Materials Science and Engineering Division and ‡Materials Measurement Science Division, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.
ACS Appl Mater Interfaces. 2016 Dec 7;8(48):33240-33249. doi: 10.1021/acsami.6b12423. Epub 2016 Nov 23.
Perfluorinated ionomers, in particular, Nafion, are a critical component in hydrogen fuel cells as the ion conducting binder within the catalyst layer in which it can be confined to thicknesses on the order of 10 nm or less. It is well reported that many physical properties, such as the Young's modulus, are thickness dependent when the film thickness is less than 100 nm. Here we utilize a cantilever bending methodology to quantify the swelling-induced stresses and relevant mechanical properties of Nafion films as a function of film thickness exposed to cyclic humidity. We observe a factor of 5 increase in the Young's modulus in films thinner than 50 nm and show how this increased stiffness translates to reduced swelling or hydration. The swelling stress was found to increase by a factor of 2 for films approximately 40 nm thick. We demonstrate that thermal annealing enhances the modulus at all film thicknesses and correlate these mechanical changes to chemical changes in the infrared absorption spectra.
全氟离子聚合物，尤其是 Nafion，是氢燃料电池的关键组成部分，它在催化剂层中作为离子传导粘结剂，可以限制在厚度为 10nm 或更薄的范围内。据报道，当薄膜厚度小于 100nm 时，许多物理性质，如杨氏模量，与薄膜厚度有关。在这里，我们利用悬臂梁弯曲法来量化 Nafion 薄膜在循环湿度下的溶胀诱导应力和相关力学性能随薄膜厚度的变化。我们观察到厚度小于 50nm 的薄膜杨氏模量增加了 5 倍，并展示了这种增加的刚度如何转化为减少的溶胀或水合作用。发现厚度约为 40nm 的薄膜的溶胀应力增加了两倍。我们证明，热退火可以提高所有薄膜厚度的模量，并将这些力学变化与红外吸收光谱中的化学变化相关联。
