Molecular modeling assisted design of new monomers utilized in fuel cell proton exchange membranes

An extensive understanding of the proton dissociation mechanism that occurs in Nafion® is of great importance in the development of an improved proton exchange membrane for use in fuel cells (PEMFC). As the proton leaves the sulfonic acid group, structural changes within the Nafion® side-chain take place. To visualize such a process, molecular modeling is particularly useful. From an experimental viewpoint, changes that occur in bonds and atomic environment can be characterized by a judicious analysis of the normal modes of vibration. Using quantum chemical modeling of the infrared spectra of Nafion®, it was shown that a model system consisting of two triflic acid (TfOH) molecules accurately predicts the process of deprotonation in Nafion® involving the addition of water molecules. This model system allows the visualization of the deprotonation events by monitoring the changes in selected frequencies. We thus observed that only the sulfonic acid groups containing the departing proton undergoes structural modification before the first proton dissociation occurs. In turn, we used this information to design new monomers that respond to these particular changes resulting from the electronegativity of fluorine atoms. The rigidity of the proposed architecture should also exhibit improved mechanical properties.

► Calculations captured key structural elements that make Nafion© an excellent PEM.
► New innovative monomers were designed that specifically incorporate these components.
► The proposed molecules are presently being synthesized.
► One of them has been prepared and reveal a pKa < −2, a very encouraging result indeed.