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Rationally Designed Self-Healing Hydrogel Electrolyte towards a Smart and Sustainable Supercapacitor Jingchen Wang, Fatang Liu, Feng Tao, and Qinmin Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07836 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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ACS Applied Materials & Interfaces

Rationally Designed Self-Healing Hydrogel Electrolyte towards a Smart and Sustainable Supercapacitor Jingchen Wang, Fatang Liu, Feng Tao, Qinmin Pan* (State Key Laboratory of Robotics and Systems, School of Chemistry and Chemical Engineering, Harbin Institute of Technology)

Corresponding author: Qinmin Pan E-mail: [email protected]

Keywords: smart supercapacitor, multifunctional hydrogel electrolyte, cryo-healable, renewable, capacitive performances

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ACS Paragon Plus Environment


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Abstract Excellent self-healability and renewability are crucial for the development of wearable/flexible energy-storage devices aiming for advanced personalized electronics. However, realizing low-temperature self-healing and harmless regeneration remains a big challenge for existing wearable/flexible energy-storage devices, which is fundamentally limited by conventional polymeric electrolytes that are intrinsically neither cryo-healable nor renewable. Here we rationally design a multifunctional polymer electrolyte based on the copolymer of vinylimidazole and hydroxypropyl acrylate, which exhibits all features solving the above-mentioned limitations. A supercapacitor comprising the electrolyte autonomously restores its electrochemical behaviors at temperatures ranging from 25 to -15 oC after multiple mechanical breakings. Interestingly, it is even able to regenerate for 5 cycles through a simple wetting process in the case of malfunction, while maintaining its capacitive properties and excellent self-healability. Our investigation provides a novel insight into designing smart and sustainable energy-storage devices that might be applied to intelligent apparel, electronic skin or flexible robot, and so on.

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Introduction The popularization of personalized electronics, such as Apple watch, Cutecircuit galaxy dress and Philips fluid smartphone, requires their energy-storage devices possessing excellent reliability and safety, since various external forces or irreversible physical damage are unavoidable during practical application.1−3 Recently, wearable/flexible energy-storage devices with self-healability, including self-healable supercapacitors6−9 or lithium ion batteries,10−15 are considered as promising candidates to meet the above requirements because they spontaneously recover electrochemical performances after physical damage like cutoff.4-15 Among the reported self-healable energy-storage devices, however, very few of them could self-heal at low temperature, which severely limits their practical application in a cold climate. Furthermore, wide application of wearable/flexible electronics inevitably produces a large quantity of waste energy-storage devices containing harmful components (e.g., heavy metals and toxic organic solvents).16,17 Today harmless recycling of spent energy-storage devices is becoming an important environmental issue but also a big technical challenge.18−20 From the standpoint of economics and environmental protection, developing renewable energy-storage devices is a green and sustainable strategy to the recycling challenge. Unfortunately, most of wearable/flexible energy-storage devices did not exhibit renewable properties. All these limitations are caused by the fact that the polymeric electrolytes used for existing wearable/flexible energy-storage devices are intrinsically neither self-healable at low temperature nor renewable, resulting in the lack of cryo-healability and regenerating capability. Therefore, it is imperative but challenging to develop a multifunctional polymeric electrolyte that intrinsically possesses renewability together with excellent self-healability at both room and subzero temperatures. Such an electrolyte should combine effective ionic conduction, high mobility of polymer chains, and reversible physical interactions between polymer chains at low temperatures. Importantly, it can retrieve the above characteristics through a facile process in the case of malfunction. All the requirements motivate us to rationally design a novel polymeric structure to realize the multifunction. The finding of such investigations will provide a new insight into 3



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the development of smart and sustainable energy-storage technologies. Here we report a multifunctional hydrogel electrolyte based on the copolymer of vinylimidazole and hydroxypropyl acrylate, which possesses all features solving the above-mentioned limitations associated with conventional polymeric electrolytes. A supercapacitor comprising the hydrogel electrolyte autonomously recovers its electrochemical behaviors at both room and subzero temperatures after multiple mechanical breakings. In the case of malfunction, it is even able to regenerate via a facile process without losing its capacitive performances and excellent self-healability. Our investigation offers a novel strategy to design multifunctional energy-storage devices that can be applied for intelligent clothes, flexible robot or wearable electronics, and so on.

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Experimental Chemicals and Materials N-vinylimidazole (VI), 2-hydroxypropyl acrylate (HPA), N, N′-methylenebisacrylamide (MBAA), ammonium persulfate (APS), and sodium nitrite (NaNO3) were purchased from Aladdin Industrial Corporation. Activated carbon (AC) powder with BET surface area of 2020.7 m2 g−1 and acetylene black (AB) were supplied by Shenzhen BTR New Energy Materials Co., Ltd (China). Synthesis of self-healable P(VI-co-HPA)/NaNO3 hydrogel electrolyte In a typical run, 0.25 mL N-vinylimidazole (VI), 0.17 mL 2-hydroxypropyl acrylate (HPA), 2 mg N, N′-methylenebisacrylamide (MBAA), 4 mg persulfate (APS) and 1.5 mL aqueous NaNO3 solution (1-3 mol L−1) were mixed under stirring in argon. The resulting mixture was then sealed in a glass vial and maintained at 60 oC for 4 h to obtain brown P(VI-co-HPA)/NaNO3 hydrogel electrolyte.21 The ionic conductivity of the electrolyte was measured through the process reported in our previous studies.14,22 The self-healability of the electrolyte was investigated by cut/healing operations. First the electrolyte was cut into halves with a blade, and the resulting pieces were then brought into contact for different duration at temperatures ranging from -15 to 25 oC. The healed electrolyte was assessed by ionic conductivity measurements, tensile experiments and optical microscopic observation. Tensile experiments A piece of the original or the healed P(VI-co-HPA)/NaNO3 hydrogel electrolyte was cut into a strip with a size of 1.0 mm × 2.0 mm × 20 mm. Two ends of the electrolyte were fixed to the clips of a microelectromechanical balance (DCAT-21, Dataphysics). Then the electrolyte was stretched at a rate of 1.0 mm s−1 and its stress-strain curves were recorded. Mechanical healing efficiency (η, %) was determined by the formula η = Th/T0 × 100, where T0 and Th are the tensile strength of the original and healed electrolytes, respectively. Assembly and electrochemical measurements of a supercapacitor prototype 5



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At first, activated carbon powder, polytetrafluoroethylene (PTFE) and acetylene black with a mass ratio of 8:1:1 was mixed to form black slurry. The resulting slurry was rolled into AC electrodes with a thickness of ~150 µm. Then two AC electrodes were coated onto the opposite of a P(VI-co-HPA)/NaNO3 hydrogel electrolyte to assemble a capacitor prototype (1.0 cm × 1.0 cm × 0.2 cm in size). The assembled capacitor was sandwiched between two nickel foams and its electrochemical behavior was evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (GCD) measurements. The specific capacitance (Cs) of the capacitor was calculated by Cs = (I × t)/(∆V × m), where I, t, ∆V, and m are current density, discharge time, voltage and the mass of activated carbon, respectively. Here the specific capacitance was the specific capacitance of a single electrode. Self-healing of the capacitor The capacitor prototype was first cut into halves with a blade, and the resulting pieces were then brought into contact for self-healing at temperatures ranging from 25 to -15 oC. The self-healability of the capacitor was evaluated by electrochemical measurements and optical microscopic observation. As the current collector of capacitor, the Ni foams were cut into halves with the capacitor together (Figure S1). Cold-resistant test of the capacitor At first, the capacitor was stored in a thermostatic chamber at subzero temperatures (0 ~ -15 oC). Then its capacitive properties were measured by CV, GCD and EIS at the temperatures. Regeneration of the capacitor First the capacitor prototype was dried at 70 oC for 5 h to remove the water it trapped. Then the dried capacitor was wetted by the droplets of de-ionized water. After wiping with a filter paper, the resulting capacitor was tested by CV, EIS and GCD measurements to evaluate its electrochemical performances. Characterizations Galvanostatic charge-discharge (GCD) test was conducted on a battery tester (Neware, China) between 0-1.0 V 6


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at a given current density. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on a CHI660E potentiostat. Fourier transform infrared (FT-IR) spectroscopy was conducted on a Shimadzu IRaffinity-1s spectrometer. Optical microscopic observation was performed on a PH50-1B43L-A/PL (Phenix, China).

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Results and discussion (a)

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Figure 1. (a) Synthesis of the P(VI-co-HPA)/NaNO3 hydrogel electrolyte. (b) Electrochemical window of the hydrogel electrolyte. (c) Ionic conductivity of the hydrogel electrolyte at temperatures ranging from 45 to -25 oC. (d) DSC diagram of the hydrogel electrolyte containing 3.0 mol L−1 NaNO3. (e) Ionic conductivity of the electrolyte after multiple drying/wetting processes. The multifunctional hydrogel electrolyte was synthesized through a radical copolymerization process in the presence of vinylimidazole (VI), hydroxypropyl acrylate (HPA), cross-linking agent methylenebisacrylamide 8


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(MBAA), and aqueous NaNO3 solution (Figure 1a).21 After reaction, the NaNO3 solution was trapped in the polymeric network constituted by covalently cross-linked P(VI-co-HPA) chains, forming a brown and transparent P(VI-co-HPA)/NaNO3 hydrogel electrolyte (Figure S2-3, Supporting Information). In this study, it is essential to use MBAA for the synthesis of our hydrogel electrolyte. When no MBAA was used in the synthetic process, a viscous liquid instead of hydrogel was obtained (Figure S4, Supporting Information). By elaborately controlling the mass ratio of P(VI-co-HPA) to NaNO3 (Figure S5-6, Supporting Information), the electrolyte exhibited an electrochemical window of ~2.0 V and maximum ionic conductivity of 59.6 mS cm−1 at 45 oC (Figure 1b-c). Its ionic conductivity was still above 16 mS cm−1 even at -15 oC (Figure 1d), exhibiting effective ionic conduction at subzero temperature. The cold-resistance is consistent with its differential scanning calorimetry (DSC) diagram that only shows exothermal peaks below -20 oC (Figure 1d). Nevertheless, the electrolyte almost lost its ionic conduction at -25 oC because of freeze. Interestingly, the hydrogel electrolyte restored its ionic conduction by simply wetting with a few droplets of water even after complete drying at 70 oC, and the restoration was repeatable for at least 5 cycles (Figure 1e). The recovery suggests that aqueous NaNO3 solution was re-trapped into the covalently cross-linked network structure after wetting due to hydrophilic nature of the P(VI-co-HPA). The P(VI-co-HPA)/NaNO3 electrolyte could spontaneously recover its configuration, mechanical properties and ionic conduction without external stimuli after multiple cuts. Here the electrolyte was first cut into halves and the resulting pieces were then placed in contact at room temperature (25 oC). The cut pieces autonomously coalesced into a single one after contact for a few min, and the healed electrolyte could support its own weight without breaking (Figure 2a). The self-healing process was quantitatively assessed by tensile measurements. Figure 2b records the stress-strain curves of the electrolyte at different healing stages. Compared with its original counterpart, the healed electrolyte restores 82.1% of tensile stress after contact for 10 min. The electrolyte was self-healable for at least 9 cycles without significant deterioration in ionic conduction (Figure 2c). The mechanism

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of the self-healing behavior was believed to originate from intermolecular hydrogen bonding between P(VI-co-HPA) chains (Figure S7, Supporting Information). (a)

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Figure 2. (a) Optical images showing self-healing of the hydrogel electrolyte at room temperature. (b) Stain-stress curves of the P(VI-co-HPA)/NaNO3 hydrogel electrolyte at different healing stages. Inset is the mechanical healing efficiency calculated from the curves. (c) Ionic conductivity of the hydrogel electrolyte after multiple cut/healing cycles at room temperature. Effect of (d) healing time and (e) temperature on the mechanical healing efficiency of the hydrogel electrolyte at subzero temperatures. The P(VI-co-HPA)/NaNO3 hydrogel electrolyte was also self-healable at subzero temperatures. Figure 2d records the stress-strain curves of the electrolyte after self-healing at -15 oC for different durations, which shows 10


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that it needs more time to recover the tensile stress and strain than that healed at 25 oC. The electrolyte only recovers 55% of tensile stress and 40% of strain after self-healing for 1 h. A maximum mechanical healing efficiency of 84.9% is reached after 3 hours of self-healing. Figure 2e displays that temperature has an important impact on the healing rate of the electrolyte. Raising the temperature from -15 to 0 oC leads to a higher mechanical healing efficiency. The cryo-healability of the electrolyte indicates that its P(VI-co-HPA) chains remain high mobility and reversible hydrogen bonding between them at the subzero temperatures. (b)

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Figure 3. (a) Illustration for the assembly of a supercapacitor prototype. (b) Optical image of the capacitor. Electrochemical performances of the capacitor at room and subzero temperatures; CV curves at (c) 25 oC and (e) subzero temperatures, GCD profiles at (d) 25 oC and (f) subzero temperatures. The resulting electrolyte was sandwiched between two activated carbon (AC) electrodes to assemble a capacitor prototype (Figure 3a). The assembled capacitor was flexible and could be readily bended without fracture (Figure 3b). Its electrochemical performances were evaluated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements. At room temperature (25 oC), the capacitor showed rectangular CV curves at 10-100 mV s−1, as well as triangular GCD profiles at the current density of 1.0-10 A g−1, exhibiting a double-layered capacitive behavior (Figure 3c-d). A specific capacitance of 90.0 and 64.0 F g−1 at 1.0 and 10 A g−1 was calculated from the GCD profiles, respectively. The capacitor also exhibited desirable capacitive performances at a higher temperature 45 oC (Figure S8) and lower temperatures ranging from 0 to -15 oC (Figure 3e-f). It displayed triangular GCD profiles with a specific capacitance of 93.2 F g-1 at 45 oC, 70.8 and 29.6 F g−1 at 0 and -15 oC, respectively. The values are about 103.6 %, 78.7 and 32.9% of that measured at 25 oC (i.e., 90.0 F g–1), respectively. The variation of capacitance is due to change of interfacial resistance caused by ionic conduction of the electrolyte at different temperatures (Figure 1c and Figure S9, Supporting Information).

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Figure 4. (a) Optical observation and resistivity measurements of the capacitor in the self-healing process. (b) Optical microscopic images of the healed region, top view (left) and side view (right). Electrochemical performances of the original and healed capacitors, (c) CV curves at 100 mV s−1, (d) GCD profiles at 1.0 A g−1, (e) EIS spectra, (f) cycling characteristics and rate capability. The capacitor exhibited self-healing capability by autonomously recovering its electrochemical performances after mechanical damage. Firstly, the capacitor was cut into halves and then they were brought into contact for self-healing at room temperature (Figure 4a). The halves spontaneously healed into a single one after contact for 5 min. The healed capacitor could support its own weight without breaking. Its AC electrodes still kept good electrical contact after healing, as evidenced by resistivity measurements and optical microscopic observation (Figure 4a-b). Further electrochemical tests showed that the healed capacitor also displayed similar CV curves and GCD profiles as its original counterpart (Figure 4c-d), suggesting the restoration of the capacitive properties. Under ambient conditions, it autonomously recovered the capacitive properties even after 9 cycles of cut/healing, 13



while without significant increase in charge transfer resistance (Rct) and solution resistance (Rs) (Figure 4e, Table S1 of Supporting Information). Consequently the healed capacitor exhibited excellent cycling stability and rate capability (Figure 4f). It still delivered a specific capacitance of 79.6 F g−1 at 1.0 A g−1 after the 20 000th cycle, which is 93.9% of that measured for its original counterpart. (a)

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Figure 5. Electrochemical performances of the capacitor after self-healing at -15 oC. (a) CV curves at 100 mV s−1, (b) GCD profiles at 1.0 A g−1, (c) EIS spectra and (d) cycling characteristics. Inset of Figure 5c is the equivalent circuit for fitting the spectra. (e) Schematic illustration of the self-healing mechanism of the capacitor. The self-healing of the capacitor was further conducted at -15 oC through the cut/healing process. After healing for 3 h, the resultant capacitor was tested by CV and GCD measurements (Figure 5a-b). The healed capacitor displayed similar CV curves and specific capacitance (31.2 F g−1 at 1.0 A g−1) as its original counterpart, exhibiting excellent cryo-healing ability. The cryo-healing behavior took place for at least 9 cycles without dramatic increase in Rct and Rs (Figure 5c, Table S2 of Supporting Information). The healed capacitor also exhibited excellent cycling stability at -15 oC (Figure 5d). To the best of our knowledge, this is the first report on the self-healing of an energy storage device at subzero temperatures. The cryo-healability will greatly improve the reliability of the device in a cold climate. The self-healing mechanism of the capacitor is illustrated in Figure 5e. As the capacitor is cut into halves, both covalent and hydrogen bonds of its P(VI-co-HPA)/NaNO3 electrolyte are broken at the cut interfaces. Once the halves contact again, the broken hydrogen bonds link together through the coordination between imidazole and hydroxyl groups. The recombination of hydrogen bonds enables the severed electrolyte to heal itself, which induces the re-contact of the broken AC electrodes covered on it. (a)

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Figure 6. Electrochemical performances of the regenerated capacitor at 25 oC, (a) CV curves at 100 mV s−1, (b) GCD profiles at 1.0 A g−1, (c) EIS spectra and (d) cycling characteristics. Interestingly, the capacitor was renewable even after complete drying. Here we first dried the capacitor in an oven to remove the water it trapped. The dried capacitor almost lost the capacitive properties because it exhibited neither CV nor GCD signals (Figure 6a). The dried capacitor was wetted by a few droplets of de-ionized water and then its capacitive performances were evaluated. The wetted capacitor displayed rectangular CV curves and triangular GCD profiles with a specific capacitance of 84.0 F g−1 at 1.0 A g−1 (Figure 6a-b), implying that the dried capacitor regenerated via the wetting. Compared with its original counterpart, the regenerated capacitor still showed small Rct and Rs (Figure 6c, Table S3 of Supporting Information). The regeneration is because the electrolyte restored the ionic conduction by re-trapping NaNO3 solution into its covalently cross-linked P(VI-co-HPA) network via wetting (Figure 1e). Note that multiple regenerations gradually decreased the specific capacitance of the resulting capacitor, but did not affect its excellent cycling stability (Figure 6a-d). After the 5th regeneration, the capacitor delivered a specific capacitance of 47.2 F g−1 at the 5000th cycle, which was about 57.6% of that measured for the original equivalent (Figure 6d). The decrease was attributed to increased Rct and Rs during the drying/wetting process (Figure 6c, Table S3 of Supporting Information).

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(c)

140 o

After healing at 25 C o After healing at -15 C

120 -1


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100 80 60 40 20 0 0

1000

2000

3000

4000

5000

Cycle number

Figure 7. Self-healability of the regenerated capacitor at 25 oC and -15 oC investigated by electrochemical measurements, (a) CV curves at 100 mV s−1, (b) GCD profiles at 1.0 A g−1 and (c) cycling characteristics at 1.0 A g−1. (d) Lighting of a LED bulb by the regenerated capacitor via cut/healing operations. The regenerated capacitor did not lost its self-healability at both room and subzero temperatures. After the cut/healing operations on the regenerated capacitor at 25 oC or -15 oC, the resulting capacitor restored its capacitive properties including voltammetric characteristics, specific capacitance and cycling stability (Figure 7a-c). Owing to the self-healability, the regenerated capacitor could light a LED bulb through the cut/healing process (Figure 7d). The restoration of the self-healability is attributed to the fact that moisture-sensitive hydrogen bonding will be re-activated in aqueous conditions.23-25 Up to date, very few energy-storage devices possessed both self-healable and renewable properties. The renewability makes easy storage and transportation of energy 17



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storage-devices, but also allows their recycling environmental friendly. The above results demonstrate that our capacitor is cold resistant, cryo-healable and renewable, which is a promising candidate for smart and sustainable energy-storage devices aiming for advanced wearable/portable electronics.

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Conclusions In summary, we first reported a multifunctional supercapacitor comprising a P(VI-co-HPA)/NaNO3 hydrogle electrolyte. The capacitor restored its capacitive performances without external stimulus after 9 cycles of cut/healing at temperatures ranging from 25 to -15 oC. More interestingly, it was renewable for at least 5 cycles through a simple wetting process even after being completely dried, while did not lose its electrochemical behaviors and self-healability. The excellent self-healability is attributed to the electrolyte keeping effective ionic conduction, high mobility of polymer chains, and reversible intermolecular hydrogen bonding at both room and subzero temperatures. On the other hand, its hydrophilic and cross-linked P(VI-co-HPA) network allows the electrolyte to retrieve the above characteristics via wetting after complete drying. The present investigation provides a strategy to develop smart and sustainable energy-storage devices aiming for wearable/flexible electronics like electronic skin or intelligent clothes.

Associated Content Optical image and FT-IR of P(VI-co-HPA)/NaNO3 hydrogel electrolyte; The effect of concentration of NaNO3 and P(VI-co-HPA) on the ionic conductivity at different temperature; Stress-strain curves of the original and urea treated P(VI-co-HPA)/NaNO3 electrolyte pieces; EIS spectra of the capacitor recorded at subzero temperatures; Optical images showing the electrical contact of the nickel foam after self-healing; Fitting results of the EIS spectra. Author contributions Jingchen Wang and Fatang Liu contributed equally to the work.

Acknowledgement The work was supported by National Natural Science Foundation of China (51473041), a self-planned task of the State Key Laboratory of Robotics and Systems of the Harbin Institute of Technology (SKLRS201604C), and 19



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the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (51521003).

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TOC

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Rationally Designed Self-Healing Hydrogel ... - ACS Publications

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