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Abstract A PEM fuel cell with the Nafion ionomer phase of the cathode catalyst layer (CL) that was exposed to hot dry gas during the hot‐pressing process showed improved performance over the whole current density range and ~ 220% peak power increase with humidified air at 80°C. This enhanced performance is attributed to the modified structure of the perfluorosulfonic acid (PFSA) ionomer layer in the CL during the MEA's hot‐pressing process. The dry gas exposure above the glass transition temperature (Tg) results in the aggregation of the ionic groups to retain the residue water molecules. This process separates the ionomer into ionic‐group‐rich domains and ionic‐group‐sparse domains. The ionic‐group‐sparse domains create hydrophobic interface and reactant transport channels with lower water content and thus higher oxygen solubility in the ionomer. Accordingly, the water‐unsaturated ionomer and its surface hydrophobicity enhance the kinetic‐controlled and concentration‐polarized regions of the fuel cell polarization curve, respectively. The surface hydrophobicity of the ionomer layer is analyzed by the contact angle measurement and XPS. The durability of the hydrophobic effect belowTgis demonstrated by boiling the treated material. Re‐treating the hydrophobic sample with humidified gas exposure aboveTgeventually exhibits hydrophilic features, further proving the manipulability of the ionic group distribution.
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Exceptional thermal conductivity increase of Nafion by hydrogen-bonded water molecules
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.
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- Award ID(s):
- 2032464
- PAR ID:
- 10524712
- Publisher / Repository:
- AIP
- Date Published:
- Journal Name:
- Applied Physics Letters
- Volume:
- 124
- Issue:
- 26
- ISSN:
- 0003-6951
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0217244
