Synergistic effects from graphene and carbon nanotubes enable flexible and robust electrodes for high-performance supercapacitors

Flexible and lightweight energy storage systems have received tremendous interest recently due to their potential applications in wearable electronics, roll-up displays, and other devices. To manufacture such systems, flexible electrodes with desired mechanical and electrochemical properties are critical. Herein we present a novel method to fabricate conductive, highly flexible, and robust film supercapacitor electrodes based on graphene/MnO(2)/CNTs nanocomposites. The synergistic effects from graphene, CNTs, and MnO(2) deliver outstanding mechanical properties (tensile strength of 48 MPa) and superior electrochemical activity that were not achieved by any of these components alone. These flexible electrodes allow highly active material loading (71 wt % MnO(2)), areal density (8.80 mg/cm(2)), and high specific capacitance (372 F/g) with excellent rate capability for supercapacitors without the need of current collectors and binders. The film can also be wound around 0.5 mm diameter rods for fabricating full cells with high performance, showing significant potential in flexible energy storage devices.     

High-performance asymmetric supercapacitors based on reduced graphene oxide/polyaniline composite electrodes with sandwich-like structure

The sandwich-like structure of reduced graphene oxide/polyaniline(RGO/PANI) hybrid electrode was prepared by electrochemical deposition. Both the voltage windows and electrolytes for electrochemical deposition of PANI and RGO were optimized. In the composites, PANI nanofibers were anchored on the surface of the RGO sheets, which avoids the re-stacking of neighboring sheets. The RGO/PANI composite electrode shows a high specific capacitance of 466 F/g at 2 m A/cm~2 than that of previously reported RGO/PANI composites. Asymmetric flexible supercapacitors applying RGO/PANI as positive electrode and carbon fiber cloth as negative electrode can be cycled reversibly in the high-voltage region of 0–1.6 V and displays intriguing performance with a maximum specific capacitance of 35.5 m F cm~(-2). Also, it delivers a high energy density of 45.5 m W h cm~(-2) at power density of 1250 m W cm~(-2). Furthermore, the asymmetric device exhibits an excellent long cycle life with 97.6% initial capacitance retention after 5000 cycles.Such composite electrode has a great potential for applications in flexible electronics, roll-up display,and wearable devices.     

NiCo2S4 nanoparticles grown on reduced graphene oxides for high-performance asymmetric supercapacitors

The nanostructures of reduced graphene oxide (rGO)/NiCo₂S₄ are prepared using the simple hydrothermal method and the thermal treatment process, which could provide good conductivity and ideal specific surface area. The rGO/NiCo₂S₄ electrode shows a maximum specific capacitance of 1059 F g⁻¹, excellent rate capability, and good cycle life. Furthermore, the three dimensional structures of rGO/MnO (3D rGO/MnO) are also synthesized by the hydrothermal method and the thermal treatment process, which have the high specific surface area and good conductivity. The rGO/MnO electrode exhibits a maximum specific capacitance of 469 F g⁻¹. A rGO/MnO//rGO/NiCo₂S₄ asymmetric supercapacitors (ASC) is assembled using 2 M KOH solution as electrolyte, rGO/NiCo₂S₄ as positive electrode and rGO/MnO as negative electrode. The rGO/MnO//rGO/NiCo₂S₄ ASC shows an energy density of 38.8 Wh kg⁻¹ at a power density of 0.4 kW kg⁻¹ and a good cycle life, which provides a possibility toward actual application in energy-storage systems.     

Self-assembled high-performance graphene oxide fibers using ionic liquid as coagulating agent

An efficient strategy for fabricating high-performance GO fibers (GFs) by using ionic liquids as coagulating agent via wet-spinning technique was reported for the first time. The interactions between the functional groups of the GO sheets and the ionic liquids cations could be tuned by choosing ionic liquids cations with designed structure, yielding GFs with varied mechanical properties. No organic solvent or postdrawing processes involved makes this process green and facile.     

Patterned growth of polyaniline nanowire arrays on a flexible substrate for high-performance gas sensing

Uniform patterning of polyaniline nanowire arrays on a wafer-sized flexible substrate is achieved by combining photolithography and in situ polymerization techniques. Chemical gas sensors based on the patterned polyaniline nanowire arrays exhibit excellent performance because of their highly ordered morphology and large specific surface area.     

Flexible pillared graphene-paper electrodes for high-performance electrochemical supercapacitors

Flexible graphene paper (GP) pillared by carbon black (CB) nanoparticles using a simple vacuum filtration method is developed as a high-performance electrode material for supercapacitors. Through the introduction of CB nanoparticles as spacers, the self-restacking of graphene sheets during the filtration process is mitigated to a great extent. The pillared GP-based supercapacitors exhibit excellent electrochemical performances and cyclic stabilities compared with GP without the addition of CB nanoparticles. At a scan rate of 10 mV s(-1) , the specific capacitance of the pillared GP is 138 F g(-1) and 83.2 F g(-1) with negligible 3.85% and 4.35% capacitance degradation after 2000 cycles in aqueous and organic electrolytes, respectively. At an extremely fast scan rate of 500 mV s (-1) , the specific capacitance can reach 80 F g(-1) in aqueous electrolyte. No binder is needed for assembling the supercapacitor cells and the pillared GP itself may serve as a current collector due to its intrinsic high electrical conductivity. The pillared GP has great potential in the development of promising flexible and ultralight-weight supercapacitors for electrochemical energy storage.     

Bismuth oxide/nitrogen-doped carbon dots hollow and porous hierarchitectures for high-performance asymmetric supercapacitors

The development of high-performance anode materials is critical for next-generation asymmetric supercapacitor devices. Herein, an advanced anode of bismuth oxide/nitrogen-doped carbon dots (Bi₂O₃/NCDs) hollow and porous hierarchitecture was successfully developed by a facile two-step strategy. It is revealed that the presence of NCDs can not only induce the assembly of Bi₂O₃ porous nanosheets to hollow microspheres, but also remarkably improve the specific capacitances of them. Attributing to the intriguing properties of NCDs and the unique structure of Bi₂O₃/NCDs composites, the Bi₂O₃/NCDs hierarchitectures deliver an enhanced specific capacitance of approximately 1046 F g⁻¹ at a current density of 1 A g⁻¹. In particularly, an asymmetric supercapacitor was constructed by using the Bi₂O₃/NCDs hierarchical microspheres as anode and Ni(OH)₂/NCDs nanosheets as cathode, which exhibits an ultrahigh energy density of about 79.9 Wh kg⁻¹ at 770.9 W kg⁻¹. These results indicate that the Bi₂O₃/NCDs hollow and porous hierarchitectures hold great potential as novel anode materials for energy storage.     

Assembly of highly stable aqueous dispersions and flexible films of nitrogen-doped graphene for high-performance stretchable supercapacitors

Here, we propose a route for production of highly stable nitrogen-doped graphene (NG) aqueous dispersions and flexible NG films. The NG dispersions are formed through sonication of NG hydrogels in methyl orange solution and stabilized through electrostatic repulsion without using any polymeric or surfactant stabilizer. The flexible NG films are formed by vacuum filtration of as-prepared NG dispersions and transferred onto stretchable and adhesive polyacrylate for the fabrication of stretchable supercapacitors. The supercapacitor exhibits high gravimetric specific capacitances of 315 F g^?1 under no strain and 300 F g^?1 under 30% applied strain at a current density of 1 A g^?1. The fabrication of aqueous dispersions and stretchable NG films makes NG materials attractive for a diversity of applications in stretchable electronics.     

纤维素纳米纤丝-还原氧化石墨烯/聚苯胺气凝胶柔性电极复合材料的制备与性能

利用高长径比的纤维素纳米纤丝（CNF）与片层结构的氧化石墨烯（GO）形成的CNF-GO复合水凝胶经抗坏血酸还原制备出CNF-还原氧化石墨烯（rGO）复合水凝胶材料。通过冷冻干燥法得到CNF-rGO复合气凝胶,并进一步通过苯胺单体在CNF-rGO复合气凝胶的孔道内原位聚合制备出CNF-rGO/聚苯胺（PANI）气凝胶柔性电极复合材料。研究了不同苯胺、CNF和GO的质量比对CNF-rGO/PANI气凝胶柔性电极复合材料的结构形貌和电化学性能的影响。结果表明,苯胺原位聚合后所得CNF-rGO/PANI复合气凝胶仍具有紧密的三维多孔网络结构。与rGO/PANI气凝胶电极复合材料相比,CNF-rGO/PANI气凝胶电极复合材料具有更理想的电容行为。当CNF与GO质量比为60∶40,PANI添加量为0.1 mol时,CNF-rGO/PANI气凝胶电极复合材料比电容可达85.9 Fg-1,且其电化学性能几乎不受弯曲程度的影响,展现出了良好的柔韧性和电化学性能。      

Low temperature reduction of graphene oxide film by ammonia solution and its application for high-performance supercapacitors

Here we demonstrate that graphene oxide (GO) film on Ni foam can be doped with nitrogen atoms and reduced directly at a lower temperature of 90?°C using ammonia solution as reducing agent and nitrogen source. The reduction and nitrogen doping of GO occur simultaneously when GO film on Ni foam is immersed into ammonia solution. The nitrogen doping can be realised and the content of N in graphene film turns out to be rather good as high as 3.60%. When used as binder-free electrode, the resulting graphene film on Ni foam delivers a gravimetric capacitance of 230 F g^?1. It also exhibites relatively an outstanding rate capability of 164 F g^?1 at 83.3 A g^?1 and better cycle stability that capacitance retention maintains at 96.7% of its initial capacitance capacitance after 2000 cycles. The method also provides a universal route for preparing a binder-free graphene-based electrode.     

Polypyrrole/titanium oxide nanotube arrays composites as an active material for supercapacitors

The authors present the first reported use of vertically oriented titanium oxide nanotube/polypyrrole (PPy) nanocomposites to increase the specific capacitance of TiO2 based energy storage devices. To increase their electrical storage capacity, titanium oxide nanotubes were coated with PPy and their morphologies were characterized. The incorporation of PPy increased the specific capacitance of the titanium oxide nanotube based supercapacitor system, due to their increased surface area and additional pseudo-capacitance.     

Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors

Electrochemical supercapacitors can deliver high levels of electrical power and offer long operating lifetimes, but their energy storage density is too low for many important applications. Pseudocapacitive transition-metal oxides such as MnO(2) could be used to make electrodes in such supercapacitors, because they are predicted to have a high capacitance for storing electrical charge while also being inexpensive and not harmful to the environment. However, the poor conductivity of MnO(2) (10(-5)-10(-6) S cm(-1)) limits the charge/discharge rate for high-power applications. Here, we show that hybrid structures made of nanoporous gold and nanocrystalline MnO(2) have enhanced conductivity, resulting in a specific capacitance of the constituent MnO(2) (~1,145 F g(-1)) that is close to the theoretical value. The nanoporous gold allows electron transport through the MnO(2), and facilitates fast ion diffusion between the MnO(2) and the electrolytes while also acting as a double-layer capacitor. The high specific capacitances and charge/discharge rates offered by such hybrid structures make them promising candidates as electrodes in supercapacitors, combining high-energy storage densities with high levels of power delivery.     

Highly ordered nanowire arrays on plastic substrates for ultrasensitive flexible chemical sensors

The development of a robust method for integrating high-performance semiconductors on flexible plastics could enable exciting avenues in fundamental research and novel applications. One area of vital relevance is chemical and biological sensing, which if implemented on biocompatible substrates, could yield breakthroughs in implantable or wearable monitoring systems. Semiconducting nanowires (and nanotubes) are particularly sensitive chemical sensors because of their high surface-to-volume ratios. Here, we present a scalable and parallel process for transferring hundreds of pre-aligned silicon nanowires onto plastic to yield highly ordered films for low-power sensor chips. The nanowires are excellent field-effect transistors, and, as sensors, exhibit parts-per-billion sensitivity to NO2, a hazardous pollutant. We also use SiO2 surface chemistries to construct a 'nano-electronic nose' library, which can distinguish acetone and hexane vapours via distributed responses. The excellent sensing performance coupled with bendable plastic could open up opportunities in portable, wearable or even implantable sensors.     

Periodically porous top electrodes on vertical nanowire arrays for highly sensitive gas detection

Nanowires of various materials and configurations have been shown to be highly effective in the detection of chemical and biological species. In this paper, we report a novel, nanosphere-enabled approach to fabricating highly sensitive gas sensors based on ordered arrays of vertically aligned silicon nanowires topped with a periodically porous top electrode. The vertical array configuration helps to greatly increase the sensitivity of the sensor while the pores in the top electrode layer significantly improve sensing response times by allowing analyte gases to pass through freely. Herein, we show highly sensitive detection to both nitrogen dioxide (NO(2)) and ammonia (NH(3)) in humidified air. NO(2) detection down to 10 parts per billion (ppb) is demonstrated and an order-of-magnitude improvement in sensor response time is shown in the detection of NH(3).