All‐carbon hybrids for high performance supercapacitors

A hybrid nanostructure with partially reduced graphene oxide (rGO) and carbon nanofibers (CNFs) was fabricated and used as supercapacitor electrodes. A straightforward, environmentally friendly, and low‐cost microwave‐assisted reduction process was developed for the synthesis of rGO/CNF hybrid structures. The fabricated supercapacitor devices showed a specific capacitance of 95.3 F g^?1 and a superior long‐term cycling stability. A capacitance retention of more than 97% after 11 000 galvanostatic charge discharge cycles was obtained. These and other results reported in this paper indicate that high‐rate, all‐carbon, rGO/CNF hybrid nanostructures are highly promising supercapacitor electrode materials.     

Preparation methods and progress of manganese dioxide/graphene based composites in supercapacitors

综述了基于MnO₂/石墨烯的二元、三元复合材料在超级电容器方面的最新研究进展。由于范德华力造成的堆叠,石墨烯实际比电容并不高。MnO₂理论比电容高达1370 F/g,但因其赝电容受MnO₂片层厚度的限制,实际比电容远小于理论值。将石墨烯和MnO₂复合,MnO₂纳米结构锚定在石墨烯纳米片之间充当间隔物,可以有效抑制石墨烯的堆叠,增强界面电荷转移,借助二者的协同效应有望实现高比电容、高电导率和良好的循环稳定性。介绍了MnO₂/石墨烯复合材料的制备方法及电化学性能。对比分析了MnO₂/石墨烯三元复合材料的电化学性能,由于金属氧化物或导电聚合物的引入,电化学性能进一步提升。最后总结指出基于MnO₂/石墨烯的多元复合材料和器件还面临着安全可靠、规模化生产以及降成本等一系列问题。随着技术的不断成熟和突破,有望在工业、交通以及日常电子器件中获得应用。     

Formation of hierarchical 3D cross-linked porous carbon with small addition of graphene for supercapacitors

In the present paper, starch was used as raw material to prepare carbon material with low-temperature hydrothermal route and hierarchical three-dimensional cross-linked porous carbon was successfully synthesized with the help of a small amount of graphene for high-performance supercapacitors. It's found that presence of graphene is a crucial condition for the formation of 3D porous carbon and graphene acts as a skeleton in the porous carbon. This kind of carbon material exhibited very high surface area of 1887.8 m² g⁻¹ and delivered excellent electrochemical performance. Its specific capacitance can reach 141 F g⁻¹ at 0.5 A g⁻¹ and more importantly, after 10,000 cycles 98.6% of initial specific capacitance can be maintained. To explore the practical application of the 3D porous carbon, an asymmetric supercapacitor coin-type device was assembled with 3D porous carbon and graphene as electrode materials in organic electrolyte. The constructed device exhibited high energy density of 48.5 Wh·kg⁻¹ at a power density of 1.5 kW kg⁻¹ and still maintains 39.625 Wh·kg⁻¹ under the high power density (15 kW kg⁻¹). These results will promote the rapid development of 3D porous carbon prepared by low-temperature route and the application in supercapacitors.     

Holey graphene/MnO2 nanosheets with open ion channels for high-performance solid-state asymmetric supercapacitors

Holey graphene/MnO₂ (HG/MnO₂) composites with open ion channels are synthesized by an electrostatic self-assembly method. The HG with rich in-plane nanopores is prepared via a mild etching reaction first, followed by modified with poly-diallyldimethylammoniumchloride (PDDA) to transfer the surface charge of HG nanosheets from negative to positive, which are eventually assembled on the negatively charged MnO₂ nanosheets via electrostatic attraction. As a result, the HG allows ions to pass through the graphene sheet, improving the ion transport channels. Besides, the electrostatic self-assembly between HG and MnO₂ enables the composite a high conductivity, providing effective electron transport pathways. The HG and MnO₂ sheets are observed to be tightly bounded by the transmission electron microscope (TEM), and the HG content in the composites is determined to be 9.6% to 20% by the thermogravimetric (TG) test. The HG/MnO₂-2 electrode with the HG content of around 14.8% displays the large specific capacitance of 219.3 F g⁻¹ at 0.5 A g⁻¹ and the high rate capacity of 134.7 F g⁻¹ at 10 A g⁻¹. Furthermore, the as-prepared solid-state asymmetric supercapacitors (SSAS) achieve a wide stable operating voltage of 1.8 V, a high energy density of 16.8 Wh kg⁻¹ at the power density of 224.6 W kg⁻¹, and low capacitance degradation of only 6.3% after 5000 cycles.     

Progress in graphene based supercapacitors

超级电容器是最具应用前景的电化学储能技术之一.目前,超级电容器的研究重点是提高能量密度和功率密度,发展具有高比表面积,电导率和结构稳定性的电极材料是关键.石墨烯因具有比表面积大,电子导电性高,力学性能好的特点而成为理想的电容材料,但石墨烯的理论容量不高,在石墨烯基电极制备过程中容易发生堆叠现象,导致材料比表面积和离子电导率下降.因此,发展合适的制备方法,对石墨烯进行修饰或与其他材料形成复合电极材料是一种有效解决途径.本文对石墨烯基电极及其在双电层电容器,法拉第准电容器和混合型超级电容器中的应用的研究进展进行归纳,重点介绍了石墨烯凝胶薄膜电极的制备过程,以促进石墨烯基电极在超级电容器构筑中应用.     

High-performance enzymatic biofuel cell based on three-dimensional graphene

Enzymatic biofuel cells are a subclass of biofuel cells, which employ enzymes to generate energy from renewable sources. In this study, 3-dimensional graphene (3DG)/glucose oxidase (GOx) bio-nanocomposite was fabricated in order to improve enzyme immobilisation and enzyme lifetime with an enhanced electron transfer rate. These enhancements are due to the unique physical properties of 3DG, e.g. high porosity, large surface area, and excellent electrical conductivity. A power density of 164 μW cm⁻² at 0.4 V was achieved from this enzymatic biofuel cell (EBFC) with an acceptable performance compared to that of the other glucose biofuel cells (GBFCs). The 3DG enhances the enzyme lifetime, decreases enzyme leaking and, due to its good conductivity, facilitates the electron harvest and transfer from the enzyme active site to the electrode. This suggests that 3DG could be used as effective support for enzyme immobilisation on the surface of the electrode in EBFC applications and related areas such as biosensors, bioreactors and implantable biofuel cells.     

Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance

Engineering multicomponent active materials as an advanced electrode with the rational designed core-shell structure is an effective way to enhance the electrochemical performances for supercapacitors. Herein, three-dimensional self-supported hierarchical CoMoO₄@CoS core-shell heterostructures supported on reduced graphene oxide/Ni foam have been rationally designed and prepared via a facile approach. The unique structure and the synergistic effects between two different materials, as well as excellent electronic conductivity of the reduced graphene oxide, contribute to the increased electrochemically active site and enhanced capacitance. The core-shell CoMoO₄@CoS composite displays the superior specific capacitance of 3380.3 F g⁻¹ (1 A g⁻¹) in the three-electrode system and 81.1% retention of the initial capacitance even after 6000 cycles. Moreover, an asymmetric device was successfully prepared using CoMoO₄@CoS and activated carbon as positive/negative electrodes. It is worth mentioning that the device delivered the high energy density of 59.2 W h kg⁻¹ at the power density of 799.8 W kg⁻¹ and the excellent cycle performance (about 91.5% capacitance retention over 6000 cycles). These results indicate that the core-shell CoMoO₄@CoS composites offers the novelty strategy for preparation of electrodes for energy conversion and storage devices.     

Preparation of activated graphene/activated carbon dry composite electrode and its application in supercapacitors

采用干法电极制备工艺成功制备了活性石墨烯/活性炭复合电极片,分别用扣式电容器和软包电容器考察活性石墨烯/活性炭复合电极的电化学性能。综合结果表明,复合电极中活性石墨烯的含量为10%（质量分数）较为合适,相较于纯活性炭电极,比容量提高了10.8%。本工作验证了活性石墨烯材料在商用超级电容器中的适用性,证实了活性石墨烯是一种非常具有实际应用价值的电极材料。但目前,活性石墨烯并未真正产业化,其成本远高于商用活性炭。在未来,如何解决活性石墨烯工程制备技术难题和降低成本是材料产业界亟待解决的难题。     

Arginine-functionalized graphene oxide for green and high-performance symmetric supercapacitors

The present paper intends to show how a graphene-based composite has effortlessly been synthesized utilizing the arginine amino acid (Arg) as a supercapacitor electrode via a green process with neither organic nor toxic substances in aqueous electrolyte (KOH 6 M). Arg amino acid molecules were used as interfacial spacer of graphene sheets to obstruct aggregation. Hence, the surface area increased and the efficient diffusion of electrolyte ions was facilitated in consequence of generating a 3D porous graphene structure. Besides, the surface chemistry of graphene is tuned as amine and carboxylic acid groups are present in the Arg structure which results in high capacitance through electrode wettability and improved diffusion of electrolyte ions into the electrode structure as well as through pseudo-faradic reactions. The as-fabricated electrode provides an exceptional specific capacitance of 295 F g⁻¹ in 1 A g⁻¹ and effective cycling stability (94% after 4000 cycles) in the design of two-electrode configuration. Considering environmental compatibility as well as the superior power and energy density (2204 W kg⁻¹ and 50 Wh Kg⁻¹, respectively), the striking ability of Arg/GO composite to become an appropriate choice for future generations of energy storage devices is proven.     

Fabrication of flexible polypyrrole/graphene oxide/manganese oxide supercapacitor

A flexible polypyrrole/graphene oxide/manganese oxide‐based supercapacitor was prepared via an electrodeposition process. The polypyrrole, graphene oxide, and manganese oxide were deposited onto a flexible and highly porous nickel foam, which acted as a current collector to enhance the electrochemical performances. The good coverage of the polypyrrole, graphene oxide, and manganese oxide onto the scaffold of the nickel foam was evidenced using field emission scanning electron microscopy and X‐ray diffraction. The manganese species, which were present in the oxidation states of Mn³⁺ and Mn⁴⁺, were shown using X‐ray photoelectron spectroscopy. The presence of Mn₂O₃ and MnO₂ polymorphs was detected using Fourier transform infrared and Raman spectroscopies. The cyclic stability of the ternary supercapacitor was consistent regardless of its geometry and curvature. In contrast, an activated carbon supercapacitor possesses limited energy storage capability compared to a ternary supercapacitor, which suppresses the electrochemical performances of activated carbon. The ternary as‐fabricated supercapacitor could retain a specific capacitance of 96.58% after 1000 cycles, and the as‐synthesized energy storage device was able to light up a light emitting diode. Copyright © 2014 John Wiley & Sons, Ltd.     

Graphene-supported ultra-small Co3O4 nanoparticles for high-performance supercapacitors

Ultra-small Co3O4 nanoparticles/graphene hybrid material had been synthesized by a facile hydrothermal route and consequent calcination process. The as-obtained ultra-small Co3O4 nanoparticles with their sizes of 5–8 nm are tightly anchored on the surface of graphene (GNS). Benefiting from the ultra-small size of Co3O4 nanoparticles, the high interconnectivity of hybrid material as well as the high conductive networks constructed by GNS, which can provide a fast and efficient transportation of electron and electrolyte ions for the overall electrode, the as-prepared hybrid material exhibits a high specific capacitance of 462 F•g1 at 5 mV•s1 compared with pure Co3O4 (193 F•g1), and retained 88.2% of its initial capacitance after 2000 cycles, indicating a promising electrode material for supercapacitors.     

A two-dimensional material for high capacity supercapacitors: S-doped graphene

In this work, sulfur-doped graphene-coated electrodes are prepared by cyclic voltammetry in different potential ranges and different cycles (from 10 to 50) for selective modification of electrodes by different functional groups. The prepared electrodes are characterized by spectroscopic, microscopic and electrochemical methods. In scanning electron microscopic analysis, formation of graphene layers and their porous structure have been determined. Electrochemical impedance spectroscopic and cyclic voltammetric analyses are also used in electrochemical characterization of the electrodes. Then, the prepared sulfur-doped graphene-coated electrodes by using cyclic voltammetry in one-step and low cost are used as electrode materials of supercapacitor for the first time in the literature. Since the mesoporous structure of the electrodes prepared in lower potential ranges increases, specific capacitance of the electrodes increases from 74 to 1833 mF cm⁻² with 10 mA cm⁻² current density. This result shows that specific capacitances of prepared electrodes are higher than those of the electrodes prepared with metal-doped in the literature.     

High electrochemical performance of metal azolate framework-derived ZnO/Co3O4 for supercapacitors

Metal azolate frameworks MAF-6 (Zn, Zn Co, and Co) were fabricated with a facile solution mixture method. Flower-like ZnO/Co₃O₄ was synthesized with MAF-6 as the template in a solvothermal condition of 90°C for 2 hours. The produced materials were characterized by XRD, EDS, SEM, and XPS. MAF-6 and the derived metal oxides acted as the electrode materials of supercapacitors. This study reported that ZnO/Co₃O₄ exhibited the maximal specific capacitance of 830.20 F/g at 1A/g, resulting from the synergy of Zn and Co and the enhanced conductivity. Furthermore, ZnO/Co₃O₄ electrode exhibited prominent rate capacitance, high electrochemical reversibility, and long life (89% capacitance was kept after 1000 cycles at 1 A/g). Given the results of this study, the prepared ZnO/Co₃O₄ electrode can be demonstrated as a potential candidate in the field of renewable energy storage.     

Two/three-dimensional reduced graphene oxide coating for porous flow distributor in polymer electrolyte membrane fuel cell

There are drawbacks to use stainless-steel plates as a flow distributor plate in fuel cell, due to some of their properties being inferior to graphite flow distributor plates in terms of electrical conductivity and corrosion resistance. To overcome these problems, many researches have been conducted to improve the properties of stainless-steel flow distributor plates through coating of carbon materials. Herein, two-dimensional Web-like graphene (WG) and self-assembled three-dimensional graphene (STG) are coated through superheat vaporization of micro-droplet method. WG is coated in porous Ni–Cr foam and STG is constructed on the flat flow distributor plate, and they exhibit the feasibility to be applied in flow distributors. Compared to uncoated Ni–Cr foam, the performance of the PEMFC system with the graphene coated foam is enhanced remarkably. Furthermore, the flow distributor plate with the STG exhibits potential to be used directly to flow distributor.     

The electrochemical properties of reduced graphene oxide film with capsular pores prepared by using oxalic acid as template

By using oxalic acid (OA) as template and reducer, a novel approach is developed to prepare reduced graphene oxide films with capsular pores (C‐rGOFs) under a hydrothermal condition. The effect of preparation conditions including concentrations of OA and reaction temperatures on the films' structure and capacitive performances has been systematically investigated. The optimal C‐rGOF shows uniform capsule‐like morphology and exhibits a density of 1.18 g cm^?3. Tested by using a two‐electrode system, the optimal film shows gravimetric specific capacitance of about 234.9 F g^?1 and volumetric specific capacitance of 277.2 F cm^?3. Additionally, the optimal film which shows good rate capability can retain 63.9% of initial capacitance at high scan rate of 1.0 V s^?1, which is much higher than that of the controlling reduced graphene oxide film (rGOF, 180.5 F g^?1, 373.6 F cm^?3 and retain only 45.0% of its initial capacitance at 1.0 V s^?1). The cells assembled by the optimal C‐rGOF exhibit maximum energy density of 7.5 Wh kg^?1, power density of 16.9 kW kg^?1, and excellent cycling stability with 91.2% capacitance retention after 21 000 cycles. It is believed that this method can be developed as a useful strategy to prepare rGO‐based materials for energy storage applications.     

Thermally annealed self-assembled three-dimensional graphene for direct construction of porous flow distributor in polymer electrolyte membrane fuel cell

Self-assembled three-dimensional graphene (STG), which is constructed via advanced boiling method (ABM) has advantages in terms of low-cost, facile procedures, and needless to use any backbone structure. Although the previously developed STG is considered as a promising material and structure to be used as a flow distributor in polymer electrolyte membrane fuel cell (PEMFC), the STG flow distributor should be advanced in the aspect of conductivity to achieve higher performance. Herein, the conductivity of STG is improved via the thermal annealing process and the advanced STG is applied in PEMFC as a flow distributor to enhance the performance of the single-cell system. Single cell constructed with the thermally annealed STG exhibits a lowered charge and ohmic resistance, which leads to the performance enhancement. These results imply that direct coating of the advanced STG can be one of the alternatives of the complex and high-cost process of graphene coating to the porous metal foam through chemical vapor deposition (CVD).     

Fabrication of novel hybrid composite La2‐xCoxCuO4 electrode for high performance supercapacitors

Hybrid composites La_2‐xCo_xCuO₄ (x = 0, 0.1, 0.2, and 0.3) are prepared using one‐step simple hydrothermal route as electrodes for supercapacitors. The effect of varying cobalt content on morphological, structural, and electrochemical properties has been explored using X‐ray diffraction, scanning electron microscopy, and cyclic voltammetry, respectively. The structural parameters obtained by X‐ray diffraction showed tetragonal phase of hybrid composite without any evident impurity phases. The analysis of morphological properties suggested a strong correlation with electrochemical properties, for instance, a relationship between fabric porous structures and electrochemically active sites for redox reactions and intercalation/de‐intercalation processes. The hybrid composite electrodes demonstrated high specific capacitance of the order of 1304 F/g at 10 mV/s scan rate and exhibited decreasing trend on increasing scan rate. Hybrid composites were also tested for their ability as an electrode of high performance supercapacitors in different aqueous electrolytes, i. e, KOH, H₂SO₄, and Na₂SO₄ to optimize the best compatible electrolyte. The composite electrode material showed excellent cyclic stability and 98% capacitance retention for 1 A/g after 2000 cycles. The remarkable performance of hybrid composite electrode entails its potential for commercial applications of supercapacitors.     

Graphene‐based ternary composites for supercapacitors

The research on electrode materials for supercapacitor application continues to evolve as the request of high‐energy storage system has increased globally due to the demand for energy consumption. Over the past decades, various types of carbon‐based materials have been employed as electrode materials for high‐performance supercapacitor application. Among them, graphene is 1 of the most widely used carbon‐based materials due to its excellent properties including high surface area and excellent conductivity. To exploit more of its interesting properties, graphene is tailored to produce graphene oxide and reduced graphene oxide to improve the dispersibility in water and easy to be incorporated with other materials to form binary composites or even ternary composites. Nowadays, ternary composites have attracted enormous interest as 2 materials (binary composites) cannot satisfy the requirement of the high‐performance supercapacitor. Thus, many approaches have been employed to fabricate ternary composites by combining 3 different types of electroactive materials for high‐performance supercapacitor application. This review focuses on the supercapacitive performance of graphene‐based ternary composites with different types of active materials, ie, conducting polymers, metal oxide, and other carbon‐based materials.     

Cauliflower‐shaped ternary nanocomposites with enhanced power and energy density for supercapacitors

The present research work aimed to study the electrochemical performance of the rGO/PPY/PANI ternary nanocomposite electrodes for supercapacitor applications. The nanocomposites have been prepared by physical blending of rGO with conducting polymers PANI and PPY in five different ratios. The prepared nanocomposites were examined by XRD, IR, Raman, SEM, and XAS characterizations, and from the results, it was found that ternary nanocomposites formed in cauliflower shape, in which PPY and PANI nanoparticles are decorated on to the rGO matrix. In addition, the electrochemical performance of the prepared nanocomposites were studied using cyclic voltammetry, galvanostatic charge‐discharge, and electrochemical impedance spectroscopic studies. The highest values of capacitance, energy density, and power density values achieved were 317.5 F/g, 254 Wh/kg, and 1508.9 W/kg for nanocomposite, respectively, as expected from the synergistic properties of two types of electrode materials resulting in the nanocomposites with hybrid and improved properties. Further, the cyclic stability was also analyzed by performing 4000 long cycles, and the retained capacitance during such long cycles indicates the high potential of rGO/PPY/PANI ternary nanocomposites as electrodes for future energy requirement.     

Compact energy storage: Opportunities and challenges of graphene for supercapacitors

碳纳米储能材料发展迅速,质量容量性能不断刷新。但通常碳纳米材料的密度较低,导致其体积比容量有限,在很多时候很难将材料水平上的优异性能反映到最终的器件上。发展高体积能量密度储能材料,在器件水平上实现致密储能,对推动储能材料和器件的实用化至关重要。作为其它sp2碳质材料的基本结构单元和一种柔性二维材料,石墨烯通过组装可以实现纳米结构致密化,在致密储能方面具有先天优势。本文以石墨烯在超级电容器中的应用为主,分别从材料、电极、器件3个层次讨论了实用化储能器件的设计原则,梳理了高体积能量密度碳基储能材料的研究进展,重点介绍了高体积容量碳电极材料的致密化设计理念,强调了从器件角度考虑储能材料设计的重要性,并对致密储能面临的机遇和挑战作了分析。