Proton-Conducting Polyoxometalates as Redox Electrolytes Synergistically Boosting the Performance of Self-Healing Solid-State Supercapacitors with Polyaniline

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Mar 2021Proton-Conducting Polyoxometalates as Redox Electrolytes Synergistically Boosting the Performance of Self-Healing Solid-State Supercapacitors with Polyaniline Dongming Cheng, Bo Li, Sai Sun, Li-Jie Zhu, Ying Li, Xing-Long Wu and Hong-Ying Zang Dongming Cheng Key Lab of Polyoxometalate, Science of Ministry of Education, Key Laboratory of Nanobiosensing and Nanobioanalysis at Universities of Jilin Province, Faculty of Chemistry, Northeast Normal University, Changchun 130024 , Bo Li Key Lab of Polyoxometalate, Science of Ministry of Education, Key Laboratory of Nanobiosensing and Nanobioanalysis at Universities of Jilin Province, Faculty of Chemistry, Northeast Normal University, Changchun 130024 , Sai Sun Key Lab of Polyoxometalate, Science of Ministry of Education, Key Laboratory of Nanobiosensing and Nanobioanalysis at Universities of Jilin Province, Faculty of Chemistry, Northeast Normal University, Changchun 130024 , Li-Jie Zhu Key Lab of Polyoxometalate, Science of Ministry of Education, Key Laboratory of Nanobiosensing and Nanobioanalysis at Universities of Jilin Province, Faculty of Chemistry, Northeast Normal University, Changchun 130024 , Ying Li , Xing-Long Wu Key Lab of Polyoxometalate, Science of Ministry of Education, Key Laboratory of Nanobiosensing and Nanobioanalysis at Universities of Jilin Province, Faculty of Chemistry, Northeast Normal University, Changchun 130024 and Hong-Ying Zang *Corresponding author: E-mail Address: [email protected] Key Lab of Polyoxometalate, Science of Ministry of Education, Key Laboratory of Nanobiosensing and Nanobioanalysis at Universities of Jilin Province, Faculty of Chemistry, Northeast Normal University, Changchun 130024 https://doi.org/10.31635/ccschem.020.202000311 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Energy storage devices with high volumetric and gravimetric capacitance are in urgent demand due to the booming market of portable and wearable electronics. Using redox-active molecules as electrolytes is a strategy to improve the capacitance and energy density of solid-state supercapacitors (SCs). In this study, polyoxometalates (POMs) are applied as proton conductors and redox mediators in polyvinyl alcohol (PVA) electrolytes, which increase the capacitance of obtained SCs with polyaniline (PANI). H3PMo12O40-loaded PANI electrodes provide pseudocapacitance with an eight-electron Faraday reaction in a charge–discharge cycle. This has rarely been reported in SCs before. The largest capacitance of SCs with H3PMo12O40 and H3PW12O40 as electrolytes is 7.69 F/cm2 (3840 F/g) based on a single electrode at 0.5 mA/cm2. In addition, POM electrolytes exhibit excellent self-healing ability, which is attributed to the rich hydrogen-bonding network between POMs and PVA. This study demonstrates that the capacitance of solid-state SCs is improved by using molecular redox-active electrolytes and showcases the potential of applying this strategy to other energy storage devices in the future. Download figure Download PowerPoint Introduction The development of flexible and recyclable energy storage devices has been attracted widespread attention due to the potential market for flexible and epidermal electronics.1–3 Supercapacitors (SCs) with solid-state electrolytes as the septum and ion conductor are considered as a promising candidate for next generation recyclable energy storage devices. SCs have been employed in the fields of flexible displays, wearable electronics, bioimplants, and sensors.4–7 However, most solid flexible SCs suffer from their low energy density.8–10 According to the equation E = 1/2 CV2,11 the energy density can be improved by increasing the capacitance of SCs.12 To increase the SCs capacitance, a few investigations have been conducted on the design of electrode materials with hierarchical porous structure or introduced heteroatoms.13–15 However, so far, the performance of SCs using traditional electrolytes is still much lower than their theoretical capacity. Therefore, pursuing novel electrolytes, such as redox mediator electrolytes, and further optimizing the combination of electrodes and electrolytes, is an advantageous strategy to maximize the performance of electrode materials. Redox-mediator electrolytes have reversible redox peaks and will contribute additional pseudocapacitance to the whole SCs with the electrodes.16–19 Matching the electrodes and redox electrolytes is a recent initiative to improve the overall capacitance of SCs. A matched combination of electrodes and redox electrolytes should have two characteristics: redox potential overlap and chemical interaction between electrodes and electrolytes. The former characteristic can synergistically improve pseudocapacitance of electrodes20; and the second characteristic can guarantee excellent electrode–electrolyte interfacial contact, thus facilitating electrolyte storage and enhancing ion migration at the electrode–electrolyte interface.21 But related research is still in its infancy due to the lack of redox mediators with both appropriate redox potential range and high ion-conducting ability in the solid state.22 Polyoxometalates (POMs) with stable and precise structure possess tunable electron–proton reservoirs and high ionic conductivity in solid state at room temperature.23–30 In the past decades, tremendous efforts have been made to prepare traditional liquid electrolytes SCs by depositing POMs on electrode materials. However, the high solubility of POMs in aqueous solution becomes the disadvantage of this type of SCs, causing degraded performance. Considering the suitable redox activity and high ionic conductivity of POMs, POMs exhibit outstanding performance as redox electrolytes with characteristics different from conventional redox electrolytes. POMs provide sufficiently high ion mobility, without the need of adding additional supporting electrolytes. POMs have the ability to undergo reversible multielectron transfer without significant structural deformation.31–33 POMs still exhibit high ionic conductivity and high redox activity in the solid state. In previous reports, Jost et al.34 used {SiW12} as an electrolyte due to its high ionic conductivity, and the capacitance of SCs showed 510 mF/cm2. Both high ionic conductivity and redox activity are vital for the capacitance of solid-state SCs. Despite possessing both of the above properties, POMs as redox-mediator electrolytes in SCs have seldom been reported. Based on the aforementioned reasons, we set up a polyaniline (PANI) symmetrical SC with POMs as a redox electrolyte to study how the multielectron transfer process between electrodes and electrolytes enhances the overall SCs capacitance. We employed an in situ electrodeposition method to fabricate porous PANI on carbon paper electrodes (CPs) for assembling solid-state SCs with a POMs gel as the redox electrolyte (illustrated in Scheme 1).35,36 After electrochemical expansion with direct current (DC), the electrodes have a porous structure that facilitates ion migration and charge transfer between the electrodes and electrolytes.37–39 Without using any adhesives, excellent electrode–electrolyte interfacial contact is obtained via electrostatic interaction between PANI and POMs, and even part of POMs electrolytes penetrate into the electrodes.40,41 The PANI electrodes synergistically work together with the POMs electrolytes to offer further pseudocapacitance for enhancing overall SCs capacitance. The H3PMo12O40 loaded PANI electrode shows an eight-electron Faraday reaction in a charge–discharge cycle at operating voltage, which is unprecedented in solid-state SCs. In addition, the rich hydrogen-bonding network between POMs and PVA endows the POMs-PVA electrolytes with self-healing ability. Scheme 1 | The preparation of electrochemically deposited PANI electrodes and the assembly process of PANI-SCs with POMs as redox electrolytes. Download figure Download PowerPoint Experimental Methods Materials preparation PVA (PVA-117, 145000), glycerol (analytical reagents (AR), 99%), and glutaraldehyde (AR, 50 wt % in H2O) were purchased from Aladdin (Shanghai, China). Phosphotungstic acid hydrate (H3PW12O40·xH2O, AR) and phosphormolybdic acid hydrate (H3PMo12O40·xH2O, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Aniline (AR, 99.5%) was purchased from Macklin Inc. (Rochelle, IL). Concentrated hydrochloric acid and concentrated sulfuric acid were purchased from Beijing Chemical Plant (Beijing, China). Ultrapure water (18.25 MΩ·cm) was obtained from an ultrapure water machine. Carbon paper (thickness = 0.3 mm) was purchased from Shanghai Hesen Electric Co., Ltd. (Shanghai, China). All reagents and chemicals were used as received without any further purification. Fabrication of POMs-gel electrolytes The POMs-gel electrolytes were prepared by dissolving a POMs electrolyte in PVA aqueous solution. Taking the H3PMo12O40 gel electrolyte as an example, typically, 2 g of 5 wt % PVA aqueous solution was added to a clean glass vial, then 0.1 g of glycerin (plasticizer) and 0.4 g of H3PMo12O40·xH2O were added, followed by stirring at room temperature for 12 h to obtain a homogeneous solution. After that, 2 µL of 25 wt % glutaraldehyde (cross-linking agent) was added into the above solution and the solution was stirred for another 5 min. The resulting solution was the H3PMo12O40 gel electrolyte. All the other gel electrolytes were prepared in the same way, and their detailed composition is shown in Supporting Information Table S1. Pretreatment of CPs The CP was cut to a size of 1 cm × 2 cm, and then ultrasonically treated in a dilute nitric acid, acetone, and ethanol solution for 0.5 h. After being dried in air, the carbon paper was covered with an insulating tape to expose a working area of 1 cm × 1 cm. To increase the specific surface area and conductivity of the CP, it was activated by the following two-step pretreatment. First, the CPs were electrochemically expanded with DC in an aqueous solution of 0.25 mM poly(styrenesulfonate) (Pss) solution with an electrostatic potential of 10 V for 0.5 h with the CP as the positive electrode and carbon rod as the negative electrode. The electrodes after this step were named as expanded CPs. Second, the CP was further treated in a three-electrode configuration in an oxygen-free 1 M LiClO4 solution for 5 min at −1.2 V versus saturated calomel electrode (SCE). The obtained CP (named as PssLi) was washed with deionized water several times to remove inorganic residues. Preparation of electrochemically deposited PANI CPs The PssLi electrode was electrodeposited in a 0.2 M aniline and 0.5 M HCl solution using a three-electrode cell (PssLi electrode as a working electrode, a SCE as a reference electrode, and a Pt sheet as a counter electrode) to prepare PANI electrodes. The electrodeposition was held for 5 min at 0.9 V versus SCE with a total charge of 5.0 C/cm2. The mass of the active material on the PANI CP was 2.0 mg/cm2. The obtained electrodes were named as PANI. Assembly of symmetrical capacitors First, the newly prepared gel electrolyte solution was dropped onto two PANI electrodes and then dried at room temperature. This operation was repeated several times to achieve proper electrolyte thickness. Second, the electrodes were affixed together and the assembled SCs were hot pressed at 60 °C for 10 min. Third, the formed device was sealed with tape and was placed in a closed container under constant relative humidity (RH) overnight before the test. Electrochemical measurements All electrochemical measurements were obtained using the CHI760E electrochemical workstation at room temperature. The electrochemical impedance spectroscopy (EIS) measurements were conducted with a voltage of 5 mV amplitude in the frequency range of 100 kHz to 0.01 Hz at an open-circuit potential. The areal or gravimetric specific capacitance (F/cm2 or F/g) of SCs based on a single electrode was calculated from the constant galvanostatic charge–discharge (GCD) curve by Equation (1) C = 2 I t / S V or C = I t / m V (1)where I is the constant discharge current (A), t is the discharge time (s), S or m is the electroactive area (cm2) or electroactive mass of the single-electrode material (g), V is the operating voltage (V). The energy density (Wh/cm2 or Wh/kg) and corresponding power density (W/cm2) were calculated from Equations (2) and (3), respectively. E = C V 2 / 2 (2) P = 3600 E / t (3) Results and Discussion To facilitate electrolyte ion transport within the electrodes, a hierarchical porous structure was fabricated on the electrode surface. CPs underwent a two-step pretreatment in the Pss solution and LiClO4 solution before PANI was electrodesposited onto CPs. According to field emission scanning electron microscopy (SEM) images (Figure 1a and Supporting Information Figure S1), CPs maintained the original structural characteristics after the two-step electrochemical expansion. After an in situ electrodesposition process, uniform PANI layers (2.0 mg/cm2) with fibrous structure formed on the surface of CPs (Figures 1b and 1c). The SEM images showed distinct pores between PANI fibers, which benefited the penetration of electrolytes into electrodes. By comparing the nitrogen adsorption–desorption isotherms of the expanded CPs with that of original ones ( Supporting Information Figure S2), it was found that the expanded CPs had nearly twice the surface area of the original CPs. The enhanced porous structure enabled a better contact between electrodes and electrolytes. In this way, electrolyte ions have efficient access to the surface of electrodes. With an extended structure, an areal capacitance of 1.09 F/cm2 at 0.5 mA/cm2 was obtained from the processed CPs. After the electrodesposition with PANI, the areal capacitance increased to 2.28 F/cm2 (1140 F/g) at 0.5 mA/cm2 ( Supporting Information Figure S3). Figure 1 | SEM images: (a) original CPs, (b) PANI electrodeposited Cps, (c) PANI fibers, (d) electrode–electrolyte interface in PANI-PMo12 electrodes, (e–i) elemental analysis mapping of obtained PANI-PMo12 electrodes. Download figure Download PowerPoint Three PANI-POMs electrodes (PANI-PW12, PANI-PMo12, and PANI-PMo12&PW12) prepared by coating the corresponding POMs-gel onto PANI electrodes were used to investigate how POMs influence the performance of PANI electrodes and the match between redox POMs electrolytes and electroactive PANI electrodes. Figures 1e–1i show the elemental analysis mapping of obtained PANI-PMo12 electrodes. It was concluded that P and Mo atoms were dispersed uniformly in the electrolytes, the electrode–electrolyte interface, as well as the electrodes. This phenomenon indicated that some electrolyte had penetrated into the electrodes, reducing the contact resistance between the electrolytes and the electrodes. The electrochemical characterization of these electrodes was performed by cyclic voltammetry (CV) and GCD using a three-electrode cell in 1 M H2SO4. As shown in Figure 2a, three PANI-POMs electrodes displayed improved performance compared with the PANI electrodes. Specifically, the PANI-PMo12 electrode had a better electrochemical performance than the PANI-PW12 electrode, which indicated that the potential of the redox reaction between PMo12 and PANI matched better than the pair of PW12 and PANI. It was because PMo12 had three pairs of redox peaks (0.44 and 0.32 V, 0.28 and 0.16 V, and 0 and −0.06 V vs SCE) at operating voltage within which PW12 did not show redox peaks. In addition, both PANI and PMo12 showed reversible electrochemical behaviors with good compatibility. Therefore, the PANI-PMo12 electrodes can contribute an eight-electron Faraday reaction in a charge–discharge cycle. Finally, when both PW12 and PMo12 were applied onto the PANI electrodes, the electrochemical performance was further enhanced, which was in agreement with the plateaus in their GCD curves (Figure 2b). The PANI-PMo12&PW12 electrodes exhibited an areal capacitance of 3.88 F/cm2 (1940 F/g) at 0.5 mA/cm2 [3.12 F/cm2 (1560 F/g) for PANI-PMo12, 2.51 F/cm2 (1250 F/g) for PANI-PW12, and 2.28 F/cm2 (1140 F/g) for PANI] (Figure 2c). The EIS spectra of these electrodes are shown in Supporting Information Figure S4, and the corresponding charge transfer resistances revealed that the introducing of POMs-gel did not block the electron transfer on the PANI electrodes. Figure 2 | Electrochemical analyses of the PANI-POMs electrodes in 1 M H2SO4: (a) CV curves at 5 mV/s. (b) GCD curves at 0.5 mA/cm2, (c) areal capacitance as a function of current density, electrochemical analyses of the PANI electrodes in 1 M H2SO4 (0, 1, 2, 5, and 10 mM PMo12), (d) CV curves at 5 mV/s, (e) GCD curves at 2 mA/cm2, (f) areal capacitance at varied current density. Download figure Download PowerPoint In general, when POMs are applied onto the PANI electrodes, both the capacitance and the current density increase. This promotion highly depends on the type of POMs, namely the redox potential of POMs. Among the PANI-POMs electrodes, the PANI-PMo12&PW12 electrodes showed the largest capacitance, because both PMo12 and PW12 can synergistically improve the capacitance of the PANI electrodes. To investigate whether PVA can effectively snap POMs without causing negative effects, PANI and PANI-POMs (PANI-PMo12) electrodes were prepared with and without PVA. Then, we compared their CV curves at 5 mV/s in 1 M H2SO4 ( Supporting Information Figure S5). The results confirmed our prediction that PVA-gel could immobilize POMs and promote the redox reaction between PMo12 and PANI. The capacitance of PANI electrodes can be greatly enhanced by introducing POMs-gel onto the electrodes. Therefore, the influence of POMs as the redox electrolytes on the performance of PANI electrodes was investigated. Because PMo12 (but not PW12) greatly improved the capacitance of PANI electrodes, we explored the effect of PMo12 concentration (0, 1, 2, 5, and 10 mM) on the capacitance of PANI electrodes in 1 M H2SO4. As shown in Figure 2d, a new redox peak at about 0.28 and 0.10 V could be found when 1 mM PMo12 was added into H2SO4. Meanwhile, the electrochemical performance of PANI electrodes increased significantly as the concentration of POMs increased. The GCD curves showed a trend similar to the CV curves (Figures 2e and 2f). So PMo12 can also improve the capacitance of PANI electrodes as the redox mediators in the electrolytes. When 10 mM PMo12 was added into the electrolytes, a large areal capacitance of 11.72 F/cm2 at 2 mA/cm2 was obtained from the PANI electrodes, which was much higher than that in 1 M H2SO4 (1.63 F/cm2 at 2 mA/cm2). This result suggested that POMs, as redox mediators in the electrolytes, could also greatly boost the capacitive activity of PANI electrodes with additional pseudocapacitance. In regard to the fact that PMo12 on the electrodes or in the electrolytes could significantly improve the capacitance of PANI electrodes, it was assumed that the capacitance can be further increased when PMo12 is both on the electrodes and in the electrolytes. As shown in Supporting Information Figure S6, we found that four pairs of redox peaks appeared in CV curves when PMo12 existed both on the electrodes and in the electrolytes, and the current density also reached its maximum value, accompanied by the largest capacitance. A synchronous promotion of the PANI electrodes with PMo12 in the electrolytes and on the electrodes was evidenced by this point. Hence, we designed a capacitor with POMs electrolytes as proton conductors and redox mediators sandwiched between two PANI electrodes for enhancing the whole capacitance of the SCs. Proton conductivity of gel electrolytes is also a key factor for the performance of solid-state SCs. Proton conductivity of PW12-gel, PMo12-gel, and PMo12&PW12-gel was tested under ambient conditions (the methods and conditions are provided in the Supporting Information). Since the acidity of PW12 was greater than that of PMo12, the PW12-gel exhibited the highest proton conductivity (15.7 mS/cm), while the PMo12-gel exhibited the lower proton conductivity (3.5 mS/cm) ( Supporting Information Figure S7). The PMo12&PW12-gel showed intermediate proton conductivity (10.2 mS/cm). Such a high proton conductivity of POMs-gel agrees with its usage as electrolytes in SCs. Then, three POMs-based solid-state SCs (PW12-SCs, PMo12-SCs, and PMo12&PW12-SCs) were assembled by hot-pressing two sheets of POMs-gel-electrolyte-coated PANI electrodes at 60 °C for 10 min. As a control, a SC (H2SO4-SCs) with sulfuric-gel as an electrolyte was assembled using the same method. The CV curves of these SCs were acquired at a scan rate of 5–200 mV/s with the operating window of −0.5–0.5 V (Figure 3a and Supporting Information Figure S8). In the test window, it was distinct that the PW12-SCs exhibited almost the same electrochemical performance as H2SO4-SCs, which was consistent with the corresponding GCD pattern of PW12-SCs and H2SO4-SCs (Figure 3b and Supporting Information Figure S9). The redox reaction between PW12 and PANI was poorly matched, leading to a lower areal capacitance (1.82 F/cm2 at 0.5 mA/cm2 based on a single electrode) than H2SO4-SCs (2.16 F/cm2). PW12 only played a role as proton conductor in the PW12-gel SCs, with effect on redox Figure | Electrochemical analysis of solid-state SCs with electrolytes PW12-gel, PMo12-gel, and (a) CV curves at 5 mV/s, (b) GCD curves at 0.5 mA/cm2, (c) areal capacitance at varied current density, (d) capacitance of after Download figure Download PowerPoint showed three pairs of redox peaks in CV and corresponding in GCD So a areal capacitance of F/cm2 based on a single electrode at 0.5 mA/cm2 than that of PW12-SCs and This is that PMo12 not only as proton also provided pseudocapacitance in the due to the good match of redox peaks between PMo12 and PANI. To the effect of PMo12 in gel on the performance of SCs, we added different PMo12 and in PMo12-gel for assembling SCs. In the in the was 0.4 As shown in the Supporting Information Figure we found that the capacitance increased with the of Therefore, we that this trend the solubility of PMo12 in PVA-gel is excellent proton conductivity of PW12 and outstanding redox activity of PMo12, the of performance about POMs can be found in the Supporting Information). As shown in Figure the gel SC the electrochemical performance and the areal capacitance of 7.69 F/cm2 at 0.5 mA/cm2 based on a single electrode with the energy density of the power density was 0.25 The energy density maintained even the areal power density was increased to 5.0 It was outstanding compared with most current SCs ( Supporting Information and S3). Considering the of the whole device current electrodes, and the maximum volumetric capacitance of was In addition, exhibited capacitance of after at a charge–discharge current of mA/cm2 (Figure Based on the above PMo12 and PW12 played their and other in the gel the potential of POMs as supporting electrolytes in SCs. As shown in Figure a was by SC devices in Figure | (a) A by SC devices in (b) process of the POMs-gel under for h. and CV and GCD curves of before and after Download figure Download PowerPoint To the interfacial contact between the electrodes and the electrolytes is in SCs. We that not only efficient chemical electrostatic interaction between POMs and PANI, also the hierarchical porous structure of the electrode was to guarantee contact between the electrodes and the electrolytes. To investigate this SCs were assembled using the same electrolytes and PANI or activated carbon CPs prepared via conventional with as The CV and GCD curves in Supporting Information Figure showed the effect of electrode structure on the SCs capacitance. Using electrodes without hierarchical porous structure, SCs exhibited much lower capacitance than that of SCs with electrodeposited PANI electrodes. Therefore, we that the additional block the pores and had negative influence on interfacial contact between the electrodes and the electrolytes. In the electrode with a porous structure is to the electrolyte onto the electrode and The self-healing ability of gel electrolytes the capacitor better and under conditions or even after being The POMs-gel exhibited a good self-healing ability under conditions because of its network structure with rich (Figure The POMs-gel was cut into two and placed together under the of The gel was after being placed at ambient temperature for h. As shown in Figures and significant performance was from the SCs capacitance of In effect between the redox activity and proton conductivity of electrolyte materials was used as a new strategy to boost the performance of solid-state SCs for the A novel solid-state SC with electrolytes sandwiched by two PANI electrodes was of the influence of POMs indicated that the redox of electrolytes and electrode materials can the performance of the SCs. PANI-PMo12 electrodes showed than most reported PANI electrodes. The PMo12 electrolytes the electrochemical performance of the PANI electrodes an eight-electron Faraday reaction to provide pseudocapacitance in a charge–discharge cycle. showed the highest areal capacitance of 7.69 F/cm2 (3840 F/g) at 0.5 mA/cm2 based on a single electrode with the energy density of In addition, the POMs-gel exhibited self-healing ability. The SC can be under ambient conditions without an performance This study a specific of using a redox-active electrolyte to improve the capacitance and energy density of SCs. This provide a strategy to be applied in other energy storage devices. 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