Xianghua Kong

Enhanced thermal conductivity of phase change materials with ultrathin-graphite foams for thermal energy storage

For thermophysical energy storage with phase change materials (PCMs), the power capacity is often limited by the low PCM thermal conductivity (κPCM). Though dispersing high-thermal conductivity nanotubes and graphene flakes increases κPCM, the enhancement is limited by interface thermal resistance between the nanofillers, among other factors such as detrimental surface scattering of phonons. Here, we demonstrate that embedding continuous ultrathin-graphite foams (UGFs) with volume fractions as low as 0.8–1.2 vol% in a PCM can increase κPCM by up to 18 times, with negligible change in the PCM melting temperature or mass specific heat of fusion. The increase in κPCM, thermal cycling stability, and applicability to a diverse range of PCMs suggests that UGF composites are a promising route to achieving the high power capacity targets of a number of thermal storage applications, including building and vehicle heating and cooling, solar thermal harvesting, and thermal management of electrochemical energy storage and electronic devices.

Covalently Connected Carbon Nanostructures for Current Collectors in Both the Cathode and Anode of Li–S Batteries

A 3D current collector made of covalently connected carbon nanostructures is presented, which can significantly improve battery performance when used as the cathode and/or anode. A Li-S cell assembled using these current collectors, with the cathode loaded with elemental sulfur and the anode loaded with lithium metal, delivers a high-rate capacity of 860 mA h g-1 at 12 C.

Graphene Synthesis via Magnetic Inductive Heating of Copper Substrates

Scaling graphene growth using an oven to heat large substrates becomes less energy efficient as system size is increased. We report a route to graphene synthesis in which radio frequency (RF) magnetic fields inductively heat metal foils, yielding graphene of quality comparable to or higher than that of current chemical vapor deposition techniques. RF induction heating allows for rapid temperature ramp up/down, with great potential for large scale and rapid manufacturing of graphene with much better energy efficiency. Back-gated field effect transistors on a SiO2/Si substrate showed carrier mobility up to ∼14 000 cm(2) V(-1) s(-1) measured under ambient conditions. Many advantages of RF heating are outlined, and some fundamental aspects of this approach are discussed.

The Origin of Improved Electrical Double‐Layer Capacitance by Inclusion of Topological Defects and Dopants in Graphene for Supercapacitors

Low-energy density has long been the major limitation to the application of supercapacitors. Introducing topological defects and dopants in carbon-based electrodes in a supercapacitor improves the performance by maximizing the gravimetric capacitance per mass of the electrode. However, the main mechanisms governing this capacitance improvement are still unclear. We fabricated planar electrodes from CVD-derived single-layer graphene with deliberately introduced topological defects and nitrogen dopants in controlled concentrations and of known configurations, to estimate the influence of these defects on the electrical double-layer (EDL) capacitance. Our experimental study and theoretical calculations show that the increase in EDL capacitance due to either the topological defects or the nitrogen dopants has the same origin, yet these two factors improve the EDL capacitance in different ways. Our work provides a better understanding of the correlation between the atomic-scale structure and the EDL capacitance and presents a new strategy for the development of experimental and theoretical models for understanding the EDL capacitance of carbon electrodes.

High Areal Capacity and Lithium Utilization in Anodes Made of Covalently Connected Graphite Microtubes

Lithium metal is an attractive anode material for rechargeable batteries because of its high theoretical specific capacity of 3860 mA h g-1 and the lowest negative electrochemical potential of -3.040 V versus standard hydrogen electrode. Despite extensive research efforts on tackling the safety concern raised by Li dendrites, inhibited Li dendrite growth is accompanied with decreased areal capacity and Li utilization, which are still lower than expectation for practical use. A scaffold made of covalently connected graphite microtubes is reported, which provides a firm and conductive framework with moderate specific surface area to accommodate Li metal for anodes of Li batteries. The anode presents an areal capacity of 10 mA h cm-2 (practical gravimetric capacity of 913 mA h g-1 ) at a current density of 10 mA cm-2 , with Li utilization of 91%, Coulombic efficiencies of ≈97%, and long lifespan of up to 3000 h. The analysis of structure evolution during charge/discharge shows inhibited lithium dendrite growth and a reversible electrode volume change of ≈9%. It is suggested that an optimized microstructure with moderate electrode/electrolyte interface area is critical to accommodate volume change and inhibit the risks of irreversible Li consumption by side reactions and Li dendrite growth for high-performance Li-metal anodes.

Surface acoustic wave mediated dielectrophoretic alignment of rolled-up microtubes in microfluidic systems

The alignment behavior of solution dispersed rolled-up microtubes by surface acoustic waves (SAW) is demonstrated. In contrast to the random alignment of rolled-up insulated silicon oxide tubes, metallic chromium tubes can be effectively aligned and assembled into “tube-chains” parallel to the SAW propagation direction. The experiments suggest that the tube orientation is mainly determined by the dielectrophoresis (DEP) forces acting on the tubes. The DEP forces arise from the induced dipole moment of the tubes in the SAW generated piezoelectric field on the LiNbO3 substrate.

Atom-Thick Interlayer Made of CVD-Grown Graphene Film on Separator for Advanced Lithium–Sulfur Batteries

Lithium-sulfur batteries are widely seen as a promising next-generation energy-storage system owing to their ultrahigh energy density. Although extensive research efforts have tackled poor cycling performance and self-discharge, battery stability has been improved at the expense of energy density. We have developed an interlayer consisting of two-layer chemical vapor deposition (CVD)-grown graphene supported by a conventional polypropylene (PP) separator. Unlike interlayers made of discrete nano-/microstructures that increase the thickness and weight of the separator, the CVD-graphene is an intact film with an area of 5 × 60 cm2 and has a thickness of ∼0.6 nm and areal density of ∼0.15 μg cm-2, which are negligible to those of the PP separator. The CVD-graphene on PP separator is the thinnest and lightest interlayer to date and is able to suppress the shuttling of polysulfides and enhance the utilization of sulfur, leading to concurrently improved specific capacity, rate capability, and cycle stability and suppressed self-discharge when assembled with cathodes consisting of different sulfur/carbon composites and electrolytes either with or without LiNO3 additive.

Non-destructive and rapid evaluation of chemical vapor deposition graphene by dark field optical microscopy

Non-destructive and rapid evaluation of graphene directly on the growth substrate (Cu foils) by dark field (DF) optical microscopy is demonstrated. Without any additional treatment, graphene on Cu foils with various coverages can be quickly identified by DF imaging immediately after chemical vapor deposition growth with contrast comparable to scanning electron microscopy. The improved contrast of DF imaging compared to bright field optical imaging was found to be due to Rayleigh scattering of light by the copper steps beneath graphene. Indeed, graphene adlayers are readily distinguished, due to the different height of copper steps beneath graphene regions of different thickness.

Highly pressure-sensitive graphene sponge fabricated by γ-ray irradiation reduction

Graphene sponge (GS) with microscale size, high mechanical elasticity and electrical conductivity has attracted great interest as a sensing material for piezoresistive pressure sensor. However, GS offering a lower limit of pressure detection with high gauge factor, which is closely dependent on the mechanical and electrical properties and determined by the fabrication process, is still demanded. Here, γ-ray irradiation reduced GS is reported to possess a gauge factor of 1.03 kPa–1 with pressure detection limit of 10 Pa and high stress retention of 76% after 800 cycles of compressing/relaxation at strain of 50%. Compared with the carbon nanotube (CNT) reinforced GS, the improved lower limit of pressure detection and gauge factor of the GS prepared by γ-ray irradiation is due to the low compression stress (0.9 kPa at stain of 50%). These excellent physical properties of the GS are ascribed to the mild, residual free, and controllable reduction process offered by γ-ray irradiation.摘要石墨烯海绵具有优异的机械弹性和电导率, 其电导率在石墨烯片相互交联形成的多孔结构被压缩和释放的过程中会发生可逆变化, 因此可作为压阻传感器的敏感材料. 然而, 由于制备方法的限制使得目前报道的石墨烯海绵的压缩模量较大, 在检测低应力时材料的电导率变化不明显, 导致压阻传感器的灵敏度和检测限不足. 本文利用γ射线辐照还原法提供的温和、 清洁的还原条件, 制备了压缩模量低的石墨烯海绵. 其作为压敏传感器的敏感材料, 实现了应变灵敏度系数为1.03 kPa−1, 检测限低至10 Pa、 机械稳定性优异(在50%应变下800次 循环最大应力保持76%)的压阻传感器.

Hot-Roll-Pressing Mediated Transfer of Chemical Vapor Deposition Graphene for Transparent and Flexible Touch Screen with Low Sheet-Resistance

Obstacles associated with graphene as transparent conductive films mainly consist of the difficulties in high-quality graphene synthesis, efficient transfer and doping of samples with lateral size of tens of centimeters for practical applications. Herein we demonstrate a hot-roll-pressing transfer technique followed by wet-chemical doping of large area graphene film grown on copper foil by chemical vapor deposition (CVD). This method enabled cost-effective and ultraclean transfer of single-layer graphene with an arbitrary size onto transparent ethylene vinyl acetate/polyethylene terephthalate (EVA/PET) substrate without any polymer residues. The sheet resistance of the single-layer graphene covered EVA/PET (graphene/EVA/PET) reached 200 Ω/sq with optical transparency of 87.3%. The graphene/EVA/PET film can be bent over 10000 cycles at a radius of 2 mm with ∼0.02% increase in sheet resistance, showing excellent mechanical flexibility for bendable electronics which was demonstrated by a capacitive-type touch screen based on the graphene/EVA/PET transparent conducting film.