Morphological features of 3DGN (a) SEM image of spherical polymer. b) SEM image of 3DGN using polymer template as carbon sources under Ni precursor and TEM image (c). Electrochemical performance of 3DGN supercapacitors. a) Cyclic voltammetric curves of the 3DNGs electrode at scan rates from 5 to 100 mV/s. b) Galvanostatic charge and discharge curves of 3DGNS at current densities from 2 A/g to 20 A/g. Electrochemical performance of i-3DGN supercapacitors. a) Cyclic voltammetric curves of the i-3DNGs electrode at scan rates from 5 to 100 mV/s. b) Galvanostatic charge and discharge curves of 3DGNS at current densities from 2 A/g to 100 A/g.
Therefore, the carbon allotrope is as active materials of the EDLC which have high electrochemical stability and large surface area.
Review of electrochemical energy storage devices
- Motivation
- Supercapacitors
- The mechanisms of energy storage
- The performance of the supercapacitors
- Electrode materials for supercapacitors
- Carbon material
- Metal oxides
- References
Finally, charge separation occurred at the electrode, and the charges were stored on the surface of each electrode. In the case of an EDLC, charges are accumulated by surface physical static forces, but a pseudocapacitor stores charges through a chemically reversible redox reaction. However, when porous materials are used as active materials, more than a hundred times more charges are stored on the surface of the electrodes, which significantly improves the capacitance.
First, in the Helmholtz model, the electrode attracts the counter ions in the electrolyte forming a monolayer on the electrode surfaces. The thickness of the thin monolayer is the approximate diameter of an ion in the electrolyte. The inner thickness zone gives rise to a rough layer consisting of the inner Helmholtz plane (IHP) and the outer Helmholtz plane (OHP).
EDLC has a high power density and long cycle stability, but it has limitations to improve the capacitance because they only store the charges on the surface of the material through electrostatic forces. The protons in electrolyte and charges in the electrodes are combined on the surface of the electrode and then the active materials are reduced10. From the mechanism, the capacitors collect the many charges on the surface of the electrode and Figure 1.6 shows the clear difference between EDLC and pseudo-capacitor.
The average pore thickness of the wall is close to the shielding length of the electric field. Due to the good properties of the SWNT, the SWNT-based electrode exhibits a high energy density of 69.4 Wh/kg. Graphene belongs to the carbon allotrope and is a structure in which carbon atoms are collected to form a two-dimensional (2D) plane.
Among the metal oxides, RuO2 is the most potential studied material of the electrode due to its wide potential window, good theoretical capacitance, good proton conductivity and long cycle life21.
Printable asymmetric supercapacitors using interconnected 3D graphene
Introduction
Carbon materials have excellent stability, but relatively low capacity. Transition metal oxides that are larger than the EDLC values benefit from the disadvantages due to poor electrical conductivity, surface area, dissolution in electrolytes, narrow potential window and low cycle stability. Recently, various types of carbon-based materials have been mainly used as electrode materials due to their high conductivity, chemical stability and huge surface area for energy storage devices. As the resistance to mass transfer increases and the active sites become clogged, this leads to poor electrocatalytic performance.
These pores allow to accelerate mass transfer, which has a huge surface area and provides many active sites and channels. 3DGNs are synthesized by a CVD process using a spherical polymer template penetrated PVA/metal precursor solution. Due to the etching procedures, 3DGNs has a high surface area (961 m2/g) due to the removal of metal particles formed by a number of micro/mesopores.
And a high-quality 3D graphene has been synthesized due to the right amount of carbon sources and metal precursor in CVD. Due to both the large surface area and high-quality graphene properties, synthesized 3DGNs are expected to be applicable in a wide range of fields, such as 3D electrodes and ESS. In this chapter, with this printing technology, we can easily adjust the shape and size of the electrode on the paper substrate and simply attach it to things (wall like wallpaper, sensors, etc.) to realize IoT.
Herein, the novelty in this study is that it is possible to fabricate high-efficiency supercapacitors using carbon materials without using expensive metals such as Au, Ag, etc. After PASs are fabricated by assembling the electrode inks, they show a wide operating potential window from 1.8 to 2.0 V and extraordinary electrochemical performances, which are the surface area, volumetric capacitance, and cycle stability represented in these devices .
Experimental section
- Materials
- Preparation of various polymer template
- Synthesis of 3DGNs
- Preparation of interconnected 3DGNs inks / adding MnO 2 inks for each electrode. 30
- Materials and electrochemical characterization
Also, because PEI is a polymer with a repeating unit included from the amine group, there is an advantage that nitrogen can be doped in 3DGNs for improved conductivity upon carbonization. KMnO4 is dissolved in DI water, and then added ethanol led to the formation of brownish precipitate MnO2. Reduced mixtures are stirred for 24 hours and then the precipitates are filtered, washed with DI water / EtOH to eliminate the by-product and non-reaction product and dried in the oven33.
The physical properties of different inks are optimized to meet the requirements of the printing inks, and the inks are applied to clean the ink cartridge. To solve clogging of the cartridge head, each electrode for ink in IPA is subjected to centrifugation (at 3000rpm for 1 hour) to remove large-sized particles and agglomerates before filling into the cartridge. The ink is injected into each colored pattern, black – negative materials / yellow – positive materials / magenta – SWCNT acting as current collector.
To fabricate the electrodes for PAS, the SWCNT solution-filled crimson ink is first printed to deposit multiple layers of SWCNTs on the substrate to act as a current collector, and then the electrodes are completely dried in the wet state. Black or yellow ink is printed onto the SWCNT film to produce multiple layers of electrodes, each ink individually printed to act as a negative or positive electrode. To make gel polymer electrolyte, molecular weight PVA is clearly dissolved in DI water at 85oC for 2 hours with vigorous stirring, and then LiCl is mixed into the PVA solution.
The morphologies of the as-prepared samples were characterized using SEM (Nano230 FE-SEM) and TEM (normal TEM, JEM-2100). The supercapacitor properties of a positive electrode and a negative electrode were evaluated using platinum mesh, Ag/AgCl electrode and different electrolytes (1M H2SO4, 6M LiCl) as counter electrode, reference electrode and electrolyte, respectively.
Results and discussion
Compared with the data from the original graphene with a strong and shape peak, 3DGNs have a broad peak representing the (002) plane of graphene. Due to the formation of pores from many volatile gases during the CVD process and the removal of the metal particles, 3DGNs have the increased surface area41. Overall, the CV curve and capacitance of supercapacitors using carbon-based electrodes can degrade significantly as the voltage scan rate increases, representing distortion of the rectangular shape.
However, the electrochemical adsorption and desorption of electrode materials on parts of the electrode materials is not well performed due to the relatively slow diffusion rate of ions from the electrode/electrolyte interface to the inner electrode materials. Therefore, most ions accumulate at the electrode/electrolyte interface and become polarized on the electrode materials. The rate performance retains 84.5% at 20 A/g (compared to 61% for 3DGNs) and can even retain about 75% of the initial capacitance at 100 A/g, which definitely results in better rate performance of the as-prepared electrode 3DGNs.
The Nyquist plot of the 3DGNs / i-3DGNs in Figure 2.6. d) displays a small semicircle at the high frequency range and a respectively flat line in the medium range. The production of a selected and printable ink for commercial printer must take into account certain rheological properties such as surface tension, viscosity, density of the printable solution and the size of the printer head. The physical properties of different inks are optimized to meet the requirements of the printable ink, and then the ink is injected into a clean ink cartridge.
On A4 paper, any ink is absorbed quickly, of which commercial inks are absorbed faster and negative/positive inks are absorbed more slowly due to the size of the active materials and numerous other impurities. The cross-sectional SEM image of the printed electrode is shown in Figure 2.9. the electrode components are pre-deposited on the surface of the A4 paper in Figure 2.9. a). The CV curves of the negative and positive electrode with 35 print numbers are shown in Figure 2.10.. b) with a variety of scan rates from 100 mV/s to 3 V/s, which increased the total potential window up to 1.8 V, Figure c ) shows the particularly gradual extension of the potential window from 0.8 V to 2.0 V at 100 mV/s. The wide potential window indicates a large energy density which is a critical advantage compared to normal symmetric supercapacitors and a key factor to satisfy the requirements of various applications.
To highlight the specific functions of A4 paper including printable asymmetric supercapacitors as integrated power sources, we produce inter-digital structural devices that are versatile aesthetic printable, designable and attachable devices.
Conclusion
34; Wearable energy-dense and power-dense supercapacitor yarns enabled by scalable graphene-metal textile composite electrodes." Nature communications. C.; Kang, L., A facile approach to the preparation of loosely packed Ni(OH)(2) ) nanofloc materials for electrochemical capacitors. 34; Design and synthesis of MnO2/Mn/MnO2 sandwich-structured nanotube arrays with high supercapacitive performance for electrochemical energy storage." Nano letters.
34; Freestanding Three-Dimensional Graphene/MnO2 Composite Networks as Ultralight and Flexible Supercapacitor Electrodes." ACS nano. 34; Factors Affecting the Electrochemical Performance of MnO2/Multiwall Carbon Nanotube Composite as Supercapacitor Electrodes." Electrochimica acta. 34; Electrodes printed by inkjet printers on A4 paper substrates for low-cost, disposable and flexible asymmetric supercapacitors." Materials used at ACS &.
Zhou, J.; Lian, J.; et al, "Ultrahigh volumetric capacitance and cycling stability of fluorine-nitrogen co-doped carbon microspheres." Communications of nature. Graphite Thin Films Consisting of Multilayered Graphene Nanograins on Sapphire Substrates Directly Grown by Alcoholic Chemical Vapor Deposition”. Wei, W.; Liang, H, "Nitrogen-doped carbon nanoplates with size-defined mesopores as a highly efficient metal-free catalyst for oxygen reduction reaction." Angewandte Chemie.
