Water sorption of poly(vinylphosphonic acid) and its influence on proton conductivity

Proton conductors based on phosphoric or phosphonic acid are considered very promising, due to their high charge carrier concentration, thermal stability and oxidation resistivity of the protogenic group. In the present work we have investigated well-characterized poly(vinylphosphonic acid) (PVPA). The material is hygroscopic, and we have studied the water content, self-condensation and proton conductivity as a function of relative humidity of air (RH). Self-condensation was determined by (31)P NMR. Our data indicate that the RH determines both: the equilibrium ratio between condensed and non-condensed phosphonic OH-groups and the water content. At low RH, condensation and water coexist in the sample. RH exceeding 40% is needed to avoid self-condensation. This corresponds to a water sorption of similar to 15 wt-% i.e.similar to 0.8 water molecules per phosphonic acid group; at higher humidity PVPA swells. Above 100 degrees C, self-condensation increases as water evaporates from the sample. This leads to condensation ratios of approximately 40% after 5 h under nitrogen atmosphere at 150 degrees C. Even under water vapor atmosphere, condensation ratios exceeding 30% were found at 150 degrees C. We proved that self-condensation is a reversible process up to temperatures as high as 250 degrees C. The proton conductivity was measured as a function of humidity and temperature under dry, humid and water vapor atmosphere. (C) 2007 Elsevier B.V. All rights reserved.