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Cite this: Nanoscale, 2014, 6, 4083
Received 7th November 2013 Accepted 15th January 2014
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Cellulose nanofiber/single-walled carbon nanotube hybrid non-woven macrofiber mats as novel wearable supercapacitors with excellent stability, tailorability and reliability† Qingyuan Niu,‡ Kezheng Gao‡ and Ziqiang Shao*
DOI: 10.1039/c3nr05929d www.rsc.org/nanoscale
Non-woven macrofiber mats are prepared by simply controlling the extrusion patterns of cellulose nanofiber/single-walled carbon nanotube suspensions in an ethanol coagulation bath, and drying in air under restricted conditions. These novel wearable supercapacitors based on non-woven macrofiber mats are demonstrated to have excellent tailorability, electrochemical stability, and damage reliability.
Wearable energy storage devices, one of the key challenges to the widespread usage of wearable electronics, have attracted increased attention in recent years.1–7 Supercapacitors, as an energy storage device, present a unique advantage for use in wearable electronics compared to batteries due to their ultrahigh power density, fast charging/discharging capacity, long cycle life, and excellent safety.8–12 Therefore, wearable supercapacitors, especially ber-shaped wearable supercapacitors,13–19 have been extensively studied as wearable energy storage devices not only because they possess common characteristics of supercapacitors but also because they are lightweight, exible, and knittable. However, in our view, apart from the abovementioned properties (especially the excellent electrochemical properties), wearable supercapacitors should also have the following characteristics: tailorability, electrochemical stability and damage reliability in the case of severe deformations or even serious destruction, which are highly signicant from a practical point of view. The capacitance performance of ber-shaped wearable supercapacitors may be completely lost when they are School of Materials science and Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China. E-mail: [email protected]; Fax: +86 010-68941797-609; Tel: +86 010-68941797-601 † Electronic supplementary information (ESI) available: Experimental, TEM image, IR spectra, and XRD spectra of cellulose nanobers, photograph of the cellulose nanober/single-walled carbon nanotube suspension, cellulose nanober/single-walled carbon nanotube non-woven macrober mat and non-woven macrober mat wearable supercapacitors. The electrochemical performance of the CNF/SWCNT hybrid ber wearable supercapacitor. Photograph of the non-woven macrober mat wearable supercapacitors integrated within textiles. See DOI: 10.1039/c3nr05929d ‡ Q. Niu and K. Gao contributed equally to this work.
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damaged. Therefore, the capacitance performance of wearable cloths woven by ber-shaped supercapacitors may be seriously affected by tailoring due to their independent ber-shaped structure. Wearable supercapacitors prepared by lm electrodes may exhibit better tailorability but be uncomfortable to wear (not allow air and sweat to pass through the wearable supercapacitors freely) compared to ber-shaped supercapacitors. Therefore, to address these issues, novel wearable supercapacitors, which not only possess excellent electrochemical properties, but also exhibit outstanding tailorability, electrochemical stability and damage reliability in the case of severe deformation or even destruction, need to be designed. In recent years, research has shown that cellulose nanobers (including natural cellulose and bacterial cellulose) can effectively help realize the inherent electrochemical properties that active materials (such as carbon nanotubes, reduced graphene oxide, and so on) can provide due to their 1D-nanostructures and good hydrophilicity.20–25 In addition, excellent electrode materials can also be prepared by carbonization of cellulose (especially bacterial cellulose).26–29 Cellulose nanobers are an efficient, low cost, biocompatible, and environmentally friendly dispersant of carbon nanotubes.30 The uniform dispersion of cellulose nanobers/carbon nanotubes may provide good wet-spinnability due to the presence of cellulose nanobers (which exhibit excellent wet-spinnability).31,32 Thus the cellulose nanober/carbon nanotube hybrid non-woven macrober mat can be prepared by simply controlling the extrusion patterns in an ethanol coagulation bath. The wearable supercapacitors prepared by these novel cellulose nanober/carbon nanotube hybrid non-woven macrober mats may not only exhibit good electrochemical properties, but also have outstanding tailorability, excellent electrochemical stability, damage reliability, and be comfortable to wear due to their structural advantages and the presence of cellulose nanobers. In this communication, we report a novel cellulose nanober/carbon nanotube hybrid non-woven macrober mat for wearable supercapacitors. The resulting wearable supercapacitors possess many outstanding properties, such as good
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electrochemical properties, outstanding tailorability, remarkable electrochemical stability and damage reliability. Cellulose nanobers (CNF) were prepared according to the methodology reported by Isogai,33–36 and detailed characterization of these cellulose nanobers is provided in Fig. S1, S2, and S3, ESI†. 0.69 g single-walled carbon nanotubes (SWCNTs) (without any modication) were added to 100 g cellulose nanober suspension (0.48 wt%) and stirred for 24 h using a magnetic stirrer. The resulting mixture was then sonicated for 3 min at a power of 700 W in an ice bath to disperse the SWCNTs in the cellulose nanober suspension (Fig. S4, ESI†). Cellulose nanober/single-walled carbon nanotube (CNF/SWCNT) hybrid non-woven macrober mats can be prepared by simply controlling the extrusion patterns of the CNF/SWCNT suspension in an ethanol coagulation bath (Fig. 1a and S5, ESI†). Finally, the extrudates were dried in air under restricted conditions (Fig. S6, ESI†) to obtain the CNF/SWCNT hybrid non-woven macrober mat (Fig. 1b). The morphology of the CNF/SWCNT hybrid non-woven macrober mat was examined using scanning electron microscopy (SEM, Fig. 1c–f). Fig. 1c and d show the surface morphology of the macrober constituting the CNF/ SWCNT hybrid non-woven macrober mat at different magnications. The macrober has a uniform diameter of 50 mm, and also shows a good porous structure, which could favour electrolyte ion diffusion into the macrober. Fig. 1e and f show the cross-sectional morphology of the macrober at different magnications. The SWCNTs are preferentially oriented along the axial direction of the macrober (extrusion direction) due to the induction of the extrusion process, which may favour electron transport along the axial direction. In addition, the CNF in the non-woven macrober mat can effectively prevent the aggregation of SWCNTs and signicantly improve the re-swell properties of the non-woven macrober mat in aqueous electrolytes because of their outstanding hydrophilicity.37 The CNF with adsorbed electrolytes may act as electrolyte nano-reservoirs, which will be benecial for the diffusion of electrolyte ions, and greatly improve the electrochemical performance of the CNF/SWCNT hybrid non-woven macrober mat.
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CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors (used as both the electrode material and charge collector) are fabricated in a traditional symmetrical two-electrode stacked conguration using H3PO4-poly(vinyl alcohol) (PVA) gel as the solid-state electrolyte and separator.38,39 The resulting wearable supercapacitor maintains the characteristics of the non-woven macrober mat (Fig. S7†), which can be very comfortable to wear. The electrochemical properties of CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors are assessed by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) at room temperature. Fig. 2a presents the CV curves of the nonwoven macrober mat wearable supercapacitors at scan rates of 5–200 mV s1 with the potential window ranging from 0 to 1 V. All of the CV curves are close to rectangular within a selected potential range, at scan rates of 5–100 mV s1, illustrating the good capacitive performance of CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors. These results may be attributed to the effective ion diffusion process in the CNF/SWCNT hybrid non-woven macrober mat, due to the porous structure and electrolyte nano-reservoir (CNF with adsorbed electrolyte) in the CNF/SWCNT hybrid non-woven macrober mat. Galvanostatic charge– discharge results of the non-woven macrober mat wearable supercapacitors at different current densities are shown in Fig. 2b. It is observed that all of the charge curves are very symmetrical to their corresponding discharge counterparts and they have excellent linear proles in a potential window ranging from 0 to 1 V, which demonstrates the outstanding capacitive characteristics of the non-woven macrober mat wearable supercapacitors with less Faradaic reactions. The Coulombic efficiency is up to 97% at a current density of 0.024 mA cm2, indicative of the excellent charge–discharge reversibility of the non-woven macrober mat wearable supercapacitors. The area capacitance (Cs) of the nonwoven macrober mat wearable supercapacitors is calculated using
Photograph of the wet (a) and dry (b) CNF/SWCNT hybrid non-woven macrofiber mat. (c, d) SEM images of the CNF/SWCNT hybrid nonwoven macrofiber mat. (e, f) Cross-sectional view of the CNF/SWCNT hybrid non-woven macrofiber mat.
Fig. 1
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(a) Typical CV curves of the non-woven macrofiber mat wearable supercapacitor at different scan rates. (b) Typical galvanostatic charge– discharge curves of the non-woven macrofiber mat wearable supercapacitor at different current densities. (c) The area capacitance of the nonwoven macrofiber mat wearable supercapacitor as a function of current density. (d) Nyquist impedance plots of the non-woven macrofiber mat wearable supercapacitor. (e) Cycling stability of the non-woven macrofiber mat wearable supercapacitor over 5000 cycles. (f) Three non-woven macrofiber mat wearable supercapacitors connected in series can illuminate a LED light.
Fig. 2
(a) CV curves of the non-woven macrofiber mat wearable supercapacitor at different bending states of 0 , 45 , 90 , 120 , and 180 (50 mV s1). (b) The durability test of the non-woven macrofiber mat wearable supercapacitor undergoing 1500 bending cycles according to CV curves at a scan rate of 50 mV s1. Inset: the photograph of the non-woven macrofiber mat wearable supercapacitor in a bending state (180 ). (c) Schematic illustration of extreme deformation. (d) CV curves with the number of extreme deformation cycles at a scan rate of 50 mV s1. (d) Normalized capacitance values versus the number of extreme deformation cycles. Fig. 3
Cs ¼
I td A V IRdrop
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(1)
where I is the discharge current, td is the discharge time, V is the highest voltage for discharge, IRdrop is the voltage drop at the beginning of the discharge, and A is the working area of the non-woven macrober mat wearable supercapacitors (Fig. S7†). The calculated area capacitance Cs from the discharge curves at different current densities is shown in Fig. 2c. The capacitance is about 5.99 mF cm2 at a current density of 0.024 mA cm2 (the areal capacitance of the CNF/SWCNT hybrid ber supercapacitor is about 3.29 mF cm2 at a current density of 0.02 mA cm2 (Fig. S8, ESI†)), which exceeds that of the pure carbonbased ber all-solid-state supercapacitor.40–42 When the current density is increased from 0.024 to 0.943 mA cm2, the area capacitance of the non-woven macrober mat wearable supercapacitor decreases gradually from 5.99 to 3.97 mF cm2. It is found that about 66.3% of the capacitance can be achieved when the current density is increased by a factor of about 39.3, indicating the good rate capabilities of the non-woven macrober mat wearable supercapacitor due to the additional diffusion pathways provided by CNF with absorbed electrolytes. The electrochemical impedance spectrum (EIS) of the non-woven macrober mat wearable supercapacitor is shown as Nyquist plots in Fig. 2d, which exhibit a typical electric double layer capacitive behavior. The equivalent series resistance is about 23.8 U, according to the intercept of the Nyquist plot with the real axis. At medium frequencies there is a short 45 angled region, which indicates the fast ion diffusion in the CNF/ SWCNT hybrid non-woven macrober mat due to the presence of CNF with adsorbed electrolytes. It is also noticeable that the Nyquist plot exhibits a straight and nearly vertical line at low frequencies indicating the ideal capacitive behavior of the non-woven macrober mat wearable supercapacitors. The cycling stability of the non-woven macrober mat wearable
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Fig. 4 Photograph of the original supercapacitor (a) and two sub-supercapacitors (b). (c) CV curves of the original supercapacitor and two subsupercapacitors at a scan rate of 50 mV s1. (d) The non-woven macrofiber mat wearable supercapacitor with different degrees of damage. (e) CV curves of the non-woven macrofiber mat wearable supercapacitor with different degrees of damage at a scan rate of 50 mV s1. (f) Capacitance retention under different degrees of damage.
supercapacitors is investigated using a cyclic voltammetry test at a scan rate of 100 mV s1, which is shown in Fig. 2e. The nonwoven macrober mat wearable supercapacitor maintains 97% of the initial capacitance aer 5000 cycles, demonstrating its excellent long-term cycling stability. Non-woven macrober mat wearable supercapacitors have an energy density of 0.702 mW h cm2 at a power density of 2.435 mW cm2. Fig. 2f shows a red light-emitting-diode (LED) being lit by a device composed of three non-woven macrober mat wearable supercapacitors connected in series, illustrating its superior performance. As a wearable energy storage device, the CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors must possess outstanding exibility without sacricing the electrochemical performance of the device. We conducted cyclic voltammetry under different bending states (even under extreme deformations) to evaluate the electrochemical stability of the non-woven macrober mat wearable supercapacitors. Fig. 3a shows the CV curves at different bending angles. Almost completely overlapping CV curves are observed, reecting that the CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors are highly exible. The bending tolerance of the non-woven macrober mat wearable supercapacitors is evaluated by long-term and repeated bending (at–bending status (see inset of Fig. 3b, bent at 180 )–at), which is shown in Fig. 3b. The non-woven macrober mat wearable supercapacitors maintain 96.0% of their initial capacitance aer 1500 bending cycles, which is signicantly better than previous work published (about 70%).17 The electrochemical stability of the non-woven macrober mat wearable supercapacitors is further investigated by repeated extreme deformation (Fig. 3c, at– crumpled non-woven macrober mat wearable supercapacitors into a ball–at). As shown in Fig. 3d and e, the shape of the CV curves (at a scan rate of 50 mV s1) are not obviously changed,
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and the capacity is maintained at around 93% aer the h extreme deformation, indicating the excellent electrochemical stability of the non-woven macrober mat wearable supercapacitors under extreme deformation. This excellent electrochemical stability of the non-woven macrober mat wearable supercapacitors could be attributed to the following two reasons. First, the presence of cellulose nanobers can confer more mechanical stability to the CNF/SWCNT hybrid nonwoven macrober mat under deformation. In addition, the layer of H3PO4-PVA gel coated on the surface of the CNF/SWCNT hybrid non-woven macrober mats will further enhance their mechanical stability. Therefore, it is very difficult for the active material-SWCNT to be delaminated from the non-woven macrober mat even in the case of extreme deformation. More importantly, the destructive stress generated during extreme deformation can perhaps be effectively dissipated by the network structure of the non-woven macrober mat. The electrochemical stability of the non-woven macrober mat wearable supercapacitors is therefore less susceptible to fatigue caused by extreme deformation. Excellent tailorability is an important basic property of the fabric. Wearable supercapacitors, as a wearable energy storage device, should possess excellent tailorability, which can realize the standardized mass production of the wearable supercapacitors. The wearable supercapacitors could then be tailored appropriately into different shapes to meet different requirements. The CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors exhibit outstanding tailorability, and can be easily cut into two pieces with a pair of scissors (Fig. 4a and b). The CV curves of the original supercapacitor and two sub-supercapacitors at a scan rate of 50 mV s1 are shown in Fig. 4c. All the CV curves exhibit a wonderful rectangular shape, showing the ideal capacitive behavior of the supercapacitor. The
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capacitances of the original supercapacitor and two sub-supercapacitors are about 32.16, 19.98, and 14.81 mF, respectively, indicating that the electrochemical performance of the non-woven macrober mat wearable supercapacitors is almost unaffected by the tailoring. To further demonstrate the wearability of non-woven macrober mat wearable supercapacitors, they are stitched onto the textile (Fig. S9a†). The wearable supercapacitor integrated textiles maintain their excellent exibility without loss of electrochemical performance (Fig. S9b and c†). Wearable supercapacitors inevitably suffer some degree of damage during use. Outstanding damage reliability is therefore an important basic performance considered when designing novel wearable supercapacitors for practical applications. To assess the damage reliability of CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors under different degrees of damage (Fig. 4d), we performed CV tests at a scan rate of 50 mV s1, and the results are shown in Fig. 4e. The CV curve of the non-woven macrober mat wearable supercapacitors can still maintain an almost rectangular shape aer suffering serious destruction. As shown in Fig. 4f, the supercapacitor maintains up to 72.4% of the initial capacitance aer undergoing serious destruction, indicating that non-woven macrober mat wearable supercapacitors possess outstanding damage reliability, which can perhaps be attributed to the unique conductive network structure of the non-woven macrober mat. The damaged non-woven macrober mat wearable supercapacitors will therefore exhibit good electrochemical properties as long as there are electron-conductive paths. In summary, the CNF/SWCNT hybrid non-woven macrober mat was prepared by simply controlling the extrusion patterns in an ethanol coagulation bath and drying under restricted conditions. CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors are fabricated using a CNF/SWCNT hybrid non-woven macrober mat as the electrode material and charge collector. Non-woven macrober mat wearable supercapacitors exhibit good electrochemical properties, outstanding tailorability, excellent electrochemical stability, and damage reliability. Our approach to non-woven macrober mat wearable supercapacitors provides a novel strategy for the large-scale preparation of wearable supercapacitors with good electrochemical properties, outstanding tailorability, excellent electrochemical stability, and damage reliability.
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Cite this: Nanoscale, 2014, 6, 4083
Received 7th November 2013 Accepted 15th January 2014
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Cellulose nanofiber/single-walled carbon nanotube hybrid non-woven macrofiber mats as novel wearable supercapacitors with excellent stability, tailorability and reliability† Qingyuan Niu,‡ Kezheng Gao‡ and Ziqiang Shao*
DOI: 10.1039/c3nr05929d www.rsc.org/nanoscale
Non-woven macrofiber mats are prepared by simply controlling the extrusion patterns of cellulose nanofiber/single-walled carbon nanotube suspensions in an ethanol coagulation bath, and drying in air under restricted conditions. These novel wearable supercapacitors based on non-woven macrofiber mats are demonstrated to have excellent tailorability, electrochemical stability, and damage reliability.
Wearable energy storage devices, one of the key challenges to the widespread usage of wearable electronics, have attracted increased attention in recent years.1–7 Supercapacitors, as an energy storage device, present a unique advantage for use in wearable electronics compared to batteries due to their ultrahigh power density, fast charging/discharging capacity, long cycle life, and excellent safety.8–12 Therefore, wearable supercapacitors, especially ber-shaped wearable supercapacitors,13–19 have been extensively studied as wearable energy storage devices not only because they possess common characteristics of supercapacitors but also because they are lightweight, exible, and knittable. However, in our view, apart from the abovementioned properties (especially the excellent electrochemical properties), wearable supercapacitors should also have the following characteristics: tailorability, electrochemical stability and damage reliability in the case of severe deformations or even serious destruction, which are highly signicant from a practical point of view. The capacitance performance of ber-shaped wearable supercapacitors may be completely lost when they are School of Materials science and Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China. E-mail: [email protected]; Fax: +86 010-68941797-609; Tel: +86 010-68941797-601 † Electronic supplementary information (ESI) available: Experimental, TEM image, IR spectra, and XRD spectra of cellulose nanobers, photograph of the cellulose nanober/single-walled carbon nanotube suspension, cellulose nanober/single-walled carbon nanotube non-woven macrober mat and non-woven macrober mat wearable supercapacitors. The electrochemical performance of the CNF/SWCNT hybrid ber wearable supercapacitor. Photograph of the non-woven macrober mat wearable supercapacitors integrated within textiles. See DOI: 10.1039/c3nr05929d ‡ Q. Niu and K. Gao contributed equally to this work.
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damaged. Therefore, the capacitance performance of wearable cloths woven by ber-shaped supercapacitors may be seriously affected by tailoring due to their independent ber-shaped structure. Wearable supercapacitors prepared by lm electrodes may exhibit better tailorability but be uncomfortable to wear (not allow air and sweat to pass through the wearable supercapacitors freely) compared to ber-shaped supercapacitors. Therefore, to address these issues, novel wearable supercapacitors, which not only possess excellent electrochemical properties, but also exhibit outstanding tailorability, electrochemical stability and damage reliability in the case of severe deformation or even destruction, need to be designed. In recent years, research has shown that cellulose nanobers (including natural cellulose and bacterial cellulose) can effectively help realize the inherent electrochemical properties that active materials (such as carbon nanotubes, reduced graphene oxide, and so on) can provide due to their 1D-nanostructures and good hydrophilicity.20–25 In addition, excellent electrode materials can also be prepared by carbonization of cellulose (especially bacterial cellulose).26–29 Cellulose nanobers are an efficient, low cost, biocompatible, and environmentally friendly dispersant of carbon nanotubes.30 The uniform dispersion of cellulose nanobers/carbon nanotubes may provide good wet-spinnability due to the presence of cellulose nanobers (which exhibit excellent wet-spinnability).31,32 Thus the cellulose nanober/carbon nanotube hybrid non-woven macrober mat can be prepared by simply controlling the extrusion patterns in an ethanol coagulation bath. The wearable supercapacitors prepared by these novel cellulose nanober/carbon nanotube hybrid non-woven macrober mats may not only exhibit good electrochemical properties, but also have outstanding tailorability, excellent electrochemical stability, damage reliability, and be comfortable to wear due to their structural advantages and the presence of cellulose nanobers. In this communication, we report a novel cellulose nanober/carbon nanotube hybrid non-woven macrober mat for wearable supercapacitors. The resulting wearable supercapacitors possess many outstanding properties, such as good
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electrochemical properties, outstanding tailorability, remarkable electrochemical stability and damage reliability. Cellulose nanobers (CNF) were prepared according to the methodology reported by Isogai,33–36 and detailed characterization of these cellulose nanobers is provided in Fig. S1, S2, and S3, ESI†. 0.69 g single-walled carbon nanotubes (SWCNTs) (without any modication) were added to 100 g cellulose nanober suspension (0.48 wt%) and stirred for 24 h using a magnetic stirrer. The resulting mixture was then sonicated for 3 min at a power of 700 W in an ice bath to disperse the SWCNTs in the cellulose nanober suspension (Fig. S4, ESI†). Cellulose nanober/single-walled carbon nanotube (CNF/SWCNT) hybrid non-woven macrober mats can be prepared by simply controlling the extrusion patterns of the CNF/SWCNT suspension in an ethanol coagulation bath (Fig. 1a and S5, ESI†). Finally, the extrudates were dried in air under restricted conditions (Fig. S6, ESI†) to obtain the CNF/SWCNT hybrid non-woven macrober mat (Fig. 1b). The morphology of the CNF/SWCNT hybrid non-woven macrober mat was examined using scanning electron microscopy (SEM, Fig. 1c–f). Fig. 1c and d show the surface morphology of the macrober constituting the CNF/ SWCNT hybrid non-woven macrober mat at different magnications. The macrober has a uniform diameter of 50 mm, and also shows a good porous structure, which could favour electrolyte ion diffusion into the macrober. Fig. 1e and f show the cross-sectional morphology of the macrober at different magnications. The SWCNTs are preferentially oriented along the axial direction of the macrober (extrusion direction) due to the induction of the extrusion process, which may favour electron transport along the axial direction. In addition, the CNF in the non-woven macrober mat can effectively prevent the aggregation of SWCNTs and signicantly improve the re-swell properties of the non-woven macrober mat in aqueous electrolytes because of their outstanding hydrophilicity.37 The CNF with adsorbed electrolytes may act as electrolyte nano-reservoirs, which will be benecial for the diffusion of electrolyte ions, and greatly improve the electrochemical performance of the CNF/SWCNT hybrid non-woven macrober mat.
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CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors (used as both the electrode material and charge collector) are fabricated in a traditional symmetrical two-electrode stacked conguration using H3PO4-poly(vinyl alcohol) (PVA) gel as the solid-state electrolyte and separator.38,39 The resulting wearable supercapacitor maintains the characteristics of the non-woven macrober mat (Fig. S7†), which can be very comfortable to wear. The electrochemical properties of CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors are assessed by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) at room temperature. Fig. 2a presents the CV curves of the nonwoven macrober mat wearable supercapacitors at scan rates of 5–200 mV s1 with the potential window ranging from 0 to 1 V. All of the CV curves are close to rectangular within a selected potential range, at scan rates of 5–100 mV s1, illustrating the good capacitive performance of CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors. These results may be attributed to the effective ion diffusion process in the CNF/SWCNT hybrid non-woven macrober mat, due to the porous structure and electrolyte nano-reservoir (CNF with adsorbed electrolyte) in the CNF/SWCNT hybrid non-woven macrober mat. Galvanostatic charge– discharge results of the non-woven macrober mat wearable supercapacitors at different current densities are shown in Fig. 2b. It is observed that all of the charge curves are very symmetrical to their corresponding discharge counterparts and they have excellent linear proles in a potential window ranging from 0 to 1 V, which demonstrates the outstanding capacitive characteristics of the non-woven macrober mat wearable supercapacitors with less Faradaic reactions. The Coulombic efficiency is up to 97% at a current density of 0.024 mA cm2, indicative of the excellent charge–discharge reversibility of the non-woven macrober mat wearable supercapacitors. The area capacitance (Cs) of the nonwoven macrober mat wearable supercapacitors is calculated using
Photograph of the wet (a) and dry (b) CNF/SWCNT hybrid non-woven macrofiber mat. (c, d) SEM images of the CNF/SWCNT hybrid nonwoven macrofiber mat. (e, f) Cross-sectional view of the CNF/SWCNT hybrid non-woven macrofiber mat.
Fig. 1
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(a) Typical CV curves of the non-woven macrofiber mat wearable supercapacitor at different scan rates. (b) Typical galvanostatic charge– discharge curves of the non-woven macrofiber mat wearable supercapacitor at different current densities. (c) The area capacitance of the nonwoven macrofiber mat wearable supercapacitor as a function of current density. (d) Nyquist impedance plots of the non-woven macrofiber mat wearable supercapacitor. (e) Cycling stability of the non-woven macrofiber mat wearable supercapacitor over 5000 cycles. (f) Three non-woven macrofiber mat wearable supercapacitors connected in series can illuminate a LED light.
Fig. 2
(a) CV curves of the non-woven macrofiber mat wearable supercapacitor at different bending states of 0 , 45 , 90 , 120 , and 180 (50 mV s1). (b) The durability test of the non-woven macrofiber mat wearable supercapacitor undergoing 1500 bending cycles according to CV curves at a scan rate of 50 mV s1. Inset: the photograph of the non-woven macrofiber mat wearable supercapacitor in a bending state (180 ). (c) Schematic illustration of extreme deformation. (d) CV curves with the number of extreme deformation cycles at a scan rate of 50 mV s1. (d) Normalized capacitance values versus the number of extreme deformation cycles. Fig. 3
Cs ¼
I td A V IRdrop
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(1)
where I is the discharge current, td is the discharge time, V is the highest voltage for discharge, IRdrop is the voltage drop at the beginning of the discharge, and A is the working area of the non-woven macrober mat wearable supercapacitors (Fig. S7†). The calculated area capacitance Cs from the discharge curves at different current densities is shown in Fig. 2c. The capacitance is about 5.99 mF cm2 at a current density of 0.024 mA cm2 (the areal capacitance of the CNF/SWCNT hybrid ber supercapacitor is about 3.29 mF cm2 at a current density of 0.02 mA cm2 (Fig. S8, ESI†)), which exceeds that of the pure carbonbased ber all-solid-state supercapacitor.40–42 When the current density is increased from 0.024 to 0.943 mA cm2, the area capacitance of the non-woven macrober mat wearable supercapacitor decreases gradually from 5.99 to 3.97 mF cm2. It is found that about 66.3% of the capacitance can be achieved when the current density is increased by a factor of about 39.3, indicating the good rate capabilities of the non-woven macrober mat wearable supercapacitor due to the additional diffusion pathways provided by CNF with absorbed electrolytes. The electrochemical impedance spectrum (EIS) of the non-woven macrober mat wearable supercapacitor is shown as Nyquist plots in Fig. 2d, which exhibit a typical electric double layer capacitive behavior. The equivalent series resistance is about 23.8 U, according to the intercept of the Nyquist plot with the real axis. At medium frequencies there is a short 45 angled region, which indicates the fast ion diffusion in the CNF/ SWCNT hybrid non-woven macrober mat due to the presence of CNF with adsorbed electrolytes. It is also noticeable that the Nyquist plot exhibits a straight and nearly vertical line at low frequencies indicating the ideal capacitive behavior of the non-woven macrober mat wearable supercapacitors. The cycling stability of the non-woven macrober mat wearable
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Fig. 4 Photograph of the original supercapacitor (a) and two sub-supercapacitors (b). (c) CV curves of the original supercapacitor and two subsupercapacitors at a scan rate of 50 mV s1. (d) The non-woven macrofiber mat wearable supercapacitor with different degrees of damage. (e) CV curves of the non-woven macrofiber mat wearable supercapacitor with different degrees of damage at a scan rate of 50 mV s1. (f) Capacitance retention under different degrees of damage.
supercapacitors is investigated using a cyclic voltammetry test at a scan rate of 100 mV s1, which is shown in Fig. 2e. The nonwoven macrober mat wearable supercapacitor maintains 97% of the initial capacitance aer 5000 cycles, demonstrating its excellent long-term cycling stability. Non-woven macrober mat wearable supercapacitors have an energy density of 0.702 mW h cm2 at a power density of 2.435 mW cm2. Fig. 2f shows a red light-emitting-diode (LED) being lit by a device composed of three non-woven macrober mat wearable supercapacitors connected in series, illustrating its superior performance. As a wearable energy storage device, the CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors must possess outstanding exibility without sacricing the electrochemical performance of the device. We conducted cyclic voltammetry under different bending states (even under extreme deformations) to evaluate the electrochemical stability of the non-woven macrober mat wearable supercapacitors. Fig. 3a shows the CV curves at different bending angles. Almost completely overlapping CV curves are observed, reecting that the CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors are highly exible. The bending tolerance of the non-woven macrober mat wearable supercapacitors is evaluated by long-term and repeated bending (at–bending status (see inset of Fig. 3b, bent at 180 )–at), which is shown in Fig. 3b. The non-woven macrober mat wearable supercapacitors maintain 96.0% of their initial capacitance aer 1500 bending cycles, which is signicantly better than previous work published (about 70%).17 The electrochemical stability of the non-woven macrober mat wearable supercapacitors is further investigated by repeated extreme deformation (Fig. 3c, at– crumpled non-woven macrober mat wearable supercapacitors into a ball–at). As shown in Fig. 3d and e, the shape of the CV curves (at a scan rate of 50 mV s1) are not obviously changed,
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and the capacity is maintained at around 93% aer the h extreme deformation, indicating the excellent electrochemical stability of the non-woven macrober mat wearable supercapacitors under extreme deformation. This excellent electrochemical stability of the non-woven macrober mat wearable supercapacitors could be attributed to the following two reasons. First, the presence of cellulose nanobers can confer more mechanical stability to the CNF/SWCNT hybrid nonwoven macrober mat under deformation. In addition, the layer of H3PO4-PVA gel coated on the surface of the CNF/SWCNT hybrid non-woven macrober mats will further enhance their mechanical stability. Therefore, it is very difficult for the active material-SWCNT to be delaminated from the non-woven macrober mat even in the case of extreme deformation. More importantly, the destructive stress generated during extreme deformation can perhaps be effectively dissipated by the network structure of the non-woven macrober mat. The electrochemical stability of the non-woven macrober mat wearable supercapacitors is therefore less susceptible to fatigue caused by extreme deformation. Excellent tailorability is an important basic property of the fabric. Wearable supercapacitors, as a wearable energy storage device, should possess excellent tailorability, which can realize the standardized mass production of the wearable supercapacitors. The wearable supercapacitors could then be tailored appropriately into different shapes to meet different requirements. The CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors exhibit outstanding tailorability, and can be easily cut into two pieces with a pair of scissors (Fig. 4a and b). The CV curves of the original supercapacitor and two sub-supercapacitors at a scan rate of 50 mV s1 are shown in Fig. 4c. All the CV curves exhibit a wonderful rectangular shape, showing the ideal capacitive behavior of the supercapacitor. The
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capacitances of the original supercapacitor and two sub-supercapacitors are about 32.16, 19.98, and 14.81 mF, respectively, indicating that the electrochemical performance of the non-woven macrober mat wearable supercapacitors is almost unaffected by the tailoring. To further demonstrate the wearability of non-woven macrober mat wearable supercapacitors, they are stitched onto the textile (Fig. S9a†). The wearable supercapacitor integrated textiles maintain their excellent exibility without loss of electrochemical performance (Fig. S9b and c†). Wearable supercapacitors inevitably suffer some degree of damage during use. Outstanding damage reliability is therefore an important basic performance considered when designing novel wearable supercapacitors for practical applications. To assess the damage reliability of CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors under different degrees of damage (Fig. 4d), we performed CV tests at a scan rate of 50 mV s1, and the results are shown in Fig. 4e. The CV curve of the non-woven macrober mat wearable supercapacitors can still maintain an almost rectangular shape aer suffering serious destruction. As shown in Fig. 4f, the supercapacitor maintains up to 72.4% of the initial capacitance aer undergoing serious destruction, indicating that non-woven macrober mat wearable supercapacitors possess outstanding damage reliability, which can perhaps be attributed to the unique conductive network structure of the non-woven macrober mat. The damaged non-woven macrober mat wearable supercapacitors will therefore exhibit good electrochemical properties as long as there are electron-conductive paths. In summary, the CNF/SWCNT hybrid non-woven macrober mat was prepared by simply controlling the extrusion patterns in an ethanol coagulation bath and drying under restricted conditions. CNF/SWCNT hybrid non-woven macrober mat wearable supercapacitors are fabricated using a CNF/SWCNT hybrid non-woven macrober mat as the electrode material and charge collector. Non-woven macrober mat wearable supercapacitors exhibit good electrochemical properties, outstanding tailorability, excellent electrochemical stability, and damage reliability. Our approach to non-woven macrober mat wearable supercapacitors provides a novel strategy for the large-scale preparation of wearable supercapacitors with good electrochemical properties, outstanding tailorability, excellent electrochemical stability, and damage reliability.
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