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Dynamics of water in a catalyst layer of a fuel cell by quasielastic neutron scattering

中性子準弾性散乱による燃料電池触媒層中の水のダイナミクス

伊藤 華苗  ; 山田 武*; 篠原 朗大*; 高田 慎一 ; 川北 至信 

Ito, Kanae; Yamada, Takeshi*; Shinohara, Akihiro*; Takata, Shinichi; Kawakita, Yukinobu

The catalyst layer (CL) in polymer electrolyte fuel cells (PEFCs) plays a critical role in the performance of a PEFC. In this study, we investigate the water dynamics in the CL using quasielastic neutron scattering (QENS) and small-angle neutron scattering. The temperature dependence of the mean square displacement $$<$$u$$^{2}$$$$>$$ shows that the freezing of water does not occur at a melting temperature in the Nafion thin film of the CL, suggesting that the water is confined in a smaller region than $$sim$$15 AA. The QENS measurements established three kinds of water in the CL: immobile water tightly connected to a sulfonic group, water in a fast mode assigned to free diffusion restricted in a sphere, and water in a slow mode described by a jump diffusion model. Assuming that these three modes were independent, the number of water molecules in each mode was estimated. On discussing the structure and dynamics elucidated in the study, we finally conclude that the coupled model of the fast and slow modes is plausible for describing the diffusion of water confined in the thin Nafion film of the CL.

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