Mengmeng
Xun
a,
Xiuting
Shi‡
a,
Haiping
Wang
a,
Xiaoyan
Li
a,
Wenxing
Miao
a,
Xiangbing
Wang
a,
Kanjun
Sun
b,
Hui
Peng
a,
Guofu
Ma
*a and
Yuxi
Xu
*c
aKey Laboratory of Eco-functional Polymer Materials of the Ministry of Education, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China. E-mail: magf@nwnu.edu.cn
bCollege of Chemistry and Chemical Engineering, Lanzhou City University, Lanzhou 730070, China
cSchool of Engineering, Westlake University, Hangzhou 310024, China. E-mail: xuyuxi@westlake.edu.cn
First published on 7th March 2024
Flexible supercapacitors (FSCs) based on hydrogel electrolytes have the advantages of high ionic conductivity, no liquid leakage, flexibility and versatility, making them the most promising power sources for wearable devices. Herein, a flexible and stretchable, ultrathin polyvinyl alcohol/carboxymethyl chitosan incorporated with a redox active ionic liquid (PVA/CMCS-[ViEtIm][Br]) hydrogel electrolyte is prepared by a facile coating and freezing/thawing method, which is used to improve the practical performance of supercapacitors. The PVA/CMCS-[ViEtIm][Br] hydrogel film has good mechanical properties. More importantly, the redox reaction caused by [ViEtIm][Br] in the hydrogel electrolyte provides a crucial pseudocapacitive contribution to supercapacitors. Thus, the flexible supercapacitor assembled with the PVA/CMCS-[ViEtIm][Br] hydrogel at a thickness of 0.1 mm has an areal specific capacitance of 314.4 mF cm−2 and an energy density of 78.6 μWh cm−2 at 540 μW cm−2, with a capacitance retention of 87.5% after 10000 charge/discharge cycles. Moreover, the flexible supercapacitor can also exhibit stable performance at different bending angles. This work provides a simple and feasible method for realizing ultra-thin flexible capacitors with high energy density.
PHEs are porous soft materials consisting of a cross-linked polymer network containing large amounts of water. Due to their high mechanical properties, flexibility, elasticity and biocompatibility, they have become widely promising electrolyte matrices for energy conversion and storage systems in recent years.23–25 Zhang et al.26 proposed the graft copolymerization of polyacrylic acid (PAAS) with carboxylated chitosan followed by strong uptake of KOH electrolyte to prepare an alkali-tolerant hydrogel electrolyte membrane with high ionic conductivity and pH responsiveness. Abbas Ali Hor et al.27 proposed a membrane consisting of ∼80 wt% ionic liquid (BMPTFSI) and redox additive ionic liquid (BMPBr) wrapped in an ∼20 wt% polymer (PVDF–HFP) matrix as a flexible and self-contained thin film PHE for SCs. The application of natural polysaccharides in the field of hydrogels has attracted wide interest from researchers due to their abundant reserves, biocompatibility, non-toxicity, renewability, degradability, and cost-saving advantages.28–30 Among the materials reported so far for PHEs, water-soluble PVA is the most widely studied and contains a large amount of –OH. Furthermore, carboxymethyl chitosan is a common natural polysaccharide material with good film-forming properties due to its rich hydrophilic groups and can be functionalized by appropriate modifications.31 1-Vinyl-3-ethylimidazolium bromide salt ([ViEtIm][Br]) is a common redox-active substance with a wide electrochemical window (generally above 3.5 V) and high electrochemical stability, which is characteristic of ionic liquids.21
Herein, we prepared a novel ionic liquid-filled poly(vinyl alcohol)/carboxymethyl chitosan-1-vinyl-3-ethylimidazolium bromide salt (PVA/CMCS-[ViEtIm][Br]) ultrathin hydrogel electrolyte film by a coating and freezing/thawing method, which is redox-active and stretchable, and assembled into a flexible supercapacitor with very good electrochemical performance and potential for practical applications. Among them, polyvinyl alcohol (PVA) is a polymer matrix, carboxymethyl chitosan (CMCS) is a natural polymer additive, and 1-vinyl-3-ethylimidazolium bromide salt ([ViEtIm][Br]) is a redox active substance for flexible ultra-thin supercapacitors. The effects of hydrogel electrolyte films with different CMCS contents, thicknesses and [ViEtIm][Br] concentrations on their electrochemical properties were investigated in detail.
For comparison, PVA/CMCS hydrogel films with different CMCS contents (0, 0.1, 0.2 and 0.3 g) were prepared by the same above-mentioned method. In addition, the PVA/CMCS-[ViEtIm][Br] hydrogel films immersed in 1 M H2SO4 solution with different concentrations of [ViEtIm][Br] (0, 1 wt%, 3 wt% and 5 wt%) were also investigated.
The thickness of the hydrogel film is measured using a thickness gauge. The PVA/CMCS-[ViEtIm][Br] hydrogel presented a homogeneous and smooth film with a thickness of 0.10 mm (Fig. 2a), which is shown in the inset. The transparent and smooth state of the gel can also be seen in Fig. S1.† The surface structure of the hydrogel was characterized using SEM. Fig. S2† shows the SEM image of the PVA hydrogel, which shows that the PVA hydrogel contains a porous structure. Fig. 2b and c show the freeze-dried PVA/CMCS and PVA/CMCS-[ViEtIm][Br] PHEs. It can be seen that the surface of the freeze-dried PVA/CMCS gel shows a large number of folds, while the surface of the freeze-dried PVA/CMCS-[ViEtIm][Br] gel shows a porous structure. Such a porous structure can facilitate the transport of ions and thus improve correlation kinetics. Fig. 2d shows the XRD patterns of PVA, CMCS and PVA/CMCS-[ViEtIm][Br] after freeze-drying. It can be observed that CMCS exhibits a weaker crystal peak at 2θ = 20.4°, indicating a typical polymer amorphous structure. PVA is a semi-crystalline polymer, which shows that an obvious crystalline peak exists at 2θ = 20°. When PVA was cross-linked with CMCS to form an interpenetrating network structure, the intensity of the peak near 2θ = 20° significantly reduced, indicating that the cross-linked copolymer became a disordered amorphous structure with low crystallinity.23 Apparently, the regularity of the original PVA is destroyed by cross-linking, which leads to the formation of amorphous copolymers. Fig. 2e shows the FT-IR plots of PVA, PVA/CMCS and PVA/CMCS-[ViEtIm][Br] after freeze-drying. In the PVA/CMCS-[ViEtIm][Br] hydrogel films, the peak at 1738.6 cm−1 corresponds to the stretching vibration of CO in –COOH and the peak at 1644.5 cm−1 corresponds to the stretching vibration of C
O in –NHCOCH3. Comparison with the characteristic peaks of PVA/CMCS shows that the CMCS chains provide –COOH and –NHCOCH3, between which hydrogen bonds can be formed. The peak at 1069.6 cm−1 is attributed to C
O and C–O–C. The peaks at 1616.5 and 1443.6 cm−1 are attributed to the tensile vibrations of C–O in –COO− on the CMCS chains, and it can form an ionic bond with the positively charged NH3+ in the CMCS chain. The peaks at 3432.2 cm−1 and 2914.4 cm−1 are attributed to the stretching vibrations of –OH and C–H in PVA.31,32 It can be noticed that the introduction of the CMCS molecular chain, which increases the –OH content, leads to broadening of the peaks at around 3430.1 cm−1 for the PVA/CMCS and PVA/CMCS-[ViEtIm][Br] curves as compared to the curve for pure PVA.28 The above information indicates the successful preparation of the PVA/CMCS-[ViEtIm][Br] PHE film.
In order to test the mechanical properties of the prepared PVA/CMCS-[ViEtIm][Br] hydrogel film, bending, twisting, and stretching tests are performed. As shown in Fig. 2f, the elongation at break of the PVA hydrogel film is 493%. With the introduction of CMCS, the elongation at break of the PVA/CMCS hydrogel film increased to 615%. The introduction of [ViEtIm][Br] further increased the elongation at break of the PVA/CMCS-[ViEtIm][Br] hydrogel film to 785%. The variation of hydrogel tensile properties with the concentrations of CMCS, thickness and [ViEtIm][Br] content is shown in Fig. S3, S4 and S5.† As the thickness of the gel film gradually increases, the elongation at break of the gel becomes larger and reaches a maximum of 615% at a thickness of 0.1 mm. This may be due to the increase in PVA and CMCS content that accompanies the increase in gel thickness, the intertwining of the CMCS molecular chains with the PVA polymer chains, and the enhanced hydrogen-bonding cross-linking between the molecular chains, which improves the elongation at break of the hydrogel films. With the increase in [ViEtIm][Br] content, the hydrogen bonding and electrostatic interactions within the gel film are enhanced, which improves the elongation at break of the hydrogel film. As shown in Fig. 2g–i, the prepared PVA/CMCS-[ViEtIm][Br] hydrogel film shows no cracks or breaks after bending and twisting, exhibiting good flexibility and mechanical properties.
The FSCs are first assembled using commercial activated carbon (AC) as the electrode material and PVA/CMCS hydrogel films with different CMCS contents (0, 0.1, 0.2 and 0.3 g) as PHEs. Fig. 3a shows the comparative CV curves of PVA/CMCS hydrogel-based FSCs containing different CMCS contents at 50 mV s−1. The CV curve area increases as the CMCS content increases, and when the CMCS content exceeds 0.2 g, the CV curve area decreases because the excess CMCS is not favorable for ion transport in PHEs. The increase in CMCS makes the 3D network voids smaller, making it difficult for water molecules and solvated electrolyte ions to enter the hydrogel network, which increases the transport resistance of electrolyte ions.32 Obviously, the area of the CV curve is the largest when the CMCS content is 0.2 g, indicating that it has the most excellent capacitance behavior. Fig. S6† shows the electrochemical properties of the hydrogel at different scan rates at a CMCS content of 0.2 g, showing good electrochemical properties. For the GCD curves in Fig. 3b, the same variation trend as in CV curves is evident. The optimal areal specific capacitance of the PVA/CMCS hydrogel-based FSC with a CMCS content of 0.2 g is 121.4 mF cm−2 at an operating voltage range of 0 to 1 V and a current density of 0.5 mA cm−2. The GCD curves at different current densities at a CMCS content of 0.2 g in Fig. S7† can also illustrate that the hydrogel has a favorable charge/discharge behavior. Meanwhile, PVA/CMCS with a CMCS content of 0.2 g shows significantly lower interfacial resistance and charge transfer resistance than other FSCs in the comparative Nyquist curve plot (Fig. 3c). Thus, PVA/CMCS with a CMCS content of 0.2 g was used as the optimal sample.
As is known, the thickness of the hydrogel electrolyte film directly affects the ion transport inside the PHEs. Fig. 3d shows the comparative CV curves of PVA/CMCS hydrogel-based FSCs with different thicknesses at 50 mV s−1. The CV curves of PVA/CMCS hydrogel-based FSCs with different thicknesses all show a rectangular-like shape, indicating an electrochemical double layer capacitance (EDLC) behavior. The CV curve area is largest when the thickness of the PVA/CMCS hydrogel film is 0.10 mm, indicating that it has the largest areal specific capacitance. Fig. 3e shows the GCD curves of PVA/CMCS hydrogel-based FSCs with different thicknesses at a current density of 0.5 mA cm−2 with an operating voltage of 0–1.0 V. The PVA/CMCS hydrogel film with a thickness of 0.10 mm presents the longest discharge time and shows a high areal specific capacitance of 127.6 mF cm−2 at 0.5 mA cm−2. Meanwhile, Fig. 3f shows the comparison of the Nyquist curves of PVA/CMCS hydrogel-based FSCs with different film thicknesses. It can be seen that the interfacial resistance (Rs) and charge transfer resistance (Rct) of FSCs gradually increase with increasing thickness, which is due to the fact that ion transport becomes more difficult with increasing thickness. By comprehensive comparison, the thickness of the PVA/CMCS hydrogel film of 0.10 mm is selected as the optimal thickness.
To investigate the effect of [ViEtIm][Br] on the electrochemical properties of FSCs, CV tests are performed for PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs with different [ViEtIm][Br] concentrations at 50 mV s−1. As shown in Fig. 3g, the CV curves of PVA/CMCS hydrogel-based FSCs show obvious rectangular-like shapes, which is due to the EDLC energy storage mechanism in carbon-based SCs. However, PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs exhibit obvious redox peaks after the introduction of [ViEtIm][Br], which is due to the occurrence of a redox reaction and the generation of pseudocapacitance. The reaction mechanism for the formation of pseudocapacitance is as follows:33
2Br− ↔ Br2 + 2e− ↔ Br3− | (1) |
As shown in Fig. 3g, the integrated area of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs with the [ViEtIm][Br] concentration of 3 wt% is larger than that of PVA/CMCS and other concentrations of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs, indicating that the former possesses a higher specific capacitance. The redox reaction resulting from the introduction of [ViEtIm][Br] provides a stable pseudocapacitance for flexible hydrogel electrolyte devices to extend the specific capacitance and energy density.34 As shown in Fig. 3h, the GCD curves of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs with [ViEtIm][Br] also demonstrate the interfacial redox activity contribution. The PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs show the longest discharge time when the [ViEtIm][Br] concentration is 3 wt%, which indicates a higher areal specific capacitance than others. Fig. 3i shows the Nyquist curves of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs with different [ViEtIm][Br] concentrations. It can be clearly seen that the introduction of [ViEtIm][Br] can increase the ionic conductivity of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs, allowing ions to quickly transfer/diffuse to the electrode surface, thereby improving the electrochemical performance of FSCs. The smallest Rs and Rct of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs exist when the [ViEtIm][Br] concentration is 3 wt%. The Rs and Rct values of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs increased when the [ViEtIm][Br] concentration was 5 wt%. This may be due to the increase in [ViEtIm][Br] content, which increases the viscosity of the electrolyte to a certain extent, leading to a decrease in ionic conductivity.35 Therefore, the [ViEtIm][Br] concentration of 3 wt% is selected as the optimal amount to be added to the PVA/CMCS-[ViEtIm][Br] hydrogels for subsequent electrochemical performance studies.
Fig. 4a shows the CV curves of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs at different scan rates from 10 to 100 mV s−1 with operating voltages from 0 to 1.0 V. It can be seen that the CV curves have similar shapes at different scan rates. The presence of distinct redox peaks in the PVA/CMCS-[ViEtIm][Br]hydrogel-based FSCs demonstrates that a reversible redox reaction occurred. Also, as the scan rate increases, their oxidation peaks are shifted in the positive direction and the reduction peaks are shifted in the negative direction, which is due to the mixed polarization of ion and electron resistance.31,36 As shown in Fig. 4b, the surface capacitive contribution of PVA/CMCS-[ViEtIm][Br]-hydrogel-based FSCs reaches 94.12% at a scan rate of 100 mV s−1, which is attributed to the occurrence of redox reactions and the generation of pseudocapacitance due to the introduction of [ViEtIm][Br]. The increase of the pseudocapacitance contribution can effectively improve the capacitance capability of supercapacitors. As follows from Fig. 4c, the contribution of the surface limiting capacitance is slightly higher than the diffusion controlled capacitance at low scan rates. As expected, the contribution of the surface capacitance increases with increasing scan rate from 64% to 94% when increasing from 10 mV s−1 to 100 mV s−1, due to the lower kinetic confinement of the electrode surface at high scan rates. The b-value is determined as the slope of the log(i) versus log(ν) plot, as shown in Fig. 4d. To estimate the capacitance effect, the type of charge storage was analyzed at different scan rates using the following equation:
i(V) = aνb | (2) |
Fig. 4e shows the GCD curves of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs in the current density range of 0.3–3 mA cm−2 to study the rate performance of FSCs. The GCD curves for PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs are no longer typically linear due to the presence of [ViEtIm][Br] redox additives, but the charge/discharge times remain relatively symmetric for all current densities, indicating good and reversible electrochemical properties. Equations are applied to calculate the energy density (EA, μWh cm−2) and power density (PA, μW cm−2) based on the data in Fig. 4e. Fig. 4f shows the areal specific capacitance of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs at different current densities. It can be clearly seen that the FSCs have high areal specific capacitances of 310.9 mF cm−2, 290.1 mF cm−2, 277.8 mF cm−2, 266.12 mF cm−2 and 249.2 mF cm−2 at 0.5, 1.0, 1.5, 2.0 and 3.0 mA cm−2, respectively. The corresponding volume specific capacities are 10.4 F cm−3, 9.7 F cm−3, 9.3 F cm−3, 8.9 F cm−3, and 8.3 F cm−3. It is shown that PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs with 3 wt% [ViEtIm][Br] content exhibit outstanding rate performance, the value of which is better than that of PVA/CMCS[ViEtIm][Br] hydrogel-based FSCs with 0, 1 wt%, and 5 wt% [ViEtIm][Br] content (Fig. S8†). The specific capacitance decreases as the current density increases in a gradient, which is due to the effect of ion diffusion at different current densities.
Fig. 4g shows the Ragone diagram of PVA/CMCS and PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs. At a power density of 540 μW cm−2, the energy density of the PVA/CMCS-[ViEtIm][Br] hydrogel-based FSC reaches a maximum of 78.6 μWh cm−2, which is much better than the energy density of 50.25 μW cm−2 of the PVA/CMCS hydrogel-based FSC. These values are greater than the other previously reported FSCs shown in Table S1,† such as the FSC based on the poly(pyrrole-co-aniline) incorporated PVA/PEG gel electrolyte (54 μWh cm−2),37 the FSC with a self-healing PVA/phytic acid hydrogel electrolyte (31.7 μWh cm−2),39 and so on.40–43 In addition, the cycling stability of the PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs is evaluated by subjecting them to 10000 charge–discharge cycles at 1 mA cm−2, and the result is shown in Fig. 4h. It can be seen that the PVA/CMCS-[ViEtIm][Br] hydrogel-based FSC has excellent cycling stability, retaining 87.5% of the initial capacitance after 10
000 cycles, and exhibits a high initial coulombic efficiency of 93.3% with virtually no losses.
The electrochemical properties of the PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs are evaluated at different bending angles for practical applications. Fig. 5a shows the CV curves of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs bent at 0°, 90° and 180° at 50 mV s−1. The CV curves have a similar shape, indicating that the structure of FSCs remains intact even when bent at 180°. Fig. 5b shows the GCD curves of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs bent at 0°, 90° and 180° at a current density of 0.5 mA cm−2. It can be seen that the curves have a high degree of overlap at different bending angles, and the discharge time of PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs did not vary greatly. As shown in Fig. 5c, the PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs retain 93.4% of the initial capacitance at 180°, which indicates that the PVA/CMCS-[ViEtIm][Br]hydrogel-based FSCs have good power output at different bending angles. Fig. 5d shows the Nyquist curves of PVA/CMCS-[ViEtIm][Br]hydrogel-based FSCs at different bending angles of 0°, 90° and 180°, which indicates that they have good ionic conductivity at different bending angles. In conclusion, PVA/CMCS-[ViEtIm][Br] hydrogel-based FSCs have excellent flexibility, can adapt to bending at multiple angles and exhibit good electrochemical properties for practical applications.
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Fig. 5 PVA/CMCS-[ViEtIm][Br] hydrogel-based supercapacitors at different bending angles (0°, 90° and 180°): (a) CV curve, (b) GCD curve, (c) capacitance retention, and (d) Nyquist curve. |
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4lp00007b |
‡ These authors contributed equally to this work. They should thus be considered co-first authors. |
This journal is © The Royal Society of Chemistry 2024 |