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Quantifying thin-film Nafion structures and their impact on PEMFC performance via neutron reflectometry and modeling
Randall, Corey R.
Randall, Corey R.
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2023
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Abstract
Proton exchange membrane fuel cells (PEMFCs) are a promising clean-energy technology; however, poor performance and durability in low-cost cells limit widespread adoption. Current literature is in agreement that some combination of kinetic and/or species transport losses associated with Nafion thin films in PEMFC cathode catalyst layers cause excessive losses. Even so, a path toward identifying and mitigating a specific limiting process has remained unclear. Therefore, this work uses a combination of modeling and experiments to further define and understand species transport mechanisms in thin-film Nafion.
Models in this work incorporate experimentally informed, first of their kind, structure-property relationships. These relationships predict how ionic conductivity and oxygen diffusion are impacted by previously observed nano-structures in humidified thin-film Nafion. Results from the models show good agreement with data from low-Pt-loaded PEMFCs, and provide crucial insight to further understand limiting phenomena. Furthermore, the models are used to predict improved PEMFC designs that mitigate high transport losses. Additionally, experiments in this work characterize water uptake and structures in carbon-supported Nafion thin films, promoting conclusions drawn from the models.
This research has led to at least four significant research contributions. The first of which is the derivation of experimentally informed thin-film Nafion structure-property relationships. Incorporating these relationships into physics-based models provides excellent agreement to experiments with low-Pt-loaded PEMFCs, without the need for imperfect assumptions about species transport and effective domain geometries. The second contribution involves exercising models to supply convincing evidence that, in addition to flooding, proton transport in PEMFC cathode catalyst layers limits cell performance. The third novel contribution is predicting future low-cost PEMFC designs that improve power densities by as much as 25%. Catalyst layers with Pt and ionomer loadings concentrated near the membrane interface showed the most promise. The final novel contribution is identifying thin-film Nafion structures -- and their impact on transport -- near carbon interfaces. Carbon-supported Nafion thin films show less interfacial structuring than Nafion at silicon interfaces. Despite this, results suggest water uptake, and presumably species mobility, in Nafion majority layers is independent of the support material.
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