Hui Peng a,**,1, Yaya Lv a,1, Ganggang Wei a, Jiezi Zhou a, Xiaojie Gao a, Kanjun Sun b, Guofu Ma a,*, Ziqiang Lei a a Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, 730070, China b College of Chemistry and Environmental Science, Lanzhou City University, Lanzhou, 730070, China
A B S T R A C T
Although the hydrogel electrolytes for flexible energy-storage device have made great progress, it still remains a huge challenge to assemble a smart supercapacitor with high ionic conductivity and excellent self-healing when suffering physical damage. Herein, a novel self-healing hydrogel electrolyte (B-PVA/KCl/GO) is designed and prepared through graphite oxide (GO) doped into a diol-borate ester bonding cross-linked poly(vinyl alcohol) network. It is found that the moderate amount of GO-doped into hydrogel (2.3 wt% GO, 47.5 mS/cm) is obviously improved ionic conductivity compared with bare B-PVA/KCl hydrogel (32.6 mS/cm). Interestingly, the B-PVA/KCl/GO hydrogel electrolyte exhibits excellent self-healing capability that can repair its original configuration with 5 min when it is cut. Moreover, the activated carbon-based supercapacitor with B-PVA/KCl/GO hydrogel electrolyte delivers high specific capacitance of 156 F g� 1 at 0.3 A g� 1 and can also restore its capacitive performances via 7 times healing cycles without external stimulus. The presented work provides a new strategy to construct a flexible and self-healing hydrogel to apply for wearable electronics, smart apparels or flexible robots.
With the rapid development of wearable electronics, such as smartwatch, portable biosensor and intelligent textiles require their energy-storage systems possessing outstanding reliability in case of mechanical damage [1–4]. Various external forces or irreversible
physical damage are unavoidable during practical application. Therefore, safety and cost should also be considered [5,6]. Recently self-healing supercapacitor and battery have been receiving more and more attention. As we all know, smart electrochemical capacitors (ECs) have several advantages, such as large specific capacitance, high power density, long cycle life and fast charging-discharging rates [7–9]. Furthermore, hydrogel electrolyte, as a crucial part, plays a fundamental role during charge-discharge process in supercapacitor. Thus, the self-mending polymers are usually used to synthesis of electrolytes and further applied in ECs , which intrinsic superior electrochemical traits are crucial important for supercapacitor. In the past decades, distinct sorts of electrolytes, such as aqueous, organic, ionic liquids and redox-active gels, have been widely used for supercapacitors. However, compared with the conventional energy storage devices using liquid electrolytes, energy storage devices based on polymer electrolytes don’t require high standard safety encapsulation materials and variable geometry shape, which may bring new design chances for energy storage devices in the future wearable electronics field . Simultaneously, it is crucial for smart electrochemical capacitors to introduce self-healability into hydrogel electrolyte, because of they can spontaneously restore their capacitive performances when they undergo physical damage or abuse like cutoff or breaking . Therefore, the self-healing hydrogel electrolyte is regarded as a promising candidate to develop for flexible supercapacitor. To construct a thin, flexible and high performance self-healable hydrogel electrolyte, polyvinyl alcohol (PVA) is commonly used as polymeric substrate for the electrolytes of smart supercapacitor due to its chemical stability, environment friendliness and easy availability . Up to date, a series of PVA-based electrolytes have been developed, including acidic PVA-H2SO4 and PVA-H3BO3, alkaline PVA-KOH, and neutral PVA-LiCl . However, most PVA-based electrolytes rarely exhibited a self-healable behavior through hydrogen bonding. At the same time, the poor self-healing feature attribute to inorganic ions (neutral salts and inorganic acids or bases), which jeopardize intermolecular hydrogen bonds between PVA chains and water molecules . To deal with these troubles, the polymer electrolytes must be tough and strong enough to support huge deformation by non-covalent interactions (hydrogen bonding , coordination , dynamic borate ester bonding , and host–guest interactions ). In addition, the polymer electrolytes should possess excellent conductivity. Currently, the polymer hydrogel electrolytes mixed with graphene oxide (GO) have been utilized to some electrochemical devices like dye-sensitized solar cells , fuel cells [21,22] and supercapacitors  due to the high ionic conductivity (2.1 � 0.2 S cm� 1) of GO. The reciprocity of additional materials and polymer matrixes will influence the aggregated state of polymer in gels and improve the ionic conductivity and stability. Furthermore, graphene oxide (GO), which owns a single-layer, ultrahigh specific surface area and plentiful oxygen-containing functional groups can act as an ionic conducting promoter to improve the ionic conductivity of gels, these oxygens bearing functional groups of GO can also significantly affect the van der Waals interactions with organic polymers [24–27]. However, to the best our knowledge, the incorporation of GO
into self-healing hydrogels as an electrolyte material in supercapacitor has not been reported so far. In this work, a high performance self-healable hydrogel electrolyte (B-PVA/KCl/GO) is prepared via doping GO into a diol-borate ester bonding cross-linked poly(vinyl alcohol)-based network, which has illustrated more fantastic self-healing ability and high conductivity compared with the B-PVA/KCl hydrogel without doping GO. The B-PVA/KCl/GO hydrogel with excellent self-healing capability is not only based on dynamic diol–borate ester bonding, but also by intermolecular hydrogen bonding between GO and polymer [28,29]. More interestingly, the smart electrochemical supercapacitor with B-PVA/KCl/GO electrolyte was fabricated and possessed more excellent electrochemical performance than that of supercapacitor with bare B-PVA/KCl hydrogel electrolyte without doping GO. 2. Experimental 2.1. Materials Polyvinyl alcohol (PVA, Aladdin Co., Ltd, China, alcoholysis ¼ 99.8–100%), potassium chloride (KCl) and borax (Na2B4O7⋅10H2O) was purchased by Tianjin Zhiyuan Reagent Co., Ltd, China, Activated carbon (AC, Shanghai Sino Tech Investment Management Co., Ltd, China, the specific surface area about 2000 m2 g� 1). Graphite flakes purchased from Sigmae Aldrich were used to prepare graphite oxide by Hummers’ method . All chemicals were commercially available and employed without further purification. 2.2. Preparation of self-healing B-PVA/KCl/GO hydrogel electrolyte The self-healable B-PVA/KCl/GO hydrogel electrolyte was prepared as follows: firstly, 1.5 g PVA was dissolved in 12 mL distilled water with stirring at 90 �C for 1 h to form transparent solution. Then the graphite oxide (GO) aqueous solution (10 mL, 6.67 mg/mL) was added to above solution until obtaining a homogeneous mixture under vigorous stirring. The content of GO in the mixed solution is about 2.3 wt% which represent the mass ratio of GO/(GO þ PVA þ KCl) but except the quality of the water and borax in the whole system. After that, the KCl solution (2.0 mol/L) and borax solution (0.1 mol/L) were added slowly under stirring. Subsequently, a small number of ammonia solution (25 wt%) was added dropwise to adjust the pH value of mixed solution to 9 under vigorous stirring until the B-PVA/KCl/GO hydrogel electrolyte was obtained. For comparison purpose, the hydrogel electrolytes with different amount of GO (0, 0.9 wt%, 3.6 wt%) were prepared as the same above procedure. 2.3. Fabrication of smart electrochemical supercapacitor The work electrode was prepared as follows: activated carbon (AC) powder, acetylene black and polyvinylidene fluoride (PVDF) were mixed in the mass ratio of 80:10:10 and dispersed in appropriate amount
of N-methyl-2-pyrrolidone (NMP) to produce a homogenous slurry. Then, the resultant slurry was coated on the nickel foam with an area of 3 cm2. The AC electrode was obtained by pressing the nickel foam under 18 MPa after being dried at 80 �C for 12 h, the loading mass of each electrode was to be 8–9 mg. For two-electrode supercapacitor device assembly, the as-prepared B-PVA/KCl/GO hydrogel was firstly cut into 20 mm � 15 mm � 2 mm in size and simultaneously used as electrolyte and separator. Subsequently, a flexible smart supercapacitor was assembled by pressing all the elements together in a sandwich configuration (electrode/gel polymer electrolyte/electrode) and encapsulated with plastic film.
3. Result and discussion In order to prepare the self-healing hydrogel electrolytes with high ionic conductivity, the different amount of GO (0, 0.9 wt%, 2.3 wt%, and 3.6 wt%) was introduced to a diol-borate ester bond cross-linked poly (vinyl alcohol)-based gel network (denoted as B-PVA/KCl/GO), which is described in the experimental section detailly. Scheme 1 illustrated the synthesis of self-healable B-PVA/KCl/GO hydrogel electrolytes. The key to self-healable hydrogels was the use of GO doped into PVA solution and subsequently converted to a hydrogel by adding borax and KCl under basic conditions. It was found that the addition of GO improved the coagulation of PVA chains caused by the salts and enhanced ionic
conductivity (47.5 mS/cm) compared with bare B-PVA/KCl hydrogel (32.6 mS/cm). The ionic conductivity of B-PVA/KCl/GO is also higher than that of the reported PVA-based hydrogel electrolytes in literature (Table S1), which may be ascribed to the presence of carboxylic acid groups that can preferentially associate with potassium ions via electrostatic interaction. On the other hand, borax provides dynamic borate-diol ester bonds between PVA chains and GO to form a three-dimensional (3D) supramolecular structure of the hydrogel. Simultaneously, the B-PVA/KCl/GO hydrogel electrolyte can spontaneously repair itself after damage. The mechanical properties the of the as-prepared B-PVA/KCl/GO hydrogel were evaluated firstly and the results as illustrated in Fig. 1. It can be seen that the B-PVA/KCl/GO hydrogel can be elastic stretched to more than 2 times length than its original shape and can recover quickly (Fig. 1a). Besides, the B-PVA/KCl/GO hydrogel can be easily multiple twisted without fracturing (Fig. 1b). Furthermore, it can be spliced into any shapes (e.g., grid shown in Fig. 1c) through small size hydrogels. The above results imply that the as-prepared B-PVA/KCl/GO hydrogel possesses the attractive flexibility. Under the compressive test (Fig. 1d–f), there is no crack or broken in the B-PVA/KCl/GO hydrogel when apply a certain external force and recover original shape quickly after removing external force, indicating that it has outstanding flexibility and compression resistance. The result may be stem from adding moderate amount of GO increasing elasticity and toughness of the hydrogel . In contrast, the bare B-PVA/KCl hydrogel can be easily broken when apply equivalent external force 300 Kpa as above Fig. S1
(Supporting Information, SI). For deep understanding the microtopography of B-PVA/KCl and B-PVA/KCl/GO hydrogels. The freeze-dried B-PVA/KCl and B-PVA/KCl/ GO hydrogels were examined by scanning electron microscopy (SEM). Fig. 2a shows the optical images of B-PVA/KCl hydrogel. The freeze-dried B-PVA/KCl hydrogel displays a smooth surface morphology (Fig. 2b) and rough cross-section morphology (Fig. 2c). Furthermore, Fig. 2d presents the optical images of B-PVA/KCl/GO hydrogel. Compared with the microtopography of bare B-PVA/KCl hydrogel, the freeze-dried B-PVA/KCl/GO hydrogel appears a relatively rough surface morphology owing to GO incorporation (Fig. 2e). The cross-section of the lyophilized B-PVA/KCl/GO hydrogel shows a 3D interwoven nets-like porous structure (Fig. 2f). This unique structure can promote the transmission of the ions and deals with salt tolerance of self-healing PVA-based hydrogel electrolytes so as to improve low self-healing ability and poor ionic conductivity . The elemental composition of the B-PVA/KCl/GO hydrogel was analyzed by energy-dispersive spectroscopy (EDS, Fig. 2g). The result reveals that the B-PVA/KCl/GO material is composed of C, O and B elements. The great amount of C and O elements are mainly from PVA and GO. The emergence of B atoms proves the existence of borax . Fig. 3a shows the Fourier transform infrared spectroscopy (FT-IR) of the B-PVA/KCl and B-PVA/KCl/GO hydrogels. There are no characteristic peaks for GO can be found from the B-PVA/KCl/GO hydrogel, which is ascribed to the similar chemical composition of PVA and GO, the intercalation homogeneous dispersion of GO into PVA matrix and
the less weight percentage of GO in the composite . Furthermore, the crosslinking of PVA and borax was confirmed by the characteristic absorption peak at 652 cm� 1 corresponding to O–B–O bending. Additionally, compared with the B-PVA/KCl hydrogel, the characteristic absorption peak of B-PVA/KCl/GO hydrogel at 652 cm� 1 is relatively weak suggesting GO was introduced into PVA molecular chains . The phase structure was examined using X-ray diffraction (XRD) characterization (Fig. 3b). For bare B-PVA-KCl hydrogel, the peak at the 2θ ? 19.8� is corresponding to PVA . Compared with the B-PVA/KCl hydrogel, a weaken crystalline peak was observed at 2θ ?19.8� for B-PVA/KCl/GO hydrogel, indicating the degree of crystallinity of the B-PVA/KCl/GO hydrogel decreases, which due to incorporating GO into PVA will lead to a decrease in crystallinity and an increase in the number of amorphous phases. The other peaks appeared are corresponding to KCl , meanwhile, these peaks in the B-PVA/KCl/GO hydrogel obviously weaken after incorporating GO into PVA solution, and the results also confirm that the GO was successfully attached into B-PVA/KCl/GO hydrogel. Furthermore, the surface chemical bonding state and compositions of B-PVA/KCl/GO hydrogel was further characterized by X-ray photoelectron spectroscopy (XPS), and the characteristic peaks of O, K, C and Cl elements were observed in the survey scan spectrum (Fig. S2a). The high resolution XPS spectrum of C1s (Fig. S2b) could be resolved into two components. The C1s peaks observed in B-PVA/KCl/GO hydrogel at 286.8 eV and 287.8 eV corresponded to carbon atoms with hydroxyl groups C–OH and carbonyl groups O–C糘/C糘, respectively. The peaks positions were in good agreement with previously reported binding energies of PVA and GO [37,38]. This results further proved the GO aqueous suspension was successfully introduced to PVA solution to form the integrated hydrogel.
Thermogravimetric analysis (TGA) of B-PVA/KCl/GO hydrogel is presented in Fig. S2c. The B-PVA/KCl/GO hydrogel has excellent thermal stability below 210 �C. It should be mentioned that the initial thermal degradation temperature of B-PVA/KCl/GO hydrogel is lower than the previous literature reports of PVA (264 �C)  and PVA-Borax hydrogel (280 �C) , which may be attributed to the addition of GO reduces the crystallinity of copolymer, leading to a decrease in thermal stability. In addition, differential scanning calorimetry (DSC) (Fig. S2d) was used to investigate the glass transition temperature (Tg) and melting temperature (Tm). It can be seen from the DSC curve that the Tg of the B-PVA/KCl/GO hydrogel is about 97 �C and Tm is about 210 �C. The Tg of B-PVA/KCl/GO hydrogel was slightly improved by the introduction of GO, which value is larger than that of previous report about pure PVA hydrogel , indicating strong interaction among the GO and a diol-borate ester bond cross-linked poly(vinyl alcohol) [41,42]. Furthermore, the ions transportation has a remarkably effect on the ionic conductivity which calculated from the Nyquist plots (Fig. 3c). It can be found that the B-PVA/KCl/GO hydrogel electrolyte with the GO content of 2.3 wt% possessed high ionic conductivity about 47.5 mS/cm. However, the ionic conductivity of the bare B-PVA/KCl hydrogel electrolyte and high content of GO (3.6 wt%) only reach 32.6 mS/cm and 24.5 mS/cm, respectively (Fig. 3d). In addition, the contents of KCl and H2O are consistent in various B-PVA/KCl/GO hydrogels. The ions transport pathway of hydrogel electrolytes with different amount of GO can be illustrated in Fig. 3e. It is shown that KCl ions can migrate via the disordered amorphous phase. The degree of crystalline of the B-PVA/ KCl/GO hydrogel decreased when low content of GO was introduced to the hydrogel electrolyte. The result can be due to the homogeneous distribution of GO with 3D network structure in the B-PVA/KCl/GO
hydrogel electrolyte, leading to an interconnected transport channel was formed between GO and the copolymer. Compared with bare B-PVA/KCl hydrogel electrolyte, the low content of GO doped into hydrogel electrolyte can significantly improve ions transport. When high content of GO was added into the hydrogel electrolyte, the ionic conductivity of the hydrogel electrolyte was deteriorated, which due to the aggregation of GO and lead to blocking and extending ions transport pathway. This phenomenon is consistent with the above test results. Thus, B-PVA/KCl/ GO with 2.3 wt% GO regarded as optimized electrolyte. Smart supercapacitors were fabricated based on activated carbon (AC) as electrode materials and using B-PVA/KCl/GO hydrogels as electrolyte and separator. All the electrochemical performances were studied at room-temperature. Fig. 4a shows the cyclic voltammetry (CV) curves of smart supercapacitors with different amounts of GO doped B-PVA/KCl/GO hydrogel electrolytes at a scan rate of 50 mV s� 1. All CV curves exhibit a similar rectangular shape, implying rapid electrochemical response and ideal capacitive behavior. Clearly, the B-PVA/ KCl/GO hydrogel with 2.3 wt% GO content exhibits the largest electrochemical active area, indicating it has the greatest specific capacitance among those hydrogel electrolytes. Fig. S3 compares the galvanostatic charge/discharge (GCD) curves of the smart supercapacitors based on B-PVA/KCl/GO hydrogel electrolytes with different GO contents at the current density of 0.5 A g� 1. A slight internal resistance (IR) drop of 0.03 V and 0.04 V respectively could be seen from the B-PVA/KCl/GO hydrogel electrolytes with low content of GO (2.3 wt% and 0.9 wt%), whereas, bare B-PVA/KCl hydrogel electrolyte and B-PVA/KCl/GO hydrogel electrolyte with high content of GO (3.6 wt%) show relatively high IR drops of 0.05 V and 0.08 V, respectively. Those results also meet well with the trend obtained from ionic conductivity results. High electrical conductivity of B-PVA/KCl/GO hydrogel electrolyte with low content of GO ensures lower IR drops. The electrochemical properties of the optimized B-PVA/KCl/GO electrolyte-based supercapacitor are further analyzed in detail. As expected, the CV curves
of supercapacitor between 0 and 1.0 V at the scan rates of 10–100 mV s� 1 are displayed rectangular shape (Fig. 4b), indicating a remarkable rate capability. In addition, all isosceles triangle-shaped GCD profiles at different current densities ranging from 0.3 to 2.0 A g� 1 are shown in Fig. 4c, exhibiting a typical reversible double-layer capacitive behavior. Fig. 4d compared the discharge capacitance between B-PVA/KCl and B-PVA/KCl/GO electrolyte-based supercapacitors at different current densities. The maximum specific capacitance of B-PVA/KCl/GO electrolyte-based supercapacitor is 156 F g� 1 at 0.3 A g� 1, outperforming the B-PVA/KCl electrolyte-based supercapacitor (130 F g� 1). Interestingly, the B-PVA/KCl/GO hydrogel could automatically repair itself when suffering physical damage. Here the self-healability of B-PVA/KCl/GO hydrogel was surveyed by cut the electrolyte into two parts firstly (Fig. 5a). Subsequently, they were recombined into an independent entity after contacting for 5 min at ambient conditions (Fig. 5b). The optical healing image was demonstrated in Fig. 5c, which shows the B-PVA/KCl/GO hydrogel can be easily bended without breaking after recovering 24 h. The cut/healing process of the B-PVA/ KCl/GO hydrogel was further confirmed by SEM at different healing times (Fig. 5d–f). One can see clearly that there was no observable gap in the healed region when healed 24 h (Fig. 5f), indicating excellent self-repaired performance of the B-PVA/KCl/GO hydrogel electrolyte, which is dominantly due to the recombination of dynamic diol-borate ester bonds . Moreover, the self-healing mechanism can be illustrated and explained in Fig. 5g. When the hydrogel was cut into two parts, the diol-borate ester bonds were cleaved at the cut interfaces. However, once the two parts contacted together again, the broken diol-borate ester bonds were prone to link together. The recombination of diol-borate ester bonds enabled the severed electrolyte to heal itself, leading to the recovery of the electronic conductivity along the hydrogel . Meanwhile, the healing behavior of the hydrogel electrolyte was quantitatively evaluated by measuring its mechanical properties. The
mechanical properties of B-PVA/KCl/GO (2.3 wt% GO) hydrogel before and after self-healing were illustrated in Fig. S4a. It was shown that the original B-PVA/KCl/GO hydrogel can be stretched up to 273.3% of initial length, while the self-healing B-PVA/KCl/GO hydrogel also can be stretched up to 226.7% of initial length, indicating a high self-healing performance and excellent tensile properties of B-PVA/KCl/GO hydrogel. The effect of healing time on the stretchability of B-PVA/KCl/GO hydrogel was showed in Fig. S4b. After healing for 45 min, the stretchability of B-PVA/KCl/GO hydrogel can reach 200% relative to the original length (3 cm). Moreover, the mechanical properties of the self-healing electrolyte are restored more perfect when the healing time was prolonged to 60 min. Table S2 summaries the self-healing mechanical properties of B-PVA/KCl/GO hydrogel and several previously reported self-healing hydrogels. We can be found that the mechanical properties of the self-healing B-PVA/KCl/GO hydrogel are comparable to those previously reported self-healing hydrogels. In addition, the effect of content of GO on the stretchability of the B-PVA/KCl/GO (2.3 wt % GO) hydrogel at the same healing time (60 min) was also investigated, the results are shown in Fig. S4c. When moderate content of GO (2.3 wt % GO) was introduced into the hydrogel, the stretchability of B-PVA/KCl/GO hydrogel was significantly better than bare B-PVA/KCl hydrogel. Specifically, the B-PVA/KCl/GO (2.3 wt% GO) hydrogel can be stretched up to 226.7% of initial length after healing for 60 min, while that of the B-PVA/KCl hydrogel is only 153.3%. This result may be due to the hydrophilicity of GO well dispersed in diol-borate ester bond cross-linked poly(vinyl alcohol)-based gel network and the hydrogen bonded was formation between the GO and polymer chains. However, the stretchability of B-PVA/KCl/GO hydrogel was declined as increase the content of GO to 3.6 wt%, which may be due to the excessive accumulation GO hindered the cross-linking between borax and poly (vinyl alcohol) in hydrogel. As an example of application, the excellent self-healability of the B-PVA/KCl/GO hydrogel electrolyte is fully utilized. We would tailor the hydrogel electrolyte into different pieces, and then they were placed together to form an alphabet “H” (Fig. 6a), which provide a facile strategy to develop special patterns energy-storage devices with flexible, self-healable and tailorable. Moreover, the smart supercapacitor prototype was assembled by using a piece of the B-PVA/KCl/GO electrolyte and two activated carbon electrodes. Subsequently, the smart supercapacitor was segmented into halves by scissors. The segmented pieces
were brought into contact for self-healing. Simultaneously, the nickel foam as current collector of smart supercapacitor was cut into halves, which was recover together with the self-healing B-PVA/KCl/GO hydrogel electrolyte owing to effective contact of nickel foams attached to the electrolyte for 15 min (Fig. S5a). The CV curves and GCD profiles of the healed supercapacitor was illustrated in Fig. 6b and c, respectively. To our surprise, the rectangular CV curves and isosceles triangular GCD profiles still can be found in B-PVA/KCl/GO electrolyte-based supercapacitor after 7 self-healing cycles at same position. Furthermore, a high specific capacitance of 119.2 F g� 1 at 1.0 A g� 1 can be retained after 7 self-healing cycles, which is slightly inferior to specific capacitance (140 F g� 1 at 1.0 A g� 1) of the original supercapacitor. The reduced capacitance is probably due to the water loss and the imperfectly contact between the electrolyte and the electrodes after the cut/ healing processes. Despite all this, the B-PVA/KCl/GO electrolyte-based supercapacitor was able to heal itself at least 7 cycles without significant degradation its capacitive properties in healing efficiency (Fig. 6d). For example, the healing efficiency of supercapacitor based on B-PVA/KCl/ GO hydrogel is 90.5% after the 5 self-healing cycles, which exceed the self-healing of MGO-PAA hydrogel (60%) , PVA/PA hydrogel (around 87%)  and PVA/H2SO4 hydrogel (80%)  during same self-healing cycles, respectively. However, the self-healing capability of the B-PVA/KCl/GO electrolyte is still slightly inferior to the dual crosslinked hydrogel electrolytes VSNPs-PAA  and poly(AMPS-co-DMAAm)/Laponite/GO  with self-healing times of 20, respectively, which may be due to the dual crosslinked hydrogel further improve its self-healing capability by combination of physical and chemical cross-linking. Electrochemical interfacial properties of the healed capacitor were investigated by EIS (Fig. 6e). It is still delivered that the solution resistance (Rs) and charge transfer resistance (Rct) were not significantly increased comparing with original counterpart, the ionic conductive of the electrolyte was not significantly deteriorated via at least 7 cutting/self-healing cycles at room (Fig. S5b), suggesting the supercapacitor can recover its configuration and ionic conduction perfectly after self-healing process. Fig. 7a represents the similar CV curves for the supercapacitor containing the resultant B-PVA/KCl/GO hydrogel electrolyte with bending angles of 0� and 180�, indicating the bending has no effect on the specific capacitance of this supercapacitor and declaring the B-PVA/KCl/ GO hydrogel electrolyte-based supercapacitor has excellent flexibility. Similar results were presented in GCD curves of B-PVA/KCl/GO hydrogel electrolyte-based supercapacitor at different bending angles (Figs. S6a and SI). EIS measurements of B-PVA/KCl/GO hydrogel electrolyte-based supercapacitor at different bending angles were shown in Fig. S6b. The Rs of supercapacitor slightly increased when increase the bending angles obtained from high-frequency region, but similar slopes at the low-frequency region, which further reflect the ideal capacitive behavior of B-PVA/KCl/GO electrolyte-based supercapacitor by bending different angles. It is obviously seen that the capacitance retention is 95% with bending angles from 0� to 180� (Fig. 7b) and the radius of bended capacitor is about 4 mm by bending 180� (the inset of Fig. 7b). Which indicates that the B-PVA/KCl/GO hydrogel electrolyte-based supercapacitor device has good flexibility and durability, as well as excellent mechanical robustness. Ragone plots of the smart supercapacitor with optimized B-PVA/ KCl/GO hydrogel electrolyte and bare B-PVA/KCl hydrogel are shown in Fig. 7c. Obviously, the energy density of the optimized B-PVA/KCl/GO hydrogel electrolyte-based supercapacitor is higher than that of B-PVA/ KCl hydrogel electrolyte-based supercapacitor. Specifically, the energy density of B-PVA/KCl/GO hydrogel electrolyte-based supercapacitor can reach 5 Wh kg� 1 at high power density of 492.7 W kg� 1 under a current density of 1 A g� 1. However, the energy density of B-PVA/KCl electrolyte-based supercapacitor is 4 Wh kg� 1 under the same current density. Impressively, the water-holding performance of the B-PVA/ KCl/GO hydrogel was evaluated by electrochemical tests of the assembled device after different stored time (Fig. S7). The CV curves of B-PVA/ KCl/GO hydrogel electrolyte-based supercapacitor were still maintained a good rectangle-like shape when after stored 12 days (Fig. S7a), indicating the B-PVA/KCl/GO hydrogel has a good water-holding performance. From the GCD curves of the B-PVA/KCl/GO hydrogel electrolyte-based supercapacitor (Fig. S7b), the discharging time of the device decreased slowly with increased of stored time, which was further verified the above CV results. Moreover, the ionic conductivity of the hydrogel electrolyte after storage for different duration times was calculate by the real axis intercepts of EIS curves (Fig. S7c). The comparison of ionic conductivity between the B-PVA/KCl hydrogel and B-PVA/KCl/GO hydrogel during different storing time are shown in Fig. S7d. The ionic conductivity of the B-PVA/KCl/GO hydrogel electrolyte dropped slightly from 47.5 mS/cm to 46.9 mS/cm during the initial 6 days, and then decreased to 32.9 mS/cm after 12 days. Compared with the B-PVA/KCl/GO hydrogel electrolyte, however, the ionic conductivity of bare B-PVA/KCl hydrogel electrolyte dropped obviously during the initial 6 days and finally dropped to 22 mS/cm over 12 days. The above results indicate that the B-PVA/KCl/GO hydrogel electrolyte keep a good stable ionic conductivity for a period of time in virtue of incorporating GO into hydrogel reduces moisture evaporation. Fig. 7d displays the cycling performance of B-PVA/KCl/GO and B-PVA/KCl hydrogel electrolyte-based supercapacitors. It can be seen that the specific capacitance of B-PVA/KCl hydrogel electrolyte-based supercapacitor remains above 90% of original specific capacitance in the initial 3000 cycles, but it reduced to 74% after the subsequent 2000 cycles. The initial stable cycle performance of B-PVA/KCl hydrogel electrolyte-based supercapacitor may be owing to the KCl well embedded in the cross-linking of borax and PVA to form integrated structure, because of the PVA/alkali metal chlorides has been verified to have excellent dissolubility and cycling stability . However, the specific capacitance sharp decline in the subsequent 2000 cycles may be due to evaporation of some of the moisture in the B-PVA/KCl hydrogel resulting in a decrease in ionic conductivity. In comparison, the specific capacitance of B-PVA/KCl/GO hydrogel-based supercapacitor remains 92% of the initial capacitance even after 5000 cycles. The good capacitance retention of B-PVA/KCl/GO hydrogel-based supercapacitor can be further ascribed to the GO-doped hydrogel electrolyte possesses high ionic conductivity and excellent compatibility. Simultaneously, the three tandem B-PVA/KCl/GO based supercapacitor units can repeatedly light white and blue light-emitting diode (LED) is shown in Fig. 7e, demonstrating the practical potential of the fabricated flexible supercapacitors for smart energy supply.
4. Conclusions In summary, we fabricated a novel self-healing hydrogel electrolyte (B-PVA/KCl/GO) based on GO-doped and diol-borate ester bonding cross-linked process. This provides a way to solve the conventional polyvinyl alcohol (PVA)-based electrolyte which is neither intrinsically stretchable nor healable properties. The B-PVA/KCl/GO hydrogel electrolyte can be easily stretched, twisted and tailored into different shapes, which provide a new tactics to construct distinct patterns supercapacitors to apply for diverse technological systems. Moreover, the B-PVA/KCl/GO hydrogel electrolyte can be provided outstanding self-healing, high ionic conductivity and superior cycling stability compared with traditional PVA-based salt electrolytes. It was revealed that the addition of GO into the B-PVA/KCl/GO hydrogel was a vital factor to improve properties of self-healing hydrogel electrolyte. This strategy may bring a new prospect to introduce nanomaterials into hydrogel electrolyte to form 3D polymer network to facilitate ions transport. In addition, the hydrogel electrolyte with eco-friendly and ideal biocompatibility could be used to smart energy-storage devices. Acknowledgements This study was supported by the National Science Foundation of China (21664012, 21703173, 51863019), the program for Changjiang Scholars and Innovative Research Team in University (IRT15R56), Basic Scientific Research Innovation Team Project of Gansu Province (1606RJIA324), University Scientific Research Innovation Team of Gansu Province (2017C-04). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.05.058. References  Y. Yue, Z. Yang, N. Liu, W. Liu, H. Zhang, Y. Ma, C. Yang, J. Su, L. Li, F. Long, Z. Zou, Y. Gao, A flexible integrated system containing a microsupercapacitor, a photodetector, and a wireless charging coil, ACS Nano 10 (2016) 11249–11257.  K. Wang, X. Zhang, C. Li, X. Sun, Q. Meng, Y. Ma, Z. Wei, Chemically crosslinked hydrogel film leads to integrated flexible supercapacitors with superior performance, Adv. Mater. 27 (2015) 7451–7457.  G. Zhou, F. Li, H. Cheng, Progress in flexible lithium batteries and future prospects, Energy Environ. Sci. 7 (2014) 1307–1338.  Y. Guo, X. Zhou, Q. Tang, H. Bao, G. Wang, P. Saha, A self-healable and easily recyclable supramolecular hydrogel electrolyte for flexible supercapacitors, J. Mater. Chem. A 4 (2016) 8769–8776.  D.P. Dubal, N.R. Chodankar, D. Kim, P. Gomez-Romero, Towards flexible solid-state supercapacitors for smart and wearable electronics, Chem. Soc. Rev. 47 (2018) 2065–2129.  Y. Yang, D. Yu, H. Wang, L. Guo, Smart electrochemical energy storage devices with self-protection and self-adaptation abilities, Adv. Mater. 29 (2017) 1703040.  T. Lv, Y. Yao, N. Li, T. Chen, Highly stretchable supercapacitors based on aligned carbon nanotube/molybdenum disulfide composites, Angew. Chem. Int. Ed. 55 (2016) 9191–9195.  J. Ye, S. Wu, K. Ni, F. Pan, J. Liu, Z. Tao, H. Ji, Y. Zhu, Direct laser writing of graphene made from chemical vapor deposition for flexible, integratable micro-supercapacitors with ultrahigh power output, Adv. Mater. 30 (2018) 1801384.  Y. Xu, Z. Lin, X. Huang, Y. Wang, Y. Huang, X. Duan, Functionalized graphene hydrogel-based high-performance supercapacitors, Adv. Mater. 25 (2013) 5779–5784.  Y. Huang, M. Zhu, Y. Huang, Z. Pei, H. Li, Z. Wang, Q. Xue, C. Zhi, Multifunctional energy storage and conversion devices, Adv. Mater. 28 (2016) 8344–8364.  G. Ma, M. Dong, K. Sun, E. Feng, H. Peng, Z. Lei, Redox mediator doped gel polymer as electrolyte and separator for high performance solid state supercapacitor, J. Mater. Chem. A 3 (2015) 4035–4041.  F. Tao, L. Qin, Z. Wang, Q. Pan, Self-healable and cold-resistant supercapacitor based on a multifunctional hydrogel electrolyte, ACS Appl. Mater. Interfaces 9 (2017) 15541–15548.  K. Liu, X. Pan, Li Chen, L. Huang, Y. Ni, J. Liu, S. Cao, H. Wang, Ultrasoft self-healing nanoparticle-hydrogel composites with conductive and magnetic properties, ACS Sustain. Chem. Eng. 6 (2018) 6395–6403.  Q. Pan, N. Tong, N. He, Y. Liu, E. Shim, B. Pourdeyhimi, W. Gao, Electrospun mat of poly (vinyl alcohol)/graphene oxide for superior electrolyte performance, ACS Appl. Mater. Interfaces 10 (2018) 7927–7934.
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