Asymmetric Micro-supercapacitors Research Articles

(Invited) Carbonaceous Hybrid Fibers for Solid-State Micro-Supercapacitors with High Volumetric Energy Density

Using the capillary chromatography column as a one-dimensional hydrothermal micro-reactor, we developed a method to produce carbon microfibers with hierarchy structures, in which reduced graphene oxide (rGO) and acid-oxidized single-walled carbon nanotubes (SWCNTs) self-assembled into an interconnected SWCNT network with rGO sheets interposed along the microfiber axis to create a mesoporous microstructure of a large specific surface area (396 m2/g) and a high electrical conductivity (102 S/cm). The resultant carbon hybrid fiber showed ultrahigh specific volumetric capacity of 305 F/cm3 (at 73.5 mA/cm3 by three-electrode cell) in H2SO4 or 300 F/cm3 (at 26.7 mA/cm3 by two-electrode cell) in a polymer gel electrolyte (PVA/H3PO4). The corresponding fiber-shaped all-solid-state flexible micro-supercapacitors with the PVA/H3PO4 electrolyte, but without binder, current collector, or separator, exhibited a long cycle life and an ultrahigh volumetric energy density of ~6.3 mWh/cm3. This value of volumetric energy density is well comparable to a 4V/500-µAh thin-film lithium battery, while the power density of our micro-supercapacitors is more than two orders of magnitude higher than the battery. Furthermore, the all-carbon hybrid fibers can be controllably functionalized to yield versatile fibers with either capacitive or Faradic characteristics for designing high-performance asymmetric fiber micro-supercapacitors. The capacitive-type N-doped rGO/SWCNT fibers obtained through in situ N-doping with urea in the flexible capillary column show tunable specific capacitances from 1.75 to 2.37 mF cm-1, while the Faradic-type fibers created by redox deposition of MnO2 on the rGO/SWCNT fibers have specific capacitances ranging from 1.76 to 3.30 mF cm-1. Both types of functionalized fibers outperform the pristine rGO/SWCNT fibers (1.5 mF cm-1), while retaining fiber geometry and mechanical flexibility. By matching the specific capacitances of one MnO2-coated rGO/SWCNT fiber as battery-type (positive) electrode and the other N-doped rGO/SWCNT fiber as capacitor-type (negative) electrode to achieve the charge balance between two fiber electrodes, we designed and constructed the fiber-based solid-state asymmetric micro-supercapacitors with polymer gelled neutral electrolyte on a flexible polymer substrate. The optimized device exhibits a stable and high voltage window of 1.8 V and excellent cycling stability (87% capacitance retention after 10,000 cycles), as well as good flexibility. These fiber-shaped flexible micro-supercapacitors can be easily integrated into miniaturized power sources for various portable flexible optoelectronics, multifunctional textiles, sensors, and energy devices, as exemplified by powering a TiO2-based UV photodetector and a commercial light-emitting-diode.

Graphene fiber-based asymmetric micro-supercapacitors

Fiber-based asymmetric micro-supercapacitor (F-asym-mSC) is assembled by core–sheath graphene fiber decorated by MnO2as the positive electrode and graphene-carbon nanotubes hybrid fiber as the negative electrode. The F-asym-mSC shows the highest energy density (11.9 μWh cm−2) for fiber-based supercapacitors and paves the way to high energy density, wearable, and flexible electronic devices.

Novel and high-performance asymmetric micro-supercapacitors based on graphene quantum dots and polyaniline nanofibers

In comparison with graphene sheets, graphene quantum dots (GQDs) exhibit novel chemical/physical properties including nanometer-size, abundant edge defects, good electrical conductivity, high mobility, chemical inertia, stable photoluminescence and better surface grafting, making them promising for fabricating various novel devices. In the present work, an asymmetric micro-supercapacitor, using GQDs as negative active material and polyaniline (PANI) nanofibers as positive active material, is built for the first time by a simple and controllable two-step electro-deposition on interdigital finger gold electrodes. Electrochemical measurements reveal that the as-made GQDs//PANI asymmetric micro-supercapacitor has a more excellent rate capability (up to 1000 V s(-1)) than previously reported electrode materials, as well as faster power response capability (with a very short relaxation time constant of 115.9 μs) and better cycling stability after 1500 cycles in aqueous electrolyte. On this basis, an all-solid-state GQDs//PANI asymmetric micro-supercapacitor is fabricated using H3PO4-polyvinyl alcohol gel as electrolyte, which also exhibits desirable electrochemical capacitive performances. These encouraging results presented here may open up new insight into GQDs with highly promising applications in high-performance energy-storage devices, and further expand the potential applications of GQDs beyond the energy-oriented application of GQDs discussed above.