(213e) High Anion Conduction in Partially Fluorinated Multiblock Copolymers

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: X_5.4Y₇,
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

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.