Exceptional thermal conductivity increase of Nafion by hydrogen-bonded water molecules

Abstract

Nafion, a widely used proton exchange membrane in fuel cells, is a representative perfluorosulfonic acid membrane consisting of a hydrophobic Teflon backbone and hydrophilic sulfonic acid side chains. Its thermal conductivity (k) is critical to fuel cell's thermal management. During fuel cell operation, water molecules inevitably enter Nafion and could strongly affect its k. In this work, we measure the k of Nafion of different water content (λ). Findings reveal that k is significantly low in a vacuum environment characterized as 0.110 W m−1 K−1, but at λ ∼1, a notable increase is observed, reaching 0.162 W m−1 K−1. Moreover, k at λ ≈ 6 is 60% higher than that of λ ∼1. This exceptional k increase is far beyond the theoretical prediction by the effective medium theory that only considers simply physical mixing. Rather this k increase is attributed to the formation of water clusters and channels with increased λ, creating thermal pathways through hydrogen bonding, thereby improving chemical connections within the Nafion structure and augmenting its k. Furthermore, it is observed that Nafion's k reaches the maximum value of 0.256 W m−1 K−1 at λ ≈ 6, with no further increase up to λ ≈ 10.5. This phenomenon is explained by the coalescence of water clusters at λ ≈ 6, forming channels that optimize heat transfer pathways and connections within the Nafion structure. Moreover, the free movement of water molecules within water channels (λ > 6) shows physical alterations in Nafion structure (significant volume increase), which have a lesser impact on k.