Scalable synthesis of hierarchically structured carbon nanotube–graphene fibres for capacitive energy storage

Abstract

Micro-supercapacitors are promising energy storage devices that can complement or even replace batteries in miniaturized portable electronics and microelectromechanical systems. Their main limitation, however, is the low volumetric energy density when compared with batteries. Here, we describe a hierarchically structured carbon microfibre made of an interconnected network of aligned single-walled carbon nanotubes with interposed nitrogen-doped reduced graphene oxide sheets. The nanomaterials form mesoporous structures of large specific surface area (396 m(2) g(-1)) and high electrical conductivity (102 S cm(-1)). We develop a scalable method to continuously produce the fibres using a silica capillary column functioning as a hydrothermal microreactor. The resultant fibres show a specific volumetric capacity as high as 305 F cm(-3) in sulphuric acid (measured at 73.5 mA cm(-3) in a three-electrode cell) or 300 F cm(-3) in polyvinyl alcohol (PVA)/H(3)PO(4) electrolyte (measured at 26.7 mA cm(-3) in a two-electrode cell). A full micro-supercapacitor with PVA/H(3)PO(4) gel electrolyte, free from binder, current collector and separator, has a volumetric energy density of ∼6.3 mWh cm(-3) (a value comparable to that of 4 V-500 µAh thin-film lithium batteries) while maintaining a power density more than two orders of magnitude higher than that of batteries, as well as a long cycle life. To demonstrate that our fibre-based, all-solid-state micro-supercapacitors can be easily integrated into miniaturized flexible devices, we use them to power an ultraviolet photodetector and a light-emitting diode.

Ultrahigh‐Power Pseudocapacitors Based on Ordered Porous Heterostructures of Electron‐Correlated Oxides

“…Despite slight decrease of the volumetric capacitance with the increase of the current density up to up to 312 A cm −3 , the NP V 2 O 3 /MnO 2 based pseudocapacitor can sustain high volumetric capacitances at ultrafast charge/discharge rates, which are ≈2–5 times higher than the best values reported for EDLCs based on the chemically converted graphene (CCG) films with a high packing density of 1.33 g cm −3 (Figure 4c) 13. This excellent rate capability is also much better than those of other carbon materials, such as single‐walled carbon nanotubes/reduced graphene oxide (SWNT/rGO) fibers,11 graphene/carbon nanotube (graphene/CNT) carpet,12 SWNTs 14 or activated carbon (AC),14 and the pseudocapacitive Ni(OH) 2 supported by NP Ni skeleton [NP Ni/Ni(OH) 2 ] 55, 56…”

“…b) Galvanostatic charge/discharge profiles at various current densities for NP V 2 O 3 /MnO 2 based pseudocapacitors. c) Volumetric capacitance of NP V 2 O 3 /MnO 2 electrodes as a function of different current densities, comparing with these of SWNT/rGO fibers,11 graphene/CNT carpets,12 liquid‐mediated CCG film,13 SWNTs,14 AC,14 NP Ni/Ni(OH) 2 ,39 and commercial supercapacitor (5.5 V/100 mF SC) 11. d) Specific capacitance of the constituent MnO 2 in the NP V 2 O 3 /MnO 2 hybrid electrode as a function of scan rate, in comparison with that of NP MnO 2 supported by CNT sponges,29 SWNT films,30 NP Au,57 and stainless steel substrate.…”

“…The volumetric power and energy densities of NP V 2 O 3 /MnO 2 ‐based pseudocapacitor are plotted in the Ragone plot ( Figure 5 ), in which the values of other energy storage devices, such as carbon‐based EDLCs8, 11, 13 and TMO‐based pseudocapacitors,6, 20, 25 as well as commercial AC supercapacitor9 and electrolytic capacitor,11 are included for comparison. At low powers (2.1 W cm −3 ), the pseudocapacitor assembled with the NP V 2 O 3 /MnO 2 electrodes has a volumetric energy density of ≈94 mWh cm −3 , which is two orders of magnitude higher than that of commercially available 2.75 V/44 mF AC supercapacitor 9.…”

“…Unlike electrochemical double‐layer capacitors (EDLCs),8, 9, 10, 11, 12 in which charge storage is achieved by nonfaradaic electrostatic adsorption in nanostructured carbons with low intrinsic capacitance (≈20 μF cm −2 carbon ),1, 13, 14 pseudocapacitors store high‐density energy on pseudocapacitive materials by fast and reversible surface redox reactions at or near the electrode/electrolyte interface 1, 2, 3, 4, 5, 6, 7, 15, 16, 17. The surface mechanisms are fundamentally distinguished from rate‐limited volumetric reactions in batteries by short charge/discharge time, high power density and long‐term cycling stability 1, 2, 3, 18.…”

Electrochemically Synthesis of Nickel Cobalt Sulfide for High‐Performance Flexible Asymmetric Supercapacitors

“…However, the commercially available supercapacitor electrodes are mainly based on activated carbon, which needs the addition of binders and conductivity enhancers and is challenging to achieve high flexibility and light weight. The ideal flexible electrodes should have energy density values close to those conventional supercapacitors, and its robust flexibility does not undermine other electrochemical characteristics, such as cycle life and rate performance 3. Although there are vast reports of carbon fiber paper,4 carbon cloth,5 metal mesh,6 CNT paper7 for flexible supercapacitors, these current collectors have high areal density range from ≈11.9 mg cm −2 (0.15 mm Torayca cloth) to ≈30 mg cm −2 (1.5 mm Ni foam).…”