High performance solid state supercapacitor based on a 2-mercaptopyridine redox-mediated gel polymer

Abstract

A novel gel polymer, polyvinyl alcohol-orthophosphoric acid-2-mercaptopyridine (PVA-H₃PO₄-PySH), is prepared through introducing redox-mediated 2-mercaptopyridine into a polyvinyl alcohol-orthophosphoric acid host, and a solid state supercapacitor is fabricated using the gel polymer as an electrolyte and a separator and using activated carbons as electrodes. The PVA-H₃PO₄-PySH gel polymer was shown to have excellent stretching and bending properties. The electrochemical properties of the supercapacitor are investigated by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy. Surprisingly, electrode specific capacitance (1128 F g⁻¹) and energy density (39.17 W h kg⁻¹) are increased by 447% by introducing PySH as the redox mediator in the PVA-H₃PO₄ gel polymer. The supercapacitor with the PVA-H₃PO₄-PySH gel polymer shows an excellent capacitance retention of 80% for over 1000 cycles. Simultaneously, the ionic conductivity of the gel polymer electrolyte increased by 92% up to 22.57 mS cm⁻¹ compared to that of the PVA-H₃PO₄ system. These improved performances are owed to the redox reaction between 2-mercaptopyridine (PySH) and 2,2′-bipyridine (PySSPy) redox couples in the PVA-H₃PO₄-PySH gel electrolyte, indicating the supercapacitor combines the double-layer characteristic of carbon-based supercapacitors and the faradaic reactions characteristic of batteries' energy-storage processes.

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1. Introduction

Flexible devices are a mainstream direction in modern electronics and related multidisciplinary fields.^1–3 Concerning flexible capacitors and batteries, the current research is mainly focused on the fabrication of flexible electrode materials.^4–6 However, electrolyte development is also a critical factor for attaining highly flexible devices.^7,8 The development of flexible energy devices, even soft robots, has quickly increased the requirement of soft electrolytes with mechanical robustness and facile ion or solute transport.^9–11 Therefore, for the special configuration design and fabrication of novel energy storage devices, a thin and flexible layer of gel polymer electrolytes (GPEs) is considered to be the most effective geometry because of the intrinsic properties of GPEs, such as its thin-film forming ability, high reliability, flexibility, facile designing, separator-free, and the relatively high ionic conductivity.^12–14 Compared with the conventional energy storage devices using liquid electrolytes, the energy storage devices based on GPEs don't require high standard safety encapsulation materials, and thus their geometric shape is variable, which may bring new design opportunities for energy storage devices in the wearable electronics field.^15,16

In order to prepare high-performance GPEs, several polymer matrix materials have been developed and investigated in recent years, including poly(ethylene oxide) (PEO), poly(vinyl alcohol) (PVA), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), poly(vinylidene fluoride) (PVDF). Among these polymers, PVA is a good candidate for GPEs because of its excellent chemical stability, mechanical properties, high ability to form transparent films, and nontoxicity.^12,17,18

Moreover, for the purpose of further improving the electrochemical performance of the GPEs, some modification methods have been utilized, such as the addition of a basic or acidic salt to enhance the conductivity because of the high dynamics of hydroxyl or proton (e.g., KOH and H₂SO₄),^12,18 and redox mediator have also been incorporated to bring additional pseudocapacitance by the quick, reversible redox reaction.^19–21 Senthilkumar et al.²² reported that 57.2% of specific capacitance and energy density was increased while introducing Na₂MoO₄ (sodium molybdate) as the redox mediator in PVA-H₂SO₄ gel electrolyte due to the redox reaction between Mo(VI)/Mo(V) and Mo(VI)/Mo(IV) redox couples in the PVA-Na₂MoO₄-H₂SO₄ gel electrolyte. The pseudocapacitive effect of p-benzenediol in PVA-H₂SO₄ was investigated by Yu et al.²³ The specific capacitance of the electrode reached 474.29 F g⁻¹ (−0.5 to 0.5 V), which was much higher than that of the PVA-H₂SO₄ system in the same conditions. Wu et al.²⁴ reported that p-phenylenediamine could be used as a redox intermedium in KOH electrolyte, and the supercapacitor with KOH + PPD electrolyte has a much higher electrode specific capacitance (605.225 F g⁻¹) than the one with conventional KOH electrolyte (144.037 F g⁻¹) at the same current density of 1 A g⁻¹. Additionally, the other redox mediators, such as potassium ferricyanide²⁵ and methylene blue²⁶ have been added into the gel electrolyte to enhance the capacitive performance of carbon-based supercapacitors. Although the above research enhanced the capacitive performance of supercapacitors, little significant research on 2-mercaptopyridine type redox mediators in the gel electrolyte have been attempted.

The stable and reversible redox reactions of 2-mercapto pyridine (PySH) have been researched and applied in the field of organic synthesis and biological sensors.^27–29 Here, we introduce the PySH into the PVA-H₃PO₄ gel polymer electrolyte for the activated carbon-based supercapacitor. Because of intrinsic redox states and quick reversible faradaic reactions, the PVA-H₃PO₄-PySH gel electrolyte simultaneously possesses higher ionic conductivity, additional pseudocapacitance and good stability, and it is expected that the electrochemical performance of the electrolyte and the corresponding supercapacitor can be remarkably improved.

2. Experimental

2.1. Materials

Polyvinyl alcohol (PVA, Aladdin Co., China, molecular weight 44.05 MW, alcoholysis: 99.8–100%), H₃PO₄ (Tianjin Baishi chemical co., China), 2-mercaptopyridine (PySH, Shanghai Saen Chemical Technology Co., China), and activated carbon (AC, Shanghai Sino Tech Investment Management Co., China) were purchased and used without further purification, unless otherwise specified. All other chemical reagents were of analytical grade.

2.2. Preparation of gel polymer

Gel polymer electrolyte was prepared by a solution-casting method by modifying the procedure from literature.³⁰ First, 1 g of PVA was dissolved in 10 mL distilled water with agitation at 80 °C for 2 h to form a homogeneous and low-viscous solution. Then, 10 mL of an aqueous solution containing H₃PO₄ (2.0 g) and PySH (0–0.20 g) was added to the above solution with constant stirring, and it was kept stirring until the formation of a gel-like solution was observed. Finally, the resultant mixture was poured into a to 90 mm plastic Petri dish, and the dish was frozen at −25 °C for 12 h and thawed at room temperature for 12 h. The freeze–thaw cycles were repeated 2 times to obtain the PVA-H₃PO₄-PySH gel polymer.

For comparison purposes, the gel polymer PVA-H₃PO₄ without PySH was also prepared under the same conditions.

2.3. Preparation of activated carbon electrode

Initially, powdered activated carbon, acetylene black, and binder polytetrafluoroethylene (PTFE) aqueous solution in the weight ratio 80 : 10 : 10 was put in agate mortar and dispersed with 0.4 mL of N-methyl-2-pyrrolidone (NMP) to form a uniform slurry.³¹ Further, the obtained slurry was coated on the stainless steel net (thickness = 0.06 mm) with an area of 1 cm² and dried at 60 °C overnight. An AC electrode was obtained, and the active material loading was (including acetylene black and polytetrafluoroethylene) 2.5 mg on each electrode.

2.4. Fabrication of supercapacitor

A two-electrode test supercapacitor was fabricated with a pair of the AC electrodes and stretchable gel polymer electrolyte in a sandwich configuration. The gel polymer simultaneously served as electrolyte and separator, and the two stainless steel nets with the electrode material were used as current collectors. The schematic diagram and model of the supercapacitor are given in Fig. 1(a) and (b).

Fig. 1 (a) Schematic diagram of a fabricated supercapacitor; (b) schematic representation for the supercapacitor model.

2.5. Characterizations

The mechanical performance of the gel polymer was tested via bending and stretching. The membrane of the gel polymer was cut into a ribbon of 0.7 cm width and 3.0 cm length and the deformation was caused under proper forces. Then, the elongation at break is tested for PVA-H₃PO₄ and PVA-H₃PO₄-PySH gel polymers by a microcomputer controlled electronic universal testing machine (WDW-2C) at the same thickness and extension rate of 5 mm min⁻¹.

All electrochemical measurements were performed on an electrochemical workstation system (Model: CHI 660D, Shanghai Chen Hua Co., Ltd) using a two-electrode system under ambient conditions. The electrochemical properties of the supercapacitor were studied through CV at different scan rates from 5 to 50 mV s⁻¹ and GCD testing was performed at current densities from 0.5 to 5 A g⁻¹ from potential of −1 to 1 V. Electrochemical impedance spectroscopy (EIS) was carried out at an open circuit potential (OCP) by applying ac potential with 5 mV of amplitude in the frequency range from 0.1 Hz to 10⁵ Hz. Measurements of cycle-life stability were performed using computer controlled cycling equipment (LAND CT2001A, Wuhan, China). The specific capacitances (C, F g⁻¹) of the supercapacitor and electrodes (C_s, F g⁻¹) were evaluated from charge–discharge curves according to the following equation:³²

C = (I × Δt)/(ΔV × m_ac) (1)

C_s = 4 × C (2)

Energy density (E, W h kg⁻¹), equivalent series resistance (ESR, Ω) and power density (P, kW kg⁻¹) of the supercapacitor were calculated according to the following equations^32,33

E = [(C × (ΔV)²)/2] × (1000/3600) (3)

ESR = iR_drop/(2 × I) (4)

P = (ΔV)²/(4 × ESR × m_ac) (5)

where I (A) is the discharge current, m_ac (g) is the weight of active material (including the acetylene black and the binder), Δt (s) is the discharge time, ΔV (V) is the operating voltage window of the supercapacitor, and iR_drop (V) is defined as the electrical potential difference between the two ends of a conducting phase during charging-discharging.

Additionally, the ionic conductivity of the gel polymer in the supercapacitor was also determined from impedance spectrum. The ionic conductivity (σ, S cm⁻¹) of the gel polymer can be calculated by the following equation:³¹

Σ = L/(R_b × S) (6)

where L (cm) is the distance between the two pieces of stainless steel net, R_b (ohm), obtained from the curves of EIS, is the bulk resistance, and S (cm²) is the contact area of the electrolyte film with stainless steel net during the experiment (manually measured for several times).

3. Results and discussions

3.1. Mechanical properties of gel polymer

The PVA-H₃PO₄-PySH gel polymer has excellent stretching and bending properties. It can be observed from Fig. 2(a) and (b) that the gel polymer ribbon can be elastically stretched to about 2 times the length of its original, and from Fig. 2(c) and (d), the gel polymer ribbon is easily twisted into a spiral and bent into a circle without fracturing. All of the ribbons are able to quickly recover their original length and shape after the external force was removed, exhibiting outstanding mechanical properties.

Fig. 2 Images of a flexible PVA-H₃PO₄-PySH gel polymer.

The data of tensile elongation at break points are obtained as 311.93% of PVA-H₃PO₄ and 392.17% of PVA-H₃PO₄-PySH gel polymers. We can infer that doping 2-mercaptopyridine could improve the flexible and tensile properties of the polymers.

3.2. Ionic conductivity of gel polymer

Fig. 3 shows the effect of the amount of PySH on the ionic conductivity of the gel polymer. As can be seen, when the amount of PySH is less than 0.17 g, the ionic conductivity rises quickly and reaches the highest value of 22.57 mS cm⁻¹ with the PySH amount of 0.17 g, but when the PySH content is beyond 0.17 g the ionic conductivity gradually decreases with increasing PySH content, which means that the ionic conductivity of the PVA-H₃PO₄ gel polymer can be improved with an appropriate doping amount of PySH. This is probably because of the quick reversible redox processes triggered by PySH, which make the ionized PySH provide more valid ions to transfer through the free volume in the polymer host, therefore, increasing the ionic conductivity of PVA-H₃PO₄ gel polymer. When the PySH amount is less than 0.17 g, PySH cannot function as a redox shuttle, and the conductivity of the gel polymer is smaller. However, higher PySH content led to the aggregation of free ions and the crystallization of PySH in the PVA-H₃PO₄ system, which impeded the ions transport process and induced the decrease of the ionic conductivity.³⁴ Therefore, 0.17 g of PySH is optimal in the PVA-H₃PO₄ gel polymer.

Fig. 3 Ionic conductivity of PVA-H₃PO₄-PySH gel electrolyte with different contents of PySH.

3.3. Cyclic voltammetry measurements

Cyclic voltammetry spectra of the supercapacitor with PVA-H₃PO₄ and PVA-H₃PO₄-PySH gel electrolytes at a scan rate of 10 mV s⁻¹ in the potential window range from −1 to 1 V are shown in Fig. 4(a). Almost rectangular behavior without visible redox peaks is observed for PVA-H₃PO₄, which indicates that the capacitance is stored by an accumulation of electrolyte ions between the electrode|electrolyte interfaces (which is known as electric double layer capacitance).³² In contrast, while using PVA-H₃PO₄-PySH as electrolyte and separator, a pair of well-defined and strong redox peaks (centered at −0.25 and 0.25 V and corresponding to the oxidation and reduction of PySH and PySSPy, respectively) appears in CV curves. The probable processes of redox reactions are presented in Fig. 5,²⁹ and this reaction makes the supercapacitor have faradaic reaction characteristic of batteries.³⁵ Because the peak potential differences of the redox pair are small and the voltammetric response on the positive sweeps is symmetric to its counterpart on the negative sweep, a reversible redox process occurs in this electrolytic system.²⁴ As we know, pseudocapacitive performance can be heightened by the redox reaction and consequently employed as an electrolyte material for supercapacitors. Obviously, the appearance of the redox peaks indicates that the addition of PySH enhances the electrode's capacitive performance, which can be inferred from the areas of CV spectra.³⁶

Fig. 4 (a) CV curves for the supercapacitors with PVA-H₃PO₄ and PVA-H₃PO₄-PySH gel electrolytes at a scan rate of 10 mV s⁻¹; (b) CV curves for the supercapacitor with PVA-H₃PO₄-PySH gel electrolyte at different scan rates from 5 mV s⁻¹ to 50 mV s⁻¹.

Fig. 5 Representation of the redox processes in the electrode|electrolyte system (black, red and yellow correspond to carbon, nitrogen and hydrogen atoms, respectively).

Fig. 4(b) represents the CV curves for the supercapacitor based on PVA-H₃PO₄-PySH gel electrolyte at different scan rates. It is observed that the CV curve area and the peak current rapidly increase with the increase of the scan rate from 5 to 50 mV s⁻¹. Moreover, a pair of redox peaks is still clearly observed even at a scan rate of 50 mV s⁻¹. The result reveals that the supercapacitor with PVA-H₃PO₄-PySH gel electrolyte has reversible redox processes and good rate ability.³⁷ In addition, from the CV curves we can find that, as the scan rate increases, the potentials of the oxidation and reduction peaks shift to more positive and negative directions, respectively. This may be due to the limitation of the ion diffusion rate to satisfy electronic neutralization during the redox reaction.³⁸

3.4. Galvanostatic charge–discharge measurements

The galvanostatic charge–discharge curves for the supercapacitor with the PVA-H₃PO₄ and PVA-H₃PO₄-PySH gel electrolyte are shown in Fig. 6. Obviously, the charge–discharge time of the supercapacitor with PVA-H₃PO₄-PySH is much longer than that of the supercapacitor with PVA-H₃PO₄. On the other hand, from the charge–discharge curves detected in the potential range of 0 to 1 V, the supercapacitor with PVA-H₃PO₄-PySH gel electrolyte exhibits nonlinear charge–discharge behavior. The inclined parts in the charging potential and the discharging potential indicate quick reversible redox reactions are occurring in the charge–discharge process,²³ which are consistent with the cyclic voltammograms in Fig. 4. Furthermore, the longer charge–discharge time, which may be due to the additional contribution of the PySH, reveals a great improvement in the electrochemical performances of the supercapacitor with PVA-H₃PO₄-PySH gel electrolytes.

Fig. 6 Galvanostatic charge–discharge curves of supercapacitors with PVA-H₃PO₄ and PVA-H₃PO₄-PySH gel electrolytes at a current density of 0.5 A g⁻¹.

According to eqn (2), for a charging–discharging current density of 0.5 A g⁻¹, discharging specific capacitance of the electrode for the supercapacitors with PVA-H₃PO₄ and PVA-H₃PO₄-PySH are determined to be 206 and 1128 F g⁻¹, respectively. Obviously, the C_s of the supercapacitor with PVA-H₃PO₄-PySH is larger than that of the supercapacitor with the PVA-H₃PO₄ gel polymer. The electrode discharging specific capacitance for the supercapacitors with the PVA-H₃PO₄-PySH is increased by 922 F g⁻¹ compared to the current PVA-H₃PO₄ system, indicating that the redox mediator PySH can greatly improve the capacitive property of the supercapacitor.

The charge–discharge curves of the supercapacitor with the PVA-H₃PO₄-PySH gel polymer at various current densities of 0.5, 0.8, 1, 2, and 3 A g⁻¹ are displayed in Fig. 7, and the C_s are calculated to be 1128, 825, 600, 368, and 300 F g⁻¹. As we can see, even when the current density is 3 A g⁻¹, the value of C_s is larger than that of the supercapacitor based on the PVA-H₃PO₄ gel polymer at lower density (206 F g⁻¹ at 0.5 A g⁻¹). It indicates clearly that a supercapacitor with PVA-H₃PO₄-PySH gel electrolyte shows superior electrochemical behaviour. The better performances may be due to the quicker ions diffusion rate and more adequate electrode|electrolyte interfacial contact, both of which are enhanced by the PySH.

Fig. 7 Galvanostatic charge–discharge curves of supercapacitors with PVA-H₃PO₄-PySH gel polymer at various current densities.

The energy and power densities were calculated from the galvanostatic charge–discharge at different current densities. According to eqn (3) and (5), energy density and power density are obtained and shown in Fig. 8. It is obvious that the supercapacitor with PVA-H₃PO₄-PySH gel polymer exhibits the highest energy density, 39.17 W h kg⁻¹, at a power density of 250 W kg⁻¹ and remains at 10.42 W h kg⁻¹ at 1500 W kg⁻¹, while the energy density and power density for the supercapacitor with PVA-H₃PO₄ gel polymer is only 7.15 W h kg⁻¹ and 250 W kg⁻¹. The supercapacitor with PVA-H₃PO₄-PySH electrolyte that has better electrochemical performances may be due to the pseudocapacitive contribution generated by the redox reactions of the redox mediator (PySH) in the supercapacitor system. Moreover, it is worth mentioning that the obtained maximum energy density of the supercapacitor with PVA-H₃PO₄-PySH is considerably higher than those of recently reported carbon-based supercapacitor using a redox-mediated gel polymer as an electrolyte, such as PVA-KOH-KI (7.80 W h kg⁻¹),¹⁹ PVA-KOH-PPD (82.56 W h kg⁻¹),²⁴ PVA-H₂SO₄-PB (10 W h kg⁻¹),²³ and PVA-KOH-K₃[Fe(CN)₆] (57.94 W h kg⁻¹).²⁵

Fig. 8 Ragone plot related to energy and power densities of the supercapacitors with PVA-H₃PO₄ and PVA-H₃PO₄-PySH gel polymer electrolytes.

3.5. EIS technique

In order to investigate the electrochemical behavior at the electrode|electrolyte interface in detail, EIS measurements were employed at open circuit potentials in the frequency range from 100 mHz to 100 kHz. The corresponding Nyquist impedance plots are shown in Fig. 9. The equivalent circuit used for fitting of the EIS plots in this work is shown in Fig. 9 by ZSimpWin software.^39,40 R_ESR is the equivalent series resistance (ESR), R_CT is the resistance of the electrode-electrolyte, C_DL is the constant phase element of double layer, and W is the Warburg element. The fitted data for all circuit elements is shown in Table 1. A possible reason for insensitivity to varying voltage scan rates is the short and equal diffusion path length of the ions in the electrolyte, as evidenced by a short Warburg region on the Nyquist plots.⁴¹ In the EIS curves, it can be seen that two gel polymers exhibit ideal electrochemical capacitance behavior, i.e., a small depressed semicircle at higher frequency, a small ∼45° inclined line at the middle frequency region (which is known as diffusive or Warburg resistance of ions with the electrode), and imaginary parts of impedance at the low frequency region are nearly linear. In addition, the supercapacitor with PVA-H₃PO₄-PySH not only has a lower inner resistance (R_i, 8.735 Ω cm²) calculated from the point of intersecting with the x-axis in the range of high frequency, but also possesses smaller interfacial charge transfer resistance (R_ct, 3.027 Ω cm²), counting from the span of the single semicircle along the x-axis from high to low frequency region. Additionally, R_b, the most important data used in the calculation formula of ionic conductivity, is obtained as follows: draw a tangent line going along the slope of EIS curves in the range of the low frequency until the line intersects with the x-axis. The point of intersection is the data we need. Clearly, the PySH additive enhances the interaction of electrolyte|electrode interface, which results in good electrochemical properties for the PVA-H₃PO₄-PySH gel electrolyte system.

Fig. 9 EIS of supercapacitors with PVA-H₃PO₄ and PVA-H₃PO₄-PySH gel polymer electrolytes.

Table 1 Fitted equivalent circuit elements of PVA-H₃PO₄-PySH gel polymer electrolytes

RESR	RCT	CDL	W
8.755 Ω	2.792 Ω	0.0001157	0.0983

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3.6. Cycle-life testing

The stability of a supercapacitor for repetitious charge–discharge cycles is one of the most electrochemical performances for its practical application. Herein, the cycle life test is carried out at a constant charge–discharge current density of 1 A g⁻¹ for 1000 cycles, and the plots of electrode specific capacitances of supercapacitors based on PVA-H₃PO₄ and PVA-H₃PO₄-PySH gel polymer as a function of charge–discharge cycles are depicted in Fig. 10. After 1000 charge–discharge cycles, the C_s of the PVA-H₃PO₄ system decreases from 176 to 158 F g⁻¹, and the C_s of the PVA-H₃PO₄-PySH system decreases from 600 to 480 F g⁻¹, still retaining 89.7% and 80.0% of the initial capacitance, respectively. It can be concluded that a redox-active PySH doping not only can increase the C_s of the supercapacitor, but also scarcely impede the stability of supercapacitors, indicating that the PVA-H₃PO₄-PySH gel polymer can be considered promising as an electrolyte and separator in the application of high-energy supercapacitors.

Fig. 10 Cycle-life of the supercapacitors with PVA-H₃PO₄ and PVA-H₃PO₄-PySH electrolytes in the long–term cycle at the charge–discharge current density of 4 A g⁻¹.

4. Conclusions

In the present work, a novel redox mediator gel polymer PVA-H₃PO₄-PySH was prepared by adding a redox additive PySH into a PVA-H₃PO₄ gel polymer electrolyte, and a solid state supercapacitor was assembled using the gel polymer as an electrolyte and a separator, and activated carbon was used as electrode. The supercapacitor with PVA-H₃PO₄-PySH gel polymer electrolyte simultaneously possesses a high ionic conductivity (22.57 mS cm⁻¹), large electrode specific capacitance (1128 F g⁻¹), high energy density (39.17 W h kg⁻¹), and an excellent cycle life that maintains 80.0% of the initial capacitance values after 1000 cycles. This result may be due to the fact that the redox reaction of PySH allows the supercapacitor to combine the double-layer characteristic of carbon-based supercapacitors and the faradaic reactions characteristic of batteries' energy-storage processes. It is strongly believed that the PVA-H₃PO₄-PySH gel polymer can be considered as a promising electrolyte and separator in the application of high-energy supercapacitors and the idea of using redox mediators has a good prospect for improving the performances of supercapacitors.

Acknowledgements

We thank the Science and Technology program of Gansu Province (no. 1308RJZA295, 1308RJZA265), the Colleges and Universities Scientific Research Program of Gansu Province (2013B-069), the PhD Scientific Research starting Program of Lanzhou City University (LZCU-BS2013-11), the program for Changjiang Scholars and Innovative Research Team in University (IRT1177), Key Laboratory of Eco-Environment-Related Polymer Materials (Northwest Normal University) of Ministry of Education, and Key Laboratory of Polymer Materials of Gansu Province.

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