Guoxin Gao

Mesoporous Co3V2O8 nanoparticles grown on reduced graphene oxide as a high-rate and long-life anode material for lithium-ion batteries

Hierarchical hybrid nanostructures based on flexible graphene sheets and ternary transition metal oxides have attracted special attention as high-performance electrode materials for next-generation lithium-ion batteries (LIBs) yet their practical application is often beset with challenges. In this work, we report a hierarchical hybrid nanocomposite of reduced graphene oxide supported mesoporous Co3V2O8 nanoparticles (rGO@Co3V2O8 NPs) through a simple hydrothermal synthesis and post-calcination. This unique hybrid architecture when used as an anode in LIBs would effectively facilitate charge transfer, maintain structural integrity and accommodate the volume variation of the electrode materials during the repeated charge/discharge processes. As a result, the hybrid rGO@Co3V2O8 NPs manifest a very stable high reversible capacity of 1050 mA h g−1 over 200 cycles at a current density of 50 mA g−1 and excellent rate capability. Importantly, even when cycled at a higher current density of 200 mA g−1, a stable reversible capacity of 899 mA h g−1 and a remarkable cycling stability could also be achieved after 600 cycles. These results indicate the potential suitability of such mesoporous nanoparticles on graphene nanostructures for high-rate and long cycle life anode materials.

One-pot synthesis of carbon coated Fe3O4 nanosheets with superior lithium storage capability

Hybrid nanosheet structures based on carbon coated metal oxides still attract promising interest as high-performance electrode materials for next-generation lithium-ion batteries (LIBs). In this study, we develop a simple one-pot solution method to synthesize large-scale flat Fe3O4 nanosheet hybrid structures coated with an amorphous carbon overlayer (denoted as Fe3O4@C NSs) followed by a thermal annealing treatment. It is found that the refluxing temperature plays an important role in adjusting the morphology of the Fe3O4@C hybrid. Increasing the temperature from 140 °C to 200 °C will lead to flower-like hybrid structures constructed by Fe3O4 nanoflakes gradually growing, rupturing, and finally evolving into flat and completely separate nanoflakes with large size at 200 °C. When evaluated as an anode material for LIBs, the hybrid Fe3O4@C NSs demonstrate a high reversible capacity of 1232 mA h g−1 over 120 cycles at a current density of 200 mA g−1, and remarkable rate capability.

Tuning phase transition and ferroelectric properties of poly(vinylidene fluoride-co-trifluoroethylene) via grafting with desired poly(methacrylic ester)s as side chains

For potential application in high energy storage capacitors with high energy density and low energy loss, three sets of poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene) [P(VDF-TrFE-CTFE)] grafted with poly(methacrylic ester)s, including poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA) and poly(butyl methacrylate) (PBMA) copolymers, are designed and investigated carefully. Due to their intermediate polarity, relatively high glass transition temperature, and excellent compatibility with PVDF chains, the poly(methacrylic ester) segments introduced could not only dramatically weaken the coupling interactions of oriented polar crystals, but could also accelerate the reversal switching of polar crystal domains along the applied electric field, which leads to well hindered remnant polarization. As a result, the displacement–electric field (D–E) hysteresis behaviors of the graft copolymers could be tuned from typical ferroelectric to either antiferroelectric or linear shape under high electric field. Meanwhile, significantly reduced energy loss and effectively improved energy discharging efficiency were obtained. Compared with PMMA and PBMA, PEMA with intermediate polarity and grafting length exhibits more suitable confinement of the F–P transition of P(VDF-TrFE-CTFE), and thus more desirable energy storage properties are observed in the resultant copolymers. These findings may help to deeply understand the ferroelectric nature of PVDF based fluoropolymers and design new energy storage capacitor materials with high discharged energy density and low energy loss.

One-step synthesis of free-standing α-Ni(OH)2 nanosheets on reduced graphene oxide for high-performance supercapacitors

In this work, a hierarchical hybrid structure of reduced graphene oxide (rGO) supported ultrathin α-Ni(OH)2 nanosheets (denoted as α-Ni(OH)2@rGO NSs) has been developed successfully via an environmentally friendly one-step solution method. The resulting product of α-Ni(OH)2@rGO NSs was further characterized by scanning electron microscope, transmission electron microscope, x-ray diffraction, Raman spectroscopy, x-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller. The ultrathin α-Ni(OH)2 nanosheets of around 6 nm in thickness are uprightly coated on the double sides of rGO substrate. When evaluated as electrodes for supercapacitors, the hybrid α-Ni(OH)2@rGO NSs demonstrate excellent supercapacitor performance and cycling stability, compared with the self-aggregated α-Ni(OH)2 powder. Even after 2000 cycles, the hybrid electrodes still can deliver a specific capacitance of 1300 F g(-1) at the current density of 5 A g(-1), corresponding to no capacity loss of the initial cycle. Such excellent electrochemical performance should be attributed to the ultrathin, free-standing, and hierarchical nanosheets of α-Ni(OH)2, which not only promote efficient charge transport and facilitate the electrolyte diffusion, but also prevent aggregation of electro-active materials effectively during the charge-discharge process.

Formation of g-C3N4@Ni(OH)2 Honeycomb Nanostructure and Asymmetric Supercapacitor with High Energy and Power Density

Nickel hydroxide (Ni(OH)2) has been regarded as a potential next-generation electrode material for supercapacitor owing to its attractive high theoretical capacitance. However, practical application of Ni(OH)2 is hindered by its lower cycling life. To overcome the inherent defects, herein we demonstrate a unique interconnected honeycomb structure of g-C3N4 and Ni(OH)2 synthesized by an environmentally friendly one-step method. In this work, g-C3N4 has excellent chemical stability and supports a perpendicular charge-transporting direction in charge-discharge process, facilitating electron transportation along that direction. The as-prepared composite exhibits higher specific capacities (1768.7 F g-1 at 7 A g-1 and 2667 F g-1 at 3 mV s-1, respectively) compared to Ni(OH)2 aggregations (968.9 F g-1 at 7 A g-1) and g-C3N4 (416.5 F g-1 at 7 A g-1), as well as better cycling performance (∼84% retentions after 4000 cycles). As asymmetric supercapacitor, g-C3N4@Ni(OH)2//graphene exhibits high capacitance (51 F g-1) and long cycle life (72% retentions after 8000 cycles). Moreover, high energy density of 43.1 Wh kg-1 and power density of 9126 W kg-1 has been achieved. This attractive performance reveals that g-C3N4@Ni(OH)2 with honeycomb architecture could find potential application as an electrode material for high-performance supercapacitors.

Construction of sandwich-type hybrid structures by anchoring mesoporous ZnMn2O4 nanofoams on reduced graphene oxide with highly enhanced capability

We have developed a sandwich-type hybrid nanostructure by anchoring foam-like zinc manganate (ZnMn2O4) on reduced graphene oxide (rGO) (rGO/ZnMn2O4 NFs) via a trisodium citrate (TSC) assisted solution reaction followed by a post-calcination treatment. The interconnected sheet-like ZnMn2O4 subunits have assembled into mesoporous nanofoams on rGO sheets with the beneficial help of TSC. When cycled at a current density of 180 mA g−1, the hybrid rGO/ZnMn2O4 NF anodes present a high discharge capacity of 945 mA h g−1 even after 150 cycles with long cycle durability and good rate capability. Such highly enhanced electrochemical performance is ascribed to the sandwich-type hierarchical foam structure effectively promoting the ion/charge transport whilst buffering volume variations upon continuous discharge/charge cycling. These results indicate that a porous anode scaffold with conductive connections is a promising structural design for rechargeable batteries with superior reversible lithium storage capability.

Synthesis of fluoropolymer containing tunable unsaturation by a controlled dehydrochlorination of P(VDF-co-CTFE) and its curing for high performance rubber applications

Fluoropolymer containing unsaturation, an important intermediate for many reactions such as radical addition and Michael addition reaction, could be either utilized to synthesize fluoropolymer with desired functions or cured for rubber applications, which has rarely been investigated because of the absence of a synthetic strategy. A facile method to synthesize fluoropolymer with tunable unsaturation via controlled dehydrochlorination of commercially available poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)) catalyzed by tertiary monoamines under mild conditions has been reported in this work. The resultant copolymers are carefully characterized with nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR), and thermal gravimetric analysis (TGA). It has been shown that the elimination could be well controlled by employing proper solvent, catalyst and reaction conditions. The typical side reactions catalyzed with amines, such as Michael addition reaction and main chain scission during the dehydrofluorination of fluoropolymer, could be avoided in the present reaction system. The kinetics results indicate that the elimination reaction is in a bi-molecular mechanism (E2), which is well recognized in strong base-catalyzed elimination of halogenated hydrocarbon. The concentration, alkalinity and steric bulk of the catalysts, the polarity and capability to absorb HCl acid of solvents, and the reaction time and temperature exhibit dominant influences on the dehydrochlorination of P(VDF-co-CTFE). The fluoropolymer containing unsaturation is readily cured with peroxide, and the crosslinked fluoropolymer exhibits excellent solvent resistance and mechanical properties.

Quick one-pot synthesis of amorphous carbon-coated cobalt–ferrite twin elliptical frustums for enhanced lithium storage capability

Hybrid carbon-coated transition metal oxides (TMOs@C) offer enhanced lithium storage capabilities, but the facile formation of TMOs@C nanocomposites remains a great challenge. Herein, we report a novel hierarchical hybrid nanostructure of carbon-coated CoFe2O4 twin elliptical frustums (CoFe2O4@C TEFs) via a quick one-pot refluxing reaction in ethylene glycol (EG) followed by an annealing treatment. When evaluated as an anode in lithium-ion batteries (LIBs), the resultant CoFe2O4@C TEF hybrids demonstrate good electrochemical performance with high reversible specific capacity, excellent rate capability and super-long life cycle. After 600 cycles at a current density of 500 mA g−1, the resultant TEFs still deliver a stable reversible discharge capacity of 875 mA h g−1. This work demonstrates the extensive potential of such simple synthetic methods towards various carbon coated transition metal oxide composites for energy conversion and storage devices.

Tunable growth of perpendicular cobalt ferrite nanosheets on reduced graphene oxide for energy storage

Ultrathin cobalt ferrite nanosheets have been successfully assembled on the surface of reduced graphene oxide (rGO) via only adjusting the volume ratio of ethanol and deionized (DI) water and a post calcination treatment. The perpendicular ultrathin cobalt ferrite nanosheets supported by rGO sheets (CoFe2O4 NSs@rGO) can be obtained when the volume ratio of ethanol and DI water is 10:30. Correspondingly, the hierarchical porous films covering the total rGO sheets will be formed nanosheets. When evaluated as the electrodes for lithium ion batteries (LIBs) and supercapacitors (SCs), the resultant CoFe2O4 NSs@rGO hybrids exhibit highly enhanced electrochemical performance. Even after 200 charge-discharge cycles at 400 mA g-1, the electrodes as the anode material for LIBs still exhibit a reversible discharge capacity of 835.6 mAh g-1. In addition, this electrode for SCs also exhibits specific capacitance of ca 1120 F g-1 after 3000 cycles. These superior results imply that CoFe2O4 NSs with novel hybrid structure of rGO could potentially lead to an excellent electrochemical performance for energy storage.

Synthesis and characterization of thermally self-curable fluoropolymer triggered by TEMPO in one pot for high performance rubber applications

The preparation of functional fluorine materials through chemical modification of commercial fluoropolymers has been recognized as an economic and convenient strategy to expand the application field of fluoropolymers. In this work, a thermally self-curable fluoroelastomer triggered by 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) has been successfully synthesized in one pot and carefully characterized. This strategy involves two competitive processes, including the coupling reaction between macroradicals and TEMPO, and the dehydrochlorination of commercially available poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)) by a route involving a three-molecule process, together with a small amount of elimination by an E2 mechanism and β-H elimination The structure and properties of the target polymer were demonstrated by nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC). The two competitive reaction processes were carefully investigated under various reaction conditions, including different reaction temperatures, reaction times, ligands, solvents, copper salt, and dosage of TEMPO. The resultant polymer is rather stable at ambient temperature and easily cured at high temperature by ‘pulling the trigger’, namely by breaking C–O or O–N bonds, and the free radicals generated in situ are responsible for initiating the crosslinking of double bonds on the polymer main chain. No other additives are required for the crosslinking of the resultant polymer, which provides a facile chemical route to prepare crosslinked fluoropolymers with high purity and excellent mechanical properties. The curing of the resultant polymer could be accomplished in several minutes at 150–160 °C without the need for a post-cure process.