Ke Zhan

Fe2O3-decorated millimeter-long vertically aligned carbon nanotube arrays as advanced anode materials for asymmetric supercapacitors with high energy and power densities

Construction of high-energy density asymmetric supercapacitors is often hindered by unsatisfactory matching between the anode and cathode. Thus, it is crucial to develop composite anodes with high specific capacitance to match that of cathodes. In this work, a novel anode material with well-dispersed Fe2O3 decorated on vertically aligned carbon nanotubes has been synthesized by a facile two-step method, consisting of supercritical carbon dioxide (SCCO2) assisted impregnation and subsequent thermal annealing. Due to the advantageous nanostructure, the Fe2O3/VACNT composites exhibit a large specific capacitance of 248 F g−1 at 8 A g−1 in 2 M KOH between −1.2 and 0 V versus SCE. An asymmetric supercapacitor operating at 1.8 V is assembled using the Fe2O3/VACNTs as the anode and the NiO/VACNTs as the cathode in a 2 M KOH aqueous electrolyte. The NiO/VACNTs//Fe2O3/VACNT asymmetric supercapacitor achieves an extremely high energy density of 137.3 W h kg−1 at a power density of 2.1 kW kg−1, and still retains 102.2 W h kg−1 at the high power density of 39.3 kW kg−1. Moreover, it also shows an outstanding cycling stability with ∼89.2% capacitance retention after 5000 cycles. The facile and effective synthesis method, as well as the superior electrochemical performance of the Fe2O3/VACNT composites, pave a way for promising applications in high-performance energy storage.

Formation of piezoelectric β-phase crystallites in poly(vinylidene fluoride)-graphene oxide nanocomposites under uniaxial tensions

A combination of chemical addition and mechanical deformation is employed to develop a nearly pure β crystalline phase in poly(vinylidene fluoride) (PVDF). The mechanical strains for the formation of pure β phase in PVDF-graphene oxide (PVDF-GO) nanocomposites are found to be only half of that for PVDF deformed at the same temperature. Based on the creep experiments, piezoelectric force microscopy and mechanical anelastic relaxation, it is shown that the attachments of PVDF molecule chains to the GO sheets caused by the interaction between the –CF2 or –CH2 groups of PVDF and the functional groups of GOs could facilitate the formation of piezoelectric β-phase crystallites in the stretched nanocomposites. A model is proposed to elucidate the easy formation of the piezoelectric β phase in the PVDF-GO nanocomposites under uniaxial tensions.

Quasi-Emulsion Confined Synthesis of Edge-Rich Ultrathin MoS2 Nanosheets/Graphene Hybrid for Enhanced Hydrogen Evolution

High-purity hydrogen produced by water splitting is considered as one of the most promising fuels to replace traditional fossil fuels. Developing highly efficient electrocatalysts toward hydrogen evolution is vital for the realization of large-scale H2 generation. Glycerol is used herein in a facile solvothermal process to synthesize edge-rich ultrathin MoS2 /reduced graphene oxide (RGO) composites. The introduction of glycerol plays an important role in the formation of such interesting structures. The MoS2 /RGO electrocatalyst exhibits excellent hydrogen evolution reaction (HER) activity and remarkable stability, owing to the rich active edges and improved electrical conductivity of the catalyst composites. This work provides new insights to engineer the structures of MoSx -based composites and thus achieves more active and efficient electrocatalysts.

Co-supported catalysts on nitrogen and sulfur co-doped vertically-aligned carbon nanotubes for oxygen reduction reaction

Co supported on nitrogen and sulfur co-doped vertically-aligned carbon nanotubes (Co/NS-CNT) has been fabricated as an efficient electrocatalyst for oxygen reduction reaction (ORR) by a two-step process involving sputtering of cobalt and subsequent annealing in a nitrogen and sulfur-containing atmosphere. The surface morphology, crystal structure and chemical composition of the samples have been investigated. Both cyclic voltammetry (CV) and rotating-disk electrode (RDE) measurements reveal that the annealing temperature has a significant impact on the ORR activity of the catalyst in both alkaline and acid electrolytes. And the best ORR performance is achieved for the catalyst annealed at 600 °C (Co/NS-CNT-600) in 0.1 M KOH solution, which exhibits an onset potential of 0.962 V and an ORR peak potential of 0.803 V. Rotating ring-disk electrode (RRDE) testing in KOH shows an electron transfer number of around 3.7, indicating a four-electron pathway-dominated ORR process. The Co/NS-CNT-600 catalyst also exhibits the best ORR catalytic activity in 0.5 M H2SO4 medium. The excellent ORR activity of the Co/NS-CNT catalyst is attributed to the synergistic effects from N and S co-doping and the increased active sites from metallic cobalt or CoS2.

Cobalt sulfide supported on nitrogen and sulfur dual-doped reduced graphene oxide for highly active oxygen reduction reaction

Cobalt sulfide nanoparticles grown on nitrogen and sulfur dual-doped reduced graphene oxide sheets (Co–S/NS-rGO) were synthesized as an efficient electrocatalyst for the oxygen reduction reaction (ORR) by a facile one-step annealing process at 400–600 °C. The catalyst synthesized at 500 °C (Co–S/NS-rGO-500) exhibits the best ORR catalytic activity compared to the other samples, together with high four-electron selectivity and excellent stability in alkaline medium. Moreover, the Co–S/NS-rGO-500 composite also manifests good ORR activity and selectivity in acid solution. The facile synthesis approach and superior ORR performance in both alkaline and acid electrolytes make the Co–S/NS-rGO catalysts promising as an alternative to commercial Pt/C catalyst for fuel cells.

Mechanisms of polarization switching in graphene oxides and poly(vinylidene fluoride) - graphene oxide films

Polarization switching in graphene oxides (GOs) and poly(vinylidene fluoride) (PVDF)–GO nanocomposite is investigated by piezoelectric force microscopy (PFM). The dynamical switching results reveal that GO films exhibit ferroelectric and piezoelectric properties with two-dimensional characteristics. Abnormal polarization switching is observed in PVDF–GO films, which is promising for electronic applications.

Bio-inspired design of hierarchical FeP nanostructure arrays for the hydrogen evolution reaction

Hierarchical FeP nanoarray films composed of FeP nanopetals were successfully synthesized via a bio-inspired hydrothermal route followed by phosphorization. Glycerol, as a crystal growth modifier, plays a significant role in controlling the morphology and structure of the FeO(OH) precursor during the biomineralization process, while the following transfer and pseudomorphic transformation of the FeO(OH) film successfully give rise to the FeP array film. The as-prepared FeP film electrodes exhibit excellent hydrogen evolution reaction (HER) performance over a wide pH range. Theoretical calculations reveal that the mixed P/Fe termination in the FeP film is responsible for the high catalytic activity of the nanostructured electrodes. This new insight will promote further explorations of efficient metal phosphoride-based catalysts for the HER. More importantly, this study bridges the gap between biological and inorganic self-assembling nanosystems and may open up a new avenue for the preparation of functional nanostructures with application in energy devices.

A two-step approach to synthesis of Co(OH)2/γ-NiOOH/reduced graphene oxide nanocomposite for high performance supercapacitors

A two-step approach was reported to fabricate cobaltous hydroxide/γ- nickel oxide hydroxide/reduced graphene oxide (Co(OH)2/γ-NiOOH/RGO) nanocomposites on nickel foam by combining the reduction of graphene oxide with the help of reflux condensation and the subsequent hydrothermal of Co(OH)2 on RGO. The microstructural, surface morphology and electrochemical properties of the Co(OH)2/γ-NiOOH/RGO nanocomposite were investigated. The results showed that the surface of the first-step fabricated γ-NiOOH/RGO nanocomposites was uniformly coated by Co(OH)2 nanoflakes with lateral size of tens of nm and thickness of several nm. Co(OH)2/γ-NiOOH/RGO nanocomposite demonstrated a high specific capacitance (745 mF/cm2 at 0.5 mA/cm2) and a cycling stability of 69.8% after 1000 cycles at 30 mV/cm2. γ-NiOOH/RGO//Co(OH)2/γ-NiOOH/RGO asymmetric supercapacitor was assembled, and maximum gravimetric energy density of 57.3 W∙h/kg and power density of 66.1 kW/kg were achieved. The synergistic effect between the highly conductive graphene and the nanoflake Co(OH)2 structure was responsible for the high electrochemical performance of the hybrid electrode. It is expected that this research could offer a simple method to prepare graphene-based electrode materials.

Investigation of nanoscale domain switching of 0.94 Na0.5 Bi0.5TiO3-0.06 BaTiO3 thin film under different temperatures by piezoresponse force microscopy

Abstract The nanoscale domain switching behavior of 0.94 Na0.5 Bi0.5TiO3-0.06 BaTiO3 (NBT-BT6) thin film under different temperatures (25°C-225°C) is investigated by piezoresponse force microscopy (PFM) via the direct observation on their domain structure. The results show that the polycrystal structure with polydomain state is detected in NBT-BT6 thin film. With the temperature increasing, 90°domain switching is observed, however some domains near the grain boundary does not change. The domain switching is attributed to the residual stress induced by the grain deformation, and the grain boundary plays an active role in pinning a preferential polarization state.