High temperature polymer electrolyte fuel cells are based on a polybenzimidazole membrane, imbibed with phosphoric acid as the electrolyte. Phosphoric acid shows a high degree of mobility within the membrane and, as was shown by Eberhardt et.al [1], can also invade the pores of the anode gas diffusion layer (GDL) after a current step from 0.2 Acm² to 0.8 Acm² and even invade the channel of the flow field, where it is removed from the fuel cell. The loss of electrolyte is a mayor degradation mode of HT-PEFCs and may also affect the performance of a HT-PEFC.

In recent years, synchrotron based operando x-ray tomographic microscopy of HT-PEFCs has been applied for visualizing and quantifying phosphoric acid redistribution in the GDL [1]. In the present work, this technique was used to quantify phosphoric acid saturation in the anode GDL after a current step (s. figure 1) and correlated it with operando electrochemical impedance spectroscopy measurement.

The synchrotron imaging parameters were optimized to enable the quantification of the acid redistribution in the GDL, the microporous layer and the catalyst layer. For the first time, phosphoric acid redistribution in the cracks of both, microporous and catalyst layers, was evaluated and quantified, allowing the determination of flooding time constants for the cracks in both layers. The redistribution of acid in both, anode and cathode catalyst layers, is quantified and compared.

References:

1. S.H. Eberhardt et.al., J. Electrochem. Soc., 162 (3), F310-F316