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Doctoral Thesis
ELECTRODE DESIGN FOR AQUEOUS ZINC-BASED RECHARGEABLE BATTERIES
Donggue Lee
School of Energy and Chemical Engineering (Battery Science and Technology)
Ulsan National Institute of Science and Technology
2021
[UCI]I804:31001-200000371651 [UCI]I804:31001-200000371651 [UCI]I804:31001-200000371651 [UCI]I804:31001-200000371651
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ELECTRODE DESIGN FOR AQUEOUS ZINC-BASED RECHARGEABLE BATTERIES
Donggue Lee
School of Energy and Chemical Engineering (Battery Science and Technology)
Ulsan National Institute of Science and Technology
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Abstract
Forthcoming smart energy era, which will find widespread use of flexible/wearable electronic devices, Internet-of-Things, electric vehicles and grid-scale energy storage systems, is in relentless pursuit of high-energy/safe rechargeable power sources. Among a variety of battery systems, aqueous Zinc (Zn)-based rechargeable batteries have garnered vigorous attention as a promising candidate beyond current state-of-the-art lithium-ion batteries because of their safety, high energy density, and cost.
Although aqueous Zn-based batteries have remarkable advantages, they are still far behind the industrialization as a rechargeable battery. The major hurdles of aqueous Zn-based rechargeable batteries are related to the degradation of cathodes and anodes, respectively, caused by parasitic reactions. In the rechargeable Zn-air batteries, the electrodes suffer from a chemical/
electrochemical corrosion causing mechanical collapse of electrode structure and detachment of electrode materials from the current collectors during charging process. It is resulting in losses of electrochemically active surface area, uniform distribution of current, and even severe leakage of electrolyte. The rechargeable and mechanically stable Zn anodes/air cathodes have been vigorously researched, however, the advanced design for the rechargeable and mechanically durable electrode architecture are still insufficient.
Besides, Zn anodes have critical obstacle associated with chemical and electrochemical reactions between Zn and aqueous electrolyte. Zn metal tends to undergo chemical corrosion in aqueous media, resulting in unwanted passivated surface. Furthermore, the higher potential of the H2 evolution reaction of water than the redox potential Zn/Zn2+ induces aqueous electrolyte decomposition accompanied by H2 gas evolution. Consequently, these interfacial parasitic reactions bring about poor reversibility of Zn redox reactions.
In this dissertation, to realize aqueous Zn-based rechargeable battery systems, a monolithic electrode architecture for Zn-air batteries and novel protective layer for extending the cycle life of Zn anodes are proposed, which lie far beyond those achievable with conventional battery technologies. As a new class of electrode design, heteronanomat architectures are introduced into the Zn anodes and air cathodes to enable the fabrication of rechargeable and mechanically durable Zn-air batteries. The multifunctional heteronanomat framework is fabricated based on polymeric nanofibers along with carbon nanotubes through one-pot fabrication processing. Thanks to the novel architectural design, unprecedented improvements in the electrochemical performance even under crumpled and folded state.
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In addition, water-repellent ion-conducting layer based on hydrophobic ionic liquid (IL) is introduced as a reliable Zn metal protective layer. The IL thin gel, consisting of hydrophobic IL solvent, Zn salts, and thiol-ene polymer compliant skeleton, prevents the access of water molecules to Zn anodes while allowing Zn2+ conduction for redox reactions. The IL-gel-skinned Zn anode enables sustainable Zn plating/stripping cyclability without suffering from water triggered interfacial parasitic reactions.
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Contents
List of Figures‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 10
Chapter I. Introduction‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 15 1.1. Introduction of Aqueous Zinc-Based Rechargeable Battery Systems‧‧‧‧‧‧‧‧‧‧‧‧‧ 15 1.2. Issues of Aqueous Zinc-Based Rechargeable Battery Systems ‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 19
Chapter II. One-Dimensional Nanofiber-based Electrode Architecture for
Rechargeable and Flexible Zinc-Air Batteries‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 22 2.1. Rechargeable and Flexible Zinc-Air Batteries Based on Multifunctional
Heteronanomat Architecture‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 22 2.1.1. Introduction‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 22 2.1.2. Experimental‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 23 2.1.3. Results and Discussion‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 25 2.1.4. Conclusion‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 33
2.2. Monolithic Heteronanomat Paper Air Cathodes Toward Rechargeable and Origami-Foldable Zinc-Air Batteries‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 34 2.2.1. Introduction‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 34 2.2.2. Experimental‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 35 2.2.3. Results and Discussion‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 37 2.2.4. Conclusion‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 42
Chapter III. Water-Repellent Ionic Liquid Skinny Gels Customized for
Rechargeable Aqueous Zinc-Ion Battery Anodes ‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 43 3.1. Introduction‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 43 3.2. Experimental‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 45 3.3. Results and Discussion‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 46
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3.4. Conclusion‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 58
List of Publications‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 59 References‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧ 60
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List of Figures
Figure 1. Ragone plot illustrating the energy and power characteristics of several Zn-based energy storage systems (Zn-air, Zn-MnO2, Ni-Zn, and Ag-Zn). Legacy Pb-acid and present Li-ion systems are included for comparison.
Figure 2. Schematic illustration of aqueous Zn-based rechargeable battery systems; (a) Zn-air battery
and specific conventional air cathode structure, (b) Zn-ion batteries using host cathode materials.
Figure 3. Schematic representation of performance-limiting phenomena that may occur on the zinc
electrode; (a) dendrite growth, (b) shape change, (c) passivation, and (d) hydrogen evolution.
Figure 4. (a) Schematic image and chemical equation of carbon corrosion and catalyst detachment. (b)
Relationship of carbon corrosion and activity loss of air cathodes.
Figure 5. Schematic representation of the chemical and electrochemical side reactions between Zn and
aqueous electrolyte.
Figure 6. (a) Schematic representation depicting the concurrent electrospraying/electrospinning- assisted fabrication of the HM Zn anode, along with its photograph. (b) Surface SEM images of the HM Zn anode. Insets show the EDS images of the Zn elements (represented by blue dots).
Figure 7. TGA profiles of the (a) HM Zn anode and (b) PEI/SWCNT mixture mat. (c) Calculation used
to estimate the Zn content in the HM Zn anode.
Figure 8. 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.
Figure 9. (a) Schematic illustration of the concurrent electrospraying/electrospinning-assisted fabrication of the HM air cathode composed of catalyst layer and GDL. (b) Schematic illustration (top) and photograph (bottom) of the bi-layered HM air cathode, wherein the catalyst layer was seamlessly integrated with the GDL. (c) Cross-sectional SEM (left) and EDS (right) images of the HM air cathode.
(d) Surface SEM image of the catalyst layer (left) and GDL (right). Inset shows the high-magnification surface SEM image of the catalyst layer.
Figure 10. Water contact angle; (a) catalyst layer and (b) GDL.
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Figure 11. Bending (radius = 5 mm, strain rate = 200 mm min−1) and twisting (rotation angle = 180°, strain rate = 30° s−1) deformation cycle of HM electrodes; (a) and (b) HM Zn anode, (c) and (d) HM air cathode. Insets are photographs of the HM electrodes (after 1000 bending and twisting cycles, respectively) and the mechanically ruptured control electrodes (Zn foil anodes and MEET air cathodes).
Figure 12. (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 charge/discharge cycling performance of the Zn-air 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 cut-off). (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).
Figure 13. (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 charge/discharge 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 charge/discharge 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 charge/discharge cycling behavior of the crumpled Zn-air cells at a current density of 0.5 mA cm–2.
Figure 14. (a) Schematic of the fabrication procedure for the MH paper air cathode, along with its photograph. Photographs showing dispersion stability of the cathode suspension: (b) without and (c) with a dispersion agent (SDS/urea = 5/5 (w/w)). (d) Cross-sectional SEM image showing the highly entangled network structure of the 1D cathode components. The inset shows a low-magnification image.
(e) Photograph of the MH paper air cathode without CNFs.
Figure 15. Cross-sectional EDS mapping images (Nd, Ba, Sr, Co, Fe, N, C, O, and F elements are
represented by the colored dots) of the MH paper air cathode.
Figure 16. Change in the relative sheet resistance (R/R0, R0 = sheet resistance prior to the deformation) as a function of deformation cycles under different deformation modes; (a) bending radius = 3 mm and deformation rate = 200 mm min−1, (b) five-folded zig-zag shape. (c) Photographs showing the mechanical flexibility; HM paper air cathode vs. control samples (commercial air cathode and carbon
12 paper).
Figure 17. (a) Discharge profiles of the MH paper air cathodes (with vs. without PTFE nanoparticles).
Insets are photographs showing the water contact angle of the cathodes. (b) EIS spectra of the MH paper air cathodes as a function of their CNF content, along with the proposed equivalent circuit model.
Figure 18. (a) Initial discharge voltages of the MH paper air cathodes (CNF content = 25 vs. 40 wt%) after exposure to ambient conditions for 8 h. Inset shows the corresponding discharge voltage profiles as a function of time. (b) Change in electrolyte content (expressed by relative mass of evaporated electrolyte) of the MH paper air cathode during the evaporation test (i.e., exposure to ambient condition).
Figure 19. (a) OCV profile of the 90° bent Zn–air battery. Inset shows its photograph. (b) Cell overpotentials reflecting the discharge–charge rate capability as a function of current density (1–20 mA cm−2). (c) Galvanostatic discharge–charge cycle profile at a current density of 1 mA cm−2. Insets show the magnified view of the initial and end parts.
Figure 20. (a) Galvanostatic discharge-charge cycle profiles (before folding vs. folded state). Inset shows a photograph of the folded Zn-air battery. (b) Photograph of the paper-airplane-shaped Zn-air battery featuring the multifolded state. The inset shows its backside view. (c) Photograph showing the successful operation of an LED lamp powered by two paper–airplane-shaped Zn-air batteries connected in series.
Figure 21. Schematic illustration depicting the fabrication of an IL skinny gel on a Zn anode, along
with its chemical structure and role as a water-repellent ion-conducting protective layer.
Figure 22. Immiscibility between various BMP+ cation-based ILs (having different anions (BF4−, OTf−, and TFSI−)) and de-ionized water (DIW). (a) Photographs and (b) 19F NMR spectra.
Figure 23. Salt-dissociating capability of various hydrophobic solvents (hexane, toluene, chloroform, butyl acetate, and BMPTFSI). (a) Photographs showing the immiscibility of the hydrophobic solvents with water. (b) Photographs showing the salt (0.3 m Zn(TFSI)2)-dissociation in the hydrophobic solvents. (c) Vis-NIR spectra of the 0.3 m Zn(TFSI)2-containing hydrophobic solvents.
Figure 24. Electrochemical characteristics of the IE (0.3 m Zn(TFSI)2 in BMPTFSI). (a) Galvanostatic Zn plating/stripping cyclability of the Zn/Zn symmetric cell at a current density of 0.1 mA cm−2. (b) C.E. of the Ti/Zn asymmetric cell at a current density of 0.1 mA cm−2.
Figure 25. Immiscibility of the IL skinny gel and its physicochemical properties. (a) Photographs of the AE (2 m ZnSO4 aqueous electrolyte), IE (0.3 m Zn(TFSI)2 in BMPTFSI), and AE/IE mixtures showing the immiscibility. (b) Raman spectra (focusing on TFSI− and OH−) of the AE, IE, and AE/IE
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mixture. (c) Raman spectra (focusing on O−H stretching modes) and photographs (insets) of the AE/IE mixture at −15 and 45℃.
Figure 26. Physicochemical characteristics of the IL gel. (a) Photographs of the IL gel before (inset) and after UV curing-driven solidification. (b) Effect of the monomer content (0, 2, and 5 wt%) on the solidification of IL gels (after the UV-curing). (c) Changes in the characteristic FT-IR peaks assigned to the thiol (S−H) groups (2575 cm−1) and acrylic C=C bonds (1610–1625 cm−1) of the thiol-ene polymer network before/after UV-curing.
Figure 27. Photographs showing the AE-repelling behavior of the IL gel, in which the IL gel-coated glass fiber separator was inserted between a bare AE (2 m ZnSO4 aqueous electrolyte, bottom) and a dye (indigo carmine)-containing AE (top).
Figure 28. (a) Cross-sectional SEM image of the ILG-Zn. (b) SEM and (c) EDS elemental mapping
images of the ILG-Zn electrode.
Figure 29. Electrochemical analysis of the ILG-Zn electrodes as a function of IL gel thickness. (a) EIS profiles. (b), (c) Galvanostatic Zn plating/stripping profiles of the AE-containing Zn/Zn symmetric cells at a current density of 0.5 mA cm−2.
Figure 30. Cross-sectional SEM images showing the stable Zn plating/stripping underneath the IL
skinny gel.
Figure 31. Chemical/electrochemical stability of the ILG-Zn with AE. (a) SEM images showing the chemical stability of the ILG-Zn (vs. bare Zn (inset)) after being stored in the AE for one week. EIS profiles of the AE-containing Zn/Zn symmetric cells as a function of elapsed time; (b) Bare Zn and (c) ILG-Zn. (d) Galvanostatic Zn stripping capacities of the AE-containing Zn/Zn symmetric cells (ILG- Zn vs. bare Zn) after being stored for one week at a current density of 0.5 mA cm−2.
Figure 32. (a) C.E. of Zn plating/stripping on ILG-coated Ti (vs. bare Ti) at a galvanostatic capacity of 0.1 mAh cm−2 and a current density of 0.1 mA cm−2. (b) Voltage profiles (at 50th cycle) of the Zn plating/stripping cycling at a current density of 0.1 mA cm−2. Bare Ti vs. ILG-coated Ti.
Figure 33. H2 gas signal of the ILG-Zn (vs. bare Zn) as a function of time, which was obtained by in- situ DEMS analysis of AE-containing Zn/Zn symmetric cells at a current density of 0.1 mA cm−2. Figure 34. (a) Galvanostatic Zn plating/stripping cyclability of the AE-containing Zn/Zn symmetric cells (ILG-Zn vs. bare Zn) at an areal capacity of 0.1 mAh cm−2 and a current density of 0.1 mA cm−2. (b) Galvanostatic Zn plating/stripping cyclability of the Zn/Zn symmetric cells under a higher areal capacity and current density; 0.5 mAh cm−2 at 1 mA cm−2 and 1 mAh cm−2 at 2 mA cm−2. (c)
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Galvanostatic Zn plating/stripping cyclability of the (ILG-Zn/ILG-Zn) symmetric cell, (ILG-Zn/bare Zn) asymmetric cell, and (bare Zn/bare Zn) symmetric cell at a current density of 0.1 mA cm−2. Figure 35. Galvanostatic Zn plating/stripping cyclability of the AE-containing Zn/Zn symmetric cells (ILG-Zn vs. bare Zn) at a higher areal capacity of 1.8 mAh cm−2 (corresponding to 90% DODZn) and current density of 0.5 mA cm−2.
Figure 36. Post-mortem analysis of the Zn electrodes after Zn plating/stripping cycling test. (a) SEM images (insets: photographs) of the bare Zn (top) and ILG-Zn (bottom), in which the Zn/Zn symmetric cells were cycled under an areal capacity of 0.1 mAh cm−2 and a current density of 0.1 mA cm−2. (b) XPS spectra (Zn 2p3/2 (left) and O 1s (right) peaks) of the bare Zn (top) and ILG-Zn (bottom). (c) TOF- SIMS mapping images (ZnO+ species) of the bare Zn (top) and ILG-Zn (bottom).
Figure 37. LEIS area scan of the (a) bare Cu and (b) ILG-coated Cu after Zn electrodeposition with an
areal capacity of 3 mAh cm−2.
Figure 38. Electrochemical performance of the Zn/MnO2 full cells (containing aqueous electrolyte). (a) CV profiles (bare Zn vs. ILG-Zn) at a scan rate of 0.1 mV s−1. (b) Galvanostatic charge/discharge profiles (bare Zn vs. ILG-Zn) at a current density of 0.1 C.
Figure 39. EIS profiles of the Zn/MnO2 full cells (bare Zn vs. ILG-Zn) after being precycled at a current density of 0.1 C; (a) Bare Zn and (b) ILG-Zn. (c) GITT profiles of the Zn/MnO2 full cells (bare Zn vs. ILG-Zn) upon repeated current stimuli (at a current density of 0.1 C and interruption time between pulses of 1 h).
Figure 40. (a) Charge/discharge cycling performance (bare Zn vs. ILG-Zn) at a current density of 2 C.
(b) Charge/discharge profiles (bare Zn vs. ILG-Zn) at the 1st and 600th cycles. (c) SEM images of the cycled ILG-Zn anode (inset: bare Zn anode).
15
CHAPTER I. Introduction
1.1. Introduction of Aqueous Zinc-based Rechargeable Battery Systems
Ever-increasing demands in energy and alert of climate change has driven and accelerated the inevitable transition from fossil fuels to clean renewable energy. Given the limited fossil-fuel resources on Earth, it is reasonable to estimate a more volatile energy economy in the near future. Even with the levelized energy cost for renewable energy sources approaching that of fossil fuels, their intermittent nature remains a challenge to widespread adoption in the global energy mix.1-2 With this in mind, the task of developing advanced energy storage systems is more urgent than ever. For centuries, batteries have been known for their excellence in converting and storing chemical energy. One of their biggest advantages over traditional forms of energy storage is the ability to be scaled down to small sizes, which has made them indispensable for portable electronic devices. Electric vehicles (EVs), which are expected to replace internal combustion engine vehicles in the coming years, are another industry where batteries have the potential to be the major form of energy storage. Most EVs today use lithium-ion batteries (LIBs), which have dominated the rechargeable battery market since their advent in the late 1990s. Going forward, the main disadvantages of LIBs are their high cost and concerns regarding both their safety and the supply of lithium and cobalt (the latter of which is most commonly used in the positive electrode).3-4 Their energy density is also limited by the capacity of the electrode materials.5 These factors have led to intense research activity involving alternative rechargeable battery technologies.
Recently, rechargeable battery systems using aqueous electrolytes possess unparalleled advantages because of their low cost, easiness of assembly, high-safety, and high ionic conductivity (10–1–6 S cm–
1), which is superior to that in organic electrolytes (10–3–10–2 S cm–1).6 Especially, the strict moisture control is not required like needed for LIBs during battery manufacturing and operation, which gives them strong price competitiveness and places them closer to large-scale energy storage applications.7 These aspects make aqueous battery systems a desired topic of studies. Among numerous candidates for aqueous batteries, aqueous zinc (Zn) batteries have recently emerged into the spotlight as promising systems that can meet today's demanding performance levels.8 Aside from the aqueous electrolyte, a distinguishing aspect of this system is the use of Zn metal as the anode. Zn offers several advantages in aqueous electrochemical systems. First, not only is it readily accessible and relatively cheap as a resource, but it is also known for its non-toxicity and chemical stability in aqueous media compared to alkali metals such as lithium or sodium.4 Second, considering the rather narrow operating window beyond which gas (H2 and O2) evolution occurs in water,9 Zn has a suitable redox potential of 0.76 V (vs. standard hydrogen electrode) for battery operation. Lastly, Zn possesses a high theoretical capacity
16
Figure 1. Ragone plot illustrating the energy and power characteristics of several Zn-based energy storage systems (Zn-air, Zn-MnO2, Ni-Zn, and Ag-Zn). Legacy Pb-acid and present Li-ion systems are included for comparison.10
(820 mA h g–1, 5854 mA h L–1) in its metallic state. In fact, the use of Zn in aqueous batteries is not a new concept. The family of aqueous Zn-based batteries, such as Zn-air, Zn-MnO2, Ni-Zn, Ag-Zn, offer a compelling alternative, with practical specific energies ranging from 80 to 475 Wh kg−1, specific power up to a continuous 800 W kg−1 (Figure 1), and the inherent, decades-validated safety advantages of operating with aqueous alkaline electrolytes.10
Zn within an aqueous metal-air battery has received far more attention due to its greater energy density (1218 W h kg−1 and 6136 W h L−1) and cell voltage, which is particularly desirable for mobile and portable devices (e.g., EVs and personal electronics).11 The Zn-air battery is typically composed of four main components, shown in Figure 2a: an air cathode, a Zn anode, an alkaline electrolyte, and a separator. Importantly, the air cathodes require a tri-phase boundary with balanced hydrophobicity and hydrophilicity to facilitate the diffusion of air and electrolyte onto the solid catalyst surface. In order to establish the tri-phase boundary, a complex multilayer air cathode that includes a catalyst layer, current collector, and gas diffusion layer (GDL)12-13 is usually required. During discharge, the Zn-air battery functions as a power generator through the electrochemical coupling of the Zn metal to the air electrode in the presence of an alkaline electrolyte with an inexhaustible cathode reactant (oxygen) from the atmosphere. The electrons liberated at the Zn travel through an external load to the air electrode, while
17
Figure 2. Schematic illustration of aqueous Zn-based rechargeable battery systems; (a) Zn-air battery and specific conventional air cathode structure, (b) Zn-ion batteries using host cathode materials.14-15
Zn cations are produced at the Zn electrode. At the same time, atmospheric oxygen diffuses into the porous air electrode and is ready to be reduced to
hydroxide ions via the oxygen reduction reaction (ORR) (forward Reaction (1)) at a tri-phase reaction site, which is the interface of oxygen (gas), electrolyte (liquid), and electrocatalysts (solid). Reaction (2) shows the overall Zn redox reaction. During charging, the Zn-air battery is capable of storing electric energy through the oxygen-evolution reaction (OER) (backward Reaction (1)), occurring at the electrode-electrolyte interface, whereas Zn is deposited at the anode surface (backward Reaction (2)).
The overall reaction (Reaction (3)) can be simply shown as Zn combining with O2 to form ZnO.
The air cathode reaction:
O2 + 2H2O + 4e− ⇔ 4OH−, E = 0.40V vs. SHE (1) The Zn anode reaction:
Zn + 4OH− ⇔ Zn(OH)42− + 2 e− ⇔ ZnO + 2OH− + H2O + 2e−, E = −1.26 V vs. SHE (2) The overall reaction:
2Zn + O2 ⇔ 2ZnO, E = 1.66 V vs. SHE (3)
18
Thermodynamically, both reactions are spontaneous and produce a theoretical voltage of 1.66 V.
However, the redox reactions of oxygen during the charging and discharging cycles are kinetically slow;
thus, electrocatalysts are often used to accelerate the process.
In addition to Zn-air batteries, aqueous Zn-ion batteries (ZIBs) using Zn metal anode and host cathode for accommodation of Zn2+ guest such as manganese/vanadium-based oxide, Prussian blue analogs, and organic compounds (Figure 2b), have gradually received increasing attention due to the diversity of potential electrolytes, low price, nontoxicity, and safety.4,16-18 In last decades, several attempts in utilizing alkaline electrolytes resulted in poor cycling performances which were attributed to the dendritic growth, dissolution of Zn, formation of insulating ZnO, and parasitic reaction generating hydrogen gas (H2) at the Zn anode (Figure 3).19-20 The high solubility of Zn in alkaline solutions can cause the Zn metal to dissolve as [Zn(OH)4]2−,21 and any non-uniformity in [Zn(OH)4]2− concentration at the Zn metal surface can lead to a non-uniform Zn plating during the charging step, resulting in dendrites.22-23 Recently, the formation of ZnO phases and Zn dendrites have been suppressed by utilizing mild acid electrolytes and a highly reversible electrochemical plating/stripping has been achieved.23-24 The reversibility of Zn anode in mild aqueous electrolyte is different from that in alkaline electrolyte, because mildly acidic electrolytes (e.g., ZnSO4) do not form passivating Zn oxides/ hydroxides on the surface according to the Pourbaix diagram.25 Thereby, aqueous ZIBs using mild acid electrolyte have been attracting increasing attention during recent decade not only in the field of grid-scale energy storage, but also for the wearable and biocompatible application.6,17
Figure 3. Schematic representation of performance-limiting phenomena that may occur on the zinc electrode; (a) dendrite growth, (b) shape change, (c) passivation, and (d) hydrogen evolution.26
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1.2. Issues of Rechargeability of Aqueous Zinc-based Battery Systems
Nevertheless, aqueous Zn-based batteries have various advantages aforementioned, they are still far behind in industrialization as a secondary battery. The major obstacles of aqueous Zn-based rechargeable batteries are related to the degradation of electrodes. In case of the electrically rechargeable Zn-air batteries, the air cathodes and Zn anodes face its own challenges. The air cathodes are typically composed of oxygen catalysts, polymeric binders (e.g., polytetrafluoroethylene (PTFE)), and conductive carbon black. Their battery performance has traditionally been a drawback due mainly to fundamental challenges in oxygen catalyzing reactions, thereby limiting the power density of Zn-air batteries.27-28 When it comes to the long-term cycle operation, carbon oxidation (i.e., corrosion) (0.207 V vs. reversible hydrogen electrode)29-30 in the air cathode during the charging reaction causes the degradation of structural stability of catalyst layer because sphere-shaped PTFE powders have the insufficient binding ability (Figure 4a). It is resulting in losses of electrochemically active surface area, uniform distribution of current, and even severe leakage of electrolyte (Figure 4b).14,31-32 The rechargeable air cathodes have been vigorously researched based on various oxygen catalytic materials, however, the advanced design for the air cathode architecture by adopting mechanically durable and free-standing structures are still insufficient.33
Figure 4. (a) Schematic image and chemical equation of carbon corrosion and catalyst detachment. (b) Relationship of carbon corrosion and activity loss of air cathodes.34-35
Furthermore, for the Zn anode, it is difficult to control the non-uniform dissolution and deposition of Zn, which is the main cause of dendrite formation and shape change.36-37 Even though dendritic Zn could be minimized in mild neutral pH (or slightly acidic) electrolyte,38 the poor reversibility and low plating/stripping Coulombic efficiency (CE) of Zn anode still challenge its practical application. The main problems associated with the low CE are the consumption of Zn and electrolyte by the chemical and electrochemical side reactions (Figure 5).39 The Zn metals tend to undergo corrosion in aqueous media, yielding unwanted passivation layers.40-41 More importantly, the higher potential of the H2
20
evolution reaction than the redox potential of Zn/Zn2+ triggers water decomposition accompanied by H2
gas evolution. As a consequence, these interfacial parasitic reactions between Zn anodes and aqueous electrolytes result in poor reversibility of Zn plating/stripping along with the consumption of Zn and electrolyte.8,42-44 Therefore, significant excessive Zn needs to be used to maintain the cycle stability, but this leads to suboptimal utilization of its theoretical capacity. Notwithstanding these problems, numerous review and perspective papers have brought about renewed interest in electrically rechargeable Zn-based aqueous batteries in this decade.4,25,40,45-48
Figure 5. Schematic representation of the chemical and electrochemical side reactions between Zn and aqueous electrolyte.
In this dissertation, focused on aqueous Zn-based rechargeable battery systems, a monolithic electrode architecture, including active materials/current collector, and novel protective layer for extending the cycle life of Zn anode are proposed, which lie far beyond those achievable with conventional battery technologies. For a new class of flexible/rechargeable Zn-air batteries, a multifunctional heteronanomat (denoted as “HM”) framework supported electrodes are achieved by one-pot concurrent electrospraying (for electrode active powders/carbon nanotubes (CNTs)) and electrospinning (for polyetherimide (PEI) nanofibers). PEI is selected due to its exceptional chemical/electrochemical resistance against corrosion and intimate intermolecular interaction with CNTs.49 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 CNTs (for electrical transport). The HM frameworks within the electrodes show unprecedented multifunctionalities as follows; (i) take on a one-dimensional (1D) binder to hold the Zn particles (in the Zn anode) and cobalt oxide (Co3O4)/CNT composite powders (in the air cathode), resulting in securing structural integrity and mechanical stability; (ii) formulate three-dimensional (3D)- bicontinuous ion/electron transport pathways.
21
As an improved design, monolithic heteronanomat (MH) air cathodes as a new electrode platform is introduced to enable origami-foldable Zn-air batteries with reliable electrochemical rechargeability. The MH paper air cathodes comprise 1D bifunctional catalyst mixtures, cellulose nanofibers (CNFs), and PTFE nanoparticles, without the use of conventional current collectors and gas diffusion layers. The 1D catalyst mixtures are composed of NdBa0.5Sr0.5-Co1.5Fe0.5O5+d double perovskite (NBSCF) nanofibers as an OER catalyst and nitrogen-doped carbon nanotubes (N-CNTs) as an ORR catalyst to ensure bifunctional electrocatalytic activities. Particularly, CNFs are introduced as a hydrophilic binder to hold the 1D catalyst mixtures and promote the formation of aqueous electrolyte channels. Besides, PTFE nanoparticles are adopted as a hydrophobic binder to prevent electrolyte flooding and provide air routes in the MH paper air cathode. The simultaneous use of CNFs (as a hydrophilic binder) and PTFE (as a hydrophobic binder) enables the formation of well-developed 3D electrolyte/air transport pathways.
Meanwhile, the highly entangled network structure (based on a fibrous mixture of NBSCF, N-CNTs, and CNFs) of the MH paper air cathode exhibits substantially improved electrochemical rechargeability and mechanical flexibility under various deformation modes.
Despite such electrode structure-level solutions, aqueous Zn-based rechargeable batteries are still have intrinsic obstacle that the chemical/electrochemical instability of Zn anodes with aqueous electrolytes, which is the foremost challenges facing RZABs. As a proof of concept, the hydrophobic ionic liquid (IL) skinny gels are proposed as a new class of water-repellent ion-conducting protective layer tailored for Zn anodes. The IL skinny gel is composed of hydrophobic IL solvent (1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI)), Zn salt (Zn(TFSI)2), and thiol- ene polymer compliant skeleton (ultraviolet (UV)-cured trimethylolpropane tris(3-mercaptopropionate) (TMPMP, “thiol”) and trimethylolpropane triacrylate (TMPTA, “ene”)). The BMPTFSI, used as a hydrophobic solvent for electrolytes,50 is expected to prevent the access of aqueous electrolytes to Zn anodes. Moreover, the BMP+ cations of the BMPTFSI, owing to their electroshielding effect,51 can contribute to uniform and stable Zn plating on Zn anodes. The thiol-ene polymers, which are known to show mechanical compliance,52 enable the IL skinny gel to be deposited conformally on the Zn surface and to maintain its structural integrity during plating/stripping cycling. Driven by the rationally designed material chemistry and structural engineering, the IL skinny gel deposited on Zn anodes can effectively suppress water-triggered interfacial parasitic reactions, such as electrolyte consumption and Zn corrosion, thereby exhibiting reliable Zn plating/stripping cyclability under 90% depth of discharge (DODZn), which are difficult to obtain with previously reported protective layers.
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CHAPTER II. One-Dimensional Nanofiber-Based Electrode Architecture for Rechargeable Zinc-air Batteries
2.1. Rechargeable and Flexible Zinc-Air Batteries Based on Multifunctional Heteronanomat Architecture
2.1.1. 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 LIBs.53-55 Among the numerous energy storage systems explored to date, 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.26,56-58 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 due to lack of oxygen catalytic activities and inappropriate electrode configuration.7 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 rechargeable/flexible Zn-air batteries, which include cable (or fiber)-type cells,59-62 nanostructured air cathodes63-64 and solid-state electrolytes.58,65-66 In the previously reported rechargeable/flexible Zn-air batteries, air cathodes were often fabricated with preformed electrically conductive substrates using complicated synthetic processes. In addition, comprehensive and quantitative analysis of mechanical integrity of the air cathodes under various deformation modes has been rarely reported.
Along with the above-mentioned air cathodes, advanced Zn anodes should be also developed in order to realize reliable rechargeable and flexible Zn-air batteries, however, little attention has been paid so far. From an electrode architecture point of view, conventional Zn anodes are fabricated by coating electrode slurries on metallic current collectors58,63 or directly using Zn metal foils.64,67-68 However, the slurry-cast and Zn metal foil electrodes show low CE because of non-uniform shape change of Zn during cycling.69-70 Especially 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.
Further, those electrodes are not sufficiently mechanically robust to withstand external deformation because of their fatigue failure.71-72 Upon repeated deformation, they often suffer from structural degradation, eventually losing their electrical conductivity.
23
Here, a new class of flexible/rechargeable Zn-air batteries based on multifunctional HM architecture is proposed to address the above-mentioned challenges. The HM framework-supported electrodes are fabricated by one-pot concurrent electrospraying (for electrode active powders/single-walled CNTs (SWCNTs)) and electrospinning (for PEI nanofibers). The feasibility of the concurrent electrospraying/electrospinning process was previously explored for the fabrication of LIBs electrodes.73 In this study, the PEI is chosen due to its exceptional chemical resistance against alkaline electrolyte solution,74 and intimate intermolecular interaction with CNTs.75
Zn powders (for the anodes) and rambutan (a type of tropical fruit)-shaped Co3O4/multi-walled CNTs (MWCNTs) 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 a 1D binder to hold the Zn particles (in the Zn anode) and rambutan-shaped Co3O4/MWCNT composite powders (in the air cathode); (ii) construct 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.
2.1.2. Experimental
Fabrication of HM Zinc 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 and NMP. The Zn powders (average particle size
= 7 mm, 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 rinsing with distilled water and drying in a vacuum oven at 60°C, the self-
24
standing HM Zn anode (thickness = 140 mm) 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 mm). 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 mm), which was seamlessly integrated with the above-prepared air catalyst layer to create the self-standing HM air cathode.
Fabrication and Electrochemical/Mechanical Characterizations of Zinc-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 mm). The film side adjacent to the air cathode had holes (diameter = 5 mm) to allow air passage to the air cathode. The galvanostatic charge/discharge cycling tests were conducted, 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).76 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 charge/discharge profiles was conducted.
Structural/Physicochemical/Electrochemical Characterizations of HM Electrodes
The morphologies of the HM electrodes were characterized by field emission scanning electron microscopy (FE-SEM) (S-4800, HITACHI) and X-ray spectroscopy (EDS) (JSM 6400, JEOL). The composition ratio of the HM electrodes was determined from TGA (SDT Q600, TA Instruments) under an air atmosphere (heating rate = 5 °C min−1). The structural stability of the ball-milled Co3O4 and
25
MWCNTs in the R-CC powders was identified using X-ray diffraction (XRD, D8 Advance, Bruker) measurements at 40 kV and 40 mA (CuKα radiation, λ = 0.154056 nm) and Raman spectroscopy (Alpha300R, WITec) with an excitation wavelength of 532 nm. The sheet resistances of the HM electrodes were measured using a four-point probe technique (CMT-SR1000N, Advanced Instrument Technology). The air permeability, i.e., the time required for air passage, of the GDL in the HM air cathode was estimated by a Gurley densometer (4110N, Gurley). The electrochemical catalytic activities (ORR and OER) of the air catalysts were investigated using a potentiostat (VMP3, Biologic Co.) with a RRDE (RRDE-3A, ALS Co.) in a 0.1 M KOH electrolyte solution (pre-saturated by oxygen or nitrogen before the ORR and OER measurements, respectively),77 and a platinum wire and a Hg/HgO electrode were employed as the counter and reference electrodes, respectively. For the working electrode, the catalyst ink was cast onto a pre-polished glassy carbon (GC) disk electrode. The catalyst ink was prepared by dispersing the catalyst powders and a carbon black additive (Ketjenblack EC- 600JD, AkzoNobel) in an ethanol/IPA mixture containing a 5 wt% Nafion solution.78 During the measurements, the disk electrode was rotating at a speed of 1600 rpm.
Preparation and Characterization of PVA/PAA GPEs
The PVA/PAA (7/3 (w/w)) solution (concentration = 7.5 wt%, solvent = water) was prepared. The obtained polymer solution was poured into a silicone mold on a Teflon plate and slowly dried at room temperature. The resulting PVA/PAA cast film was annealed at 140°C for 3 h in a vacuum oven to allow the crosslinking reaction,76 yielding a crosslinked PVA/PAA film with a thickness of ~ 30 mm. The crosslinked PVA/PAA film was immersed in a 6 M KOH solution for 1 h to obtain an ion-conductive GPE film. The crossover of the Zn(OH)42– ions through the GPE film was quantitatively monitored as a function of elapsed time using inductively coupled plasma atomic emission spectrometry (ICP-OES, Varian).76
2.1.3. 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 6a. The morphological analysis showed that the Zn powders were densely packed in the HM frameworks (consisting of highly interconnected PEI nanofibers and SWCNTs) over wide areas (Figure 6b), 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
26
with alkaline electrolyte. These results exhibited the successful formation of 3D bicontinuous ion/electron transport pathways in the HM Zn anode.
Figure 6. (a) Schematic representation depicting the concurrent electrospraying/electrospinning-assisted fabrication of the HM Zn anode, along with its photograph. (b) Surface SEM images of the HM Zn anode.
Insets show the EDS images of the Zn elements (represented by blue dots).
From the thermogravimetric analysis (TGA) profiles and associated calculation, the Zn content in the HM Zn anode was estimated to be 92 wt% (Figure 7). The other anode components (PEI and SWCNT) were completely removed above 600 °C (Figure 7b), which indicated that the weight increase above 600 °C in Figure 7a corresponds to the formation of Zn oxide (ZnO).79 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).
Figure 7. TGA profiles of the (a) HM Zn anode and (b) PEI/SWCNT mixture mat. (c) Calculation used to estimate the Zn content in the HM Zn anode.
Co3O4 is widely used as a bifunctional ORR/OER catalyst material, however, it suffers from low electrical conductivity.56,80-81 To overcome this limitation, adding carbon black additives into air
27
cathodes or modifying the cathodes with conductive agents56,82 have been suggested. Here, the rambutan-shaped, micron-sized Co3O4/MWCNT composite powders (i.e., R-CC powders) was synthesized by ball-milling pristine Co3O4 powders (an inset of Figure 8a) and MWCNTs with a mass ratio of 10/0.05 (= Co3O4/MWCNT). The obtained R-CC powders are MWCNT-embedded secondary Co3O4 particles (Figure 8a). The microstructure of the R-CC powders was further elucidated. No significant difference in the characteristic XRD patterns (assigned to spinel Co3O4 structure83) was observed between the pristine Co3O4 and R-CC powders (Figure 8b). Furthermore, the Raman peaks (Figure 8c) reveal that the D/G ratios84 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.78,85 Note that the R-CC powders showed the superior ORR performance than the pristine Co3O4 powders without deteriorating oxygen evolution reaction (OER) activity (Figure 8d), representing the results identical to that of previous studies.
Figure 8. 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.
28
Figure 9. (a) Schematic illustration of the concurrent electrospraying/electrospinning-assisted fabrication of the HM air cathode composed of catalyst layer and GDL. (b) Schematic illustration (top) and photograph (bottom) of the bi-layered HM air cathode, wherein the catalyst layer was seamlessly integrated with the GDL. (c) Cross-sectional SEM (left) and EDS (right) images of the HM air cathode. (d) Surface SEM image of the catalyst layer (left) and GDL (right). Inset shows the high-magnification surface SEM image of the catalyst layer.
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 GDL.55,86-
88 Conventional air cathodes are fabricated by casting catalyst pastes or suspensions on porous current collectors such as carbon paper and metallic foam.86,89 However, this cathode structure is vulnerable to structural disruptions (i.e., detachment of catalyst materials and delamination between the catalyst layer and GDL) upon electrochemical charge/discharge and mechanical deformation. Herein, the HM air cathode based on seamless integration of a catalyst layer and a GDL was fabricated. The detailed fabrication procedure for the HM air cathode is illustrated in Figure 9a. The catalyst layer of the HM
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air cathode was fabricated through concurrent electrospraying (for the R-CC powders/SWCNTs) and electrospinning (for the PEI nanofibers) process. On top of the catalyst layer, a GDL was continuously fabricated using the same concurrent electrospraying (for SWCNTs) and electrospinning (for the PEI nanofibers) process, resulting in a bi-layered air cathode (Figure 9a). The schematic of the air cathode and its components is shown in Figure 9b. The aforementioned electrospraying/electrospinning process allowed for seamless unitization of the catalyst layer with the GDL in the HM air cathode (Figure 9c).
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. A composition ratio of the Co3O4/(SWCNT + MWCNT)/PEI was estimated to be 81/11/8 (w/w/w) from the TGA measurement and the selective PEI removal results in the same manner as in HM Zn anode.
Figure 9d 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. As a result, facile air passage (Gurley value = 8 s 100 cc–1) was allowed through the GDL. The each PEI nanofibers were coated with the SWCNTs due to the strong intermolecular affinity based on the p-p stacking interactions,75 thus acting as electron conductive channels (@ 10 W □–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.90 The water contact angles (Figure 10) 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.
Figure 10. Water contact angle; (a) catalyst layer and (b) GDL.
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Figure 11. Bending (radius = 5 mm, strain rate = 200 mm min−1) and twisting (rotation angle = 180°, strain rate = 30° s−1) deformation cycle of HM electrodes; (a) and (b) HM Zn anode, (c) and (d) HM air cathode.
Insets are photographs of the HM electrodes (after 1000 bending and twisting cycles, respectively) and the mechanically ruptured control electrodes (Zn foil anodes and MEET air cathodes).
To evaluate the structural stability, the mechanical flexibility of the HM Zn anode and air cathode was quantitatively investigated under various deformation modes. The change in the electrical resistance of the HM electrodes (Zn anode and air cathode) 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 11) for 1000 cycles, a negligible change was observed in the electrical resistance of the HM electrodes, indicating that the structural stability of the HM electrodes was maintained after 1000 bending cycles. In comparison, the control electrodes (Zn foil and commercial MEET air cathode) mechanically ruptured in the early deformation stage (prior to 200 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 structural stability.
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 of the deposition of unwanted, irreversible ZnO on
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air cathodes.26,37,91 In this work, a thermally crosslinked PVA/PAA GPE film (thickness = 28 mm, Figure 12a) was prepared as a permselective membrane to outperform the commercial polyolefin separator
Figure 12. (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 charge/discharge cycling performance of the Zn-air 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 cut-off). (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).
(Celgard 3501, thickness = 24 mm) that is widely used in alkaline batteries. Details on the thermal- crosslinking reaction of PVA/PAA were reported in the previous work.76 Figure 12b-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 12b). To verify this advantageous effect, galvanostatic charge/discharge 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 12c). 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
