Compositing effects for high thermoelectric performance of Cu2Se-based materials.

Thermoelectric materials can realize direct conversion between heat and electricity, showing excellent potential for waste heat recovery. Cu2Se is a typical superionic conductor thermoelectric material having extraordinary ZT values, but its superionic feature causes poor service stability and low mobility. Here, we reported a fast preparation method of self-propagating high-temperature synthesis to realize in situ compositing of BiCuSeO and Cu2Se to optimize the service stability. Additionally, using the interface design by introducing graphene in these composites, the carrier mobility could be obviously enhanced, and the strong phonon scatterings could lead to lower lattice thermal conductivity. Ultimately, the Cu2Se-BiCuSeO-graphene composites presented excellent thermoelectric properties with a ZTmax value of ~2.82 at 1000 K and a ZTave value of ~1.73 from 473 K to 1000 K. This work provides a facile and effective strategy to largely improve the performance of Cu2Se-based thermoelectric materials, which could be further adopted in other thermoelectric systems.

Enhancing Thermoelectric Properties of (Cu2Te)1-x-(BiCuTeO)x Composites by Optimizing Carrier Concentration.

Because of the high carrier concentration, copper telluride (Cu2Te) has a relatively low Seebeck coefficient and high thermal conductivity, which are not good for its thermoelectric performance. To simultaneously optimize carrier concentration, lower thermal conductivity and improve the stability, BiCuTeO, an oxygen containing compound with lower carrier concentration, is in situ formed in Cu2Te by a method of combining self-propagating high-temperature synthesis (SHS) with spark plasma sintering (SPS). With the incorporation of BiCuTeO, the carrier concentration decreased from 8.1 × 1020 to 3.8 × 1020 cm-3, bringing the increase of power factor from ~1.91 to ~2.97 µW cm-1 K-2 at normal temperature. At the same time, thermal conductivity reduced from 2.61 to 1.48 W m-1 K-1 at 623 K. Consequently, (Cu2Te)0.95-(BiCuTeO)0.05 composite sample reached a relatively high ZT value of 0.13 at 723 K, which is 41% higher than that of Cu2Te.

Porous-Carbon Aerogels with Tailored Sub-Nanopores for High Cycling Stability and Rate Capability Potassium-Ion Battery Anodes.

Developing advanced electrode materials for potassium-ion batteries (PIBs) is an emerging research area in recent years; so far, several strategies such as heteroatom doping into carbon, increasing interlayer spacing, or creating amorphous region in graphite have been investigated. Here, we studied the effect of sub-nanopores in a porous-carbon aerogel with a pore size distribution centered at around 0.8 nm and achieved outstanding PIB performance including long cycling stability (particularly at small current densities for prolonged charge/discharge period) and high rate capability with enhanced retentions. Mechanism studies reveal very high contribution from surface capacitive potassium (K)-ion storage (more than 90%) to the total capacity, and theoretical calculations show that 0.8 nm sub-nanopores lead to substantially low barrier for K-ion transport and storage, with ultrasmall diffusion energy and negligible lattice change. Sub-nanopore engineering, as demonstrated here, may be adopted to develop highly efficient and stable porous-carbon-based structures for applications in advanced energy storage systems and electrochemical catalysis.

Membrane adsorbers with ultrahigh metal-organic framework loading for high flux separations.

Metal-organic frameworks (MOFs) with high porosity and designable functionality make it possible to access the merits of high permeability and selectivity. However, scalable fabrication methods to produce mixed matrix membranes (MMMs) with good flexibility and ultrahigh MOF loading are urgently needed yet largely unmet. Herein, we report a thermally induced phase separation-hot pressing (TIPS-HoP) strategy to roll-to-roll produce 10 distinct MOF-membranes (loadings up to 86 wt%). Ultrahigh-molecular-weight polyethylene interweaving the MOF particles contributes to their mechanical strength. Rejections (99%) of organic dyes with a water flux of 125.7 L m-2 h-1 bar-1 under cross-flow filtration mode. The micron-sized channels between the MOF particles translate into fast water permeation, while the porous MOFs reject solutes through rapid adsorption. This strategy paves ways for developing high-performance membrane adsorbers for crucial separation processes. As a proof-of-concept, the abilities of the membrane adsorbers for separating racemates and proteins have been demonstrated.

Large-area graphene-nanomesh/carbon-nanotube hybrid membranes for ionic and molecular nanofiltration.

Nanoporous two-dimensional materials are attractive for ionic and molecular nanofiltration but limited by insufficient mechanical strength over large areas. We report a large-area graphene-nanomesh/single-walled carbon nanotube (GNM/SWNT) hybrid membrane with excellent mechanical strength while fully capturing the merit of atomically thin membranes. The monolayer GNM features high-density, subnanometer pores for efficient transport of water molecules while blocking solute ions or molecules to enable size-selective separation. The SWNT network physically separates the GNM into microsized islands and acts as the microscopic framework to support the GNM, thus ensuring the structural integrity of the atomically thin GNM. The resulting GNM/SWNT membranes show high water permeance and a high rejection ratio for salt ions or organic molecules, and they retain stable separation performance in tubular modules.

Highly Dispersed Catalytic Co3S4 among a Hierarchical Carbon Nanostructure for High-Rate and Long-Life Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries are next-generation energy storage systems with high energy density, and the rate performance is a very important consideration for practical applications. Recent approaches such as introducing catalytic materials to facilitate polysulfide conversion have been explored, yet the results remain unsatisfactory. Here, we present an optimized Li-S electrode featured by a large amount of highly dispersed Co3S4 nanoparticles (∼10 nm in size) throughout a hierarchical carbon nanostructure hybridized from ZIF-67 and carbon nanotube (CNT) sponge. This enables homogeneous distribution and close contact between infiltrated sulfur and Co3S4 nanoparticles within the ZIF-67-derived N-doped carbon nanocubes, leading to effective chemical interaction with polysulfides, maximum catalytic effect and enhanced lithium ion diffusion, while the built-in three-dimensional CNT network ensures high electrical conductivity of the electrode. As a consequence, the Li-S battery exhibits both extraordinary rate performance by maintaining a capacity of ∼850 mAh g-1 at very high charge/discharge rate (5 C) and long-term cycling stability with 85% retention after 1000 cycles at 5 C (an average capacity fading rate of only 0.0137% per cycle).

Controlled Air-Etching Synthesis of Porous-Carbon Nanotube Aerogels with Ultrafast Charging at 1000 A g-1.

Supercapacitors are energy storage systems capable of fast charging and discharging, thus generating superior power density. Porous carbon with high surface area and tunable pore size represents a promising candidate to construct ultrafast supercapacitors; so far, most porous carbon-based electrodes can only be charged to a moderate current density (100-200 A g-1), also with significant capacitance loss at increasing rate. Here, it is shown that a 3D aerogel consisting of interconnected 1D porous-carbon nanotubes (PCNs) can serve as a freestanding supercapacitor electrode with excellent rate performance. As a result, the PCN aerogel electrodes achieve 1) ultrafast charging at current densities up to 1000 A g-1 (corresponding to a charge period of 16 ms), which is the highest value among other porous carbon-based supercapacitors, 2) superior cycling stability at high charging rates (88% capacitance retention after 105 cycles at 1000 A g-1). Mechanism study reveals favorable kinetics including a centralized pore size distribution at 0.8 nm which is a dominant factor to allow high-rate charging, a low and linear IR drop, and a metallic feature of 1D PCNs by theoretical calculation. The results indicate that 1D PCNs with controlled porous structures have potential applications in ultrafast energy conversion and storage.

Hyperporous Sponge Interconnected by Hierarchical Carbon Nanotubes as a High-Performance Potassium-Ion Battery Anode.

Recently, commercial graphite and other carbon-based materials have shown promising properties as the anode for potassium-ion batteries. A fundamental problem related to those carbon electrodes, significant volume expansion, and structural instability/collapsing caused by cyclic K-ion intercalation, remains unsolved and severely limits further development and applications of K-ion batteries. Here, a multiwalled hierarchical carbon nanotube (HCNT) is reported to address the issue, and a reversible specific capacity of 232 mAh g-1 , excellent rate capability, and cycling stability for 500 cycles are achieved. The key structure of the HCNTs consists of an inner CNT with dense-stacked graphitic walls and a loose-stacked outer CNT with more disordered walls, and individual HCNTs are further interconnected into a hyperporous bulk sponge with huge macropore volume, high conductivity, and tunable modulus. It is discovered that the inner dense-CNT serves as a robust skeleton, and collectively, the outer loose-CNT is beneficial for K-ion accommodation; meanwhile the hyperporous sponge facilitates reaction kinetics and offers stable surface capacitive behavior. The hierarchical carbon nanotube structure has great potential in developing high-performance and stable-structure electrodes for next generation K and other metal-ion batteries.

Robust and Stable Cu Nanowire@Graphene Core-Shell Aerogels for Ultraeffective Electromagnetic Interference Shielding.

Cu nanowires (CuNWs) are considered as a promising candidate to develop high performance metal aerogels, yet the construction of robust and stable 3D porous structures remains challenging which severely limits their practical applications. Here, graphene-hybridized CuNW (CuNW@G) core-shell aerogels are fabricated by introducing a conformal polymeric coating and in situ transforming it into multilayered graphene seamlessly wrapped around individual CuNWs through a mild thermal annealing process. The existence of the outer graphene shell reinforces the 3D bulk structure and significantly slows down the oxidation process of CuNWs, resulting in improved mechanical property and highly stable electrical conductivity. When applied in electromagnetic interference shielding, the CuNW@G core-shell aerogels exhibit an average effectiveness of ≈52.5 dB over a wide range (from 8.2 to 18 GHz) with negligible degradation under ambient conditions for 40 d. Mechanism analysis reveals that the graphene shell with functional groups enables dual reflections on the core-shell and a multiple dielectric relaxation process, leading to enhanced dielectric loss and energy dissipation within the core-shell aerogels. The flexible core-shell-structured CuNW@G aerogels, with superior mechanical robustness and electrical stability, have potential applications in many areas such as advanced energy devices and functional composites.

Single Carbon Fibers with a Macroscopic-Thickness, 3D Highly Porous Carbon Nanotube Coating.

Carbon fiber (CF) grafted with a layer of carbon nanotubes (CNTs) plays an important role in composite materials and other fields; to date, the applications of CNTs@CF multiscale fibers are severely hindered by the limited amount of CNTs grafted on individual CFs and the weak interfacial binding force. Here, monolithic CNTs@CF fibers consisting of a 3D highly porous CNT sponge layer with macroscopic-thickness (up to several millimeters), which is directly grown on a single CF, are fabricated. Mechanical tests reveal high sponge-CF interfacial strength owing to the presence of a thin transitional layer, which completely inhibits the CF slippage from the matrix upon fracture in CNTs@CF fiber-epoxy composites. The porous conductive CNTs@CF hybrid fibers also act as a template for introducing active materials (pseudopolymers and oxides), and a solid-state fiber-shaped supercapacitor and a fiber-type lithium-ion battery with high performances are demonstrated. These CNTs@CF fibers with macroscopic CNT layer thickness have many potential applications in areas such as hierarchically reinforced composites and flexible energy-storage textiles.

MOF-Derived ZnO Nanoparticles Covered by N-Doped Carbon Layers and Hybridized on Carbon Nanotubes for Lithium-Ion Battery Anodes.

Metal-organic frameworks (MOFs) have many promising applications in energy and environmental areas such as gas separation, catalysis, supercapacitors, and batteries; the key toward those applications is controlled pyrolysis which can tailor the porous structure, improve electrical conductivity, and expose metal ions in MOFs. Here, we present a systematic study on the structural evolution of zeolitic imidazolate frameworks hybridized on carbon nanotubes (CNTs) during the carbonization process. We show that a number of typical products can be obtained, depending on the annealing time, including (1) CNTs wrapped by relatively thick carbon layers, (2) CNTs grafted by ZnO nanoparticles which are covered by thin nitrogen-doped carbon layers, and (3) CNTs grafted by aggregated ZnO nanoparticles. We also investigated the electrochemical properties of those hybrid structures as freestanding membrane electrodes for lithium ion batteries, and the second one (CNT-supported ZnO covered by N-doped carbon) shows the best performance with a high specific capacity (850 mA h/g at a current density of 100 mA/g) and excellent cycling stability. Our results indicate that tailoring and optimizing the MOF-CNT hybrid structure is essential for developing high-performance energy storage systems.

Rational Design of Hierarchical Carbon/Mesoporous Silicon Composite Sponges as High-Performance Flexible Energy Storage Electrodes.

Nanostructuring silicon (Si) and combining Si with carbon shells have been studied in recent Li-ion battery electrodes, yet it remains a grand challenge to overcome the low electrical conductivity and associated volume change of Si. Here, by first coating a mesoporous SiO2 (meso-SiO2) onto carbon nanotube (CNT) networks and then converting it into a meso-Si layer covered by carbon, we obtained a freestanding, highly porous composite sponge electrode consisting of three-dimensionally interconnected sandwiched carbon-Si-CNT skeletons. In this hierarchical structure, the macropores among the sponge connect to mesopores in the meso-Si layer so that Li+ diffusion is facilitated, whereas the underlying CNT networks serve as conductive paths for electrons transport. Meanwhile, the outer carbon coating on meso-Si could buffer the volume expansion and prevent material shedding. As a result, our sandwiched carbon-Si-CNT electrodes exhibit large specific capacity, high rate capability and long cycle life. The combination of carbon-wrapped meso-Si and CNT sponges might be a potential strategy for developing efficient electrodes in various energy storage systems.

Graphene Oxide Glue-Electrode for Fabrication of Vertical, Elastic, Conductive Columns.

Graphene has a planar atomic structure with high flexibility and might be used as ultrathin conductive glues or adhesion layers in electronics and other applications. Here, we show that graphene oxide (GO) sheets condensed from solution can act as a pure, thin-layer, nonpenetrating glue for fabrication of vertical architectures anchored on rigid and flexible substrates. Carbon nanotube (CNT) sponges are used as a porous template to make polymer-reinforced composite columns, to achieve both high conductivity and elastic behavior. These vertical columns are fixed on a substrate by reduced GO sheets as an electrode and exhibit reversible resistance change under large-strain compression for many cycles. Similar to the CNT gecko feet, we disclose high adhesion forces at the CNT-GO and GO-SiO2 interfaces by mechanical tests and theoretical calculation. Three-dimensional CNT, graphene, and nanowire networks with our GO glue-electrodes have potential applications as energy storage electrodes, flexible sensors, functional composites, and vertical interconnects.

Highly Stable Carbon Nanotube/Polyaniline Porous Network for Multifunctional Applications.

Three-dimensional carbon nanotube (CNT) networks with high porosity and electrical conductivity have many potential applications in energy and environmental areas, but the network structure is not very stable due to weak inter-CNT interactions. Here, we coat a thin polyaniline (PANI) layer on as-synthesized CNT sponge to obtain a mechanically and electrically stable network, and enable multifunctional applications. The resulting CNT/PANI network serves as stable strain sensors, highly compressible supercapacitor electrode with enhanced volume-normalized capacitance (632 F/cm3), and reinforced nanocomposites with the PANI as intermediate layer between the CNT fillers and polymeric matrix. Our results provide a simple and controllable method for achieving high-stability porous networks composed of CNTs, graphene, or other nanostructures.

Controlled Synthesis of Core-Shell Carbon@MoS2 Nanotube Sponges as High-Performance Battery Electrodes.

Heterogeneous inorganic nanotube structures consisting of multiwalled carbon nanotubes coated by long, continuous MoS2 sheets with tunable sheet number are synthesized using a carbon-nanotube sponge as a template. The resulting 3D porous hybrid sponges have potential applications as high-performance freestanding anodes for Li-ion batteries with excellent specific capacity and cycling stability.

Blown-Bubble Assembly and in Situ Fabrication of Sausage-like Graphene Nanotubes Containing Copper Nanoblocks.

We use a blown-bubble method to assemble Cu nanowires and in situ fabricate graphene-based one-dimensional heterostructures, including versatile sausage-like configurations consisting of multilayer graphene nanotubes (GNTs) filled by single or periodically arranged Cu nanoblocks (CuNBs). This is done by first assembling Cu nanowires among a polymer-based blown-bubble film (BBF) and then growing graphene onto the nanowire substrate using the polymer matrix as a solid carbon source by chemical-vapor deposition. The formation of sausage-like GNT@CuNB nanostructures is due to the partial melting and breaking of embedded Cu nanowires during graphene growth, which is uniquely related to our BBF process. We show that the GNT skin significantly slows the oxidation process of CuNBs compared with that of bare Cu nanowires, and the presence of stuffed CuNBs also reduces the linear resistance along the GNTs. The large-scale assembled graphene-based heterostructures achieved by our BBF method may have potential applications in heterojunction electronic devices and high-stability transparent conductive electrodes.

Direct Oil Recovery from Saturated Carbon Nanotube Sponges.

Oil adsorption by porous materials is a major strategy for water purification and industrial spill cleanup; it is of great interest if the adsorbed oil can be safely recovered from those porous media. Here, direct oil recovery from fully saturated bulk carbon nanotube (CNT) sponges by displacing oil with water in controlled manner is shown. Surfactant-assisted electrocapillary imbibition is adopted to drive aqueous electrolyte into the sponge and extrude organic oil out continuously at low potentials (up to -1.2 V). More than 95 wt % of oil adsorbed within the sponge can be recovered, via a single electrocapillary process. Recovery of different oils with a wide range of viscosities is demonstrated, and the remaining CNT sponge can be reused with similar recovery capacity. A direct and efficient method is provided to recover oil from CNT sponges by water imbibition, which has many potential environmental and energy applications.

Double polymer sheathed carbon nanotube supercapacitors show enhanced cycling stability.

Pseudo-materials are effective in boosting the specific capacitance of supercapacitors, but during service their degradation may also be very strong, causing reduced cycling stability. Here, we show that a carbon nanotube sponge grafted by two conventional pseudo-polymer layers in sequence can serve as a porous supercapacitor electrode with significantly enhanced cycling stability compared with single polymer grafting. Creating conformal polymer coatings on the nanotube surface and the resulting double-sheath configuration are important structural factors leading to the enhanced performance. Combining different polymers as double sheaths as reported here might be a potential route to circumvent the dilemma of pseudo-materials, and to simultaneously improve the capacitance and stability for various energy storage devices.

Blown Bubble Assembly of Graphene Oxide Patches for Transparent Electrodes in Carbon-Silicon Solar Cells.

Graphene oxide (GO) sheets have a strong tendency to aggregate, and their interfaces can impose limitations on the electrical conductivity, which would hinder practical applications. Here, we present a blown bubble film method to assemble GO sheets with a uniform distribution over a large area and further interconnect individual GO sheets by transforming the bubble film into graphitized carbon. A conventional polymer was used to facilitate the bubble blowing process and disperse GO sheets in the bubble. Then, the bubble film was annealed on a Cu substrate, resulting in a highly transparent reduced GO (RGO)-carbon hybrid structure consisting of RGO patches well adhered to the carbon film. We fabricated RGO-carbon/Si solar cells with power conversion efficiencies up to 6.42%, and the assembled RGO patches hybridized with carbon film can form an effective junction with Si, indicating potential applications in thin film electronic devices and photovoltaics.

Comparison of Nanocarbon-Silicon Solar Cells with Nanotube-Si or Graphene-Si Contact.

Nanocarbon structures such as carbon nanotubes (CNTs) and graphene (G) have been combined with crystalline silicon wafers to fabricate nanocarbon-Si solar cells. Here, we show that the contact between the nanocarbon and Si plays an important role in the solar cell performance. An asymmetrically configured CNT-G composite film was used to create either CNT-Si dominating or G-Si dominating junctions, resulting in obviously different solar cell behavior in pristine state. Typically, solar cells with direct G-Si contacts (versus CNT-Si) exhibit better characteristics due to improved junction quality and larger contact area. On the basis of the composite film, the obtained CNT-G-Si solar cells reach power conversion efficiencies of 14.88% under air mass 1.5, 88 mW/cm2 illumination through established techniques such as acid doping and colloidal antireflection. Engineering the nanocarbon-Si contact is therefore a possible route for further improving the performance of this type of solar cells.