Porous graphene materials for advanced electrochemical energy storage and conversion devices

Cellulose nanofibril/reduced graphene oxide/carbon nanotube hybrid aerogels for highly flexible and all-solid-state supercapacitors

A novel type of highly flexible and all-solid-state supercapacitor that uses cellulose nanofibril (CNF)/reduced graphene oxide (RGO)/carbon nanotube (CNT) hybrid aerogels as electrodes and H2SO4/poly(vinyl alcohol) (PVA) gel as the electrolyte was developed and is reported here. These flexible solid-state supercapacitors were fabricated without any binders, current collectors, or electroactive additives. Because of the porous structure of the CNF/RGO/CNT aerogel electrodes and the excellent electrolyte absorption properties of the CNFs present in the aerogel electrodes, the resulting flexible supercapacitors exhibited a high specific capacitance (i.e., 252 F g(-1) at a discharge current density of 0.5 A g(-1)) and a remarkable cycle stability (i.e., more than 99.5% of the capacitance was retained after 1000 charge-discharge cycles at a current density of 1 A g(-1)). Furthermore, the supercapacitors also showed extremely high areal capacitance, areal power density, and energy density (i.e., 216 mF cm(-2), 9.5 mW cm(-2), and 28.4 μWh cm(-2), respectively). In light of its excellent electrical performance, low cost, ease of large-scale manufacturing, and environmental friendliness, the CNF/RGO/CNT aerogel electrodes may have a promising application in the development of flexible energy-storage devices.

Graphene/phase change material nanocomposites: light-driven, reversible electrical resistivity regulation via form-stable phase transitions

Innovative photoresponsive materials are needed to address the complexity of optical control systems. Here, we report a new type of photoresponsive nanomaterial composed of graphene and a form-stable phase change material (PCM) that exhibited a 3 orders of magnitude change in electrical resistivity upon light illumination while retaining its overall original solid form at the macroscopic level. This dramatic change in electrical resistivity also occurred reversibly through the on/off control of light illumination. This was attributed to the reversible phase transition (i.e., melting/recrystallization) behavior of the microscopic crystalline domains present in the form-stable PCM. The reversible phase transition observed in the graphene/PCM nanocomposite was induced by a reversible temperature change through the on/off control of light illumination because graphene can effectively absorb light energy and convert it to thermal energy. In addition, this graphene/PCM nanocomposite also possessed excellent mechanical properties. Such photoresponsive materials have many potential applications, including flexible electronics.

Spongy nitrogen-doped activated carbonaceous hybrid derived from biomass material/graphene oxide for supercapacitor electrodes

A nitrogen doped and activated material with spongy-like structure containing a low cost carbon derived from the waste agricultural material and graphene oxide is synthesized via facile thermal treatment for supercapacitor applications.

Porous graphene–carbon nanotube hybrid paper as a flexible nano-scaffold for polyaniline immobilization and application in all-solid-state supercapacitors

Three-dimensional porous graphene–carbon nanotube hybrid papers were obtained as a conductive, flexible and free-standing nano-scaffold for PANI immobilization.

Anti-stacking dense conversion of solid organic sodium salt particles into graphene with excellent electrode performance

Graphene frameworks can be densely synthesized from a rapid decomposition of common solid organic sodium salts.

Laser irradiated self-supporting and flexible 3-dimentional graphene-based film electrode with promising electrochemical properties

Rapid preparation of self-supporting and flexible 3-dimentional graphene-based film electrode with promising electrochemical properties by HI reduction and laser irradiation.

Porous reduced graphene oxide paper as a binder-free electrode for high-performance supercapacitors

Porous rGO exhibited an excellent high specific capacity of 173.5 F g⁻¹, good cycling stability, and 28.5 W h kg⁻¹ and power density of 4000 W kg⁻¹, far beyond those of pure stacked rGO paper.

Graphene-based materials for flexible supercapacitors

The recent advances in developing graphene-based materials for flexible supercapacitors are summarized in this review.

Micelle-template synthesis of nitrogen-doped mesoporous graphene as an efficient metal-free electrocatalyst for hydrogen production

Synthesis of mesoporous graphene materials by soft-template methods remains a great challenge, owing to the poor self-assembly capability of precursors and the severe agglomeration of graphene nanosheets. Herein, a micelle-template strategy to prepare porous graphene materials with controllable mesopores, high specific surface areas and large pore volumes is reported. By fine-tuning the synthesis parameters, the pore sizes of mesoporous graphene can be rationally controlled. Nitrogen heteroatom doping is found to remarkably render electrocatalytic properties towards hydrogen evolution reactions as a highly efficient metal-free catalyst. The synthesis strategy and the demonstration of highly efficient catalytic effect provide benchmarks for preparing well-defined mesoporous graphene materials for energy production applications.

NiO/nanoporous graphene composites with excellent supercapacitive performance produced by atomic layer deposition

Nickel oxide (NiO) is a promising electrode material for supercapacitors because of its low cost and high theoretical specific capacitance of 2573 F g(-1). However, the low electronic conductivity and poor cycling stability of NiO limit its practical applications. To overcome these limitations, an efficient atomic layer deposition (ALD) method is demonstrated here for the fabrication of NiO/nanoporous graphene (NG) composites as electrode materials for supercapacitors. ALD allows uniform deposition of NiO nanoparticles with controlled sizes on the surface of NG, thus offering a novel route to design NiO/NG composites for supercapacitor applications with high surface areas and greatly improved electrical conductivity and cycle stability. Electrochemical measurements reveal that the NiO/NG composites obtained by ALD exhibited excellent specific capacitance of up to ∼ 1005.8 F g(-1) per mass of the composite electrode (the specific capacitance value is up to ∼ 1897.1 F g(-1) based on the active mass of NiO), and stable performance after 1500 cycles. Furthermore, electrochemical performance of the NiO/NG composites is found to strongly depend on the size of NiO nanoparticles.

Electrodeposition of porous graphene networks on nickel foams as supercapacitor electrodes with high capacitance and remarkable cyclic stability

We describe a facile, low-cost, and green method to fabricate porous graphene networks/nickel foam (PG/NF) electrodes by electrochemical deposition of graphene sheets on nickel foams (NFs) for the application of supercapacitor electrodes. The electrodeposition process was accomplished by electrochemical reduction of graphene oxide (GO) in its aqueous suspension. The resultant binder-free PG/NF electrodes exhibited excellent double-layer capacitive performance with a high rate capability and a high specific capacitance of 183.2 mF cm(-2) at the current density of 1 mA cm(-2). Moreover, the specific capacitance maintains nearly 100% over 10,000 charge-discharge cycles, demonstrating a remarkable cyclic stability of these porous supercapacitor electrodes.

PACS

82.47.Uv (Electrochemical capacitors); 82.45.Fk (Electrodes electrochemistry); 81.05.Rm (Fabrication of porous materials).

Three-dimensional Fe2O3 nanocubes/nitrogen-doped graphene aerogels: nucleation mechanism and lithium storage properties

We developed a solvothermal-induced self-assembly approach to construct three dimensional (3D) macroscopic Fe₂O₃ nanocubes/nitrogen-doped graphene (Fe₂O₃-NC/GN) aerogel as anode materials for lithium-ion batteries (LIBs). The Fe₂O₃ nanocubes with length of ~50 nm are homogeneously anchored on 3D GN frameworks and as spacers to separate the neighboring GN sheets. Based on intensively investigations on the early stages of formation process, it is discovered that a non-classical nanoparticle-mediated crystallization process and a subsequent classical ion-mediated growth dominate the nanocube formation. This is totally different from the commonly recognized classical atom-mediated crystallization and ripening mechanism. Benefitting from the unique structures and characteristics, the optimized Fe₂O₃-NC/GN aerogel exhibits excellent rate capability, outstanding long-term cyclic stability at high current densities, which are outperforming most of Fe₂O₃/GS hybrid electrodes. These results suggest us to in-depth understand the detailed crystallization process, and rational design and precisely control the morphologies of nanocrystals on graphene for high performance energy applications.

A honeycomb-like porous carbon derived from pomelo peel for use in high-performance supercapacitors

A cost-effective approach to obtain electrode materials with excellent electrochemical performance is critical to the development of supercapacitors (SCs). Here we report the preparation of a three-dimensional (3D) honeycomb-like porous carbon (HLPC) by the simple carbonization of pomelo peel followed by KOH activation. Structural characterization indicates that the as-prepared HLPC with a high specific surface area (SSA) up to 2725 m(2) g(-1) is made up of interconnected microporous carbon walls. Chemical analysis shows that the HLPC is doped with nitrogen and also has oxygen-containing groups. Electrochemical measurements show that the HLPC not only exhibits a high specific capacitance of 342 F g(-1) and 171 F cm(-3) at 0.2 A g(-1) but also shows considerable rate capability with a retention of 62% at 20 A g(-1) as well as good cycling performance with 98% retention over 1000 cycles at 10 A g(-1) in 6 M KOH. Furthermore, an as-fabricated HLPC-based symmetric SC device delivers a maximum energy density of ∼9.4 Wh kg(-1) in the KOH electrolyte. Moreover, the outstanding cycling stability (only 2% capacitance decay over 1000 cycles at 5 A g(-1)) of the SC device makes it promising for use in a high-performance electrochemical energy system.

Fabrication of nitrogen-doped holey graphene hollow microspheres and their use as an active electrode material for lithium ion batteries

Nitrogen-doped holey graphene hollow microspheres (NHGHSs), synthesized through a template sacrificing method, were utilized as an anode material for lithium ion batteries (LIBs). Because of their specific microspherical hollow structure comprising nitrogen-doped holey graphene (NHG), the NHGHSs can exhibit reversible capacities of ∼ 1563 mAh g(-1) at a low rate of 0.5 C and ∼ 254 mAh g(-1) at a high rate of 20 C, which are significantly higher than the discharge capacity of the pristine graphene and other graphene-based carbonaceous materials. These, along with their good cycling stability, clearly demonstrate the great potential of using the NHGHSs as the anode material for LIBs of both high energy and power densities. We believe that the high specific surface area, holey structure of nitrogen-doped graphene, specific microspherical hollow structure, and increased interlayer spacing between the NHG nanosheets in their hollow walls are the main origins of their high electrochemical performance.

Graphene frameworks promoted electron transport in quantum dot-sensitized solar cells

Graphene frameworks (GFs) were incorporated into TiO2 photoanode as electron transport medium to improve the photovoltaic performance of quantum dot-sensitized solar cells (QDSSCs) for their excellent conductivity and isotropic framework structure that could permit rapid charge transport. Intensity modulated photocurrent/photovoltage spectroscopy and electrochemical impedance spectroscopy results show that the electron transport time (τ(d)) of 1.5 wt % GFs/TiO2 electrode is one-fifth of that of the TiO2 electrode, and electron lifetime (τ(n)) and diffusion path length (Ln) are thrice those of the TiO2 electrode. Results also revealed that the GFs/TiO2 electrode has a shorter electron transport time (τ(d)), as well as longer electron lifetime (τ(n)) and diffusion path length (Ln), than conventional 2D graphene sheets/TiO2 electrode, thus indicating that GFs could promote rapid electron transfer in TiO2 photoanodes. Photocurrent-voltage curves demonstrated that when incorporating 1.5 wt % GFs into TiO2 photoanode, a maximum power conversion efficiency of 4.2% for QDSSCs could be achieved. This value was higher than that of TiO2 photoanode and 2D graphene sheets/TiO2 electrode. In addition, the reasons behind the sensitivity of photoelectric conversion efficiency to the graphene concentration in the TiO2 were also systematically investigated. Our results provide a basic understanding of how GFs can efficiently promote electron transport in TiO2-based solar cells.

Flexible energy-storage devices: design consideration and recent progress

Flexible energy-storage devices are attracting increasing attention as they show unique promising advantages, such as flexibility, shape diversity, light weight, and so on; these properties enable applications in portable, flexible, and even wearable electronic devices, including soft electronic products, roll-up displays, and wearable devices. Consequently, considerable effort has been made in recent years to fulfill the requirements of future flexible energy-storage devices, and much progress has been witnessed. This review describes the most recent advances in flexible energy-storage devices, including flexible lithium-ion batteries and flexible supercapacitors. The latest successful examples in flexible lithium-ion batteries and their technological innovations and challenges are reviewed first. This is followed by a detailed overview of the recent progress in flexible supercapacitors based on carbon materials and a number of composites and flexible micro-supercapacitors. Some of the latest achievements regarding interesting integrated energy-storage systems are also reviewed. Further research direction is also proposed to surpass existing technological bottle-necks and realize idealized flexible energy-storage devices.

The selective formation of graphene ranging from two-dimensional sheets to three-dimensional mesoporous nanospheres

This research presents a template-free solvothermal method which offers selective preparation of graphene ranging from two-dimensional sheets to 3-dimensional nanospheres. The thus prepared nanospheres have size-defined mesopores with a huge surface area and, after doping with nitrogen, exhibited stronger electrocatalytic activity toward oxygen reduction than commercial Pt/C catalysts.

Evolution of cellulose into flexible conductive green electronics: a smart strategy to fabricate sustainable electrodes for supercapacitors

The integration of cellulose with electronic elements could form green electronics with the merits of the biopolymer and conductive polymer.

MnO2nanoflakes grown on 3D graphite network for enhanced electrocapacitive performance

The highly conductive 3D graphite network prepared by CVD serves as an excellent skeleton to grow uniform MnO₂nanoflakes with an enhanced capacitive performance.