Physicochemical investigation of phosphoric acid doped poly(2,5-benzimidazole) as electrolyte membrane for fuel cells

Phosphoric acid doped polybenzimidazole membranes are a promising candidate for use as electrolyte membrane in high temperature polymer electrolyte membrane fuel cells (HT-PEFC). They present high thermal stability and excellent proton conductivity at low humidity [1]. Although polybenzimidazole membranes are routinely doped with phosphoric acid, few studies on the exact nature of the acid inside the membrane have been published. It is not known whether the phosphoric acid is present as ortho- or pyrophosphoric acid at all operating conditions and how it interacts with the membrane at a high operating temperature of 160 °C. In this study, we investigated cross-linked poly(2,5-benzimidazole) (ABPBI) membranes obtained from FuMA-Tech GmbH which were doped in 85 % phosphoric acid. Conductivity measurements of the doped membrane under dry conditions show a maximum conductivity at 160 °C. At higher temperatures, the conductivity decreases as the dehydration of the orthophosphoric acid to the less conductive pyrophosphoric acid becomes relevant. Thermogravimetric analyses of pure phosphoric acid and doped membranes help to assess the amount of dehydrated phosphoric acid as well as the content of water and phosphoric acid inside the doped membrane. The thermal signals show two distinct weight losses between 30 °C and 200 °C, whose onset varies with the doping level of the membrane. An explanation for this effect, besides limited heat transfer, could be the bonds formed between the phosphoric acid molecules and the imidazole rings of the polymer. While free phosphoric acid molecules, found in pure phosphoric acid or in highly doped membranes, are free to condensate to pyrophosphoric acid molecules as soon as the activation energy is reached, bonded molecules need more energy to first break the bond and then dehydrate. To further consolidate these results, the structure of the as-received and phosphoric acid doped membranes were analyzed and compared by means of Raman spectroscopy [2]. They confirm the presence of hydrogen bonds between the phosphoric acid and the imidazole group of the membrane, thus supporting the results obtained by thermogravimetric analysis. [1] Q. Li, J. O. Jensen, R. F. Savinell, and N. J. Bjerrum, Progress in Polymer Science, 34, 449 (2009) [2] F. Conti, A. Majerus, V. Di Noto, C. Korte, W. Lehnert, and D. Stolten, Phys. Chem. Chem. Phys., 14, 10022 (2012) SUMMARY We present a physicochemical investigation of phosphoric acid doped polybenzimidazole membranes, including conductivity measurements, thermogravimetric analysis and Raman spectroscopy.