Saito Morihiro, Hayamizu Kikuko, Okada Tatsuhiro
National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Center 5, Ibaraki 305-8565, Japan.
J Phys Chem B. 2005 Mar 3;109(8):3112-9. doi: 10.1021/jp045624w.
To clarify the mechanisms of transport of ions and water molecules in perfluorosulfonated ionomer membranes for fuel cells, the temperature dependence of their transport behaviors was investigated in detail. Two types of Flemion membranes having different equivalent weight values (EW) were utilized along with Nafion 117 as the perfluorinated ionomer membranes, and H-, Li-, and Na-form samples were prepared for each membrane by immersion in 0.03 M HCl, LiCl, and NaCl aqueous solutions, respectively. The ionic conductivity, water self-diffusion coefficient (D(H)(2)(O)), and DSC were measured in the fully hydrated state as a function of temperature. The ionic conductivity of the membranes was reflected by the cation transport through the intermediary of water. Clearly, H(+) transports by the Grotthuss (hopping) mechanism, and Li(+) and Na(+) transport by the vehicle mechanism. The differences of the ion transport mechanisms were observed in the activation energies through the Arrhenius plots. The D(H)(2)(O) in the membranes exhibited a tendency similar to the ionic conductivity for the cation species and the EW value. However, no remarkable difference of D(H)(2)(O) between H- and the other cation-form membranes was observed as compared with the ionic conductivity. It indicates that water in each membrane diffuses almost in a similar way; however, H(+) transports by the Grotthuss mechanism so that conductivity of H(+) is much higher than that of the other cations. Moreover, the D(H)(2)(O) and DSC curves showed that a part of water in the membranes freezes around -20 degrees C, but the nonfreezing water remains and diffuses below that temperature. This fact suggests that completely free water (bulk water) does not exist in the membranes, and water weakly interacting with the cation species and the sulfonic acid groups in secondary and higher hydration shells freezes around -20 degrees C, while strongly binding water in primary hydration shells does not freeze. The ratio of freezing and nonfreezing water was estimated from the DSC curves. The D(H)(2)(O) in the membranes was found to be influenced by the ratio of freezing and nonfreezing water. DFT calculation of the interaction (solvation) energy between the cation species and water molecules suggested that the water content and the ratio of freezing and nonfreezing water depend strongly on the cation species penetrated into the membrane.
为阐明用于燃料电池的全氟磺酸离聚物膜中离子和水分子的传输机制，详细研究了其传输行为的温度依赖性。使用了两种具有不同当量重量值（EW）的Flemion膜以及作为全氟离子交换膜的Nafion 117，并通过分别浸入0.03 M HCl、LiCl和NaCl水溶液中为每种膜制备了H型、Li型和Na型样品。在完全水合状态下测量离子电导率、水自扩散系数（D(H)(2)(O)）和DSC作为温度的函数。膜的离子电导率通过阳离子通过水的中介进行传输来反映。显然，H(+)通过Grotthuss（跳跃）机制传输，而Li(+)和Na(+)通过载流子机制传输。通过阿仑尼乌斯图观察到离子传输机制在活化能方面的差异。膜中的D(H)(2)(O)表现出与阳离子种类和EW值的离子电导率相似的趋势。然而，与离子电导率相比，未观察到H型膜与其他阳离子型膜之间D(H)(2)(O)有显著差异。这表明每种膜中的水几乎以相似的方式扩散；然而，H(+)通过Grotthuss机制传输，因此H(+)的电导率远高于其他阳离子。此外，D(H)(2)(O)和DSC曲线表明，膜中的一部分水在约-20℃时冻结，但未冻结的水仍存在并在该温度以下扩散。这一事实表明膜中不存在完全自由的水（大量水），与阳离子种类和二级及更高水合层中的磺酸基团弱相互作用的水在约-20℃时冻结，而与一级水合层中强结合的水不冻结。从DSC曲线估计了冻结水和未冻结水的比例。发现膜中的D(H)(2)(O)受冻结水和未冻结水比例的影响。阳离子种类与水分子之间相互作用（溶剂化）能的密度泛函理论计算表明，水含量以及冻结水和未冻结水的比例强烈依赖于渗透到膜中的阳离子种类。
