Fen Xu

Synthesis of N-doped hierarchical carbon spheres for CO2 capture and supercapacitors

N-doped hierarchical carbon spheres have been synthesized via a soft template and hydrothermal method using melamine as a nitrogen source. The obtained carbon spheres possess a high nitrogen content and well-developed porosity. These carbon spheres are examined as absorbents for CO2 capture and as electrode materials for supercapacitors. Due to the high nitrogen content and hierarchical pore size distribution, the carbons show high CO2 uptakes of 2.2–4.4 mmol g−1 at 298 K and 1 bar. Furthermore, we observe that the carbon spheres exhibit excellent performance as supercapacitor electrodes with a high specific capacitance of 356 F g−1 at a current density of 0.2 A g−1. These carbon spheres as promising materials will exhibit excellent performance in various fields.

Fabrication and characterization of a novel nanoporous Co–Ni–W–B catalyst for rapid hydrogen generation

A highly active nanoporous Co–Ni–W–B alloy has been prepared using chemical reduction in an ethanol solution and tested as a novel catalyst for hydrolysis of ammonia borane. Compared with the alloy prepared in an aqueous solution, the as-prepared alloy shows a much higher surface area and hydrogen generation rate.

Preparation and thermophysical properties of a novel form-stable CaCl2·6H2O/sepiolite composite phase change material for latent heat storage

In this work, CaCl2·6H2O/sepiolite was successfully designed and synthesized as a novel form-stable composite phase change material by vacuum impregnation method, using sepiolite as a sustainer and CaCl2·6H2O as phase change material. Scanning electron microscope and Fourier transform infrared spectroscopy measurements display that CaCl2·6H2O is filled into the porous structure of sepiolite by physical interactions. Phase transformation behavior and thermal stability were revealed from differential scanning calorimetry and thermogravimetric analysis, respectively. Results show that the melting enthalpy of the composite phase change material containing 70% CaCl2·6H2O can achieve 87.9xa0Jxa0g−1, and the composite PCM has a good thermal stability in the temperature range from 25 to 100xa0°C. Meanwhile, the crystal structure of CaCl2·6H2O is maintained in the porous structure of sepiolite observed by X-ray diffraction. It means that sepiolite reduces the super-cooling of CaCl2·6H2O which ensures the good phase change behavior of the composites. These results exhibit that the CaCl2·6H2O/sepiolite composite phase change material possesses high latent heat. Moreover, low cost of the sepiolite enables the composites to be a good candidate for latent heat storage.

Synthesis of three-dimensional graphene aerogel encapsulated n-octadecane for enhancing phase-change behavior and thermal conductivity

We prepared a series of three-dimensional graphene aerogel (3D-GA) encapsulated n-octadecane (OD) composite phase change materials (PCMs) through both solution and vacuum impregnation to ensure that a homogeneous dispersion of OD in the porous structure of 3D-GA was present. At the same time, we also investigated the micro-structure, thermal storage properties, and thermal conductivity of the composite PCMs. We used scanning electron microscopy and Fourier transform infrared spectroscopy to demonstrate that OD was encapsulated effectively in the porous structure of 3D-GA and that the composite PCMs were prepared successfully. Differential scanning calorimetry (DSC) results confirmed that the composite PCMs possess good phase change behavior, fast thermal-response rates and excellent thermal cycling stability. The melting enthalpy and crystallization enthalpy can reach 195.70 J g−1 and 196.67 J g−1, respectively, and have almost no change for 60 DSC thermal cycles. Temperature–time curves suggested that the composite PCMs have excellent thermal regulation properties, and their temperature can be maintained in the range of 21–27 °C for about 640 s in a heating procedure. Thermal conductivity analysis indicated that the thermal conductivities of the composite PCMs are improved significantly by the highly thermally conductive 3D-GA. All these results demonstrated that the composite PCMs possess good comprehensive properties that can be used widely in energy storage systems.

Simple synthesis of graphene-doped flower-like cobalt–nickel–tungsten–boron oxides with self-oxidation for high-performance supercapacitors

In this study, we devised an easy and simple approach to synthesize a composite of flower-like cobalt–nickel–tungsten–boron oxides (Co–Ni–W–B–O) that were doped with reduced graphene oxide (rGO); the composite was designed for supercapacitor applications. A Co–Ni–W–B alloy was first deposited on rGO through one-pot chemical reduction in an ethanol solution at room temperature. The resulting Co–Ni–W–B alloy self-oxidized in air on the rGO surface. The Co–Ni–W–B–O/rGO composites resembled three-dimensional flowers with a high surface area; they also exhibited superior electrochemical performance when compared to most previously reported electrodes based on nickel–cobalt oxides. Furthermore, the Co–Ni–W–B–O/rGO composite prepared in an ethanol solution showed much higher electrochemical performance than the composite prepared in water. The Co–Ni–W–B–O/rGO electrode showed an ultrahigh specific capacitance of 1189.1 F g−1 at 1 A g−1 and exhibited a high energy density of 49.9 W h kg−1 along with remarkable cycle stability (10u2006000 cycles with 80.7% capacitance retention at 15 A g−1), which is promising for its application in energy storage devices.

Self-assembly synthesis of nitrogen-doped mesoporous carbons used as high-performance electrode materials in lithium-ion batteries and supercapacitors

Nitrogen-doped mesoporous carbons (NMCs) have been employed as electrode materials for energy storage devices such as lithium-ion batteries (LIBs) and supercapacitors due to their high accessible porosity and nitrogen content. However, how to maintain the structural stability of NMCs with abundant nitrogen atoms in their 2D honeycomb lattice is still a significant challenge. Herein, NMCs with a high nitrogen content of 18.86 wt% have been successfully synthesized via a self-assembly route in the presence of guanine as a nitrogen source. Furthermore, the nitrogen content and porosity of NMCs can be tuned by controlling the experimental conditions. Benefiting from the high N content, appropriate specific surface area (455 m2 g−1), and high accessible porosity, NMC exhibits superior electrochemical performance. This NMC anode for LIBs can retain a discharge capacity as high as 610 mA h g−1 with a coulombic efficiency of 98.5% after 50 cycles. In addition, it also displays good potential towards supercapacitor application with a specific capacitance of 227 F g−1 (in 6 M KOH) at a current density of 0.5 A g−1. The enhanced electrochemical performance could be attributed to both the low charge transfer resistance and the incremental electrochemical activity resulting from the existence of optimized nitrogen atoms.

Thermochemical studies of Rhodamine B and Rhodamine 6G by modulated differential scanning calorimetry and thermogravimetric analysis

Heat capacity is a very important thermodynamic parameter of any compound. How to precisely measure the heat capacity is a long-sought task. In this study, the low-temperature molar heat capacities of Rhodamine B (225–320xa0K) and Rhodamine 6G (225–440xa0K) were determined on a modulated differential scanning calorimetry (MDSC). There is no thermal anomaly or phase transition observed for Rhodamine B from 225 to 320xa0K. However, a phase transition appears for Rhodamine 6G with a peak temperature of 373xa0K, and the onset temperature is at 349.4xa0±xa00.4xa0K. The molar enthalpy ΔHtrans and molar entropy ΔStrans for this phase transition are determined to be 2.310xa0±xa00.011xa0kJxa0mol−1 and 6.611xa0±xa00.032xa0Jxa0K−1xa0mol−1, respectively. According to the measured molar heat capacities, the entropy and enthalpy referenced to the temperature of 298.15xa0K for Rhodamine B and Rhodamine 6G are derived. Moreover, thermogravimetric analysis (TG) is employed to investigate their thermal stability. The initial decomposition temperature for Rhodamine B and Rhodamine 6G is 326.5 and 492.1xa0K, respectively. There are two steps of mass loss observed for Rhodamine 6G and three steps for Rhodamine B.

Preparation and thermal performance of n-octadecane/expanded graphite composite phase-change materials for thermal management

Series of n-octadecane/expanded graphite composite phase-change materials (PCMs) with different mass ratio were prepared using n-octadecane as PCMs, expanded graphite as multi-porous supporting matrix through vacuum impregnation method. Microstructure, crystallization properties, energy storage behavior, thermal cycling property and intelligent temperature-control performance of the composite PCMs were investigated. Results show that the composite PCMs have a good energy storage property. The melting enthalpy and crystallization enthalpy can reach 164.85 and 176.51xa0Jxa0g−1, respectively. Furthermore, the good thermal conductivity of expanded graphite reduces the super-cooling degree of n-octadecane and endows the composite PCMs with fast thermal response rate and excellent thermal cycling stability. As a result, the phase-change temperatures and phase-change enthalpy almost have no change after 50 thermal-cooling cycles. The test of intelligent temperature-control performance shows that the electronic radiator filled with the composite PCMs possesses a high intelligent temperature-control performance, and its temperature can sustain in the range of 22–27.5xa0°C for about 6120xa0s. These results indicate that the prepared composite PCMs possess good comprehensive property and can be widely used in energy storage and thermal management systems.

Organic carbon gel assisted-synthesis of Li1.2Mn0.6Ni0.2O2 for a high-performance cathode material for Li-ion batteries

Lithium-rich layered oxide Li1.2Ni0.2Mn0.6O2 with a stable network flake structure has been synthesized through a facile resorcinol–formaldehyde (RF) organic carbon gel-assisted method. The as-prepared sample used as a cathode material in lithium ion batteries (LIBs) was characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and electrochemical measurements. The stable network flake structure is assembled through a dense stack of nanoparticles with an average size of 50–200 nm. As an active material for LIB cathodes, the Li1.2Ni0.2Mn0.6O2 sample shows excellent rate capacities and cycling stability, and delivers a high initial discharge capacity of 273.3 mA h g−1 at 0.1C (1C = 200 mA g−1) between 2.0 V and 4.8 V. When the discharge rate is increased to 2C, an initial capacity of 196.7 mA h g−1 is obtained. After 150 cycles, a discharge capacity of 183.7 mA h g−1 and a high capacity retention of 93.4% are yielded at a rate of 2C.

Metal-Organic Frameworks/Carboxyl Graphene Derived Porous Carbon as a Promising Supercapacitor Electrode Material

A UiO-66-NO2/carboxyl graphene composite (UiO-66-NO2/CXYG) was synthesized using a simple solvothermal reaction. The composite was then calcined to obtain series of all-carbon mixture of carbonized UiO-66-NO2/reduced carboxyl graphene (CUiO-66-NO2/rCXYG). The obtained carbon materials were characterized by X-ray diffraction (XRD), nitrogen sorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM). Then their electrochemical properties were systematically tested. The results indicated that UiO-66-NO2/CXYG derived Carbon-700 as an electrode for the electrochemical capacitor exhibited a high specific capacitance of 302 F g-1 in 6 M KOH at a current density of 0.15 A g-1, even 170 F g-1 at a high current of 10 A g-1 and good stability (retaining 94% capacitance after 5000 cycles). These UiO-derived porous carbon materials may offer a new insight into the various fields, such as fuel cells, supercapacitors and lithium batteries.