Investigations on the H3PO4-Uptake of Polybenzimidazole type Polymers using RAMAN Spectroscopy — Correlations between Adsorption Process and Electrolyte - Polymer Molecular Interactions

An important application of phosphoric acid doped polybenzimidazol (PBI) is the use as proton conducting electrolyte membrane in high temperature polymer electrolyte fuel cells (HT-PEFC). At typical operation temperatures of 120 to 200 °C and very low humidity ionic conductivities of 10-1 to 10-2 S cm-1 can be measured. The H3PO4-doping process of different PBI-type materials were still investigated in se¬ve¬ral experimental studies, e.g. [1,2]. Unfortunately, no general applicable model for the kinetic of the adsorption process available which is able to describe the whole acces¬sible range of doping degrees has been published yet. The correlation between the interactions between H3PO4 molecules and polymer chains, the ad-sorp¬tion iso¬therm as well as the polycondensation equilibria of H3PO4 and the corresponding implications on the pro¬ton conductivity are also not finally illuminated. We have investigated the ad¬sorption process of H3PO4 on a com¬mer¬cial cross-linked PBI deri¬vative (Fuma¬pem AM-55). A num¬ber of mem¬bra¬nes has been pre¬pared at dif¬fe¬rent do¬ping levels and analysed to elucidate the ad¬sorp¬tion process of H3PO4 as func¬tion of temperature and con¬cen¬tra¬tion. Karl-Fischer-, pH-titration and RAMAN spec¬tro¬scopy are used to cha¬rac¬terise the mem¬branes [3,4]. The ad¬sorp¬tion equi¬li¬bria of the up¬take pro¬cess have been ana¬ly¬sed with dif¬fe¬rent ki¬ne¬tic mo¬dels for own and for lite¬ra¬ture data on non-cross¬lin¬ked m-PBI and AB-PBI. The be¬ha¬viour of all PBI-type poly¬mers can be de¬scri¬bed sa¬tis¬fac¬torily with a BET-like ad¬sorp¬tion iso¬therm. Using the RAMAN data, re-gions in the isotherm can be cor¬re¬la¬ted to the pro¬tonation of the poly¬mer chains, for¬ma¬tion of H-bonds di¬rec¬tly to the chains and to still adsorbed H3PO4 mole¬cules. [1] Q. Li et al., Solid State Ionics, 2004, 168, 177-185 [2] J. A. Asensio et al., Chem. Soc. Rev., 2010, 39, 3210-3239 [3] F. Conti et al., submitted to Fuel Cells, Jan. 2014 [4] C. Korte et al., to be submitted 2014