Uncovering Alternate Pathways to Nafion Membrane Degradation in Fuel Cells with First-Principles Modeling

Abstract

<jats:p>Polymer electrolyte membrane fuel cells (PEMFCs) represent promising energy storage solutions, but challenges remain to maximize their utility. Nafion is frequently employed as the PEMFC membrane material, but degradation of Nafion can limit the life of PEMFCs. Using hybrid density functional theory (DFT), we carry out reaction pathway analysis on a range of candidate degradation pathways on both pristine and defect-containing models of Nafion. Degradation of pristine Nafion initiated by hydrogen radicals involves moderate (ca. 20 kcal/mol) barriers lower than alternative pathways initiated by hydroxyl radicals. We propose a new pathway for continued Nafion degradation after initial H radical attack in the presence of H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;. This pathway has a modest barrier and provides a mechanistic basis for the production of experimentally observed trifluoroacetic acid and hydrogen fluoride. Our work suggests inherent limits to mechanistic studies that use hydroxyl radical as the sole radical source to model Nafion degradation under operating conditions. We observe that hydroxyl radical-only degradation mechanisms have barriers competitive with hydrogen radical species only for initiation at carboxylic acid defects on the main chain or sulfonic acid functional groups on the Nafion side chain. We confirm our observations with DFT by comparison to correlated wavefunction theory. Our study highlights the importance of thorough first-principles modeling to identify the most probable, low energy pathways for materials degradation.</jats:p>

Role of H3O· Radical in the Degradation of Fuel Cell Proton-Exchange Membranes

“…Therefore, we chose the ωB97XD method and the 6-311++G(2d,p) basis set to be used in the following stages due to the faster computational speed than the MP2 method. 10 Zhao et al, 12 and Bajaj et al, 15 respectively, and the other rows are from our calculations. Please note that due to the differences in the model compound and whether the zero-point vibrational energy correction (ZPVE) was included, there are small energy differences between literature values and our values, even for the same method.…”

“…Density functional theory (DFT) calculations from previous reports, as well as in this work, show that these radicals can attack via different reaction pathways. 9− 15 The following degradation reactions are found to have relatively lower degradation barriers (ΔE ≠ ) and could be the candidates for the dominant degradation pathway for backbone and side chain degradation: 10,12…”