Ultralow Interfacial Thermal Resistance of Graphene Thermal Interface Materials with Surface Metal Liquefaction.

Developing advanced thermal interface materials (TIMs) to bridge heat-generating chip and heat sink for constructing an efficient heat transfer interface is the key technology to solve the thermal management issue of high-power semiconductor devices. Based on the ultra-high basal-plane thermal conductivity, graphene is an ideal candidate for preparing high-performance TIMs, preferably to form a vertically aligned structure so that the basal-plane of graphene is consistent with the heat transfer direction of TIM. However, the actual interfacial heat transfer efficiency of currently reported vertically aligned graphene TIMs is far from satisfactory. In addition to the fact that the thermal conductivity of the vertically aligned TIMs can be further improved, another critical factor is the limited actual contact area leading to relatively high contact thermal resistance (20-30 K mm2 W-1) of the "solid-solid" mating interface formed by the vertical graphene and the rough chip/heat sink. To solve this common problem faced by vertically aligned graphene, in this work, we combined mechanical orientation and surface modification strategy to construct a three-tiered TIM composed of mainly vertically aligned graphene in the middle and micrometer-thick liquid metal as a cap layer on upper and lower surfaces. Based on rational graphene orientation regulation in the middle tier, the resultant graphene-based TIM exhibited an ultra-high thermal conductivity of 176 W m-1 K-1. Additionally, we demonstrated that the liquid metal cap layer in contact with the chip/heat sink forms a "liquid-solid" mating interface, significantly increasing the effective heat transfer area and giving a low contact thermal conductivity of 4-6 K mm2 W-1 under packaging conditions. This finding provides valuable guidance for the design of high-performance TIMs based on two-dimensional materials and improves the possibility of their practical application in electronic thermal management.

Aqueous Organic Zinc-Ion Hybrid Supercapacitors Prepared by 3D Vertically Aligned Graphene-Polydopamine Composite Electrode.

A three-dimensional vertical-aligned graphene-polydopamine electrode (PDA@3DVAG) composite with vertical channels and conductive network is prepared by a method of unidirectional freezing and subsequent self-polymerization. When the prepared PDA@3DVAG is constructed as the positive electrode of zinc-ion hybrid supercapacitors (ZHSCs), excellent electrochemical performances are obtained. Compared with the conventional electrolyte, PDA@3DVAG composite electrode in highly concentrated salt electrolyte exhibits better multiplicity performance (48.92% at a current density of 3 A g-1), wider voltage window (-0.8~0.8 V), better cycle performance with specific capacitance from 96.7 to 59.8 F g-1, and higher energy density (46.14 Wh kg-1).

Preparation and Electrochemical Performance of Three-Dimensional Vertically Aligned Graphene by Unidirectional Freezing Method.

Three-dimensional vertically aligned graphene (3DVAG) was prepared by a unidirectional freezing method, and its electrochemical performances were evaluated as electrode materials for zinc-ion hybrid supercapacitors (ZHSCs). The prepared 3DVAG has a vertically ordered channel structure with a diameter of about 20-30 µm and a length stretching about hundreds of microns. Compared with the random structure of reduced graphene oxide (3DrGO), the vertical structure of 3DVAG in a three-electrode system showed higher specific capacitance, faster ion diffusion, and better rate performance. The specific capacitance of 3DVAG reached 66.6 F·g-1 and the rate performance reached 92.2%. The constructed 3DVAG zinc-ion hybrid supercapacitor also showed excellent electrochemical performance. It showed good capacitance retention up to 94.6% after 3000 cycles at the current density of 2 A·g-1.

Multiscale Structural Modulation of Anisotropic Graphene Framework for Polymer Composites Achieving Highly Efficient Thermal Energy Management.

Graphene is usually embedded into polymer matrices for the development of thermally conductive composites, preferably forming an interconnected and anisotropic framework. Currently, the directional self-assembly of exfoliated graphene sheets is demonstrated to be the most effective way to synthesize anisotropic graphene frameworks. However, achieving a thermal conductivity enhancement (TCE) over 1500% with per 1 vol% graphene content in polymer matrices remains challenging, due to the high junction thermal resistance between the adjacent graphene sheets within the self-assembled graphene framework. Here, a multiscale structural modulation strategy for obtaining highly ordered structure of graphene framework and simultaneously reducing the junction thermal resistance is demonstrated. The resultant anisotropic framework contributes to the polymer composites with a record-high thermal conductivity of 56.8-62.4 W m-1 K-1 at the graphene loading of ≈13.3 vol%, giving an ultrahigh TCE per 1 vol% graphene over 2400%. Furthermore, thermal energy management applications of the composites as phase change materials for solar-thermal energy conversion and as thermal interface materials for electronic device cooling are demonstrated. The finding provides valuable guidance for designing high-performance thermally conductive composites and raises their possibility for practical use in thermal energy storage and thermal management of electronics.

Valence Alignment of Mixed Ni-Fe Hydroxide Electrocatalysts through Preferential Templating on Graphene Edges for Enhanced Oxygen Evolution.

Engineering the metal-carbon heterointerface has become an increasingly important route toward achieving cost-effective and high-performing electrocatalysts. The specific properties of graphene edge sites, such as the high available density of states and extended unpaired π-bonding, make it a promising candidate to tune the electronic properties of metal catalysts. However, to date, understanding and leveraging graphene edge-metal catalysts for improved electrocatalytic performance remains largely elusive. Herein, edge-rich vertical graphene (er-VG) was synthesized and used as a catalyst support for Ni-Fe hydroxides for the oxygen evolution reaction (OER). The hybrid Ni-Fe/er-VG catalyst exhibits excellent OER performance with a mass current of 4051 A g-1 (at overpotential η = 300 mV) and turnover frequency (TOF) of 4.8 s-1 (η = 400 mV), outperforming Ni-Fe deposited on pristine VG and other metal foam supports. Angle-dependent X-ray absorption spectroscopy shows that the edge-rich VG support can preferentially template Fe-O units with a specific valence orbital alignment interacting with the unoccupied density of states on the graphene edges. This graphene edge-metal interaction was shown to facilitate the formation of undersaturated and strained Fe-sites with high valence states, while promoting the formation of redox-activated Ni species, thus improving OER performance. These findings demonstrate rational design of the graphene edge-metal interface in electrocatalysts which can be used for various energy conversion and chemical synthesis reactions.

Metal-Level Thermally Conductive yet Soft Graphene Thermal Interface Materials.

Along with the technology evolution for dense integration of high-power, high-frequency devices in electronics, the accompanying interfacial heat transfer problem leads to urgent demands for advanced thermal interface materials (TIMs) with both high through-plane thermal conductivity and good compressibility. Most metals have satisfactory thermal conductivity but relatively high compressive modulus, and soft silicones are typically thermal insulators (0.3 W m-1 K-1). Currently, it is a great challenge to develop a soft material with the thermal conductivity up to metal level for TIM application. This study solves this problem by constructing a graphene-based microstructure composed of mainly vertical graphene and a thin cap of horizontal graphene layers on both the top and bottom sides through a mechanical machining process to manipulate the stacked architecture of conventional graphene paper. The resultant graphene monolith has an ultrahigh through-plane thermal conductivity of 143 W m-1 K-1, exceeding that of many metals, and a low compressive modulus of 0.87 MPa, comparable to that of silicones. In the actual TIM performance measurement, the system cooling efficiency with our graphene monolith as TIM is 3 times as high as that of the state-of-the-art commercial TIM, demonstrating the superior ability to solve the interfacial heat transfer issues in electronic systems.

High-Stacking-Density, Superior-Roughness LDH Bridged with Vertically Aligned Graphene for High-Performance Asymmetric Supercapacitors.

The high-performance electrode materials with tuned surface and interface structure and functionalities are highly demanded for advanced supercapacitors. A novel strategy is presented to conFigure high-stacking-density, superior-roughness nickel manganese layered double hydroxide (LDH) bridged by vertically aligned graphene (VG) with nickel foam (NF) as the conductive collector, yielding the LDH-NF@VG hybrids for asymmetric supercapacitors. The VG nanosheets provide numerous electron transfer channels for quick redox reactions, and well-developed open structure for fast mass transport. Moreover, the high-stacking-density LDH grown and assembled on VG nanosheets result in a superior hydrophilicity derived from the tuned nano/microstructures, especially microroughness. Such a high stacking density with abundant active sites and superior wettability can be easily accessed by aqueous electrolytes. Benefitting from the above features, the LDH-NF@VG can deliver a high capacitance of 2920 F g-1 at a current density of 2 A g-1 , and the asymmetric supercapacitor with the LDH-NF@VG as positive electrode and activated carbon as negative electrode can deliver a high energy density of 56.8 Wh kg-1 at a power density of 260 W kg-1 , with a high specific capacitance retention rate of 87% even after 10 000 cycles.

Vertical-Aligned Li2 S-Graphene Encapsulated within a Carbon Shell as a Free-Standing Cathode for Lithium-Sulfur Batteries.

Construction of a novel matrix with both high conductivity and an excellent confinement effect for polysulfides is of great importance for developing high-performance lithium-sulfur (Li-S) batteries. In this work, we have developed a double-modification strategy to integrate lithium sulfide (Li2 S) into a conductive composite network consisting of vertical graphene (VG) arrays and an amorphous carbon shell, forming an integrated cathode (VG/Li2 S-C). Facile liquid-solution/evaporation methods in combination with chemical vapor deposition were successfully adopted for preparation of the above cathode. Due to the enhanced electrical conductivity and noticeable blocking effect for the shuttle of polysulfides, the binder-free flexible VG/Li2 S-C cathode exhibits high rate performance and reinforced cycles (656.2 mAh g-1 after 100 cycles). The pronounced electrochemical performance is ascribed to the unique architecture with a coherent conductive network of VG and the carbon shell, which not only provides a conductive network for fast reaction kinetics, but also forms a durable protective shield to suppress the shuttle of polysulfides. Our research further demonstrates the synergistic effectiveness by means of inner and outer carbon matrixes for electrochemical enhancement of Li-S batteries.

Vertically Aligned Graphene Sheets Membrane for Highly Efficient Solar Thermal Generation of Clean Water.

Efficient utilization of solar energy for clean water is an attractive, renewable, and environment friendly way to solve the long-standing water crisis. For this task, we prepared the long-range vertically aligned graphene sheets membrane (VA-GSM) as the highly efficient solar thermal converter for generation of clean water. The VA-GSM was prepared by the antifreeze-assisted freezing technique we developed, which possessed the run-through channels facilitating the water transport, high light absorption capacity for excellent photothermal transduction, and the extraordinary stability in rigorous conditions. As a result, VA-GSM has achieved average water evaporation rates of 1.62 and 6.25 kg m-2 h-1 under 1 and 4 sun illumination with a superb solar thermal conversion efficiency of up to 86.5% and 94.2%, respectively, better than that of most carbon materials reported previously, which can efficiently produce the clean water from seawater, common wastewater, and even concentrated acid and/or alkali solutions.

M3C (M: Fe, Co, Ni) Nanocrystals Encased in Graphene Nanoribbons: An Active and Stable Bifunctional Electrocatalyst for Oxygen Reduction and Hydrogen Evolution Reactions.

Transition metal carbide nanocrystalline M3C (M: Fe, Co, Ni) encapsulated in graphitic shells supported with vertically aligned graphene nanoribbons (VA-GNRs) are synthesized through a hot filament chemical vapor deposition (HF-CVD) method. The process is based on the direct reaction between iron group metals (Fe, Co, Ni) and carbon source, which are facilely get high purity carbide nanocrystals (NCs) and avoid any other impurity at relatively low temperature. The M3C-GNRs exhibit superior enhanced electrocatalystic activity for oxygen reduction reaction (ORR), including low Tafel slope (39, 41, and 45 mV dec(-1) for Fe3C-GNRs, Co3C-GNRs, and Ni3C-GNRs, respectively), positive onset potential (∼0.8 V), high electron transfer number (∼4), and long-term stability (no obvious drop after 20 000 s test). The M3C-GNRs catalyst also exhibits remarkable hydrogen evolution reaction (HER) activity with a large cathodic current density of 166.6, 79.6, and 116.4 mA cm(-2) at an overpotential of 200 mV, low onset overpotential of 32, 41, and 35 mV, small Tafel slope of 46, 57, and 54 mV dec(-1) for Fe3C-GNRs, Co3C-GNRs, and Ni3C-GNRs, respectively, as well as an excellent stability in acidic media.

ZnO anchored on vertically aligned graphene: binder-free anode materials for lithium-ion batteries.

ZnO has been regarded as a promising anode material for the next-generation lithium-ion battery. Unfortunately, the structure broken caused by the volume change of ZnO and the capacity degression due to the irreversible electrochemical reaction of ZnO still remain two major challenges. Here, we design a novel kind of in situ growth binder-free ZnO-based anodes via ZnO anchored on vertically aligned graphene. The composite anode retains physical integrity post cycling. Especially, the good conductivity of graphene and the ultrasmall size of ZnO particles help to produce a completely reversible electrochemical reaction of ZnO-based anode. The composite material exhibits a high capacity (810 mAh g(-1)), long cycle life, good cycle stability, and fast charge/discharge rate.

Embedded into graphene Ge nanoparticles highly dispersed on vertically aligned graphene with excellent electrochemical performance for lithium storage.

Decreasing particle size has always been reported to be an efficient way to improve cyclability of Li-alloying based LIBs. However, nanoparticles (NPs) tend to agglomerate and evolve into lumps, which in turn limits the cycling performance. In this report, we prepared a unique nanostructure, graphene-coated Ge NPs are highly dispersed on vertically aligned graphene (Ge@graphene/VAGN), to avoid particle agglomeration and pulverization. Remarkable structure stability of the sample leads to excellent cycling stability. Upon cycling, the anode exhibits a high capacity of 1014 mAh g(-1), with nearly no capacity loss in 90 cycles. Rate performance shows that even at the high current density of 13 A g(-1), the anode could still deliver a higher capacity than that of graphite.