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

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 H2O2. 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 wave function theory. Our study highlights the importance of thorough first-principles modeling to identify the most probable, low-energy pathways for materials degradation.聚合物电解质膜燃料电池（PEMFCs）是极具应用前景的能源存储方案，但要最大化其实际应用效能仍面临诸多挑战。纳菲昂膜（Nafion）常被用作PEMFC的核心膜材料，但纳菲昂膜的降解失效会直接缩短PEMFC的服役寿命。本研究采用杂化密度泛函理论（DFT），针对完整纳菲昂膜与含缺陷纳菲昂膜的两类模型，对一系列潜在降解路径开展反应路径分析。由氢自由基引发的完整纳菲昂膜降解过程，其能垒约为20 kcal/mol，属于中等能垒，低于羟基自由基引发的其他降解路径的能垒。本研究提出了一种全新的降解路径：在过氧化氢（H₂O₂）存在的工况下，初始氢自由基攻击纳菲昂膜后，其降解过程可继续进行。该路径的能垒适中，可为实验中观测到的三氟乙酸与氟化氢的生成过程提供机理解释。本研究表明，若仅以羟基自由基作为唯一自由基源，来模拟运行工况下纳菲昂膜的降解过程，此类机理研究存在固有局限性。本研究发现，仅以羟基自由基为引发源的降解机理，其能垒仅在两种场景下可与氢自由基路径相媲美：一是在主链的羧酸缺陷位点处引发降解，二是在纳菲昂膜侧链的磺酸官能团位点处引发降解。本研究通过与关联波函数理论的对比计算，借助DFT验证了上述观测结果的可靠性。本研究强调，需通过全面的第一性原理建模，才能精准识别材料降解过程中最具可行性的低能反应路径。