Composite phase change material based on reduced graphene oxide/expanded graphite aerogel with improved thermal properties and shape-stability

Composite phase change materials (PCMs) based on reduced graphene oxide/expanded graphite (rGO/EG) aerogel were prepared by hydrothermal self-assembly and impregnation method. The morphology, chemical structure, thermal properties, and shape-stability of the composite PCMs based on rGO/EG aerogel were examined. The results show that rGO sheets form a three-dimensional (3D) network structure and EG particles are attached to rGO sheets and uniformly interspersed in the aerogel. The oxygen-containing functional groups remaining in rGO/EG aerogel promote heterogeneous crystallization of paraffin, leading to increased latent heat. The 3D thermally conductive pathway provided by rGO/EG aerogel improves the composite PCM's thermal conductivity up to 0.79 W·m⁻¹·K⁻¹, which is about 4 times of that of pure paraffin. The leakage of composite PCMs is remarkably improved at very high percentage of paraffin. Simulative light-thermal experiments reveal that the composite PCMs have the ability of conversion and storage of light-thermal energy. In short, 3D network structure of rGO, with the aid of EG, endows the composite PCMs with improved thermal properties, good shape-stability, and light-thermal storage performance.     

Polymer/graphite oxide composites as high-performance materials for electric double layer capacitors

A single graphene sheet represents a carbon material with the highest surface area available to accommodating molecules or ions for physical and chemical interactions. Here we demonstrate in an electric double layer capacitor the outstanding performance of graphite oxide for providing a platform for double layer formation. Graphite oxide is generally the intermediate compound for obtaining separated graphene sheets. Instead of reduction with hydrazine, we incorporate graphite oxide with a poly(ethylene oxide)-based polymer and anchor the graphene oxide sheets with poly(propylene oxide) diamines. This polymer/graphite oxide composite shows in a “dry” gel-electrolyte system a double layer capacitance as high as 130 F g⁻¹. The polymer incorporation developed here can significantly diversify the application of graphene-based materials in energy storage devices.     

Electrochemical performances of graphene nanosheets prepared through microwave radiation

Graphene nanosheets (GNSs) are prepared by oxidation and rapid expansion of graphite using microwave radiation. The prepared GNSs are characterized using various characterization tools. Morphological characterization using field emission scanning electron microscopy (FE-SEM) and transmission electron microscopic (TEM) studies revealed that the GNSs prepared through microwave radiation are crumbled with scrolled and entangled nature. Raman spectroscopy and X-ray diffraction (XRD) studies confirmed the graphitic crystalline structure of the synthesized GNSs. The surface composition and functional groups introduced on the surface of GNSs due to microwave radiation are corroborated using X-ray photoelectron spectroscopy (XPS) and Fourier transform Infrared Spectroscopy (FT-IR). Electrochemical characterization studies of GNSs based anode materials showed an enhanced lithium storage capacity and fine cycle performance. The initial discharge capacity of GNS electrode is 580 mAh g⁻¹ that decreases to 420 mAh g⁻¹ at 50th cycle. Electrochemical impedance spectral analysis reveals that the exchange current density of GNSs increases with the charge-discharge cycle numbers exhibiting the peculiar electrochemical performance.     

Formation of hierarchical 3D cross-linked porous carbon with small addition of graphene for supercapacitors

In the present paper, starch was used as raw material to prepare carbon material with low-temperature hydrothermal route and hierarchical three-dimensional cross-linked porous carbon was successfully synthesized with the help of a small amount of graphene for high-performance supercapacitors. It's found that presence of graphene is a crucial condition for the formation of 3D porous carbon and graphene acts as a skeleton in the porous carbon. This kind of carbon material exhibited very high surface area of 1887.8 m² g⁻¹ and delivered excellent electrochemical performance. Its specific capacitance can reach 141 F g⁻¹ at 0.5 A g⁻¹ and more importantly, after 10,000 cycles 98.6% of initial specific capacitance can be maintained. To explore the practical application of the 3D porous carbon, an asymmetric supercapacitor coin-type device was assembled with 3D porous carbon and graphene as electrode materials in organic electrolyte. The constructed device exhibited high energy density of 48.5 Wh·kg⁻¹ at a power density of 1.5 kW kg⁻¹ and still maintains 39.625 Wh·kg⁻¹ under the high power density (15 kW kg⁻¹). These results will promote the rapid development of 3D porous carbon prepared by low-temperature route and the application in supercapacitors.     

两种石墨烯修饰的复合热界面材料的制备及热性能的研究

采用化学氧化还原法制备的石墨烯和化学气相沉积法制备的三维网状石墨烯共同作为导热填料改性环氧树脂,研究导热填料质量分数的变化对环氧树脂热导率的影响,并进一步测定复合热界面材料的热导率在高温下的稳定性。结果表明:当石墨烯-三维网状石墨烯的质量分数为0.2(石墨烯和三维网状石墨烯的比例为1∶9)时,可使环氧树脂的热导率提高2 400%;三维网状石墨烯的三维网状结构和石墨烯的表面官能团对复合热界面材料的热性能具有显著地影响;三维网状石墨烯为声子提供了快速传输通道,而石墨烯的表面官能团能促进环氧树脂与石墨烯之间形成良好的接触,降低界面热阻,在石墨烯和三维网状石墨烯的协同作用下可提高热界面材料的热导率。此外,可以通过优化导热填料的尺寸,提高复合热界面材料热导率的稳定性。     

Crystalline polymer functionalized non-oxidized graphene flakes for high gas barrier composites

High gas barrier polyethylene (PE) composites containing non-oxidized graphene flakes (GFs) were fabricated through solution blending. GFs were synthesized through ultrasonic-assisted electrochemical exfoliation of graphite. Then, GFs were functionalized via directly inducing PE molecular crystallization along their surface under proper temperature and PE/GFs ratio. The composites with only 0.5 wt% GFs gave a helium leak rate of 6.29 × 10^?8 Pa m³/s at 0.6 MPa, outperforming pure PE with a decrease of 83.4%. In addition, the interface interaction between PE and GFs is very weak, and the cohesive energy is only 0.1448 J m^?2. Therefore, at high GFs concentrations (>0.5 wt%), the continuous weak interfaces between the GFs interconnected networks and PE will reduce the effective diffusion path to a certain extent, leading to the reduction of gas barrier efficiency. In general, this method offers great promise for barrier materials that can be utilized in lightweight high-pressure gas storage vessels.     

Enhanced photocatalytic activities of three-dimensional graphene-based aerogel embedding TiO2 nanoparticles and loading MoS2 nanosheets as Co-catalyst

A novel graphene-based three-dimensional (3D) aerogel embedded with two types of functional nanomaterials had been prepared by a facile one-pot hydrothermal process. During the hydrothermal reaction, graphene, TiO₂ nanoparticles and MoS₂ nanosheets were self-assembled into the 3D interconnected networks aerogel, where the uniformly dispersed TiO₂ nanoparticles were densely anchored onto the graphene nanosheets and decorated with the ultrathin MoS₂ nanosheets. The UV–vis DRS and PL spectra measurement shows that the MoS₂/P25/graphene aerogel exhibits enhanced light absorption and efficient charge separation properties. As a new photocatalyst, the photocatalytic activity was evaluated by photoelectrochemical test and photodegradation methyl orange (MO) under UV irradiation, an improvement of photocurrent was observed, as 6 times higher for MoS₂/P25/graphene aerogel (37.45 mA/cm²) than pure P25 at +0.6 V, and the fastest photodegradation of MoS₂/P25/graphene aerogel was found within 15 min. The improved photocatalytic activity is attributed to the porous structure, good electrical conductivity and the maximization of accessible sites of the unique 3D graphene aerogel, the increasing active adsorption sites and photocatalytic reaction centers for the introduction of MoS₂ nanosheets, and the positive synergetic effect between the three components in this hybrid. This work demonstrates that the as-prepared MoS₂/P25/graphene aerogel may have a great potential application in photoelectrochemical hydrogen production and pollution removal.     

Facile synthesis of three‐dimensional graphene networks by magnetron sputtering for supercapacitor electrode

Three‐dimensional (3D) graphene network deposition on Ni foam without any conductive agents and polymer binders was successfully synthesized by radio frequency magnetron sputtering at low temperature. The direct and close contact between graphene and Ni foam is beneficial to the enhanced conductivity of the electrode, as well as the improvement of ion diffusion/transport into the electrode. As a result, the 3D graphene network deposition on Ni foam electrode delivered a high specific capacitance of 122.0 F g⁻¹ at 1.0 A g⁻¹ and excellent cycle stability with capacitance retention of 99.0% after 1000 charge–discharge cycles. The work shows a new way to facile synthesis of 3D graphene network for various applications in the future. Copyright © 2016 John Wiley & Sons, Ltd.     

Layered MoS2–graphene composites for supercapacitor applications with enhanced capacitive performance

Layered molybdenum disulfide (MoS₂)–graphene composite is synthesized by a modified l-cysteine-assisted solution-phase method. The structural characterization of the composites by energy dispersive X-ray analysis, X-ray powder diffraction, Fourier transform infrared spectroscopy, XPS, Raman, and transmission electron microscope indicates that layered MoS₂–graphene coalescing into three-dimensional sphere-like architecture. The electrochemical performances of the composites are evaluated by cyclic voltammogram, galvanostatic charge–discharge and electrochemical impedance spectroscopy. Electrochemical measurements reveal that the maximum specific capacitance of the MoS₂–graphene electrodes reaches up to 243 F g⁻¹ at a discharge current density 1 A g⁻¹. The energy density is 73.5 Wh kg⁻¹ at a power density of 19.8 kW kg⁻¹. The MoS₂–graphene composites electrode shows good long-term cyclic stability (only 7.7% decrease in specific capacitance after 1000 cycles at a current density of 1 A g⁻¹). The enhancement in specific capacitance and cycling stability is believed to be due to the 3D MoS₂–graphene interconnected conductive network which promotes not only efficient charge transport and facilitates the electrolyte diffusion, but also prevents effectively the volume expansion/contraction and aggregation of electroactive materials during charge–discharge process. Taken together, this work indicates MoS₂–graphene composites are promising electrode material for high-performance supercapacitors.     

P-doped 3D graphene network supporting uniformly vertical MoS2 nanosheets for enhanced hydrogen evolution reaction

In order to optimize the conductivity of molybdenum disulfide (MoS₂) and promote its large-scale application as a catalyst for hydrogen evolution, MoS₂ is usually used to form composites with conductive materials, but these hybrid materials suffer from scare active sites, overlapping and complicate process. In this work, phosphoric acid is used as a builder of stereoscopic structures, which can not only twist graphene sheets into a P-doped three-dimensional (3D) graphene network but also promote surface electron transport between graphene sheets. Without adding additional framework materials such as carbon nanotubes or nickel foam, stereoscopic MoS₂/graphene structures are formed with a large number of twisted graphene sheets to support the vertical growth of MoS₂ and expose the edge sites of MoS₂, showing a low Tafel slope about 35 mV dec⁻¹, a high current density of 900 mA cm⁻² at about 300 mV and a robust stability over 2000 cycles. Thus, this work shows a possibility to synthesize an efficient catalyst on a large-scale for hydrogen evolution reaction, which can promote the realization of hydrogen economy.     

Improvement of electrochemical characteristics of natural graphite negative electrode coated with polyacrylic acid in pure propylene carbonate electrolyte

In order to improve the negative electrode characteristics of a graphite electrode in a propylene carbonate (PC)-containing electrolyte, we have prepared a graphite negative electrode coated with a water-soluble anionic polymer as a binder for composite graphite electrodes. The electrochemical characteristics of the coated graphite were evaluated by cyclic voltammetry and charge–discharge cycle tests. The coated graphite negative electrode showed a stable Li⁺ ion intercalation/deintercalation reaction without the exfoliation of the graphene layers caused by the co-intercalation of the PC solvent in the LiClO₄/PC solution. The charge–discharge characteristic of the coated graphite negative electrode in a PC-containing electrolyte was almost the same as that in ethylene carbonate-based electrolyte.     

Engineering of N,P,S-Triple doped 3-dimensional graphene architecture: Catalyst-support for “surface-clean” Pd nanoparticles to boost the electrocatalysis of ethanol oxidation reaction

The engineering of robust electrocatalysts for the ethanol oxidation reaction (EOR) with cost-natural, superior electrocatalytic activity, and stability, is crucial for the scaled-up applications of direct ethanol fuel cells. Herein, a facile bottom-up hydrothermal strategy has been implemented to synthesize N,P,S triple-doped 3-dimensional (3D) graphene architectures (N,P,S-3DG) with interconnected, hierarchical porous structure, followed by Pd nanoparticles were uniformly decorated onto the N,P,S-3DG via solvothermal approach. As fabricated hybrid nanocatalyst, labeled as Pd@N,P,S-3DG, is of charming physicochemical characteristics including large electrochemically active specific surface area, interconnected hierarchical pore network, a satisfactory percentage of heteroatom dopants, uniform distribution of Pd nanoparticles, as well as superior electrocatalytic performance metrics such as high catalytic activity, long-term stability, and tolerance to poisoning. The characterizations have confirmed the strong electrostatic interaction between the Pd nanoparticles and carbonaceous support material, thereby leading to homogeneously anchoring Pd nanoparticles onto 3D architecture and forming of novel active sites as well as synergistically boosting the EOR catalytic activity. The Pd@N,P,S-3DG has offered an enlarged electrochemically active surface area (50.3 m² g^?1), an enhanced catalytic current density of 1784 mA mg^?1_Pd, and outstanding long-term stability, thereby distinctly transcending those of commercial carbonaceous material-supported Pd catalysts. The work is of great importance since it may pave the way for the rational design of low-cost high-performance carbonaceous-based nano-electrocatalysts to be utilized in large-scale energy applications.     

Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts

A promising electrocatalyst material composed of 2D layered MoS₂-WS₂ heterostructure hierarchically assembled into a 3D highly interconnected macroporous network of graphene was facilely fabricated. This in-situ synthesis method involves hydrothermal reaction followed by moderate thermal annealing which guarantees the uniform distribution of the MoS₂-WS₂ heterojunctions within graphene matrix. The presence of 3D conductive and porous graphene network and the combined merits of MoS₂ and WS₂ endow the resulting 3D MoS₂-WS₂/graphene nanohybrids with unique conductivity pathways and channels for electrons and with outstanding electrocatalytic performance towards enhanced hydrogen evolution reaction (HER). This 3D nanohybrid delivered the small overpotential of 110 mV, and the small Tafel slope of 41 mV per dec, demonstrating high HER activity. Furthermore, the resulting nanohybrids exhibit excellent stability as very trivial drop in the current density was noticed even after 2000 cycles. The superior electrocatalytic performance of 3D MoS₂-WS₂/graphene over other non-precious metal electrocatalysts is accredited to the robust synergism of 2D MoS₂-WS₂ with 3D graphene that offer ample active sites and improved conductivity for HER. The proposed approach can be further extended to modify other layered transition metal dichalcogenides with hierarchical 3D porous structure as a competent electrocatalysts for HER.     

Solid-state synthesis of amorphous TiB2 nanoparticles on graphene nanosheets with enhanced catalytic dehydrogenation of MgH2

A bi-component catalyst TiB₂/GNSs (GNSs is the abbreviation of graphene nanosheets) is synthesized by a solid-state method. Microstructural characterizations based on SEM (scanning electron microscopy), TEM (transmission electron microscopy) and N₂ physisorption show that the size of TiB₂/GNSs catalyst is at nanoscale (20–30 nm) with a surface area of 84.69 m² g⁻¹. The TiB₂/GNSs nanoparticles ball milled with MgH₂ and exhibit enhanced catalytic effects on the dehydrogenation properties of MgH₂ compares to TiB₂ and GNSs individually. DSC (differential scanning calorimetry) measurements confirm that the peak desorption temperature of MgH₂-5 wt%TiB₂/GNSs composites can be lowered more than 44 °C than the pure as-milled MgH₂. And the dehydrogenation kinetics of TiB₂/GNSs-doped MgH₂ is severalfold acceleration compares to the pure as-milled MgH₂. It is proposed that the TiB₂/GNSs nanoparticles could significantly enhance the intimate interface between TiB₂/GNSs and hydride, therefore, provide more active “catalytic sites” and H “diffusion channels” to reduce the dehydrogenation temperature and improve the dehydrogenation kinetics of MgH₂. The synergistic effect of nano-GNSs and TiB₂ nanoparticles contributes to the highly efficient for dehydrogenation of MgH₂-5wt%TiB₂/GNSs composites.     

Fabrication of electrochemically interconnected MoO3/GO/MWCNTs/graphite sheets for high performance all-solid-state symmetric supercapacitor

Binder-free MoO₃/GO/MWCNTs/graphite sheets were successfully fabricated via electrodeposition of graphene oxide and functionalized multi walled carbon nanotube onto graphite sheets followed by electrodeposition of molybdenum oxide. The capacitive behavior of the MoO₃/GO/MWCNTs/G sheet was found to be superior with respect to those of MoO₃/MWCNTs/graphite and MoO₃/GO/G sheets. The high wettability, interconnected structure and synergetic effects between MoO₃, GO and MWCNTs made the MoO₃/GO/MWCNTs/G sheet exhibited a high areal capacitance of 103 mF cm⁻² at current density of 0.7 mA cm⁻² in 1.0 M KCl. An all-solid-state symmetric supercapacitor device prepared by using the MoO₃/GO/MWCNTs/G sheet as both positive and negative electrodes showed high cell voltage of 2.5 V and remarkable cycle life of 86.8% retention after 2000 cycles, suggesting the possibility for practical applications in energy storage device.     

Review on use of phase change materials in battery thermal management for electric and hybrid electric vehicles

The battery electric vehicle is evolving and has the potential to replace conventional internal combustion‐based vehicles in the future. Batteries are the major power source of these vehicles. A thermal management system is required for a battery to attain effective operation and long life in all environmental conditions. Although several types of thermal management system are available, there remains a need to address various issues like high power consumption, narrow optimum temperature range and operation in varying climates. Phase change materials can assist in resolving these issues. In this paper, battery thermal management systems for electric and hybrid electric vehicles are reviewed, and challenges and opportunities for battery electric vehicles are discussed. Cooling strategies used in various thermal management systems are explained. Applications of and issues regarding the use of phase change materials in thermal management systems are also reviewed. Potential bottlenecks that need to be addressed in electric vehicle technology are explained, as are important achievement milestones and trends regarding the growth of the electric vehicle industry. It is shown that using graphite can increase thermal conductivity of PCMs by up to 70 W m^{‐ 1}K^{‐ 1}. Some commercially available passive thermal management systems for batteries use wax and graphite, which can increase the driving range of an electric scooter from 30 km to 55 km. Copyright © 2016 John Wiley & Sons, Ltd.     

Functional interface of polymer modified graphite anode

Graphite electrodes were modified by polyacrylic acid (PAA), polymethacrylic acid (PMA), and polyvinyl alcohol (PVA). Their electrochemical properties were examined in 1 mol dm⁻³ LiClO₄ ethylene carbonate:dimethyl carbonate (EC:DMC) and propylene carbonate (PC) solutions as an anode of lithium ion batteries. Generally, lithium ions hardly intercalate into graphite in the PC electrolyte due to a decomposition of the PC electrolyte at ca. 0.8 V vs. Li/Li⁺, and it results in the exfoliation of the graphene layers. However, the modified graphite electrodes with PAA, PMA, and PVA demonstrated the stable charge–discharge performance due to the reversible lithium intercalation not only in the EC:DMC but also in the PC electrolytes since the electrolyte decomposition and co-intercalation of solvent were successfully suppressed by the polymer modification. It is thought that these improvements were attributed to the interfacial function of the polymer layer on the graphite which interacted with the solvated lithium ions at the electrode interface.     

Synthesis of three-dimensional graphene architectures by using an environmental-friendly surfactant as a reducing agent

Polyvinylpyrrolidone (PVP) modified reduced graphene aerogel (PVP-GA) was prepared through hydrothermal reduction and chemical reduction of graphene oxide (GO) in the presence of PVP. The structure of PVP-GA was characterized using XRD, FTIR, SEM, TEM and BET specific surface area. The results showed that GO was reduced effectively in the presence of PVP and the PVP molecules were absorbed onto the basal plane of reduced graphene oxide (rGO) through non-covalent interactions. The preparation process was carried out in aqueous media at 120 °C for 10 h without using any toxic reducing agent, thereby making it a facile, environmental-friendly and economical approach for the synthesis of a three-dimensional (3D) interconnected graphene macrostructure. The as-prepared monolithic 3D graphene exhibits a loose and high porous structure, which may find potential applications for adsorbent, shock absorber, catalyst carrier and hydrogen storage materials. In addition, it confirmed that simultaneous hydrothermal reduction and chemical reduction routes would be of value for preparing 3D nonporous graphene-based materials via the use of cheap and environmentally-friendly PVP as a reducing agent.     

Electrochemical performance of Li4Ti5O12/carbon nanotubes/graphene composite as an anode material in lithium-ion batteries

A three-dimensional Li₄Ti₅O₁₂/carbon nanotubes/graphene composite (LTO-CNT-G) was prepared by ball-milling method, followed by microwave heating. The as-prepared LTO-CNT-G composite as anode material in lithium-ion battery exhibited superior rate capability and cycle performance under relative high current density compared with that of Li₄Ti₅O₁₂/CNTs (LTO-CNT) and Li₄Ti₅O₁₂/graphene (LTO-G) composites. Graphene nanosheets and CNTs were used to construct 3D conducting networks, leading to faster electron transfer and lower resistance during the lithium ion reversible reaction, which significantly enhanced the electrochemical activity of LTO-CNT-G composite. The synergistic effect of graphene and CNTs can greatly improve the rate capability and cycling stability of Li₄Ti₅O₁₂-based anodes. The LTO-CNT-G composite exhibited a high initial discharge capacity of 172 mAh g^?1 at 0.2 C and 132 mAh g^?1 at 20 C, as well as an excellent cycling stability. The electrochemical impedance spectroscopy demonstrated that the LTO-CNT-G composite has the smallest charge-transfer resistance compared with the LTO-CNT and LTO-G composites, indicating that the fast electron transfer from the electrolyte to the LTO-CNT-G active materials during the lithium ion intercalation/deintercalation owing to the three-dimensional networks of graphene and CNTs.