Abstract
Synchrotron based X-ray tomographic microscopy was used to image the redistribution of phosphoric acid in HT-PEFC due to electrolyte migration from cathode to anode. The acid migration rate, transference number of the hydrogen phosphate ion and flooding of the anode gas diffusion layer (GDL) was analyzed for MEAs with different membrane acid doping levels (24–36 mgcm−2) and membrane materials (imbibed m-polybenzimidazole (PBI) and polyphosphoric acid (PPA) processed p-PBI). The most influential factors for the acid migration rate are current density and the amount of free acid in the membrane. High doping level of the membrane and current density above 0.4 A cm−2 significantly increase the migration rate. From the migration rates apparent transference numbers for the hydrogen phosphate anions in the order 10−5 to 10−4 are calculated at the high current densities. Besides the membrane properties, also the influence of the microstructure of the porous transport layers was analyzed. Most probably cracks in the catalyst and microporous layers facilitate the migration of acid into the anode GDL.
Highlights
High temperature polymer electrolyte fuel cells (HT-PEFC) are operating at temperatures up to 200◦C using phosphoric acid (PA) doped polybenzimidazole (PBI) based membranes
This has already been reported for phosphoric acid fuel cells (PAFC) where the phosphoric acid is confined in a silicon carbide matrix[9] and has recently been been shown for cells with PBI membranes produced by the polyphosphoric acid (PPA) process.[10]
X-ray tomographic microscopy (XTM) measurements were performed in time intervals of 10–30 min to analyze the dynamics of the migration process
Summary
It has recently been observed that under high current density operation (0.8 A cm−2), HT-PEFC exhibit extensive acid migration to the anode GDL.[10]. The spatial resolution of the imaging (2.3 μm voxel size) prevents the segmentation of PA droplets in the pores of the MPL and the catalyst layer. High Pt attenuation and scattering result in unreliable segmentation of PA droplets in the cracks of MPL and catalyst layer. The different components of the MEA, membrane in the center (light gray), catalyst layers (bright gray/white), the porous GDL (gray: carbon fibers; dark gray: empty pores) and the flow field with three parallel channels, are clearly visible.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
