All-in-one and bipolar-membrane-free acid-alkaline hydrogel electrolytes for flexible high-voltage Zn-air batteries

https://doi.org/10.1016/j.cej.2021.132718Get rights and content

Highlights

  • An All-in-one and membrane-free acid-alkaline hydrogel electrolyte is proposed.

  • A simplified structure with intimate electrode–electrolyte interface is achieved.

  • The fabricated flexible ZAB exhibits an unprecedentedly high voltage of 2 V.

  • The “two flavors in one meat” design is available for other dual-electrolyte batteries.

Abstract

The low operating voltage of 1.4 V limits the widespread application of flexible Zn-air batteries (ZABs) in wearable electronics. However, a high-voltage flexible ZAB has not been achieved yet, which results from the few choices of fitted flexible electrolytes. Now we propose a novel, universal, and simple strategy to design all-in-one and membrane-free acid-alkaline flexible electrolytes based on thermo-reversible Pluronic® F127 hydrogels. Benefiting from the unique sol–gel transition property of Pluronic® F127 hydrogel, the acid and alkaline can be decoupled but integrated simultaneously in one hydrogel. Surprisingly, the as-developed ZAB achieves an unprecedentedly high voltage of 2 V, surpassing all the existing flexible ZABs. Meanwhile, this battery exhibits remarkable high-voltage stability of 37 h and a large area capacity of 1.35 mAh cm−2 without the use of costly bipolar membranes. Our work presents a pioneering example for flexible high-voltage ZAB and may further inspire other designs of flexible high-voltage aqueous batteries and decoupled dual-electrolyte batteries.

Graphical abstract

A novel all-in-one and bipolar-membrane-free hydrogel electrolyte is designed for flexible high-voltage Zn-Air batteries (ZABs). The battery exhibits an unprecedentedly high voltage of 2 V, surpassing all the reported flexible ZABs.

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Introduction

Flexible aqueous Zn-based batteries with hydrogel electrolytes are ideal power sources for wearable electronic devices owing to their intrinsic safety and low cost [1], [2], [3], [4], [5]. Among them, flexible ZABs are especially outstanding and attract tremendous interest for their high theoretical energy density (1084 Wh kg−1) and stable discharge platforms [6], [7]. Regretfully, they deliver a low operating voltage of 1.4 V because of their low cathodic potential in alkaline environments, which lowers the battery energy density and limits their widespread application [8], [9], [10], [11]. Therefore, there is an urgent need to develop flexible high-voltage ZABs; however, which have not been reported yet.

Recently, the voltage of liquid-state ZABs was successfully elevated to over 2 V by decoupling the liquid electrolyte into acid catholyte and alkaline anolyte, separated by a bipolar membrane for preventing neutralization [12], [13]. This way, the Zn anode can retain stability in alkaline, and the air cathode in acid exhibits a higher potential [14], [15]. The following reactions explained for the voltage promotion:

Anode in alkaline anolyte:Zn + 4OH- = Zn(OH)42- + 2e- Eθ = -1.199 V vs SHE

Cathode in alkaline catholyte:O2(g) + 2H2O + 4e- = 4OH- Eθ = 0.401 V vs SHE

Cathode in acid catholyte:O2(g) + 4H+ + 4e- = 2H2O Eθ = 1.229 V vs SHE

For conventional flexible sandwich-structure ZABs (Fig. 1a), the self-standing hydrogel electrolyte is employed to replace the liquid-state alkaline electrolyte to achieve battery flexibility [16], [17], [18]. Inspired by the double-meat sandwich (Fig. 1b), it is theoretically feasible to pile up the individual acid and alalkaline hydrogel electrolytes, which directly applies the acid-alkaline decoupling strategy from liquid-state ZABs to achieve flexible high-voltage solid-state ZABs [19]. However, ZABs with this cumbersome multiple-layer structure may easily delaminate during repeated daily bending, leading to increased resistance at the electrode–electrolyte and electrolyte-electrolyte interfaces, even battery performance degradation [20], [21]. Moreover, the use of bipolar membrane inevitably increases the battery cost and further complicates the battery structure [22]. Hence, there should be a transformative shift on hydrogel electrolyte design for flexible high-voltage batteries but very challenging.

Herein, for the first time, we develop a novel “two flavors in one meat” structure (Fig. 1c), that is, an all-in-one and membrane-free acid-alkaline hydrogel electrolyte (AAHE) based on a thermo-reversible Pluronic® F127 hydrogel. By virtue of the unique sol–gel transition property of Pluronic® F127 hydrogel, the acid and alkaline can be decoupled but integrated simultaneously in one hydrogel and maintain high stability, thus avoiding the use of an expensive bipolar membrane. Furthermore, this all-in-one design dramatically simplifies the multiple-layer structure and contributes to integrated electrolyte-electrolyte and electrode–electrolyte interfaces. Surprisingly, the fabricated flexible ZABs exhibited an unprecedented high voltage of 2 V, surpassing all the existing flexible ZABs. Meanwhile, this battery exhibits remarkable high-voltage stability of 37 h and a large area capacity of 1.35 mAh cm−2.

Section snippets

Materials

Potassium hydroxide (85%, pellets), sulfuric acid (98%, liquid), potassium sulfate (99%, powder), zinc sulfate heptahydrate (99%, powder), Copper(II) sulfate pentahydrate (98%, powder), Sodium stannate trihydrate (95%, powder), and Pluronic® F127 (PEO100-PPO65-PEO100, Critical Micelle Concentration (CMC): 950–1000 ppm, powder) for preparing hydrogel electrolytes were purchased from SIGMA-ALDRICH. The dye Congo Red (Dye content 35%, powder) and Methylene Blue (Dye content 82%, powder) used for

Results and discussion

The reported ZAB is composed of a Zn anode, an Pt/C air cathode, and the key for battery flexibility and high voltage, the newly developed AAHE (Fig. 2a) [23], [24]. Unlike the widely used self-standing hydrogels (e.g., polyvinyl alcohol, abbr. PVA), AAHE is made of thermo-reversible Pluronic® F127 hydrogels which are sol state at subzero temperature but transform to viscous gel state at room temperature (Fig. 2b) [25]. Take advantage of this unique sol–gel transition property, the flexible

Conclusion

In summary, we propose a novel, simple, universal, and effective “two flavors in one meat” strategy to design acid-alkaline hydrogel electrolytes for flexible ZABs, which achieve an unprecedentedly high voltage of 2 V and large area capacity of 1.35 mAh cm−2. The all-in-one and membrane-free structure of AAHE significantly simplify the battery structure, and intimate contacts between AAHE and electrodes further contribute to the battery stable discharge even after multiple times bending.

CRediT authorship contribution statement

Siyuan Zhao: Conceptualization, Methodology, Investigation, Writing - original draft, Supervision. Tong Liu: Investigation, Writing–review & editing, Supervision. Yawen Dai: Investigation, Validation. Yang Wang: Validation. Zengjia Guo: Investigation. Shuo Zhai: Validation. Jie Yu: Validation. Chunyi Zhi: Writing–review & editing, Supervision. Meng Ni: Funding acquisition, Writing–review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work is supported by a grant from Collaborative Research Fund (CRF) (Project no. C5031-20G) of Research Grant Council, University Grants Committee, HK SAR. We also thank the help from Dr. Jinhye Bae, Jiayu Zhao, and Minghao Li from University of California, San Diego.

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