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Selective Ion Transporting Polymerized Ionic Liquid Membrane Separator for Enhancing Cycle Stability and Durability in Secondary Zinc Air Battery Systems Ho Jung Hwang, Won Seok Chi, Ohchan Kwon, Jin Goo Lee, Jong Hak Kim, and Yong Gun Shul ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07841 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 15, 2016

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ACS Applied Materials & Interfaces

Selective Ion Transporting Polymerized Ionic Liquid Membrane Separator for Enhancing Cycle Stability and Durability in Secondary Zinc Air Battery Systems Ho Jung Hwanga‡, Won Seok Chib‡, Ohchan Kwonb, Jin Goo Leeb, Jong Hak Kimb* and YongGun Shula,b*.

a

New energy and battery engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-ku,

Seoul 120-749, Republic of Korea b

Department of Chemical and Biomolecular Engineering, Yonsei University,262 Seongsanno,

Seodaemun-gu, Seoul 120-749, South Korea

ACS Paragon Plus Environment

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ABSTRACT

Rechargeable secondary zinc air batteries with superior cyclic stability were developed using commercial polypropylene (PP) membrane coated with polymerized ionic liquid as separators. The anionic exchange polymer was synthesized copolymerizing 1-[(4-ethenylphenyl)methyl]-3butyl-imidazolium hydroxide (EBIH) and butyl methacrylate (BMA) monomers by free radical polymerization for both functionality and structural integrity. The ionic liquid induced copolymer was coated on a commercially available PP membrane (Celguard 5550). The coat allows anionic transfer through the separator and minimizes the migration of zincate ions to the cathode compartment, which reduces electrolyte conductivity and may deteriorate catalytic activity by the formation of zinc oxide on the surface of the catalyst layer. Energy dispersive xray spectroscopy (EDS) data revealed the copolymer coated separator showed less zinc element in the cathode, indicating lower zinc crossover through the membrane. Ion Coupled Plasma Optical Emission Spectroscopy (ICP-OES) analysis confirmed over 96% of zincate ion cross over was reduced. In our charge/discharge setup, the constructed cell with the ionic liquid induced copolymer casted separator exhibited drastically improved durability as the battery life increased more than 281% compared to the pure commercial PP membrane. Electrochemical Impedance Spectroscopy (EIS) during the cycle process elucidated the premature failure of cells due to the zinc cross over for the untreated cell and revealed a substantial importance must be placed in zincate control.

KEYWORDS zinc air battery, separator, durability, ionic liquid, anion exchange membrane.


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1. Introduction The degree of progress in portable electronics has been astonishing in last few decades. Such advances fuelled a perpetual search for a mobile energy source that is more efficient, compact and convenient.1 As a result, currently the most proliferated technology of the market is the lithium-ion battery. While effective to a certain extent, it is not without its barriers. Insufficient energy densities, safety concerns and increasing natural resource costs all became motives for persuading researchers to seek for an upgraded power supply. 2 In this context, metal air batteries have recently received much appropriate spotlight. Harnessing almost infinite energy source from air, the system’s superior power density of 1084 Wh/Kg and 5200 Wh/kg for Zn, Li-air respectively, has been one of the key factors for its regained interest.

3

Out of the various

candidates for metal air batteries, zinc has been a leading contender. Relative abundance, minimum impact on environment, low toxicity are aspects that facilitates zinc based system as the next post-lithium technology.3-5 Moreover, the concept is consistent with the scientific field as primary zinc-air batteries already withhold a long history of commercialization dating back from the 1930s.1, 4-5 Despite its undeniable tremendous potential, secondary zinc-air batteries are still in its infant stages of development and thus much of the research has been focused on tackling the system’s lacking recyclability.6-20 The Zn-Air battery system consist of 4 components: the zinc anode, air cathode, electrolyte, and the separator. Extensive efforts has been placed into engineering durable Oxygen Reduction/Evolution Reaction (ORR/OER) air catalysts as the overall performance of the cell is strongly related to such dual-functionality of the cathode.7, 11-12, 14, 16, 18-20 Some setups employed tri-electrode system where the charging and discharging process occur at separate electrodes for


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higher cycle stability.8 The Zn anode also went under rigorous investigations. Morphological changes such as dendrite growth during charging process has been a vital factor in the cycle stability of the cell and thus, were thoroughly modelled and studied.13, 15, 17 While majority of the attention has been bestowed upon the electrodes, not enough consideration has been presented for the electrolyte and the separator. The ideal separator has the characteristics of high ionic conductivity, high electrical resistance, and satisfactory stability in alkaline solutions.

21

Currently, the most widely utilized membrane is the commercialized

microporous polyolefin films. The closely ordered micropores of the separator, which act as a passage for electrolyte flooding through the membrane giving it a high ionic conductivity. 4, 21-22 However, one substantial trade-off that this certain architecture brings is the zincate ion crossover. These free migrating ions diffuse by a concentration gradient occurring from the anode compartment to the cathode compartment. After which the electrolyte of the air electrode is saturated, and eventual precipitation of zinc oxide particles poisons the catalyst surface. To address these drawbacks, separator has to prevent zincate ion transportation from zinc anode to catalytic cathode, maintaining free movement of ions (K+ and OH–) in aqueous electrolyte. However, commercial polypropylene separator penetrates most of flowing materials in electrolyte through pores between layered-fibrous structures. This provokes zincate ion nonselective mechanism in zinc air battery device, leading to reduced cell durability. Dewi et al, addressed these issues by employing membranes with high anionic selectivity which significantly raised the discharge capacity, yet no cyclic stability was considered.6 Kim and coworkers developed a selective separator by impregnating Nafion in to a PVA/PAA polymer mat fabricated by electrospinning. While drastic improvements in the longevity was monitored the mechanism of degradation were not studied in detail.10


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Herein we report a facile method to minimize zincate ion migration by utilizing novel anionic exchanging coating on commercial membranes by copolymerization of ionic liquid monomers including hydroxide group and butyl methacrylate. Ionic liquid, including cation and anion simultaneously in one fluidic chemical as organic salts, has great tremendous physicochemical advantages such as high conductive, non-flammable, environmental-friendly, thermal resistant etc.11, 23-26 Ionic liquids can be tuned by selective combination of cation and anion for widely broad application.24,

27-28

Among them, we considered anion as hydroxide group to transport

KOH electrolyte well, but also cation as imidazolium ring, owing to the resonance effect to decrease charge density of a cation and diminish interaction with the hydroxide ions, with functional end group for polymerization. Pure ionic liquid based anion exchange membrane cannot be obtained, because of their weak physical strength, which is not robust enough to fabricate membrane with high stability. Mechanically endurable and polymerizable chemical monomers, i.e. methacrylate, styrene and acrylonitrile, also have to be induced to develop hydrophobicity in copolymer, which is not soluble in aqueous electrolyte, for anion exchange membrane in zinc air battery. 5, 29-35

2. Experimental 2.1. Materials 1-Buthylimidazole, 4-vinylbenzylchloride, butyl methacrylate (BMA), azobisisobutyronitrile (AIBN), sodium hydroxide (NaOH), dimethylformamide (DMF), acetonitirile (CH3CN), ethyl acetate (EtOAc), 5% Nafion solution were purchased from Aldrich and used as received without further purification. Potassium hydroxide (KOH) and Isopropanol(IPA) were purchased from


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Ducksan and also were used as received without further purification. 20% Pt and Ir on Vulcan were both purchased from Premetek Co. and 2.2. Ionic liquid induced membrane separator fabrication 2.2.1 Synthesis of of 1-[(4-ethenylphenyl)methyl]-3-butyl-imidazolium chloride, EBIC 1-Butylimidazole (125 mmol) was dissolved in CH3CN (30 mL) in a 100-mL round bottom flask equipped with a stir bar. 4-vinylbenzyl chloride (19.5 mL, 139 mmol) was then added, and the reaction was heated at 50 oC while stirring overnight. The reaction was stopped at this time, and the reaction mixture was poured into diethylether (Et2O, 250 mL). The ionic product was precipitated, and the mixture was placed in a freezer for several hours. The Et2O phase was decanted, and the product, EBIC, was dried in an oven overnight.

2.2.2 Synthesis of 1-[(4-ethenylphenyl)methyl]-3-butyl-imidazolium hydroxide, EBIH First, 1-[(4-ethenylphenyl)methyl]-3-butyl-imidazolium chloride was dissolved in deionized H2O (125 mL). The aqueous phase was washed with EtOAc (3 x 100 mL). Sodium hydroxide (NaOH, 125.0 mmol) was added to the aqueous phase, and an oily liquid was observed to separate immediately. The mixture was stirred for 1 h. The oily phase was extracted into EtOAc (250 mL) and washed with deionized H2O (3 x 100 mL). The organic phase was dried over anhydrous MgSO4 and the remaining solvent was completely removed under vacuum at 40 oC overnight to obtain the final product, EBIH. The yield was approximately 80%.

2.2.3 Synthesis of PEBIH-PBMA copolymer The two monomers EBIH (1 g) and BMA (2 g) were dissolved in 8 ml of DMF, followed by the addition of 1 mg of AIBN. A tube containing the monomers was tightly sealed, purged with


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nitrogen gas and immersed in an oil bath at 90 oC for 24 h. The resulting solution was precipitated in diethyl ether. The final product was dried in a vacuum oven overnight.

2.2.4 Fabrication of PEBIH-PBMA copolymer thin film The PEBIH-PBMA copolymer was dissolved in ethanol at a 20 wt% concentration for high viscous solution preparation. Then, the solution was coated on the polypropylene (Celgard 5550) separator membrane using the RK Control coater (Model 101, Control RK Print-Coat Instruments Ltd., UK). After drying the membrane at 50 oC completely overnight, the membrane was cut with 1 × 1 cm2 active area for separator in zinc air battery.

2.3. Electrochemical analysis 2.3.1 Air cathode fabrication Pt and Ir carbon composite was chosen as the cathode catalyst as it is the readily selected benchmark materials respectively, for ORR and OER in previous literatures.8,

16

Also, its

commercial availability was taken into account for its qualification as the standard catalyst material. 20% Pt/C and Ir/C were mixed equal weight parts to form a bi-functional catalytic composite. 0.9 g of catalyst on Vulcan was dispersed in 100 ml of isopropanol with 6.5 ml of 5wt% Nafion by ultrasonication of 1 hr. The catalyst ink was spray coated on to a fibrous carbon paper to achieve a metal loading of 1 mg/cm2. The coated carbon paper was further dried in a 100 oC oven for 30 mins.

2.3.2 Zn-air battery test


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Battery cycle test were performed by a homemade cell as depicted in Figure S1. For the anode, a pure zinc plate was cut and fitted to match the cell specifications along with all the other components including the separator and the cathode. After the battery was securely constructed, it was loaded with 1 ml of 6 M KOH electrolyte in both compartments separated by the membrane prior to test. The design of the cell is specified for an active cathode area of 1 cm2. No additional oxygen other than that of natural air had been introduced, while high humidity was maintained to minimize electrolyte evaporation during the cycling. All cycle test were performed using a standard battery cycler system (WBCS3000, WonATech, Seoul, Korea). Versatile multichannel potentiostat system (VMP3B-10, Bio-logic, Claix, France) was employed for Impedance data acquisition.

Electrochemical Impedance Spectroscopy (EIS) test were

conducted with the fully assembled cell both prior and after the discharge-charge experiment. The EIS data were obtained at 1.0 V vs Ref at the frequency range of 0.1 Hz ~ 0.1 MHz with 6 points per decade. While the applied AC amplitude was 14.14 Vrms. The spectra was fitted with the EC-Lab software version 10.18. The cell was later decomposed to recollect the separator and the electrolyte for further characterization

2.4. Characterization FT-IR spectra of the samples were collected using an Excalibur Series FTIR (DIGLAB Co., Hannover, Germany) instrument between the frequency ranges of 4000–400 cm-1 using an ATR facility. 1H NMR (600 MHz high-resolution, Avance 600 MHz FT NMR spectrometer, Bruker, Ettlingen, Germany) measurements were performed to analyze the copolymer composition. The thermal behavior of the copolymer was analyzed using a differential scanning calorimeter (DSC 8000, Perkin Elmer, USA) operated at a heating rate of 10 °C/min in air. The polymer was


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heated from –80 °C to 200 °C and then recooled to –80 °C, and the sample was again heated from –80 °C to 200 °C as second sequence. The second scanning data were used to determine the thermal transition of the copolymer. TGA was conducted with a simultaneous DTA/TGA analyzer by TA instruments (USA) at a heating rate of 10 °C/min under an air atmosphere. WAXS analysis was carried out on a Rigaku 18 kW rotating anode X-ray generator with Cu-K radiation (λ=1.5405 nm) operated at 40 kV and 300 mA. The 2-theta angular region between 5° and 80° was explored at a scan rate of 4°/min with grazing incidence mode at incidence angle = 0.5o. SAXS data were obtained with the 4C SAXS II beamline at the Pohang Light Source (PLS), Korea. TEM pictures were obtained from a JEOL (JEM1010, Japan) microscope operating at 80 kV. A field emission scanning electron microscope (FE-SEM, JSM-7001F, JEOL Ltd., Japan) was used to characterize the cross-section morphology of the polymer separator membranes both after and before the battery cycle test. The EDS analysis was further performed using the same equipment. The recollected electrolytes were tested for metal contents by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES, PerkinElmer, Optima 8300).

3. Results and discussion

A

B


9


C

O O

OH N N

CH2CH2CH2CH3

H3C

BMA

EBIH

y

x O O

AIBN, DMF 90oC, 1day OH N N

CH2CH2CH2CH3

CH3

PEBIH-PBMA

Figure 1. (A) Synthesis of 1-[(4-ethenylphenyl)methyl]-3-butyl-imidazolium chloride, EBIC, (B) Synthesis of 1-[(4-ethenylphenyl)methyl]-3-butyl-imidazolium hydroxide, EBIH, (C) Synthesis of PEBIH-PBMA copolymer.

B

A

1560

Absorbance (a.u.)

Absorbance (a.u.)

3380

1-butylimidazole

4-vinylbenzyl chloride

EBIC

1630

EBIH 1717 1638

BMA

1721 3392

1560

1630

PEBIH-PBMA

EBIH 4000

3000

2000

4000

1000

3000

-1

2000

1000 −1

Wavenumber (cm )

Wavenumber (cm )

C

D 100

DMSO

y

x

PEBIH-PBMA

O

80

Weight loss (wt%)

O

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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OH N N CH3

CH2CH2 CH2CH 3

b

a

a

BMA:EBIH = 7:3

b

H2O

60

40 BMA 20 EBIH 0

8

7

3

2

Chemical shift (ppm)

1

0

200

400

600

800

o

Temperature ( C)


10

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Figure 2. Chemical characterization of synthesized copolymer (A) FT-IR spectrum of EBIC and EBIH, (B) FT-IR spectrum of PEBIH-PBMA copolymer, of PEBIH-PBMA copolymer , (C) NMR of PEBIH-PBMA copolymer, (D) TGA of PEBIH-PBMA copolymer.

The ionic liquid (IL) monomer, EBIC was synthesized as a step 1 as illustrated in Figure 1A as previous reports36-37. After synthesis of EBIC, the chloride anions were converted to hydroxide anions by ion-exchange with NaOH salt in an aqueous solution for an hour as a step 2 to prepare EBIH (Figure 1B). The FT-IR spectra of the starting materials (1-butylimidazole, 4vinylbenzyl chloride), intermediate (EBIC) and final IL product (EBIH) were measured and were displayed in Figure 2A. A sharp peak at 1560 cm–1 was observed in EBIH, attributable to the C=N stretching vibration of the imidazole ring. Moreover, a weak peak at 1630 cm–1 was appeared, owing to vinyl group for feasible polymerization producing random copolymer with other functional chemicals. As compared with EBIC, EBIH exhibited broad absorption bands at 3380 cm–1, assigned to the O-H stretching vibration of hydroxide anion, indicating successful ion exchange for OH– ion conductor. The synthesis of PEBIH-PBMA copolymers via free radical polymerization with AIBN initiator was illustrated in Figure 1C as a step 3. To control copolymer solubility, double weight amount of BMA, compared to EBIH, was introduced in polymerization reaction. The FT-IR spectra of EBIH, BMA monomers and PEBIH-PBMA copolymer were presented in Figure 2B. The weak absorption band at 1560 cm–1 was observed attributable to C=N stretching vibration mode of imidazole group in EBIH. Also, the broad absorption band of O-H peaks was shifted from 3380 cm–1 to 3392 cm –1 after polymerization resulted from strong interaction between hydroxide ions by addition of BMA monomers. The sharp absorption peak at 1721 cm–1 was found, because of C=O stretching bond in BMA. The


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C=C stretching vibration bonds at 1630 cm–1 in EBIH monomer completely disappeared in absorption band of PEBIH-PBMA, which demonstrates successful polymerization. The successful copolymerization of PEBIH-PBMA also confirmed using

1

H NMR

spectroscopy to understand accurate monomers grafting weight ratio. The 1H NMR spectra of PEBIH-PBMA was presented in Figure 2C. The strong peak at … ppm denoted as “a” was assigned to the three protons in the end methyl group in butyl chain, whereas relative lower intense peak at … ppm named as “b” was owing to imidazole ring bonding. The actual mass composition of PEBIH-PBMA copolymer was calculated by comparing integral areas of peak “a” with peak “b”. As a result, grafting degree of PEBIH-PBMA copolymers was determined as 3:7, which had high BMA mass content in copolymer. However, when it was recalculated with molecular weight, PEBIH-PBMA contained comparable mole ratio, nearly 1:1. TGA analysis is powerful tool to figure out PEBIH-PBMA copolymer thermal stability. In Figure 2D, the PEBIH-PBMA copolymer was decomposed from 50 oC to 800 oC at 10 oC/min temperature rising rate. First, water molecules, which interacted with OH– ions in EBIH, were detached from PEBIH-PBMA copolymer at temperature range from 50 oC to 200 oC. Weakly bound water in PEBIH-PBMA copolymer evaporated at low temperature from 50 oC to 100 oC, whereas strongly bound water evaporated at high temperature from 100 oC to 200 oC. The BMA chain mainly degraded from 200 oC to 400 oC, whereas EBIH chain decomposed from 400 oC to 600 oC. This is due to high thermal durability of ionic liquid resulted from strong ionic interaction. The PEBIH-PBMA copolymer was completely decomposed up to 600 oC at air atmosphere.


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A

B -0.15

Heat flow (W/g)

PEBIH-PBMA

o

36 C

-0.30

o

85 C

-0.45 0

50

100

150

o

Temperature ( C)

D

C 2000

100

31.4nm

log(I(q))*q (a.u.)

1500

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

10

500

0 10

20

30

40

50

1 0.0

0.1

0.2

o

2 theta ( )

0.3

0.4

0.5

0.6

0.7

-1

q (nm )

Figure 3. Microphase-separated structure analysis of PEBIH-PBMA copolymer (A) DSC analysis, (B) TEM image, (C) XRD pattern (D) SAXS analysis.

Glass transition temperature (Tg) of PEBIH-PBMA copolymer was determined by DSC measurement. The DSC curve of PEBIH-PBMA was depicted in Figure 3A. Two Tgs were observed, where at 36 oC from EBIH chain and 85 oC from BMA chain, respectively. The presence of two distinct Tgs explained microphase-separated structure of PEBIH-PBMA. The TEM analysis of PEBIH-PBMA copolymer was investigated by 0.5 wt% polymer solution in ethanol casting onto copper grid. Figure 3B displayed microphase-separated morphology of PEBIH-PBMA due to differences in electron density between EBIH and BMA. Dark regions represented EBIH chains arose from high electron density of ionic liquids, whereas bright regions indicated BMA chains.


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The XRD pattern of PEBIH-PBMA copolymer through gazing incidence mode to enhance low peak intensity was exhibited in Figure 3C. The PEBIH-PBMA showed broad amorphous peak at a 2θ values of approximately 19.8 o, indicating d-spacing value as 4.49 Å. SAXS measurement was conducted for PEBIH-PBMA copolymer to understand intra-domains spacing value, which demonstrates microphase-separated morphology (Figure 3D). Mostly, block copolymers was analysed to figure out polymer morphology structure through SAXS research from the scattering vector ratios of q/q*, where q is for the higher order peaks with respect to q* (the primary peak), e.g., q/q* = 1, 2, 3, 4… for lamellar, q/q* = 1, 3 , 4 , 7 , 9 ... for cylindrical, and q/q* = 1, 2 , 3 , 4 , 5 ... for spherical morphologies.38-40 However, broad one peak position with q values of 0.18 nm-1 was observed in Figure 3D. This revealed disordered structure between PEBIH domain and PBMA domain, because PEBIH-PBMA was randomly polymerized via free radical polymerization. The PEBIH-PBMA appeared 34.9 nm intra-domains spacing average value, which was shown PEBIH dark region, as confirmed by microphase-separated TEM image (Figure 3B). This microphase-separated morphology would provide efficient pathways, where OH– ion facilitated transport comfortably.


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–

OH 2-

Zn(OH)

4

Figure 4. Schematic sketch of ion selectively penetrating configuration in zinc air battery.

Figure 5. Water contact angle for unmodified PP(A) and PEBIH-PBMA coated PP(B).

Contact angle

0sec

1sec

2sec

3sec

4sec

5sec

Bare PP separator

100º

96º

90º

72º

63º

60º


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PEBIH-PBMA coated PP separator

131º

131º

131º

131º

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131º

131º

Table 1. Summary of water contact angle for unmodified PP separator and PEBIH-PBMA coated PP separator.

The PEBIH-PBMA coated separator has to distinctively ion transport, which zincate ion would be prohibited and OH– was rapidly transported without difficulty during the charge/discharge process (Figure 4). The PEBIH-PBMA copolymer retained high soluble capability corresponding to OH–, because of their essential component, ionic liquid as ion carrier. Contact angle between the separators and KOH (6M) aqueous electrolyte was conducted to characterize how the electrolyte could easily penetrate through separator as shown in Figure 5 and the parameters are summarized in Table 1. Unmodified polypropylene film obviously presented fast penetration of electrolyte, owing to their excessively large-sized pore structure between randomly arranged polymer fiber, where electrolyte flows freely. First, when aqueous electrolyte dropped onto polypropylene film, contact angle between separator and electrolyte was 100o. However, electrolyte was infiltrated into porous polypropylene separator as time goes by, which 40o contact angle value was reduced after 5 seconds. Contact angle could not be clearly detectable after 6s as it was almost penetrated and has low contact angle. Upon coating PEBIHPBMA copolymer onto polypropylene separator, contact angle was maintained as 131o even after several minutes on account of evenly distributed PEBIH-PBMA copolymer, which was not soluble in aqueous solution. Based on this optional ion transportation membrane, just OH– ion can be facilitated.


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Items

OCV

Initial Energy efficiency

Final Energy Coulomb Efficiency efficiency

Cycle

Unit

V

%

%

%

#

Bare PP separator

1.44

59.4

51.2

99.8

37

PEBIH-PBMA coated PP separator

1.45

60.8

41.8

99.9

107

Table 2. Summary of discharge and charge test results.

In order to examine the effectiveness of the PEBIH-PBMA coated separator on the cell longevity, constant current cycle method was employed. While a battery system’s durability can be considered in multiple aspects and may regard different components of the total system, the result of a stronger durability can be witnessed simply and essentially by a prolonged operation time. Each cycle lasts 1 hr, consisting of a 30 min, 10 mA discharge and a 30 min 10 mA charge step. Terminal voltages of 2.3 V and 0.8 V were set to ensure no excess overvoltage were applied to the cell. Both the constructed cells showed similar initial properties. Only miniscule differences of 0.1 V in OCVs and 1.4% in first cycle energy efficiency were exhibited. The details of the initial battery properties are summarized in Table 2.


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A 2.5

B

2.5

2.0

Voltage (V)

Voltage (V)

2.0

1.5

1.5

1.0

1.0

0

10

20

30

40

0

25

50

Time (hr)

75

100

125

Time (hr)

D

70

65

Energy Efficiency (%)

C Energy Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

55

50

45

65

60

55

50

45

40 0

10

20

30

0

25

Cycle number (#)

50

75

100

Cycle number (#)

Figure 6. Discharge and charge polarization curves and energy efficiency plots of unmodified (A,C) and PEBIH-PBMA coated PP (B,D) separator under the constant current mode at 10mA/cm2.

Despite such comparable early characteristics, the durability of the coated membrane cell remarkably surpassed that of the pristine separator. The unmodified separator lasted 37 cycles while the PEBIH-PBMA coated membrane survived for 104 cycles which is a significant increase of over 281% in battery durability (Figure 6). This result implies that the minimization of the zincate ion cross over results in an improvement of the battery life and in turn becomes a key factor for extending resilience of the system. The final energy efficiency of the cell was 41.8% for the copolymer coated separator and 51.4% for the pure commercial separator. Higher terminal energy efficiency of the unmodified membrane cell is a strong suggestion that the cell


18

Page 19 of 34

has prematurely deceased before the actual degradation of the air cathode catalyst. This can be interpreted with closer examination of the cycle graphs A, B in Figure 6. Apart from the radical contrast of cyclic stability of the two cells, although the coated membrane displays equal overpotential increase during both the discharge and charge steps, the pristine separator does not. While ORR characteristics remain invariant, only the OER process exhibit difficulties portrayed as overpotential growth, resulting the above mentioned higher terminal voltage. The OER catalyst is expected to be more sensitive toward zincate ion exposure. The water formation during the OER process consequences in a decreased KOH level which lowers the solubility of the zincate ion at the surface of the catalyst thus facilitates the formation of the zinc oxide.

30000

Zn Concentration (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PP w/ PEBIH-PBMA PP 20000

10000

0 0

50

100

Cycle Number (#)

Figure 7. Zinc concentration versus cycle graph.

Bare PP separator Cycle

Anode

Cathode

PEBIH-PBMA coated PP separator Anode

Cathode

mg/L 10

20901.238

16488.801

19352.144

1345.703


19


20

28583.682

23172.490

16457.380

2112.411

37

18170.626

27631.816

19083.224

3844.960

80

-

-

30938.924

9874.711

107

-

-

46594.352

12688.074

Table 3. ICP analysis Zn content of the electrolyte solution after cycling test.

In order to monitor the effectiveness of the coated membrane, ICP analysis was used to monitor the zinc concentration levels of the cathode electrolyte. Electrolyte samples were taken subsequently at 10, 20, 37th cycle for the unmodified commercial separator and 10, 20, 37, 80, 104th cycle for the PEBIH-PBMA coated separator. Although the anode compartment zinc level does not vary drastically according to the separators, given in Figure 7 and Table 3, the cathode zinc concentration represents extreme reduction of cross over for the coated separator. At the 37th cycle, for the cathode compartment of the coated separator, only 13.9% of zincate ion was present compared to the uncoated counter. This indicates that the zincate ion level of the cathode electrolyte holds significant influence on the overall durability of the battery.

B 14

14 Before 10 cyc 20 cyc Full

12

12 10

-Im(Z) (Ohm)

10

150

Before 10 cyc 20 cyc 37 cyc 80 cyc

8 6 4

Before 10 cyc 20 cyc 37 cyc 80 cyc full

100

-Im(Z) (Ohm)

A

-Im(Z) (Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 34

8

50

0 0

50

100

150

Re(Z) (Ohm)

6 4 2

2

0

0 0

2

4

6

8

10

12

14

0

2

4

Re(Z) (Ohm)

6

8

10

12

14

Re(Z) (Ohm)


20

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Cycle Impedance spectra of the unmodified (A) and PEBIH-PBMA coated (B) separator

Comparative impedance spectroscopy measurements were performed at same cycles accordingly with the ICP sample acquirement. Five element, equivalent circuit similar to previous reports have been proposed. 7, 41-42 R1 is the sum of the contact resistances, arising from the electrolyte and various battery components. R2 is given by the solid and electrolyte interface resistances, including the separator and the electrolyte boundary. R3 is associated with the charge transfer resistance of the air cathode during the electrochemical reactions. The parameter results are summarized in Figure 8. As presented in the figures, clear tendencies of the resistance for both the cells can be witnessed. The definite values of R1 and R3 for the PEBIH-PBMA coated separator is larger than that of the pristine separator. This is due to the reduced ionic conductivity of the membrane from the copolymer, blocking majority of the micropores. The anionic coating, although allows hydroxyl ions to transfer, the overall flux of ions is expected to be less than that of a free open channel as it exist in uncoated separators. While the cell is continuously cycled, R1 gradually increases indicating the conductivity of the electrolyte solution decreases as the zincate ion concentration is increased. Further on, precipitation of insulating zinc oxide layers on the separator also degrades the conductivity of the membrane undermining the battery longevity.10


21


A

B

10

R3 of PP w/ PEBIH-PBMA R3 of PP

6

Re(Z) (Ohm)

8

Re(Z) (Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34

6

4

2

4

2

R1 of PP w/ PEBIH-PBMA R1 of PP

0

0 0

20

40

60

80

0

20

Cycle number (#)

40

60

80

Cycle number (#)

Figuire 9. (A) R1 and (B) R3 Resistance graphs.

In order to further understand the mechanisms of the cell failure, R3 resistance was plotted according to cycle as seen in Figure 9. Both the pristine and copolymer coated membrane show similar trends, as the maximum resistance of ORR kinetics is reached at cycle 10, and display a gradual decrease. This is due to the activation of the cathode catalyst. From Figure S2, the CV diagrams of the catalyst with the same composition exhibit similar observations, as the maximum activity of the catalyst is achieved after 25 cycles. For the unmodified membrane, the R3 of the last cycle reveals no degradation characteristics, which is a clear illustration of the cell’s premature failure due to zincate ion crossover rather than the deactivation of catalytic ability of the cathode. After 80 cycles, the PEBIH-PBMA coated membrane’s R3 rises and after its termination, full degradation of the catalyst can be readily seen. This directs that the cell with the coating has elongated its durability until the actual degradation of the air-cathode catalyst can occur. During the early stages of battery discharge-charge cycling, the control of ion species is crucial in order to harness the full potential of the cell. Figure S3 is the chronological impedance spectra of a cell cycled without the separator. It also depicts similar trends as a constant increase in R1 is observed and a successive increase-decrease step in R3 is observed. The sharp increase


22

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in R1 can be monitored for the spectra taken after the cell’s termination indicating that the resistance arising from the electrolyte is the key factor in battery cyclic stability. The surface and cross-sectional FE-SEM images of pristine polypropylene separator and PEBIH-PBMA deposited polypropylene separator were presented in Figure S4. The pure commercial polypropylene separator retained big pores between interconnected polymer branches, where ions involved in electrolyte could freely transport separator from anode to cathode and reversely as well. Furthermore, as we could see in cross-sectional FE-SEM image, PEBIH-PBMA copolymer developed thin film onto separator, which reduces ionic pathway resistance shown from the EIS results.

Separator

Elements 10 cyc

20cyc

37cyc

80cyc

104cyc

PP

IL

PP

IL

PP

IL

IL

IL

Zn

3.93

0.2

2.04

0.2

1.73

0.3

0.4

0.3

C

41.43

84.8

54.52

78.7

79.41

62.5

69.9

75.3

O

17.58

13.4

17.42

17.4

11.46

29.3

23.7

18.9

K

37.06

1.6

26.01

3.8

7.39

7.8

5.9

5.6

Table 4. Summary of EDX scanning results of unmodified PP separator and PEBIH-PBMA coated PP separator.


23


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Page 24 of 34

Cathode Surface Elements 10 cyc

20cyc

37cyc

80cyc

104cyc

PP

IL

PP

IL

PP

IL

IL

IL

Zn

2.56

0.2

5.53

0.1

2.88

0.1

0.7

0.6

C

44.00

77.4

71.46

35.9

72.40

37.6

30.9

67.4

O

16.70

11.6

6.37

41.5

6.56

42.5

45.7

11.0

F

2.54

5.2

5.22

0.8

7.25

1.3

0.9

4.6

K

31.05

5.1

5.04

21.5

7.63

18.4

21.8

15.6

Table 5. Summary of EDX scanning results of the cathode surface for unmodified PP separator and PEBIH-PBMA coated PP separator.


24

Page 25 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. EDS profile mapping of Zinc unmodified PP(A) and PEBIH-PBMA coated PP(B) separator by cycle.


25


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Page 26 of 34

Figure 11. EDS profile mapping of Zinc unmodified PP(A) and PEBIH-PBMA coated PP(B) separator battery cathode by cycle.

EDS data were obtained to further elaborate the effectiveness of the polymer coating. Both the cathode and the separator were sampled after the same cycle as ICP and EIS measurements. As expected, from the EDS results summarized in Table 4 and Table 5, drastic reduction of zinc presence is monitored. For the cross sectional analysis of the separator as shown in Figure 10, at all cycles the unmodified setup has far more zinc content compared to the polymer coated membrane. This is a clear portrayal of the exclusion effect of the functionality of the ionic liquid induced polymer and its success to minimized zincate ion cross over through the membrane. Moreover, less agglomeration of zinc source alleviates possibilities of ion conductivity deterioration by zinc oxide formation.10 Similar drastic difference can also be monitored for the


26

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cathode surface as observed in Figure 11. Benefits of the polymer coating is not simply focused on the separator and the electrolyte condition. It extends to enhance the durability of the cathode by effectively minimizing any possible catalytic poisoning by zinc oxide. The selective ion transport effectiveness of the polymer coating is resulted by the preferred solubility of hydroxyl ions in the ionic liquid copolymer and the size exclusion effect. While the micropores of the commercial membrane allows nonspecific ionic flow, the drastic blockage of these cavities by the PEBIH-PBMA copolymer outcomes the radical differences monitored in Table 3 to 5. The radii of the zincate ion consisting of 4 hydroxyl groups and a zinc ion is far larger than a single hydroxyl ion. The relative movement easiness of hydroxyl ions compared to that of the zincate ions due to size variances in the ionic channels is the major factor in the optional transport. By calculating the diffusion coefficient of the zincate ion similarly done by Kim et al10, the selectiveness of the membrane can be indirectly quantified. Given by the following equation the diffusion coefficient of the zincate ions can be calculated. ‫ܽܥ‬ ‫ܣܦ‬ ݈݊ ൬ ൰= ‫ݐ‬ ‫ ܽܥ‬− ‫ܾܥ‬ ܸܾ‫ܮ‬ Here Ca, Cb is the concentration of zincate ions in the anode and cathode compartments respectively (mol/L), A is the area of the membrane exposed to the electrolyte (m2), L is the thickness of the separator (m) and t is the time (s) let to diffuse. Ca,Cb were all acquired by the ICP results for 10 hr cycled cell. The exposed area of the membrane is restricted to 1cm2 by cell design. No thickness increase from the coating was monitored by SEM imagery from its original factory specification of 25 µm. The diffusion coefficient D (m2/s) results for the copolymer coated membrane and the pristine membrane are 5.00 * 10-7 (m2/s) and 1.08 * 10-5 (m2/s) respectively. This is a clear illustration of that the copolymer membrane has a selectivity over two magnitudes compared to the commercial untreated membrane.


27


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Page 28 of 34

4. Conclusions Secondary Zn-air batteries with high cyclic stability was developed by utilizing ionic liquid induced polymer coated PP separators. The anionic polymer coating was synthesized by copolymerizing EBIH and BMA monomers for both functionality and structural integrity. It was found that the zincate ion cross over is a strong factor in the rise of polarization. The polymer coating had minimized the porosity of the membrane to limit zincate ion migration while maintaining its ionic conductivity. Battery test result exhibited, a drastic 281% increase in lifetime, and with 1.4% slightly higher initial energy efficiency. EDS and ICP analysis revealed that the improved durability is due to the successful grounding of the metal ion at the anode side. In the case of pristine PP membranes, analysis of the EIS spectra revealed that the battery prematurely deceased due to the zincate ions rather than the deactivation of the cathode catalyst. This insight reveals that the key factor in determining the longevity of the systems is heavily weighted not only the durability of the cathode catalyst, but also the performance of the separator. Future works should be focused upon manufacturing membranes with high ion transport selectivity.

ASSOCIATED CONTENT The following files are available free of charge. Figure S1. Zinc air battery components Figure S2. CV profiles of the Pt/Ir catalyst


28

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure S3. Impedance spectra of a cell without any separator Figure S4. SEM images of PEBIH-PBMA copolymer coated PP separator (A) Unmodified PP separator, (B) PEBIH-PBMA coated PP separator. (PDF)

AUTHOR INFORMATION Corresponding Author * To whom correspondence should be addressed Tel: +82-2-2123-5757; +82-2-2123-2758, Fax: +82-2-312-6401 E-mail: [email protected], [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENT This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning(NRF-2015M1A2A2056833) and Mobienflex Co., Ltd..


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Page 30 of 34

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