Revealing the Mechanism of Bilayer Heterogeneous Polyelectrolytes to Suppress the Self‐Discharge of Symmetric Supercapacitors

The electrochemical supercapacitors with high power density, long cycle life, and excellent safety represent one of the most promising energy storage devices for flexible and portable electronics, but their spontaneously rapid drop of open‐circuit voltage (self‐discharge) greatly limits their wide applications. Herein, a series of bilayer heterogeneous polyelectrolyts (BHPs) consisting of a polyanion complex and a polycation complex are designed, to regulate the self‐discharge performance of supercapacitors. The BHP‐based supercapacitors possess comparable energy storage properties with those of devices based on traditional homogeneous polymer electrolyte, but exhibit a unique and noteworthy suppressed effect on the self‐discharge performance of devices. The experimental results and theoretical simulation reveal that the zeta potential difference between the used polyanion/polycation complexes has effect on the self‐discharge rate of BHP‐based supercapacitor, and the electrostatic interaction between polyelectrolytes and the mobile counterions also greatly affects the self‐discharge performance of devices. Herein, the effect of molecular structures and their interaction of polyelectrolyte complexes in BHPs on the electrochemical performance of the resultant supercapacitors are systemmatically investigated, which provides a general strategy to design novel polymer electrolytes to build high‐performance flexible supercapacitors with long self‐discharge time.

electrodes with the same effect. Some efforts were made to slowdown the self-discharge process of supercapacitors by modifing separator or using ion-exchange membrane. [20,24] In view of electrolyte, the self-discharge of supercapacitors could be suppressed by adding functional additive in electrolytes [25,26] or using ionic liquid electrolytes. [27,28] Polymer gel electrolytes used for flexible two-electrode supercapacitors have been widely investigated. [29,30] Conventional polymer electrolytes by mixed insulating polymers (e.g., polyvinyl alcohol, PVA) with ionic conducting compounds, however, their derived supercapacitors also suffer from the problem of rapid self-discharge, [31,32] the same with devices based on liquid electrolytes. Due to their unique chemical structure and properties, polyelectrolyte (polycation and polyanion) complexes containing dissociated movable ions are considered as the promising candidate electrolyes for supercapacitors with high electrochemical performance and suppressed self-discharge rate, which has been attracted increasing attention in recent years. [33][34][35] Recently, we for the first time introduced the bilayer heterogeneous polyelectrolyte (BHP) building by a layer of polycation complex and a layer of polyanion complex to construct two-electrode supercapacitors, which exhibited interesting suppressed effect to slowdown the self-discharge process of devices. [36,37] However, the insightful mechanism of how do the polyelectrolyte complexes affect on the electrochemical performance and self-discharge behavior of the BHP-based supercapacitors is still unclear. In this work, we designed a series of bilayer heterogeneous polyelectrolytes by using various polycation and polyanion complexes with different structures and properties and systematically investigated their effects on the electrochemical performance and self-discharge behaviors from the experimental and theoretical views. The experimental results showed that all the supercapacitors with positive electrode at the side of polyanion during charging process exhibited much longer self-discharge time than the devices with positive electrode at the side of polycation, while their capacitances and energy densities were almost the same. The theoretical simulation revealed that there was a built-in electric field (BEF) between the charged polyanions and polycations in the charged supercapacitor, which had strongly electrostatic interaction with the movable ions acculated on/in the electrodes. The intensity of electrostatic interaction depended on the different movable ions and the dissociated abilities of polyelectrolyte complexes.

Results and Discussion
Aiming to insightfully investigate the effect of different polyelectrolyte complexes on the self-discharge performance of supercapacitors, we first built BHP-based symmetric supercapacitors with using electric double-layer electrode of carbon nanotubes (CNTs). On the one hand, polydiallyl dimethyl ammonium chloride (PDDACl), polyallylamine (PAAOH), and polyethyleneimine (PEIOH) (Figure 1a) with different positive macromolecular backbones and counter ions were selected as the additive of polycation complexes. On the other hand, sodium polystyrene sulfonate (PSSNa), sodium polymethacrylate (PMAANa), poly 2-acrylamido-2-methylpropane sulfonic acid (PAMPSH), and polyacrylic acid (PAAH) (Figure 1b) were chozen as the additive of polyanion complexes. The surface-charged capacity and the surface potential of these polyelectrolyte complexes were evaluated by zeta potential. Figure 1c shows that PDDACl, PAMPSH, and PSSNa possessed higher absolute potentials than other polyelectrolyte complexes, suggesting their higher surface-charged capacities and ion dissociations. Similarly, the aqueous solution of PDDACl, PAMPSH, and PSSNa exhibited higher ionic conductivity (Figure 1d) than other polyelectrolyte complexes, which further reflected the different ion dissociations of the selected polyelectrolyte complexes.
The polyelectrolyte complexes were served as additive by dissolving in an aqueous solution of polyvinyl alcohol, which functioned as the matrix to facilitate the formation of films. The heterogeneous polyelectrolytes were formed by directly pressing one electrode coated with PVA solution containing a polyanion complex with another electrode coated with PVA solution containing a polycation complex during the assembling process of supercapacitors. In order to visualize the heterogeneous structure of bilayer polyelectrolyte, we colored the polycation (e.g., PDDACl) layer with solid green and the polyanion (PSSNa) layer with methyl orange, respectively. From Figure 1e, compact contact interface between two layers was clearly observed in the bilayer heterogeneous polyelectrolyte by using fluorescent microscope. Importantly, the bilayer polyelectrolyte well maintained its heterogeneous structure without obvious phase separator (Figure 1f ) even it was placed outside at room temperature for one month, indicating excellent structural stability. Figure 1g shows the current rectification of the device assembed by sandwiching two carbon naontube films with different BHPs in between. It can be calculated that the PDDACl/PSSNa heterogeneous polyelectrolyte had a current rectification ratio of 12.7, which was much higher than the heterogeneous polyelectrolyte of PEIOH/PSSNa (8.3) and PAAOH/PSSNa (6.0). The high current rectification ratio of PDDACl/PSSNa heterogeneous polyelectrolyte can be attributed its high degree of ions dissociation and large concentration of movable ions in the opposite polyelectrolytes. For comparison, the homogeneous bilayer of pure PVA polymer electrolyte did not present the rectification effect.
On the basis of absolute zeta potential of polyelectrolyte complexes, we first used polycationic complex of PDDACl and other polyanionic complexes with different zeta potentials to build bilayer heterogeneous polyelectrolytes in symmetric supercapacitors with using CNTs film as electrodes during assembling process of devices. The effect of different polyanionic complexes on the electrochemical performance and self-discharge behavior of BHP-based symmetric supercapacitors was investigated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). From Figure 2a,b and S1, Supporting Information, the electrochemical performance of all BHP-based supercapacitors almost unchanged as the electrode at either polyanion complex or polycation complex was served as positive during charging process of any device. The specific capacitances of the BHP-based supercapacitors were about 2.7 mF cm À2 , which were comparable with those of supercapacitors based on homogeneous electrolytes ( Figure S2a-c, Supporting Information). The result indicated that the bilayer heterogeneous polyelectrolytes can effectively mitigate the diffusion rate of movable small anions and cations throughout the BHP in the charged symmetric electric doublelayer supercapacitors, but did not change the energy storage mechanism and capacity of the devices. As we know, the devices with using homogeneous polyelectrolytes exhibited the same self-discharge behaviors as one electrode was acted as either positive or negative during charging process. In case of the BHP-based supercapacitors, the self-discharge time of any device charged with electrode at the side of polyanion complex as positive was nearly one time longer than that of device charged with electrode at the side of polycation complex as positive (Figure 2c). From Figure 2d, the self-discharge time of the PSSNa/PDDACl BHP-based supercapacitor was at least two times higher than those of devices based on homogeneous electrolytes, which indicated that the BHP had an efficiently suppressing effect on the self-discharge behaviors of supercapacitors.
Our further researches revealed that the contributions of BHP suppressing the self-discharge performance of supercapacitors not only came from the difference of zeta potential between polyanion and polycation complexes but also highly depended on the interaction forces between counter ions acculated on/in the electrodes and the charged polyelectrolytes in the open-circuit charged supercapacitors. From the simulated results in the charged supercapacitor based on BHP of PDDACl/PAMPSH (Figure 2e), the electrostatic repulsion between Cl À and PAMPS À was 224.76 kcal mol À1 , but electrostatic repulsion was only 0.17 kcal mol À1 between PDDA þ and H þ , which cannot www.advancedsciencenews.com www.small-structures.com efficiently restrict mobile ions migrating from both electrodes to the electrolyte layer. For BHP of PDDACl/PSSNa-based supercapacitor, the electrostatic repulsion between Cl À and PSS À was about 300.47 kcal mol À1 , while the electrostatic repulsion between PDDA þ and Na þ was about 303.18 kcal mol À1 , the comparable electrostatic repulsions at two sides can efficiently restrict mobile ions migrating from two charged electrodes to the electrolyte layer. As a result, the supercapacitor with using BHP of PDDACl/PSSNa possessed the longest self-discharge time (24 530 s) and the lowest leakage current (1.06 μA, Figure S3, Supporting Information) than the devices based on other three types of BHPs.
To further verify the feasibility of the above strategy, a series of BHPs constructed by polyanion complex of PSSNa with other polycation complexes were investigated to regulate the self-discharge performance of supercapacitors. As Figure S4, Supporting Information, shows, all the supercapacitors based on different BHP almost exhibited the same electrochemical performance as the electrode at the side of PSSNa was served either positive or negative electrode during charging process. For any BHP built by different polyanion and polycation complexes, the self-discharge time of the supercapacitor charged with the electrode at the side of PSSNa served as positive was much longer than that of the device charged with the electrode at the side of PSSNa as negative (Figure 3a), which was highly in accordance with the above demonstration. More specifically, the supercapacitor based on BHP of PSSNa/PDDACl possessed the longest self-discharge time than the devices with using BHPs of  PSSNa/PEIOH and PSSNa/PAAOH, which can be attributed to the large zeta potential difference between PSSNa and PDDACl, and the strong electrostatic repulsion between the small counterions and the charged polyelectrolytes ( Figure 3b). The calculated results (Figure 3b) showed that there was one electrode/ electrolyte interface that had relatively low electrostatic repulsion between counterions and the charged polyelectrolytes in the charged supercapacitors based on BHPs of PSSNa/PEIOH and PSSNa/PAAOH, meanwhile the electrostatic repulsions were strong with a comparative value for two electrode/ electrolyte interfaces at both sides of PSSNa/PDDACl-based supercapacitor. Therefore, the supercapacitor based on BHP of PSSNa/PDDACl showed much longer self-discharge time and lower leakage current ( Figure S5, Supporting Information) than those of supercapacitors while using BHPs of PSSNa/PEIOH and PSSNa/PAAOH.
Except the inevitable leaking current generated during device assembling process, the self-discharge of electric double-layer supercapacitors is mainly caused by the charge rearrangement, which is driven by potential-controlled model and/or diffusioncontrolled model. [22] With the supercapacitor based on BHP of PSSNa/PDDACl as an example, we simulated its self-discharge data at different experimental temperatures by the diffusioncontrolled model (V∝t 1/2 ) and the potential-driving model (lnV∝t) to reveal the self-discharge mechanism of these novel electrolytes-based supercapacitors. As Figure S6, Supporting Information, shows, the simulated curves were not well overlapped with the experimental data, which indicated that diffusion-controlled model dominated the self-discharge process, accompanied by potential-driving model. Figure 3c-e shows that both potential-driving model and diffusion-controlled model contributed to the self-discharge of the BHP-based supercapacitors.  It can be seen that the potential-driving time increased from 18% at 0°C to 30% at 25°C and 45% at 75°C, due to the accelerated ion diffusion rate at high temperature. Molecular dynamics simulations (MDS) were performed to deeply understand how did the polyanion complex and polycation complex in BHPs affect the self-discharge performance of supercapacitors based on CNTs electrodes. As Figure 4a shows, the mobile small anions (Cl À ) and cations (Na þ ) were accumulated on the surfaces of positive and negative electrodes, respectively, with the electrode at the side of electrolyte containning polycation complex of PDDACl served as positive during charging process. In the open-circuit charged supercapacitor after  withdrawing the external power supply, the existing strong electrostatic attraction between the charged polycations of PDDA þ and their counterions of Cl À at the positive side (also between charged polyanions of PSS À and their counterions of Na þ at the negetive side) will accelerate the diffusion of mobile small ions from the surface of electrodes to their corresponding charged polyelectrolytes. The accelerated diffusion of small ions caused rapid charge redistribution from the charged electrodes to electrolytes, resulting in fast self-discharge process. As the electrode at the side of electrolyte containing polyanion complex of PSSNa was functioned as positive during charging process (Figure 4b), the mobile small anions (Cl À ) and cations (Na þ ) will be accumulated at the surfaces of positive and negetive electrodes, respectively. In this case, strong electrostatic repulsive force was formed between the charged polyelectrolyte of PSS À and small ions of Cl À at the positive side (also between charged polyelectrolyte of PDDA þ and Na þ at the negetive side), which will largely slow and even restrain the diffusion of mobile small ions from the charged electrodes into electrolytes. As a result, the charged supercapacitor in open circuit exhibited a quite long self-discharge process. The simulation results ( Figure S7, Supporting Information) showed that the same effect was also existing in the BHP-based supercapacitors with using other polyelectrolyte complexes. Based on the simulation results, it can be clearly seen that a built-in electric field (BEF) was constructed between the charged polyanions and polycations in the charged supercapacitors (Figure 4c,d). As the direction of BEF is the same with that of electric double layer (EDL), the BEF will restrain the diffusion of charges from electrodes into the electrolytes (Figure 4c), resulting in slow self-discharge of supercapacitors. If the direction of BEF is contrary to that of EDL, the BEF will accelerate the diffusion of charges from electrodes into the electrolytes (Figure 4d), which will promote self-discharge process of supercapacitors. With using the unique bilayer heterogeneous polyelectrolyte, high-performance flexible symmetric supercapacitors with long self-discharge time were also developed, with using composite electrodes consisting of pseudocapacitvie material of manganese dioxide and electric double-layer capacitive material of CNTs. During charging/discharging process, intercalationdeintercalation of electrolyte cations (A þ ) on the surface and in the bulk of MnO 2 nanosheet (MnO 2 þ A þ þ e À ⇌ MnOOA) can greatly enhance the electrochemical performance of resultant supercapacitors. [38] Uniform MnO 2 nanosheets were grown on CNTs film to fabricate MnO 2 /CNTs composite ( Figure S8, Supporting Information) through an electrochemical deposition approach. The CV curves (Figure 5a) with nearly rectangular shapes at different scan rates and the GCD curves (Figure 5b) with symmetrically triangular shapes at different current densities indicated that the BHP-based supercapacitor exhibited ideally capacitive behavior. Calculated from GCD curves, the specific capacitance and energy density of the supercapacitor were 43 F g À1 and 3822 mwh kg À1 at current density of 0.1 A g À1 , respectively, which were comparable with other MnO 2 / CNTs-based supercapacitor with using homogeneous polymer electrolyte. [39][40][41] The specific capacitance of supercapacitor devices could be further enhanced by using polyelectrolytes with ionic liquid solvent or other electrode materials with high pseudocapacitance. The BHP-based supercapacitor with using MnO 2 /CNTs composite electrodes exhibited a capacitance retention of 60.5% as the discharge current density increased from 0.1 to 1.0 A g À1 ( Figure S9, Supporting Information), indicating good rate performance. In addition, the BHP-based supercapacitor even possessed comparable capacitance retention ( Figure S10, Supporting Information) with that of device with using conventional aqueous electrolyte containing 1.0 M NaCl. The electrochemical impedance spectrum (EIS) had two semicircles in the high frequency region and a straight diagonal line in the low frequency region ( Figure S11, Supporting Information). The series resistance (R s ) of the supercapacitor was about 13.16 Ω, deriving from the the electrolyte, separator, and electrodes. The nearly straight line at low frequency responded to the Warburg impedance once again revealed the ideally capacitive behavior of our BHP-based supercapacitor. [42,43] In addition, the BHP-based supercapacitor can maintain approximately 95% of its original capacitance after 5000 charge/discharge cycles ( Figure S12, Supporting Information), indicating excellent cyclic performance. Figure 5c shows that the supercapacitor exhibited much slower self-discharge time (79 030 s) from 0.8 to 0.4 V as the device charged with the electrode at the side of polyanion complex served as positive than that (33 590 s) of device charged with the electrode at the side of polycation complex as the positive. The result was the same with that of the electric double-layer supercapacitor demonstrated earlier, which revealed that it is an efficient strategy to suppress self-discharge performance of pseudocapacitive supercapacitors by designing the novel heterogeneous polyelectrolytes. The mechanism of suppressing effect of BHP on the self-discharge of pseudocapacitive supercapacitors could be ascribed to that of the confined ions and/or charges by BHP will somewhat decrease the rate of redox reaction of pseudocapacitive materials, resulting in slow self-discharge of the charged devices. As desired, the self-discharge time of the BHP-based supercapacitor was 21 times longer than that of supercapacitor fabricated by the mostly used PVA electrolyte. Accordingly, the leakage current of the homogeneous PVA electrolyte-based supercapacitor is much higher than that of the heterojunction-based supercapacitor (Figure S13a, Supporting Information). As Figure 5d shows, the self-discharge performance of our BHP-based supercapacitor is also superior to that of most supercapacitors reported previously. [20,[44][45][46][47][48][49][50] The mechanical flexibility of BHP-based supercapacitor has also been investigated. Figure 5e showed that the supercapacitor can maintain its original capacitance of 97.5% as it was bent from 0 to 180 degrees. Importantly, the supercapacitor exhibited a capacitance retention up to 97% (Figure 5f ) even the device was repeatedly bent to 135 degrees for 5000 times, suggesting the excellent flexible stability of the BHP-based supercapacitor. The output voltage and current could be easily tuned by connecting several devices in series and in parallel ( Figure S13b,c, Supporting Information). For instance, output voltage of 2.4 V can be realized by connecting three supercapacitors in series, which could successfully lit a light-emitting diode with operating voltage of 2.0 V (Figure 5h). Even if the connected devices in series were curled up, they still work normally. These devices can work without performance decay even they were bent to any degree, which indicated the BHP-based supercapacitors with excellent flexible stability.

Conclusion
In summary, we constructed a series novel BHPs, which were consisted of a polycation complex and a polyanion complex dissolved in the same matrix solution of PVA, respectively, to suppress the self-discharge behavior of symmetric supercapacitors. Specifically, three types of polycation complexes and four types of polyanion complexes with different structures and zeta potentials were selected to build the BHPs, and the effects of structures and properties of polyelectrolyte complexes on the self-discharge performance of the resulting supercapacitors were systematically investigated from both experimental and theoretical views. The results revealed that the self-discharge rates of BHP-based supercapacitors not only relied on the zeta potential difference between the used polyanion and polycation complexes but also highly depended on the electrostatic interaction forces between counterions and the charged polyelectrolytes at the interface of electrode and electrolyte. Among the series BHPs, there were large and proportionable electrostatic repulsive forces between the movable counterions and polyelectrolyte at the interface of electrode and electrolyte at both sides in the charged supercapacitor based on BHP of PSSNa/PDDACl, which can efficiently restrict the diffusion and redistribution of small counterions accumulated on/in electrodes into the electrolyte, resulting in slow self-discharge process. Furthermore, the same polymer matrix of PVA used in BHPs can well prevent the phase separation between two layers, which endowed the BHP-based supercapacitors with excellent mechanical flexibility and stability. This work insightfully revealed the working principle of BHP to suppress the self-discharge of supercapacitors, which could provide an important guideline to design novel electrolytes to build high-performance energy storage devices beyond supercapacitors in the future.

Experimental Section
Preparation of MnO 2 /CNTs Composite Films: MnO 2 /CNTs composite films were fabricated through an electrochemical deposition method in a three-electrode system. The optimal ratio was previously optimized by our group as following. [36] 1) The pure CNTs films were immersed in concentrated nitric acid for 12 h to remove impurities and then were treated with oxygen plasma for 5 min to improve the hydrophilicity of CNTs films. 2) The treated CNTs film was used as the working electrode for electrochemical deposition, where a platinum sheet and a saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. Electrolyte was an aqueous solution consisting of MnSO 4 (0.05 M), CH 3 COONa (0.05 M), and ethanol (10%, v/v). The electrochemical deposition was conducted at a constant current density of 5.0 mA cm À2 for 35 min. The prepared MnO 2 /CNTs composite films were rinsed several times with deionized water and then dried in a vacuum oven at 60°C for 2 h. The mass loading of MnO 2 in MnO 2 /CNTs composite films was 0.4 mg cm À2 .
Preparation of Bilayer Heterogeneous Polyelectrolyte-Based Supercapacitors: First, the mass percentage of each polyelectrolyte complex was determined to ensure that the concentration of ionized small ions in the matrix aqueous solution of PVA was with the same order of magnitudes (e.g., 15 wt% PDDACl, 15 wt% PSSNa, 6 wt% PAAH, 3.75 wt% PAAOH, 3.75 wt% PEIOH, 18.75 wt% PAMPSH, 10 wt% PMAANa). Then, the known contents of polyelectrolyte complexes were dissolved in aqueous solution of polyvinyl alcohol (5 wt%), respectively. The electrolyte solution containing polycation complex (30 μL) and the electrolyte solution containing polyanion complex (30 μL) were drop-coated on the electrodes (CNTs film or CNTs/MnO 2 composite film), respectively. After that, the electrodes coated with electrolyte were placed in a vacuum drying oven for 1 h in order to make electrolyte efficiently infiltrate into electrodes. Finally, two electrodes coated with polycation and polyanion electrolytes were pressed together, during which process the bilayer heterogeneous polyelectrolyte was in situ constructed.
Characterization: The gel electrolytes containning polycation complex and polyanion complex were colored with solid green and methyl orange, respectively, to observe the heterogeneous structure of BHPs by fluorescent microscope. The assembled supercapacitors were sliced longitudinally with a frozen section machine (Leica RM 2265) and then the interface of the heterojunction electrolyte was characterized in an Olympus BX 51 fluorescent microscope. The morphology of materials was characterized by field scanning electron microscopy (FESEM, Hitachi S-4800) at an accelerating voltage of 5 kV. Renishaw Raman spectrometer (514 nm laser) are used to characterize the structure of electrode materials. The electrochemical performance measurements were recorded with an electrochemical systems (CHI 760 E, Shanghai Chenhua). For the self-discharge measurements, the supercapacitors were charged to 0.8 V at 0.13 mA cm À2 (0.26 mA cm À2 ) and maintained at 0.8 V for 30 min.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.