Within an ionomer, cations, including Ce3+, associate (ion pair) with the negatively charged sulfonate groups to varying degrees and have been observed to diffuse in-plane on the centimeter length scale during ten to100 hours of fuel cell operation. More rapid migration of cations into the electrode layers has been observed over the µm length scale of MEA thickness. The movement of cations within the MEA is governed by local water content (λ = n H2O/-SO3-), water content gradients (Δλ) and electrical potential gradients. The observed in-plane movement of Ce3+ and other cations such as Co2+ can lead to large cation concentration gradients within a fuel cell MEA. This 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 isotropic diffusion coefficients of Ce3+ and Co2+ within NRE211 membrane over a wide range of uniform temperature and relative humidity values. The cation concentrations have been monitored using a variety of analytical techniques including FTIR, x-ray fluorescence and UV spectroscopy (Figure 1). The derived diffusion coefficients show a strong dependence on both temperature and relative humidity value (water content) and are consistent with the macroscale redistribution of cations seen in operating fuel cells. Quantitative structure activity relationships and cation movement under RH gradients will also be discussed.