High temperature polymer electrolyte membrane (HT-PEM) fuel cells typically work at 120-200\ub0C and are mainly based on phosphoric acid (PA) swollen basic polymer membranes like phosphoric acid doped polybenzimidazole. An overview can be found in [1-7]. Although PA is a widely used material even outside the field of electrochemical transformers, only little is known about the thermodynamical but also physical properties at temperatures above 100\ub0C. Especially the correlation between different parameters provided from different sources in literature is often demanding due to different experimental approaches.
In this work, an alternative approach is used simulating directly the conditions inside an operational HT-PEM. The obtained physical and thermodynamic data are critically compared to results from other laboratories
