Abstract
In this research work, Nickel was in situ synthesized on polyester fabric using facile method. For this purpose, electroless plating method was used. In the first step, samples were sensitized using copper acetate. Hydrazine were used as reducing agent. In the same bath, Nickle acetate was added and in presence of hydrazine, the Nickle nano particles were deposited on the surface of polyester fibers. The morphological properties of nano particles on the surface were investigated using Scanning Electron Microscope (SEM). The electrical resistance of samples after metal plating were measured by two-point probe. The rubbing and wash fastness of samples through electrical resistance were also studied. The results show that, Ni plated sample has very high resistance to rubbing. Even after 8000 cycles of rubbing using Martindale system, the electrical resistance of rubbed samples reaches to 40 Ω/sq. But it should be mentioned that, the wash fastness of prepared samples is not satisfactory. The main purpose of this research was, investigating on the storage properties of Ni plated polyester fabric. The electrochemical properties of samples were investigated using cyclic voltammetry (CV) and galvanostatic charging–discharging (GCD) measurements (Autolab PGSTAT 302 N). The Ni plated polyester in this work, showed, a specific capacitance of 450 mF/cm2 by GCD test at the current density of 7.5 mA/cm2. Also the integrated area for the CV curves related to rubbed samples, is large enough. The resultant Ni plated fabric provides a promising substrate to prepare textile-based electrodes for flexile supercapacitors.
Introduction
Electronic fabrics, also identified as e-textiles or smart fabrics, can attain both the purpose of normal clothes, such as protection from the outside environment, and electronic functions such as sensing, implantable medical devices, data processing, and energy storage [1,2]. Conventional energy storage devices such as the electrochemical secondary battery are not well-matched with fabrics and inadequate in numerous areas like poor cycling stability and low power density. Also the heat produced by commercially available batteries can damage the human skin [3]. Certainly, the detection of new material or processing that guarantees the ability of high energy-storing properties combined with flexibility can open up a new era of user wearable electronics [4–6].
Among different energy storage devices, supercapacitors (SC), also called electrochemical capacitors, have concerned extreme consideration owing to their favorable features, such as high power density, long life cycle, wide working temperature range, easy maintenance and high safety [7–12]. The active materials used to prepare the electrodes of supercapacitors have a vast impact on their electrochemical properties [13]. Carbon materials, conductive polymers and transition metal oxides, are active materials for supercapacitors which can be synthesized in the form of various nanostructures to obtain high specific capacitance. Transition metal oxides (pseudo supercapacitors) have much higher theoretical specific capacitance than that of the other active materials [14–17]. Nickel oxide and nickel hydroxide are competent as supercapacitor electrode materials, including high theoretical capacitance, low cost and low environmental impact [18–21]. NiO is promising candidate for pseudocapacitors due to its easy availability, high theoretical capacitance, cost effectiveness, good thermal and chemical stability and pseudocapacitive behavior. The common necessities for NiO based electrode in supercapacitor applications are the followings: I) the oxide must be electrically conductive. II) The metal should occur in two or more oxidation states, which can coexist over a potential range. III) It should possess high specific surface area. IV) On reduction, electrolyte ions can freely intercalate into the oxide lattice and de-intercalate the lattice on oxidation. The performance of nickel oxide based pseudo-capacitor electrode materials is determined by the redox reaction of NiO or Ni(OH)2 in alkaline electrolytes [22].
The main components to be considered for the fabrication of flexible electrode is the substrate which to be coated with electro-active material. Substrate should have the superior properties like high electrical conductivity, good mechanical strength, porous structure, flexible as fabric and non-corrosive in nature [23,24]. In general, textile materials like cotton, wool, nylon, polyester, polyacrylonitrile and other exhibit a high electrical resistivity of 1010–1015 Ω/sq, much higher than the desired resistivity of materials for textronic applications, which usually should be lower than 104 Ω/sq for textronic sensors and lower than 102 Ω/sq for electrodes and wires [2]. Finding novel electroactive materials with higher electrical conductivity is greatly anticipated to indemnify the shortfalls of transition metal hydroxides/oxide for improved electrochemical stability. In fact, the relatively high electrical conductivity and remarkable electrochemical activity of electroactive materials help to improve the electron transportation within the material to the current collector [25]. Substrate should be able to hold the electro-active material without any binder and also roles as a current collector. Different methods have already been used to coat the electro-active material on different kind of substrates; such as spin coating and dip coating. However, a misfortune with these techniques is the poor adhesion between the substrate and electro-active material. This causes etching of the material during the electrode handling and electrochemical tests [23,26]. To avoid these problems, flexible electrodes were prepared in this research work using a simple chemical route and instead of conventional substrates like paper, polymer and metal sheets; innovative and cheap material like polyester fabric was used. Nickel was in situ synthesized on polyester fabrics using simple electro less plating method. In this research for sensitizing and improving the electrical conductivity of the polyester fabric, copper NPs were first synthesized on the polyester and then Ni NPs were in situ synthesized in the same bath on polyester fabrics. The electrochemical performance of prepared flexible electrode was investigated.
Experimental
In this research work, Nickel was coated on polyester fabric by using a facile chemical electro less plating method. 100% polyester woven fabric (80 gm−2 and warp/weft density of 31/21 yarns per cm) has been used in this research work. Copper acetate, Nickel acetate and hydrazine were supplied from Fluka Company, Switzerland. Potassium hydroxide (KOH) and polyvinyl alcohol (PVA) were obtained from Merck, Darmstadt, Germany. All other regents were of a commercial grade of purity.
Prior to the electro less plating, the polyester fabric was initially cleaned in washing bath with a solution of 1 g/l nonionic detergent for 30 min at 50°C and then rinsed with distilled water to remove impurities followed by drying at room temperature. Copper NPs were first synthesized on the polyester surface to activate and sensitize the fabric and then Ni NPs were in situ synthesized in the same bath on polyester fabrics. For this purpose, the cleaned fabrics were soaked in copper acetate (0.01–0.3% w/v) and heated to 70°C. By introducing hydrazine (2.5% v/v) to the solution, instantaneously the color of the solution changed from blue to brown. After 10 min, by adding nickel acetate (0.5–1.5% w/v) to the solution, the color turns to black. The electro less plating process was continued for 30 min at 80°C. Finally, Ni plated polyester was washed with distilled water and dried at room temperature.
The electrical resistance of the prepared samples was tested via two-point probe. The electrical resistance of samples in 10 different locations was measured and the average of electrical resistivity was reported. The treated sample with the lowest electrical resistance was selected and the further analyses were done on this sample.
The surface morphology of untreated and Ni plated fabrics was studied using a scanning electron microscope (SEM, Tescan Vega 3, made in Czech Republic). Before SEM analysis, the fabrics were gold coated using sputter coater. Also the atomic content of elements on the surface of Ni-plated polyester was determined using an energy-dispersive X-ray spectroscopy (EDX) unit connected to the SEM. Also the crystalline structure of plated sample was studied using X-Ray Diffraction method. To confirm the configuration of copper and nickel NPs on the treated fabrics, X-ray diffractometer (type Inel, Equinox 3000) with Cu-kα irradiation source (λ = 1.54190 A) was applied. After preliminary experiments, it was found that, 0.1% (w/v) of copper acetate and 1% (w/v) of Nickel acetate is enough for making polyester to be highly electrical conductive. So this sample was chosen for working electrode, cause of higher electrical conductivity.
The adhesion properties of Ni plated polyester were compared using abrasion (Martindale 2000, Shirley LTD) and rubbing tests. We acted upon the European standard ISO 105-X12: Textiles – tests for color fastness-color fastness to rubbing. Samples were rubbed by dry wiping fabric for 2000, 4000 and 8000 cycles. The surface morphology of the rubbed surfaces was studied using SEM. Furthermore, the Energy Dispersive X-ray (EDX) unit connected to the SEM microscope was used to determine the percentage of atomic contents of elements present in the prepared plated fabrics and the amounts of remained Cu and Ni on the surface of rubbed samples was compared. Also plated polyester sample were washed for 5, 10 and 20 times with 1 g/l nonionic detergent according to ISO 6330:2000. The effect of washing on electrical resistivity of samples was studied.
The electrochemical properties of the as-obtained Ni plated polyester were investigated under a three electrode cell configuration at room temperature. The specific capacitance of plated polyester was determined by cyclic voltammetry (CV) and galvanostatic charging–discharging measurements (Autolab PGSTAT 302 N). The Ni plated textile acted directly as the working electrode, which was soaked in a 1 M KOH solution. The reference and counter electrode were Ag/AgCl and platinum, respectively. As it was mentioned before, after preliminary experiments, it was found that, 0.1% (w/v) of copper acetate and 1% (w/v) of Nickel acetate is enough for making polyester to be highly electrical conductive. So this sample was chosen for working electrode, cause of higher electrical conductivity. A fixed voltage window of 0.0–0.6 V was used to test the electrochemical performances. Areal capacitances (Ca) were calculated using equation (1)
Flexible all-solid SCs was assembled by two same pieces of Ni-plated polyester electrodes. The preparation of polyvinyl alcohol (PVA)/KOH gel electrolyte and the assembly of solid–state symmetric SC device were prepared as follows. Typically, 1 g PVA and 1 g KOH were added to 10 mL DI water, followed by heating at 85°C under vigorous stirring until the solution became clear. The obtained viscous solution was dropped onto two pieces of Ni-plated polyester samples with size of (2 × 1 cm2). Then these two electrodes were assembled together. After the electrolyte solidified to a gel, the two electrodes were further pressed to form a flexible solid-state SC. The electrochemical properties of the SC device were measured using a two-electrode system.
Results and discussion
In this research work, after preliminary experiments, it was concluded that, 0.1% (w/v) of copper acetate and 1% (w/v) of nickel acetate with 2.5% Hydrazine is enough for obtaining higher electrical conductivity on polyester fabric. The electrical resistivity of the plated fabrics was checked at room temperature by two probes digital multi meter. The average electrical resistivity for the mentioned sample is 0.7 Ω/sq and this amount is less than the other prepared samples with different concentration of copper acetate and nickel acetate. As it is seen in Figure 1, the prepared Ni plated fabric can be used as a part of electrical circuit for turning on the LED.
Photo of turning on the LED using the Ni plated fabric.
In Figure 2, the X-ray diffraction pattern of the polyester fabric shows maxima at 2θ values of 17.5, 23 and 25 [27] and strong diffraction peaks at 43.40, 44.47, 52.12, 76.44, 93.13 and 98.48° represent (111), (200), (220), (311), and (222) Bragg’s reflections of crystal structure of Ni nanoparticles. Also peaks at 43.40, 44.47, 50.43 and 74.06° are assigned as (111), (200) and (220) crystal planes of the face-centered cubic (fcc) phase of Cu nanoparticles on the fabric. Moreover, the list of the metal treated fabric diffraction peaks well-adjusted to the reference nickel card No03-065-2865 and copper card No 01-070-3039 revealing the produced and resultant particles are pure cubic nickel and copper. [28] The results show two separate phases of copper and nickel metals and divergent of the chance of alloy formation. Here, hydrazine guides to aminolysis of polyester and Cu and Ni acetate can be hydrolyzed in aqueous solution and
The XRD pattern regarding Ni plated polyester.
From the widths of the peaks at 2θ of 44.55° and using Scherrer's equation, the crystallite size of the NPs on polyester was calculated to be 23.83 nm. The synthesized NPs purity was verified with X-ray diffraction peaks since no extra peaks coming up from the impurities.
The SEM images of the untreated, rubbed and Ni plated fabrics are shown in Figures 3 and 4. As shown in Figure 3, the surface of the untreated fabric is smooth. Some nano particles can be seen which they can be attributed to impurities. Also after 8000 cycles of rubbing, some cracks can be found. According to Figure 4, some particles are hedgehog-like and most of them are rod-like. Synthesized NPs covered on the polyester homogeneously as an effectual and valuable reason for gaining the electrical conductivity of the treated fabric. The SEM images of rubbed Ni plated polyester with different magnification are shown in Figure 5. It is seen that, after rubbing, some parts of nano particles were removed from the surface of fibers. By 8000 cycles of rubbing, less amount s of NPs remains on the surface. The EDX results confirm it.
The SEM images of untreated polyester before and after 8000 cycles of rubbing.
The SEM images of Ni-plated polyester with different magnifications.
The SEM images of Ni-plated polyester after 2000, 4000 and 8000 cycles of rubbing.
By EDX analysis, the amounts of copper and nickel on the plated fabrics before and after rubbing, were compared and results are shown in Figure 6. Weight percentage of copper and nickel in plated fabric before rubbing is 18 and 61.4% respectively. As it is seen in Figure 6, these amounts for 2000 cycles rubbed sample reach to 5.4 and 43.3%. After 4000 cycles of rubbing, low amounts of Copper and Nickle remain on the surface. The achieved data are in good agreement with SEM results. After 8000 times of rubbing, no copper remains on the surface of polyester fabric.
The EDX results for (a) Ni plated polyester before rubbing (b) 2000 cycles rubbed sample, (c) 4000 cycles rubbed sample and (d) 8000 cycles rubbed sample.
The results related to electrical resistivity of samples after rubbing and washing are shown in Table 1. As it is seen, after 2000 cycles of rubbing the electrical resistivity of plated polyester sample reach to 3 (Ω/sq) and after 4000 cycles of rubbing this amount reach 7 (Ω/sq). However, as it was expected after 8000 rubbing cycles, the electrical resistivity increases to 40 (Ω/sq). It shows that the rubbing fastness of the prepared electrode is very high and the prepared electrode is highly electrical conductive. But the results show low washing fastness for the obtained plated polyester. After 5, 10 and 15 times of washing the average electrical resistivity are 550, 1000 and 2000 (Ω/sq), respectively.
Rubbing and washing fastness properties of plated polyester samples toward electrical resistivity.
The electrochemical performance of flexible plated polyester was firstly investigated in a three-electrode cell with the electrolyte of 1 M KOH. Figure 7 shows the CV curves of our flexible electrode within 0–0.6 V at different scan rates (5–60 mV s−1). All of the curves present essentially the same shape with a pair of well-clear redox peaks, and the peak currents are slowly increased with increasing the scan rate. The redox peaks can be attributed to the reversible Faradaic reaction between Ni(II) and Ni(III), where Ni(OH)2 is created throughout the electrochemical activation process representing their pseudocapacitive features [29–31].
The CV curve for Ni-plated polyester in 3 electrode system with different scan rates (5–60 mV/S).
The reduction and oxidation peaks are almost symmetric in each curve, representing the brilliant reversibility and superior conductivity of Ni-plated polyester electrode. The capacitances were also calculated from GCD. To evaluate the rate capability of plated polyester, GCD curves under various current densities were carried out and are presented in Figure 8. Galvanostatic charge/discharge (GCD) of Ni-plated polyester electrodes were tested by applying current densities of 7.5, 10, 15 and 30 mA/cm2. As expected, the plated polyester demonstrated a much longer charging and discharging profile in current density of 7.5 mA/cm2. By increasing the current density to higher amount the charge and discharge profile will be shorter. Also it shows that GCD profiles represent battery-type performance, namely two obvious potential plateaus, pointing out that Faradaic redox reactions occur during the charge and discharge processes, which is in good agreement with CV results. It can be seen that each discharge curve has an apparent potential plateau corresponding to the reduction peak in CV (Ni3+→Ni2+). Such good performance should be attributed to the intimate encapsulation of tiny activated Ni nanoparticles in the polyester fabric, which ensures both fast electron transport and rapid access to electrolyte (OH-) [32–34]. When the current density increase from 7.5 to 30 mA/cm2, areal specific capacitance of Ni plated polyester decrease from 450 mF/cm2 to 350 mF/cm2 (Figure 9).
The GCD curves for Ni-plated polyester in different current densities.
The areal capacitance of Ni-plated polyester in different current densities.
As it was mentioned in experimental part, the electrochemical performance of rubbed samples for 4000 and 8000 cycles, were also investigated at various scan rates from 1 to 10 mV/S. The CV curves related to 4000 and 8000 cycles rubbed samples are shown in Figures 10 and 11.
CV curves for 4000 cycles rubbed sample in different scan rates.
CV curves for 8000 cycles rubbed samples in different scan rates.
The integrated areas of the CV curves of Ni-Plated Polyester are larger than that of the rubbed samples suggesting the capacitance of Ni plated polyester is higher than that of rubbed samples. But this deference is not significant. As shown in Figures 10 and 11, the peak currents of Faradaic redox reaction increase with the scan rate and the CV curves retain the original shape, implying that the electron transfer kinetics on prepared electrode are very fast even after rubbing. However, at higher scan rate, the oxidation peaks shift to the positive potential while the reduction peaks move to the negative range, suggesting a larger diffusion resistance at higher scan rates. The CV curves for Flexible all-solid SCs which has been assembled by two same pieces of Ni-plated polyester electrodes and gel electrolyte is shown in Figure 12. The integrated areas of the CV curves are enough high. It shows that prepared symmetric SC has enough capacitance. But it should be mentioned that, the wash fastness of prepared electrodes is not enough satisfactory. After 5 times of washing the electrical conductivity of samples decrease noticeably (Table 1). Also the CV curves related to washed samples is shown in Figure 13. As it is seen, no oxidation and reduction peak can be found in the curves and the integrated area in these peaks are very short as compared to rubbed samples.
The CV curves for prepared symmetric SC.
CV curves for (a) 5th cycles washed samples (b) 10th cycles washed samples and (c) 15th cycles washed samples in different scan rates.
Conclusion
Nickel clothes were good current collectors for supercapacitors which have been reported in literature [14]. Besides current collectors, nickel cloths with large specific surface area also exhibited relative large capacitance. The active material on nickel cloth is pure nickel and partially nickel oxide, because the surface of nickel layer can be in situ oxidized and converted to nickel hydroxide and nickel oxide during the electrochemical charge-discharge process [34]. In the other research works, the used substrate for preparation of electrode is mostly carbon based products, such as graphene, carbon fibers, carbon nanotubes and carbon black. Preparation of these electrode type materials is not easy and is not cost effective. In the present project, we try to produce, fabric type electrode witch does not involve any carbon base material. The process for preparation of this electrode is very easy and cost effective. It is possible to make it semi industrial easily. and the nickel plated polyester in this work showed a specific capacitance of 450 mF/cm2 by GCD test at the current density of 7.5 mA/cm2. Also the results related to electrochemical measurement showed that, the rubbing fastness of the prepared electrode is highly satisfactory. Even after 8000 cycles of rubbing, the integrated area of CV curves is high enough large. By increasing the scan rates, the area of CV curves will be larger. It shows that, the specific capacitance of the prepared electrode after 8000 cycles of rubbing is high. Also the integrated area for the CV curves related to prepare symmetric SC, is large enough. But is should be mentioned that, the wash fastness of prepared electrodes is not satisfactory. After 5 times of washing, the electrochemical properties of washed samples decrease and the area of CV curves in various scan rates, reduces, noticeably. However, it should be mentioned that, the application of this electrodes are in electronic devices which do not need many times of washing.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Assoc. Prof. Dr. Sheila Shahidi has received grant from TUBITAK 2221 Fellowship program for visiting Scientists and Scientists on Sabbatical leave.
