Microgel enhanced double-network hydrogel electrode with high

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Letter

Microgel enhanced double-network hydrogel electrode with high conductivity and stability for intrinsically stretchable and flexible all-gel-state supercapacitor Yu Zhao, Shuo Chen, Jian Hu, Jiali Yu, Gengchao Feng, Bo Yang, Cuihua Li, Ning Zhao, Caizhen Zhu, and Jian Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Applied Materials & Interfaces

Microgel enhanced double network hydrogel electrode with high conductivity and stability for intrinsically stretchable and flexible all-gel-state supercapacitor Yu Zhao†, Shuo Chen†, Jian Hu‡, Jiali Yu†, Gengchao Feng§, Bo Yang†, Cuihua, Li†, Ning Zhao#, Caizhen Zhu*† and Jian Xu# †

College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518060,

China ‡

State Key Lab for Strength and Vibration of Mechanical Structures, Department of Engineering

Mechanics, Xi’an Jiaotong University, Xi’an 710049, China §

ShenZhen Surg Science Medical Tech. Co., Ltd. Shenzhen 518012, China

#

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy

of Sciences, Beijing 100190, PR China KEYWORDS:

All-gel-state

supercapacitors,

intrinsically

stretchable

electrode,

high

conductivity, high stability, tough hydrogel.

ACS Paragon Plus Environment

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ABSTRACT. In the present work, a new strategy is proposed to simultaneously enhance the toughness and electrochemical performance of the hydrogel with conductive microgel to form microgel-reinforced double network hydrogel. In this hydrogel, the conductive microgel is crosslinked to form the first network, which can dissipate energy to improve mechanical performance and stabilize the conductive network to improve the electrochemical performance. These hydrogels show excellent mechanical properties and good conductivity. When these hydrogels were assembled to all-gel-state intrinsically flexible and stretchable supercapacitor, it delivers outstanding capacitance. The strategy put forward here can extend the application scope of the hydrogel with multifunction.

In recent years, intensive efforts have been devoted to developing high-performance flexible electronics due to their wide application in the wearable devices, such as supercapacitors1, 2, sensors for health monitors3 and soft robots4, 5. For the wearable device, not only flexibility but also stretchability is an essential requirement. Flexible and stretchable supercapacitor plays an increasingly important role in wearable device for their fast rate of charge-discharge, long cycling lifetime and high capacitance. Unfortunately, conventional electrodes are not suitable as smart wearable device due to its poor flexibility, weak mechanical strength or low electrochemical stabilities6. To make the supercapacitor stretchable, non-coplanar buckled7, wavy8 and coplanar serpentine structure9 have been designed. These kinds of stretchable supercapacitors are usually composed of the intrinsically stretchable electrolyte and a flexible electrode. These flexible electrodes are always fabricated by directly coating active materials, carbon and carbon nanotube slurries on flexible substrates such as plastic10,

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, textile12 or


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paper13-15. Although the supercapacitors mentioned above use flexible electrodes, they still face some drawbacks with poor electrochemical cycling stability, nonbiomimetic ability and high resistance, which limits their applications for the flexible electronic device. The hydrogel is a soft mater that maintains a three-dimensional structure, owing to its interconnected porous structures, the contact between electrolyte and conductive polymers improved by providing excellent electrochemical kinetics and significant structural stability, which has been shown that the hydrogel can be used in a wide range of applications such as cell scaffolds16, artificial tissue17, soft robotics18, smart actuators19, and sensors20, 21. Recently, some studies have been proven that hydrogel polyelectrolyte can be used in supercapacitors as membrane materials, which exhibits high capacitance and ductility, owing to the good ionic conductivity and stretchability of hydrogel22, 23. However, these hydrogels are rarely be used as electrode materials in supercapacitors due to the poor electrochemical performance, which hinders the development of supercapacitors. Therefore, it is still a challenge to develop a universal strategy to create stretchable hydrogel electrode with high conductivity and stability for all-gel-state supercapacitor. To the best of our knowledge, fully intrinsic stretchable supercapacitor has not been reported yet. The stretchable supercapacitor requires sustainability of high deformation, high conductivity, and electroactivity to fulfill the future application. Herein, a new strategy was put forward to fabricate a fully intrinsic stretchable supercapacitor with all outstanding properties above.

We

have

synthesized

a

conductive

microgel

by

adding

Poly

(3,

4-

ethylenedioxythiophene): polystyrene sulfonate (PEDOT) solution to the microgel pre-solution to increase the conductivity of hydrogels. Then we crosslinked those conductive microgels with polyacrylamide (PAAm) and PEDOT to form the first network in the hydrogel. By the


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polymerization of the second network, these conductive microgel-reinforced double network (MR-DN) hydrogel exhibits excellent mechanical and electrochemical performance. The high mechanical performance is originated from the fracture of the microgel network during the deformation. The microgel network plays a role in the sacrificial bonds to dissipate energy, which is similar to the fracture mechanism of other DN hydrogels24. Then it worked as a “cluster” to catch the second network when the bulk hydrogels underwent a large deformation. Meanwhile, the electrochemical performance of first network and second network is improved by introducing PEDOT (Scheme 1). The polypyrrole (PPy) was electrodeposited in a network to improve energy storage capacity25,

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further increase its conductivity, and the infrared

spectroscopy (FT-IR) result is shown in Figure S1. These hydrogels exhibit good flexibility ( ε > 500%), high toughness ( Wext. > 2.4 MJ/m3), high conductivity ( > 3.3 S cm-1) and high specific capacity ( > 220 F g-1). More interestingly, the hydrogel electrode and assembled supercapacitor display excellent electroactivity and stability, with the specific capacitance retained up to 94% and 83% of its initial value after 1000 chargedischarge cycles, respectively. This hydrogel has potential applications in flexible, stretchable smart energy-storage devices. PEDOT is an excellent conductive and water-dispersible polymer, which can dramatically improve the conductivity of the hydrogel27. Intuitively, the conductivity of the DN hydrogel will increase with the increasing content of PEDOT. However, we found that the polymerization of PAAm may be hindered by the PEDOT due to the increasing viscosity of the solution. Therefore, the concentration of PEDOT is an important parameter to the toughness of the hydrogel. To balance the relationship between the conductivity and the toughness of the hydrogel, we investigated the effect of the content of PEDOT on the mechanical performance of microgel-


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reinforced PEDOT/PAAm single network (MR-PEDOT/PAAm SN) hydrogels by tuning PEDOT concentrations. The tensile stress-strain curves of MR-PEDOT/PAAm SN hydrogels with the content of PEDOT was shown in Figure S2a. Attributed to the soft and ductile PAAm hydrogel matrix, all MR-PEDOT/PAAm SN hydrogels displayed poor mechanical properties like a common PAAm hydrogel with the fracture stress and fracture strain less than 0.1 MPa and 100%, respectively. As shown in Figure S2b, the elastic modulus decreases with the increasing concentration of PEDOT. But the volume of hydrogels changes regardlessly as the content of PEDOT increased (Figure S2c). The results indicate that PEDOT significantly influenced the process of the AAm polymerization. PEDOT hindered the hydrogel to form a homogeneous network structure, leading to the decrease in the average density of polymer chains. In addition, with the increase of PEDOT concentration, a large proportion of the hydrogel space was occupied by PEDOT during the polymerization of the second network, which reduced the concentration of PAAm in and around the microgels. DN hydrogel becomes tough only when its structure satisfies two conditions24: (i) the first network is a densely cross-linked, rigid, and brittle network, whereas the second network is sparsely cross-linked, soft, and ductile; (ii) the molar concentration of the second network is 20-30 times higher than that of the first network. Thus, the tough microgelreinforced double network hydrogel requires a first PAAm network to trap all microgels, and more second network PAAm chains are impregnated into the microgels to satisfy the large molar ratio of the second network to the first network28. In other words, the high concentration of PEDOT leads to the decreasing of the toughness of microgel-reinforced PEDOT/PAAm double network (MR-PEDOT/PAAm DN) hydrogel. Furthermore, the bulk ionic conductivities of hydrogels are around 2.8 S cm-1, which did not change with PEDOT concentration. (Figure S2d).


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By tuning the PEDOT concentration in microgel, we found that the optimized PEDOT concentration is 0.5 wt%, which was further used to prepare conductive tough double network hydrogel. Figure 1a shows the comparison of tensile stress-strain curves of PEDOT/PAAm SN

hydrogel, MR-PEDOT/PAAm SN hydrogel, MR-PEDOT/PAAm DN hydrogel and MRPEDOT/AAm/PPy DN hydrogel. PEDOT/PAAm SN hydrogel and MR-PEDOT/PAAm SN hydrogel were low in toughness, and the fracture stress and fracture strain were less than 0.1 MPa and 200%, respectively. In contrast, the fracture stress and fracture strain of hydrogel after the formation of the second network were remarkably high compared to single network hydrogels (0.91 MPa and 570%, respectively). Furthermore, the PPy was deposited to the MRPEDOT/PAAm DN hydrogel to form a dense network (MR-PEDOT/PAAm/PPy DN hydrogel), which further enhanced the mechanical performance of hydrogel (σf>1.1 MPa, ε>570%). The results showed clearly that these hydrogels were reinforced via the formation of the second network. The work of extension Wext., a parameter that expresses one aspect of toughness of hydrogels, increased dramatically from 0.029~0.032 MJ/m3 for single network hydrogel to 2.43~2.94 MJ/m3 for double network hydrogel (Figure 1b). Such excellent mechanical performance enables our hydrogel electrode to meet some mechanically extreme conditions. Electrochemical measurements were conducted to compare the performances of the MRPEDOT/PAAm hydrogels as supercapacitor electrodes. Due to the small specific capacity contribution of PEDOT, the MR-PEDOT/PAAm SN hydrogel displayed a negligible current response when it was scanned over the voltage range from −0.7 to 0.7 V at a scan rate of 100 mV s−1 as shown in Figure 2a. Voltammograms deviate from a rectangular shape because of slow ionic transport derived from the porous structure of hydrogel29. By comparisons, the hydrogels


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electrodeposited PPy displayed a high current larger than that of the MR-PEDOT/PAAm gel, which represents the current response increased and a higher specific capacitance after deposited PPy,suggesting an excellent capacitance after deposited PPy. Electrochemical impedance spectroscopy (EIS) provided analysis of the specific role of hydrogels in the charge-storage mechanisms. According to the intercept of the Nyquist plot with the real axis, the PEDOT/PAAm/PPy SN hydrogel, MR-PEDOT/PAAm SN hydrogel, MRPEDOT/PAAm/PPy SN hydrogel and MR-PEDOT/PAAm/PPy DN hydrogel displayed a bulk resistance of 21Ω, 27Ω, 16Ω and 10Ω in KCl electrolyte, respectively (Figure 2b). It is obviously found that the Nyquist plot exhibits a straight and nearly vertical line at low frequencies of hydrogel with PPy, indicating the ideal capacitive behavior of the hydrogel supercapacitors. Moreover, the hydrogel with PPy exhibited smaller bulk resistances than that without PPy, suggesting its excellent conductive behavior of PPy. This phenomenon could be explained by the poor conductivity of PAAm network hindered the continuity of conductive PEDOT polymer, while the electrodeposited PPy could improve the continuity of electroactive materials as “bridge” (Figure S3). The similar tendency of hydrogel conductivity was shown in Figure 2c. The galvanostatic charge-discharge (GCD) curves of MR-PEDOT/PAAm SN hydrogel, MR-PEDOT/PAAm/PPy SN hydrogel, and MR-PEDOT/PAAm/PPy DN hydrogel were obtained at various current densities (Figure S5-S7), and the specific capacitance of these hydrogels calculated from the GCD curves at 0.5 to 10 A g-1 was shown in Figure 2d. The maximal specific capacitances were 200 F g-1, 130 F g-1, 220 F g-1and 220 F g-1 for PEDOT/PAAm/PPy SN hydrogel, MR-PEDOT/PAAm SN hydrogel, MR-PEDOT/PAAm/PPy SN hydrogel and MR-PEDOT/PAAm/PPy DN hydrogel at a current density of 0.5 A g-1,


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respectively. And the areal and volumetric capacitances of MR-PEDOT/PAAm/PPy DN hydrogel are 19.25 mF/cm2 and 770 mF/cm3, respectively. Although both PEDOT and PPy contribute to the capacitance, it is clearly found that the contribution from PPy is much higher (about 1.5 times) than PEDOT. More importantly, these hydrogel electrodes can endure fast current change rates, which maintained 67%, 67%, 83% and 79.5% of its initial capacitance with the increased current density up to 10 A g-1, respectively. This property arose from the good ionic conductivity of the hydrogel and effective electrochemical dynamic processes in the electrodes. According to the mentioned above, the PPy played multiple roles in improving the specific capacitance: 1) its capacitive and Faradaic currents contributed to the charge storage; 2) PPy accelerated the electron transport and improved the collection of the charge. As the results, all hydrogel electrode not only exhibited excellent conductive properties as soft conducting materials30 but also showed good electroactivity. To further investigate the stability of MR-PEDOT/PAAm/PPy DN hydrogel as supercapacitor electrode, the long-term cyclic voltammetry was conducted. The hydrogel exhibited excellent repeatability (Figure S8), as a contrast, the hydrogel without microgel cannot keep the original tracks (Figure S9). The specific capacitance of the hydrogel electrode maintained more than 90% of its initial value after 1000 cycles. But the capacitance of the hydrogel electrode without microgels rapidly decreased to less than 70% of its initial value (Figure 3a). Meanwhile, we found that the value of the resistance increased slightly after 1000 cycles (Figure 3b), from 24Ω to 33Ω. This little change may be attributed to the degradation of PPy after long-term charge-discharge cycles. These results indicated that the microgel played an important role in maintaining the specific capacitance. During the process of electrodeposition, the chains of PPy penetrated into the microgel. Therefore, the microgel network was able to


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restrain the movement of the PPy chains, which is beneficial for the stability of the supercapacitor during the charge-discharge cycles. In addition, the hydrogel electrode exhibited outstanding flexibility, the hydrogel electrode can be maintained more than 90% of its initial specific capacitance after 1000 times cyclic deformation at a strain of 100%. (Figure 3c). To further examine the flexibility and stretchability of the MR-PEDOT/PAAm/PPy DN hydrogel electrode, different deformation mode was repeatedly exerted, such as twisting, crumpling, tying a knot, as shown in Figure 3e. The hydrogel electrode does not break and can recover to its initial state after 100 times such deformation, delivering an excellent flexibility of the hydrogel electrode. Moreover, attributed to the high conductivity of hydrogel electrode, one commercial lightemitting diode (LED) could be lightened with the stretchable electrode (Figure 3d, movie S1). To examine the practical application of the MR-PEDOT/PAAm/PPy DN hydrogel electrode for stretchable and flexible supercapacitors, a symmetric intrinsically stretchable supercapacitor device was assembled by sandwich structure, with two MR-PEDOT/PAAm/PPy DN hydrogels as electrodes and PAAm/KCl hydrogel as the electrolyte. The CV diagrams (Figure 4a) at different scan rates presented a nearly rectangular curve, indicating a rapid current response to voltage reversal. GCD curves (Figure S10) shows a typical charge-discharge pattern for supercapacitors. After the calculation based on the GCD curves, the specific capacitance was calculated to be 120 F g-1 at a current density of 0.5 A g-1.For wearable applications, a supercapacitor is not only required to be highly stretchable but also needs to be highly flexible. To demonstrate the flexibility of the supercapacitor, the capacitance of the supercapacitor was tested under the bending state. The device maintained its original electrochemical properties while the supercapacitor was bent from 0° to 180° (Figure 4b). The


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hydrogel supercapacitor developed in this study can fulfill the harsh requirements endure a large curvature bending. Figure S11 shows the electrochemical perfomance of supercapacitor as flexible electroics, which was streached 1000 times at 100% strain. Meanwhile the capacitance of supercapacitor decreased with the increasing of stretch strain (Figure S12). The hydrogel supercapacitor was able to light a LED light were formed to a series circuit, and the light could be lightened with the stretch supercapacitor as a conductor. (Figure 4e, movie S2) To investigate the long-term stability of the intrinsically stretchable supercapacitor, the stretchable supercapacitor experienced 1000 times charge-discharge cycles. After those cycles, the specific capacitance maintained about 85% of its initial value (Figure 4c), larger than that of the PPy composite supercapacitor30. The decrease may be ascribed to the poor interfacial connection between electrolyte and electrode, detachment in the form of gaps during the longterm charge-discharge cycles, leading to the decrease of the ionic conductivity. The resistance slightly increased from 18Ω to 32Ω after the 1000 charge-discharge cycles (Figure 4d) due to the degradation of PPy polymer. Therefore, the super-stretchability, high strength and high conductivity of the MRPEDOT/PAAm/PPy DN hydrogel electrode is a good material to fabricate an intrinsically superstretchable and flexible all-gel supercapacitor. In summary, we have designed a new kind of double network hydrogel, whose mechanical performance and electrochemical performance are enhanced by a crosslinked conductive microgel simultaneously. These microgel reinforced double network hydrogel possesses intrinsical stretchability, flexibility, high strength, good toughness, high conductivity. After assembled to intrinsically stretchable and flexible supercapacitor, the supercapacitor also exhibited excellent electroactivity, conductivity, and electrochemical performances. Embedded


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microgel played multiple roles in mechanical and electrochemical performance. Firstly, the high mechanical strength and toughness of MR-PEDOT/PAAm/PPy DN hydrogels were ascribed to the sacrificial bonds of the crosslinked microgel network which will fracture during the process of the deformation. Secondly, microgel plays a role to stabilize PPy, leading to the high stability of capacitance during the charge-discharge cycle and the process of the deformation. Lastly, the stacked all-gel-state supercapacitor possessed stretchability, high capacitance, good cycle performance and stability. It is expected that the tough hydrogel electrode with good electrochemical performance can expand the application of hydrogel as stretchable electronics. The strategy to prepare functional double network hydrogel put forward here should be applicable to other systems to extend the application scope of the hydrogel with multifunction.


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Scheme 1. The procedure of preparation of conductive MR-PEDOT/PAAm/PPy SN hydrogel and MR-PEDOT/PAAm/PPy DN hydrogel synthesized from AAm monomer, PEDOT and PPy as electrochemically active materials. (a) The fabrication process of conductive microgel. (b) The fabrication process of conductive MR-PEDOT/PAAm/PPy SN hydrogel. (c) The fabrication process of conductive MR-PEDOT/PAAm/PPy DN hydrogel.

Figure 1. Tensile stress−strain curves (a) and the work of extension (b) of different hydrogels. The first hydrogel network synthesized using 1M PAAm and 0.07 g/mL microgel, while the AAm monomer in the second hydrogel network is 4 M.


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Figure 2. (a) Cyclic voltammograms at a scan rate of 100 mV s−1, (b) Nyquist plots, (c) conductivity spectra and (d) Specific capacitances at various current densities of 0.5, 1, 3, 5, 7.5 and 10 A g-1 for four different hydrogels in 1M KCl. The inset of Fig. 2b shows the corresponding expanded plots.


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Figure 3. (a) The capacitance retention for PEDOT/PAAm/PPy DN hydrogel, MRPEDOT/PAAm/PPy DN hydrogel. (b) EIS curves for the electrode before and after cycling, (c) Specific capacitance of MR-PEDOT/PAAm/PPy DN hydrogel, which is stretched 1000 times at 100% strain. (d) Photo of LED light connected by a hydrogel electrode as a conductor under the stretch state. (e) MR-PEDOT/PAAm/PPy DN hydrogels are twisted, crumpled and tied a knot.


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Figure 4. Electrochemical performances of a textile hydrogel supercapacitor. (a) CV curves at the scan rates of 5, 10, 20, 50 and 100 mV s-1. (b) Cyclic voltammograms at a scan rate of 100 mV s−1 for a hydrogel supercapacitor under bending from 0° to 180°. (c) Capacitance retention of supercapacitor calculated from CV curves at a scan rate of 100 mV s-1. (d) Analysis of EIS for


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the electrode before and after 1000 times cycles. (e) The photo of LED lighted by hydrogel supercapacitors. ASSOCIATED CONTENT Supporting Information. A listing of the contents of each file supplied as Supporting Information should be included. The following files are available free of charge. Experimental details，water contents, and mechanical properties of the hydrogels, SEM images of hydrogels, electrochemical performance of the hydrogel electrode and hydrogel supercapacitor (PDF). Video of the LED light connected by a hydrogel electrode as a conductor under the stretch state (Movie S1 avi). Video of the LED light lighted by a hydrogel supercapacitor (Movie S2 avi). AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions †Yu Zhao and Shuo Chen contributed equally. Funding Sources This

work

was

supported

by

National

Natural

Science

Foundation

of

China

(51673117,51574166), the Science and Technology Innovation Commission of Shenzhen (JCYJ20150529164656097,

JSGG20160226201833790,

JCYJ20160520163535684,


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JCYJ20160422144936457,

JCYJ20170818093832350,

JCYJ20170818112409808,

JSGG20170824112840518) Notes The authors declare no competing financial interest.

ABBREVIATIONS PEDOT, Poly (3, 4-ethylenedioxythiophene): polystyrene sulfonate; PAAm, Polyacrylamide; PPy, Poly pyrrole; DN, double network; MR-DN, microgel-reinforced double network; LED, light-emitting diode. REFERENCES [1]

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Microgel enhanced double-network hydrogel electrode with high

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