The water vapor equilibrium in the phosphoric acid – water system

摘要

For high temperature polymer electrolyte fuel cells (HT-PEFCs), phosphoric acid (PA, H3PO4) is still the state-of-the-art electrolyte material, since it shows excellent proton conductivity [1] in the temperature range between 373 K and 473 K. Another advantage is the chemical compatibility with polymers which is exceptional considering the acidity of the acid (pH < 0 unless highly diluted). However, care has to be taken that strongly dehydrating conditions have to be avoided when using PA in HT-PEFCs, since enhanced degradation of the catalyst [2] appears for these conditions. This can be explained with the formation of PA condensates [3], which are formed at low water contents and high temperatures, typically above 390 K. It is therefore crucial to avoid the formation of PA condensates by adjusting the operating conditions of a HT-PEFC. In this work, PA is exposed at a constant water vapor pressure of 10 kPa until equilibration is reached at different temperatures in the range between 333 K and 433 K. The equilibration time and the evolution of the electrical parameters during the equilibration process are discussed with respect to the water content at the end of the tests. The results not only show the formation of PA condensates for temperatures above 413 K, but also indicate the existence of a local PA hemihydrate structure occurring in the liquid before condensation occurs.