Elucidating Cation Transport Properties in Nafion Membranes and Electrode Ionomer Network via X-ray Fluorescence Imaging

Membranes and electrode ionomers in proton exchange membrane fuel cells are prone to cation contamination, leading to a reduction in performance. Despite the importance, the characteristics of cation mobility within membranes and ionomer thin films remain poorly understood. Here, we investigate Co 2+ transport properties in membranes and electrode ionomers using synchrotron X-ray fluorescence imaging. Specifically, the samples are doped with a controlled Co 2+ exchange and the samples are subsequently subject to hydrogen pump operation under fixed humidity and potential gradient. A 1-D model is developed based on the Nernst-Planck relation, which predicts the diffusion and mobility coefficients of Co 2+ . We also develop a characterization platform, termed Electrode Ionomer Network (EIN), for studying cation transport properties in electrode ionomers that are inherently tortuous and non-uniform. We observe that an increase in relatively humidity from 40 to 75% increases both the diffusion and mobility in Nafion membranes (by factors of 2.2 and 7.1, respectively), determined via fitting the Nernst-Planck relation to our experimental data. Despite the identical humidity conditions, Co 2+ become less mobile in EINs relative to membrane (lower by 67% and 44% for diffusivity and mobility, respectively), which are attributed to confinement effects, and the tortuous and disconnected ionomer network in the electrode. Our results provide insights that can help predict cation concentration distributions across membrane-electrode assemblies for hydrogen fuel cell applications.