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Investigation of Water-Molecule Network in Hydrated Polymer Electrolyte Membranes by Raman Spectroscopy: Effect of Polymer Structure and Cation Impregnation

Low relative humidity (RH) and high temperature operation possibly gives a large impact on cost reduction of polymer electrolyte membrane fuel cell (PEMFC) systems for automobile applications. However, these severe operating conditions causes lower proton conductivity in the membrane, resulting in intensive deterioration of the cell performance. Perfluorinated sulfonic acid (PFSA) membranes have been used in PEMFCs to their higher proton conductivity and durability. More recently, hydrocarbon (HC) membranes have been developed as targeted to further cost reduction of the membrane, but their proton conductivity is still insufficient especially in low RH condition. Higher proton conductivity in proton exchange membranes used in PEMFCs is generally regarded as a result of effective proton-hopping process via formation of hydrogen bonding network of water molecules in hydrated membranes [1]. It has been demonstrated that Raman spectroscopy is a powerful tool to investigate watermolecule network (water cluster) in a Nation® membrane [2,3]. Density function theory (DFT) analysis confirmed that lower frequency shift of OH vibration peaks in Raman spectra corresponded to formation of watermolecule network. In this study, we applied Raman spectroscopy to investigate water-molecule network in both PFSA and HC membranes under different RH conditions in order to obtain a fundamental insight in formation of watermolecule network that was possibly affected by polymer structure. We also performed measurement of Raman spectra on PFSA membrane in which two different cations, Sodium ion (Na) or potassium ion (K), was impregnated to examine an influence of physicochemical properties of cations on formation of water-molecule network in the membrane. Figure 1 and 2 show Raman spectra of a PFSA membrane and a HC membrane at different relative humidity. In all measurements, we used an in-house flow cell designed for Raman measurement [2, 3] to control relative humidity surrounding a membrane. We used a Nafion115 membrane as a PFSA membrane and a HC membrane. As reported in our previous study, OH vibration peak intensity (3000 to 3800 cm) increased with relative humidity in both membranes. At RH=30% and 60%, OH vibration peaks (A’, B’ and C’) in the HC membrane was separately identified. This suggests that water-molecule network in the HC membrane under lower RH condition was not well established, but was more isolated than those in the PFSA membrane in which the OH vibration peaks (A, B and C) was likely to overlap each other. With increase in RH up to 95% and more, the HC membrane showed broad OH vibration peaks as observed in the PFSA membrane. We also confirmed that under these RH conditions, the HC membrane showed better proton conductivity in the same level as the PFSA membrane. This suggests that formation of water-molecule network is correlated with proton conductivity and that poor formation of watermolecule network in the HC membrane at low RH condition results in less proton conductivity. We also examined effect of cation on watermolecule network in the PFSA membrane. Figure 3 showed Raman spectra in H-, Kand Na-form PFSA membrane under the same hydration. Kand Na-form PFSA membranes were prepared by soaking the membrane with 1M aqueous solution. The K-form and the Na-form membrane showed less formation of watermolecule network than the H-form. This indicates that dissociation of cation from sulfonic acid in the Na-form and the K-form membrane was deteriorated, because sodium and potassium are more interacted with the sulfonic acid. It was also observed that Na-form membrane showed less water-molecule network than the K-form. This is possibly related with ionic radius of cations where ionic radius of sodium ion is slightly different with hydronium ion and thus hindered hydrogen bonding network in the membrane. References [1] K.D. Kreuer, S.J. Paddison, E. Spohr, M. Schuster, Chem. Rev. 104 (2004) 4637. [2] Y. Tabuchi et al., J.Power Sources,196(2011),652. [3] Y. Tabuchi et al., ECS Trans., 33(2010), 1045.