Computational study of proton transfer in restricted sulfonic acid for proton exchange membrane fuel cell

Abstract

Interest in the use of fuel cell as highly efficient, clean energy conversion device has been rapidly increasing over the past twenty years. Currently, proton exchange membrane fuel cells (PEMFC) are regarded as the paramount type of fuel cell due to their wide range of applicability. Perfluorosulfonic acid (PFSA) ionomer Nafion® by DuPont remains the typical membrane in PEMFC under development today, despite well recognized drawbacks which include limitations in thermal stability. Recent studies have found that although the polymer-zeolite composite membranes have lower value of proton conductivity than Nafion®, polymer-zeolite composites show a more stable performance at high temperature. In this study, the proton transfer mechanism of zeolite functionalized sulfonic acid with water molecules is investigated using density functional theory (DFT) calculation at PM3/ONIOM(B3LYP/6-311G(d,p):PM3) level of theory. The systems were constructively built up by modifying the crystal structure of Linde Type A (LTA) zeolite functionalized sulfonic acid side chains, by varying the degree of separation of sulfonic acid side chains (2T, 3T and 4T) as well as the alkyl chain length (n=3, 5, 7) in order to study the effect of proton transfer at different distance and different chain length. Extensive searches for minimum energy conformations from 1 to 6 explicit water molecules revealed that 2T distance gives the best results for propyl sulfonic acid side chain, meanwhile 4T distance gives the best result for pentyl and heptyl sulfonic acid side chains, indicated by the minimum water molecules required to initiate second proton dissociation. The results have shown several agreements with previous calculation regarding polymeric fragments where partial dissociation of the protons in the fragments occurs at water contents of less than 3 H2Os/SO3H. Furthermore, we found that water distributions that facilitate a higher degree of dissociation and separation of the protons are important factors in stabilizing the fragments.