453357 High Anion Conduction in Partially Fluorinated Multiblock Copolymers

Monday, November 14, 2016: 4:19 PM
Plaza B (Hilton San Francisco Union Square)
Lisha Liu, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, John Ahlfield, School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA and Paul A. Kohl, Georgia Institute of Technology, Atlanta, GA

High Anion Conduction in Partially Fluorinated Multiblock Copolymers

Lisha Liu*†, John M. Ahlfield, and Paul A. Kohl

School of Materials Science and Engineering,

School of Chemical and Biomolecular Engineering

Georgia Institute of Technology

Atlanta, Georgia 30332-0100

Anion Exchange Membranes (AEM) are used in fuel cells, electrolyzers and electrodialysis devices. Alkaline conditions provide a route to overcoming fundamental issues with acid-based devices. The fuel cell and electrolyzer issues include the high cost of platinum catalysts, complex water management, and sluggish electrochemical reactions. However, the performance of AEM fuel cells is not as good as that of Proton Exchange Membrane (PEM) fuel cells partially because of the limitations of current anion exchange membranes, such as low ionic conductivity, poor stability at high pH, and high water uptake.

A series of partially fluorinated multiblock copoly(arylene ether)s with long head-group tether were synthesized for use in AEM fuel cells and electrolyzers. The multiblock copolymers were synthesized via polycondensation of hydroxyl-terminated oligomers and fluoro-terminated oligomers. The resulting multiblock structure has one hydrophilic block and one hydrophobic block. It was designed so that nanophase-separation occurs and efficient conductive channels are formed with low water uptake. Multiblock copolymers with different block lengths and ion exchange capacity (IEC) were synthesized to maximize ion conductivity and explore the relationship between chemical structures and membrane properties. Table 1 shows the membrane properties.  A non-linear relationship was found between the number of head-groups on a hydrophilic block and the conductivity. Doubling the number of head-groups more than doubled the hydroxide conductivity. Hydroxide conductivity as high as 119 mS/cm at 80°C have been observed with a specific size block size: X5.4Y7, where X is the hydrophobic block and Y is the hydrophilic block. The hydrophobicity of the backbone has allowed synthesis of polymers with minimal water uptake. The ratio of conductivity-to-water uptake shows that less water is absorbed compared to conventional materials. The number of waters per ion within the polymer is as low as 4. This is reflected in the measurement of the amount of free-water and bound-water. No conductivity loss was observed after soaking the membrane in 1M NaOH solution at 60°C for over 600 hours.

Financial support from US Office of the Deputy Assistant Secretary of the Army for Defense Exports and Cooperation (DASA-DE&C) is gratefully acknowledged.

Table 1. Summary of properties of membranes

Block copolymer

Molecular Weight (GPC)

IEC (Ion Exchange Capacity) (meq/g)

OH- Conductivity (mS/cm)

Water Uptake

(%)

R.T.

40˚C

60˚C

80˚C

Y8-1

18K

1.18

13.12

17.91

24.34

36.05

35.85

X3.1Y3.6-1

88.6K

0.66

16.41

26.37

35.55

51.50

5.56

X5.4Y7-1

68.2K

0.73

14.17

21.08

27.94

34.72

8.00

X5.4Y7-2

68.2K

1.30

38.21

58.29

96.07

119.70

50.77

X3.1Y8-2

55.9K

1.56

23.13

45.41

66.41

94.03

26.67

X3.1Y3.6-2

66.0K

1.19

25.79

41.75

59.03

85.00

25.00

X5.9Y5-2

59.0K

1.10

22.14

33.56

50.29

66.70

19.57

1.      X-hydrophobic block; Y-hydrophilic block; subscripted number-block lengths; 1 or 2-tether amount)

2.      IEC (Ion Exchange Capacity) was calculated via 1H NMR results.

3.      OH- conductivity was measured by four-probe conductivity cell.

4.      Water uptake was measured at room temperature.


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