Materials and Process Simulation Center, California Institute of Technology, MC 139-74, Pasadena, California 91125, USA.
J Am Chem Soc. 2011 Dec 14;133(49):19857-63. doi: 10.1021/ja2074642. Epub 2011 Nov 15.
We report results of quantum mechanics (QM) mechanistic studies of Nafion membrane degradation in a polymer electrolyte membrane (PEM) fuel cell. Experiments suggest that Nafion degradation is caused by generation of trace radical species (such as OH(●), H(●)) only when in the presence of H(2), O(2), and Pt. We use density functional theory (DFT) to construct the potential energy surfaces for various plausible reactions involving intermediates that might be formed when Nafion is exposed to H(2) (or H(+)) and O(2) in the presence of the Pt catalyst. We find a barrier of 0.53 eV for OH radical formation from HOOH chemisorbed on Pt(111) and of 0.76 eV from chemisorbed OOH(ad), suggesting that OH might be present during the ORR, particularly when the fuel cell is turned on and off. Based on the QM, we propose two chemical mechanisms for OH radical attack on the Nafion polymer: (1) OH attack on the S-C bond to form H(2)SO(4) plus a carbon radical (barrier: 0.96 eV) followed by decomposition of the carbon radical to form an epoxide (barrier: 1.40 eV). (2) OH attack on H(2) crossover gas to form hydrogen radical (barrier: 0.04 eV), which subsequently attacks a C-F bond to form HF plus carbon radicals (barrier as low as 1.00 eV). This carbon radical can then decompose to form a ketone plus a carbon radical with a barrier of 0.86 eV. The products (HF, OCF(2), SCF(2)) of these proposed mechanisms have all been observed by F NMR in the fuel cell exit gases along with the decrease in pH expected from our mechanism.
我们报告了量子力学（QM）对全氟磺酸膜在聚合物电解质膜（PEM）燃料电池中降解机制的研究结果。实验表明，只有在存在 H₂、O₂和 Pt 的情况下，Nafion 降解才会导致痕量自由基（如 OH(●)、H(●)）的产生。我们使用密度泛函理论（DFT）构建了各种可能反应的势能面，这些反应涉及 Nafion 暴露于 H₂（或 H(+)）和 O₂存在 Pt 催化剂时可能形成的中间体。我们发现，HOOH 在 Pt(111)上化学吸附形成 OH 自由基的势垒为 0.53 eV，而化学吸附的 OOH(ad)形成 OH 自由基的势垒为 0.76 eV，这表明 ORR 过程中可能存在 OH，尤其是当燃料电池开启和关闭时。基于 QM，我们提出了两种 OH 自由基攻击 Nafion 聚合物的化学机制：（1）OH 攻击 S-C 键形成 H₂SO₄和一个碳自由基（势垒：0.96 eV），然后碳自由基分解形成环氧化物（势垒：1.40 eV）。（2）OH 攻击 H₂ crossover 气体形成氢自由基（势垒：0.04 eV），随后攻击 C-F 键形成 HF 和碳自由基（势垒低至 1.00 eV）。这个碳自由基可以分解形成酮和一个碳自由基，其势垒为 0.86 eV。这些提出的机制的产物（HF、OCF₂、SCF₂）都已通过 F NMR 在燃料电池出口气体中观察到，同时也观察到了我们的机制所预期的 pH 值下降。
