Cross-linked Anion Exchange Membranes Composed ...

ECS Transactions, 66 (3) 99-104 (2015) 10.1149/06603.0099ecst ©The Electrochemical Society

Cross-linked Anion Exchange Membranes Composed of Imidazolium Salt for Alkaline Fuel Cell Feifei Songa, Shuli Chena, Ying Gaoa, Yuyu Liub*, Jinli Qiaoa* College of Environmental Science and Engineering, Donghua Universtiy, 2999 Ren’min North Road, Shanghai 201620, China b Multidisciplinary Research on the Circulation of Waste Resources, Graduate School of Environmental Studies, Tohoku University. 6-6-11 Aoba-Aramaki-Aza, Aoba-ku, Sendai, 980-8579, Japan a

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Two novel cross-linked anion exchange membranes (AEMs) composed of PVA or Chitosan by incorporation of PMViC-Co-VP as ion conductors for alkaline fuel cells (AFCs) were prepared by a simple solution-casting method. The anion conductivity (OH-) and water uptake of the as-prepared membranes were investigated. It is found that the OHconductivities could reach 0.0086S cm-1 for both PVA/PMViC-Co-VP/GA and Chitosan/PMViC-Co-VP/GA membranes, and gradually increased with increasing temperature up to 80oC. Meamnwhile, the water uptake (WU) of these two kinds membranes are aslo less than 100% after immerion in different concentrations of KOH (1-8M) solution. In addition, both PVA (Chitosan)/PMViC-Co-VP/GA membranes posses good mechanical property, where the PVA/PMViC-Co-VP/GA (1:0.5:0.1 by mass) membrane shows the tensile strength of 31.11 MPa with elongation at break of 10.45%.

Introduction

The alkaline membrane fuel cells (AMFCs), which use the anion-exchange membranes(AEMs) as electrolytes have attracted considerable attention owing to their higher reaction kinetics, lower fuel crossover, reduced CO poisoning, and use of non-precious metal catalysts(1). In the development of AMFCs, the AEMs are obviously the key issues to make a breakthrough in AMFC performances. However, the conductivity and stability of AAEMs are still far less than commercial available membranes, for example, the Nafion which has commonly been used to proton-exchange membrane fuel cells (PEMFCs) (2). More recently, imidazolium-based AEMs are of interest due to the five-membered heterocyclic ring and π conjugated structure of the imidazolium cation, which is expected to have good

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ECS Transactions, 66 (3) 99-104 (2015)

stability in alkaline condition (3,4). Based on this conception, we here report novel series of alkaline anion-exchange membranes: Chitosan/poly (3-methyl-1-vinylimidazolium chloride)-Co-(1-vinylpyrrolidone) membranes (Chitosan/PMViC-Co-VP/GA) and PVA/poly (3-methyl-1-vinylimidazolium chloride)-Co-(1-vinylpyrrolidone) (PVA/PMViC-Co-VP/GA) membranes. PVA has film-forming capacity, hydrophilic properties, and a high density of reactive chemical functions that are favorable for cross-linking by irradiation, chemical, or thermal treatments (5). Chitosan is a cyclo-aliphatic polymer, and it contains active amino groups (-NH2) and hydroxyl groups (-OH) thus possessing high hydrophilicity. In addition, the good film forming property, high mechanical strength, and chemical resistance make Chitosan very promising membrane material (6,7). PMViC-Co-VP, a water soluble imidazolium-type quaternized copolymer, thus can offer conductive anions (OH-) as charge carriers (8). This research work is to synthesize anionic imidazolium-type membranes aided by PVA or Chitosan chemical cross-linking, subsequent adding PMViC-Co-VP, and finally by ion exchange with KOH solution in different concentrations. The membranes’ properties including OH- conductivity, water uptake and mechanical property were investigated.

Experimental Materials and membrane preparation The membranes were prepared by a solution-casting method, where 1g chitosan (degree of deacetylation = 80.0-95.0，supplied by Sinopharm Chemical Reagent Co. Ltd. China) was dissolved in 50 mL of 2% aqueous solution of acetic acid. Meanwhile, PVA (99% hydrolyzed, average molecular weight Mw = 86,000-89,000; Aldrich) was fully dissolved in water to make a 10% solution at 70oC. Then PMViC-Co-VP (supplied by Aldrich) was mixed with the above chitosan solution or PVA solution, respectively. At last, 0.5 mg GA was added in two types of composite solutions for cross-linking reaction. Membranes were obtained with a thickness about 50-120 µm. Fig. 1 shows the digital pictures of the two prepared PVA/PMViC-Co-VP/GA membrane and Chitosan/PMViC-Co-VP/GA membrane. It can be seen that PVA/PMViC-Co-VP/GA membrane is wholly transparent while the Chitosan/PMViC-Co-VP/GA membrane shows brown color. Both of membranes are self-standing and flexible.

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Fig. 1 Digital pictures of two imidazolium-type membranes, (a) PVA/PMViC-Co-VP/GA membrane ;(b) Chitosan/PMViC-Co-VP/GA membrane. Polymer composition: PVA (Chitosan)/ PMViC-Co-VP /GA = 1: 0.5:0.1 by mass. Ionic conductivity and water uptake The ionic conductivity (for OH- form) of the formed membranes was measured by an AC impedance technique using an electrochemical impedance analyzer (CHI760), where the AC frequency was scanned from 100 kHz to 0.1Hz at a voltage amplitude of 100 mV. Fully hydrated membranes were sandwiched in Teflon conductivity cell equipped with Pt foil contacts on which Pt black was plated. The membranes were in contact with water throughout the measurements. The impedance was measured by placing the cell in a temperature range of 25-80oC. The ionic conductivity (S cm-1) was calculated according to the following equation: [1] Where l is the length of the membrane between two potential sensing platinum wires, R is the membrane resistance, Wand Tare the width and the thickness of the membrane, respectively. The swelling of the membranes was evaluated by the water uptake (WU) of the membranes (g g-1), which was estimated from the mass change before and after the complete dryness of the membrane. A dry membrane was swelled in D.I. water for a day, then the surface water was wiped carefully with a filter paper, and it was immediately weighed. After drying the sample overnight in a vacuum oven at 60oC, the water uptake (WU), was calculated using the expression: /

[2]

Where Wwet and Wdry are the mass of fully hydrated membrane, and of the dry membrane, respectively. Membrane stability

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Tensile evaluation was performed on a universal material testing machine (H5K-S, Hounsfield) under ambient condition (room temperature, ~50% relative humidity) at a speed of 5 mm min-1 with a 1000 N sensor loaded. The mean value was obtained from at least three strip samples with the size of 1×5 cm.

Results and Discussion Ionic conductivity and water uptake The PVA (Chitosan)/PMViC-Co-VP/GA membranes were rendered conducting by soaking in KOH solution to convert it from Cl- form into OH- form. In order to clarify any interaction between the KOH concentration and PVA (Chitosan)/PMViC-Co-VP/GA membranes, it was assessed by soaking the membranes in different concentrations of KOH (1-8 M) solution at room temperature for 24 h. Fig. 2(a) displays the OH- conductivity and WU of Chitosan/PMViC-Co-VP/GA alkaline membranes (1:0.5:0.1 in mass) as a function of KOH concentration for ion-exchange. It can be seen that the OH- values of Chitosan/PMViC-Co-VP/GA membrane decreased with the KOH concentration, and reached 8.8×10-3 S cm-1 at ambient temperature when the KOH concentration was 1M, at this stage, its WU was 43.2%. Fig. 2(b) shows the OH- conductivity and WU of PVA/PMViC-Co-VP/GA membranes (also 1:0.5:0.1 in mass) as a function of KOH concentration for ion-exchange. As shown in Fig. 2(b), the OH- values of PVA/PMViC-Co-VP/GA membrane reached 8.8×10-3 S cm-1 at ambient temperature when the KOH concentration was 2M, at this stage, its WU was 32.1%. It is also found that the WU of this two membranes are less than 100% when immered in different concentrations of KOH (1-8M) solution.

6

0.5 4

0

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8 KOH concentration / mol L-1

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-1

-1

2

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OH Conductivity Water uptake

3

1.0

σ x 10 / S cm

3

8

1.5

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OH Conductivity Water uptake

Water uptake / g g

σ x 10 / S cm

10

1.5

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Water uptake / g g

-1

10

0 0

2

4

6

8 KOH concentration / mol L-1

0.0 10

Fig. 2 OH- conductivity and WU of (a) PVA/PMViC-Co-VP/GA-OH- membranes and (b) Chitosan/PMViC-Co-VP/GA-OH- membranes as a function of KOH concentration for ion-exchange. Polymer compositions for both are 1:0.5:0.1 in mass. Temperature dependences of OH- conductivity

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14

(b) PVA/PMViC-Co-VP

log(σ / S cm-1)

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σ x 10 / S cm

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-4.4

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10 8 6

Chitosan/PMViC-Co-VP

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50 60 o Temperature/ C

70

PVA/PMViC-Co-VP

Chitosan/PMViC-Co-VP

-5.2

80

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3.0 3.1 3.2 1000 / T(K-1)

3.3

Fig. 3 (a) Temperature dependence of OH- conductivity and (b) the lnσ vs. 1000/T plot for PVA (Chitosan)/PMViC-Co-VP/GA-OH- membranes. Polymer compositions for both are 1:0.5:0.1 in mass. The temperature dependences of the ionic conductivity for PVA (Chitosan)/ PMViC-Co-VP/GA-OH- membranes are shown in Fig. 3. As shown in Fig. 3(a), the ionic conductivities of the OH- form membranes varied from (6–10) ×10−3 S cm−1 in D.I. water within the temperature range between 30-80◦C. Membranes exhibited an increment in ionic conductivity with the temperature increasing. The dependence of OH- conductivity on temperature for low temperature polymer electrolytes is typically taken as Arrhenuis type:

−

[3]

Where σ0 is a pre-exponential factor, Ea is the apparent activation energy, and T is the thermodynamic temperature in K. The Ea was estimated from the linear regression of ln (σ) vs. 1000/T as shown in Fig. 3(b), assuming an Arrhenius behavior. Thus, the values of Ea obtained from the Arrhenius plots are 7.92 kJ mol-1 for PVA/ membrane and 7.72 kJ mol-1 for PMViC-Co-VP/GA-OHChitosan/PMViC-Co-VP/GA-OH membrane, respectively. Both are close to 10 kJ mol-1, indicating that the Grotthus mechanism and vehicle mechanism both existed in the membranes (9). However, the Grotthus mechanism becomes more predominant, in which the ion transport proceeds through the hydrogen bond. Membrane stability The mechanical properties of composite membranes were tested and showed in Table 1. It can be seen that both PVA/PMViC-Co-VP/GA membrane and Chitosan/PMViC-Co-VP/GA exhibit good mechanical stability. Here, the Chitosan/PMViC-Co-VP/GA (1:0.5:0.1 by mass) membrane showed the tensile strength of 10.45 MPa with elongation at break of 8.37%. Compared to Chitosan/PMViC-Co-VP/GA membrane, the mechanical properties of PVA/PMViC-Co-VP/GA membrane are much better, where the tensile strength of

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PVA/PMViC-Co-VP/GA membrane reached 31.11MPa with elongation at break of 10.45%, similar to those of the previous reports on AEMs (10,11).

Table 1 The mechanical property of PVA (Chitosan)/PMViC-Co-VP/GA membranes (t = 30 ±2oC ) Membranes

TS (MPa)

Eb (%)

YM (MPa)

PVA/PMViC-Co-VP/GA (1: 0.5:0.1 mass) Chitosan/PMViC-Co-VP/GA(1: 0.5:0.1 mass)

31.11 10.45

10.46 8.37

297 124

Acknowledgements This work is supported by the National Natural Science Foundation of China (21173039), the Innovation Program of the Shanghai Municipal Education Commission (14ZZ074), the Graduate degree Thesis Innovation Foundation of Donghua University (EG2015018), and College of Environmental Science and Engineering, State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile Industry, Donghua University, Shanghai 201620, China. All the financial supports are gratefully acknowledged.

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