Poly(vinyl Alcohol) Borate Gel Polymer Electrolytes Prepared by

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Polyvinyl Alcohol Borate Gel Polymer Electrolytes Prepared by Electrodeposition and Their Application in Electrochemical Supercapacitors Mengjin Jiang, Jiadeng Zhu, Chen Chen, Yao Lu, Yeqian Ge, and Xiangwu Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11984 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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ACS Applied Materials & Interfaces

Polyvinyl Alcohol Borate Gel Polymer Electrolytes Prepared by Electrodeposition and Their Application in Electrochemical Supercapacitors Mengjin Jiang,†,‡,* Jiadeng Zhu,‡ Chen Chen,‡ Yao Lu,‡ Yeqian Ge,‡ and Xiangwu Zhang‡,*

†

College of Polymer Science and Engineering, Sichuan University, Chengdu, 610065, China

‡

Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and

Science, North Carolina State University, Raleigh, NC 27695-8301, USA

ACS Paragon Plus Environment

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ABSTRACT: Gel polymer electrolytes (GPEs) have been studied for preparing flexible and compact electrochemical energy storage devices. However, the preparation and use of GPEs are complex and most GPEs prepared through traditional methods do not have good wettability with the electrodes, which retard them from achieving their performance potential. In this study, these problems are addressed by conceiving and implementing a simple, but effective method of electrodepositing polyvinyl alcohol potassium borate (PVAPB) GPEs directly onto the surfaces of active carbon electrodes for electrochemical supercapacitors. PVAPB GPEs serve as both the electrolyte and the separator in the assembled supercapacitors, and their scale and shape are determined solely by the geometry of the electrodes. PVAPB GPEs have good bonding to the active electrode materials, leading to excellent and stable electrochemical performance of the supercapacitors. The electrochemical performance of PVAPB GPEs and supercapacitors can be manipulated simply by adjusting the concentration of KCl salt used during the electrodeposition process. With a 0.9 M KCl concentration, the as-prepared supercapacitors deliver a specific capacitance of 65.9 F g-1 at a current density of 0.1 A g-1, and retain more than 95% capacitance after 2000 charge/discharge cycles at current density of 1 A g-1. These supercapacitors also exhibit intelligent high voltage self-protection function due to the electrolysis-induced crosslinking effect of PVAPB GPEs.

KEYWORDS: gel polymer electrolyte, electrodeposition, supercapacitor, polyvinyl alcohol, potassium chloride


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1. INTRODUCTION Compact, ultrathin, flexible and wearable energy storage devices have recently gained attention from both public and research communities.1,2 Various structures have been designed to construct these elaborate devices, and it is worth noting that almost all of these designs use gel polymer electrolytes (GPEs) to fulfill their performance aim.3-5 The popularity of GPEs in compact and flexible energy storage devices can be ascribed to their unique properties. Firstly, GPEs are soft and flexible quasi-solid materials, which help avoid the shortcomings of liquid electrolytes, such as leaking, volatilization, inner corrosion, etc. Secondly, the solvent-containing gel-network structure of GPEs enables high ionic conductivity, which is difficult to achieve in solid electrolytes.6-8 Last but not least, GPEs can also function as the separators in batteries and supercapacitors, which make it possible to prepare ultra-thin devices.9,10 Normally, there are two approaches to prepare and apply GPEs in batteries or supercapacitors. The first approach is to prepare GPE membranes and then sandwich them between the electrodes. This method is easy to operate, but the quasi-solid GPEs cannot penetrate into the electrodes to wet the active materials. The devices prepared by this method usually have low energy densities and high internal resistances.11,12 The second approach is to first prepare gelable electrolyte solutions by mixing crosslinkable polymers or monomers with liquid electrolytes. The gelable electrolyte solutions are then added into the device cases preloaded with separators and electrodes to carry out the gelation reaction. In this method, the liquid gelable electrolyte solutions are able to penetrate into the voids and cracks of the electrodes to wet the surfaces of


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active materials prior to gelation, which can significantly reduce the electrode-electrolyte interfacial resistances. In addition, the gelation process is accomplished during the device assembling, which enhances the stability, reliability and durability of the assembled devices. However, separators are still needed in this approach, and the residual reactants after gelation often have negative effect on the device performance.13,14 GPEs can be categorized into aqueous GPEs and organic GPEs based on the solvent type. Though organic GPEs have wider electrochemical windows, aqueous GPEs are attractive due to their high ionic conductivity, environmental friendliness, low raw material cost, and simple preparation process.15,16 Among the various polymers used for preparing aqueous GPEs, polyvinyl alcohol (PVA) is the most commonly used aqueous GPE polymer because of its excellent chemical stability and durability. PVA-based GPEs, such as PVA-KOH, PVA-H2SO4, PVA-H3PO4, etc., have been reported by several researches groups.17-21 However, in these reports, PVA-based GPEs were prepared or applied based on the traditional approaches discussed above. The quasi-solid PVA GPE membranes are not suitable for the electrodes with thick active material layers, and they are normally prepared with large thickness to avoid short-circuiting during assembling. The above-mentioned drawbacks of GPEs can be potentially overcome by utilizing one unique property of PVA. Unlike most other polymers, PVA can be gelated by boric acid or borates under alkali condition to form polyvinyl alcohol borate (PVAB) hydrogel.22,23 Zhitomirsky and coworkers found that the increased pH value caused by electrolysis of water was able to initiate


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this gelating process of PVA.24,25 As a result, the PVAB hydrogel can be prepared directly on the surface of the electrodes by the electrodeposition (ED) method, which is a novel approach and has not been reported in literature. It is apparent that preparing PVAB GPEs directly on electrode surface by the ED process can potentially solve most existing problems in the traditional approaches. For example, the ED solution can easily penetrate into the electrode materials, and the gelation process does not need any monomers or organic crosslinker. The ED process is independent of the size and shape of the electrodes, and the resultant GPE membranes can cover the entire electrode surfaces uniformly without the need of separators. The process of direct electrodeposition on electrode surfaces also gives an opportunity to prepare ultrathin GPEs. In addition, the ED process is a material and energy saving technique with high efficiency. In this study, we utilized the unique gelation property of PVA to design a new approach for preparing PVAB GPEs in-situ onto the working electrodes by the ED method, as shown in Figure 1. The ED solution, consisting of XY salt (X+ are cations whose electron accepting ability is weaker than H+, and Y- are any kind of anions), PVA, and boric acid, is placed between the anode (graphite or other inert metal electrodes) and the cathode (active carbon or other supercapacitor electrodes). When this XY salt-containing solution is electrolyzed, the pH value increases on the surface of the cathode, followed by the formation of borate ions B(OH)4-. The newly-formed borate ions crosslink the PVA molecules to form a 3-dimensional network, and at the same time, the X+ cations move to the cathode surface under the effect of electric field, combine with the borate anions, and stabilize the crosslinked centers. Finally, a stable PVAB gel


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membrane with uniform thickness is formed on the surface of the cathode. This PVAB gel membrane is generated directly from the ED solution, and hence it contains abundant water and XY salt, and can function as a GPE. Theoretically, any water-soluble salts whose cations have weaker electron accepting ability than H+ can be used in this ED process (Supporting Information, Figure S1a). As a preliminary study, we used potassium chloride (KCl) as the XY salt for the preparation of polyvinyl alcohol potassium borate (PVAPB) GPEs on active carbon (AC) electrodes. The AC electrodes covered with as-prepared PVAPB GPEs were assembled to form AC/AC supercapacitors. The structure of the PVAPB GPEs and the electrochemical performance of the supercapacitors were studied in this work.


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2. EXPERIMENTAL SECTION Materials: AC electrode (DD001Z670, composed of 90 wt% active carbon, 5 wt% PTFE, and 5 wt% Carbon ECP600) was provided by Liaoning Brother Electronics Technology Co., Ltd. China. PVA (DP = 1700±50, alcoholysis degree > 99%), industry degree, was provided by Sinopec Sichuan Vinylon Works, China. Boric acid (for electrophoresis, ≥ 99.5%), potassium chloride (for molecular biology, ≥ 99.0%) and graphite rod (diameter: 6mm, 99.995%) were all purchased from Sigma-Aldrich. Glass microfiber membrane (GF/A, WhatmanTM) was used as the separator for the control sample. Water used in the experiments was deionized. Preparation of ED solutions: PVA (6 g) and boric acid (1.05 g) were dissolved in 250 mL DI water at 95 oC for 2h, followed by the addition of different amounts of KCl (6.71, 10.06, 13.42, 16.77, and 20.13 g). After the solutions were cooled down to 50 oC, additional DI water was added to obtain ED solutions with KCl concentrations of 0.3, 0.45, 0.6, 0.75, and 0.9 M. ED solutions with KCl concentrations higher than 0.9 M were also prepared. However, phase separation occurred in these high-concentration ED solutions and no stable GPEs could be formed. Preparation of PVAPB GPEs: The ED process was conducted with a DC power supply (VOLTEQ HY20010EX) at 2.5 V for 2 min, by using graphite rod as the anode and AC electrode as the cathode. The distance between the cathode and the anode was 5±0.5 cm. The AC electrodes were punched into round slices (diameter: 1.27 cm, active material: 68.4±0.5 mg pc-1), and were attached onto stainless steel current collectors prior to ED process.


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Structure characterization of PVAPB GPEs: The morphology of PVAPB GPEs and corresponding electrodes were observed with optic microscope (OM, Nikon SMZ1000) and scanning electron microscope (SEM, FEI Verios 460L). The GPE samples for OM observation were at the wet gel state while those for SEM experiments were vacuum dried and gold plated. The PVAPB GPEs were also analyzed with Fourier transform infrared spectroscopy (FTIR, Nicolet Nexus 470) and X-ray diffraction (XRD, WAXD, Rigaku Smartlab). Prior to FTIR and XRD analyses, soft PVAPB GPEs were scraped off from the electrodes, and dried in the vacuum oven. The dried GPE films contained abundant KCl salt and became brittle after drying. The dried, brittle GPE films were then grounded into the powder form. The thermo gravimetric analysis (TGA, Perkin Elmer, Pyris 1) was carried out in air atmosphere with a heating rate of 20 o

C min-1 to determine the weight percentages of each components in PVAPB GPEs. For

comparison, FTIR and TGA were also performed on a PVA film. The PVA film was obtained by solution casting method, and dried before testing. Fabrication of supercapacitors: Electrodes covered with PVAPB GPEs were assembled into coin-type CR2032 cells to form symmetric supercapacitors. During the cell assembling, the two working electrodes were placed with the sides covered with PVAPB GPEs facing each other. No additional separator or liquid electrolyte was added during cell assembling. For comparison, control supercapacitor samples were prepared using 0.9 M KCl solution as the liquid electrolyte and glass microfiber membrane as the separator.


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Performance evaluation of supercapacitors: The galvanostatic charge-discharge behaviors of supercapacitors were tested with a battery testing station (BT-2000, Arbin Instrument) in the voltage range from 0.05 V to 1.15 V. The charge-discharge curves of the twentieth cycle with the current density of 0.1 Ag-1 were recorded to calculate the specific capacitances of supercapacitors according to the equation: = 2 ∙ Δ⁄ ∙ Δ , where is specific capacitance (F g-1), is discharge current (A), Δ is discharge time (s), is the weight of active carbon on one electrode (g), and Δ is the discharge voltage range (V). The rate performance of supercapacitors was recorded at current densities of 0.1, 0.2, 0.5, 1, and 2 A g-1. The cycling performance of supercapacitors was tested at current densities of 1 A g-1 up to 10000 cycles. Cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements of supercapacitors were carried out with an electrochemical work station (Gamry Reference 600). CV was conducted at scan rates of 1, 2, 5, 10, and 20 mVs-1 in the voltage range from 0.05 to 1.15 V. LSV was carried out at a scan rate of 10 mV s-1 in the voltage range from 0 to 3 V. EIS was performed at frequencies ranging from 10-1 to 106 Hz, and the results were fitted by software Z-View 2. The self-discharge behavior of supercapacitors was tested with Arbin battery testing station. During the test, samples were first charged to 1.15 V with a current density of 0.1 A g-1, and then the voltage loss was recorded under open circuit for 24 h. All electrochemical performance tests were performed at ambient temperature (25 oC).


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3. RESULTS AND DISCUSSION Figure 2a shows the images of an AC electrode before and after the ED process using a 0.9 M KCl solution. During the ED process, the AC electrode was placed on a stainless steel current collector. It is seen from Figure 2a that a semi-transparent gel layer was formed on the surface of the electrode after electrodeposition. This gel layer is composed of PVAPB, water and KCl salt, and the possible electrode reactions of the ED process are shown as below. Anode:

Cathode:

(PVAPB) After electrodeposition, the AC electrode was peeled off from the current collector. As shown in Figure 2b, one side of the AC electrode was completely covered with a PVAPB GPE layer, and the entire structure was soft and flexible, which could be used directly in preparing flexible supercapacitors. Figure 2c shows the optical microscope (OM) image of the cross-section of the AC electrode covered with PVAPB GPE. The GPE layer attached tightly to the surface of the AC electrode with a thickness of about 120 µm. The thickness of the GPE layer is readily controllable by selectively adjusting the ED condition such as deposition time and solution


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concentration. The dried PVAPB GPE was also observed by SEM. Figures 2d, e and Figures 2f, g show the SEM images of the cross-section and the surface of dried PVAPB gel, respectively. It is seen that the dried PVAPB gel layer has a porous structure, which is beneficial for ion diffusion. KCl crystal granules are also observable on the surface of PVAPB, as indicated in the red circles in Figure 2f, demonstrating that the KCl salt has been incorporated in the PVAPB gel during electrodeposition. Figures 3a and b present FTIR and XRD spectra of the PVAPB GPE prepared from 0.9 M KCl ED solution. In the FTIR spectrum, the peaks at 3325 cm-1 and 1330 cm-1 are attributed to the O-H stretching and bending vibrations of PVA, respectively. The peak at 2917.3 cm-1 is assigned to the stretching vibration of C-H bonds. The peak at 1422.8 cm-1 belongs to the -CH2- bending vibration, and the peak at 1104.5 cm-1 ascribes to the stretching vibration of C-O. All these peaks correspond to the characteristic adsorption bands of PVA. However, the peaks of B-O-C cannot be observed from the spectrum probably due to its relatively low concentration. The absence of crystal band of PVA (near 1140 cm-1, showed in the FTIR spectrum of pure PVA film) indicates that PVA is mainly in its amorphous state in the as-prepared PVAPB GPE.26 In the XRD spectrum, the peaks at 2θ of 28.3°, 40.5°, 50.1°, 58.6° and 66.3° are all attributed to the KCl crystal (JCPDS card No. 41-1476), and only a very weak peak at 19.6° belongs to the PVA phase (JCPDS card No.53-1847).27 These results show that the PVAPB GPE contains large amount of KCl salt and PVA is mainly in its amorphous state in the PVAPB GPE, which is desirable for effective ion transfer.


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TGA tests were carried out to determine the proportion among H2O, PVAPB and KCl in the PVAPB GPE. Figure 4 shows the TGA curves of PVAPB GPE (prepared from 0.9 M KCl ED solution) and PVA. It is seen that PVA starts to decompose in air at 230 oC and achieve 100% weight loss at 650 oC, indicating that PVA can be completely thermo-decomposed. Due to the relatively low borax concentration, the thermostability of PVAPB in the gel electrolyte is comparable to that of pure PVA. Therefore, the weight loss of PVAPB GPE before 230 oC can be ascribed to the removal of water molecules (either free H2O or combined H2O), and the residual weight after 650 oC is mainly caused by the remaining KCl salt. According to the TGA result, the mass ratio among H2O, PVAPB and KCl is about 87.4 : 5.5 : 7.1. This indicates that PVAPB GPE contains a large amount of KCl solution, absorbed in the crosslinked gel. As a result, the density of PVAPB GPE prepared from 0.9 M KCl ED solution was measured to be 1.04 g cm-3, which is comparable to that of the ED solution. AC electrodes electrodeposited with PVAPB GPEs with different KCl concentrations were assembled into supercapacitors. The assembling process is shown in Figure 5a. In the assembled supercapacitors, PVAPB GPEs serve as both the electrolyte and the separator. The PVAPB GPB-integrated AC electrodes were in the free-standing form with no substrate support, and no additional electrolyte and separator was used in the assembling process. The galvanostatic charge/discharge curves of AC/AC supercapacitors at 20th cycles are demonstrated in Figure 5b. It is seen that with increase in KCl concentration, both the charge and discharge processes take longer time. The specific capacitances of supercapacitors prepared from ED solutions containing


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0.3, 0.45, 0.6, 0.75 and 0.9 M KCl are 49.9, 55.5, 61.25, 64.1 and 65.9 F g-1, respectively. It is obvious that supercapacitors using PVAPB GPEs prepared with higher KCl concentrations have higher specific capacitances. For comparison, the result for a control supercapacitor sample using 0.9 M KCl aqueous solution as the electrolyte and glass microfiber membrane as the separator is also shown in Figure 5b. It is seen that although a 0.9 M KCl solution is used as the electrolyte in the control supercapacitor sample, its specific capacitance (52.7 F g-1) is only slightly higher than that of the supercapacitor using PVAPB GPE prepared with a lower KCl concentration of 0.3 M. This demonstrates that the introduction of electrodeposited PVAPB GPEs can significantly improve the charge/discharge performance of supercapacitors. From Figure 5b, it is seen that an obvious potential drop exists at the top of each charge/discharge curve, which is called IR drop and represents the internal resistance of the device. Evidently, the supercapacitors using PVAPB GPEs prepared with higher KCl concentrations have reduced internal resistances since the electrolytes with higher salt concentrations typically possess higher ionic conductivities. The electrochemical behavior was also evaluated by cyclic voltammetry (CV) analysis in a voltage range from 0.05 V to 1.15 V at scan rate of 1 mV s-1 for supercapacitors using PVAPB GPEs prepared from ED solutions with different KCl concentrations, as shown in Figure 5c. All six supercapacitors present nearly rectangular-shaped CV curves with no apparent redox peaks, which indicate that the charges are accumulated on the interfaces between the electrolyte and electrode and these supercapacitors are truly electric double layer capacitors. The supercapacitors


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using PVAPB GPEs prepared with higher KCl concentrations exhibit higher capacitances, which is in good agreement with the galvanostatic charge/discharge result. PVAPB GPEs in this study are aqueous electrolytes. It is well known that the main drawbacks of most aqueous electrolytes are that they often have narrow electrochemical windows due to the electrolysis of water. In order to examine the electrochemical windows of PVAPB GPEs, linear sweep voltammetry (LSV) tests were carried out and the results are showed in Figure 5d. It is seen that the currents of all PVAPB GPEs are stable at low voltages. With continuous increase in voltage, the currents begin to rise gradually and redox peaks start to appear at 2 V. For the control sample, irregular peaks with strong current fluctuations appear when the voltage is greater than 2.2 V, indicating the dramatic electrolysis reaction of the aqueous electrolyte. However, for PVAPB GPEs, the redox peaks are relatively weak and wide, and the currents do not increase dramatically at high voltages. Thus, it is obvious that PVAPB GPEs are more electrochemically stable than the liquid KCl solution electrolyte used in the control sample. It is well known that the formation of gas bubbles is a phenomenon that is associated with the electrolysis of aqueous electrolyte. From the dynamic aspect, suppressing the bubble formation is a way to improve the stability of the electrolyte. In this work, PVAPB GPEs were prepared directly on the surfaces of the electrodes, and hence they are able to penetrate into the electrodes to cover and wrap the active particles tightly, which prevent the bubbles from forming on the active particle surfaces. Additionally, PVAPB GPEs can also minimize the water electrolysis effect. When the electrolysis of water is induced at high voltages, hydroxyl ions are generated on


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the electrode surfaces and enhance the crosslinking in PVAPB. The increased crosslinking density reduces the ionic conductivity of PVAPB GPEs, which retards the electrolysis of water. This unique feature, in fact, endows the supercapacitors with self-protection function at high charging voltage. The EIS data expressed as Nyquist plots over the frequency range of 10-1 to 106 Hz for the AC/AC supercapacitors are given in Figure 4e. The intercept of the semicircle with the real axis represents the solution resistance (Rs) of the electrolyte in contact with the electrodes. The diameter of the semicircle represents the resistance of the charge transfer (Rct).28 The Nyquist plots of the supercapacitor using PVAPB GPE prepared from 0.9 M KCl ED solution and the control sample were also fitted by using software Z-View 2.15 The equivalent circuit model and the fitting curves are showed in Supporting Information Figure S2. The Rs and Rct values of the control sample are 0.47 Ω and 2.31 Ω, and those of the supercapacitor using PVAPB GPE are 0.54 Ω and 0.22 Ω. Results show that the Rct values of supercapacitors using PVAPB GPEs are much lower than that of the control sample, while the Rs of the control sample is at the same level with that of supercapacitor using PVAPB GPE prepared from 0.9 M KCl ED solution. This demonstrates that the use of PVAPB GPEs with the separator-free structure can effectively reduce the interfacial charge transfer resistance. With the increase of KCl concentration used in PVAPB GPE preparation, the Rs of the resultant supercapacitor decreases rapidly while the Rct presents a tendency of slow reduction. This indicates that PVAPB GPEs prepared using higher salt concentrations are more suitable for use in supercapacitors.


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The supercapacitor using PVAPB GPE prepared from 0.9 M KCl ED solution was disassembled after 20 charge-discharge cycles to examine the cross-section of the electrode/GPE/electrode structure using SEM (Figure 6). It is seen that compared with the AC electrode layers, the PVAPB GPE layer is extremely thin (about 6~8 µm), which is not only thinner than the original thickness (about 240 µm) but also thinner than those of most commercial separators.29 This indicate that during the supercapacitor assembling process, the PVAPB GPE was compressed into a thin and compact layer. This thin GPE structure makes it more suitable for preparing compact, paper-like supercapacitors. The ionic conductivity of PVAPB GPE inside the supercapacitor can be calculated by = /( × ) , where L is the thickness of the GPE layer, A is the contact area between the GPE layer and the electrode (1.27 cm2), and Rb is the bulk resistance of the GPE layer, which equals the Rs value obtained from EIS study. The median thickness of the PVAPB GPE in the supercapacitor was 7 µm. Therefore, the ionic conductivity of PVAPB GPE prepared from ED solution containing 0.9 M KCl was calculated to be 1.02 mS cm-1, which is comparable to those of liquid electrolytes.30 The thickness, ionic conductivity and sheet resistivity of the PVAPB GPE are compared with those of seven different PVA based GPEs reported in literature (Supporting Information, Table S1). The ionic conductivity of PVAPB GPE is higher than LiPVAOB and LiClO4-SiO2-g-HBPAE/PVA reported by Y.S. Zhu et al. and X.L. Hu et al. respectively,31,32 but it is lower than those of other five reported PVA GPEs.17-21 It must be noted that the thicknesses of these five PVA GPEs are all one or two magnitude larger than that of our


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PVAPB GPE. As a result, the sheet resistivity of the PVAPB GPE is the lowest among all these GPEs. The extremely small thickness together with the reasonable ionic conductivity of the PVAPB GPE could lead to the low internal resistance, which is useful for the development of supercapacitors with better rate performance and higher capacitance. Among all PVAPB GPEs studied in this work, the PVAPB GPE prepared using 0.9 M KCl shows the highest specific capacitance, lowest internal resistance and good electrochemical stability. The rate performance, cycling stability and self-discharge behavior of the supercapacitor using this PVAPB GPE are shown in Figure 7. It is seen that the rate performance, cycling stability and self-discharge stability of the supercapacitor using PVAPB GPE are superior to those of the control sample. With increase in current density, the specific capacitance of the supercapacitor using PVAPB GPE decreases slowly while that of the control sample experiences rapid decrease. The supercapacitor using PVAPB GPE still exhibits relatively large discharge specific capacitances of about 45 and 30 F g-1 at 1 and 2 A g-1, which are about 70% and 47% of the capacitance (64 F g-1) at 0.1 A g-1, respectively (Figure 7a). This good rate performance should attribute to the ultra-low Rct value of PVAPB GPE, which reduces the energy loss, especially at high current densities. In addition, the capacitance retention of the supercapacitor using PVAPB GPE at 1 A g-1 is greater than 95% even after 2000 charge/discharge cycles (Figure 7b), which demonstrates the good electrochemical stability of PVAPB GPE in the supercapacitor. The self-discharge of the supercapacitor using PVAPB GPE is obviously slower than that of the control sample. After 24 h, the voltage of the supercapacitor


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using PVAPB GPE is still higher than 0.85 V, while the control sample only remains 0.46 V (Figure 7c). The CV tests at different scan rates and long term charge-discharge cycling tests were also performed to further illustrate the stability of the supercapacitor using PVAPB GPE (Supporting Information, Figure S3). The results show that the supercapacitor exhibits good and stable electric double layer capacitive behavior even at high scan rates. The capacitance retention rate after 10000 cycles is as high as 90%, and the coulombic efficiency in the entire cycling process maintains more than 99.5%.

4. CONCLUSIONS In summary, we conceived a method for preparing new polyvinyl alcohol borate aqueous gels by electrodeposition, which allows the uniform deposition of PVAPB GPE membranes directly onto active carbon electrodes. These PVAPB GPEs are mainly composed of PVAPB, potassium chloride and water, and can serve as both the electrolyte and the separator in supercapacitors. Supercapacitors fabricated using PVAPB GPEs have lower internal resistance, higher specific capacitance and better rate/cycling performance than the supercapacitor using corresponding liquid electrolyte. Additionally, the PVAPB GPEs provide supercapacitors with an excessive voltage protection function due to their unique crosslinking structure. It is, therefore, demonstrated that PVAPB GPEs are promising electrolyte candidates for supercapacitors. Theoretically, all water-soluble salts whose cations have weaker electron accepting ability than H+ can be adopted to prepare PVAB GPEs by this method (Supporting Information, Figure S1),


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and hence PVAB GPEs can be prepared using different salts to be applied in other electrochemical devices or for other purposes.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Photograph of PVAB GPEs electrodeposited on activated carbon electrodes, charge-discharge performance of supercapacitors using PVAB GPEs, equivalent circuit model and fitting curves, CV curves at different scan rates and long-term cycling performance, properties of PVA GPEs reported in literature. AUTHOR INFORMATION Corresponding Author *E-mail:[email protected]; *E-mail:[email protected] Notes The authors declare no competing financial interest.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Funding Sources Outstanding Young Scholar Visiting Fund of Sichuan University


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ACKNOWLEDGMENT The authors appreciate the financial support from Outstanding Young Scholar Visiting Fund of Sichuan University and the use of Analytical Instrumentation Facility (AIF) at North Carolina State University.


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+

-

e-

e-

H2 Y2 or O2 + H+ HO Y OH X+ YX+ B X+ YB(OH) 3 Y + OHB(OH)3 HO * HO X YY Y+ Y + X+ Y X + X * X X+ + X+ X O O Y Y B- X+ Y- OH OH O O B(OH)3 Y- B(OH)3 X+ YX+ O O Y Y + X HO B- X+ * YHO Y O O + Y- X+ m X B(OH) n Y3 X+ PVA-B-X+ Gel + + + + 2+ 2+ X : Li , Na , K , Mg , Ca , etc Electrode material Y-: Cl-, Br-, I-, F-, NO3-, SO42-, etc Current collector *

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Schematic of electrodeposition of PVAB GPE.


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Figure 2. (a) Surface appearance of AC electrode before and after electrodeposition of PVAPB GPE; (b) Photograph of an AC electrode coated with PVAPB GPE showing its flexibility; (c) Optical microscope image of the cross-section of the AC electrode coated with PVAPB GPE; (d, e) SEM images of the cross-section of PVAPB GPE layer; and (f, g) SEM images of the surface of PVAPB GPE layer. PVAPB GPE was prepared from 0.9 M KCl ED solution.


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(a)

(b)

28.3

—PVAPB GPE 40.5

1330.2 1422.8

50.1

19.6

58.6

66.3

—PVA JCPDS Card No.53-1847 —KCl JCPDS Card No.41-1476

1104.5

2917.3

Pure PVA Film PVAPB GPE

3325.2

4000

Intensity

1140

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3000

2000

1000

10

20

Wavenumbers (cm-1)

30

40

50

60

70

2θ°

Figure 3. (a) FTIR spectrum of PVAPB GPE and pure PVA film; and (b) XRD spectrum of PVAPB GPE and JCPDS cards No. 53-1847 and No. 41-1476. PVAPB GPE was prepared from 0.9 M KCl ED solution.


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100

Pure PVA Film PVAPB GPE

80

Weight % (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40

20

12.61%

7.10%

0 0

100

200

300

400

500

600

700

Temperature (°C) Figure 4. TGA curves of PVAPB GPE and PVA in air atmosphere. PVAPB GPE was prepared from 0.9 M KCl ED solution.


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Figure 5. (a) Assembling process of a supercapacitor; (b) Charge/discharge curves of supercapacitors at current density of 0.1 A g-1; (c) CV curves of supercapacitors with scan rate of 1 mV s-1; (d) LSV curves of supercapacitors with scan rate of 10 mV s-1; and (e) Nyquist plots of supercapacitors. Supercapacitors in (b, c, d, e) used PVAPB GPEs prepared from ED solutions with different KCl concentrations.


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Figure 6. SEM imagines of the cross-section of electrode/PVAPB GPE/electrode after 20 charge-discharge cycles. PVAPB GPE was prepared from 0.9 M KC ED solution.


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50

(a)

60 50

0.1A/g 0.2A/g

40

0.5A/g

30

1A/g

20

2A/g

Charge Discharge

10

(b)

PVAPB GPE Specific capacitance(F/g)

PVAPB GPE Control sample

70

Specific capacitance(F/g)

40

30

Charge Discharge current density: 1A/g

20

10

Control sample 0

0 0

10

20

30

40

50

0

500

1000

Cycle Index

1500

2000

Cycle Index

1.2

(c)

PVAPB Control Sample

1.0

Voltage(V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

0.6

0.4 0

5

10

15

Time(h)

20

Figure 7. (a) Rate, (b) cycling, and (c) self-discharge tests of the supercapacitor using PVAPB GPE prepared from 0.9 M KCl ED solution. The control sample used 0.9 M KCl solution as the liquid electrolyte and glass microfiber membrane as the separator.


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Table of Contents Graphic


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Poly(vinyl Alcohol) Borate Gel Polymer Electrolytes Prepared by

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