Abstract
Synchrotron based X-ray tomographic microscopy (XTM) is used for imaging and quantifying the redistribution of phosphoric acid (PA) in high temperature polymer electrolyte fuel cells (HT-PEFC) in-operando. The main focus of this work is the redistribution of phosphoric acid under dynamic load conditions. Therefore, two different load cycling protocols were applied and the transient redistribution within the fuel cell components was imaged. XTM, for the first time, revealed that the examined PBI based membrane system exhibits extensive electrolyte migration from cathode to anode under high current density operation. PA flooding of anode gas diffusion layer (GDL) and flow field channels occurred. Implications for technical applications and fuel cell degradation are discussed. Quantification of the migrated electrolyte is made by correlating in-operando grayscale values to ex-situ reference samples.
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 fuel cell technology has long been in the focus of research[1] due to advantages based on the higher operation temperature as compared to standard PEFCs, operating between 60–90◦C
A process which is well known from earlier work on phosphoric acid fuel cells (PAFC), where PA imbibed in a porous silicon carbide separator remains mobile
This section is divided into a presentation of the imaging results of electrolyte redistribution for two current cycling protocols followed by a quantification of redistributed PA
Summary
24 24 24 24 electrodes consist of Pt/Vulcan XC-72 supported platinum catalyst with a loading of 1 mgPtcm−2 on anode and cathode, respectively, coated onto SGL 38 carbon paper gas diffusion layers (GDL) including a micro-porous layer (MPL). First a median filter (2D, 3px) was applied, and a grayscale threshold segmentation window was applied once to obtain the carbon fibers/binder structure of the GDL and once to obtain the phosphoric acid phase. A calibration experiment was performed where MEAs doped with 40, 60, 85 and 100 wt% H3PO4 were imaged at room temperature In these images the grayscale value of the phosphoric acid phase (without carbon) was analyzed. Based on this information and a correction for the temperature dependent PA density the composition of the PA electrolyte in the in-operando experiment can be determined
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