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Flexible and low temperature resistant double network alkaline gel polymer electrolyte with dual-role KOH for supercapacitor
Journal of Power Sources 414 (2019) 201–209
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Flexible and low temperature resistant double network alkaline gel polymer electrolyte with dual-role KOH for supercapacitor
T
Xiaoyi Hua, Lidan Fanb, Gang Qina,∗, Zhongshuo Shena, Juan Chena, Mengxiao Wanga, Jia Yanga, Qiang Chena,∗∗ a b
School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003, China School of Civil Engineering, Henan Polytechnic University, Jiaozuo, 454003, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
playing two roles of ionic do• KOH nator and cross-linking agent in the AGPE.
tensile property and ionic • Outstanding conductivity of PVA/carrageenan AGPE.
properties of supercapacitor • Superior under deformation and low temperature.
A R T I C LE I N FO
A B S T R A C T
Keywords: Kappa-carrageenan Alkaline gel polymer electrolyte Flexible supercapacitor Low temperature resistant
A green physical route for producing novel multifunctional polyvinyl alcohol/kappa-carrageenan alkaline gel polymer electrolyte is developed, which is based on double-network physical cross-linked hydrogel. The alkaline gel polymer electrolyte has excellent tensile stress (2.22 MPa), stretchability (12.43 mm/mm) and ionic conductivity (0.21 S/cm). In this study, KOH is served as not only ionic donator but also cross-linking agent. A supercapacitor is assembled from polyvinyl alcohol/kappa-carrageenan alkaline gel polymer electrolyte and activated carbon electrodes, which exhibits ideal electrochemistry behavior. The electrode specific capacitance is 470 F/g at 0.5 A/g, and retains above 95% after 2000 charge/discharge cycles. Additionally, the electrochemical performance of supercapacitor under different tensile deformations (≤200%), bending angles (0–230°) and temperatures (≥-40 °C) displays a slightly reduction, indicating that the supercapacitor possesses good flexibility and low temperature resistance. These findings are desirable for applications in supercapacitor devices of flexibility or low working temperature.
1. Introduction Supercapacitor as one of hopeful electrochemical energy storage/ conversion devices has aroused great concerns, which exhibit many advantages such as rapid charging/discharging rate, environment-
∗
friendly feature, and long cycling life [1–7]. In supercapacitor, electrolyte plays a critical role, whose ionic conductivity and potential range can influence supercapacitor performance [3,8]. Gel polymer electrolyte (GPE), especially alkaline gel polymer electrolyte (AGPE), is deemed to be more promising because of electrode protection [9], no
Corresponding author. Correponding author. E-mail address: [email protected] (G. Qin).
∗∗
https://doi.org/10.1016/j.jpowsour.2019.01.006 Received 18 October 2018; Received in revised form 10 December 2018; Accepted 3 January 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
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leakage, flexibility, high ionic conductivity, stable electrochemical performances, and so on [10,11,13–16]. Among the various polymer matrixes for AGPE, polyvinyl alcohol (PVA) has been widely researched due to chemical stability, high conductivity, nontoxic, good filmforming property and low cost [8,12,17]. Recently, flexible, stretchable and wearable energy storage devices have gained attention from both public and research communities [4,11,17–25]. To meet the application requirements, the AGPE need to be strong and stretchable enough to support huge deformation. However, AGPEs suffer from the poor mechanical properties and there are few reports to improve them. Many works have been developed to improve toughness and strength of the hydrogels, such as nanocomposite [20], double network (DN) [21,25], interpenetrating polymer network [26], and slide-ring hydrogels [27]. Among these, DN hydrogels exhibit outstanding mechanical properties, which is correlated to their strong network entanglement. The DN hydrogel is an interpenetrating network of two different polymer networks with asymmetric structures: a rigid and brittle first network, and a soft and stretchable second network [28]. As acted on by a force, the first network provides a “sacrificial bond” to disperse external stress, meanwhile the second one provides elasticity to the hydrogel [28,29]. Therefore, we consider that the DN structure can be an effective method to improve the mechanical properties of PVA AGPE. Nowadays, many natural materials have been applied for hydrogel like agar [30–32], xanthan [33] and kappa-carrageenan (KC) [33–35]. KC are a family of linear sulfated polysaccharides extracted from red seaweeds (Fig. S1), and is hydrophilic, biocompatible, biodegradable, non-toxic, and gel-forming [33]. KC can be cross-linked through ionic association between K+ and double helix units formed at low temperature. In this study, we prepare a novel DN PVA/KC (PK) AGPE, which is composed of the first network of KC through K+ cross-linking [35], and the second network of PVA via the freezing-thawing cycle [30,36]. This PK AGPE is fully physical hydrogel and the system is clean without extra chemical cross-linking agent and initiator. In the PK AGPE, KOH plays two roles of ionic donator and cross-linking agent of KC, preventing the mechanical performance from weakening after the absorbing too much aqueous electrolyte to increase the conductivity. So, high tensile stress (2.22 MPa), stretchability (12.43 mm/mm) and good conductivity (0.21 S/cm) are obtained at the meantime. The supercapacitor is assembled using the PK AGPE as electrolyte and separator between two activated carbon electrodes, which shows high capacitance retention when PK10 AGPE is stretched and bended. Besides, the operating temperature of this supercapacitor can be varied over a very wide range (≥-40 °C), and even at −40 °C, the flexibility remains stable. Thus, the novel multifunctional PK AGPE has a broad prospect in the devices requiring flexibility or low working temperature.
2. Experimental section 2.1. Materials Polyvinyl alcohol (PVA 1799, molecular weight about 75000, alcoholysis: 99%) was purchased from Shanghai Aladdin Biochemical Technology Co., China. Kappa-carrageenan (KC) and KOH were obtained from TCI (Shanghai) Chemical Industry Development Co., China and Tianjin Mainland Chemical Reagent Plant, China, respectively. Activated carbon (AC) was provided by Shanghai Sino Tech Investment Management Co., China. Poly-tetrafluoroethylene (PTFE) aqueous solution (60 wt%) and acetylene black were provided by Heluelida Power Materials Co., China. Foam nickel was from Nanjing Huiyu Energy Co., China.
2.2. Preparation of alkaline gel polymer electrolyte The PK AGPE was prepared by freezing-thawing and soaking method. Firstly, PVA and KC were dissolved in 20 mL distilled water at 90 °C under continuous stirring to obtain a homogeneous solution. Then, the solution was poured into a mold followed the removing of the bubbles by ultrasonic oscillation. Afterwards, when cooling to room temperature (RT), the samples underwent the freezing/thawing cycle (freezing at −20 °C for 20 h followed by thawing at RT for 4 h). Then, the PK AGPE was soaked in KOH solution (1 M, 2 M, 3 M, 5 M, 7 M and 9 M) for 24 h. In this work, the total concentration of PVA and KC in water was kept 10 wt%, and PK AGPEs were designed as PKx, where x indicated the weight percentage of KC in the total amount of KC and PVA (e.g. PK10 stood for AGPE containing 10 wt% KC in total PVA and KC).
2.3. Preparation of activated carbon electrode and assembly of supercapacitor To prepare the electrode, the activated carbon (80 wt%), acetylene black (10 wt%) and binder PTFE aqueous solution (10 wt%) were fully mixed in ethanol to form a uniform slurry at RT. After ultrasonic vibration and stirring, the mixture was coated onto a foamed nickel sheet. After drying at 100 °C for 24 h, the electrode sheet was obtained by pressing with 20 MPa [3,37]. A self-made mold was used as Device 1 (Fig. 1a) to assemble supercapacitor, and a PK AGPE membrane was sandwiched between two activated carbon electrodes. We also prepared the Device 2 (Fig. 1b) without mold, to test the effects of tensile deformation, bending and low temperature on the electrochemical performance of supercapacitors. The AGPE played the role of electrolyte and separator in supercapacitor. For comparison, the supercapacitor with PVA AGPE was also assembled.
Fig. 1. Schematic representation of the supercapacitor (a) Device 1 and (b) Device 2. 202
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each other and form an ordered structure. Furtherly, the micro crystallite regions come into being and act as physical cross-link points. Some of the PVA segments are activated after thawing at RT, and then, they have the ability to rearrange in the next freezing process to improve crystallization, followed by the formation of gel [30,36]. Next, the gel is soaked in KOH solution and the KC network is acquired through ionic association between K+s and double helix units. Completely different from other chemically cross-linked and hybrid crosslinked DN gels, the fully physical DN gels are obtained through such ionic cross-linking and freezing-thawing method without the addition of initiator and cross-linking agent. The tensile tests were performed to quantitatively examine the mechanical properties of PK AGPE in response to the changes in weight ratio of KC. As shown in Fig. 3a and b, the mechanical properties of gels have strong dependence on KC content. When the KC content reaches 10 wt%, the tensile stress remarkably enhances from 1.1 MPa to 1.9 MPa, compared with pure PVA AGPE. These results reveal that the first network (KC network) contributes much more to the strength of the PK AGPE. Meanwhile, a high stretchability is achieved and the elongation at break reaches 14.8 mm/mm for the PK10 AGPE (Fig. 3b). However, the PVA and KC mixture solution becomes very viscous when the KC content exceeds 10 wt%, which may cause the inhomogeneity and a lot of tiny bubbles, so that the stress at break decreases. The conductivity and the water content exhibit similar rising trends with KC content increasing from 0 to 20 wt% as shown in Fig. 3c. The conductivity and the water content vary from 0.11 to 0.22 S/cm (Fig. S2) and from 54 to 64% according to Eq. (1) and Eq. (2). This outcome is possibly ascribed to the reasons: on the one hand, because KC has a certain hygroscopic property, the water content increases during soaking and more KOH solution is absorbed in the AGPE; on the other hand, the addition of KC can hinder the crystallization of PVA and enlarge the amorphous region as proved by Fig. 3d, thereby the number of the ion transport tunnels increase. As shown in XRD patterns, the peaks at 2θ = 19.4° and 41° belong to the crystalline peaks of PVA. However, the intensity of them dramatically decreases with increasing the KC content in the PK AGPE. It is difficult for pure PVA AGPE to enhance the conductivity and mechanical properties at the same time. However, as KC increases, the water content and conductivity get improved, but the mechanical properties don't deteriorate. On the contrary, because of cross-linking of K+ to KC, the PK AGPE forms the DN structure and enhances the mechanical properties after soaking in KOH solution. Besides, in a practical application, an electrolyte must have high conductivity, and sufficiently wide electrochemical stability window. It can be seen from Fig. S3 that these curves between −1.0 and + 1.0 V have almost no Faraday current response, indicating that the change of KC content has no obvious effect on the electrochemical window. The conductivity of PK10 AGPE reaches 0.19 S/cm which is lower than KOH solution electrolyte because the cross-linked network can hinder ion migration. However, this result is close to or even higher than many articles [38,39], which is high enough for practical applications (usually ≥ 10−3 S/cm [40]), at the same time, good mechanical properties are obtained. So, PK10 AGPE has been chosen as the optimum for the subsequent experiments. Fig. 4a shows the influence of the changes in KOH concentration on the mechanical properties of PK AGPE. PK hydrogel has poor mechanical properties (0.11 MPa, 2 mm/mm) without cross-linking of K+. However, the tensile stress increases from 0.25 to 2.22 MPa with the KOH concentration increasing from 1 to 7 M. Moreover, a high stretchability is obtained with the elongation at break of 12.43 mm/mm at the 7 M KOH (Fig. 4b). The results reveal that the immobilized K+ functions as cross-linking agent which greatly increases the crosslinking degree of KC and the mechanical properties of hydrogel. Besides, the conductivity of PK AGPEs was systematically investigated with various KOH concentrations as shown in Fig. 4c. With increasing KOH concentration from 1 to 9 M, the conductivity of PK
2.4. Characterization measurements The mechanical properties of AGPE were evaluated by a universal testing machine (Changchun Intelligent Instrument Equipment Co., Ltd. China) with the tensile speed of 100 mm/min. The AGPE was analyzed by Smart-lab X-ray diffractometer from Rigaku Corporation, Japan, with the scanning range of 10–80° and the scanning speed of 10°/min. The surface morphology and energy dispersive spectrometer (EDS) of PK AGPE membrane were investigated by a FESEM (FEI Quanta 250, Brock AG, German) at 15 kV. Before observation, the membrane samples were fractured in liquid nitrogen, freeze-dried and sputtered with gold. The water content could be estimated using the following equation: water content (%)= (wt-wd)/wt × 100%
(1)
where Wt is the wet weight of membranes before drying, and Wd is the dry weight of membranes after drying in a vacuum oven at 60 °C. The measurement of the bulk resistance of PK AGPE was carried out by electrochemical impedance spectroscopy (EIS) using an electrochemical workstation (Parstat2273, Princeton Applied Research Co., USA). The AGPE was sandwiched by two stainless steels (SS) with the mode of SS/PK/SS in the frequency range between 0.1 Hz and 100 kHz with a perturbation of 5 mV/rms. The electrochemical stability window was determined using the cyclic voltammetry (CV) method and was also measured by electrochemical workstation in the potential of −1.5 V to 1.5 V at a scan rate of 10 mV/s. Ionic conductivity was calculated according to the following equation: σ = L/(A × Rb)
(2)
where L, A and Rb are the thickness, area, and bulk resistance, respectively. 2.5. Performance evaluation of supercapacitor EIS measurement of supercapacitor was conducted in the frequency range from 0.1 Hz to 100 kHz. Galvanostatic charge/discharge cycling was tested using a battery test instrument (CT 2001A, LAND Technology Co. Ltd., China) with different current densities in the potential range of 0.09–0.9 V. The CV of the supercapacitor was performed at various scan rates of 5–100 mV/s in the potential range of 0.09–0.9 V. The supercapacitor specific capacitance (C, F/g) and electrode specific capacitance (Cs, F/g) could be calculated from charge/ discharge curve as follows [3,15,37]: C=(I × △t) /(△V × m)
(3)
CS = 4 × C
(4)
where I is discharge current, △t is the discharge time, △V is discharge voltage range, and m denotes the mass of the activated carbon in two electrodes. All electrochemical measurements of supercapacitor were carried out using two-electrode system. 3. Results and discussion The natural materials are gaining increasing interest due to the high biocompatibility and biodegradability without toxic by-products. In this present study, PK AGPE was prepared by a green and simple method using water as solvent, and the procedure of preparation of the physically cross-linked gel is shown in Fig. 2 schematically. The PK AGPE is synthesized via a sequential network formation technique. In the initial step, KC and PVA are dissolved together in hot water, and the KC forms helix bundles during cooling to RT [35]. The PVA network is achieved by the freezing-thawing method via the formation of large numbers of micro crystallites [30]. The molecules of the PVA solution is “frozen” in the freezing process, which allow the molecular chains to contact with 203
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Fig. 2. Schematic illustrations of hydrogel network structures of PK AGPE.
Fig. 3. (a) Tensile stress-strain curves of PK AGPE containing various KC contents. Effect of KC content on the (b) tensile stress and elongation at break, (c) ionic conductivity (σ) and water content, (d) XRD patterns of PK AGPE containing various KC contents. (RT, three freezing-thawing cycles, KOH 5 M).
pore walls and cobweb-like fiber networks, indicating that the addition of KC has an obvious effect on the structure. The rigid KC molecular chain can change the heterogeneity of the PVA molecular chain network, and the two networks form a fine double network structure that greatly improves the mechanical properties. Due to such structure, water uptake improves and more KOH electrolyte can be absorbed and stored in AGPE, which is beneficial to the ionic conductivity. According to the EDS for sulfur element shown in Fig. S5, sulfur element uniformly distributes throughout the membrane, suggesting the homogeneous blending at the EDS scale of KC and PVA. Electrochemical impedance measurements were performed in order to observe the resistivity behavior of the supercapacitor fabricated by PVA and PK10 AGPE as shown in Fig. 6a. Nyquist plots show that the ohmic resistance [7] (Rs 0.41 Ω) of supercapacitor using PK10 AGPE is much lower than that of the PVA AGPE (Rs 0.68 Ω) due to the higher
AGPE gradually enhances from 0.11 to 0.23 S/cm (calculated based on Fig. S4). The high ionic conductivity stems mainly from the contribution of excess KOH diffusion into the system, which results in the higher concentration of conductive ions in the membrane [41]. Considering both the mechanical (2.22 MPa) and electrical performance (0.21 S/ cm), the optimal KOH concentration is determined as 7 M for the following study. As shown in Fig. 4d–f, PK10 AGPE exhibits extraordinary mechanical properties, which can withstand high-level deformations of knotting (Fig. 4d), compression (Fig. 4e), and sustaining a 500 g weight (Fig. 4f) without any observable damage. From SEM images, it can be observed that the PVA AGPE and PK10 AGPE both represent porous structures (Fig. 5a and b), nevertheless, with significant differences. It can be clearly seen from Fig. 5a that PVA AGPE exhibits smooth and thick pore walls. Compared to pure PVA AGPE, PK10 AGPE possesses more irregular morphology with thinner 204
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Fig. 4. (a) Typical stress-strain curves of PK10 AGPE. Effect of KOH concentration on the (b) tensile stress and elongation at break, and (c) ionic conductivity (σ). (RT, three freezing-thawing cycles), PK10 AGPE is highly tough and flexible with the ability to withstand different deformations of (d) knotting, (e) compression and (f) sustaining a 500 g weight.
the current density becomes larger. However, the electrode specific capacitance can still achieve to 430 F/g with current density of 2.5 A/g, which remains approximate to 85% of the highest electrode specific capacitance (518 F/g) at 0.2 A/g. It is indicated that the PK10 AGPE supercapacitor has good electrochemical stability under high current density. Fig. 6f presents the cyclic voltammetry curves of the PK10 AGPE supercapacitor at various scan rates. As increasing of scan rates from 5 to 100 mV/s, the cyclic voltammetry curves remain close to the standard rectangular, confirming an excellent stability. The cycling stability is an important property for the practical application of supercapacitor, which is carried out with the galvanostatic charge/discharge measurement from 0.09 to 0.9 V at 1 A/g for 2000 cycles. Fig. 6g illustrates that the PK10 AGPE supercapacitor retains above 95% of its capacitance after 2000 charge/discharge cycles, suggesting a good cycling stability. Based on the above, it can be concluded that the PK10 AGPE could be a hopeful candidate for supercapacitor. In order to check the influence of tensile deformation on electrochemical properties, the galvanostatic charge/discharge measurement and the capacitance retention rate of PK10 AGPE supercapacitor were evaluated at various stretching state of PK10 AGPE (0, 100, 150, 200, 400 and 600%). Fig. 7a presents that the PK10 AGPE supercapacitor shows similar galvanostatic charge/discharge curve shape and iRdrop under small stretching state (≤200%), and about 93% of its initial capacitance (88.6 F/g, Fig. S6) is retained under 200% stretching state (Fig. 7b). The result confirms outstanding capacitance performances and highly stable mechanical properties of PK10 AGPE. Further stretching (≥400%) causes an obvious increase in iRdrop, which leads to reduction of capacitance. The electrochemical performance in the bending test at different angles was measured. Fig. 7c–g show that Nyquist plots of five bending states from 0° to 230° present an inconspicuous enlargement of resistance. The almost overlap cyclic voltammetry curves for the PK10 AGPE supercapacitor at different bending angles are obtained from Fig. 7h, presenting the approximate rectangular cyclic voltammetry responses and rectangular area. This points out that the bending has almost no influence over the cyclic voltammetry responses. There are the as-measured galvanostatic charge/discharge curves in Fig. 7i, which show a negligible change at different bending angles. Under flat and bending at 230°, the electrode specific capacitance are 88.6 F/g and 88.2 F/g, respectively (Fig. S7), which are nearly equal, revealing good
Fig. 5. SEM images of (a) PVA and (b) PK10 AGPE.
ionic conductivity of PK10 AGPE. The capacitance behavior of two supercapacitors was investigated by galvanostatic charge/discharge and cyclic voltammetry. Fig. 6b presents the charge/discharge curves of supercapacitor fabricated by PVA and PK10 AGPE at constant current of 0.5 A/g. It is seen that, according to Eq. (3) and Eq. (4), electrode specific capacitance of the PK10 AGPE supercapacitor (470 F/g) is higher than that of the PVA AGPE supercapacitor (360 F/g). Moreover, the PK10 AGPE supercapacitor shows a smaller internal resistance drop (iRdrop) compared to the PVA AGPE supercapacitor, which is consistent with Fig. 6a. Cyclic voltammetry curves of the supercapacitors with PVA and PK10 AGPE at scan rate of 10 mV/s in the potential window of 0.09–0.9 V are presented in Fig. 6c. The two curves show rectangles without any obvious redox peaks, which implies that the accumulation of charge mainly occurs between the electrode and the electrolyte, and a typical doublelayer capacitance performance is presented. Furthermore, the PK10 AGPE supercapacitor has higher capacitance, which is in accord with the charge and discharge results. The galvanostatic charge/discharge curves of PK10 AGPE supercapacitor under current densities from 0.2 A/g to 2.5 A/g (Fig. 6d) are provided, which is a reliable method to evaluate the electrochemical capacitance of materials. In general, the shape of the charge/discharge curves is triangle form with a quite small iRdrop, indicating that the supercapacitor has excellent rate capability. The electrode specific capacitances at various current densities are plotted in Fig. 6e. It is evident that the capacitance of the supercapacitor slowly decreases when 205
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Fig. 6. Comparison of (a) Nyquist plots, (b) galvanostatic charge/discharge curves at 0.5 A/g, (c) cyclic voltammetry curves at 10 mV/s of PVA AGPE supercapacitor and PK10 AGPE supercapacitor, (d) galvanostatic charge/discharge curves and (e) electrode specific capacitances of PK10 AGPE supercapacitor at various current densities, (f) cyclic voltammetry curves of PK10 AGPE supercapacitor at various scan rates, (g) capacitance retention rate at 1 A/g as a function of charge/discharge cycles of PK10 AGPE supercapacitor. (Measured with Device1).
angle stability and ideal flexibility of the PK10 AGPE supercapacitor. Meanwhile, the cycling stability test of PK10 AGPE supercapacitor was carried out for 100 times repeatedly bending at 90° and extending (Fig. 7j). The capacitance retention keeps over 90%, suggesting that the
supercapacitor has a good bending stability and can endure mechanical bending without significant loss of electrode specific capacitance. This loss may be due to incomplete contact between the electrolyte and the electrode, or the separation of activated carbon from nickel foam sheet 206
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Fig. 7. (a) Galvanostatic charge/discharge curves and (b) the capacitance retention rate of PK10 AGPE supercapacitor at various stretching states at 0.2 A/g, comparison of (c-g) Nyquist plots, (h) galvanostatic charge/discharge curves at 0.2 A/g, (i) cyclic voltammetry at 20 mV/s of PK10 AGPE supercapacitor at different angles, (j) capacitance retention rate of PK10 AGPE supercapacitor at 0.2 A/g after 100 bending cycles (bending angle = 90°). (Measured with Device 2).
verify the practical application of the supercapacitor, the tandem device can light up a red LED (the lowest working potential is about 2V) (Fig. 8g).
during the bending process [42]. Considering its practical application in cold climates, the electrochemical property of the supercapacitor was also examined under low temperatures. Fig. 8a displays that the Rs of PK10 AGPE supercapacitor from RT to −40 °C presents a slightly upward trend, which is caused by slowing the migration of ions of the electrolyte between the two electrodes with decreasing of temperature. However, even at −40 °C, the supercapacitor still shows a relatively low Rs (0.34 Ω). This may be because: on the one hand, exceed KOH forms hydrated ions, which lower the freezing point of PK10 AGPE; on the other hand, the network structure of the PK10 AGPE chains disrupts the crystalline growth of ice [43]. With the decrease in temperature, the area enveloped by the cyclic voltammetry curves reduces at 20 mV/s (Fig. 8b), because the incremental resistance to ionic transport causes the shape of the cycle to deviate from a quasi-rectangle. The galvanostatic charge/discharge curves of PK10 AGPE supercapacitor at 0.2 A/g behave as nearly triangular shapes shown in Fig. 8c, which means an almost ideal capacitance behavior within this wide temperature range. At low temperature, the charge/discharge time reduces and iRdrop enlarges gradually, which matches well with the Nyquist plots. The electrode specific capacitance (Fig. 8d) diminishes slightly with the temperature going down, and its electrode specific capacitance is 89 F/g (Fig. S8) at −40 °C, which is 83% of that measured at RT (107 F/g). Besides, the supercapacitor is still flexible at −40 °C (Fig. 8e). In addition, to light a red LED, the potential window was expanded from 0.9 V for a single device to 2.7 V for three devices in series. The tandem device was evaluated by galvanostatic charge/discharge measurements as shown in Fig. 8f. It can be seen that the charge/discharge time is almost unchanged, revealing consistent performances of the connected devices and the ideal capacitance behaviors. Lastly, to better
4. Conclusions In summary, we have developed a nontoxic and simple method to design strong, stretchable and high conductive PVA and KC composite AGPE. The DN network is proved a valid approach to enhance the mechanical properties of AGPE. The PK DN network AGPE demonstrates ideal mechanical properties as well as electrochemical properties. As a typical case, high tensile stress of 2.22 MPa, large deformation of 12.47 mm/mm and good ionic conductivity of 0.21 S/cm are obtained for the PK10 AGPE. Both the stretchable and electrochemical properties of PK10 AGPE are expected to be significant for practical applications, and a solidstate supercapacitor has been assembled. The superior electrochemical performances are displayed, such as, low Rs (0.41 Ω), and high electrode specific capacitance (470 F/g) at constant current of 0.5 A/g and favorable cycling stability. Furthermore, when the PK10 AGPE is highly stretched and bended, the capacitance retention of the supercapacitor still maintains above 90%. Besides, the capacitance retention as well as flexibility remains stable even at −40 °C. Thus, the novel multifunctional PK AGPE can be utilized in wide application such as the equipment of flexibility or low working temperature. Acknowledgments G. Q. and Q. C. are grateful for financial support from National Nature Science Foundation of China (51403056, 21504022), Henan Province (144300510041, 2017GGJS049, NSFRF1605, 2016GGJS-039, 207
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Fig. 8. Electrochemical performances of the supercapacitor at low temperature, (a) Nyquist plots, (b) cyclic voltammetry curves at 20 mV/s, (c) galvanostatic charge/ discharge curves at 0.2 A/g, (d) capacitance retention rate at 0.2 A/g at different temperatures and (e) photograph of PK10 AGPE supercapacitor at −40 °C, (f) Galvanostatic charge/discharge curves of single device and three devices in series of PK10 AGPE supercapacitor (0.1 A/g) and (g) photograph of an LED lit up by the PK10 AGPE supercapacitors. (Measured with Device 2).
17HASTIT006), Foundation of Henan Educational Committee (16B430004), Henan Polytechnic University (B2012-052, 72105/001 and 672517/005), Young Backbone Teachers Program of Henan Polytechnic University (2016XQG-07).
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Flexible and low temperature resistant double network alkaline gel polymer electrolyte with dual-role KOH for supercapacitor
T
Xiaoyi Hua, Lidan Fanb, Gang Qina,∗, Zhongshuo Shena, Juan Chena, Mengxiao Wanga, Jia Yanga, Qiang Chena,∗∗ a b
School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003, China School of Civil Engineering, Henan Polytechnic University, Jiaozuo, 454003, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
playing two roles of ionic do• KOH nator and cross-linking agent in the AGPE.
tensile property and ionic • Outstanding conductivity of PVA/carrageenan AGPE.
properties of supercapacitor • Superior under deformation and low temperature.
A R T I C LE I N FO
A B S T R A C T
Keywords: Kappa-carrageenan Alkaline gel polymer electrolyte Flexible supercapacitor Low temperature resistant
A green physical route for producing novel multifunctional polyvinyl alcohol/kappa-carrageenan alkaline gel polymer electrolyte is developed, which is based on double-network physical cross-linked hydrogel. The alkaline gel polymer electrolyte has excellent tensile stress (2.22 MPa), stretchability (12.43 mm/mm) and ionic conductivity (0.21 S/cm). In this study, KOH is served as not only ionic donator but also cross-linking agent. A supercapacitor is assembled from polyvinyl alcohol/kappa-carrageenan alkaline gel polymer electrolyte and activated carbon electrodes, which exhibits ideal electrochemistry behavior. The electrode specific capacitance is 470 F/g at 0.5 A/g, and retains above 95% after 2000 charge/discharge cycles. Additionally, the electrochemical performance of supercapacitor under different tensile deformations (≤200%), bending angles (0–230°) and temperatures (≥-40 °C) displays a slightly reduction, indicating that the supercapacitor possesses good flexibility and low temperature resistance. These findings are desirable for applications in supercapacitor devices of flexibility or low working temperature.
1. Introduction Supercapacitor as one of hopeful electrochemical energy storage/ conversion devices has aroused great concerns, which exhibit many advantages such as rapid charging/discharging rate, environment-
∗
friendly feature, and long cycling life [1–7]. In supercapacitor, electrolyte plays a critical role, whose ionic conductivity and potential range can influence supercapacitor performance [3,8]. Gel polymer electrolyte (GPE), especially alkaline gel polymer electrolyte (AGPE), is deemed to be more promising because of electrode protection [9], no
Corresponding author. Correponding author. E-mail address: [email protected] (G. Qin).
∗∗
https://doi.org/10.1016/j.jpowsour.2019.01.006 Received 18 October 2018; Received in revised form 10 December 2018; Accepted 3 January 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
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leakage, flexibility, high ionic conductivity, stable electrochemical performances, and so on [10,11,13–16]. Among the various polymer matrixes for AGPE, polyvinyl alcohol (PVA) has been widely researched due to chemical stability, high conductivity, nontoxic, good filmforming property and low cost [8,12,17]. Recently, flexible, stretchable and wearable energy storage devices have gained attention from both public and research communities [4,11,17–25]. To meet the application requirements, the AGPE need to be strong and stretchable enough to support huge deformation. However, AGPEs suffer from the poor mechanical properties and there are few reports to improve them. Many works have been developed to improve toughness and strength of the hydrogels, such as nanocomposite [20], double network (DN) [21,25], interpenetrating polymer network [26], and slide-ring hydrogels [27]. Among these, DN hydrogels exhibit outstanding mechanical properties, which is correlated to their strong network entanglement. The DN hydrogel is an interpenetrating network of two different polymer networks with asymmetric structures: a rigid and brittle first network, and a soft and stretchable second network [28]. As acted on by a force, the first network provides a “sacrificial bond” to disperse external stress, meanwhile the second one provides elasticity to the hydrogel [28,29]. Therefore, we consider that the DN structure can be an effective method to improve the mechanical properties of PVA AGPE. Nowadays, many natural materials have been applied for hydrogel like agar [30–32], xanthan [33] and kappa-carrageenan (KC) [33–35]. KC are a family of linear sulfated polysaccharides extracted from red seaweeds (Fig. S1), and is hydrophilic, biocompatible, biodegradable, non-toxic, and gel-forming [33]. KC can be cross-linked through ionic association between K+ and double helix units formed at low temperature. In this study, we prepare a novel DN PVA/KC (PK) AGPE, which is composed of the first network of KC through K+ cross-linking [35], and the second network of PVA via the freezing-thawing cycle [30,36]. This PK AGPE is fully physical hydrogel and the system is clean without extra chemical cross-linking agent and initiator. In the PK AGPE, KOH plays two roles of ionic donator and cross-linking agent of KC, preventing the mechanical performance from weakening after the absorbing too much aqueous electrolyte to increase the conductivity. So, high tensile stress (2.22 MPa), stretchability (12.43 mm/mm) and good conductivity (0.21 S/cm) are obtained at the meantime. The supercapacitor is assembled using the PK AGPE as electrolyte and separator between two activated carbon electrodes, which shows high capacitance retention when PK10 AGPE is stretched and bended. Besides, the operating temperature of this supercapacitor can be varied over a very wide range (≥-40 °C), and even at −40 °C, the flexibility remains stable. Thus, the novel multifunctional PK AGPE has a broad prospect in the devices requiring flexibility or low working temperature.
2. Experimental section 2.1. Materials Polyvinyl alcohol (PVA 1799, molecular weight about 75000, alcoholysis: 99%) was purchased from Shanghai Aladdin Biochemical Technology Co., China. Kappa-carrageenan (KC) and KOH were obtained from TCI (Shanghai) Chemical Industry Development Co., China and Tianjin Mainland Chemical Reagent Plant, China, respectively. Activated carbon (AC) was provided by Shanghai Sino Tech Investment Management Co., China. Poly-tetrafluoroethylene (PTFE) aqueous solution (60 wt%) and acetylene black were provided by Heluelida Power Materials Co., China. Foam nickel was from Nanjing Huiyu Energy Co., China.
2.2. Preparation of alkaline gel polymer electrolyte The PK AGPE was prepared by freezing-thawing and soaking method. Firstly, PVA and KC were dissolved in 20 mL distilled water at 90 °C under continuous stirring to obtain a homogeneous solution. Then, the solution was poured into a mold followed the removing of the bubbles by ultrasonic oscillation. Afterwards, when cooling to room temperature (RT), the samples underwent the freezing/thawing cycle (freezing at −20 °C for 20 h followed by thawing at RT for 4 h). Then, the PK AGPE was soaked in KOH solution (1 M, 2 M, 3 M, 5 M, 7 M and 9 M) for 24 h. In this work, the total concentration of PVA and KC in water was kept 10 wt%, and PK AGPEs were designed as PKx, where x indicated the weight percentage of KC in the total amount of KC and PVA (e.g. PK10 stood for AGPE containing 10 wt% KC in total PVA and KC).
2.3. Preparation of activated carbon electrode and assembly of supercapacitor To prepare the electrode, the activated carbon (80 wt%), acetylene black (10 wt%) and binder PTFE aqueous solution (10 wt%) were fully mixed in ethanol to form a uniform slurry at RT. After ultrasonic vibration and stirring, the mixture was coated onto a foamed nickel sheet. After drying at 100 °C for 24 h, the electrode sheet was obtained by pressing with 20 MPa [3,37]. A self-made mold was used as Device 1 (Fig. 1a) to assemble supercapacitor, and a PK AGPE membrane was sandwiched between two activated carbon electrodes. We also prepared the Device 2 (Fig. 1b) without mold, to test the effects of tensile deformation, bending and low temperature on the electrochemical performance of supercapacitors. The AGPE played the role of electrolyte and separator in supercapacitor. For comparison, the supercapacitor with PVA AGPE was also assembled.
Fig. 1. Schematic representation of the supercapacitor (a) Device 1 and (b) Device 2. 202
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each other and form an ordered structure. Furtherly, the micro crystallite regions come into being and act as physical cross-link points. Some of the PVA segments are activated after thawing at RT, and then, they have the ability to rearrange in the next freezing process to improve crystallization, followed by the formation of gel [30,36]. Next, the gel is soaked in KOH solution and the KC network is acquired through ionic association between K+s and double helix units. Completely different from other chemically cross-linked and hybrid crosslinked DN gels, the fully physical DN gels are obtained through such ionic cross-linking and freezing-thawing method without the addition of initiator and cross-linking agent. The tensile tests were performed to quantitatively examine the mechanical properties of PK AGPE in response to the changes in weight ratio of KC. As shown in Fig. 3a and b, the mechanical properties of gels have strong dependence on KC content. When the KC content reaches 10 wt%, the tensile stress remarkably enhances from 1.1 MPa to 1.9 MPa, compared with pure PVA AGPE. These results reveal that the first network (KC network) contributes much more to the strength of the PK AGPE. Meanwhile, a high stretchability is achieved and the elongation at break reaches 14.8 mm/mm for the PK10 AGPE (Fig. 3b). However, the PVA and KC mixture solution becomes very viscous when the KC content exceeds 10 wt%, which may cause the inhomogeneity and a lot of tiny bubbles, so that the stress at break decreases. The conductivity and the water content exhibit similar rising trends with KC content increasing from 0 to 20 wt% as shown in Fig. 3c. The conductivity and the water content vary from 0.11 to 0.22 S/cm (Fig. S2) and from 54 to 64% according to Eq. (1) and Eq. (2). This outcome is possibly ascribed to the reasons: on the one hand, because KC has a certain hygroscopic property, the water content increases during soaking and more KOH solution is absorbed in the AGPE; on the other hand, the addition of KC can hinder the crystallization of PVA and enlarge the amorphous region as proved by Fig. 3d, thereby the number of the ion transport tunnels increase. As shown in XRD patterns, the peaks at 2θ = 19.4° and 41° belong to the crystalline peaks of PVA. However, the intensity of them dramatically decreases with increasing the KC content in the PK AGPE. It is difficult for pure PVA AGPE to enhance the conductivity and mechanical properties at the same time. However, as KC increases, the water content and conductivity get improved, but the mechanical properties don't deteriorate. On the contrary, because of cross-linking of K+ to KC, the PK AGPE forms the DN structure and enhances the mechanical properties after soaking in KOH solution. Besides, in a practical application, an electrolyte must have high conductivity, and sufficiently wide electrochemical stability window. It can be seen from Fig. S3 that these curves between −1.0 and + 1.0 V have almost no Faraday current response, indicating that the change of KC content has no obvious effect on the electrochemical window. The conductivity of PK10 AGPE reaches 0.19 S/cm which is lower than KOH solution electrolyte because the cross-linked network can hinder ion migration. However, this result is close to or even higher than many articles [38,39], which is high enough for practical applications (usually ≥ 10−3 S/cm [40]), at the same time, good mechanical properties are obtained. So, PK10 AGPE has been chosen as the optimum for the subsequent experiments. Fig. 4a shows the influence of the changes in KOH concentration on the mechanical properties of PK AGPE. PK hydrogel has poor mechanical properties (0.11 MPa, 2 mm/mm) without cross-linking of K+. However, the tensile stress increases from 0.25 to 2.22 MPa with the KOH concentration increasing from 1 to 7 M. Moreover, a high stretchability is obtained with the elongation at break of 12.43 mm/mm at the 7 M KOH (Fig. 4b). The results reveal that the immobilized K+ functions as cross-linking agent which greatly increases the crosslinking degree of KC and the mechanical properties of hydrogel. Besides, the conductivity of PK AGPEs was systematically investigated with various KOH concentrations as shown in Fig. 4c. With increasing KOH concentration from 1 to 9 M, the conductivity of PK
2.4. Characterization measurements The mechanical properties of AGPE were evaluated by a universal testing machine (Changchun Intelligent Instrument Equipment Co., Ltd. China) with the tensile speed of 100 mm/min. The AGPE was analyzed by Smart-lab X-ray diffractometer from Rigaku Corporation, Japan, with the scanning range of 10–80° and the scanning speed of 10°/min. The surface morphology and energy dispersive spectrometer (EDS) of PK AGPE membrane were investigated by a FESEM (FEI Quanta 250, Brock AG, German) at 15 kV. Before observation, the membrane samples were fractured in liquid nitrogen, freeze-dried and sputtered with gold. The water content could be estimated using the following equation: water content (%)= (wt-wd)/wt × 100%
(1)
where Wt is the wet weight of membranes before drying, and Wd is the dry weight of membranes after drying in a vacuum oven at 60 °C. The measurement of the bulk resistance of PK AGPE was carried out by electrochemical impedance spectroscopy (EIS) using an electrochemical workstation (Parstat2273, Princeton Applied Research Co., USA). The AGPE was sandwiched by two stainless steels (SS) with the mode of SS/PK/SS in the frequency range between 0.1 Hz and 100 kHz with a perturbation of 5 mV/rms. The electrochemical stability window was determined using the cyclic voltammetry (CV) method and was also measured by electrochemical workstation in the potential of −1.5 V to 1.5 V at a scan rate of 10 mV/s. Ionic conductivity was calculated according to the following equation: σ = L/(A × Rb)
(2)
where L, A and Rb are the thickness, area, and bulk resistance, respectively. 2.5. Performance evaluation of supercapacitor EIS measurement of supercapacitor was conducted in the frequency range from 0.1 Hz to 100 kHz. Galvanostatic charge/discharge cycling was tested using a battery test instrument (CT 2001A, LAND Technology Co. Ltd., China) with different current densities in the potential range of 0.09–0.9 V. The CV of the supercapacitor was performed at various scan rates of 5–100 mV/s in the potential range of 0.09–0.9 V. The supercapacitor specific capacitance (C, F/g) and electrode specific capacitance (Cs, F/g) could be calculated from charge/ discharge curve as follows [3,15,37]: C=(I × △t) /(△V × m)
(3)
CS = 4 × C
(4)
where I is discharge current, △t is the discharge time, △V is discharge voltage range, and m denotes the mass of the activated carbon in two electrodes. All electrochemical measurements of supercapacitor were carried out using two-electrode system. 3. Results and discussion The natural materials are gaining increasing interest due to the high biocompatibility and biodegradability without toxic by-products. In this present study, PK AGPE was prepared by a green and simple method using water as solvent, and the procedure of preparation of the physically cross-linked gel is shown in Fig. 2 schematically. The PK AGPE is synthesized via a sequential network formation technique. In the initial step, KC and PVA are dissolved together in hot water, and the KC forms helix bundles during cooling to RT [35]. The PVA network is achieved by the freezing-thawing method via the formation of large numbers of micro crystallites [30]. The molecules of the PVA solution is “frozen” in the freezing process, which allow the molecular chains to contact with 203
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Fig. 2. Schematic illustrations of hydrogel network structures of PK AGPE.
Fig. 3. (a) Tensile stress-strain curves of PK AGPE containing various KC contents. Effect of KC content on the (b) tensile stress and elongation at break, (c) ionic conductivity (σ) and water content, (d) XRD patterns of PK AGPE containing various KC contents. (RT, three freezing-thawing cycles, KOH 5 M).
pore walls and cobweb-like fiber networks, indicating that the addition of KC has an obvious effect on the structure. The rigid KC molecular chain can change the heterogeneity of the PVA molecular chain network, and the two networks form a fine double network structure that greatly improves the mechanical properties. Due to such structure, water uptake improves and more KOH electrolyte can be absorbed and stored in AGPE, which is beneficial to the ionic conductivity. According to the EDS for sulfur element shown in Fig. S5, sulfur element uniformly distributes throughout the membrane, suggesting the homogeneous blending at the EDS scale of KC and PVA. Electrochemical impedance measurements were performed in order to observe the resistivity behavior of the supercapacitor fabricated by PVA and PK10 AGPE as shown in Fig. 6a. Nyquist plots show that the ohmic resistance [7] (Rs 0.41 Ω) of supercapacitor using PK10 AGPE is much lower than that of the PVA AGPE (Rs 0.68 Ω) due to the higher
AGPE gradually enhances from 0.11 to 0.23 S/cm (calculated based on Fig. S4). The high ionic conductivity stems mainly from the contribution of excess KOH diffusion into the system, which results in the higher concentration of conductive ions in the membrane [41]. Considering both the mechanical (2.22 MPa) and electrical performance (0.21 S/ cm), the optimal KOH concentration is determined as 7 M for the following study. As shown in Fig. 4d–f, PK10 AGPE exhibits extraordinary mechanical properties, which can withstand high-level deformations of knotting (Fig. 4d), compression (Fig. 4e), and sustaining a 500 g weight (Fig. 4f) without any observable damage. From SEM images, it can be observed that the PVA AGPE and PK10 AGPE both represent porous structures (Fig. 5a and b), nevertheless, with significant differences. It can be clearly seen from Fig. 5a that PVA AGPE exhibits smooth and thick pore walls. Compared to pure PVA AGPE, PK10 AGPE possesses more irregular morphology with thinner 204
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Fig. 4. (a) Typical stress-strain curves of PK10 AGPE. Effect of KOH concentration on the (b) tensile stress and elongation at break, and (c) ionic conductivity (σ). (RT, three freezing-thawing cycles), PK10 AGPE is highly tough and flexible with the ability to withstand different deformations of (d) knotting, (e) compression and (f) sustaining a 500 g weight.
the current density becomes larger. However, the electrode specific capacitance can still achieve to 430 F/g with current density of 2.5 A/g, which remains approximate to 85% of the highest electrode specific capacitance (518 F/g) at 0.2 A/g. It is indicated that the PK10 AGPE supercapacitor has good electrochemical stability under high current density. Fig. 6f presents the cyclic voltammetry curves of the PK10 AGPE supercapacitor at various scan rates. As increasing of scan rates from 5 to 100 mV/s, the cyclic voltammetry curves remain close to the standard rectangular, confirming an excellent stability. The cycling stability is an important property for the practical application of supercapacitor, which is carried out with the galvanostatic charge/discharge measurement from 0.09 to 0.9 V at 1 A/g for 2000 cycles. Fig. 6g illustrates that the PK10 AGPE supercapacitor retains above 95% of its capacitance after 2000 charge/discharge cycles, suggesting a good cycling stability. Based on the above, it can be concluded that the PK10 AGPE could be a hopeful candidate for supercapacitor. In order to check the influence of tensile deformation on electrochemical properties, the galvanostatic charge/discharge measurement and the capacitance retention rate of PK10 AGPE supercapacitor were evaluated at various stretching state of PK10 AGPE (0, 100, 150, 200, 400 and 600%). Fig. 7a presents that the PK10 AGPE supercapacitor shows similar galvanostatic charge/discharge curve shape and iRdrop under small stretching state (≤200%), and about 93% of its initial capacitance (88.6 F/g, Fig. S6) is retained under 200% stretching state (Fig. 7b). The result confirms outstanding capacitance performances and highly stable mechanical properties of PK10 AGPE. Further stretching (≥400%) causes an obvious increase in iRdrop, which leads to reduction of capacitance. The electrochemical performance in the bending test at different angles was measured. Fig. 7c–g show that Nyquist plots of five bending states from 0° to 230° present an inconspicuous enlargement of resistance. The almost overlap cyclic voltammetry curves for the PK10 AGPE supercapacitor at different bending angles are obtained from Fig. 7h, presenting the approximate rectangular cyclic voltammetry responses and rectangular area. This points out that the bending has almost no influence over the cyclic voltammetry responses. There are the as-measured galvanostatic charge/discharge curves in Fig. 7i, which show a negligible change at different bending angles. Under flat and bending at 230°, the electrode specific capacitance are 88.6 F/g and 88.2 F/g, respectively (Fig. S7), which are nearly equal, revealing good
Fig. 5. SEM images of (a) PVA and (b) PK10 AGPE.
ionic conductivity of PK10 AGPE. The capacitance behavior of two supercapacitors was investigated by galvanostatic charge/discharge and cyclic voltammetry. Fig. 6b presents the charge/discharge curves of supercapacitor fabricated by PVA and PK10 AGPE at constant current of 0.5 A/g. It is seen that, according to Eq. (3) and Eq. (4), electrode specific capacitance of the PK10 AGPE supercapacitor (470 F/g) is higher than that of the PVA AGPE supercapacitor (360 F/g). Moreover, the PK10 AGPE supercapacitor shows a smaller internal resistance drop (iRdrop) compared to the PVA AGPE supercapacitor, which is consistent with Fig. 6a. Cyclic voltammetry curves of the supercapacitors with PVA and PK10 AGPE at scan rate of 10 mV/s in the potential window of 0.09–0.9 V are presented in Fig. 6c. The two curves show rectangles without any obvious redox peaks, which implies that the accumulation of charge mainly occurs between the electrode and the electrolyte, and a typical doublelayer capacitance performance is presented. Furthermore, the PK10 AGPE supercapacitor has higher capacitance, which is in accord with the charge and discharge results. The galvanostatic charge/discharge curves of PK10 AGPE supercapacitor under current densities from 0.2 A/g to 2.5 A/g (Fig. 6d) are provided, which is a reliable method to evaluate the electrochemical capacitance of materials. In general, the shape of the charge/discharge curves is triangle form with a quite small iRdrop, indicating that the supercapacitor has excellent rate capability. The electrode specific capacitances at various current densities are plotted in Fig. 6e. It is evident that the capacitance of the supercapacitor slowly decreases when 205
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Fig. 6. Comparison of (a) Nyquist plots, (b) galvanostatic charge/discharge curves at 0.5 A/g, (c) cyclic voltammetry curves at 10 mV/s of PVA AGPE supercapacitor and PK10 AGPE supercapacitor, (d) galvanostatic charge/discharge curves and (e) electrode specific capacitances of PK10 AGPE supercapacitor at various current densities, (f) cyclic voltammetry curves of PK10 AGPE supercapacitor at various scan rates, (g) capacitance retention rate at 1 A/g as a function of charge/discharge cycles of PK10 AGPE supercapacitor. (Measured with Device1).
angle stability and ideal flexibility of the PK10 AGPE supercapacitor. Meanwhile, the cycling stability test of PK10 AGPE supercapacitor was carried out for 100 times repeatedly bending at 90° and extending (Fig. 7j). The capacitance retention keeps over 90%, suggesting that the
supercapacitor has a good bending stability and can endure mechanical bending without significant loss of electrode specific capacitance. This loss may be due to incomplete contact between the electrolyte and the electrode, or the separation of activated carbon from nickel foam sheet 206
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Fig. 7. (a) Galvanostatic charge/discharge curves and (b) the capacitance retention rate of PK10 AGPE supercapacitor at various stretching states at 0.2 A/g, comparison of (c-g) Nyquist plots, (h) galvanostatic charge/discharge curves at 0.2 A/g, (i) cyclic voltammetry at 20 mV/s of PK10 AGPE supercapacitor at different angles, (j) capacitance retention rate of PK10 AGPE supercapacitor at 0.2 A/g after 100 bending cycles (bending angle = 90°). (Measured with Device 2).
verify the practical application of the supercapacitor, the tandem device can light up a red LED (the lowest working potential is about 2V) (Fig. 8g).
during the bending process [42]. Considering its practical application in cold climates, the electrochemical property of the supercapacitor was also examined under low temperatures. Fig. 8a displays that the Rs of PK10 AGPE supercapacitor from RT to −40 °C presents a slightly upward trend, which is caused by slowing the migration of ions of the electrolyte between the two electrodes with decreasing of temperature. However, even at −40 °C, the supercapacitor still shows a relatively low Rs (0.34 Ω). This may be because: on the one hand, exceed KOH forms hydrated ions, which lower the freezing point of PK10 AGPE; on the other hand, the network structure of the PK10 AGPE chains disrupts the crystalline growth of ice [43]. With the decrease in temperature, the area enveloped by the cyclic voltammetry curves reduces at 20 mV/s (Fig. 8b), because the incremental resistance to ionic transport causes the shape of the cycle to deviate from a quasi-rectangle. The galvanostatic charge/discharge curves of PK10 AGPE supercapacitor at 0.2 A/g behave as nearly triangular shapes shown in Fig. 8c, which means an almost ideal capacitance behavior within this wide temperature range. At low temperature, the charge/discharge time reduces and iRdrop enlarges gradually, which matches well with the Nyquist plots. The electrode specific capacitance (Fig. 8d) diminishes slightly with the temperature going down, and its electrode specific capacitance is 89 F/g (Fig. S8) at −40 °C, which is 83% of that measured at RT (107 F/g). Besides, the supercapacitor is still flexible at −40 °C (Fig. 8e). In addition, to light a red LED, the potential window was expanded from 0.9 V for a single device to 2.7 V for three devices in series. The tandem device was evaluated by galvanostatic charge/discharge measurements as shown in Fig. 8f. It can be seen that the charge/discharge time is almost unchanged, revealing consistent performances of the connected devices and the ideal capacitance behaviors. Lastly, to better
4. Conclusions In summary, we have developed a nontoxic and simple method to design strong, stretchable and high conductive PVA and KC composite AGPE. The DN network is proved a valid approach to enhance the mechanical properties of AGPE. The PK DN network AGPE demonstrates ideal mechanical properties as well as electrochemical properties. As a typical case, high tensile stress of 2.22 MPa, large deformation of 12.47 mm/mm and good ionic conductivity of 0.21 S/cm are obtained for the PK10 AGPE. Both the stretchable and electrochemical properties of PK10 AGPE are expected to be significant for practical applications, and a solidstate supercapacitor has been assembled. The superior electrochemical performances are displayed, such as, low Rs (0.41 Ω), and high electrode specific capacitance (470 F/g) at constant current of 0.5 A/g and favorable cycling stability. Furthermore, when the PK10 AGPE is highly stretched and bended, the capacitance retention of the supercapacitor still maintains above 90%. Besides, the capacitance retention as well as flexibility remains stable even at −40 °C. Thus, the novel multifunctional PK AGPE can be utilized in wide application such as the equipment of flexibility or low working temperature. Acknowledgments G. Q. and Q. C. are grateful for financial support from National Nature Science Foundation of China (51403056, 21504022), Henan Province (144300510041, 2017GGJS049, NSFRF1605, 2016GGJS-039, 207
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Fig. 8. Electrochemical performances of the supercapacitor at low temperature, (a) Nyquist plots, (b) cyclic voltammetry curves at 20 mV/s, (c) galvanostatic charge/ discharge curves at 0.2 A/g, (d) capacitance retention rate at 0.2 A/g at different temperatures and (e) photograph of PK10 AGPE supercapacitor at −40 °C, (f) Galvanostatic charge/discharge curves of single device and three devices in series of PK10 AGPE supercapacitor (0.1 A/g) and (g) photograph of an LED lit up by the PK10 AGPE supercapacitors. (Measured with Device 2).
17HASTIT006), Foundation of Henan Educational Committee (16B430004), Henan Polytechnic University (B2012-052, 72105/001 and 672517/005), Young Backbone Teachers Program of Henan Polytechnic University (2016XQG-07).
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