In this study, the low-humidity proton conductance and degradation mechanisms are theoretically investigated for Nafion®, which is the most frequently used proton exchange membrane.

The proton conductance mechanism of Nafion membrane was first explored to reveal the cause for its high proton conductance under low-humidity conditions (Ref. 1). Nafion membrane gets an advantage in proton exchange membranes in fuel cells mainly due to its high low-humidity proton conductance. The actual proton conductance of Nafion at low humidity was explored by comparing to that of hydrocarbon multiblock sulfonated poly(arylene ether sulfone)s (SPE) membrane. Using long-range corrected density functional theory (LC-DFT) that one of the authors (TT) developed (Ref. 2), we calculated the proton dissociation energies from the sulfonic acid groups and investigated the subsequent proton conductance mechanism at low humidity. As a result, we found that these membranes have negligible difference in the proton dissociation ability inside the hydrated water clusters but show a remarkable difference in the conductivity of protonated water clusters in disconnected hydrogen bond networks. Note that low-humidity membranes are known to contain the regions of disconnected hydrogen bond networks (Ref. 3), in which Grotthuss mechanism assuming connected hydrogen bond networks does not proceed. Though vehicle mechanism assuming the diffusion of protonated water molecules (or clusters) seems to explain the proton conductance in the disconnected networks, we found that protonated water molecules and clusters need very large dissociation energies (about 400 kJ/mol). As an alternative, we suggested the relay mechanism (Fig. 1), in which protonated water clusters are relayed between the sulfonic acid groups through the doubly-hydrated structures. The calculated results showed that protonated water clusters require only about 100kJ/mol to dissociate in this relay mechanism.

The vibrational spectra of hydrated Nafion membrane was also investigated to reveal the dependence of the proton conductance on humidity in further detail by a collaborative experimental and theoretical infrared (IR) spectroscopy study (Ref. 4). The experimental time-resolved attenuated total reflection Fourier transform IR spectroscopic results showed a unique IR peak intensity dependence on the hydration number. Based on the experimental results, we theoretically explored the dependence of the vibrational spectra of Nafion membrane on the hydration number by using LC-DFT. As a result, we found that this unique IR peak intensity dependence is correctly reproduced for the singly-hydrated but for the doubly-hydrated Nafion, in which protons are initially detached from the sulfonic acid groups in the geometry optimizations. This strongly supports the above-mentioned relay mechanism of the proton conductance.

The degradation mechanism of Nafion membrane was also explored focusing on the H₂O₂-induced dissociation of the ether-linkages of the side chains. Many conventional studies on the degradation of membranes in fuel cells have assumed the principal mode of the degradation involves OH radicals. However, recent experimental studies have raised questions concerning this assumption. For example, a Fenton test, which is supposed to provide OH radicals, have revealed that the ratio of the degradation fragment molecules is considerably different from that of actual operating conditions (Ref. 5). Using LC-DFT, we found an H₂O₂-induced degradation mechanism, which is consistent with experimental results. In this mechanism, the ether-linkage of the side chains of Nafion is decomposed with a relatively low activation energy to produce (H₂O)_lHO₃S-CF₂-CF₂-O-OH (l is the hydration number) as a key fragment. We also elucidated the subsequent decomposition reaction mechanisms, though these mechanism need metal ions to proceed. The vibrational spectra for the decomposed Nafion are found to be consistent with the experimental IR spectra. This mechanism is supposed to proceed together with OH radical-induced degradation mechanism.

This work was supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cells (SPer-FC)” project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

1. T. Tsuneda, R. K. Singh, K. Miyatake, and M. Watanabe, Chem. Phys. Lett., 608, 11 (2014).
2. H. Iikura, T. Tsuneda, T. Yanai, and K. Hirao, J. Chem. Phys., 115, 3540 (2001).
3. G. Gebel, Polymer, 41, 5829 (2000); Macromolecules, 46, 6057 (2013).
4. R. K. Singh, K. Kunimatsu, K. Miyatake, and T. Tsuneda, Macromolecules, 49, 6621 (2016).
5. T. Kinumoto, M. Inaba, Y. Nakayama, K. Ogata, R. Umebayashi, A. Tasaka, Y. Iriyama, T. Abe, and Z. Ogumi, J. Pow. Sour., 158, 1222 (2006).

Figure 1. Schematic diagram for the proton conductance in Nafion membrane in fuel cells.