Structure-Property Relationships for Thin-Ilm Nafion: Model-Predictive Design of Low-Pt PEMFC Catalyst Layers
Authors: DeCaluwe Steven C. , Randall Corey R. 
Published: December 2024

Proton exchange membrane fuel cells (PEMFCs) are a promising means to decarbonize the global vehicle fleet, but commercialization is limited by costly Pt catalysts, among other challenges. PEMFC designs with low Pt loading typically suffer anomalous performance losses. Previous studies have agreed that poor performance with low Pt-loading is attributed to slow transport through the nano-thin Nafion ionomer found in the catalyst layer (CL). Although it is known that the properties of the Nafion thin films in the CL vary from those previously measured for thicker membranes, research has not yet definitively connected CL polymer properties with PEMFC limitations in low Pt-loaded cells, nor found a means to address the limitations for low cost, higher-performance PEMFCs.

This presentation covers parallel efforts to identify and address limiting processes in low-Pt-loaded PEMFC cathode CLs. We begin with an extensive investigation of thin-film Nafion structure-property relationships, as an analog for CL Nafion thin-films. We model nm-scale transport in thin-film Nafion to develop structure-property relationships for thin-film Nafion, based on in situ neutron reflectometry [2], and conductivity measurements [3] on thin film Nafion. These include equilibrium measurements for varying thicknesses and substrates, as well as novel in operando measurements of thin-film Nafion with an H2O flux across the film. Results suggest that conductivity in CL Nafion is a function of not just temperature and relative humidity (as in bulk Nafion membranes), but also film thickness and proximity to the substrate. These insights result in a model that calculates the conductivity as a function of temperature, local water uptake, film thickness, and underlying substrate (Pt or carbon).

Simultaneously, we have conducted continuum-scale numerical simulations of PEMFC catalyst layers, to determine the necessary level of detail for predictive yet efficient numerical simulation for device design. Incorporating these structure-property models into a continuum-scale PEMFC cathode half-cell model to locally resolve proton conductivity and O2 diffusion improves model fits to PEMFC data with varying Pt loading [4]. The fits provide new insights into the coupling between kinetic, mass transport, and Ohmic losses in low-Pt-loaded PEMFC cathodes and show that ohmic losses related to proton transport in thin film Nafion play a key role in low-Pt loaded cells. Based on these new fits to low-Pt loaded cells, we present a parametric study of PEMFC CL design parameters with low Pt loading. Incorporating multi-scale modeling of Nafion transport processes enhances the predictive model capabilities and unlocks new insights into CL design. In particular, results suggest that functionally graded catalyst layer designs could significantly improve PEMFC power density with low Pt loading [4].

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