Proton Exchange Membranes for Fuel Cells with High Proton Conductivity and Membrane Selectivity by Formation Proton Conducting Channels
Date Issued
2009
Date
2009
Author(s)
Su, Yu-Huei
Abstract
Proton conductivity is one of the major properties of proton exchange membranes (PEMs) for direct methanol fuel cells (DMFCs). This work explores an approach to effectively enhance the proton conductivities as well as other properties of cost-effective PEMs for DMFCs by understanding how the chemical structures, physical morphologies, and the state of water influence the membrane’s ability to transport protons, methanol, and water. The results would help to facilitate development of new materials with favorable transport properties for fuel cells use. The discussion in this study is divided into two parts.n the first part, the effect of chemical structures and physical morphologies of PEMs on their proton conductivities is examined using poly(vinylidene fluoride) (PVDF) possessing linear and highly branched polystyrene sulfonic acid side chains. For polymers with similar ion exchange values, both types of PVDF-g-PSSA graft copolymers show similar water uptakes and bound water contents. However, the samples with highly branched PSSA side chains exhibit higher proton conductivity, lower methanol permeability, and higher selectivity compared to the linear analogues. Incorporation of highly branched side chains effectively increases the properties of the PEMs for DMFCs because of the formation of agglomerate PSSA domains, which promote proton conducting but depress methanol permeation through the PEMs. n the second part, incorporating silica nanoparticles (SNP) and sulfonated silica nanoparticles (SA-SNP) into polyelectrolyte membranes with high degree of sulfonation permits investigation of their effect on membrane properties. Sulfonated poly(phthalazinone ether ketone) (sPPEK) with a degree of sulfonation of 1.23 and poly(arylene ether ether ketone ketone) (SPAEEKK) are used as polymer matrixes. The nanoparticles homogeneously disperse in the polymer matrixes. All of the nanocomposite membranes exhibit improved swelling behavior, enhanced thermal stability, and reduced methanol crossover through the membrane. When sPPEK membrane with 5 phr SNP is operated at a high methanol concentration in the feed (3 M) for a single cell test, it shows an open cell potential of 0.6V and an optimum power density of 52.9 mW/cm2 at a current density of 264.6 mA/cm2. The cell performance is better than that of both the pristine sPPEK membrane and the Nafion®117, although the density of sulfonic acid groups in the nanocomposite membranes is lower than others.owever, the sPPEK membrane with 7.5 phr SA-SNP exhibits low methanol crossover, high bound-water content, and a proton conductivity 3.6-fold higher than that of the pristine sPPEK membrane. We also utilize SA-SNP as additives to modify sulfonated poly(arylene ether ether ketone ketone) (SPAEEKK) and to investigate the reasons for the increase in proton conductivity by incorporating SA-SNP into sulfonated PEMs. It is found that the interaction between the sulfonic acid groups of SA-SNP and those of SPAEEKK combined with hydrophilic–hydrophobic phase separation induces the formation of proton-conducting channels, as evidenced by TEM images, which contributes to the increase in the proton conductivity of the polymer / SA-SNP nanocomposite membrane. Therefore, the SPAEEKK/SA-SNP nanocomposite membrane shows a high selectivity, which is 2.79-fold higher than the selectivity of Nafion®117. The improved selectivity of the SPAEEKK/SNP nanocomposite membrane demonstrates potential for the approach of providing hydrocarbon-based PEMs as alternatives to Nafion in direct methanol fuel cells.
Subjects
porton conductivity
graft copolymer
nanocomposite
microphase separation
methanol permeability
DMFCs
Type
thesis
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