

A hierarchical flower-like TiO2/MPCNFs web was cost-effectively fabricated by electrospinning, solvothermal treatment and calcination. The flower-like TiO2/MPCNFs as a free-standing anode possessed superior cycling reversibility and rate capability to nano-particulate and micro-particulate TiO2/MPCNFs because of its unique multiporous micro-/nano-architecture for synergistic lithium storage.