Advanced asymmetrical supercapacitors based on graphene hybrid materials

Supercapacitors operating in aqueous solutions are low cost energy storage devices with high cycling stability and fast charging and discharging capabilities, but generally suffer from low energy densities. Here, we grow Ni(OH)₂ nanoplates and RuO₂ nanoparticles on high quality graphene sheets in order to maximize the specific capacitances of these materials. We then pair up a Ni(OH)₂/graphene electrode with a RuO₂/graphene electrode to afford a high performance asymmetrical supercapacitor with high energy and power density operating in aqueous solutions at a voltage of ∼1.5 V. The asymmetrical supercapacitor exhibits significantly higher energy densities than symmetrical RuO₂-RuO₂ supercapacitors or asymmetrical supercapacitors based on either RuO₂-carbon or Ni(OH)₂-carbon electrode pairs. A high energy density of ∼48 W·h/kg at a power density of ∼0.23 kW/kg, and a high power density of ∼21 kW/kg at an energy density of ∼14 W·h/kg have been achieved with our Ni(OH)2/graphene and RuO₂/graphene asymmetrical supercapacitor. Thus, pairing up metal-oxide/graphene and metal-hydroxide/graphene hybrid materials for asymmetrical supercapacitors represents a new approach to high performance energy storage.      

Facile growth of homogeneous Ni(OH)<Subscript>2</Subscript> coating on carbon nanosheets for high-performance asymmetric supercapacitor applications

The growth of a Ni(OH)₂ coating on conductive carbon substrates is an efficient way to address issues related to their poor conductivity in electrochemical capacitor applications. However, the direct growth of nickel hydroxide coatings on a carbon substrate is challenging, because the surfaces of these systems are not compatible and a preoxidation treatment of the conductive carbon substrate is usually required. Herein, we present a facile preoxidation-free approach to fabricate a uniform Ni(OH)₂ coating on carbon nanosheets (CNs) by an ion-exchange reaction to achieve the in situ transformation of a MgO/C composite to a Ni(OH)₂/C one. The obtained Ni(OH)₂/CNs hybrids possess nanosheet morphology, a large surface area (278 m²/g), and homogeneous elemental distributions. When employed as supercapacitors in a three-electrode configuration, the Ni(OH)₂/CNs hybrid achieves a large capacitance of 2,218 F/g at a current density of 1.0 A/g. Moreover, asymmetric supercapacitors fabricated with the Ni(OH)₂/CNs hybrid exhibit superior supercapacitive performances, with a large capacity of 198 F/g, and high energy density of 56.7 Wh/kg at a power density of 4.0 kW/kg. They show excellent cycling stability with 93% capacity retention after 10,000 cycles, making the Ni(OH)₂/CNs hybrid a promising candidate for practical applications in supercapacitor devices.

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Controllable synthesis of triangular Ni(HCO<Subscript>3</Subscript>)<Subscript>2</Subscript> nanosheets for supercapacitor

Triangular Ni(HCO₃)₂ nanosheets were synthesized via a template-free solvothermal method. The phase transition and formation mechanism were explored systematically. Further investigation indicated that the reaction time and pH have significant effects on the morphology and size distribution of the triangular Ni(HCO₃)₂ nanosheets. More interestingly, the resulting product had an ultra-thin structure and high specific surface area, which can effectively accelerate the charge transport during charge–discharge processes. As a result, the triangular Ni(HCO₃)₂ nanosheets not only exhibited high specific capacitance (1,797 F·g^-1 at 5 A·g^-1 and 1,060 F·g^-1 at 50 A·g^-1), but also showed excellent cycling stability with a high current density (~80% capacitance retention after 5,000 cycles at the current density of 20 A·g^-1).

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One-step strategy to graphene/Ni(OH)2 composite hydrogels as advanced three-dimensional supercapacitor electrode materials

Graphene-based three-dimensional (3D) macroscopic materials have recently attracted increasing interest by virtue of their exciting potential in electrochemical energy conversion and storage. Here we report a facile one-step strategy to prepare mechanically strong and electrically conductive graphene/Ni(OH)₂ composite hydrogels with an interconnected porous network. The composite hydrogels were directly used as 3D supercapacitor electrode materials without adding any other binder or conductive additives. An optimized composite hydrogel containing ～82 wt.% Ni(OH)₂ exhibited a specific capacitance of ～1,247 F/g at a scan rate of 5 mV/s and ～785 F/g at 40 mV/s (～63% capacitance retention) with excellent cycling stability. The capacity of the 3D hydrogels greatly surpasses that of a physical mixture of graphene sheets and Ni(OH)₂ nanoplates (～309 F/g at 40 mV/s). The same strategy was also applied to fabricate graphene-carbon nanotube/Ni(OH)₂ ternary composite hydrogels with further improved specific capacitances (～1,352 F/g at 5 mV/s) and rate capability (～66% capacitance retention at 40 mV/s). Both composite hydrogels obtained here can deliver high energy densities (～43 and ～47 Wh/kg, respectively) and power densities (～8 and ～9 kW/kg, respectively), making them attractive electrode materials for supercapacitor applications. This study opens a new pathway to the design and fabrication of functional 3D graphene composite materials, and can significantly impact broad areas including energy storage and beyond.      

Stable ultrahigh specific capacitance of NiO nanorod arrays

Previously reported examples of electrochemical pseudocapacitors based on cheap metal oxides have suffered from the need to compromise between specific capacitance, rate capacitance, and reversibility. Here we show that NiO nanorod arrays on Ni foam have a combination of ultrahigh specific capacitance (2018 F/g at 2.27 A/g), high power density (1536 F/g at 22.7 A/g), and good cycling stability (only 8% of capacitance was lost in the first 100 cycles with no further change in the subsequent 400 cycles). This resulted in an improvement in the reversible capacitance record for NiO by 50% or more, reaching 80% of the theoretical value, and demonstrated that a three-dimensional regular porous array structure can afford all of these virtues in a supercapacitor. The excellent performance can be attributed to the slim (< 20 nm) rod morphology, high crystallinity, regularly aligned array structure and strong bonding of the nanorods to the metallic Ni substrate, as revealed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD).      

Ionic liquid-assisted one-step hydrothermal synthesis of TiO2-reduced graphene oxide composites

We have demonstrated a facile and efficient strategy for the fabrication of soluble reduced graphene oxide sheets (RGO) and the preparation of titanium oxide (TiO₂) nanoparticle-RGO composites using a modified one-step hydrothermal method. It was found that graphene oxide could be easily reduced under solvothermal conditions with ascorbic acid as reductant, with concomitant growth of TiO₂ particles on the RGO surface. The TiO₂-RGO composite has been thoroughly characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetric analysis. Microscopy techniques (scanning electron microscopy, atomic force microscopy, and transmission electron microscopy) have been employed to probe the morphological characteristics as well as to investigate the exfoliation of RGO sheets. The TiO₂-RGO composite exhibited excellent photocatalysis of hydrogen evolution.      

Ni@N-doped graphene nanosheets and CNTs hybrids modified separator as efficient polysulfide barrier for high-performance lithium sulfur batteries

Lithium-sulfur batteries (LSBs) have been regarded as one of the most promising energy storage systems to break through the upper limit of lithium-ion batteries. However, the rampant diffusions of soluble lithium polysulfides (LiPSs) in the electrolyte induced the shuttle effect between anode and cathode, resulting in low sulfur utilization, low energy efficiency and short cycling life. Herein, we prove the rational design and construction of Ni nanoparticles filled in vertically grown N-doped bamboo-like carbon nanotubes (CNTs) on graphene nanosheets (Ni@NG-CNTs) as efficient polysulfide barrier for high-performance LSBs. The unique design integrates graphene nanosheets and CNTs into hierarchical architectures with one-dimensional (1D) CNTs, two-dimensional (2D) ultrathin nanosheets and abundant carbon nanocages. This design provides large surface area for lithium polysulfides (LiPSs) adsorption, accelerates electron transport and enhances electrochemical redox of LiPSs. Benefiting from the unique structural features, the LSBs with the Ni@NG-CNTs as polysulfide barrier keep high reversible specific capacities of 309.1 and 265.0 mAh·g⁻¹ at 5 and 10 C rates after 500 cycles. This work provides a new strategy for constructing self-assembled hybrids of CNTs and graphene nanosheets with abundant carbon nanocages for high-performance LSBs.

     

A molecular understanding of the gas-phase reduction and doping of graphene oxide

Chemical reduction of graphene oxide represents an important route towards large-scale production of graphene sheets for many applications. Thus far, gas-phase reactions have been demonstrated to efficiently reduce graphene oxide, but a molecular understanding of the reaction processes is largely lacking. Here, using molecular dynamics simulations, we compare the reduction of graphene oxide in different environments. We find that NH₃ affords more efficient reduction of hydroxyl and epoxide groups than H₂ and vacuum annealing partly due to lower energy barriers. Various reduction paths of oxygen groups in NH₃ and H₂ are quantitatively identified. Furthermore, we show that with the combination of vacancies and oxygen groups, pyridinic- or pyrrolic-like nitrogen can readily be incorporated into graphene. All of these nitrogen configurations lead to n-doping of the graphene. Our results are consistent with many previous experiments and provide insights towards doping engineering of graphene.      

Different growth behaviors of ambient pressure chemical vapor deposition graphene on Ni(111) and Ni films: A scanning tunneling microscopy study

Graphene growth on the same metal substrate with different crystal morphologies, such as single crystalline and polycrystalline, may involve different mechanisms. We deal with this issue by preparing graphene on single crystal Ni(111) and on ∼300 nm thick Ni films on SiO₂ using an ambient pressure chemical vapor deposition (APCVD) method, and analyze the different growth behaviors for different growth parameters by atomically-resolved scanning tunneling microscopy (STM) and complementary macroscopic analysis methods. Interestingly, we obtained monolayer graphene on Ni(111), and multilayer graphene on Ni films under the same growth conditions. Based on the experimental results, it is proposed that the graphene growth on Ni(111) is strongly templated by the Ni(111) lattice due to the strong Ni-C interactions, leading to monolayer graphene growth. Multilayer graphene flakes formed on polycrystalline Ni films are usually stacked with deviations from the Bernal stacking type and show small rotations among the carbon layers. Considering the different substrate features, the inevitable grain boundaries on polycrystalline Ni films are considered to serve as the growth fronts for bilayer and even multilayer graphene.      

Controllable synthesis of Co3O4 from nanosize to microsize with large-scale exposure of active crystal planes and their excellent rate capability in supercapacitors based on the crystal plane effect

Co₃O₄ nanorods, nanobelts, nanosheets and cubic/octahedral nanoparticles have been successfully synthesized with tunable size from the nanoscale to the microscale, accompanied by a variation in the nature of the exposed crystal planes. The products are formed by thermal treatment of Co(CO₃)_0.5(OH)·_0.11H₂O nanorod, nanobelt, nanosheet and nanocubic/nanooctahedral precursors at 250 °C. Detailed characterization, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photo-electron spectroscopy (XPS), and nitrogen adsorption and desorption isotherms, revealed that the as-prepared nanorods, nanobelts, and nanosheet Co₃O₄ samples are single crystalline and mesoporous in nature with a predominance of exposed high-energy (110) crystal planes. They exhibited excellent electrochemical properties in supercapacitors, showing higher capacitance and better rate capability than conventional cubic/octahedral Co₃O₄ nanoparticles having exposed low-energy (100) and (111) planes. No decay in capacitance was observed when the scan rate was increased from 5 mV/s to 100 mV/s, or from 1 A/g to 10 A/g. The maximum value of the specific capacitance was calculated to be 162.8 F/g and the capacitance retention reached as high as 90%. Their excellent performance in supercapacitors is believed to result from the large-area exposure of active (110) crystal planes. The Co₃O₄ nanosheets showed the best performance due to their larger surface area and ability to provide a better pathway for charge transfer, and are promising electrode materials for application in practical supercapacitors.      

Hierarchical ferric-cobalt-nickel ternary oxide nanowire arrays supported on graphene fibers as high-performance electrodes for flexible asymmetric supercapacitors

Fiber-based supercapacitors (FSCs) are new members of the energy storage family. They present excellent flexibility and have promising applications in lightweight, flexible, and wearable devices. One of the existing challenges of FSCs is enhancing their energy density while retaining the flexibility. We developed a facile and cost-effective method to fabricate a highly capacitive positive electrode based on hierarchical ferric-cobalt-nickel ternary oxide nanowire arrays/graphene fibers and a negative electrode based on polyaniline-derived carbon nanorods/graphene fibers. The elegant microstructures and excellent electrochemical performances of both electrodes enabled us to construct a highperformance flexible asymmetric graphene fiber-based supercapacitor device with an operating voltage of 1.4 V, a specific capacitance up to 61.58 mF·cm^–2, and an energy density reaching 16.76 μW·h·cm^–2. Moreover, the optimal device presents an outstanding cycling stability with 87.5% initial capacitance retention after 8,000 cycles, and an excellent flexibility with a capacitance retention of 90.9% after 4,000 cycles of repetitive bending.

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Solvothermal-assisted exfoliation process to produce graphene with high yield and high quality

Monolayer and bilayer graphene sheets have been produced by a solvothermal-assisted exfoliation process in a highly polar organic solvent, acetonitrile, using expanded graphite (EG) as the starting material. It is proposed that the dipole-induced dipole interactions between graphene and acetonitrile facilitate the exfoliation and dispersion of graphene. The facile and effective solvothermal-assisted exfoliation process raises the low yield of graphene reported in previous syntheses to 10 wt%–12 wt%. By means of centrifugation at 2000 rpm for 90 min, monolayer and bilayer graphene were separated effectively without the need to add a stabilizer or modifier. Electron diffraction and Raman spectroscopy indicate that the resulting graphene sheets are high quality products without any significant structural defects.      

Shape control of CoO and LiCoO<Subscript>2</Subscript> nanocrystals

Shape control of nanocrystals has become a significant subject in materials science. In this work, we describe a convenient way to achieve morphology-controllable synthesis of CoO nanocrystals including octahedrons and spheres as well as LiCoO₂ polyhedrons and spheres. In particular, we explain the formation of CoO octahedrons exposing only high-energy (111) facets using theoretical calculations; these should also be a useful tool for directing future face-controlled preparation of other nanocrystals. More importantly, the as-obtained LiCoO₂ nanocrystals showed different electrochemical performance depending on their morphology, indicating that Li-insertion/deintercalation dynamics might be crystal face-sensitive.      

Carbon quantum dot-induced self-assembly of ultrathin Ni(OH)<Subscript>2</Subscript> nanosheets: A facile method for fabricating three-dimensional porous hierarchical composite micro-nanostructures with excellent supercapacitor performance

Significant efforts have been directed towards the preparation and application of porous hierarchically structured materials owing to their large surface area, rich active sites, and enhanced mass transport and diffusion. In this study, a simple and cost-effective method for the carbon quantum dot (CQD)-induced assembly of two-dimensional ultrathin Ni(OH)₂ nanosheets into a three-dimensional (3D) porous hierarchical structure was developed. The electrostatic forces between the CQDs and cations drove the self-assembly of the 3D CQDs/Ni(OH)₂ hierarchical structures. As a new type of structure-directing agent, the CQDs played dual roles in tuning the morphology of the products and improving the supercapacitor performance. The multilevel CQDs/Ni(OH)₂ micro-nanostructures had a large specific surface area and rich porosity. Owing to their unique structures and the conductivity of the CQDs, an optimized asymmetric supercapacitor using the CQDs/Ni(OH)₂ exhibited a maximum specific capacity of 161.3 F·g^–1 and a high energy density of 57.4 Wh·kg^–1. This study introduces a potential method for the fabrication of many other 3D hierarchical structures with great potential for applications in various fields.

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Sequential assembly of metal-free phthalocyanine on few-layer epitaxial graphene mediated by thickness-dependent surface potential

Due to strong interactions between epitaxial graphene and SiC(0001) substrates, the overlayer charge density induced by the interface charging effect is much more attenuated than that of exfoliated graphene on SiO₂. We report herein a quantitive detection of the charge properties of few-layer graphene by surface potential measurements using electrostatic force microscopy (EFM). A minor difference in surface potential is observed to mediate a sequential assembly of metal-free phthalocyanine (H₂Pc) on monolayer, bilayer and trilayer graphenes, as demonstrated by scanning tunneling microscopy (STM). In order to understand this, we further executed density functional theory (DFT) calculations which showed higher adsorption energies for Pc on thinner graphenes. In this case, we attribute the unique growth behavior of Pc to its variable adsorption energies on few-layer graphene, and in turn the layer charge variations from the viewpoint of energy minimizations. This work is expected to provide fundamental data useful for related nanodevice fabrications.      

Morphology-controlled fabrication of sulfonated graphene/polyaniline nanocomposites by liquid/liquid interfacial polymerization and investigation of their electrochemical properties

A novel morphology-controlled strategy has been developed to fabricate sulfonated graphene/polyaniline (SGEP) nanocomposites by liquid/liquid interfacial polymerization. Sulfonated graphene (SGE) sheets were synthesized and used as both a macromolecular acid dopant and substrate for the polymerization of polyaniline (PANI), affording the SGEP nanocomposites. The morphology of PANI in the nanocomposites can be controlled to be either nanorods or nanogranules by varying the synthesis conditions. The morphology of SGEP and the shape of PANI can be tuned by adding an additional dopant and varying the amount of SGE used, and this had a significant influence on the electrochemical performance of the nanocomposites as supercapacitor electrode materials. The SGEP nanocomposite with PANI nanorods exhibited a specific capacitance of 763 F/g with a capacity retention of 96% after 100 cycles and good rate properties. Composites obtained with HCl as an additional acid dopant with two different ratios of SGE to PANI showed higher specific capacitances of 793 and 931 F/g, but lower capacity retention after 100 cycles of 77% and 76%, respectively.      

Configuration-sensitive molecular sensing on doped graphene sheets

We show by molecular dynamics simulations that configuration-sensitive molecular spectroscopy can be realized on optimally doped graphene sheets vibrated by an oscillatory electric field. High selectivity of the spectroscopy is achieved by maximizing Coulombic binding between the detected molecule and a specific nest, formed for this molecule on the graphene sheet by substituting selected carbon atoms with boron and nitrogen dopants. One can detect binding of different isomers to the nest from the frequency shifts of selected vibrational modes of the combined system. As an illustrative example, we simulate detection of hexanitrostilbene enantiomers in chiral nests formed on graphene.      

Template-free synthesis of ordered mesoporous NiO/poly(sodium-4-styrene sulfonate) functionalized carbon nanotubes composite for electrochemical capacitors

We report the first example of a practical and efficient template-free strategy for synthesizing ordered mesoporous NiO/poly(sodium-4-styrene sulfonate) (PSS) functionalized carbon nanotubes (FCNTs) composites by calcining a Ni(OH)₂/FCNTs precursor prepared by refluxing an alkaline solution of Ni(NH₃) _x ²⁺ and FCNTs at 97 °C for 1 h. The morphology and structure were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Thermal decomposition of the precursor results in the formation of ordered mesoporous NiO/FCNTs composite (ca. 48 wt% NiO) with large specific surface area. Due to its enhanced electronic conductivity and hierarchical (meso- and macro-) porosity, composite simultaneously meets the three requirements for energy storage in electrochemical capacitors at high rate, namely, good electron conductivity, highly accessibleelectrochemical surface areas owing to the existence of mesopores, and efficient mass transport from the macropores. Electrochemical data demonstrated that the ordered mesoporous NiO/FCNTs composite is capable of delivering a specific capacitance (SC) of 526 F/g at 1 A/g and a SC of 439 F/g even at 6 A/g, and show a degradation of only ca. 6% in SC after 2000 continuous charge/discharge cycles.      

Vapour-phase graphene epitaxy at low temperatures

We report an epitaxial growth of graphene, including homo- and hetero-epitaxy on graphite and SiC substrates, at a temperature as low as ∼540 °C. This vapour-phase epitaxial growth, carried out in a remote plasma-enhanced chemical vapor deposition (RPECVD) system using methane as the carbon source, can yield large-area high-quality graphene with the desired number of layers over the entire substrate surfaces following an AB-stacking layer-by-layer growth model. We also developed a facile transfer method to transfer a typical continuous one layer epitaxial graphene with second layer graphene islands on top of the first layer with the coverage of the second layer graphene islands being 20% (1.2 layer epitaxial graphene) from a SiC substrate onto SiO₂ and measured the resistivity, carrier density and mobility. Our work provides a new strategy toward the growth of graphene and broadens its prospects of application in future electronics.      

Facile synthesis and application of Ag-chemically converted graphene nanocomposite

An in situ chemical synthesis approach has been employed to prepare an Ag-chemically converted graphene (CCG) nanocomposite. The reduction of graphene oxide sheets was accompanied by generation of Ag nanoparticles. The structure and composition of the nanocomposites were confirmed by means of transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray diffraction. TEM and AFM results suggest a homogeneous distribution of Ag nanoparticles (5–10 nm in size) on CCG sheets. The intensities of the Raman signals of CCG in such nanocomposites are greatly increased by the attached silver nanoparticles, i.e., there is surface-enhanced Raman scattering activity. In addition, it was found that the antibacterial activity of free Ag nanoparticles is retained in the nanocomposites, which suggests they can be used as graphene-based biomaterials.