Flexible Hydrogel Electrolyte with Superior Mechanical Properties

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Energy, Environmental, and Catalysis Applications

A Flexible Hydrogel Electrolyte with Superior Mechanical Properties Based on Poly (vinyl alcohol) and Bacterial Cellulose for the Solid-state Zinc-air Batteries Nana Zhao, Feng Wu, Yi Xing, Wenjie Qu, Nan Chen, Yanxin Shang, Mingxia Yan, Yuejiao Li, Li Li, and Renjie Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00758 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 23, 2019

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ACS Applied Materials & Interfaces

A Flexible Hydrogel Electrolyte with Superior Mechanical Properties Based on Poly (vinyl alcohol) and Bacterial Cellulose for the Solid-state Zinc-air Batteries Nana Zhao,a Feng Wu,a,b Yi Xing,a Wenjie Qu,a Nan Chen,a Yanxin Shang,a Mingxia Yan,a Yuejiao Li ,a,b* Li Lia,b and Renjie Chena,b*

a. School of Material Science & Engineering, Beijing Institute of Technology, Beijing 100081, China. b. Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China *E-mail: [email protected] (L.Y.J.). *E-mail:[email protected] (C.R.J.). KEYWORDS: flexible, bacterial cellulose, dual network, superior mechanical strength, electrolytes

ABSTRACT ： Flexible solid-state zinc-air batteries are promising energy technologies for lowcost, superior performance and safety. However, flexible electrolytes are severely limited by their poor mechanical properties. Here, we introduce flexible hydrogel electrolytes (BPCE) based on bacterial cellulose microfibers (BC) and poly (vinyl alcohol) (PVA) by an in-situ synthesis.

1 Environment ACS Paragon Plus

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Originating from the hydrogen bonds among BC microfibers and PVA matrix, these composites form load bearing percolating dual network, mechanical strength of BPCE have increased 9 times (from 0.102 MPa of pristine PVA to 0.951 MPa of 6-BPCE). 6-BPCE show extremely high ionic conductivities (80.8 mS cm-1). In addition, the solid-state zinc-air batteries can stably cycle over 440 h without large discharge and charge polarizations equipped with zinc anode and Co3O4@Ni cathode. Moreover, flexible solid-state zinc-air batteries can cycle well at any bending angles. As flexible electrolytes, they open up a new opportunity for the development of superior performance, flexible, rechargeable zinc-air batteries.

Introduction The flexible consumer electronics, which can be portable, bendable and wearable, have brought great convenience to daily life, and become more significance for the modern society.1-6 To fulfill these advanced flexible electronics, it is essential to produce the flexible power source that have high energy density and excellent safety. Among all the energy storage systems, the rechargeable zinc-air batteries are viewde as the most promising candidates, since they provide a high theoretical specific energy (1086 Whkg-1)7,8 and an excellent rate capability. The zinc-air batteries use the oxygen from atmosphere as cathode materials, the zinc metal as anode materials, and the alkaline solutions as the electrolytes, the mechanism of which is based on the electrochemical reaction: (2Zn + O2⇌ZnO). Therefore, this system can possess several advantages, for example, environmental friendliness, low cost and ease of fabrication.8-14 The alkaline solutions are widely used in zinc–air batteries, owing to their benefits such as high ionic conductivity and low viscosity.8,15-23 However, the alkaline electrolytes are still at infant stage due to their leakage problem in flexible batteries.19,24 The flexible hydrogel electrolytes can solve this intrinsic problem, which not only can retain the physicochemical properties of aqueous electrolytes and act as separators, and thus prevent battery internal short-circuiting, but also can meet the requirements of the flexible batteries.25-27 Up to now, Poly(vinyl alcohol) (PVA) has been widely utilized as the host polymer in alkaline gel electrolytes (AGE)


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in Zn-air batteries, owing to its good flexibility, and hydrophilicity as the electrolytes reservoirs.28-37 However, its merits are severely impeded by the poor mechanical properties, due to the flexible applications needing to be stretched and folded repeatedly. To address these challenges, one approach is to introduce celluloses into the PVA matrix to enhance strength. The celluloses are not only biodegradable natural materials, but also have high thermal/chemical stability. So far, numerous kinds of celluloses have been utilized to improve the strength in the flexible hydrogel electrolytes. High crystallinity, high purity and high hygroscopicity of bacterial celluloses (BC) make them outstanding reinforcement materials in polymer composites38-42. However, the hydrogel electrolytes base BC are rarely reported for now in zinc-air batteries. Herein, flexible BC/PVA composite hydrogel electrolytes (BPCE) with a superior mechanical strength was successfully designed and synthesized by an in-situ method, the optimal tensile strength of which (0.951 Mpa ) is 9 times more than the pristine PVA electrolyte (0.102 MPa). The superior mechanical strength is attributed to the dual network structure of PVA matrix and BC microfibers, achieved by hydrogen bonding between the BC microfibers and PVA polymer. Meanwhile, the homodispersed BC microfibers can make BPCE’ pores sizes smaller than pure PVA. Therefore, the BPCE with microporous dual network structure can trap much water, and exhibit an ultra-high ionic conductivity of 80.8 mS cm-1 at room temperature. Particularly, zinc–air battery with the BC/PVA hydrogel electrolytes and a Co3O4@Ni cathode achieves a superior long cycling performance, which can stably cycle over 440 h without large discharge and charge polarizations. In addition, the flexible solid-state zinc-air battery can cycle well at any bending angles, demonstrating the enormous potential in practical flexible wearable applications.

Experimental Preparation of BPCE membrane The synthetic process of BPCE is shown in Figure 1, 1g PVA (M.W.145000-180000, Across) and BC powders (Beijing Guan Lan, China) (0.02/0.04/0.06g) were added into 10 mL deionized water followed by intensive stirring at 100°C until the PVA and BC were dissolved. Then, 1 mL aqueous alkaline solutions of KOH (18M, Aldrich) and Zn(CH 3COO)2 (0.6M, Alfa) was added droplet into the above



mixture to afford homogeneous solutions. The as-prepared solutions were transferred into the watch glass and kept in a freeze drier at -45 °C for 12 h. Afterward, the temperature was increased to room temperature for 6 h. This freezing-thawing process was repeated three times to further enhance the crosslinking degree. The PVA materials modified with 2wt%, 4wt%, 6wt% concentration of BC are denoted as 2-BPCE, 4-BPCE, 6-BPCE, respectively.The PVA membrane without adding bacterial cellulose (PCE) was also fabricated for the comparison by the same method.

Mechanical measurements The tensile properties were tested by a commercial test machine (Tensile Tester: AGS-J 5KN/1KN Shimadzu) with a stretching speed of 10 mm min-1 at room temperature. According to GB/T 528-2009 method, the sample was prepared into a dumbbell shape (4 mm × 20 mm).

Fabrication of solid-state rechargeable zinc–air battery Co3O4 nanosheets was used as the cathode. Through a simple hydrothermal reaction and calcination treatment, Co3O4 nanosheets directly grown on nickel foam (Co3O4@Ni) were obtained. A polished zinc foil (the thickness is about 0.2mm) was utilized as the anode, a BPCE membrane was filled between the cathode and the anode. All the tested membranes were pre-wetted by 6.0 M KOH and 0.2 M Zn(CH3COO)2 mixed solutions before the battery assembly. It should be noted that the solid-state rechargeable zinc–air batteries were assembled in an open-air environment without special atmosphere protection.

Characterization and electrochemical measurements Scanning electron microscopy (SEM, FEI Quanta 250, USA) was used for material morphology analysis. X-ray diffraction (XRD, Rigaku Ultima IV-185, Japan) was carried out using a Cu Kα radiation over a 2θ range of 10–80° at a scan rate of 5° min−1. Ionic conductivity of the composite membrane was examined by electrochemical impedance spectroscopy (EIS) on a blocking stainless steel cell from 100 kHz to10 mHz with an amplitude of 5 mV. The battery electrochemical evaluation was tested under


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atmospheric air. The cycling stability of rechargeable zinc–air batteries is tested at current density of 0.5 mAcm-2 with each cycle consisting of 20 min for discharge followed by 20 min charge.

Results and discussion The related synthetic process of resultant BPCE membrane is described in Figure 1. In brief, precursor solutions were obtained by dissolving the BC and PVA in deionized H 2O. Due to the large surface area of microfibers, much hydrogen bonding can be expected. And, the hydroxyl groups on the BC microfibers surface interact favorably with PVA, thus these microfibers were almost homogeneous and steadily dispersed in the matrix. Therefore, these BPCE membrane combine the advantages of both and shows good mechanical properties, which are an important index for flexible electrolyte, especially in practical energy storage systems. A series of tensile tests were carried out to examine the mechanical properties of hydrogels membranes. Figure 2a-b show mechanical properties of the BPCE with different BC contents and PCE. As the amount of BC increases from 0% to 6wt%, the tensile strength quickly increases from 0.102 MPa to 0.951 MPa, the tensile strain increases from 209% to 443%, indicating that the hydrogels’ mechanical performances improve. When the BC content increases, the dissolution of the polymer composite in deionized water becomes very difficult, thus we used 6 wt% as the optimum content. The mechanical strength of these hydrogels membranes is enhanced with the BC content due to H-bonding interaction between PVA matrix and BC microfibers, which can form the percolating dual network. When stress is applied, this percolating network can maximize the stress transfer through the hydrogen bonding and help to increase the strength of composite membranes.43 Meanwhile, both bulk PVA and BC microfibers are hydrophilic in nature, they interact well at the molecular level, leading to good interface compatibility, thus, the strength and toughness of the composite membranes can be improved synergistically with appropriate amount of bacterial cellulose microfibers. Moreover, we compared the conductivities of electrolyte membranes from PCE to 6-BPCE and there were no obvious changes (Figure 2c), Therefore, BC microfibers in BPCE membrane doesn't affect the conductivity.



The frameworks of the BPCE membrane dehydrated were observed by the scanning electron microscopy (SEM). The PCE framework is constructed by the PVA molecular chains with large pores (Figure 3a), they were just long polymer chains. With the BC concentration increasing, BC microfibers began to appear in the PVA matrix (Figure 3b-d). BC microfibers tend to build smaller pore structures than pure PVA. When the amount of BC is 6 wt%, polymer long chains and microfibers intertw ined form the strong framework that means dual network, as illustrated in Figure 3d. And the pores of the dual network structure are more compact than the PCE membrane presented in Figure 3b-d. The formation of dual network structure is ascribed to the abundant hydroxyl groups on the surface of BC microfibers, which can interact with the PVA, allowing the BC microfibres to be stably and evenly dispersed in the PVA matrix. BPCE membrane can be folded and bent in any angles, it can be restored to its original size (Figure 3e) and made into any shape (Figure 3f), which reflects its superior flexibility. The active cathode material is an important component in rechargeable zinc–air batteries. In this work, the synthesized Co3O4@Ni has characteristic peak of Co 3O4 (Figure 4a). Figure 4b-c show the nanosheets are ordered, interconnected and form an open microporous structure, and thus render sufficient catalytic sites for ORR/OER in rechargeable zinc-air batteries. The EDX elemental mapping of the Co3O4 is shown in Figure 4c1-c2, which shows a uniform distribution of Co and O elements on the surface of Ni. The long-term cycling stability is very important for rechargeable zinc-air batteries. During discharge, external oxygen gas reacts at the cathode and generates hydroxyl ions. Then the hydroxyl ions migrate into the anode and yield zincate ions (Zn(OH) 42-). The electrons released by the anode travel through the outer circuit to the cathode. The zincate ions undergo spontaneous decomposition to form zinc oxide, and water in turn enter the electrolyte. During charge, the zinc oxide reacts with water at the anode, and the water can be recycled back on the cathode with oxygen releasing 14. Figure 5 reveals the cycling stability of solid-state rechargeable zinc-air battery based on 6-BPCE membrane, the measurement was carried out at a current density of 0.5 mA cm -2 with each cycle per 40 min under atmospheric air. As we can see, the initial discharge and charge voltages are about 1.24 and 1.82 V. After 650 cycles over 440


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h, the discharge and the charge voltages slightly change to 1.18 and 1.90 V. The zinc-air battery using the 6-BPCE showed an excellent rechargeability. The battery using the aqueous alkaline electrolyte (AE) was also fabricated for comparison. Performance loss of the battery of aqueous alkaline caused by large polarizations was observed in Figure S1a (battery deteriorated obviously after 130 h).The excellent cycling performance of 6-BPCE is mainly ascribed to the following factors: (1) the high water-holding capacity, during the entire battery cycling, there was no water loss. However, due to the open system, water evaporation is inevitable, this phenomenon is more pronounced in aqueous alkaline electrolytes. This dual network microporous structure can effectively capture large amounts of water and prevent water from evaporating during long-term cycling; (2) during the overall battery operation process, oxygen bubbles and zinc dendrites bring internal pressure to the membrane. This 6-BPCE solid electrolyte has superior mechanical strength tolerate the periodic stress; (3) protection of zinc anode, we disassembled the recycled batteries and observed the surface morphology of the zinc anode and found that the 6-BPCE membrane has a protective effect on the zinc anode. The zinc anode of the aqueous alkaline electrolyte battery has already been corroded showing large vacancies, while 6 -BPCE’ zinc anode was basically intact (Figure.S1b-c); (4) zinc-air batteries have enough catalyst active sites for ORR and OER, and thus presented a superior electrochemical performance due to these hierarchical mesoporous/macroporous Co 3O4 ultrathin nanosheets. On the basis of the promising batteries performance demonstrated above, we produced a flexible device with the 6-BPCE membrane (Figure 6a). It is worth noting that at any given bending angles, the open circuit voltage remains virtually unchanged (Figure 6b). Figure 6c shows that powering for electronic displays is a good demonstration of the application of flexible zinc-air batteries, that is, three flexible single cells (size 4.2 cm*1.6 cm) were connected in series to an external circuit, which can power a blue LED. Moreover, the flexible Zn-air battery can be cycling well at stable potentials, even when bended to different angles, particularly, no significant discharge and charge polarizations was detected even at angle up to 180° (Figure 6d). These results reveal that the as-obtained BPCE membranes are attractive for rechargeable and flexible Zn-air batteries, combining basic research and practical electronic devices with smart, flexible energy storage and conversion devices.

Conclusions 7 Environment ACS Paragon Plus


In summary, we developed a highly flexible and conductive BC/PVA membrane with a microporous structure by an in-situ method as a solid electrolyte for flexible, rechargeable zinc–air batteries. Bacterial cellulose microfibers are added to enhance mechanical properties into PVA membrane. Thank to the unique load bearing percolating dual network, these BC/PVA composite membranes exhibit superior mechanical strength and toughness. By adding 6wt% BC, the tensile strength of the entire BC/PVA membrane is increased by 9 times and the elongation at break is doubled. In addition, the superior conductivity of 80.8 mS cm−1 at room temperature was obtained. The batteries using the 6-BPCE membranes exhibited more excellent rechargeability and stability compared to aqueous alkaline electrolytes, besides, the 6-BPCE membrane also can protect zinc anode. The flexible 6-BPCE zinc-air battery delivered superior performance even bending at different angles. The new 6BPCE membrane boost the advent of next-generation flexible electrochemical energy conversion.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China “New Energy Project for Electric Vehicle” (2016YFB0100204), the National Natural Science Foundation of China (21373028), the Joint Funds of the National Natural Science Foundation of China (U1564206), Major achievements Transformation Project for Central University in Beijing, and Beijing Key Research and Development Plan (Z181100004518001).

Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website at DOI:

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FIGURES PVA

BC

KOH + Zn(Ac) 2

Stirred at 100 °C

Stirred at 100 °C

Freezing and thawing

: PVA chain

: BC microfiber

Figure 1. Schematic of the synthesis route to the BPCE. The dual network BPCE was synthesized by BC and PVA in situ.



Figure 2. (a) Photographs of solid state electrolyte membrane’ elongation. (b)Stress-strain curve of composite membranes. (c) Ionic conductivity of the BPCE membranes.

Figure 3. (a) SEM image of the PCE membrane. (b) SEM image of 2- BPCE membrane. (c) SEM image of the 4- BPCE membrane. (d) SEM image of the 6- BPCE membrane. (e) A photograph of 6- BPCE membrane having excellent flexibility. (f) A photograph of shape-conformable 6- BPCE membrane electrolyte.


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(a)

(b)

200 μm

(c)

(c1)

Co (c2)

50µm

O

Figure 4. (a) XRD patterns of the Co3O4 grown on Ni foam. (b)(c)SEM images of the cathode sample at different magnifications viewed from the top. (c1−c2) EDX elemental mappings of Co,O.

Figure 5. cycling performance of the 6-BPCE at a current density of 0.5 mAcm-2 with a 40 min per cycle period.



Figure 6. (a) Schematic diagram of a flexible zinc–air battery device. (b) Open circuit voltage of flexible device at different bending angles. (c) A demonstration of the flexible device to power a LED . (d) Galvanostatic charge and discharge cycling of the flexible zinc–air battery using at a current density of 4 mAcm-2 with a 40 min per cycle period.


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For Table of Contents Only

Air electrode

Zinc electrode

Electrolyte membrane


Flexible Hydrogel Electrolyte with Superior Mechanical Properties

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