Highly conductive PANI/ATMP/AgNO₃ composite hydrogel electrodes for all-hydrogel-state supercapacitors

Abstract

High electronic conductivity and excellent specific capacitance still remain challenges for the practical application of hydrogel electrodes. Herein, a novel polymer composite hydrogel electrode, PANI/PVA/ATMP/AgNO₃ (PPA-Ag), was successfully constructed through in situ polymerization of aniline (ANI) in a solution of polyvinyl alcohol (PVA), amino trimethylene phosphonic acid (ATMP) and silver nitrate (AgNO₃). The incorporation of AgNO₃ was expected to enhance the electronic conductivity and specific capacitance of PPA hydrogel electrodes. The AgNO₃ incorporated PPA-Ag hydrogel electrodes exhibited a superior specific capacitance (510 F g⁻¹ at 0.5 A g⁻¹), which was much higher than that without AgNO₃ incorporation (317 F g⁻¹ at 0.5 A g⁻¹). Additionally, the PPA-Ag hydrogel electrodes also showed excellent flexibility (93.43% capacitance retention after 200 cycles of bending) and excellent cycling stability (81.41% of initial capacitance after 10 000 cycles). The energy storage mechanism originated from the three-dimensional porous structure of the hydrogel, the multiple redox structures of polyaniline, and the Ag⁺/Ag of silver nitrate. The all-hydrogel-state supercapacitor was assembled based on the PPA-Ag hydrogel electrodes, which delivered a high energy density of 13.3 W h kg⁻¹ at a power density of 125 W kg⁻¹. Meanwhile, the supercapacitors can also maintain above 77% of the initial capacitance after 10 000 charge–discharge cycles. This work constructed polymer hydrogel electrodes with high electronic conductivity and excellent specific capacitance for flexible supercapacitors, which demonstrated great potential within the field of flexible energy storage.

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Introduction

Flexible supercapacitors (FSCs) have shown promising potential in flexible energy storage devices due to their advanced features, such as fast charge–discharge rate, high performance, and long cycling life.^1–3 Electrode materials are crucial for flexible supercapacitors.⁴ Currently, most flexible supercapacitors are assembled from conventional rigid electrodes and hydrogel electrolytes, which are deficient in flexibility.^5,6 Reasonable design of electrode materials with excellent performance is an effective approach to improve the performance of flexible supercapacitors.⁷ Specifically, conductive polymer hydrogels (CPHs) equipped with superior electrochemical properties and excellent flexibility are ideal candidate materials for FSC electrodes.^8–10 To date, the three most commonly used conductive polymer hydrogels are polyaniline (PANI),^11,12 polypyrrole (PPy)^13,14 and polythiophene (PEDOT)^15,16 hydrogels.

Polyaniline hydrogel electrodes have attracted intense attention and become research hotspots due to their distinct chemical and physical properties, including high conductivity, superior specific capacitance, and excellent environmental stability.^17,18 For instance, Li et al.¹⁹ prepared a PANI/PVA hydrogel as an electrode by a low-temperature polymerization strategy. All-solid-state supercapacitors with PANI/PVA hydrogel electrodes showed excellent specific capacitances of 260 F g⁻¹ and 650 mF cm⁻², a significantly high energy density of 27.5 W h kg⁻¹, and excellent stability. Dopants are the critical factor affecting the electrical conductivity of PANI.²⁰ Different acidic dopants were introduced into PANI hydrogels, which may improve their performance and enhance their application in the field of flexible supercapacitors.²¹ Dou et al.²² introduced phytic acid (PA) as an organic dopant to prepare PANI/HQ hydrogel electrodes with a favorable specific capacitance of 642 F g⁻¹ at 0.5 A g⁻¹. Additionally, the as-prepared PANI/PA/HQ hydrogel electrodes also displayed excellent flexibility (93.3% capacitance retention after 500 cycles of bending) and superior cycling stability (81.5% after 10 000 cycles). The all-hydrogel-state FSCs assembled with PANI/PA hydrogel electrodes delivered a high energy density of 12 W h kg⁻¹ at a power density of 125 W kg⁻¹. Nandi et al.²³ employed folic acid (FA) to synthesize PANI-based hydrogels, in which FA acted as a dopant and cross-linker. The PANI/FA gel electrodes exhibited a superior specific capacitance (646 F g⁻¹ at 1 A g⁻¹) as well as excellent rate capability (403 F g⁻¹ at 20 A g⁻¹). Besides, the obtained hydrogel electrodes exhibited long-term stable cycling performance with 74% capacitance retention after 5000 times. The energy density was estimated to be 57.25 W h kg⁻¹ at a power density of 400 W kg⁻¹. Although much research progress has been made on PANI hydrogel electrodes, the conductivity and specific capacitance properties still remain challenges to satisfy the severe requirements of practical applications.

An investigation revealed that the introduction of silver nitrate is an effective method to improve the electrochemical properties of PANI electrode materials.²⁴ On one hand, the presence of AgNO₃ introduced an additional Ag⁺/Ag redox pair to improve specific capacitance.²⁵ On the other hand, the uniform distribution of silver could enhance the conductivity of the PANI electrodes.²⁶ For example, Das et al.²⁷ introduced AgNO₃ to develop an Ag-PANI nanocomposite for high-performance supercapacitor electrodes. The nanocomposite exhibited a conductivity of 4.24 S cm⁻¹ at room temperature and a maximum specific capacitance of 528 F g⁻¹ at a scan rate of 5 mV s⁻¹. Additionally, the nanocomposite demonstrated superior energy and power density compared to other materials. Patil et al.²⁸ employed AgNO₃ to synthesize an Ag-PANI electrode by chemical polymerization techniques and obtained the highest specific capacitance of 512 F g⁻¹ at 5 mV s⁻¹ scan rate, energy density of 50.01 W h kg⁻¹ at 1 mA cm⁻², and a significant increase in conductivity at 0.9 wt% doping of Ag. Chen et al.²⁹ synthesized Ag-PANI composites for electrode materials in energy storage applications. The Ag-PANI electrodes exhibited excellent specific capacitance as high as 615 F g⁻¹ at 1 A g⁻¹, much higher than that of PANI (316 F g⁻¹). Moreover, they also showed higher electrical conductivity (18.5 S cm⁻¹). To summarize, PANI/AgNO₃ powders have received great attention as feasible electrode materials for supercapacitors. However, the PANI/AgNO₃ hydrogel has rarely been reported as a conducting electrode for supercapacitor application up to now.

Amino trimethylene phosphonic acid (ATMP) with three phosphate groups can be employed as a novel dopant to improve the electrochemical properties of polyaniline hydrogel electrodes.³⁰ Moreover, the introduction of AgNO₃ into PANI hydrogel electrodes was an effective method to construct conductive polymer composite hydrogel electrodes with excellent electrochemical properties for FSCs. Therefore, our main motivation of this work was to synthesize a PANI/PVA/ATMP/AgNO₃ composite hydrogel, and the electrochemical performance of PANI hydrogel electrodes was improved by the combined effect of both ATMP and AgNO₃ for their practical applications.

Herein, we constructed a novel (PANI/PVA/ATMP/AgNO₃) PPA-Ag composite hydrogel electrode through in situ polymerization of aniline (ANI) in a mixture of polyvinyl alcohol (PVA), amino trimethylene phosphonic acid (ATMP) and silver nitrate (AgNO₃), in which ATMP acted as an organic doping acid. The presence of AgNO₃ introduced an additional Ag⁺/Ag redox pair. Meanwhile, the uniform distribution of silver within the PPA-Ag hydrogel network facilitated rapid electron conduction, thereby enhancing the conductivity of the PPA-Ag hydrogel from 11.29 to 16.33 S m⁻¹. The obtained PPA-Ag15 composite hydrogel electrode had a high specific capacitance of 510 F g⁻¹ at 0.5 A g⁻¹ and a good flexibility of 93.43% capacitance retention after 200 bending cycles. The effect of silver nitrate content on the electrochemical properties of the PPA-Ag composite hydrogel electrodes and the possible formation mechanism were investigated. The kinetics of energy storage suggests that the PPA-Ag15 hydrogel electrode shifted from a diffusion-controlled process to a surface capacitive process as the scan rate increased from 5 mV s⁻¹ to 100 mV s⁻¹. All-hydrogel state supercapacitors were assembled based on the PPA-Ag hydrogel electrodes, which delivered a high energy density of 13.3 W h kg⁻¹ at a power density of 125 W kg⁻¹. Meanwhile, the supercapacitors could also maintain above 77% of the initial capacitance after 10 000 charge–discharge cycles.

Experimental section

Materials

Polyvinyl alcohol (PVA, hydrolysis degree 99%, polymerization degree 1750) and ammonium persulfate (APS) were purchased from Sinopharm Chemical Reagent Co. Aniline (ANI), silver nitrate (AgNO₃) and amino trimethylene phosphonic acid (ATMP) were purchased from Aladdin Chemical Company Limited, China. All reagents in this experiment were used directly without further treatment. Deionized water (DI) was used throughout the experiments.

Synthesis of PPA-Ag composite hydrogel electrodes

The conductive polymer composite hydrogel was prepared by the one-pot method. 1 g of PVA and a certain mass of AgNO₃ were added to 10 mL of deionized water and stirred at 95 °C for 1 h to obtain a completely dissolved PVA solution, which was recorded as solution A. After cooling to room temperature, 0.56 g of ANI and 1.18 g of ATMP were added simultaneously and stirred well at room temperature. 1.37 g APS was dissolved in 10 mL deionized water and recorded as solution B. Solution A was transferred to an ice bath, and solution B was added to it when the temperature dropped to 2–4 °C. The resulting solution was polymerized with rapid mixing and constant stirring. After 10 minutes of polymerization reaction, the mixture was poured into a Petri dish and covered with carbon cloth as a collector, and then the Petri dish was transferred to a cryostat for freeze–thaw cycles, which were performed for a period of 12 hours for a total of three times to obtain the PPA-Ag hydrogel. For comparison, the PPA-Ag hydrogel electrodes prepared with different concentrations of AgNO₃ were labeled as PPA-0, PPA-Ag5, PPA-Ag10, PPA-Ag15, and PPA-Ag20. Table S1† presents the specific amounts of each component required for the preparation of the PPA-0 and PPA-Ag hydrogels.

Fabrication of all-hydrogel-state FSCs

The all-hydrogel-state FSCs with a sandwich structure were assembled with two identical PPA-Ag hydrogel electrodes sandwiched with a PVA/H₂SO₄ hydrogel electrolyte. The PVA/H₂SO₄ hydrogel electrolytes were prepared by mixing 1 g of PVA with 10 mL of 1 M H₂SO₄.

Results and discussion

The preparation process of the PANI/PVA/ATMP/AgNO₃ (PPA-Ag) composite hydrogel is shown in Fig. 1. The PPA-Ag composite hydrogels were successfully synthesized by in situ polymerization of aniline in an aqueous solution of PVA, ATMP and AgNO₃. In the PPA-Ag hydrogel system, PANI was utilized as the primary electroactive material for energy storage, exhibiting a variety of redox states.³¹ The conversion between these redox states enabled the realization of pseudo-capacitive energy storage. Besides, ATMP functioned as a cross-linking agent and protonating dopant, facilitating the protonation of PANI.³² The process enabled the PPA-Ag hydrogel to possess high electrical conductivity under conditions of strong oxidation.³³ AgNO₃ was doped into PPA-Ag hydrogel electrodes to provide additional Ag⁺/Ag redox pairs, which could enhance the electronic conductivity and further improve the electrochemical performance of PPA-Ag hydrogel electrodes.³⁴ Additionally, AgNO₃ acted as an oxidizing agent for the polymerization process, promoting the polymerization of the aniline monomer.³⁵ Physical crosslinking (hydrogen bonds/coordination bonds/electrostatic interaction) played a significant role in the formation of PPA-Ag hydrogels. Hydrogen bonds were formed between the hydroxyl group (–OH) of PVA chains.³⁶ In addition, hydrogen bonds were also formed between the hydroxyl group (–OH) of PVA and the phosphate group (–POOH) of ATMP. Electrostatic interactions were established between the multiple phosphonate groups (–POOH) of the ATMP and imine groups (C–NH⁺=C) on the protonated PANI chains, which could facilitate cross-linking of multiple PANI chains.³³ This process conferred an interconnecting structure to PPA-Ag composite hydrogels, rendering them highly conductive. Coordination bonds were created between Ag⁺ and the oxygen atoms of the hydroxyl group (–OH) on PVA chains.²⁵ The formation of PPA-Ag hydrogels was enabled by the establishment of physical crosslinking, which allowed the development of hydrogels with remarkable flexibility. Fig. S1† shows the physical photographs of the PPA-Ag hydrogel before and after gelation.

Fig. 1 Schematic illustration for the preparation process of PPA-Ag composite hydrogels.

The chemical structure of the PPA-0 and PPA-Ag composite hydrogels was investigated by Fourier transform infrared spectroscopy (FTIR), and the results are shown in Fig. 2. In the infrared spectra of the PPA-0 hydrogel, the characteristic peaks at 3236 cm⁻¹, 2916 cm⁻¹, and 1087 cm⁻¹ corresponded to the stretching vibrations of O–H, C–H, and C–O bonds in the PVA chains, respectively.²² Two sharp peaks located at 1288 cm⁻¹ and 1236 cm⁻¹ were the C–N stretching vibration of the benzene ring in the polyaniline chain.³⁷ The characteristic peak at 1141 cm⁻¹ was the stretching vibration of N=Q=N (Q is the quinone ring) in polyaniline.³⁸ The broad peak located at 821 cm⁻¹ indicated the bending vibration of the C–H bond in 1,4-disubstituted benzene,³⁹ and the characteristic peaks at 1569 cm⁻¹ and 1442 cm⁻¹ belonged to the quinone ring and benzene ring expansion vibration, respectively, which indicated that the PANI in the composite hydrogel was in the state of semi-oxidized doping (emeraldine salt).⁴⁰ The absorption peak at 1176 cm⁻¹ corresponded to the vibration of the P=O bond, which indicated that ATMP was successfully doped into the PPA-0 hydrogel. The characteristic peak at 1637 cm⁻¹ may be attributed to the vibrational bending of residual water molecules (H₂O).^41,42 For the PPA-Ag hydrogel, the characteristic peaks of PPA-0 were all present in the samples of PPA-Ag, and some of the peaks were shifted and attenuated, while new peaks appeared in PPA-Ag. For the PPA-Ag composite hydrogel, the characteristic band at 1357 cm⁻¹ was attributed to the vibration of the N–O bond in NO₃⁻.³⁵ The corresponding characteristic peak of the O–H bond on the PVA chain was red-shifted to 3220 cm⁻¹, which may be due to the breakage of the O–H bond.⁴³ In addition, the intensity of the characteristic peak of the C–O bond stretching vibration at 1087 cm⁻¹ was slightly weakened, which confirmed the complexation of Ag⁺ with the oxygen atom of the O–H group.⁴⁴ The FTIR spectra indicated that the PPA-Ag composite hydrogels were successfully synthesized. Segmented FTIR is shown in Fig. S2.†

Fig. 2 FTIR spectra of PPA-0 and PPA-Ag composite hydrogels.

To investigate the microstructure composite hydrogels, PPA-Ag and PPA-0 hydrogels were analyzed by SEM. Fig. 3a–c show the SEM images of the pure PPA-0 hydrogel, from which it could be seen that the PPA-0 hydrogel exhibited a typical three-dimensional porous morphology characteristic (not regular) of hydrogel materials. The formation of this porous morphology could be attributed to the physical cross-linking of the PVA chains during the freeze–thaw cycling process.⁴⁵ The three-dimensional porous structure facilitated sufficient and rapid ion/electron penetration from the electrolyte into the hydrogel electrode, the storage sites of electrolyte ions were increased, and the internal space utilization of the electrode was improved, thus developing the capacitance performance of the electrodes.⁴⁶ In addition, ATMP not only had electrostatic interactions with PANI chains, but also could form physical cross-links with PVA chains through hydrogen bonding, which could build a more complex cross-link network in the PVA skeleton. After incorporating AgNO₃, the microstructure of the PPA-Ag composite hydrogel was obtained similar to that of the PPA-0 hydrogel, but the pore structure was more regular and ordered (Fig. 3d–f), which was more favorable for ion diffusion and electron transfer.⁴⁷ To confirm the presence of the elements C, O, N, P, and Ag corresponding to each component in the composite hydrogel, the PPA-Ag composite hydrogel was analyzed by EDS spectroscopy. As shown in Fig. 3g, the silver elements were distributed uniformly, indicating that silver nitrate had been successfully incorporated into the hydrogel. The uniformly distributed C and O elements corresponded mainly to the PVA in PPA-Ag composite hydrogels. The N element corresponded to the PANI chains in the PPA-Ag composite hydrogel, and the P element corresponded to the small molecule doping acid ATMP, confirming that ATMP successfully doped and crosslinked the PANI chains to form a more complex network structure. SEM and EDS results confirmed that PPA-Ag composite hydrogels with typical three-dimensional porous morphology were successfully prepared, while Ag particles were uniformly distributed in the hydrogel. The tubular structure of the hydrogel cross-section is illustrated in Fig. S3.†

Fig. 3 SEM results of (a–c) PPA-0 and (d–f) PPA-Ag hydrogels, and EDS results of (g) PPA-Ag hydrogels.

The electrochemical properties of PPA-Ag composite hydrogel electrodes were investigated by cyclic voltammetry (CV), constant current charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS). All tests were performed in a three-electrode system with 1 M sulfuric acid as an electrolyte. The potential window was selected between −0.2 and 0.8 V.

Fig. 4a shows the CV curves of PPA-0 and PPA-Ag composite hydrogel electrodes with different AgNO₃ loadings at a scan rate of 5 mV s⁻¹. The CV curves of all samples exhibited non-rectangular closed curves, which showed a characteristic of pseudocapacitive materials. These curves displayed a pair of oxidation/reduction peaks at potentials of 0.4–0.5 V. This was attributed to redox conversions from leucoemeraldine to emeraldine and from emeraldine to pernigraniline.^48,49 For PPA-Ag hydrogel electrodes, a new pair of redox peaks appeared in the CV curves at potentials of 0–0.2 V due to the addition of AgNO₃. The intensity of the redox peaks exhibited a notable increase with the rise in AgNO₃ content, which resulted from the redox transition of silver ions within the PPA-Ag hydrogel electrode system at this potential. This contributed to the enhancement of the specific capacitance of the hydrogel electrodes. Formula S1† demonstrates a positive correlation between the integral area and specific capacitance. The calculations demonstrated that the integration area of all PPA-Ag hydrogel electrodes was larger than that of PPA-0 hydrogel electrodes, which indicated capacitance enhancement with the addition of AgNO_3. The specific capacitance values of the PPA-Ag hydrogel electrodes (PPA-0, PPA-Ag5, PPA-Ag10, PPA-Ag15, PPA-Ag20) calculated from the CV curves at 5 mV s⁻¹ scan rate were 300, 328, 350, 404, and 361 F g⁻¹, respectively (Table S2†). In particular, the specific capacitance of PPA-Ag hydrogel electrodes was higher than that of the PPA-0 hydrogel electrodes. This might be attributed to the enhanced pseudocapacitance obtained by the redox reactions of Ag⁺ at the electrode/electrolyte interface.³⁴ The GCD curves of PPA-Ag composite hydrogel electrodes with different AgNO₃ loadings at a current density of 0.5 A g⁻¹ are presented in Fig. 4b. The GCD curves of all PPA-0 and PPA-Ag electrode samples showed approximately symmetrical triangles with a pair of charge/discharge platforms near the 0.4–0.5 V potential window. These results indicated that polyaniline underwent a reversible redox transition, which was consistent with the results of the CV curves. According to Formula S2,† the discharge time of the GCD curve was a crucial factor in determining the specific capacity of the electrode materials. The discharge time of the PPA-Ag composite hydrogel electrodes exhibited an initial increase and subsequent decrease with increasing silver content. Among the electrodes, the PPA-Ag15 composite hydrogel electrode was discharged for the longest duration, indicating PPA-Ag15 had the highest specific capacitance. The specific capacitance of the PPA-Ag composite hydrogel electrodes with various AgNO₃ contents was calculated using Formula S2,† and the results are presented in Fig. 4c. The mass-specific capacitance values of the PPA-0, PPA-Ag5, PPA-Ag10, PPA-Ag15, and PPA-Ag20 electrodes were 317, 443, 467, 510, and 493 F g⁻¹, respectively (Table S3†), at a current density of 0.5 A g⁻¹. These results demonstrated that PPA-Ag15 exhibited the highest charge storage capacity among all hydrogel electrode samples. This might be because the addition of AgNO₃ increased the conductivity of the PPA-Ag hydrogel electrodes, which promoted electron diffusion and prevented the accumulation of electrons on the hydrogel surface, thus increasing the pseudocapacitance of the PPA-Ag hydrogel electrodes.⁵⁰ Furthermore, Fig. 4c illustrates that the PPA-Ag15 composite hydrogel electrode exhibited the highest-mass specific capacitance at all current densities (0.5, 1, 2, 3, 5 A g⁻¹), which was consistent with the GCD curves. When the current density was increased from 0.5 A g⁻¹ to 5 A g⁻¹, the specific capacitance values of the PPA-Ag15 composite hydrogel electrode exhibited a decreasing trend, but still maintained a mass specific capacitance of 390 F g⁻¹ at a high current density of 5 A g⁻¹, as high as 76.5% of the initial mass-specific capacitance (at 0.5 A g⁻¹).

Fig. 4 (a) CV curves, (b) GCD curves, (c) specific capacitance, and (d) EIS plots, the inset shows the fitted circuit diagram of the electrochemical performance of the PPA-Ag15 hydrogel electrode. (e) Electronic conductivity, (f) CV curves, (g) linear fitting curve of peak current versus square root of scan rate, (h) GCD curves, and (i) cycle stability (3 A g⁻¹) of PPA-Ag hydrogels.

Fig. 4d shows the Nyquist curves of the PPA-0 and PPA-Ag composite hydrogel electrodes in the frequency range of 0.01 Hz to 10 kHz. The curves consisted of high-frequency, mid-frequency, and low-frequency regions, which represent the electrical transfer process, diffusion process, and capacitive behavior of the electrodes, respectively.^51–53 The Nyquist curves of the PPA-Ag composite hydrogel electrodes were fitted with ZView software to calculate their R_s and R_ct values. In the high-frequency region, the R_s values of PPA-0, PPA-Ag5, PPA-Ag10, PPA-Ag15, and PPA-Ag20 were 0.75, 0.73, 0.73, 0.70, and 0.71 Ω, respectively. The mid-frequency region was composed of a semicircle whose diameter was the interfacial charge transfer resistance (R_ct).⁵⁴ For the PPA-0 hydrogel electrode, the R_ct value was 1.72 Ω. After the incorporation of AgNO₃, the R_ct values of the PPA-Ag hydrogel electrodes (PPA-Ag5, PPA-Ag10, PPA-Ag15, PPA-Ag20) decreased to 1.43, 1.33, 1.12, and 1.17 Ω, respectively (Table S4†). PPA-Ag exhibited a lower R_ct value than PPA-0, which showed that the incorporation of AgNO₃ could effectively reduce the R_ct of the hydrogel electrodes. This might be attributed to the uniform distribution of the high-conductivity of silver within the PPA-Ag hydrogel, which significantly enhanced the overall conductivity and charge transfer rate of the PPA-Ag hydrogel electrodes. The small R_s value facilitates the diffusion of electrolyte ions into the internal pores of electrodes, while the relatively small R_ct value is beneficial for charge transfer, thereby ensuring the satisfactory capacitive performance of the electrodes.^55,56 Among all PPA-Ag hydrogel electrodes, the PPA-Ag15 hydrogel electrode exhibited the smallest R_ct and R_s values, suggesting the high electrical properties of the PPA-Ag15 hydrogel electrode. The straight lines observed in the low-frequency region were primarily indicative of the capacitive properties of the hydrogel electrode.⁵⁷ Among all the PPA-Ag hydrogel samples, the PPA-Ag15 composite hydrogel electrode exhibited the greatest slope value, indicating that it exhibits the lowest electrolyte ion diffusion impedance and the fastest ion diffusion rate. In conclusion, the PPA-Ag15 composite hydrogel electrode exhibited superior capacitance properties among all PPA-Ag hydrogel samples, suggesting that the incorporation of AgNO₃ could significantly enhance the electrochemical performance of the PPA hydrogel electrodes.

To further investigate the effects of AgNO₃ on the electronic conductivity of PPA-Ag composite hydrogels, four-probe experiments were performed for all the samples (Fig. 4e). The sample is shown in Fig. S4.† The electronic conductivity of PPA-Ag composite hydrogels can be substantially improved by the incorporation of AgNO₃ (11.29, 14.81, 15.97, and 16.33 S m⁻¹ for PPA-0, PPA-Ag5, PPA-Ag10, and PPA-Ag15, respectively), whereas there was no discernible change in the conductivity of the PPA-Ag20 composite hydrogel upon further addition of AgNO₃ (16.31 S m⁻¹). The PPA-Ag15 composite hydrogel electrode had the highest electronic conductivity. The high electrical conductivity of the PPA-Ag15 composite hydrogel electrode could be attributed to two main factors: First, as a small organic dopant acid, ATMP could dope and cross-link multiple PANI chains to construct a three-dimensional continuous conductive pathway, which was conducive to the rapid transport of charge carriers.^58,59 Second, the isotropic distribution of highly conductive Ag inside the composite hydrogel network facilitated rapid electron conduction on the internal surface of the hydrogel, further increasing the electronic conductivity of the composite hydrogels. This could enhance the specific capacitance of the PPA-Ag composite hydrogel electrodes and the energy density of the hydrogel-based flexible supercapacitor.⁶⁰ The PPA-Ag15 composite hydrogel could acted as a conductor to illuminate a blue light bulb and the brightness does not change significantly under different deformation conditions. This indicated that the PPA-Ag composite hydrogel possesses excellent electrical conductivity and flexibility (Fig. S5†).

To further explore the electrochemical performance of the PPA-Ag15 hydrogel electrode, the CV curves under scan rates of 5, 10, 20, 30, 50, and 100 mV s⁻¹ are shown in Fig. 4f. The shape of the peaks exhibited a minimal change, demonstrating the enhanced rate capability of the PPA-Ag15 composite hydrogel electrode. With increasing scan rate, the anodic (oxidation) peak current was positively shifted whereas the cathodic (reduction) peak current was negatively shifted due to the pseudocapacitance of the PANI.⁶¹ It was observed that the peak oxidation/reduction current exhibited a significant linear relationship with the square root of the scan rate. The linear fitting results are shown in Fig. 4g. This result confirmed that the oxidation–reduction reaction across the interface of the electrode/electrolyte was quasi-reversible.⁶²Fig. 4h shows the GCD curves of the PPA-Ag15 hydrogel electrode at various current densities. The specific capacitance values of the PPA-Ag15 hydrogel electrode were 510, 460, 430, 413, and 390 F g⁻¹ at current densities of 0.5, 1, 1.5, 2.5, and 5 A g⁻¹ respectively. The specific capacitance showed a declining trend with increasing current density. The reason behind this phenomenon was that the ions in the electrolyte accelerate and are unable to fully access the active sites on the electrode surface with the increase of the current density. Thus, the redox reaction was shorter and incomplete at high current densities, which might result in a reduction in capacitance during the charging and discharging processes.⁶³ In addition, the charge–discharge curves of the PPA-Ag15 composite hydrogel electrode remained highly symmetric when the current density was increased from 0.5 A g⁻¹ to 5 A g⁻¹, the charge–discharge times were almost equal, and there was no significant increase in the voltage drop, which indicated that the PPA-Ag15 composite hydrogel electrode had good electrochemical reversibility and excellent coulombic efficiency. Fig. 4i shows the cycling test result of the PPA-Ag15 composite hydrogel electrode at 3 A g⁻¹. The capacity retention and coulombic efficiency of the PPA-Ag15 composite hydrogel electrode displayed a slight increase during the initial stage of the charge/discharge cycle. The underlying effects of the increase in capacity retention and coulombic efficiency can be explained from the following two aspects. First, the activation of the PANI chains during short-term cycles enhanced the specific capacitance of the electrodes.⁶⁴ Second, the degradation products of PANI generated in situ during cycles serve as additional electroactive materials, thereby enhancing the specific capacitance.⁶⁵ After 10 000 charging and discharging cycles, the PPA-Ag15 composite hydrogel electrode could still maintain 81.41% of the initial specific capacitance with a high coulombic efficiency of 94.22%. The superior cycling stability of the PPA-Ag composite hydrogel electrodes can be explained from two aspects. First, the hierarchical structure with uniform macropores and aligned channels was favorable for the fast ion diffusion and reduced the damage caused by swelling and shrinking of the PANI hydrogel during prolonged cycling. Second, the highly conducting interconnected hydrogel network could provide electron transfer pathways, which could significantly alleviate electron accumulation on the electrode surface.

In summary, the satisfactory electrochemical properties of the PPA-Ag15 composite hydrogel electrode may be ascribed to the synergy effect offered by the pseudocapacitance of the PANI hydrogel, the doping effect of ATMP, and the high electrical conductivity and redox properties of silver. PANI, as a typical pseudocapacitive material, can provide large specific capacitance. ATMP could crosslink multiple protonated PANI chains, which helped to create additional conductive pathways and enhanced the electrochemical performance of the PPA-Ag hydrogel electrode. In addition, the superior hydrophilicity of the PANI hydrogel contributed by ATMP could enable intimate contact between the hydrogel electrode and hydrogel electrolyte. AgNO₃ could further improve the charge storage capacity of the composite hydrogel electrodes by introducing additional Ag⁺/Ag redox pairs. Furthermore, the uniform distribution of Ag in the PPA-Ag hydrogel network facilitated rapid electron conduction, which improved the conductivity of the PPA-Ag hydrogel and the utilization of the electroactive material (PANI) in the hydrogel electrode. Hence, PANI, ATMP, and AgNO₃ endowed the PPA-Ag hydrogel electrodes with remarkable electrochemical properties. Table 1 illustrates that the PPA-Ag hydrogel electrode exhibits superior electrochemical performance in comparison to other electrodes.

Table 1 Comparison of the electrochemical performance of electrode materials

Electrode materials	Specific capacitance	Current density/scan rate	Cycle life	Ref.
PANI/PHQ/RGO	356 F g^−1	0.5 A g^−1	94% (1000)	66
PANI/PVA	311.3 F g^−1	0.5 A g^−1	76% (10 000)	67
PANI/PVA	351 F g^−1	1 A g^−1	85.7% (1000)	68
PANI/CNT/PVA	389.5 F g^−1	0.5 A g^−1	83.87% (5000)	30
PANI/CNT	196.5 mF cm^−2	0.5 mA cm^−2	71.4% (5000)	69
PANI/HA	369 F g^−1	0.5 mV s^−1	85% (1000)	70
PANI/GO	311 F g^−1	0.5 A g^−1	88.5% (10 000)	71
PANI/FA/Ag	646 F g^−1	1 A g^−1	74% (5000)	23
PANI/CNT/Ag	528 F g^−1	5 mV s^−1	94% (1000)	27
PANI/Ag	512 F g^−1	5 mV s^−1	82% (2000)	28
PANI/SSCNTs/Ag	615 F g^−1	1 A g^−1	95% (1000)	29
PVA/PANI/ATMP/Ag	510 F g^−1	0.5 A g^−1	81.41% (10 000)	This work

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To further explore the charge storage mechanism of the PPA-Ag15 hydrogel electrode, the dependence of the current response of the CV curve at different scan rates is shown in Fig. 5a, which can be expressed by the following formulae:

i = av^b (1)

or

log i = a + b log v (2)

where “i” is the current value at a specific voltage; “v” is the scan rate; when the slope values b are 0.5 and 1, the electrode electrochemical behaviors can be classified as the pure diffusion process and pure surface capacitance process, respectively.⁷² The results of fitting the linear correlation between the logarithm of the current and the logarithm of the scan rate at different voltages of the CV curve of the PPA-Ag15 composite hydrogel electrode are shown in Fig. 5a. The calculated b values for the two pairs of redox peaks were 0.691, 0.658 and 0.802. From the results of fitting the peaks currents versus scanning rates, the values of b were between 0.5 and 1, suggesting that the electrochemical process of the PPA-Ag15 hydrogel electrode was controlled by both the surface capacitive process and the diffusion-controlled process.^57,73

Fig. 5 (a) The linear dependence of the scan rate on the peak current: (b and c) CV curves at 5 and 100 mV s⁻¹ scan rates, in which the shadowed area is the surface-capacitive (d) contribution rate and (e and f) specific capacitance of surface-capacitive and diffusion-controlled currents of the PPA-Ag15 hydrogel electrode.

Thus, the overall current is divided into diffusion-controlled current and surface capacitive current,⁷⁴ according to the formula below:

i = k₁v + k₂v^1/2 (3)

where “i” is the total CV response current, and the current consists of the surface capacitive current (k₁v) and the diffusion-controlled current (k₂v^1/2). By calculating the integral area ratio of the surface capacitance current to the diffusion control current at various scan rates, the proportion of the surface capacitance current contribution can be concluded. And it also can be expressed as:

Fig. 5b and c illustrate the integral area ratio of the surface capacitance current to the diffusion control current for a sweep speed of 5 and 100 mV s⁻¹, from which we can get the proportion of surface capacitive current. Combined with Fig. S6,† the percentage of surface capacitive current was calculated for varying sweep speeds, with the results presented in Fig. 5d. It was observed that the percentage of surface capacitive current increased significantly from 41.83% to 90.62% when the scan rate was increased from 5 mV s⁻¹ to 100 mV s⁻¹. As the scan rate increased, the charge storage mechanism of the PPA-Ag15 hydrogel electrode transformed from a predominantly diffusion-controlled to a predominantly surface capacitive effect due to the rapid surface redox reaction kinetics. The specific values of diffusion-controlled current and surface capacitive current of the PPA-Ag15 hydrogel electrode at different scan rates are given in Fig. 5e. It was observed that the surface capacitance of the PPA-Ag15 hydrogel electrode remained constant at 169 F g⁻¹, while the diffusion control current decreased from 235 F g⁻¹ to 18 F g⁻¹ as the scanning rate increased from 5 mV s⁻¹ to 100 mV s⁻¹. In three-electrode systems, the surface capacitance was mainly derived from the fast and reversible redox reactions occurring at the hydrogel electrode/electrolyte interface,⁷⁵ while the diffusion capacitance was a result of the dynamic diffusion and adsorption/desorption of ions from the electrolyte in the hydrogel electrode network structure. At high scan rates, the slower ion diffusion was significantly limited, while the faster pseudocapacitance redox reaction was almost unaffected. This was demonstrated by the reduction in diffusion capacitance with increasing scan rate, while the surface capacitance remained unaltered. Similarly, the specific capacitance provided by the surface capacitance current was quantified as a percentage of the diffusion control current by integrating over all the composite hydrogel electrodes at a scan rate of 5 mV s⁻¹, and the results are shown in Fig. 5f. The percentage of specific capacitance provided by the surface capacitance current of the PPA-0, PPA-Ag5, PPA-Ag10, PPA-Ag15, and PPA-Ag20 gel electrodes was 35.18%, 35.40%, 40.92%, 41.83% and 47.95%, respectively. For the PPA-Ag composite hydrogel, the source of surface capacitance contained the oxidation/reduction reaction of PANI, in addition to redox pairs provided by Ag⁺/Ag. As a result, the surface capacitance percentage of the PPA-Ag composite hydrogel electrodes demonstrated a notable upward trend with increasing amounts of AgNO₃.

The energy storage mechanism of the PPA-Ag15 hydrogel electrode is proposed in Fig. 6. The structural formula of polyaniline exhibited three stable states: the fully oxidized state (pernigraniline base, PNB), the intermediate oxidized state (emeraldine base, EB) and the fully reduced state (leucoemeraldine base, LEB). The intermediate oxidation state was the most stable. Furthermore, doped polyaniline exhibited a range of oxidation states, allowing it to exist stably in the form of polyaniline salts.⁷⁶ The ability of polyaniline to undergo reversible transitions between different oxidation states was the reason why it can be used as an active material for electrodes. During the charging and discharging processes, PANI experienced doping/dedoping and oxidation/reduction transitions at the electrode/electrolyte interface. This transition process was accompanied by electron gain and loss, thus possessing the ability to store charge.³¹ Under acid conditions, the atoms on the imine sites can be protonated by proton doping, and the PANI could undergo a reversible redox reaction after doping. When the oxidation reaction occurred, the electrolyte ions were transferred into the polymer backbone. In contrast, when the reduction reaction occurred, the ions were released from the backbone to the electrolyte again. Thus, polyaniline underwent redox transformation during the charging and discharging processes between the pernigraniline base, emeraldine base/salts, and leucoemeraldine base/salt. In particular, the conductive PANI hydrogel possessed a typical 3D network structure. This porous structure facilitated the diffusion and transmission of electrolyte ions, which increased the active sites of the reaction between the hydrogel electrode and electrolyte ions.⁷⁷ AgNO₃ acted as the electroactive material and made an extra contribution to the pseudocapacitance of the PPA-Ag hydrogel electrodes. After incorporating the AgNO₃, a second redox system was formed in the PPA-Ag15 hydrogel electrode by the reversible transfer of e⁻ from Ag⁺ to Ag in H₂SO₄ electrolyte aqueous solution (Ag⁰_n ⇌ Ag_n⁺ + e⁻).³⁴ The additional redox system at the electrode/electrolyte interface improved the pseudocapacitance and electrochemical performance of the PPA-Ag hydrogel electrodes.⁷⁸ In summary, PANI and silver had a positive synergistic effect, promoting each other to improved the specific capacitance of the PPA-Ag15 hydrogel electrode. The structural transition between the various redox states of polyaniline is illustrated in Fig. S7.†

Fig. 6 Schematic diagram of the energy storage mechanism of the PPA-Ag hydrogel electrode.

In order to investigate the flexibility of the PPA-Ag15 composite hydrogel electrode, the PPA-Ag15 hydrogel electrode was subjected to 200 bending cycles and the electrochemical properties were tested (Fig. 7). Fig. 7a presents the CV curves of the PPA-Ag15 hydrogel electrode before and after bending cycles. It can be seen that the shape of the CV curves before and after bending was highly consistent and the area was slightly reduced, indicating that the charge storage process of the PPA-Ag15 hydrogel electrode remained unaltered and continued to exhibit pseudocapacitive properties. This indicated that the PPA-Ag15 hydrogel electrode exhibits excellent flexibility. Fig. 7b shows the GCD curves of the PPA-Ag15 hydrogel electrode before and after 200 times bending, and it can be seen that the discharge time is only slightly shortened after 200 times bending, indicating that it had excellent capacitance retention. Fig. 7c demonstrates the comparison of the specific capacitance of the PPA-Ag15 composite hydrogel electrode before and after bending at 0.5, 1, 2, 3, and 5 A g⁻¹ current density. It was noticeable that the PPA-Ag15 composite hydrogel electrode retained an excellent capacitance retention rate of 93.43% after 200 instances of bending at a current density of 0.5 A g⁻¹. Moreover, even under high current density (5 A g⁻¹), its specific capacitance remained at 90.22% of the primary value. The electron transport and ion diffusion before/after bending cycles were studied by EIS measurement (Fig. 7d). The EIS curves before and after bending exhibit a high degree of overlap, indicating that the equivalent series resistance (R_s), charge transfer resistance (R_ct), and electrolyte ion diffusion impedance of the PPA-Ag15 hydrogel electrodes did not substantially change following the bending cycle. This indicated that the PPA-Ag15 hydrogel electrode exhibited excellent flexibility. The digital photos of the PPA-Ag15 composite hydrogel under bending, twisting, and holding a weight of 200 g are shown in Fig. S8.† In conclusion, the PPA-Ag15 hydrogel electrode displays excellent flexibility, which rendered it a promising candidate in the field of flexible energy storage.

Fig. 7 (a) CV curves, (b) GCD curves, (c) specific capacitances, and (d) EIS plots of the PPA-Ag15 hydrogel electrode before/after 200 bending cycles.

To further explore the practical performance of PPA-Ag hydrogel electrodes, flexible symmetric all-solid-state supercapacitors with a sandwich-like structure were assembled by the layer-by-layer gelation strategy using the PPA-Ag hydrogel as the electrode and PVA/H₂SO₄ as the hydrogel electrolyte as shown in Fig. 8a. The assembly of supercapacitors with a sandwich structure exhibits a low contact resistance between the electrodes and the electrolyte, which facilitated the rapid transport of ions.

Fig. 8 (a) Structure, (b) CV curves, (c) GCD curves of the PPA-Ag15 hydrogel based FSC, (d) Nyquist plot and (e) Ragone plot of PPA-Ag hydrogel based all-hydrogel-state FSCs; (f) cycling stability of the PPA-Ag15 hydrogel based FSC, (g) CV curves, (h) GCD curves, (i) Ragone plots and (j) EIS plots of the PPA-Ag15 hydrogel based FSC at different bending angles.

The CV curves of the symmetric PPA-Ag15∥PPA-Ag15 device demonstrated nearly rectangular shapes with a set of redox peaks at 5, 10, 20, 30, 50, and 100 mV s⁻¹ (Fig. 8b). The potential difference between the reduction and oxidation peaks increased gradually with increasing scan rate, resulting in a slight increase in the rate of polarization, while the similar shape maintains well because of its good rate behavior.

The GCD profiles (Fig. 7c) exhibited triangular shapes with negligible iR drops, indicating that the symmetric device can be steadily charged to 0.8 V even at a low current density of 0.5 A g⁻¹, suggesting a low internal resistance and high rate capability. Meanwhile, the assembly of supercapacitors based on PPA-Ag15 hydrogel electrodes exhibited a low equivalent series resistance (R_s) and charge transfer resistance (R_ct) of 1.10 and 2.11 Ω, respectively (Fig. 8d). Fig. 8e shows the Ragone plot of the PPA-Ag15 all-hydrogel-state flexible supercapacitor, from which it could be seen that the PPA-Ag15 composite hydrogel electrode-based device has the highest energy density of 13.3 W h kg⁻¹ at a power density of 125.0 W kg⁻¹ among all devices. Compared with other studies, the PPA-Ag15 hydrogel electrode-based supercapacitor exhibited excellent electrochemical performance (Table 2).

Table 2 Summary of the capacitive performance of supercapacitors

Electrode materials	Energy density	Power density	Cycle stability	Ref.
PEDOT:PSS/GO	75.21 μW h cm^−2	136 mW cm^−2	89% (10 000)	79
PANI/PVA/TiO2	2.8 W h kg^−1	125.0 W h kg^−1	86.9% (10)	80
PANI/CNT/PVA	12.8 W h kg^−1	125 W kg^−1	77.31% (1000)	30
PANI/PVA	7.8 W h kg^−1	200 W kg^−1	87% (3000)	64
PANI/PyHCP	0.57 W h kg^−1	320.63 W kg^−1	90.7% (2000)	81
PANI/CF	2.5 W h kg^−1	300 W kg^−1	86.1% (1000)	82
PANI/PVA/HQ	12 W h kg^−1	125 W kg^−1	77.6% (10 000)	22
PANI/FA/Ag	57.25 W h kg^−1	400 W kg^−1	—	23
PANI/CNT/Ag	187.73 W h kg^−1	1100 W kg^−1	—	27
PANI/Ag	50.01 W h kg^−1	3250 W kg^−1	—	28
PANI/CF/ExG/Ag	3.6 W h kg^−1	1377.80 W kg^−1	—	50
PPA-Ag	13.3 W h kg^−1	125 W kg^−1	77.01% (10 000)	This work

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The specific capacitance of the PPA-Ag15 hydrogel-based device remained at the initial 77.01% after 10 000 charge/discharge cycles at a current density of 3 A g⁻¹, indicating good cycling stability. Furthermore, its coulombic efficiency was retained at 97.72% (Fig. 8f). Consequently, PPA-Ag15 hydrogel electrode-based supercapacitors exhibited excellent electrochemical performance and have considerable potential for application in the field of energy storage.

To further examine the flexibility and stability of the PPA-Ag hydrogel electrode-based supercapacitor, its electrochemical performance was evaluated at bending angles of 0, 60, and 120°(Fig. 8g–j). The CV curves of PPA-Ag15 hydrogel electrode-based flexible supercapacitors with different bending angles were obtained at 5 mV s⁻¹, as shown in Fig. 8g. The CV curves exhibited a high degree of overlap at bending angles of 0 and 60°. The CV curve retained its shape even in the devices subjected to 120° large-angle bending, with only a slight decrease in area, indicating that the PPA-Ag15 hydrogel electrode-based supercapacitor exhibited good flexibility. A similar conclusion can be obtained for the GCD curves of PPA-Ag15 hydrogel electrode-based supercapacitors at bending angles of 0, 60, and 120° (Fig. 8h).

At a current density of 0.5 A g⁻¹, the PPA-Ag15 hydrogel electrode-based flexible supercapacitors could maintain 98.5% of the initial specific capacitance at a bending angle of 60°, and even if the bending angle is increased to 120°, the energy density of the device still maintained 84.21% of the initial value (Fig. 8i). The EIS diagrams of the PPA-Ag15 hydrogel electrode-based flexible supercapacitors at different bending angles are presented in Fig. 8j. It can be seen that there were small changes in charge-transfer resistance after bending, meanwhile, the typical straight sloping line can be maintained, confirming the good flexibility and capacitive behavior. The stable electrochemical performance enabled the assembled supercapacitor based on the PPA-Ag15 composite hydrogel electrode to exhibit significant prospects in new-generation flexible storage applications.

Conclusions

To summarize, a conducting polymer composite hydrogel, PPA-Ag, was successfully constructed by in situ polymerization of ANI, in which ATMP served as an organic doping acid. The presence of AgNO₃ introduced an additional Ag⁺/Ag redox pair. Meanwhile, the uniform distribution of silver within the PPA-Ag hydrogel network facilitated rapid electron conduction, thereby enhancing the conductivity of the PPA-Ag hydrogel from 11.29 to 16.33 S m⁻¹. The obtained PPA-Ag composite hydrogel electrode had a high specific capacitance of 510 F g⁻¹ at 0.5 A g⁻¹ and a superior flexibility of 93.43% capacitance retention after 200 bending cycles. After 10 000 charging and discharging cycles, the PPA-Ag15 composite hydrogel electrode could still maintain 81.41% of the initial specific capacitance. The symmetric all-hydrogel-state supercapacitor based on the PPA-Ag15 hydrogel electrode demonstrated a maximum high energy density of 13.3 W h kg⁻¹ at a power density of 125 W kg⁻¹. The specific capacitance of the supercapacitors maintained the initial 77.01% after 10 000 charge/discharge cycles at a current density of 3 A g⁻¹. Consequently, PPA-Ag hydrogel electrodes with excellent flexibility and electrochemical properties demonstrated a broad range of applications within the field of flexible energy storage.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and ESI.†

Author contributions

Cheng Zhao: investigation, validation, methodology, writing – original draft, writing – review & editing. Shixiang Zhou: writing – review. Jie Ma: conceptualization. Cong Liu: methodology. Jiading Zhu: formal analysis. Shifang Ye: data curation. Zhe Xin: writing – review. Jiantao Cai: writing – review. Peizhong Feng: writing – review. Xueyu Tao: funding acquisition, writing – review & editing, project administration.

Conflicts of interest

We declare that we have no conflicts of interest in this work.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (52173015).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07437h

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