Thermal Transport in Three-Dimensional Foam Architectures of Few-Layer Graphene and Ultrathin Graphite

At a very low solid concentration of 0.45±0.09 vol %, the room-temperature thermal conductivity (κ_GF) of freestanding graphene-based foams (GF), comprised of few-layer graphene (FLG) and ultrathin graphite (UG) synthesized through the use of methane chemical vapor deposition on reticulated nickel foams, was increased from 0.26 to 1.7 W m^–1 K^–1 after the etchant for the sacrificial nickel support was changed from an aggressive hydrochloric acid solution to a slow ammonium persulfate etchant. In addition, κ_GF showed a quadratic dependence on temperature between 11 and 75 K and peaked at about 150 K, where the solid thermal conductivity (κ_G) of the FLG and UG constituents reached about 1600 W m^–1 K^–1, revealing the benefit of eliminating internal contact thermal resistance in the continuous GF structure

Crystalline Copper Phosphide Nanosheets as an Efficient Janus Catalyst for Overall Water Splitting

Hydrogen is essential to many industrial processes and could play an important role as an ideal clean energy carrier for future energy supply. Herein, we report for the first time the growth of crystalline Cu₃P phosphide nanosheets on conductive nickel foam (Cu₃P@NF) for electrocatalytic and visible light-driven overall water splitting. Our results show that the Cu₃P@NF electrode can be used as an efficient Janus catalyst for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). For OER catalysis, a current density of 10 mA/cm² requires an overpotential of only ∼320 mV and the slope of the Tafel plot is as low as 54 mV/dec in 1.0 M KOH. For HER catalysis, the overpotential is only ∼105 mV to achieve a catalytic current density of 10 mA cm^–2. Moreover, overall water splitting can be achieved in a water electrolyzer based on the Cu₃P@NF electrode, which showed a catalytic current density of 10 mA/cm² under an applied voltage of ∼1.67 V. The same current density can also be obtained using a silicon solar cell under ∼1.70 V for both the HER and the OER. This new Janus Cu₃P@NF electrode is made of inexpensive and nonprecious metal-based materials, which opens new possibilities based on copper to exploit overall water splitting for hydrogen production. To the best of our knowledge, such high performance of a copper-based water oxidation and overall water splitting catalyst has not been reported to date

Nitrogen-Doped Hollow Carbon Nanospheres for High-Performance Li-Ion Batteries

N-doped carbon materials is of particular attraction for anodes of lithium-ion batteries (LIBs) because of their high surface areas, superior electrical conductivity, and excellent mechanical strength, which can store energy by adsorption/desorption of Li⁺ at the interfaces between the electrolyte and electrode. By directly carbonization of zeolitic imidazolate framework-8 nanospheres synthesized by an emulsion-based interfacial reaction, we obtained N-doped hollow carbon nanospheres with tunable shell thickness (20 nm to solid sphere) and different N dopant concentrations (3.9 to 21.7 at %). The optimized anode material possessed a shell thickness of 20 nm and contained 16.6 at % N dopants that were predominately pyridinic and pyrrolic. The anode delivered a specific capacity of 2053 mA h g^–1 at 100 mA g^–1 and 879 mA h g^–1 at 5 A g^–1 for 1000 cycles, implying a superior cycling stability. The improved electrochemical performance can be ascribed to (1) the Li⁺ adsorption dominated energy storage mechanism prevents the volume change of the electrode materials, (2) the hollow nanostructure assembled by the nanometer-sized primary particles prevents the agglomeration of the nanoparticles and favors for Li⁺ diffusion, (3) the optimized N dopant concentration and configuration facilitate the adsorption of Li⁺; and (4) the graphitic carbon nanostructure ensures a good electrical conductivity

Low-Temperature Chemical Vapor Deposition Growth of Graphene from Toluene on Electropolished Copper Foils

A two-step CVD route with toluene as the carbon precursor was used to grow continuous large-area monolayer graphene films on a very flat, electropolished Cu foil surface at 600 °C, lower than any temperature reported to date for growing continuous monolayer graphene. Graphene coverage is higher on the surface of electropolished Cu foil than that on the unelectropolished one under the same growth conditions. The measured hole and electron mobilities of the monolayer graphene grown at 600 °C were 811 and 190 cm²/(V·s), respectively, and the shift of the Dirac point was 18 V. The asymmetry in carrier mobilities can be attributed to extrinsic doping during the growth or transfer. The optical transmittance of graphene at 550 nm was 97.33%, confirming it was a monolayer, and the sheet resistance was ∼8.02 × 10³ Ω/□

Nanoporous Ni(OH)₂ Thin Film on 3D Ultrathin-Graphite Foam for Asymmetric Supercapacitor

Nanoporous nickel hydroxide (Ni(OH)₂) thin film was grown on the surface of ultrathin-graphite foam (UGF) <i>via</i> a hydrothermal reaction. The resulting free-standing Ni(OH)₂/UGF composite was used as the electrode in a supercapacitor without the need for addition of either binder or metal-based current collector. The highly conductive 3D UGF network facilitates electron transport and the porous Ni(OH)₂ thin film structure shortens ion diffusion paths and facilitates the rapid migration of electrolyte ions. An asymmetric supercapacitor was also made and studied with Ni(OH)₂/UGF as the positive electrode and activated microwave exfoliated graphite oxide (‘a-MEGO’) as the negative electrode. The highest power density of the fully packaged asymmetric cell (44.0 kW/kg) was much higher (2–27 times higher), while the energy density was comparable to or higher, than high-end commercially available supercapacitors. This asymmetric supercapacitor had a capacitance retention of 63.2% after 10 000 cycles

Ultrathin Graphite Foam: A Three-Dimensional Conductive Network for Battery Electrodes

We report the use of free-standing, lightweight, and highly conductive ultrathin graphite foam (UGF), loaded with lithium iron phosphate (LFP), as a cathode in a lithium ion battery. At a high charge/discharge current density of 1280 mA g^–1, the specific capacity of the LFP loaded on UGF was 70 mAh g^–1, while LFP loaded on Al foil failed. Accounting for the total mass of the electrode, the maximum specific capacity of the UGF/LFP cathode was 23% higher than that of the Al/LFP cathode and 170% higher than that of the Ni-foam/LFP cathode. Using UGF, both a higher rate capability and specific capacity can be achieved simultaneously, owing to its conductive (∼1.3 × 10⁵ S m^–1 at room temperature) and three-dimensional lightweight (∼9.5 mg cm^–3) graphitic structure. Meanwhile, UGF presents excellent electrochemical stability comparing to that of Al and Ni foils, which are generally used as conductive substrates in lithium ion batteries. Moreover, preparation of the UGF electrode was facile, cost-effective, and compatible with various electrochemically active materials

From 1D Polymers to 2D Polymers: Preparation of Free-Standing Single-Monomer-Thick Two-Dimensional Conjugated Polymers in Water

Recently, investigation on two-dimensional (2D) organic polymers has made great progress, and conjugated 2D polymers already play a dynamic role in both academic and practical applications. However, a convenient, noninterfacial approach to obtain single-layer 2D polymers in solution, especially in aqueous media, remains challenging. Herein, we present a facile, highly efficient, and versatile “1D to 2D” strategy for preparation of free-standing single-monomer-thick conjugated 2D polymers in water without any aid. The 2D structure was achieved by taking advantage of the side-by-side self-assembly of a rigid amphiphilic 1D polymer and following topochemical photopolymerization in water. The spontaneous formation of single-layer polymer sheets was driven by synergetic association of the hydrophobic interactions, π–π stacking interactions, and electrostatic repulsion. Both the supramolecular sheets and the covalent sheets were confirmed by spectroscopic analyses and electron microscope techniques. Moreover, in comparison of the supramolecular 2D polymer, the covalent 2D polymer sheets exhibited not only higher mechanical strength but also higher conductivity, which can be ascribed to the conjugated network within the covalent 2D polymer sheets

Growth Mechanism and Controlled Synthesis of AB-Stacked Bilayer Graphene on Cu–Ni Alloy Foils

Strongly coupled bilayer graphene (<i>i.e.</i>, AB stacked) grows particularly well on commercial “90–10” Cu–Ni alloy foil. However, the mechanism of growth of bilayer graphene on Cu–Ni alloy foils had not been discovered. Carbon isotope labeling (sequential dosing of ¹²CH₄ and ¹³CH₄) and Raman spectroscopic mapping were used to study the growth process. It was learned that the mechanism of graphene growth on Cu–Ni alloy is by precipitation at the surface from carbon dissolved in the bulk of the alloy foil that diffuses to the surface. The growth parameters were varied to investigate their effect on graphene coverage and isotopic composition. It was found that higher temperature, longer exposure time, higher rate of bulk diffusion for ¹²C <i>vs</i> ¹³C, and slower cooling rate all produced higher graphene coverage on this type of Cu–Ni alloy foil. The isotopic composition of the graphene layer(s) could also be modified by adjusting the cooling rate. In addition, large-area, AB-stacked bilayer graphene transferrable onto Si/SiO₂ substrates was controllably synthesized

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

Breaking Low-Strain and Deep-Potassiation Trade-Off in Alloy Anodes via Bonding Modulation for High-Performance K‑Ion Batteries

Alloy anode materials have garnered unprecedented attention for potassium storage due to their high theoretical capacity. However, the substantial structural strain associated with deep potassiation results in serious electrode fragmentation and inadequate K-alloying reactions. Effectively reconciling the trade-off between low-strain and deep-potassiation in alloy anodes poses a considerable challenge due to the larger size of K-ions compared to Li/Na-ions. In this study, we propose a chemical bonding modulation strategy through single-atom modification to address the volume expansion of alloy anodes during potassiation. Using black phosphorus (BP) as a representative and generalizing to other alloy anodes, we established a robust P–S covalent bonding network via sulfur doping. This network exhibits sustained stability across discharge–charge cycles, elevating the modulus of K–P compounds by 74%, effectively withstanding the high strain induced by the potassiation process. Additionally, the bonding modulation reduces the formation energies of potassium phosphides, facilitating a deeper potassiation of the BP anode. As a result, the modified BP anode exhibits a high reversible capacity and extended operational lifespan, coupled with a high areal capacity. This work introduces a new perspective on overcoming the trade-off between low-strain and deep-potassiation in alloy anodes for the development of high-energy and stable potassium-ion batteries