Sulfonation mechanisms of aromatic polymers for PEMFC applications

Abstract

The sulfonated aromatic polymers are competitive proton conducting materials for proton exchange membrane fuel cells because of their relatively low costs and adequate performances compared with the perfluorinated sulfonic acid ionomers. These polymeric materials can be economically synthesized by the sulfonation of existing aromatic polymers; however, the inadequate sulfonation degree has prevented the further optimization of their overall performances. In order to improve the overall performances of sulfonated aromatic polymeric proton conducting materials, the sulfonation mechanisms of typical aromatic polymers, including bisphenol A type polysulfone (BPA-PSF), 4,4'-biphenol type polysulfone (44BP-PSF), poly(ether ether ketone) (PEEK), and poly(ether sulfone) (PES), are studied in terms of transition states and activation energies using density functional theory calculations with satisfactory results. It is concluded that the sulfonations are favored at the ortho-position of direct arylene-arylene linkage and ether bond linkage, but retarded by the bulky sulfonyl linkage, dimethyl methylene linkage, and carbonyl linkage. It is also concluded that the solvation effect from polar solvent reduces the activation barrier more efficiently than that from nonpolar solvent.

Polysulfone with Different Degrees of Sulfonation: Simple Method with Acetyl Sulfate

“…In the reactions with AES, in structures like bisphenol A (poly(arylene ethers), as the one contained in the PSU, the insertion of the group -SO3H occurs preferably in the ortho position related to the C-O bonds of the ether group of the monomeric part of bisphenol A, that is, in the meta position to the alkyl group (position 2 in Figure 2), due to the lower steric hindrance and the mesomeric effect, predominant in this case, the inductive effect caused by the alkyl group in this activated aromatic ring, exactly by the alkyl group [22,26,29,30,42]. In the case of PSU, the deactivating group (sulfone) causes the reaction rate to be lower, however, both the orientation position of the sulfone group and the C-O group are coincident, that is, sulfonation occurs in position 7 (Figure 2) [42]. Designating the PSU-SA when occurs AES in more than one ring and can involve both positions (2 and 7) and PSU-SB when occurs the substitution in only one single ring (mainly in position 2)…”

“…The polymer shows different mere units throughout its structure, therefore the spectrums seen in Figure 6 result in that diversity with chains with the group -SO3H inserted in position 2 in one of its meres in position 7 and so on. That diversity and complexity make it hard to make an individual assignment for each proton [26,29,42]. By observing the integrals of proton 2, it is possible to conclude that sulfonation preferably occurs at this site, as its integration reduces from 4 protons to approximately 1.91 on PSU-SA.…”