Convective Transport of Ce3+ in Nafion® Membrane

Abstract

Proton exchange membrane fuel cells (PEMFC) are often loaded with various metal cations (Ce3+, Co2+, etc.) within the ion-conducting membrane. These proton-displacing cations can be introduced at beginning-of-life (BOL) for chemical degradation mitigation purposes or may passively accumulate over run time via Pt-alloy catalyst dissolution or other contamination sources.1 Within the proton conducting ionomer phase of the membrane electrode assembly (MEA), the metal cations ion pair with the sulfonate anions of the polymer. During operation over less than 100 hours, the metal cations are observed to migrate or redistribute on cm-length scales in the plane of the MEA. The movement of cations within the MEA plane is governed by cation charge, ionic radius, local water content (λ = n H2O/-SO3 -) and water content gradients (Δλ). Previous studies have shown that the cation isotropic concentration-gradient diffusion coefficients vary strongly with the membrane hydration level.2 The resulting redistribution of cations can have a significant impact on cell performance and mitigation of chemical degradation processes. Accordingly, there is great interest in understanding and quantifying the factors that govern cation movement to optimize fuel cell efficiency and durability.We will report the in-plane convective migration of Ce3+ and Co2+ in Nafion® membranes at 80 °C under the influence of controlled λ gradients. The cation migration studies were performed using a two-chamber RH cell as shown in Figure 1a. The test design can investigate the migration of both discreet (positions 2-4) and continuous uniform cation distributions (position 1), allowing a comprehensive understanding of the migration physics.Figure 1b depicts the convective migration of Ce3+ in Nafion® membrane (NR211, 12 mol%) over 288 hours of treatment in the two-chamber cell at 80 °C. Significant net migration of Ce3+ from the wet (95%RH) to the dry chamber (50% RH) is observed, including wet-side Ce3+ depletion and dry-side concentration. In addition to the convective migration, isotropic concentration gradient diffusion coefficient measurements were obtained from positions 3 and 4 (Figure 1a).Using the fundamental diffusion coefficients determined at various RH values, a robust 1-D transient model of the convective transport has been developed that captures experimental observations of cation migration across a wide range of membrane hydration gradients and cation concentrations. References A. Kusoglu and A. Weber, Chem Rev. 117, 987, (2017)A. M. Baker, S. K. Babu, K. Chintam, A. Kusoglu, R. Mukundan, and R. L. Borup, ECS Trans., 92 (8), 429 (2019) Figure 1. Depiction of the convective cation migration apparatus and representative data. The two-chamber device (a) accommodates four test samples that can support a range of t=0 cation distributions. Experimental 80°C time series data of a uniform 12 mol% Ce3+ Nafion® sample subjected to 95 and 50% RH in the wet and dry chambers, respectively. Figure 1