Fabrication and Characterisation of Energy StorageFibres
Ruirong Zhang1, Yanmeng Xu1, David Harrison1, John Fyson1, Fulian Qiu1,Darren Southee2 and Anan Tanwilaisiri1
1Cleaner electronics, School of Engineering and Design, Brunel University, London, UK
2Loughborough Design School, Loughborough University, Leicestershire, UK
Abstract—Fibresupercapacitors weredesigned andmanufactured using a dip coating method.Their electrochemical properties were characterizedusing a VersaSTAT 3 workstation. Chinese inkwith afine dispersion of carbon and binder was coated as the electrode material. The specific capacitance per unit length of acopper fibresupercapacitorwith the length of 41 cm reached34.5 mF/cm. When this fibre supercapacitor was bent on rods with a diameter of 10.5cm, the specific capacitance per lengthwas 93% of the original value(without bending). It proved that these fibresupercapacitors have a good flexibility and energy storage capacity.
Keywords-supercapacitors; chinese ink; energy storage fibre; gel electrolyte; flexible
I. Introduction
Recently, supercapacitors (also named electrochemical capacitors) have attracted much attentionas energy storage devices. Because of the high power density, long cycle life and high reversibility [1], supercapacitorshave been considered as a promising high power energy sourcefor delivering peak power demands in electrical vehicles, hybrid electric vehicles,portable electronic devices and emergency power supplies.Electrochemical energy can be stored in two ways in supercapacitors [1-4]. In electrical double-layer capacitors (EDLCs), there are no faradic reactions in the charge storage process, the capacitance arises from the charge separation at the electrode/electrolyte interface. Pseudocapacitorsare another kind of supercapacitors, in which, there is a faradic reaction in the energy storage process.
In recent years, there has been great interest indevelopingflexible, lightweight, low-cost and environmentally friendly energy storage devices.Supercapacitors can deliver higher powerthanbatteries and conventional capacitors. However, many existing supercapacitors are stilltoo bulky and heavy for intended applications.It is a challenge to develop highly efficient miniaturizedflexible supercapacitorsfor future energy storage.Recently, some attempts have been made to manufacture flexible andweaveablesupercapacitors [5-10]. JoonhoBae etal.developeda kind of novel fibre supercapacitor using ZnOnano-wires as electrodes, which showed a high specific capacitance of 2.4 mF/cm2 and 0.2 mF/cm using PVA/H3PO4 as the gel-electrolyte[6].Yongping Fu et al. developed a novel flexible fibre supercapacitor which consisted of two fibre electrodes using Chinese ink as the active materials, a helical spacer wire and an electrolyte showed good capacitance [9]. Coaxialfibresupercapacitors,using Chinese ink as active materials,were designed,manufacturedand characterised.The results have been discussed in this study. The particular novelty or worth reported in this paper is the significant increase in specific capacitance (from 0.5 mF/cm to 34.5 mF/cm), achieved through increasing the number of coatings for the carbon electrodes from 4 to 24.
II.Experimental
A.Materials
Phosphoric acid (H3PO4, dry) and polyvinyl alcohol (PVA, MW 146,000-186,000, >99% hydrolyzed) were used without further purification. The gel electrolyte was made by dissolving0.8 g H3PO4and 1 g PVA in 10 mL deionized water. Copper fibre (50 µm in diameter) was usedas the core conductor material. Commercial Chinese ink was used as the active coating material.
B.Design of the structure of the energy storage fibre
Based on the working mechanism [1], fibresupercapacitors have been designed. As shown in Figure 1, the coaxial single fibresupercapacitors consist of five layers. The central metal fibre and the outer layer of silver coating are current collectors. Two active layers made of Chinese inkare separated by a gel electrolyte layer and served as electrodes. The energy is stored by the accumulation of electrical charges at the boundary layers between the two electrodes and electrolyte.
Figure 1. 3-D schematic of four coating layers on the metal fibre.
C.Manufacturing method
Figure 2 schematically shows the experimental setup of the coating method for the energy storage fibre. A reelofmetal fibre and two pulleys are fixed horizontally onto a plate.Aweight is clamped to the bottom end of the core wire to keep the wire straight and in alignment with the coating vessel. A motor with a two-direction controllerwasused to connect the reel to control the coating speed.When the coating process is carried out, coating liquid isfilled in the coating vessel. During the movement of the core metal wire,the wire drags the coating liquid with it,the solvent is vapourised and thecoating materials depositon the wire.
D.Characterisation of the electrochemical properties
The electrochemical performance of the energy storagefibredeveloped was studied by cyclic voltammetry (CV),galvanostatic charge/discharge and electrochemical impedance spectroscopy using a VersaSTAT 3 electrochemical workstation. The morphology of the cross-section of the supercapacitor wasstudied by optical microscopy.
Figure 2. The schematic of coating method.
A CV test is carried out by applying a positive (charging) voltage sweeping(scan rate) in a specific voltage range and then reversing (discharging) the voltage sweep polarity immediately after the maximum voltage range is achieved. The electrochemical behaviour of a supercapacitorcould be evaluated based on the corresponding current response against the applied voltage by the equations as:
(1)
(2)
C is the capacitance, and CL is the specific capacitance per unit length. Qtotal is the supercapacitor’s charge in coulombs. The charge is automatically calculated by the workstation software.L is the length of the fibresupercapacitors.
III.RESULTS AND DISCUSSION
Following the schematic of fibresupercapacitors (as shown in Figure 1), the first energy storage fibre was made usingthe commercial Chinese ink (coating 4 times) as the active material.
The CV curve of an initialfibresupercapacitor sample with the length of 35 cm is illustrated in Figure 3. The capacitance and specific capacitance per unit length calculated by Equations (1) and (2)were 18 mF and 0.5 mF/cm. This showsthat this kind of energy storage fibreis functional, although the capacitance was not ideal.This was considered to be caused by an uneven structure of the supercapacitor, which is revealed in Figure 4, so there was not enough active material deposited.
A typical cross-section of a copper fibre supercapacitor is shown in Figure 4. The five layers structure can be seen clearly. The copper fibre core is easy to see. The second Chinese ink layer was not uniform. Gel electrolyte layer as the third layer was complete but the thickness was not uniform. The fourth layer was also a Chinese ink layer and was also uneven.
Figure 3. The Cyclic voltammogram of an initial fibre supercapacitor sample.
Figure 4. Optical image of the cross-section of a fibre supercapacitor.
A new fibre supercapacitorwith the length of 41 cmwas madebased on the technical manufacturing skills gained from many times of laboratory trials. The ink layer was deposited 24 times in this fibresupercapacitor, which was 6 times more than the initial trial sample. The specific capacitance of the new samplewasabout 34.5 mF/cm, which is 69 times that of the trial sample. Generally, when the number of coatings is increased substantially, the thickness of ink layerincreasesdramatically asdoes the amountof the active materials in each of the electrodes. This shouldenlarge the electrical storage capacity of the fibre supercapacitor.
The flexibility of the fibresupercapacitorswas also studied. The specific capacitance of the 41 cm long supercapacitorwas examined when the fibrewasbent on a glass rodwith different curvatures. The diameters of the glass rods were10.5, 3.0 and 1.5 cm. Figure 5 shows the CV curves of the fibresupercapacitor bent at different curvatures.It can be seen the CV curves were the same when the fibre supercapacitor was bent using the glass rods with the diameters of 1.5 and 3.0 cm.When the fibresupercapacitor was bent on the glass rod with the diameter of 10.5 cm, the specific capacitance decreased slightlyto 32.2 mF/cm, which was about 93% of the originalstraight sample. When the diameter of rods decreased to 3.0 and 1.5 cm, the specific capacitance changed to 26.2 mF/cm for both cases,which was 76% of the straight sample. The comparison of the specific capacitance of the fibresupercapacitor at different bending conditions with the original sample is shown in Figure 6. These resultsshow that the fibresupercapacitors are able to work at the severe bending condition, indicating the fibresupercapacitors are flexible enough to be woven into other fabrics to make smart textiles.
Figure 5. CV curves of the 41 cm long fibre supercapacitorat different testing conditions (straight and bent with different curvatures).
Figure 6Comparison of the specific capacitance per length of the fibre supercapacitancebeing bent with different curvatures with the original straight sample.
IV. CONCLUSIONS
The coaxial single fibresupercapacitors were successfully manufactured and characterisedin this study. Chinese ink was used as electrode materials.A coating method was designed and setup to produce the fibresupercapacitors. The initial sample made proved the fibresupercapacitor designed to be functional. When the coating layers increased substantially, the amount of active materials deposited on the electrodes increased dramatically.Thisimproved the electrical storage of the fibresupercapacitor tens of times.The specific capacitance of the supercapacitors at various bending conditions was examined. The results showed that the supercapacitors manufactured using the present methodkept a good electrochemical performance at the severe bending conditions, which indicated that the supercapacitors designed and made in this study have a very good flexibility. Thiscouldbe used as a flexible energy storage and woven into other fabrics to make smart textile materials.
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