Novel Triple Tertiary Amine Polymer-Based Hydrogen Bond Network

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Energy, Environmental, and Catalysis Applications

A Novel Triple Tertiary Amine Polymer Based Hydrogen Bond Network Inducing Highly Efficient Proton Conducting Channels of Amphoteric Membrane for High-Performance Vanadium Redox Flow Battery Huaqing Zhang, Xiaoming Yan, Li Gao, Lei Hu, Xuehua Ruan, Wenji Zheng, and Gaohong He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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ACS Applied Materials & Interfaces

A Novel Triple Tertiary Amine Polymer Based Hydrogen Bond Network Inducing Highly Efficient Proton Conducting Channels of Amphoteric Membrane for High-Performance Vanadium Redox Flow Battery Huaqing Zhanga, Xiaoming Yana,b, Li Gaoa, Lei Hua, Xuehua Ruana, Wenji Zhenga, Gaohong Hea,b* a

State Key Laboratory of Fine Chemicals, School of Petroleum and Chemical Engineering, Dalian University of Technology, 2 Dagong Road, Panjin, LN 124221, China b Panjin Industrial Technology Institute, Dalian University of Technology, 2 Dagong Road, Panjin, LN 124221, China

Abstract A novel amphoteric membrane was designed by blending triple tertiary amine grafted poly (2,6-dimethyl-1,4-phenylene oxide) (PPO-TTA) with sulfonated poly (ether ether ketone) (SPEEK) for vanadium redox flow batteries. An “acid-base pair” effect is formed by the combination of tertiary amine group and sulfonic group, and extra nonbonding amine groups could be protonated. Both of them construct hydrogen bond network, which facilitates proton conduction and also hinders vanadium permeability due to lowered swelling ratio and Donnan effect. All these contribute to improve the ion selectivity of the membrane while maintaining ionic conductivity. Compared with other amphoteric and SPEEK-based membranes, the membrane exhibits excellent performance. The amphoteric membrane containing 15% PPO-TTA exhibits an ultra-low vanadium permeability of 3.4×10−9 cm2 s−1 and a low area resistance of 0.39 Ω cm−2. Consequently, the cell assembled with this membrane shows excellent performances far superior to SPEEK and Nafion 212. The coulombic efficiency and energy efficiency of the cell are 94.3-98.3% and 90.3-77.1% at 40-200 mA cm−2, respectively, and have no significant reductions after 200 cycles. This performance are at a high level among the amphoteric and SPEEK based membranes reported in recent years. The cell’s open circuit voltage is maintained for up to 165 h. In addition, the membrane’s chemical stability is improved by the effective barrier to vanadium ion. Keywords: Amphoteric membrane, Tertiary amine, Acid-base pair, Hydrogen bond network, Vanadium redox flow battery



Corresponding author:

Tel.: +86 411 84986291; E-mail: [email protected] (Dr. Xiaoming Yan); [email protected] (Dr. Gaohong He) 1

ACS Paragon Plus Environment

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1 Introduction The renewable new energy (i.e. solar cell and wind energies) have attracted attentions due to growing environmental pollution problem and energy consumption1. In order to improve the stability of these new energy sources, an advanced energy storage technology with high efficiency and reliability is urgently needed2-3. Vanadium redox flow battery (VRFB) is promising because of some advantages, such as fast response, low manufacturing cost and high energy efficiency4-6. As a key component of VRFB, the ion exchange membrane plays an important role in preventing vanadium ion cross-permeation and providing a connected ion channel to form the battery circuit7. The requirements for an ideal ion exchange membrane for VRFB include excellent chemical stability, high ionic conductivity and ionic selectivity, and low cost8-10. Proton exchange membranes (PEMs) are first applied to VRFBs, of which Nafion series membranes have been most widely used due to their good stability and high conductivity. Unfortunately, severe vanadium ion penetration and high price hinder their commercial application11-12. In recent years, the sulfonated non-fluorinated membranes with aromatic structures, such as sulfonated polysulfone (SPSf), sulfonated poly (ether ether ketone) (SPEEK) and sulfonated polyimide (SPI), have been considered as ionic exchange membranes for VRFB because of their low cost and good electrochemical activity13-16. Among them, SPEEK is a excellent materials which have been extensively studied due to their good mechanical properties, high ion conductivity and low cost17-21. However, the high ionic conductivity of these membranes depends on the high degree of sulfonated (DS), which results in excessive swelling and aggravation of vanadium permeation22. In addition, the chemical stability of sulfonated non-fluorinated membranes needs to be further improved23. On the other hand, the anion exchange membranes (AEMs) have positively charged 2


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ion exchange group, so that the ion selectivity of AEMs can be remarkably improved owing to the Donnan repelling effect on vanadium ions24-25. However, the ion conductivity of the AEMs is usually low, which greatly reduces the voltage efficiency of the battery26. In order to improve the “trade-off” between high ionic conductivity and ionic selectivity, the amphoteric membranes have been developed27-30. Containing both immobilized anion and cation exchange groups, amphoteric membranes can possess the advantages of PEMs and AEMs31-32. The positively charged groups such as quaternary ammonium and imidazolium have been frequently used as the anion exchange groups33-34. Recently, it has been reported that the uncharged amine groups (i.e. –N(CH3)2 and –NH2) can act as proton acceptor to form the “acid-base pair” structure with sulfonic group as proton donor35-36. Different from the electrostatic interaction between the sulfonic group and the quaternary ammonium group, the acid-base pair is formed through the hydrogen bond, which is conducive to proton conduction37-38. Moreover, nonbonding amine group can be protonated in the acidic media, promoting proton conduction and blocking the vanadium penetration39-40. Therefore, amphoteric ion membranes based on uncharged amines are expected to exhibit promising performance in the application of VRFB. Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), a polymer with good chemical and mechanical stability, exhibits excellent performances in both fuel cell and flow battery applications41-43. In addition, PPO is easily Brominated or acylated for further modification to load the desired groups44-46. And it can be used to introduce the tertiary amine

groups47.

In

this

work,

a

triple

tertiary

amine

grafted

poly

(2,6-dimethyl-1,4-phenylene oxide) (PPO-TTA) was synthesized. Then a novel amphoteric membrane was fabricated by blending PPO-TTA with SPEEK, as shown in Scheme 1. 3



Scheme 1. Preparation of the triple tertiary amine polymer based amphoteric membrane

The conduction mechanism of the triple tertiary amine polymer based amphoteric membrane is shown in Scheme 2. In the acidic environment, the tertiary amine group can be protonated and producing the acid-base pair with sulfonic group48. The hydrogen bond crosslinked network structure of “acid-base pairs” narrowed the ion channels, and hampered the vanadium penetration. Meanwhile, the efficient hydrogen bond network contributed to proton conduction. In addition, extra nonbonding protonated tertiary amine groups with positive charge repelled the vanadium ions and facilitated the construction of hydrogen bond networks, further enhanced proton conduction and impeded the penetration of vanadium ions. Compared to pristine SPEEK and Nafion 212, the amphoteric membrane prepared here can significantly improve their ionic selectivity while maintaining excellent ionic conductivity. As a result, the assembled sing cell showed superior energy efficiency and capacity retention. Furthermore, the chemical stability of amphoteric membrane was also significantly enhanced in virtue of the barrier effect on vanadium ions.

4


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Scheme 2. Conduction mechanism of the triple tertiary amine polymer based amphoteric membrane

2 Experimental 2.1. Materials Poly (ether ether ketone) (PEEK, VESTAKEEP®) was obtained from Evonik Degussa (China) Co. Ltd. Poly (2,6-dimethyl−1,4-phenylene oxide) (PPO, GFN2-701) was purchased from SABIC Innovative Plastics (USA). Dichloroethane was dried by soaking molecular sieves. Concentrated sulfuric acid (analytical reagent, 98 wt%), anhydrous AlCl3, 4-fluorobenzoyl chloride, ethanol, N,N-dimethylacetamide (DMAc), 2,4,6-tri(dimethylaminomethyl)-phenol (TDAP), Cs2CO3 and dimethyl formamide (DMF) were obtained by commercial purchase. The purity of all chemicals used was analytical grade. 2.2 Synthesis of 4-Fluorobenzoylated PPO (PPO-BzF) 3 g of PPO was added to 30 mL of dichloroethane under vigorous stirring condition to from the PPO solution. In nitrogen atmosphere, put 2.42 g of anhydrous AlCl3 into a three-neck bottle containing 25 mL of dichloroethane and stirred violently. Then slowly added 1.83 mL of 4-fluorobenzoyl chloride under the condition of ice/water bath. After that, the PPO solution was added and reacted for 10 h at 60 °C. After the reaction finished, poured the reaction solution into ethanol to obtain the PPO-BzF. The obtained polymer was washed repeatedly and dried in vacuum at room temperature. 5



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2.3 Synthesis of Triple Tertiary Amine Grafted PPO (PPO-TTA) Dissolved 1 g of PPO-BzF in 30 mL of DMAc. Then 1 g of TDAP and 1.6 g of Cs2CO3 were added. Raised the temperature to 110 °C and reacted for 10 h. During the reaction, nitrogen was bubbled in the solution to facilitate the removal of water. The final PPO-TTA was obtained by pouring the reaction mixture into excess methanol. Then washed the product thoroughly with water and methanol before vacuum drying. The synthesis procedures of the polymers are shown in Scheme 3. *

O

O C

O

*

n

50℃

*

H2SO4

O

O C

O

*

n

SO3H

O

*

O

n

*

OH

Cl

F

AlCl3

N

Ox

*

O

N N

*

n-x

Cs2CO3

O

Ox

*

O

*

n-x

O

F

O N

N

N

Scheme 3. Synthesis procedure of the SPEEK and PPO-TTA

2.4 Synthesis of Sulphonated Poly(ether ether ketone) (SPEEK) 3 g of PEEK was dissolved by 60 mL of 98% sulfuric acid in a violent agitation. When the polymer was completely dissolved, kept the temperature at 50 °C. After a period of reaction, poured the reaction solution into the ice water. The obtained SPEEK product was repeatedly washed and fully dried. 2.5 Preparation of Triple Tertiary Amine Polymer Based Amphoteric Membranes PPO-TTA and SPEEK with the total mass of 0.15 g were dissolved in 1 mL and 3 6


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mL of DMF, respectively. And these two solutions were then mixed and casted onto a square glass plate with a side length of 5cm and dried at 50 °C. After the solvent was completely volatilized, the membrane was stripped off. Immersed the membrane in 1 M H2SO4 solution to allow full ion exchange, and then removed the residual sulphuric acid. For convenience, the amphoteric membrane is expressed as SPEEK/PPO-TTA-x%, where x% is PPO-TTA content in the membrane. 2.6 Characterizations The synthesis of SPEEK, PPO-BzF and PPO-TTA were confirmed by 1H NMR spectrometer (Bruker AVANCEIIIHD 500). The resonance frequency is 500 MHz. The DS of SPEEK and the DA (degree of acylation) of PPO-BzF was calculated according to the formulas: DS=4A(HE)/A(HA,A’), DA=3A(H4)/A(H2), respectively. where the A(Hx) is the integral areas of Hx peak. The morphologies of the membranes were photographed with a digital camera. The content and distribution of various elements in the membranes were analyzed through energy-dispersive X-ray spectrum (EDS) obtained by SEM (Nova NanoSEM 450). The microphase separation structure of the membrane was observed by AFM (Dimension Icon, Bruker Co.). The ion exchange capacity (IEC) of membrane were measured by the typical titration. The water uptake (WU) was obtained by the difference between the weights in wet and dry states. The swelling ratio (SR) was measured by the difference between the lengths in wet and dry states. The membrane’s ionic conductivity was measured by the the impedance analyzer (ivium-n-Stat) from 1 to 105 Hz at 25 °C in pure water. The area resistance (AR) and VO2+ permeability were obtained as described in these literatures49-50 The chemical stability was determined by the morphology change and weight loss of the membrane after soaking in strong oxidizing electrolyte (0.1 M VO2+ 7



in 3 M H2SO4) for 40 days. In VRFB single cell test, the effective area is 9 cm2. 50 mL of electrolyte (1.5 M V2+/V3+ in 3.0 M H2SO4) in the negative side and same amount of solution (1.5 M V4+/V5+ in 3.0 M H2SO4) in the positive. The carbon felt treated in a muffle furnace at 400 °C for 8 h was used as the electrode. The thickness of the electrode is 5 mm and the compression ratio of the electrode is 1. The battery performance test was performed using the CT2001 battery testing system (5V/3A) (LANHE, China) at 40 to 200 mA cm-2 current densities, and 1.65 V and 0.8 V were set as the cut-off voltages. The cycling performance was tasted at 80 mA cm−2 current density. The self-discharge test started when the battery was charged to 50%, and stopped at the state that the OCV (open circuit voltage) is below to 0.8 V. 3 Result and discussion 3.1 Structure and Composition SPEEK’s 1H NMR spectra is shown in Fig. 1(a). The characteristic peak of hydrogens on benzene ring containing sulfonic group deviates appears at 7.57 ppm (HE), confirming the SPEEK was successfully synthetized. The DS of SPEEK is 78%. 1H NMR spectra of PPO, PPO-BzF, and PPO-TTA are presented in Fig. 1(b-d). In the PPO spectrum (Fig. 1(b)), the characteristic peaks at 2.09 ppm (H2) and 4.67 ppm (H1) are ascribed to the protons on the methyl and benzene rings respectively. After the Friedel-Crafts acylation reaction, It can be observed from Fig. 1(c) that a portion of the H1 signal is offset to the 6.12 ppm (H1’) due to the introduction of carbonyl groups, and two new peaks corresponding to the aromatic hydrogen atoms of 4-fluorobenzoyl group appear at 7.98 ppm (H3) and 7.13 ppm (H4). After further triple tertiary amine functionalization, as shown in Fig. 1(d), H4 peak can be observed shifts to 6.77 ppm (H4’) because of the introduction of TDAP and the five characteristic peaks of TDAP 8


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appear at 3.23 ppm (H5), 7.37 ppm (H6), 3.43 ppm (H7), 2.14 ppm (H8) and 2.25 ppm Oct16-2017.90.fid Speek Zhanghuaqing DMSO H

(H9), respectively. These signals confirm the successful synthesis of PPO-BzF and PPO-TTA. The degree of functionalization (DF) of PPO-TTA is 50%. B A O A' B'

D C

(a)

* O

O

C

HA,A'

n*

B A

SO3H E

Jun04-2018.140.fid PPO Zhanghuaqing CDCl3 H

A' B'

HC

HE

HD

HB'

HB

8.1

8.0

7.9

7.8

7.7

7.6

7.5 δ/ppm

Jun04-2018.130.fid PPO-BZF Zhanghuaqing CDCl3 H

7.4

(b)

7.3

7.2

7.1

7.0

H2

2

1

6.9

O n*

* 1

H1

2

CHCl3

(c)

9.0

8.5

8.0

7.5

7.0

6.5

6.0

CHCl3

Jul19-2017.70.fid PPO-TA Zhanghuaqing CDCl3 H

2

15.0 5.5 δ/ppm O

H4

(d)

8.5

8.0

7.5

7.0

6.5

2

O

O4.5 x

3

9.0

Fig. 1

1H

8.5

8.0

H6

7.5

H4 H1

7.0

6.5

4 5

6.0

H2

2 4.0

1

2

O

N 8 6

5 N 8

6 7

H1'

1.5

H8

O 3.5 * n-x

3.0

2

2.5

2.0

1.5

H9

4

3

H3

2.0

2

1

* 5.5 5.0 δ/ppm

8

2.5

F

H1'

6.0

3.0

4 4

1' 9.0

3.5

O n-x * 1

32 3

H3

2

Ox

*

H1

1 4.0

4.5

5.5 5.0 δ/ppm

N 9

8

9

4.5

H2 H5 H7

4.0

3.5

3.0

2.5

2.0

1.5

NMR spectra of: (a) SPEEK, (b) PPO, (c) PPO-BzF, (d) PPO-TTA

The FTIR spectra of SPEEK, PPO-TTA and SPEEK/PPO-TTA-15% membrane are shown in Fig. 2(a). In the spectra of PP0-TTA, the peak centered at around 1358 cm−1 can be ascribed to the stretching vibration of C-N group in tertiary amine. This characteristic peak can also be found in the spectra of SPEEK/PPO-TTA-15% membrane, simultaneously the asymmetric and symmetrical stretching vibration signal 9



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bands of O=S=O groups in the sulfonic groups at 1217 cm−1 and 1074 cm−1 are observed, which proves the successful preparation of amphoteric membrane. Moreover, we can observed the weakening and broadening of the band at 1217 cm−1, it may be due to the influence of hydrogen bonds and acid-base pairs formed in the membrane. To further identify the hydrogen bond and acid-base pair in SPEEK/PPO-TTA amphoteric membrane, the corresponding XPS spectra of SPEEK/PPO-TTA membranes are shown in Fig. 2(b). The characteristic peaks of N1s in tertiary amine group appear at around 399.1 ev. In addition, there is another signal peak in 401.9 ev, which attributes to the oxidation of N in protonated tertiary amine group. These indicate the formation of hydrogen bonds and acid-base pairs in amphoteric membranes. The distribution of N element in the amphoteric membranes was presented in Fig. 2(c-f). Because the signal of N element is relatively weak especially when the content is low, the content of N element shown in figure 4 deviates to a certain extent from the actual value and is not strictly proportional. But we can still observe that the N element in the membranes is uniformly distributed, and its concentration increases with increasing

PPO-TTA

content.

These

results

indicate

that

SPEEK/PPO-TTA amphoteric membranes were successfully prepared.

10


homogeneous

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Fig. 2 The (a) FTIR spectra of PP0-TTA, SPEEK and SPEEK/PPO-TTA-15% membrane; (b) XPS N1s core level spectra resolving results of SPEEK/PPO-TTA-15% and EDS element mapping of N in amphoteric membranes: (c) SPEEK/PPO-TTA-5%, (d) SPEEK/PPO-TTA-10%, (e) SPEEK/PPO-TTA-15%, (f) SPEEK/PPO-TTA-20%

3.2 Morphologies and Properties The photographs and SEM images of the membranes are shown in Fig. 3. All the prepared membranes are flexible and transparent macroscopically. In the SEM images of the surface and cross section, it can be observed that all of them have uniform and compact structure, which indicates the good compatibility of the two materials and the successful preparation of the amphoteric membranes. In addition, it can be noticed that the transparency of the membranes decreases slightly and the surface and cross section become rougher when the PPO-TTA content raises. The possible reason is that the excessive interaction between the tertiary amine and sulfonic group may cause the slight precipitation of the polymers during the membrane formation process.

11



Fig. 3 Digital photographs and surface, cross section SEM images of different membranes: (a) SPEEK, (b) SPEEK/PPO-TTA-5%, (c) SPEEK/PPO-TTA-10%, (d) SPEEK/PPO-TTA-15%, (e) SPEEK/PPO-TTA-20%

The microphase separation structures of different membranes are illustrated by AFM in Fig. 4. The dark region represents the hydrophilic phase containing sulfonic acid and 12


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tertiary amine clusters, while the bright domain corresponds to the hydrophobic phase formed by the aggregation of polymer aromatic backbones. Apparently, as seen from Fig. 4(a) to Fig. 4(e), with the increase of tertiary amine group content in the blend membrane, the hydrophilic ion channel of the membrane gradually becomes narrower and denser. It can be explained by the formation of "acid-base pair" between tertiary amine group and sulfonic group. It can be speculated that the introduction of PPO-TTA could reduce the vanadium ion permeability of the membrane.

Fig. 4 AFM phase images of the membranes: (a) SPEEK, (b) SPEEK/PPO-TTA-5%, (c) SPEEK/PPO-TTA-10%, (d) SPEEK/PPO-TTA-15%, (e) SPEEK/PPO-TTA-20%

The IEC, SR and WU of the SPEEK/PPO-TTA, SPEEK and Nafion 212 membranes are listed in Table 1. The Nafion membrane needs to be pretreated before the test51-52. The Nafion 212 was treated using the methods in the previous paper42. With the increase of PPO-TTA content from 0% to 20%, the IEC decreases from 1.98 to 1.51 mmol g−1 due 13



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to the decline of SPEEK mass ratio. At the same time, because of the interaction between tertiary amine and sulfonic group, both SR and WU of the membrane are reduced. The lowered swelling ratio is favorable to the vanadium penetration resistance and stability of the membrane. Table 1 IEC, SR and WU of the membranes Membrane Nafion 212 SPEEK SPEEK/PPO-TTA -5% SPEEK/PPO-TTA -10% SPEEK/PPO-TTA -15% SPEEK/PPO-TTA -20%

Thickness (μm) 59 43 46 45 44 46

IEC (mmol g−1) 0.96 1.98 1.89 1.77 1.65 1.51

Swelling ratio (%) 15.6 17.9 16.5 15.1 13.2 11.3

Water uptake (%) 36.2 46.1 41.8 37.7 33.1 31.1

The area resistance of different membranes are shown in Table 2 and Fig. 5(a). With the increase of PPO-TTA content, the proton conductivity of the membrane decreases gradually because of smaller IEC and narrower proton transport channel. An interesting phenomenon is that the conductivity in pure water declines from 64.3 to 53.7 mS cm−1 with the raise of PPO-TTA, while the area resistance in 0.5 M H2SO4 shows only a slight increase from 0.37 to 0.40 Ω cm2. This phenomenon indicates that even though the proton transport channels of the amphoteric membranes become narrower, they still exhibit excellent proton conductivity in acidic media. It is possibly attributed to the promotion of efficient hydrogen bond networks based on the “acid-base pair” and protonated tertiary amine groups in acidic media. In order to characterize the VO2+ permeability of different membranes, the VO2+ permeability value was shown in Table 2. And the relationship between the concentration of VO2+ permeating through membranes into MgSO4 solution and the time was described in Fig. 5(b). Obviously, the VO2+ permeability of amphoteric 14


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membranes is much slighter than those of Nafion 212 and SPEEK membranes. At low PPO-TTA contents of 5-15%, the ion cross contamination of amphoteric membrane is gradually mitigated owing to narrowed ion channels and the Donnan repelling effect of protonated tertiary amine groups. The VO2+ permeability of SPEEK/PPO-TTA-15% membrane is only 3.4×10−9 cm2 s−1, which is 5 and 19 times lower than that of the original SPEEK (1.81×10−8 cm2 s−1) and Nafion 212 (6.46×10−8 cm2 s−1), respectively. However, when the content of PPO-TTA increases to 20%, the VO2+ permeability rises instead. This is probably because that the precipitation of polymers during membrane formation causes some defects when the PPO-TTA content is too high. Table 2 Proton conductivity, AR and VO2+ permeability Membrane Nafion 212 SPEEK SPEEK/PPO-TTA -5% SPEEK/PPO-TTA -10% SPEEK/PPO-TTA -15% SPEEK/PPO-TTA -20%

Proton conductivity (mS cm-1)

Area resistances (Ω cm2)

VO2+ permeability (10-8 cm2 s-1)

118 64.3 62.9 59.3 57.2 53.7

0.24 0.37 0.35 0.38 0.39 0.40

6.46 1.81 0.82 0.53 0.34 0.66

Fig. 5 The (a) area resistance and (b) VO2+ ion concentration as a function of time in the diffusion cell of different membrane

15



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3.3. Cell Performances The battery performances of Nafion212, SPEEK and SPEEK/PPO-TTA at 40-200mA cm−2 are described in Fig. 6. Obviously, Fig. 6(a) presents that the cells with the SPEEK/PPO-TTA amphoteric membranes possess the higher coulombic efficiency (CE) than Nafion212 and SPEEK because of low vanadium permeability. The CE of SPEEK/PPO-TTA membrane increases with the rise of PPO-TTA content from 5% to 15%, and the highest CE is 94.8%-98.3% at 40-200 mA cm−2, which is much higher than those of Nafion 212 (75.9%-93.2%) and SPEEK (79.7%-94.0%). The CE of SPEEK/PPO-TTA-20% membrane is lower than those of SPEEK/PPO-TTA-10% and SPEEK/PPO-TTA-15%, because of its higher vanadium permeability. As seen in Fig. 6(b), SPEEK/PPO-TTA amphoteric membranes exhibit close voltage efficiencies (VEs) compared with SPEEK membranes. It can be attributed to the promotion effects of hydrogen bond network and protonated tertiary amine groups on proton conduction. The energy efficiency (EE) of batteries with different membranes are seen in Fig. 6(c). The SPEEK/PPO-TTA-15% membrane exhibits the highest EE of 90.28% at 40 mA cm−2, and it still has an EE of 77.11 % at 200 mA cm-2. SPEEK/PPO-TTA-10% and SPEEK/PPO-TTA-15% show higher EEs than Nafion 212 and SPEEK at 40-200 mA cm−2. The EE of SPEEK/PPO-TTA-15% is a little lower than that of SPEEK/PPO-TTA-10% at large current density, but the EE of SPEEK/PPO-TTA-15% is a little higher than that of SPEEK/PPO-TTA-10% when the current density is lower than 120 mA/cm2. The CE of SPEEK/PPO-TTA-15% is higher than that of SPEEK/PPO-TTA-10%,

but

its

VE

is

slightly

lower

than

that

of

SPEEK/PPO-TTA-10%. It suggested that with the increase of current density, the difference of VE has more influence on EE. As shown in Fig 6(d), self-discharge phenomenon of VRFB systems with different membranes were also investigated. The 16


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OCV of the cell with SPEEK/PPO-TTA-15% remained above 0.8 V for 165 h, much longer than those with SPEEK (46 h) and Nafion 212 (30 h). It suggests that the amphoteric membrane possesses excellent vanadium penetration resistance.

Fig. 6 The cell performances of SPEEK, SPEEK/PPO-TTA, and Nafion212 membranes: (a) CE, (b)

VE, (c) EE, (d) OCV As shown in Fig. 7, compared with other type membranes, the VRFB performances of the SPEEK/PPO-TTA-15% membrane is at a high level among the amphoteric and SPEEK based membranes reported in recent years. Although the experimental environment varies in different laboratory, it can still shows that the membrane is good to some extent. This is attribute to its simultaneous high CE and VE ensured by the “acid-base pair” effect and hydrogen bond network structure.

17



Fig. 7 The battery performances of different amphiprotic and SPEEK based membranes reported in recent years16-21, 27-29, 32-35, 37, 39, 42, 48, 53-72

The cycle performance of SPEEK/PPO-TTA-15% membrane was tested at 80 mA cm−2, and the electrolyte was replaced after 100 cycles to eliminating the effect of capacity attenuation. Fig. 8(a) shows that all CE, VE and EE have no significant decline after 200 cycles. In Fig. 8(c), it can be seen that the VRFB performances of the membrane after cycle test were comparable to those before the cycle test. The Fig 8(d) shows the surface SEM image of the membrane after cycle test, with no significant damage observed. These results indicates that the membrane has excellent chemical stability under oxidizing environment. The capacity retentions of VRFB using Nafion, SPEEK and SPEEK/PPO-TTA are compared in Fig. 8(b). The electrolyte capacity of battery using the SPEEK and Nafion 212 membrane decreased rapidly and had been too low to continue testing after 50 cycles. In order to maintain consistency with the contrast data, the capacity attenuation of 50 cycles of SPEEK/PPO-TTA-15% membrane is given in the diagram. It can be observed that the cell with SPEEK/PPO-TTA membrane exhibits lower capacity decay than SPEEK and 18


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Nafion212 due to the better vanadium penetration resistance. Its capacity retention is 48% after the 50 cycles, higher than those of Nafion (30.5%) and SPEEK (32.1%).

Fig. 8 (a) The cycling performance of the battery with SPEEK/PPO-TTA-15% of 80 mA cm-2; (b) the capacity retention of different membranes; (c) Comparison of battery performance before and after cyclic testing; (d) Surface SEM image of the membrane after cyclic testing

After 40 days of oxidation, as shown in Fig. 9, the SPEEK membrane is completely eroded into pieces. As the content of PPO-TTA increases, the erosion gradually decreases. When PPO-TTA content is more than 15%, the membrane’s morphology is perfectly maintained. And the mass loss also decreases from 18.4% to 3.6% with increasing PPO-TTA content from 0 to 20%. These all results clearly indicate that the addition of PPO-TTA can effectively improve the chemical stability of the membrane. In order to further investigate the effect of blending ratio on chemical stability of amphoteric membranes, the micromorphology, conductivity and mechanical properties retention of the membranes after being immersed in oxidizing electrolyte solution (1.5 M VO2+/3.0 M H2SO4) for 14 days were tested. From the surface SEM images (Fig. 19



10(a-e)), we can observe that the SPEEK membrane is seriously damaged, and with the increase of PPO-TTA content, the surface morphology of the membrane becomes more and more uniform. The conductivity and mechanical property retentions. With the increase of PPO-TTA content from 0 to 20, the conductivity and mechanical retention rate of the membranes increased from 59.4% to 90.4% and from 30.3% to 88.1%, respectively, as shown in Fig. 10(f) and Fig. 10(g).

Fig. 9 The (a) The morphology changes and (b) weight variations of different membranes after 40 days in electrolyte solution

Fig. 10 The surface SEM image of different membranes after being immersed for 14 days: (a) SPEEK, (b) SPEEK/PPO-TTA-5%, (c) SPEEK/PPO-TTA-10%, (d) SPEEK/PPO-TTA-15%, (e) SPEEK/PPO-TTA-20%; and (f) conductivity retention of membranes after being immersed for 14 days; (g) mechanical retention of membranes after being immersed for 14 days 20


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The stability enhancement may be attributed to the narrowed ionic channels and the repulsive effect of the protonated tertiary amine group on vanadium ion in the acidic environment, which effectively prevents the highly oxidizing VO2+ from attacking the polymer. In order to verify this hypothesis, the VO2+ distribution in the membranes was determined by EDS after immersion in the electrolyte solution for 72 h. As shown in Fig. 11, with the raise of PPO-TTA content, the number of VO2+ entering into the membrane decreases obviously, which validates that the effective rejection of protonated tertiary amine group to VO2+ protects the polymer against the destruction.

Fig. 11 The EDS element mapping of V in different membranes after 72 h in electrolyte solution: (a) SPEEK, (b) SPEEK/PPO-TTA-5%, (c) SPEEK/PPO-TTA-10%, (d) SPEEK/PPO-TTA-15%, (e) SPEEK/PPO-TTA-20%

Conclusions This work provides a novel triple tertiary amine polymer based amphoteric membrane with promising prospect in VRFB applications. Due to the hydrogen bond network and “acid-base pair” is constructed in the membrane, which efficiently facilitates the proton conduction and hinders the vanadium penetration, the membrane shows excellent performance among different types of amphoteric and SPEEK based 21



membrane. The SPEEK/PPO-TTA-15% membrane exhibits very slight vanadium permeability (3.4×10−9 cm2 s−1) which is much lower than that of the original SPEEK (1.81×10−8 cm2 s−1) and Nafion 212 (6.46×10−8 cm2 s−1). Meanwhile, the membrane still maintains a comparable low area resistance (0.39 Ω cm-2), even though the proton transport channels become narrower and the IEC diminished. As a result, The VRFB assembled with SPEEK/PPO-TTA-15% membrane exhibits high CE of 94.28-98.29% and EE of 90.28-77.11% at 40-200 mA cm-2, respectively, which VRFB performances is at a high level among the amphoteric and SPEEK based membranes reported in recent

years. No performance reduction is observed after 200 cycles. In addition, in virtue of low VO2+ permeability, the OCV of the assembled VRFB can be maintained for up to 165 h, and the discharge capacity attenuation is also lighter than those of SPEEK and Nafion 212. Moreover, the amphoteric membrane’s chemical stability is improved. All these results indicate that the triple tertiary amine polymer based amphoteric membrane is promising in VRFB applications. Acknowledgements This work was supported by National Natural Science Foundation of China (grant no. U1808209), National Key Research and Development Program of China (grant no. 2016YFB0101203) and Fundamental Research Funds for the Central Universities (grant no. DUT18JC40). References 1. Liu, C.; Li, F.; Ma, L. P.; Cheng, H. M., Advanced Materials for Energy Storage. Adv. Mater. 2010, 22 (8), E28-E62. 2. Larcher, D.; Tarascon, J., Towards greener and more sustainable batteries for electrical energy storage. Nat. Chem. 2015, 7 (1), 19-29. 3. Zakeri, B.; Syri, S., Electrical energy storage systems: A comparative life cycle cost analysis. Renewable Sustainable Energy Rev. 2015, 42 (C), 569-596. 4. Gong, K.; Fang, Q.; Gu, S.; Li, S. F. Y.; Yan, Y., Nonaqueous redox-flow batteries: 22


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Table of Contents Graphic 144x79mm (300 x 300 DPI)


Novel Triple Tertiary Amine Polymer-Based Hydrogen Bond Network

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