Nanostructured Polymer electrolyte membranes for fuel cell applications: Structure vs properties
This dissertation explores various topics within the theme of nanostructured polymer electrolyte membranes having controlled morphology, and their resulting properties. Chapter 1 gives an introduction to the field of Polymer electrolyte membranes (PEM) in its current state, and an overview of the work done. In chapter 2, relatively inexpensive block copolymer ionomers of fluorinated poly(Isoprene)-block-sulfonated poly(Styrene) (FISS) with various sulfonation levels, in both the acid form and the cesium neutralized form, have been cast into membranes of desired random phase separated morphology. The morphology of these membranes were characterized by TEM and USAXS, as well as water uptake, proton conductivity and methanol permeability from 20 to 60°C. The transport properties increased with increasing sulfonation and temperature for all samples. The acid form samples absorbed more water than the cesium samples with a maximum swelling recorded at 60°C for the acid sample with 50mol% sulfonation. Methanol permeability for the latter sample was more than an order of magnitude less than Nafion 112 but so was the proton conductivity at 20°C within the plane of the membrane. Across the plane of the membrane this sample had half the conductivity of Nafion 112 at 60°C.
In chapter 3, neutron and x-ray scattering techniques have been used to study the structural evolution of FISS materials as they have evolved from the dry state to the water soluble state. A dilation of the nanometer-scale hydrophilic domains have been observed as hydration has been increased, with higher swelling for the higher sulfonated sample or upon hydrating at higher temperatures. Furthermore a decrease in the order in these phase separated structures is reduced upon swelling. The glass transition temperature of the fluorinated blocks decreased upon hydration, and at the highest hydration levels loosely bound water was evident. Thermal and dynamic mechanical characterization of these materials have shown that there is a high degree of softening beyond the 45°C glass transition temperature. Finally highly sulfonated samples have shown the formation of spherical micelles, even at concentrations as low as 0.05 mg/ml. The sizes of these micelles range from 13–13.5 nm, with the higher concentration solutions having smaller radius of gyration, possibly due to crowding effects.
In chapter 4, Ionomers from the cesium salt (20 mol%) of fluorinated Poly(Isoprene)-block-sulfonated Poly(Styrene) have been spun cast into membranes and annealed under an electric field of ∼40 V/um at 130°C for 24 hours. This resulted in the transformation of the morphology from a random phase separated state to one preferentially oriented in the direction of the electric field but with smaller domain sizes. The effect of this change in morphology was a 2.5 times increase in the ionic conductivity, as measured by electrochemical impedance spectroscopy, at all humidity conditions measured. This can be attributed to the increased connectivity of the ionic domains.
In chapter 5, The applicability of electrospun nanostructure with high surface to volume ratios for PEM application is presented. To this end, sulfonated poly(ether ether ketone) has been electrospun and electrosprayed by varying concentration in DMF, yielding isotropic fibrous mats and beads. The glass transition temperatures of these materials have been shown to be higher those of the unsulfonated precursors and they increase with increasing sulfonation, due to hydrogen bonding induced rigidity. The presence of sulfonic acid groups on the surface has been confirmed by means of x-ray photoelectron spectroscopy, with sulfur representing 3% of the surface elemental composition. These acid groups on the surface of internal fibers, help to form a 3 dimensional network of conducting channels in the voids of the fibrous mats upon hydration. This in turn has led to an improvement of conductivity from 0.033 S/cm for void-less solution cast membranes to 0.040 S/cm for the electrospun fibrous mats.
0795: Materials science