Dianlong Wang

Well-Dispersed High-Loading Pt Nanoparticles Supported by Shell-Core Nanostructured Carbon for Methanol Electrooxidation

Shell-core nanostructured carbon materials with a nitrogen-doped graphitic layer as a shell and pristine carbon black particle as a core were synthesized by carbonizing the hybrid materials containing in situ polymerized aniline onto carbon black. In an N-doped carbon layer, the nitrogen atoms substitute carbon atoms at the edge and interior of the graphene structure to form pyridinic N and quaternary N structures, respectively. As a result, the carbon structure becomes more compact, showing curvatures and disorder in the graphene stacking. In comparison with nondoped carbon, the N-doped one was proved to be a suitable supporting material to synthesize high-loading Pt catalysts (up to 60 wt %) with a more uniform size distribution and stronger metal-support interactions due to its high electrochemically accessible surface area, richness of disorder and defects, and high electron density. Moreover, the more rapid charge-transfer rates over the N-doped carbon material are evidenced by the high crystallinity of the graphitic shell layer with nitrogen doping as well as the low charge-transfer resistance at the electrolyte/electrode interface. Beneficial roles of nitrogen doping can be found to enhance the CO tolerance of Pt catalysts. Accordingly, an improved performance in methanol oxidation was achieved on a high-loading Pt catalyst supported by N-doped carbon. The enhanced catalytic properties were extensively discussed based on mass activity (Pt utilization) and intrinsic activity (charge-transfer rate). Therefore, N-doped carbon layers present many advantages over nondoped ones and would emerge as an interesting supporting carbon material for fuel cell electrocatalysts.

A three-dimensional porous LiFePO4 cathode material modified with a nitrogen-doped graphene aerogel for high-power lithium ion batteries

A composite cathode material consisting of (010) facet-oriented LiFePO4 nanoplatelets wrapped in a nitrogen-doped graphene aerogel is reported. Such a composite possesses a 3D porous structure with a BET surface area as high as 199.3 m2 g−1. In this composite, the nitrogen-doped graphene aerogel combined with its interconnected porous networks provides pathways for rapid electron transfer and ion transport, while the thin LFP nanoplatelets with large (010) surface area enhance the active sites and shorten the Li+ diffusion distances. As a result, a high rate capability (78 mA h g−1 at 100 C) as well as a long life cycling stability (89% capacity retention over 1000 cycles at 10 C) are achieved.

Nitrogen-doped magnetic onion-like carbon as support for Pt particles in a hybrid cathode catalyst for fuel cells

Pt and non-precious metal catalysts were combined to build a hybrid cathode for fuel cell application, with the aim of dramatically reducing the amount of Pt and increasing the overall catalytic performance. An active nitrogen-doped magnetic onion-like graphitic carbon material (N-Me-C) was synthesized by pyrolyzing a hexamethylene diamine-Me (Me: Co and Fe) complex. The N-Me-C materials proved capable of effectively catalyzing the oxygen reduction reaction (ORR), as evidenced by rotating disk/ring electrode (RDE/RRDE) data showing significant positive shifts of onset and half-wave (E½) potentials and a drop of H2O2 yield, when compared to traditional carbon supporting materials. In the hybrid cathode catalyst, ultra-low loading Pt nanoparticles (2 wt%) were subsequently anchored to the N-Me-C support through a chemical reduction method. The configuration using ultra-low Pt loading is advantageous for mitigating particle agglomeration and improving Pt utilization due to isolated particle distributions and smaller particle sizes. Electrochemical and fuel cell data confirmed that the use of the N-Me-C support leads to a significant enhancement of ORR catalytic activity. It is quite significant that the 2% Pt/N-Me-C cathode with an ultra-low Pt loading of 0.04 mg-Pt cm−2 is effective in generating a current density of ca. 0.14 and 0.59 A cm−2 at cell voltages of 0.80 and 0.65 V operated in a H2-air cell, respectively. The corresponding mass activity (A mg-Pt−1) was increased by factors of 1.4 and 3.5 at 0.65 V, when compared with 2% Pt/C and commercial E-TEK 20% Pt/C cathodes. Extensive physical and electrochemical characterization revealed that the significant improvement in mass activity is mainly attributable to the non-precious ORR active sites on M-Me-C, and also partially to the beneficial support effect of nitrogen doping associated with stronger support-metal interactions and smaller particle sizes.

Improvement of the electrochemical performance of carbon-coated LiFePO4 modified with reduced graphene oxide

In this work, carbon-coated LiFePO4 was further modified with reduced graphene oxide (RGO) using an ultrasonic-assisted rheological phase method coupled with carbothermal treatment. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and electrochemical methods were used to characterize the materials properties. The results showed that the composite material consisting of carbon-coated LiFePO4 and RGO sheets possesses a unique and effective three-dimensional “sheet-web” structure. In the structure, the LiFePO4 particle size can be maintained at nanosize to form abundant voids between the nanoparticles while the RGO sheets are significantly beneficial for Li+ diffusion. As a result, the electrochemical properties of the composite material have been greatly improved. A sample with 5 wt% RGO exhibited high specific capacity and superior rate performance with the discharge capacities of 160.4 mA h g−1 at 0.2 C and 115.0 mA h g−1 at 20 C. The sample also showed an excellent cycling stability with only about 10% capacity decay at 10 C after 1000 cycles.

A three dimensional SiOx/C@RGO nanocomposite as a high energy anode material for lithium-ion batteries

A co-modification strategy to improve the electrode performance of SiO-based materials through the use of a carbon coating layer and reduced graphene oxide (RGO) network has been developed. The as-synthesized SiOx/C@RGO nanocomposites showed excellent specific capacity, cycling performance and rate capability when used as an anode in lithium-ion batteries.

Ultrafast preparation of three-dimensional porous tin–graphene composites with superior lithium ion storage

Three-dimensional porous Sn–graphene composites have been prepared on Ni foam by an easy, binder-free, low-cost and ultrafast electrophoretic deposition method. The structure and morphology of the as-prepared composite material are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, high resolution transmission electron microscopy, field emission scanning electron microscopy and elemental analysis. The lithium storage performance of the three-dimensional porous Sn–graphene anode is evaluated by cyclic voltammetry, galvanostatic charge–discharge cycling, and electrochemical impedance spectroscopy measurements. Results show that the composite material with a graphene content of 6.1 wt% delivers a reversible capacity of 552 mA h g−1 after 200 cycles at a current density of 500 mA g−1. The synthetic approach presented in this work may provide a facile strategy for the preparation of three-dimensional porous metal–graphene composites.

The composite electrode of LiFePO4 cathode materials modified with exfoliated graphene from expanded graphite for high power Li-ion batteries

In recent years, copious papers have reported the fruitful modifications of LiFePO4-based composites and exhibited excellent electrochemical performance in terms of rate capability and cycling stability. Besides, the optimization of bulk electrodes are essential to keep pace with composites, by enhancement of the electronic and ionic transport to further improve the power performance of an electrode. Therefore, in this work, a facile strategy is adopted to fabricate a composite electrode with NMP as a solvent, containing large-size multilayer graphene, which is prepared by the exfoliation of economical expanded graphite under high power ultrasound in isopropyl alcohol. Active LiFePO4 nanograins, as well as conductive additives, are attached to the superior conductive graphene and thereby fast pathways are established as a “highway” for electronic transport in the bulk electrode. As a result, this composite electrode exhibits a lower polarization at high-rate charge–discharge processes. The operating flat voltage of 20 C rate is maintained at more than 3.0 V in one minute and its discharge capacity is up to 107.8 mA h g−1, representing a better energy density and power density.

The synergy effect on Li storage of LiFePO4 with activated carbon modifications

In this work, composite electrodes containing lithium iron phosphate (LiFePO4) and activated carbon (AC) were prepared by physically mixing LiFePO4 and AC with polyvinylidene fluoride (PVDF) as a binder and acetylene black (AB) as an electrically conductive agent. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), nitrogen sorption, four-probe conductivity and vibrating densitometer techniques were employed to characterize samples. The characterization results showed that the presence of AC increased the electrical conductivity, reduced the tap density, and modified the porosity of the resultant composite electrode materials. Electrochemical data demonstrated that the composite electrode displayed a significantly improved electrochemical performance in comparison with the pure LiFePO4 electrode. An electrode with 5 wt% AC exhibited specific discharge capacities of 70 mA h g−1 at 20 C and 100 mA h g−1 at 10 C without significant capacity decay after 400 cycles. Galvanostatic charge–discharge and cyclic voltammetry results revealed that energy was stored via both charge adsorption and lithium intercalation/deintercalation owing to the presence of both AC and LiFePO4 in the composite electrode. Electrochemical impedance spectroscopy (EIS) was used to investigate the charge–discharge kinetics and mechanism of the composite electrode. The EIS results demonstrated that the two different active materials (LiFePO4 and AC) displayed synergy in terms of both material structure and energy storage, contributing to the observed excellent electrochemical performance.

Carbon-coated single-crystalline LiFePO4 nanocomposites for high-power Li-ion batteries: the impact of minimization of the precursor particle size

In this work, a high-energy ball mill technique is designed to deal with bulk precursors and achieve particle size minimization. A large amount of the nano-sized precursor is achieved in a narrow particle size distribution of ca. 95 nm. We confirm that the dimensional size of the precursor has a significant influence on the final LiFePO4 particle size and that small grains of the precursor probably form single-crystalline nanoparticles during the calcination process. After carbothermal reduction, the carbon-coated single-crystalline LiFePO4 nanocomposites (nano-CS–LFP) are easily synthesized. Benefiting from the decreasing particle size, the specific surface area of nano-CS–LFP is up to 48.0 m2 g−1, which implies a higher interfacial contact area between the active particles and the electrolyte, as well as an increase in its capacitance capability. Besides, cyclic voltammetry curves of nano-CS–LFP reveal a better capability of reversible reactivity and a lower polarization. Galvanostatic charge–discharge results exhibit excellent rate performance with a discharge capacity of ca. 100 mA h g−1 at 10 °C and a stable cycling property with a capacity retention of ca. 90% after 1000 cycles. In addition, the rapid charge–discharge test over 60 seconds indicates an excellent pulse performance with a high current in a short time period. The combination of the merits of carbon coating and particle size minimization is responsible for the above improvements. Finally, this facile preparation strategy is favorable for the industrial production of economical LiFePO4 materials from lab synthesis.

A facile hydrothermal synthesis of a reduced graphene oxide modified cobalt disulfide composite electrode for high-performance supercapacitors

In this study, a 3D reduced graphene oxide modified CoS2 composite electrode (CoS2/RGO) is synthesized by a facile hydrothermal approach. The dimensions of the CoS2 nanoparticles in CoS2/RGO are effectively reduced due to the geometric confinement of RGO, and a novel, large-scale wave-like structure is formed. This leads to an enlarged specific surface area and improved conductivity and could thus be favourable for both fast electron and ion transport. As a consequence, CoS2/RGO displays better electrochemical properties than the pure individual components. When the mass ratio of CoCl2·6H2O and GO as the raw materials is 1 : 2, the obtained CoS2/RGO composite electrode (CoS2/RGO-2) delivers the highest capacitance of 930.3 F g−1 at 2 A g−1 and retains a capacitance as high as 677.9 F g−1 as the current density increases up to 20 A g−1. Moreover, in order to obtain high energy and power densities, a high-voltage asymmetric supercapacitor has been designed and constructed using the optimized CoS2/RGO-2 composite electrode as the positive electrode and activated carbon (AC) as the negative electrode material. Such a device with an operational voltage of 1.6 V can achieve a remarkable energy density of 45.7 W h kg−1 at a power density of 797.0 W kg−1, in addition to the superior rate capability and prominent stability towards long time charge–discharge cycles.