Toward Flexible and Wearable Zn-Air Batteries from Cotton Textile Waste.

Considering the environmental problems caused by a large amount of cotton textile waste and its possible applications in flexible electrodes, it is very promising to reuse the cotton textile waste as an electrode material, reducing the cost of flexible electrodes and alleviating environmental problems. In this work, we present a rechargeable flexible Zn-air battery based on cotton textile waste, which employs Ni-metallized cotton textile waste (NMCTW) as a flexible substrate for Zn anodes and air cathodes. The transparent NiFe hydroxide thin film horizontally grown on the surface of the NMCTW substrate was synthesized in situ by the electrodeposition method, which exhibits excellent catalytic activity because of the high surface area of the two-dimensional (2D) thin film, large contact area between the thin film and substrate, and fast charge transport of the 2D thin-film structure. In view of the high catalytic performance of the NiFe hydroxide thin film, it was used as the catalytic material of the air cathode for the flexible Zn-air battery. The assembled Zn-air battery based on cotton textile waste demonstrated a good rate performance and outstanding charge and discharge cycling stability. The assembled Zn-air battery was applied to power the light-emitting diode, which exhibits exceptional flexibility and stable output power even under severe mechanical bending deformation, proving the feasibility for its application in flexible electronics.

Introduction

Flexible electronics have attracted tremendous research interest in recent years owing to their distinguished advantages such as portability, lightweight, flexibility, and potentially wearability.[1−9] They offer various novel applications for wearable devices, such as flexible bracelets, smartphones, and humanlike display skins.[10−13] To achieve the flexibility of an integrated electronic system, well-matched flexible power supplies are recognized as one of the key components.[1,14] Meanwhile, taking into account the increasing need for continuous power supplies for electronic devices, it is also imperative to develop high energy density storage/conversion devices. Therefore, it is highly urgent to develop a flexible power source with high energy density.

To date, a large number of energy storage/conversion systems have been studied, such as solar cells,[15−19] fuel cells,[20−24] supercapacitors,[25−31] lithium-ion batteries,[32−36] and metal–air batteries.[37−42] Among them, metal–air batteries possess high theoretical energy densities, being regarded as the promising candidates as power suppliers for next-generation flexible electronics.[2,6,43,44] Especially, Zn–air batteries are gaining ever-growing attention owing to their low price, good safety, environmental benignity, and resource richenss.[2,6,7,45] Theoretically, a Zn–air battery has a high energy density (1084 W h kg–1) that is about 3–5 times higher than the traditional lithium-ion battery.[8,44,46−48] Nevertheless, typical Zn–air batteries are usually not suitable for wearable and flexible electronics because of their rigid configuration. Zn–air batteries usually consist of an electrolyte, a separator, and Zn anodes and air cathodes. As the key components, flexible electrodes are particularly important for achieving high-performance flexible Zn–air batteries.

Until now, considerable efforts have been devoted to the high-performance flexible electrode materials, such as metal materials,[49,50] polymers,[51,52] metal oxides/nitrides/sulfides,[13,53,54] and carbon materials.[13,32−34,55] Especially, carbon materials such as carbon nanotubes, graphene papers, carbon cloth, carbon fibers, and carbon films have been receiving extensive attention because of their good mechanical robustness, higher power density, long-lasting cycle, and good electrical conductivity.[10,13,55−58] For instance, Fu et al.[59] fabricated the CoO0.87S0.13/nitrogen-doped graphene nanomesh hybrid as the air cathode catalyst in Zn–air batteries, and the electrode exhibited comparable catalytic activity and excellent stability than the commercial Ir/C and Pt/C. Xu et al.[11] synthesized the porous carbon nanotube sheet electrode by chemical vapor deposition, which demonstrated outstanding charge/discharge properties at 2 A g–1. However, expensive carbon nanotubes or graphene used to construct flexible electrodes will increase the cost of the device, hampering the commercial application. On the other hand, a large amount of cotton textile waste has been generated because of the increase in cotton textile consumption in recent decades.[60,61] Generally, cotton textile waste is landfilled or incinerated, resulting in serious environmental problems.[62,63] If the cotton textile waste is used as a flexible electrode substrate, it can turn waste into treasure, reducing the cost of flexible electrode materials and alleviating environmental problems. To date, there has been no report on fabricating flexible electrodes for wearable and flexible Zn–air batteries based on cotton textile waste. Therefore, it is highly desirable to develop a new strategy to prepare flexible electrodes based on cotton textile waste and explore their electrochemical performance toward practical flexible applications.

Herein, we demonstrate a novel flexible Zn–air battery employing the Ni-metallized cotton textile waste (NMCTW) as a flexible substrate for Zn anode- and air cathode-active materials. The binder-free flexible electrodes are obtained by directly electrodepositing the Zn metal and NiFe hydroxide thin-film catalyst on NMCTW. The as-obtained flexible electrode with NiFe hydroxide thin-film catalyst exhibits excellent mechanical strength/flexibility, good rate capability, and good cycling stability. The assembled flexible Zn–air battery demonstrates practical flexible application and exhibits excellent mechanical flexibility and electrochemical performance even under severe deformation conditions, opening up unprecedented opportunities for constructing a next-generation high-performance flexible power source toward flexible and wearable electronics.

Results and Discussion

Figure shows the fabrication procedure of a Zn-coated electrode and a NiFe hydroxide electrode. First, the cleaned cotton textile waste was metallized by Ni electroless deposition, and the color of the cleaned cotton textile waste changes from white to silver gray with metallic luster (observed from the optical photographs in Figure ), indicating the deposition of Ni on the cotton textile waste. Next, the Zn-coated cotton textile waste electrode with silver white color and the NiFe hydroxide electrode with deep silver gray color were successfully fabricated through Zn electrodeposition and NiFe hydroxide electrodeposition based on the as-obtained NMCTW, respectively. In addition, both Zn-plated cotton textile waste and NiFe hydroxide electrode possess high flexibility, and the Zn deposition layers and NiFe hydroxide have strong adhesion to cotton textile waste even under mechanical deformation (Figure ) and vigorous sonication. This would be beneficial to realize a Zn–air battery with high flexibility and mechanical strength.

Figure 1

Fabrication procedures of the Zn-coated electrode and NiFe hydroxide electrode.

Figure shows the scanning electron microscopy (SEM) image and energy-dispersive spectroscopy (EDS) spectrum of NMCTW. It can be seen that the cotton textile waste is uniformly covered by Ni after the deposition (Figure a). The EDS mapping of Ni element further indicates the presence of metallic Ni particles. The Ni particles evenly dispersed on the surface of the cotton textiles waste substrate (Figure b–d) act as a flexible conductive electrode substrate.

Figure 2

(a) SEM image, (b) EDS spectrum, (c) enlarged SEM images, and (d) corresponding elemental mapping of NMCTW.

Figure shows the SEM and EDS results of Zn-plated cotton textile waste. It can be observed that the Ni deposition layer is uniformly covered by Zn. Simultaneously, the Zn-plated cotton textile waste still maintains its fabric characteristics (Figure a). The EDS elemental mapping of Zn-coated cotton textile waste shows that only Zn metal element is detected, suggesting that Ni metal is completely covered by the thick Zn layer (Figure b–d).

Figure 3

(a) SEM image, (b) EDS spectrum, (c) enlarged SEM images, and (d) corresponding elemental mapping of Zn-plated cotton textile waste.

Figure shows the SEM images of the surface morphology of NiFe hydroxides prepared at −0.95 V [vs saturated calomel electrode (SCE)] for different electrodeposition times. When the electrodeposition time is 250 s, a small amount of NiFe hydroxides sporadically grow on the surface of NMCTW, exhibiting nanocluster structure (Figure a,b). As the electrodeposition time increases to 500 s, more NiFe hydroxides are uniformly formed on the surface (Figure c). The magnified views of NiFe hydroxides show that nearly transparent NiFe hydroxide thin films are horizontally grown on the surface. Some small wrinkles were also observed on the surface (Figure d), which is a typical feature of NiFe hydroxides[64−66] and has been reported in our previous studies on electrodeposited NiFe hydroxides.[67] The formation of such wrinkles is related to the presence of flexible and thin-layered double hydroxides.[68] As the electrodeposition time further increases to 1000 s, a thick porous NiFe hydroxide layer is formed on the surface of NMCTW (Figure f), and some NiFe hydroxides are detached because of very thick deposits (Figure e), which may cause poor catalytic performance. Among the as-obtained NiFe hydroxides with various surface morphologies, the two-dimensional (2D) thin-film structure possesses good flexibility, which can alleviate the battery surface stress under mechanical bending.[3] This feature is very beneficial for electrode materials of flexible batteries.

Figure 4

SEM images of the surface morphology of NiFe hydroxides prepared at −0.95 V (vs SCE) for different electrodeposition times of (a) 250, (c) 500, and (e) 1000 s. (b,d,f) are the magnified views of (a,c,e), respectively.

To gain insights into the elemental composition distribution and structural information of the as-obtained NiFe hydroxide thin-film electrode, EDS mapping and X-ray diffraction (XRD) analysis were performed. As shown in Figure a, the EDS mapping of NiFe hydroxide thin film displays a homogeneous distribution of Ni, Fe, and O elements, implying the uniform growth and high dispersion of NiFe hydroxides on NMCTW. In the XRD pattern of hydroxide thin-film electrode (Figure b), the peaks at around 44° and 52° are assigned to the coated Ni.[69] The peaks at approximately 18°, 26°, and 48° are related to the diffraction from cotton textile waste.[70] The peaks labeled (003), (006), (012), and (110) are well consistent with literature data for NiFe hydroxides (JCPDS card 51-0463),[71−73] indicating the formation of NiFe hydroxides.

Figure 5

(a) EDS mapping and (b) XRD of the NiFe hydroxide thin-film electrode electrodeposited at −0.95 V (vs SCE) for 500 s.

To assess the electrocatalytic property of the as-obtained NiFe hydroxides, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) of the as-obtained NiFe hydroxides with different surface morphologies were tested in O2-saturated 0.1 M KOH aqueous solution by linear sweep voltammetry. As shown in Figure a, the current density of ORR observed at −0.8 V (vs SCE) for NiFe hydroxide thin films is 1.77 mA cm–2, which is distinctly higher than those of NiFe hydroxide nanoclusters (0.17 mA cm–2) and thick porous NiFe hydroxide (0.51 mA cm–2) (Figure a), illustrating the better ORR catalytic activity of NiFe hydroxide thin films. A similar trend is also observed in Figure b for OER catalytic activity of as-obtained NiFe hydroxides. The NiFe hydroxide thin-film electrode exhibits the highest OER current density of 14.62 mA cm–2 at 0.8 V (vs SCE), which is 8.7–32.5 times higher than those of the NiFe hydroxide nanoclusters (0.45 mA cm–2) and thick porous NiFe hydroxide (1.65 mA cm–2), indicating the superior OER catalytic activity of the NiFe hydroxide thin-film electrode. Obviously, the NiFe hydroxide thin-film electrode possesses the better ORR and OER catalytic activities. The origin of the better ORR and OER catalytic activities for the NiFe hydroxide thin-film electrode could be mainly attributed to the thin-film structure effect and good conductivity.[67,74] On the one hand, the as-obtained NiFe hydroxide possesses 2D thin-film structure (Figure b). This structure has large surface area and high percentage of coordinated unsaturated surface atoms, which could provide a large number of catalytic active sites for the ORR and OER.[75−78] In contrast, the NiFe hydroxide nanocluster electrode has small amount of NiFe hydroxides grown sporadically on the surface of NMCTW (Figure b), which results in the fewer active sites for the ORR and OER catalytic activity. On the other hand, because NiFe hydroxide thin films are horizontally grown on the surface of NMCTW substrate (Figure b), the NiFe hydroxide thin-film electrode has a large contact area between the NiFe hydroxide thin film and the NMCTW substrate. This will lead to the low interface resistance and thus facilitate catalytic performance, which has also been reported in our previous work.[67] Besides, the fast charge transport of 2D thin-film structure is also the key to enhance the ORR/OER kinetics during catalytic reactions. Compared to NiFe hydroxide thin-film electrode, the thick porous NiFe hydroxide electrode has the poor conductivity because of its large thickness, which has a negative effect on the ORR/OER catalytic performance.[74]

Figure 6

Curves of (a) ORR and (b) OER of the as-obtained NiFe hydroxides with different surface morphologies.

Apart from better ORR/OER catalytic performance, NiFe hydroxide thin films could accommodate the mechanical deformation of the cell, which makes them suitable for the practical application in flexible Zn–air batteries. In the following battery test, the as-obtained NiFe hydroxide thin-film electrode is used to assemble the flexible Zn–air batteries based on cotton textile waste. Figure a shows the cross-sectional SEM image of the assembled Zn–air batteries based on cotton textile waste. The configuration of the battery presents a sandwich structure, including a NiFe hydroxide air electrode layer, a Zn-coated electrode layer, and a hydrogel electrolyte layer (Figure a). To investigate the galvanostatic discharge performance of the assembled Zn–air batteries based on cotton textile waste, the discharge curves at different current densities were measured. As shown in Figure b, as the discharge current density was gradually increased from 0.2 to 3.0 mA cm–2, the discharge voltage plateaus change from 1.25 to 0.68 V, indicating that the battery can work at a relatively wide range of discharge current densities. Figure c shows the galvanostatic discharge curve at 1 mA cm–2. Clearly, the galvanostatic discharge voltage plateaus of the battery decrease slightly from 0.94 to 0.87 V and last for 4.6 h during the discharge process, indicating good discharge stability. The voltage decreases sharply at the end of discharge, demonstrating the discharge failure of the battery. The reactions occurring on the Zn electrode during the discharge can be expressed as follows[43]

Figure 7

(a) Cross-sectional SEM image and galvanostatic discharge curves of the assembled Zn–air battery with 1 cm2 at (b) different current densities and (c) 1 mA cm–2.

On the one hand, the consumption of OH– in the electrolyte and the formation of the insoluble ZnO on the surface of electrolyte will lead to an increase in resistance of battery and finally cause the discharge failure of the Zn–air battery. On the other hand, the excessive consumption of the Zn electrode may result in the end of the discharge of the Zn–air battery.[3,8] The gravimetric energy density calculated was 547 W h kg–1, which is close to the reported value of the Zn–air battery in the previous literature.[8]

To investigate the discharge/charge cycling performance, the continuous galvanostatic discharge/charge was tested at 1 mA cm–2 with the cycle period of 20 min of discharge and then 20 min of charge. As shown in Figure a, the assembled Zn–air battery based on cotton textile waste demonstrates excellent stability without the significant potential change during the whole testing period, evidencing good stability of charge and discharge process of the assembled Zn–air battery based on cotton textile waste. The assembled battery exhibits the discharge and charge potential of approximately 0.94 and 1.99 V at 1 mA cm–2, respectively. The potential gap between charge and discharge was about 1.05 V, which is close to that (1.1 V) of commercial Pt/C- and Ir/C-mixed catalyst-based rechargeable Zn–air batteries,[79] implying the good catalytic activity of NiFe hydroxide. In addition, the failure time of the Zn–air battery during the cycling test can be observed, about 8 h, displaying a long cycle life. Figure b shows the discharge stability of the prototype of the assembled Zn–air battery based on cotton textile waste under different mechanical bending deformation angles. It can be seen that the assembled Zn–air battery based on cotton textile waste was mechanically bent to different angles ranging from 0° to 180°, and no significant loss in the discharge performance was observed, demonstrating exceptional flexibility and good discharge stability of the assembled Zn–air batteries based on cotton textile waste.

Figure 8

(a) Galvanostatic charge and discharge cycling curve of rechargeable Zn–air battery at 1 mA cm–2, with the cycle period of 20 min of discharge and then 20 min of charge. (b) Galvanostatic discharge curve of the assembled Zn–air battery based on cotton textile waste under different mechanical bending deformation angles.

To explore the potential application, a demonstration of the red light-emitting diode (LED) powered by the assembled flexible Zn–air batteries based on cotton textile waste was conducted. As shown in Figure a, three flexible Zn–air batteries based on cotton textile waste connected in series can light a red LED. Even when assembled flexible Zn–air batteries were mechanically bent to a small angle of 45° (Figure b) or extremely folded to 0° (Figure c), it could still power the red LED, indicating the stable power output and considerable flexibility of the Zn–air batteries based on cotton textile waste.

Figure 9

Photographs of the LED powered by the assembled flexible Zn–air batteries based on cotton textile waste mechanically bent into different angels of (a) 180°, (b) 45°, and (c) 0°.

Conclusions

In summary, for the first time, we report a novel and flexible rechargeable Zn–air battery based on cotton textile waste, which employs NMCTW as a flexible substrate for Zn anode- and air cathode-active materials, respectively. The binder-free flexible electrode with a transparent NiFe hydroxide thin film horizontally grown on the surface of NMCTW substrate was successfully synthesized, exhibiting excellent mechanical strength/flexibility and good catalytic activity. The assembled Zn–air batteries based on cotton textile waste display a good rate capability and an outstanding charge and discharge cycling stability. In practical applications of powering for LED, the cotton textile waste-based Zn–air batteries show exceptional flexibility and stable output power, even under extreme mechanical bending conditions.

Experimental Section

Synthesis and Characterization of Electrodes Based on Cotton Textile Waste

The cotton textile waste is from waste lab coat, which was ultrasonically cleaned in acetone for 15 min and repeatedly rinsed with deionized water, and then dried by argon gas flow. The Ni-coated cotton textiles were fabricated following the procedure. First of all, the cotton textile waste was immersed in 37% HCl solution containing 53 mM SnCl2 for 20 min at room temperature. The as-obtained textile was then dipped in 0.5 g L–1 of PdCl2 + 20 mL L–1 37% HCl aqueous solution for activation. Next, the activated cotton textile waste was immersed into the mixture solution of 0.34 M NH4Cl + 27 mM C6H5Na3O7·2H2O + 97 mM NiSO4 + 0.14 M NaH2PO2·H2O for electroless plating of Ni. Finally, the as-obtained Ni-plated cotton textile waste was rinsed with distilled water to remove the residual plating solution and dried at 50 °C for 30 min. On the basis of the as-obtained NMCTW, the Zn-coated anode were prepared by electroplating at the current density of 30 mA cm–2 for 30 min in an aqueous solution of 250 g L–1 of ZnSO4·7H2O + 30 g L–1 Na2SO4 + 5 g L–1 Al2(SO4)3, and the air electrode with NiFe hydroxide catalyst was prepared by electrochemical deposition at −0.95 V (vs SCE) for 250, 500, and 1000 s, respectively, in an aqueous solution of 3 mM Ni(NO3)2 + 3 mM Fe(NO3)3.

The surface morphology and composition of the as-obtained Zn anode and the air electrode with nickel iron hydroxide catalyst were characterized by a scanning electron microscope (S-4800, Hitachi, Japan) equipped with an X-ray EDS system. The crystalline structures of the as-obtained electrodes were measured by XRD (Cu Kα radiation, D/max 2200/PC, Rigaku, Japan).

Electrochemical Tests

Electrochemical tests were conducted in a three-electrode cell configuration using the electrochemical workstation (CHI 760E, Shanghai Chenhua, China). A Pt plate and an SCE acted as the counter electrode and reference electrode, respectively. The NMCTW electrodes covered with a NiFe hydroxide catalyst were directly used as the working electrode in the absence of a binder and/or a conductivity promoter. The catalytic performance of NiFe hydroxide was evaluated by linear sweep voltammetry in O2-saturated 0.1 M KOH aqueous solution at 5 mV s–1.

Assembly and Tests of the Cotton Textile Waste-Based Flexible Zn–Air Battery

The sandwich-structured flexible Zn–air battery device was assembled with the flexible electrodes of the Zn-plated cotton textile waste and the NiFe hydroxide face-to-face separated by the poly(vinyl alcohol) (PVA)–KOH alkaline polymer electrolyte. The electrolyte was fabricated following the procedure:[80] 1.6 g of PVA was first dissolved in 16 mL of distilled water under continuous stirring at 85 °C for about 90 min, until a clear viscous solution was obtained. Subsequently, 4 mL of aqueous solution containing 2 g of KOH solution was slowly dropped into the viscous solution under continuous stirring for approximately 30 min, until the viscous solution became clear. A thin KOH–PVA film hydrogel electrolyte was prepared by cross-linking at −8 °C for 2 h in a Petri dish. The KOH–PVA film of the sandwich-structured flexible Zn–air battery acted as both the separator and electrolyte.

A multichannel battery testing system (LAND CT2001A, Wuhan, China) was used to test the galvanostatic discharge and the discharge/charge cycling of the cotton textile waste-based flexible Zn–air battery. Galvanostatic discharge test was performed at different discharge current densities. The discharge/charge performance test was carried out, in which each cycle consists of 20 min of discharge, followed by 20 min of charge. The energy density was calculated according to the following equation:where the mass of consumed Zn electrode is 11.6 mg. All of the tests were carried out under ambient atmosphere condition.