The ERC membrane with the ion size exclusion-based permselectivity opens a new membrane-driven possibility that brings us closer to rechargeable Zn-air batteries. Structural/electrochemical characterization of a Zn-air cell incorporating the electrolyte-raised PVA film (after 1st cycle): (a) a photograph (top part) of the disassembled cell and (b) an FE-SEM image (bottom part) of the PVA membrane inside the cell showing poor structural stability during cell operation; (c) charge/discharge profiles showing the internal short-circuit fault (after 1st cycle). A schematic illustration underlying the transport superiority (ie, size-exclusion-based permselectivity far beyond that available with conventional microporous polyolefin separators) of ERC membrane as an alternative separator membrane for rechargeable Zn air cells.
Principle of Zn-air batteries
Recently, Zn-air rechargeable batteries have been researched for portable and EV applications, but commercial applications are still hampered by several problems, including non-uniform Zn dissolution, ZnO precipitation, Zn anode dendrite formation, and bifunctionality of insufficient (oxygen reduction reaction). ORR) and also oxygen evolution reaction (OER)) air cathode. Microporous polyolefinic membranes can be commonly used as separators in a Zn-air battery, such as polypropylene (PP) such as Celgard 4560, Celgard 5550 (Figure 1.2a, b)12. Wu and co-workers suggested that sulfonation of nonwoven polyolefin spacers improves their hydrophilicity and as a result increases the ionic conductivity of a spacer (Figure 1.3c, d). 27–38 mW cm−2.
Introduction
These newly formed ZnO powders at the air cathode can act as an inert resistive layer to ion/electron conduction and thereby cause capacity loss by cycling.2, 4-5 Considering these electrochemical reactions both at the anode and at the cathode, the phenomena of ion transport through the separation membranes must be carefully controlled in search of suppression of Zn(OH)4. Although polyolefin separator membranes have many favorable attributes (including electrical insulation, chemical/electrochemical stability, and high ionic conductivity) suitable for use in Zn-air batteries, they fail to prevent soluble Zn(OH)4. Furthermore, microporous polyolefin separators have limitations in slowing down the growth of Zn dendrites, which can eventually reach the counter-electrode through electrolyte-filled pores, possibly causing internal short-circuit failure of the cells.
In addition, an anion exchange membrane based on polysulfonium has been reported as a single ion conducting membrane,19 however experimental data on the suppression of Zn(OH)4. However, in a strongly basic state, Zn2+ ions react immediately with OH ions, resulting in the formation of Zn(OH)4. Here, as a facile and scalable strategy to address the aforementioned separation membranes and electrochemical performance of Zn air cells.
A basic architecture of the ERC membrane was inspired by porous substrate-reinforced ionic conductors for use in proton exchange membrane fuel cells (PEMFCs).24-26. Compared to typical electrospun fiber materials such as polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN) and polyethylene terephthalate (PET), PEI exhibits excellent chemical resistance to strong basic electrolytes used for Zn-air batteries. These unique structural/physicochemical features of the ERC membrane (i.e., the combined effect of PEI nanomat and PVA GPE) enable the suppression of Zn(OH)4.
To our knowledge, this is the first report showing the unprecedented advantages of separator membranes (in the form of ion size exclusion-based permselectivity), which act as a kind of pseudo-electrochemically active cell component that facilitates the development of rechargeable Zn-air batteries.
Experimental
- Structural design and fabrication of ERC membranes
- Structural/physicochemical characterization of ERC membranes
- Characterization of transport phenomena in ERC membranes
- Electrochemical characterization of rechargeable Zn-air batteries
The morphological analysis of membranes was performed by a field emission scanning electron microscope (FE-SEM, FEI) equipped with an energy dispersive spectrometer (EDS). The functional groups of membranes were elucidated using an FT-IR spectrometer (ALPHA-P, Bruker) with a spectral resolution of 4 cm-1. The electrolyte uptake (ΔW (%) = [(Wwet − Wdry)/Wdry] × 100) of membranes was analyzed by measuring the weight difference of membranes before/after immersion in the 6M KOH electrolyte solution at room temperature as a function of soaking time .24-25 Next, the electrolyte uptake-induced dimensional change of membranes was also estimated using the equation of ΔA (area-based, %) = [(Awet − Adry)/Adry] × 100 and ΔV (volume-based ) , %) = [(Vwet − Vdry)/Vdry.
The ionic conduction of (KOH-electrolyte-swollen) separation membranes was investigated using an AC impedance analysis (VSP classic, Bio-Logic) over a frequency range of 1 - 106 Hz at room temperature. For this measurement, a unit cell (2032 coin type) was fabricated by simulating a coin cell configuration used for traditional lithium-ion batteries, 14, 29 with the separator membrane sandwiched between two stainless steels and serving as a kind of blocking electrode. Figure 2.2a). In the right ventricle, samples were taken regularly at a predetermined time interval (= 12 hours) and then the time-dependent concentration variation of Zn(OH)4.
A Zn air cell was fabricated using rectangular shaped acrylic plastic (bottom cap) and stainless steel (top cap) housing containers (dimension (length x width) = 6.5 x 5.0 (cm x cm)).22 Assembly diagram of the Zn-air cell air cell was shown in Supporting Information Figure 2.2b. The stainless steel plate with 38 small holes (diameter = 0.1 cm) was used as a top cover to allow air to enter the cell. Here, to minimize evaporation-induced water loss in the electrolyte solution without hindering air infiltration into the cells during cell operation, a pair of hydrophobic three-layer (polypropylene/polyethylene/polypropylene) microporous separation membranes (Celgard 2320) were placed between the air cathode. and top cover.
The performance of the cell was examined using a potentiostat/galvanostat (Classic VSP, Bio-Logic) at room temperature, where the cell was discharged and charged at a constant current density of 10 mA cm-2 under a voltage range of 0.0 ( charge cut-off) - 2.0 V (charge cut-off).22, 35 After reaching the charge cut-off voltage, cells were subjected to constant voltage (CV) mode for 5 h before resuming the discharge reaction.
Results and discussion
Fabrication and structural/physicochemical characterization of ERC membranes
The structural uniqueness of the ERC membrane was further elucidated by analyzing the FT-IR and XRD profiles. As a result, the KOH-treated PEI nanomat facilitated the impregnation of polar PVA solution (Figure 2.6b). The comparison of XRD profiles between the samples (Figure 2.5b) is another evidence to demonstrate the presence of PEI nanomat and PVA in the ERC membrane.
The ERC membrane presents a higher tensile modulus than the PVA film and Celgard3501, although its tensile strength at break is lower than that of Celgard3501. In particular, the comparison of the tensile properties between the ERC membrane and the PVA film highlights the beneficial effect of the PEI nanomat as a mechanically reinforced framework. Structural analysis of ERC membrane (focusing on PEI nanomat and PVA matrix): (a) FT-IR peaks; (b) XRD profiles.
Tensile properties of ERC membrane, PVA film and Celgard3501: (a) stress-strain curve;. b) summary of the most important tensile properties of the membranes. In contrast, the significant improvement in dimensional change (ΔA ~ 0 % & ΔV ~ 50 %) and also chemical tolerance was observed with the ERC membrane (Figure 2.10c) due to the presence of PEI nanosize (PEI film itself barely absorbed liquid electrolyte ) (ΔW ~ 0 %, Figure 2.8)), showing its chemical inertness against KOH electrolyte solution). This result demonstrates the beneficial effect of the PEI nanosize as an effective reinforcing framework on structural/dimensional stability of the ERC membrane.
The volume change (ΔV ~ 50%), which arises exclusively from the swelling in the thickness direction, can be attributed to the asymmetric morphology of the ERC membrane (Figure 2.3c). Further studies will be conducted to improve the volume expansion and chemical tolerance of the ERC membrane. Consistent with the thymol blue indicator results, the ERC membrane significantly prevents Zn(OH)4.
Advantageous effects of permselective ERC membranes on electrochemical performance of Zn-air cells
Structural information on Zn powder (used for Zn gel anode) and air cathode of Zn-air cells. A noticeable difference between the ERC membrane and Celgard3501 was found in the discharge capacity in the 2nd cycle (Figure 2.15b,e). Future studies will aim to elucidate the effect of membrane structure and properties on the initial discharge capacity of Zn-air cells.
To visualize this beneficial effect of the ERC membrane, the operating time of a pinwheel connected to the Zn-air cell was measured during the 2nd discharge reaction (Figure 2.15c,f). This is another piece of evidence to underline the contribution of the ERC membrane to facilitate the development of rechargeable Zn air cells. Electrochemical performance of Zn-air cells assembled with Celgard3501 ((a)-(c)) or ERC membrane ((d)-(f)): (a), (d) discharge profiles at 1st discharge reaction; (b), (e) charge/discharge profiles after 2nd cycle; (c), (f) photographs showing the operating time of a pinwheel connected to the Zn-air cell.
In comparison, the ERC membrane remarkably suppressed the formation of ZnO powders on the air cathode (Figure 2.16b). This result is consistent with the cycle capacity retention of Zn-air cells (shown in Figure 2.15). The unique functionality of the ERC membrane as an alternative separator membrane for potential use in rechargeable Zn air cells is conceptually illustrated in Scheme 2.1.
This result demonstrates the exceptional selectivity of the ERC membrane based on ion size exclusion, far beyond those achievable with conventional microporous polyolefin separators.
Conclusion
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Acknowledgement
