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Flexible/Rechargeable Zn-Air Batteries based on Multifunctional Heteronanomat Architecture Donggue Lee, Hyun-Woo Kim, Ju-Myung Kim, Ka-Hyun Kim, and Sang-Young Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05215 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Materials & Interfaces

Flexible/Rechargeable Zn-Air Batteries based on Multifunctional Heteronanomat Architecture Donggue Lee,† Hyun-Woo Kim,† Ju-Myung Kim,† Ka-Hyun Kim,‡,§,* Sang-Young Lee†,*

†Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea ‡

KIER-UNIST Advanced Center for Energy, Korea Institute for Energy Research, Ulsan

44919, Korea §Division of Energy & Optical Technology Convergence, Cheongju University, Cheongju 28503, Korea

Correspondence and requests for materials should be addressed to K.-H. Kim‡ (email: [email protected]), and S.-Y. Lee† (email: [email protected])

KEYWORDS: Heteronanomat electrode structure; Electrospinning/electrospraying; Mechanical flexibility; Electrochemical rechargeability; Zn-air batteries

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ABSTRACT The increasing demand for advanced rechargeable batteries spurs development of new power sources beyond currently the most widespread lithium-ion batteries. Here, we demonstrate a new class of flexible/rechargeable zinc (Zn)-air batteries based on multifunctional heteronanomat architecture as a scalable/versatile strategy to address this issue. In contrast to conventional electrodes that are mostly prepared by slurry casting techniques, heteronanomat (denoted as “HM”) framework-supported electrodes are fabricated through one-pot concurrent electrospraying (for electrode powders/single-walled carbon nanotubes (SWCNTs)) and electrospinning (for polyetherimide (PEI) nanofibers) process. Zn powders (in anodes) and rambutan-shaped cobalt oxide (Co3O4)/multi-walled carbon nanotube (MWCNT) composite powders (in cathodes) are used as electrode active materials for proof-of-concept. The Zn (or Co3O4/MWCNT) powders are densely packed and spatially bound by the all-fibrous HM frameworks that consist of PEI nanofibers (for structural stability)/SWCNTs (for electrical conduction) networks, leading to formation of threedimensional (3D) bicontinuous ion/electron transport channels in the electrodes. The HM electrodes are assembled with crosslinked polyvinyl alcohol/polyvinyl acrylic acid gel polymer electrolytes (acting as zincate ion crossover-suppressing, permselective separator membranes). Benefiting from its unique structure and chemical functionalities, the HMstructured Zn-air cell significantly improves mechanical flexibility and electrochemical rechargeability, which are difficult to achieve with conventional Zn-air battery technologies.


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1. Introduction The ongoing surge in demand for smart electronics, such as flexible/wearable devices and the Internet of Things (IoTs), has spurred the relentless pursuit of advanced rechargeable power sources with high energy densities and reliable electrochemical performances, which are beyond the current state-of-the-art lithiumion batteries.1-3 Among the numerous energy storage systems explored to date, zinc (Zn)-air batteries have recently garnered considerable attention as a promising candidate to address the aforementioned issues due to their theoretically high energy density (1086 Wh kg–1), nonflammable aqueous electrolytes, ecological benefits and low cost.4-7 Despite these noteworthy advantages, only primary Zn-air batteries have been commercialized, and the development of electrochemically rechargeable Zn-air batteries lags behind practically meaningful levels. In addition to the rechargeability concern, the use of liquid-state electrolytes and metallic foil current collectors, together with fixed form factors based on button- or stack-type cell designs, poses serious obstacles to flexible battery applications. Several approaches have been undertaken to develop flexible Zn-air batteries, which include cable (or fiber)-type cells,8-11 nanostructured air cathodes12,13 and solidstate electrolytes.6,14,15 In the previously reported flexible Zn-air batteries, air cathodes were often fabricated with preformed electrically conductive substrates using complicated synthetic processes.5,10,12 In addition, comprehensive and quantitative analysis of mechanical flexibility of the air cathodes under various deformation modes has been rarely reported. Along with the above-mentioned air cathodes, flexible Zn anodes should be also developed in order to realize reliable flexible Zn-air batteries, however, little attention has been paid so far. From an electrode architecture point of view, conventional Zn 3 ACS Paragon Plus Environment


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anodes are fabricated by coating electrode slurries on metallic current collectors6,12 or directly using Zn metal foils.13,16,17 However, the metallic current collectors and Zn metal foils are not sufficiently mechanically robust to withstand external deformation because of their fatigue failure.18,19 Upon repeated deformation, they often suffer from mechanical rupture, eventually losing their electrical conductivity. In slurry-cast electrodes, insufficient adhesion with the metallic current collectors could give rise to detachment of anode materials (such as Zn powders and conductive additives), resulting in an abrupt increase of cell resistance and even internal short-circuit failure. Here, we present a new class of flexible/rechargeable Zn-air batteries based on multifunctional

heteronanomat

architecture

to

address

the

above-mentioned

challenges. The heteronanomat (denoted as “HM”) framework-supported electrodes are

fabricated

by

one-pot

concurrent

electrospraying

(for

electrode

active

powders/single-walled carbon nanotubes (SWCNTs)) and electrospinning (for polyetherimide

(PEI)

nanofibers).

The

feasibility

of

the

concurrent

electrospraying/electrospinning process was previously explored for the fabrication of lithium-ion battery electrodes.20 In this study, the PEI is chosen due to its exceptional chemical resistance against alkaline electrolyte solution,21 and intimate intermolecular interaction with CNTs.22 Zn powders (for the anodes) and rambutan (a type of tropical fruit)-shaped cobalt oxide (Co3O4)/multi-walled carbon nanotube (MWCNT) composite powders (for the cathodes) are used as electrode active materials for the proof-of-concept. The electrode active powders are densely/uniformly dispersed and tightly bound by the HM frameworks that consist of highly interconnected PEI nanofibers (contributing to structural stability) and SWCNTs (for electrical transport). The HM frameworks in the electrodes show unprecedented multifunctionalities as follows: (i) act as an one4 ACS Paragon Plus Environment

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dimensional (1D) binder to hold the Zn particles (in the Zn anode) and rambutanshaped Co3O4/MWCNT composite powders (in the air cathode); (ii) construct threedimensional (3D)-bicontinuous ion/electron transport pathways; and (iii) secure structural integrity and mechanical flexibility. In addition to these HM electrodes, potassium hydroxide (KOH) electrolyte-swollen crosslinked polyvinyl alcohol (PVA)/polyvinyl acrylic acid (PAA) gel polymer electrolytes (GPEs) are fabricated as a zincate ion (Zn(OH)42–) crossover-suppressing, permselective separator membrane, which plays a significant role in achieving the electrochemical rechargeability. Driven by the structural uniqueness and material functionalities of the HM electrodes and permselective PVA/PAA gel polymer electrolytes, the resultant Zn-air batteries show exceptional improvements in electrochemical performance (stable cycling behavior until 1500 min) and mechanical deformability (even under crumpled state), which lie far beyond those attainable with conventional Zn-air battery technologies.

2. Experimental 2.1. Fabrication of the HM Zn anodes and HM air cathodes The HM Zn anode was fabricated by concurrent electrospraying (for the Zn powders (as active materials) and SWCNTs (as electrical conduction networks)) and electrospinning (for the PEI nanofiber) through two different nozzles. The PEI solution was prepared by dissolving 25 wt% PEI (Ultem 1000) in a 1/1 (w/w) solvent mixture of dimethylacetamide (DMAc) and NMP. The Zn powders (average particle size = 7 µm, Alfa Aesar) were mixed with 0.3 wt% SWCNT (TUBALL) in sodium dodecylbenzenesulfonate (SDBS)-containing water to obtain the Zn powder/SWCNT suspension. The processing conditions were 15 kV with a feed rate of 10 mL h−1 for electrospraying and 11 kV with a feed rate of 0.4 mL h−1 for electrospinning. After 5 ACS Paragon Plus Environment


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rinsing with distilled water and drying in a vacuum oven at 60°C, the self-standing HM Zn anode (thickness = 140 µm) was obtained. Co3O4 powders (average particle size ≅ 50 nm, Sigma-Aldrich) were chosen as air catalyst materials. The Co3O4 powders and MWCNTs (as a conductive additive, Nanocyl) were dispersed in distilled water (Co3O4/MWCNT = 10/0.05 (w/w)) and subjected to ball milling for 2 h, yielding rambutan-shaped Co3O4/MWCNT composite (referred to as “R-CC”) powders. The R-CC powders were mixed with SWCNTs in a water/isopropyl alcohol (IPA) (88/12 (w/w)) solvent, in which 1 wt% SDBS was used as a dispersing additive for the SWCNTs. The R-CC/SWCNT mixture suspension and the PEI solution were subjected to concurrent electrospraying/electrospinning, respectively, to produce the air catalyst layer (thickness = 96 µm). The processing conditions were 13 kV with a feed rate of 15 mL h−1 for the electrospraying and 9 kV with a feed rate of 0.4 mL h−1 for the electrospinning. On top of the obtained air catalyst layer, the SWCNT aqueous suspension (incorporating 1 wt% SDBS) and the PEI solution were concurrently electrosprayed and electrospun to fabricate a GDL layer (thickness = 80 µm), which was seamlessly integrated with the above-prepared air catalyst layer to create the selfstanding HM air cathode. 2.2 Fabrication and electrochemical/mechanical characterizations of the Zn-air cells The HM Zn anode was pre-wetted with a 6 M KOH solution prior to the cell assembly. The prepared HM Zn anode, PVA/PAA GPE film and HM air cathode were stacked in-series and sealed with a paraffin film (thickness = 130 µm). The film side adjacent to the air cathode had holes (diameter = 5 mm) to allow air passage to the air cathode. The galvanostatic discharge/charge cycling tests were conducted, and each 6 ACS Paragon Plus Environment

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cycle period was 10 min (5 min discharge followed by 5 min charge) in the voltage range of 0.6 (discharge cut-off) – 2.4 V (charge cut-off).23 The mechanical flexibility of the HM Zn anode, HM air cathode and Zn-air cells was quantitatively investigated using a universal tensile tester (DA-01, Petrol LAB) under various deformation modes. The change in the electrical resistance of the electrodes was monitored as a function of bending (bending radius = 5 mm and deformation rate = 200 mm min–1) and twisting cycle (angle = 180° and deformation rate = 30° s–1). The Zn-air cells were subjected to bending (bending radius = 5 mm and deformation rate = 200 mm min–1) and twisting (angle = 100° and deformation rate = 30° s–1) deformation, and an in situ analysis of their galvanostatic discharge/charge profiles was conducted.

3. Results and discussion The self-standing HM Zn anode was fabricated by one-pot concurrent electrospraying (for Zn powders/SWCNTs) and electrospinning (for PEI nanofibers) technique, which is schematically illustrated in Figure 1a. From the thermogravimetric analysis (TGA) profiles and associated calculation, the Zn content in the HM Zn anode was estimated to be 92 wt% (Figure S1a and c). The other anode components (PEI and SWCNT) were completely removed above 600 °C (Figure S1b), which indicated that the weight increase above 600 °C in Figure S1a corresponds to the formation of zinc oxide (ZnO).24 The composition ratio of Zn/SWCNT/PEI in the anode was 92/6/2 (w/w/w), in which the PEI/SWCNT ratio was determined by measuring the weight change before/after the selective removal of PEI (using N-methyl-2-pyrrolidinone (NMP) as the solvent). The morphological analysis showed that the Zn powders were densely packed in the HM frameworks (consisting of highly interconnected PEI nanofibers and 7 ACS Paragon Plus Environment


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SWCNTs) over wide areas (Figure 1b) and in the through-thickness direction (Figure 1c), in which the SWCNTs contribute to facilitating electron transport in the HM Zn anode. In addition to the SWCNT-mediated electronic pathways, the interstitial voids were formed between the components of the HM Zn anode, and the voids will act as ion-conduction channels after being filled with alkaline electrolyte. These results exhibited the successful formation of 3D bicontinuous ion/electron transport pathways in the HM Zn anode. The mechanical flexibility of the HM Zn anode was quantitatively investigated under various deformation modes. The change in the electrical resistance of the anodes was monitored as a function of bending (bending radius = 5 mm and deformation rate = 200 mm min–1) and twisting cycle (angle = 180° and deformation rate = 30° s–1). Upon repeated bending and twisting (Figure 1d and e) for 1000 cycles, a negligible change was observed in the electrical resistance of the HM Zn anode. Moreover, the structural stability of the HM Zn anode (Figure S2) was maintained after 1000 bending cycles. In comparison, the control anode (Zn foil) mechanically ruptured in the early deformation stage (prior to 100 cycles). This result showed that the all-fibrous HM architecture based on the PEI nanofibers and SWCNTs plays a decisive role in achieving the exceptionally high mechanical flexibility. Co3O4 is widely used as a bifunctional ORR/OER catalyst material, however, it suffers from low electrical conductivity.4,25,26 To overcome this limitation, adding carbon black additives into air cathodes or modifying the cathodes with conductive agents4,27 have been suggested. Here, we synthesized rambutan-shaped, micron-sized Co3O4/MWCNT composite powders (i.e., R-CC powders) by ball-milling pristine Co3O4 powders (an inset of Figure 2a) and MWCNTs with a mass ratio of 10/0.05 (= Co3O4/MWCNT). The obtained R-CC powders are MWCNT-embedded secondary 8 ACS Paragon Plus Environment

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Co3O4 particles (Figure 2a). The microstructure of the R-CC powders was further elucidated. No significant difference in the characteristic X-ray diffraction (XRD) patterns (assigned to spinel Co3O4 structure28) was observed between the pristine Co3O4 and R-CC powders (Figure 2b). Furthermore, the Raman peaks (Figure 2c) reveal that the D/G ratios29 of the pristine MWCNTs and R-CC powders are negligibly different (1.18 for pristine MWCNTs and 1.17 for R-CC powders). These results exhibited that the structural characteristics of Co3O4 and MWCNTs were not impaired by the ball-milling process. The electrochemical catalytic activity of the R-CC powders was investigated using a rotating ring disk electrode (RRDE) test. The incorporation of electrical conductive agents into bifunctional air catalysts is known to improve oxygen reduction reaction (ORR) activity.30,31 Note that the R-CC powders showed the superior ORR performance than the pristine Co3O4 powders without deteriorating oxygen evolution reaction (OER) activity (Figure 2d), representing the results identical to that of previous studies. As far as we know, the facile fabrication of promoting catalytic activity of air catalyst has not been proposed so far, and has only been introduced to lithium ion batteries active materials.32 Future works will be needed to elucidate effect of MWCNT content on the electrochemical catalytic activity. The air catalyst cathodes in rechargeable Zn-air batteries should be designed to ensure bifunctional (i.e., ORR/OER) electrochemical activity and facile air passage/electron transport through a gas diffusion layer (GDL).3,33-35 Conventional air cathodes are fabricated by casting catalyst pastes or suspensions on porous current collectors such as carbon paper and metallic foam.33,36 However, this cathode structure is vulnerable to structural disruptions (i.e., detachment of catalyst materials and delamination between the catalyst layer and GDL) upon mechanical deformation. We, 9 ACS Paragon Plus Environment


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herein, fabricated the HM air cathode based on seamless integration of a catalyst layer and a GDL. A schematic of the air cathode and its components is shown in Figure 3a. The catalyst layer of the HM air cathode was fabricated through concurrent electrospraying (for the R-CC powders/SWCNTs) and electrospinning (for the PEI nanofibers) process. The composition ratio of the Co3O4/(SWCNT + MWCNT)/PEI was estimated to be 81/11/8 (w/w/w) from the TGA measurement (Figure S3) and the selective PEI removal results. Meanwhile, a control catalyst layer, which did not incorporate the PEI nanofibers, failed to create a self-standing film (Figure S4), which further underscored the advantageous effect of the PEI nanofibers on the structural integrity. On top of the catalyst layer, a GDL was fabricated using the same concurrent electrospraying and electrospinning process, resulting in a bi-layered air cathode (Figure 3a). The detailed fabrication procedure for the HM air cathode is illustrated in Figure S5. The aforementioned electrospraying/electrospinning process allowed for seamless unitization of the catalyst layer with the GDL in the HM air cathode (Figure 3b). The N elements (represented by blue dots), which originated from the PEI nanofibers, were uniformly distributed throughout the thickness direction of the air cathode. Meanwhile, the Co elements (represented by red dots) from the Co3O4 powders were exclusively observed in the catalyst layer. Figure 3c shows the surface morphology of the catalyst layer (left) and GDL (right). In the catalyst layer, the R-CC powders were densely packed between the PEI nanofibers and SWCNTs. Note that the 3D bicontinuous electron (SWCNT networks)/ion (interstitial voids) transport pathways were successfully constructed. The GDL of the HM air cathode showed a highly porous mat structure, which was similar to that of a GDL in a commercial MEET air cathode (Figure S6). As a result, facile air passage (Gurley value = 8 s 100 cc–1) was allowed through the GDL. The 10 ACS Paragon Plus Environment

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each PEI nanofibers were coated with the SWCNTs (Figure S7) due to the strong intermolecular affinity based on the π-π stacking interactions,22 thus acting as electron conductive channels (≅ 10 Ω □–1). To achieve satisfactory electrochemical performance of air cathodes, the catalyst layers should ensure intimate contact between the electrolytes and air while ensuring electronic conduction. However, during the cell operation, air cathodes often suffer from unwanted electrolyte flooding. As a consequence, the air cathode pores are blocked, and air transport is seriously impeded.37 The water contact angles (Figure S8) of the air catalyst layer and GDL in the HM air cathode were 120° and 145°, respectively. This hydrophobic nature of the HM air cathode is expected to mitigate the flooding concern. To investigate the mechanical flexibility of the HM air cathode, the change in its electrical resistance upon bending (Figure 3d) (bending radius = 5 mm and deformation rate = 200 mm min–1) and twisting (Figure 3e) (angle = 180° and deformation rate = 30° s–1) stresses was monitored as a function of deformation cycles. For the HM air cathode, an appreciable change in the electrical resistance was not observed, whereas the commercial MEET air cathode (a control sample) was mechanically broken and lost its electrical conductivity. Notably, the HM air cathode did not have cracks or defects in the structure after 1000 bending cycles (Figure S9), which verified that the all-fibrous HM architecture based on the highly networked PEI nanofibers and SWCNTs contributes to the superior mechanical flexibility. In regard to ion transport, an essential requirement for reliable electrochemical rechargeability of Zn-air cells is preventing the zincate ion (Zn(OH)42−) from crossing through the separator membranes. The Zn(OH)42− crossover is known as a main cause 11 ACS Paragon Plus Environment


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of the deposition of unwanted, irreversible ZnO on air cathodes.7,38,39 In this work, a thermally crosslinked PVA/PAA GPE film (thickness = 28 µm, Figure 4a) was prepared as a permselective membrane to outperform the commercial polyolefin separator (Celgard 3501, thickness = 24 µm) that is widely used in alkaline batteries. Details on the thermal-crosslinking reaction of PVA/PAA were reported in the previous work.23 Figure 4b-d shows the superior electrochemical properties and permselectivity of the PVA/PAA GPE over those of the Celgard 3501. The Zn(OH)42− ion crossover was substantially reduced compared with that of the Celgard 3501 membrane

(Figure

4b).

To

verify

this

advantageous

effect,

galvanostatic

discharge/charge profiles of the Zn-air cells (Zn foil anodes/MEET air cathodes) containing the PVA/PAA GPE were examined as a function of cycle number (Figure 4c). The cells were repeatedly discharged and charged at a constant current density of 20 mA cm–2, and each cycle period was 10 min (5 min discharge followed by 5 min charge) in the voltage range of 0.6 (discharge cut-off) – 2.4 V (charge cut-off). Herein, the current density of 20 mA cm–2 was used to expedite the generation of Zn(OH)42− ions in the electrolytes, which is effective in clearly identifying the membrane effect on the cycling performance. In comparison to the Celgard 3501, the PVA/PAA GPE exhibited a significant improvement in the cycling performance. After the cycling test, the cells were disassembled, and their air cathodes were analyzed using SEM and energy dispersive X-ray spectroscopy (EDS) characterizations to detect ZnO byproducts (resulting from the Zn(OH)42− crossover). The air cathode assembled with the PVA/PAA GPE was nearly intact without any serious ZnO contamination (Figure 4d), confirming the effective prevention of the Zn(OH)42− crossover.


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The above-prepared PVA/PAA GPE was sandwiched between the HM Zn anode and HM air cathode, and sealed with a paraffin film to fabricate a pouch-type the Znair cell (Figure 5a). The all-fibrous HM architecture in the electrodes was anticipated to allow 3D bicontinuous ion/electron conduction while also securing the structural integrity under external deformation. Figure 5b shows the discharge rate capability of the Zn-air cell as a function of discharge current density. As expected, the voltage plateau decreased as the current density increased. Note that a high discharge voltage over 1.0 V was observed up to a discharge current density of 10 mA cm–2. In addition, a galvanostatic discharge/charge cycling test (5 min of discharging followed by 5 min of charging at a current density of 0.5 mA cm–2) was conducted (Figure 5c). Herein, the relatively low current density of 0.5 mA cm–2 was used to mitigate the conversion of dissolved Zn(OH)42− ions to unwanted ZnO byproducts that exert detrimental influence on cycling performance. Future works will be devoted to optimizing the fabrication/structure of the pouch-type cell and examining the effect of current density on its cycling performance. The HM cell showed stable cycling behavior without a detectable increase in the cell polarization until a cycle time of 1500 min. This decent cycling performance is due to the combined effect of the HM architecture-enabled facile ion/electron transport in the electrodes and the prevention of Zn(OH)42− crossover through the permselective PVA/PAA GPE layer. The HM frameworks of the electrodes played a key role in the mechanical flexibility of the resulting Zn-air cells. The Zn-air cells were subjected to various deformation modes (such as bending and twisting), and an in situ analysis of their galvanostatic discharge/charge profiles was conducted to quantitatively elucidate the mechanical flexibility. During both the bending (radius = 5 mm and strain rate = 200 mm min–1) and twisting (rotation angle = 100° and strain rate = 30° s−1) modes, 13 ACS Paragon Plus Environment


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normal and stable discharge/charge cycling behavior (at a current density of 0.5 mA cm–2) was observed without mechanical rupture (Figure 5d). To highlight the mechanical flexibility, the Zn-air cell was severely crumpled, and its electrochemical activity was examined. Figure 5e and Movie S1 show that the crumpled cell (two cells were connected in series.) successfully operated an LED lamp without internal shortcircuit failure and delivered stable discharge/charge cycling behavior (at a current density of 0.5 mA cm–2). To the best of our knowledge, this is the first report of a crumpled Zn-air cell with reliable electrochemical rechargeability. To further highlight these excellent results of the HM-structured Zn-air cell, their mechanical flexibility and electrochemical performance were compared with those of previously reported flexible Zn-air cells (Table S1), underscoring the advantageous effect of the HM architecture of the Zn anode and air cathode.

4. Conclusions In summary, we presented the flexible/rechargeable Zn-air batteries based on multifunctional heteronanomat (HM) architecture. The HM Zn anode and HM air cathode in the Zn-air battery were fabricated through the one-pot, concurrent electrospraying/electrospinning process. The all-fibrous HM frameworks in the anode and air cathode consisted of highly interconnected PEI nanofibers (for structural stability) and SWCNTs (for electrical conduction). The electrode active materials (Zn powders for the anode and R-CC powders for the air cathode) were uniformly/densely packed throughout the electrodes and spatially bound by the HM frameworks, leading to the generation of 3D bicontinuous ion/electron transport channels. Notably, the HM electrodes maintained their electrical conductivity after repeated deformation cycles. The HM electrodes were assembled with the crosslinked PVA/PAA GPE (acting as a 14 ACS Paragon Plus Environment

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permselective separator membrane to suppress the Zn(OH)42– crossover), eventually producing the Zn-air cell. Due to the structural/chemical uniqueness described above, the resultant Zn-air cell showed exceptional improvements in the mechanical deformability (even under crumpled state) and electrochemical rechargeability, which far exceeded those attainable with conventional Zn-air battery technologies. The multifunctional HM framework strategy, which can be readily combined with various electrode active materials, holds great promise as a versatile and scalable nanoarchitecture platform for the development of high-performance flexible metal-air and other alkaline batteries.

ASSOCIATED CONTENT Supporting Information. Material and characterization details, experimental procedures, supporting characterization data, electrochemical analysis data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: (S.Y.L) [email protected], (K.H.K) [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Basic Science Research Program (NRF-2012-M1A2A2029542, 2018R1A2A1A05019733) and Wearable Platform Materials Technology Center 15 ACS Paragon Plus Environment


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(2016R1A5A1009926) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning. This work was also supported by the Development Program of the Korea Institute of Energy Research (KIER) (Grant No. B82421).

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(15) Fu, J.; Lee, D. U.; Hassan, F. M.; Yang, L.; Bai, Z.; Park, M. G.; Chen, Z. Flexible HighEnergy Polymer-Electrolyte-Based Rechargeable Zinc-Air Batteries. Adv. Mater. 2015, 27, 5617-5622. (16) Xu, N. N.; Qiao, J. L.; Zhang, X.; Ma, C. Y.; Jian, S. A.; Liu, Y. Y.; Pei, P. C. Morphology Controlled La2O3/Co3O4/MnO2-CNTs Hybrid Nanocomposites with Durable Bi-Functional Air Electrode in High-Performance Zinc-Air Energy Storage. Appl. Energy 2016, 175, 495-504. (17) Li, Z. H.; Shao, M. F.; Yang, Q. H.; Tang, Y.; Wei, M.; Evans, D. G.; Duan, X. Directed Synthesis of Carbon Nanotube Arrays Based on Layered Double Hydroxides Toward Highly-Efficient Bifunctional Oxygen Electrocatalysis. Nano Energy 2017, 37, 98-107. (18) Alic, J.; Asimow, R. Growth Of Tensile and Fatigue Cracks in Metal Foils. Eng. Fract. Mech. 1974, 6, 223-230. (19) Alic, J.; Asimow, R. Direct Observation of Tensile and Fatigue Cracks. Eng. Fract. Mech. 1972, 4, 915-923. (20) Kim, J.-M.; Park, C.-H.; Wu, Q.; Lee, S.-Y. 1D Building Blocks-Intermingled Heteronanomats as a Platform Architecture for High-Performance Ultrahigh-Capacity Lithium-Ion Battery Cathodes. Adv. Energy Mater. 2016, 6, 1501594. (21) Wang, G. G.; Weng, Y. M.; Chu, D.; Xie, D.; Chen, R. R. Preparation of Alkaline Anion Exchange Membranes Based on Functional Poly(ether-imide) Polymers for Potential Fuel Cell Applications. J. Membr. Sci. 2009, 326, 4-8. (22) Oh, Y. S.; Jung, G. Y.; Kim, J. H.; Kim, J. H.; Kim, S. H.; Kwak, S. K.; Lee, S. Y. Janus-Faced, Dual-Conductive/Chemically Active Battery Separator Membranes. Adv. Funct. Mater. 2016, 26, 7074-7083. (23) Kim, H. W.; Lim, J. M.; Lee, H. J.; Eom, S. W.; Hong, Y. T.; Lee, S. Y. Artificially Engineered, Bicontinuous Anion-Conducting/-Repelling Polymeric Phases as a 18 ACS Paragon Plus Environment

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Figure 1. Concurrent electrospraying/electrospinning-assisted fabrication of the HM Zn anode and its structural/electrochemical/mechanical characteristics. (a) Schematic representation depicting the concurrent electrospraying/electrospinning-assisted fabrication of the HM Zn anode, along with its photograph. (b) Surface and (c) cross-sectional SEM images of the HM Zn anode. Insets show the EDS images of the Zn elements (represented by blue dots). Change in the electrical resistance of the HM Zn anode and Zn foil anode as a function of (d) bending (radius = 5 mm, strain rate = 200 mm min−1) and (e) twisting (angle = 180° and deformation rate = 30° s–1) cycle. Insets are photographs of the HM Zn anode (after 1000 bending and twisting cycles, respectively) and mechanically ruptured Zn foil anode (after 76 bending and 74 twisting cycles, respectively).



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Figure 2. Characterizations of R-CC powders. (a) SEM image of R-CC powders (inset shows the morphology of the pristine Co3O4 powders). (b) XRD patterns (assigned to spinel Co3O4 structure). (c) Raman spectra of the MWCNT, pristine Co3O4 and R-CC powders. (d) Voltammograms of the ORR/OER disk current for the R-CC and pristine Co3O4 powders. The RRDE measurements (scan rate = 10 mV s−1) were conducted at a rotation rate of 1600 rpm.


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Figure 3. Concurrent electrospraying/electrospinning-assisted fabrication of the HM air cathode and its structural/electrochemical/mechanical characteristics. (a) Schematic illustration (top) and photograph (bottom) of the bi-layered HM air cathode, wherein the catalyst layer was seamlessly integrated with the GDL. (b) Cross-sectional SEM (left) and EDS (right) images of the HM air cathode. (c) Surface SEM image of the catalyst layer (left) and GDL (right). Inset shows the high-magnification surface SEM image of the catalyst layer. (d) Bending (radius = 5 mm, strain rate = 200 mm min−1) and (e) twisting (rotation angle = 180°, strain rate = 30° s−1) cycle. Insets are photographs of the HM air cathode (after 1000 bending and twisting cycles, respectively) and the mechanically ruptured control MEET air cathode (after 263 bending and 54 twisting cycles, respectively).



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Figure 4. Fabrication and permselective transport phenomena of the crosslinked PVA/PAA GPE film. (a) Photographs of the PVA/PAA mixture solution (before thermal crosslinking) and the PVA/PAA GPE film (after thermal crosslinking). (b) Zn(OH)42− ion crossover through the crosslinked PVA/PAA GPE film and Celgard 3501 membrane as a function of elapsed time. (c) Galvanostatic discharge/charge cycling performance of the Znair cells (Zn foil anodes/MEET air cathodes) assembled with different membranes (crosslinked PVA/PAA GPE film and Celgard 3501 membrane). The Zn-air cells were cycled at a constant current density of 20 mA cm−2 for each 10 min cycle period (5 min discharge followed by 5 min charge) in the voltage range of 0.6 (discharge cut-off) – 2.4 V (charge cutoff). (d) Surface SEM (inset) and EDS images of the air cathodes after the cycling test: crosslinked PVA/PAA GPE film (left) vs. Celgard 3501 membrane (right). 24 ACS Paragon Plus Environment

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Figure 5. Fabrication of the multifunctional HM-structured Zn-air cells and their electrochemical/mechanical properties. (a) Photographs and schematic illustration of the Zn-air cells (HM Zn anode, HM air cathode and crosslinked PVA/PAA GPE film). (b) Galvanostatic discharge profiles at different discharge current densities. (c) Galvanostatic discharge/charge cycling performance at a constant current density of 0.5 mA cm–2 for each 10 min cycle period (5 min discharge followed by 5 min charge). (d) In situ analysis of the galvanostatic discharge/charge cycling profiles during the bending (radius = 5 mm, strain rate = 200 mm min–1) and twisting (rotation angle = 100°, strain rate = 30° s−1) modes. (e) Photograph showing the operation of an LED powered by the crumpled Zn-air cells. (two cells were connected in series.) Inset is the galvanostatic discharge/charge cycling behavior of the crumpled Zn-air cells at a current density of 0.5 mA cm–2.



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